ISO (the International Organization for Standardization) and IEC (the International Electrotechnical Commission) form the specialized system for worldwide standardization. National bodies that are member of ISO or IEC participate in the development of International Standards through technical committees established by the respective organization to deal with particular fields of technical activity. ISO and IEC technical committees collaborate in fields of mutual interest. Other international organizations, governmental and non- governmental, in liaison with ISO and IEC, also take part in the work.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.

In the field of information technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1. Draft International Standards adopted by the joint technical committee are circulated to national bodies for voting. Publication as an International Standard requires approval by at least 75% of the national bodies casting a vote.

International Standard ISO/IEC 9899 was prepared by Joint Technical Committee ISO/IEC JTC 1, ``Information Technology'', subcommittee 22, ``Programming languages, their environments and system software interfaces''. The Working Group responsible for this standard (WG14) maintains a site on the World Wide Web at http://www.dkuug.dk/JTC1/SC22/WG14/ containing additional information relevant to this standard such as a Rationale for many of the decisions made during its preparation and a log of Defect Reports and Responses.

This edition replaces the previous edition, ISO/IEC 9899:1990, as amended and corrected by ISO/IEC 9899/COR1:1994, ISO/IEC 9899/COR2:1995, and ISO/IEC 9899/AMD1:1995. Major changes from the previous edition include:

-- restricted character set support in <iso646.h> (originally specified in AMD1)

-- wide-character library support in <wchar.h> and <wctype.h> (originally specified in AMD1) -- restricted pointers

-- variable-length arrays

-- flexible array members

-- complex (and imaginary) support in <complex.h>

-- type-generic math macros in <tgmath.h>

-- the long long int type and library functions

-- increased translation limits

-- remove implicit int

-- the vscanf family of functions

-- reliable integer division

-- universal character names

-- extended identifiers

-- binary floating-point literals and printf/scanf conversion specifiers

-- compound literals

-- designated initializers

-- // comments

-- extended integer types in <inttypes.h> and <stdint.h>

-- remove implicit function declaration

-- preprocessor arithmetic done in intmax_t/uintmax_t

-- mixed declarations and code

-- integer constant type rules

-- integer promotion rules

-- vararg macros

-- additional math library functions in <math.h>

-- floating-point environment access in <fenv.h>

-- IEC 60559 (also known as IEC 559 or IEEE arithmetic) support -- trailing comma allowed in enum declaration

-- %lf conversion specifier allowed in printf

-- inline functions

-- the snprintf family of functions

-- boolean type in <stdbool.h>

-- idempotent type qualifiers

-- empty macro arguments

-- new struct type compatibility rules (tag compatibility)

-- _Prama preprocessing operator

-- standard pragmas

-- __func__ predefined identifier

-- VA_COPY macro

-- additional strftime conversion specifiers

-- LIA compatibility annex

-- deprecate ungetc at the beginning of a binary file

-- remove deprecation of aliased array parameters

Annexes D and F form a normative part of this standard; annexes A, B, C, E, G, H, I, J, the bibliography, and the index are for information only. In accordance with the ISO/IEC Directives, Part 3, this foreword, the introduction, notes, footnotes, and examples are for information only. Introduction

With the introduction of new devices and extended character sets, new features may be added to this International Standard. Subclauses in the language and library clauses warn implementors and programmers of usages which, though valid in themselves, may conflict with future additions.

Light editing:

With the introduction of new devices and extended character sets, new features may be added to this International Standard. Subclauses in the language and library clauses warn implementors and programmers of usages THAT, though valid NOW, may conflict with future additions.

Medium editing:

With the introduction of new devices and extended character sets, new features may be added to this International Standard. Subclauses in the language and library clauses warn implementors and programmers of usages that, though valid now, may conflict with future additions.

I rely on William Strunk and E. B. White, "The Elements of Style," MacMillan, 1979. That idiosyncratic and opinionated book works for me /just because/ of the idiosyncracies and opinions of White and his teacher. Some think White one of the contemporary masters of prose style in English. S&W, like K&R1/2, gains much of its weight from its slimness.

(1) "that" vs. "which" S&W p.59: "That" is the defining, or restrictive pronoun, "which" the nondefining, or non-restrictive. See Rule 3.

The lawn mower that is broken is in the garage. (Tells which one)

The lawn mower, which is broken, is in the garage. (Adds a fact about the only lawn mower in question)

The use of which for that is common in written and spoken language ("Let us now go even unto Bethlehem, and see this thing which is come to pass.") Occasionally which seems preferable to that, as in the sentence from the Bible. But it would be a convenience to all if these two pronouns were used with precision. The careful writer, watchful for small conveniences, goes which- hunting, removes the defining whiches, and by so doing improves his work.

Certain features are obsolescent, which means that they may be considered for withdrawal in future revisions of this International Standard. They are retained because of their widespread use, but their use in new implementations (for implementation features) or new programs (for language [6.11] or library features [7.26]) is discouraged.

This International Standard is divided into four major subdivisions:

-- the introduction and preliminary elements (clauses 1-4);

-- the characteristics of environments that translate and execute C programs (clause 5);

-- the language syntax, constraints, and semantics (clause 6);

-- the library facilities (clause 7).

Examples are provided to illustrate possible forms of the constructions described. Footnotes are provided to emphasize consequences of the rules described in that subclause or elsewhere in this International Standard. References are used to refer to other related subclauses. Recommendations are provided to give advice or guidance to implementors. Annexes provide additional information and summarize the information contained in this International Standard. A bibliography lists documents that were referred to during the preparation of the standard.

The language clause (clause 6) is derived from ``The C Reference Manual''.

The library clause (clause 7) is based on the 1984 /usr/group Standard.

1. Scope

This International Standard specifies the form and establishes the interpretation of programs written in the C programming language.1) It specifies

-- the representation of C programs;

-- the syntax and constraints of the C language;

-- the semantic rules for interpreting C programs;

-- the representation of input data to be processed by C programs;

-- the representation of output data produced by C programs;

-- the restrictions and limits imposed by a conforming implementation of C.

This International Standard does not specify

-- the mechanism by which C programs are transformed for use by a data-processing system;

-- the mechanism by which C programs are invoked for use by a data-processing system;

-- the mechanism by which input data are transformed for use by a C program;

-- the mechanism by which output data are transformed after being produced by a C program;

-- the size or complexity of a program and its data that will exceed the capacity of any specific data- processing system or the capacity of a particular processor;

-- all minimal requirements of a data-processing system that is capable of supporting a conforming implementation.

2. Normative references

The following normative documents contain provisions which, through reference in this text, constitute provisions of this International Standard. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. However, parties to agreements based on this International Standard are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. For undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC maintain registers of currently valid International Standards.

ISO/IEC 646:1991, Information technology -- ISO 7-bit coded character set for information interchange.

ISO/IEC 2382-1:1993, Information technology -- Vocabulary -- Part 1: Fundamental terms.

ISO 4217:1995, Codes for the representation of currencies and funds.

ISO 8601:1988, Data elements and interchange formats -- Information interchange -- Representation of dates and times.

ISO/IEC 10646:1993, Information technology -- Universal Multiple-Octet Coded Character Set (UCS).

IEC 60559:1989, Binary floating-point arithmetic for microprocessor systems, second edition (previously designated IEC 559:1989).

3. Terms and definitions

For the purposes of this International Standard, the following definitions apply. Other terms are defined where they appear in italic type or on the left side of a syntax rule. Terms explicitly defined in this International Standard are not to be presumed to refer implicitly to similar terms defined elsewhere. Terms not defined in this International Standard are to be interpreted according to ISO/IEC 2382-1.

3.1

alignment requirement that objects of a particular type be located on storage boundaries with addresses that are particular multiples of a byte address

3.2

argument actual argument actual parameter (deprecated) expression in the comma-separated list bounded by the parentheses in a function call expression, or a sequence of preprocessing tokens in the comma-separated list bounded by the parentheses in a function-like macro invocation

3.3

bit unit of data storage in the execution environment large enough to hold an object that may have one of two values

NOTE It need not be possible to express the address of each individual bit of an object.

3.4

byte addressable unit of data storage large enough to hold any member of the basic character set of the execution environment

NOTE 1 It is possible to express the address of each individual byte of an object uniquely.

NOTE 2 A byte is composed of a contiguous sequence of bits, the number of which is implementation-defined. The least significant bit is called the low-order bit; the most significant bit is called the high-order bit.

3.5

character bit representation that fits in a byte

3.6

constraints restrictions, both syntactic and semantic, by which the exposition of language elements is to be interpreted

3.7

correctly rounded result a representation in the result format that is nearest in value, subject to the effective rounding mode, to what the result would be given unlimited range and precision

3.8

diagnostic message message belonging to an implementation-defined subset of the implementation's message output

3.9

forward references references to later subclauses of this International Standard that contain additional information relevant to this subclause

3.10

implementation a particular set of software, running in a particular translation environment under particular control options, that performs translation of programs for, and supports execution of functions in, a particular execution environment

3.11

implementation-defined behavior unspecified behavior where each implementation documents how the choice is made

EXAMPLE An example of implementation-defined behavior is the propagation of the high-order bit when a signed integer is shifted right.

3.12

implementation limits restrictions imposed upon programs by the implementation

3.13

locale-specific behavior behavior that depends on local conventions of nationality, culture, and language that each implementation documents

EXAMPLE An example of locale-specific behavior is whether the islower function returns true for characters other than the 26 lowercase Latin letters.

3.14

multibyte character sequence of one or more bytes representing a member of the extended character set of either the source or the execution environment

NOTE The extended character set is a superset of the basic character set.

3.15

object region of data storage in the execution environment, the contents of which can represent values

NOTE When referenced, an object may be interpreted as having a particular type; see 6.3.2.1.

3.16

parameter formal parameter formal argument (deprecated) object declared as part of a function declaration or definition that acquires a value on entry to the function, or an identifier from the comma-separated list bounded by the parentheses immediately following the macro name in a function-like macro definition

3.17

recommended practice specifications that are strongly recommended as being in keeping with the intent of the standard, but that may be impractical for some implementations

3.18

undefined behavior behavior, upon use of a nonportable or erroneous program construct, of erroneous data, or of indeterminately valued objects, for which this International Standard imposes no requirements

NOTE Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).

EXAMPLE An example of undefined behavior is the behavior on integer overflow.

3.19

unspecified behavior behavior where this International Standard provides two or more possibilities and imposes no requirements on which is chosen in any instance

EXAMPLE An example of unspecified behavior is the order in which the arguments to a function are evaluated.

Forward references: bitwise shift operators (6.5.7), expressions (6.5), function calls (6.5.2.2), the islower function (7.4.1.6), localization (7.11).

4. Conformance

In this International Standard, ``shall'' is to be interpreted as a requirement on an implementation or on a program; conversely, ``shall not'' is to be interpreted as a prohibition.

If a ``shall'' or ``shall not'' requirement that appears outside of a constraint is violated, the behavior is undefined. Undefined behavior is otherwise indicated in this International Standard by the words ``undefined behavior'' or by the omission of any explicit definition of behavior. There is no difference in emphasis among these three; they all describe ``behavior that is undefined''.

A program that is correct in all other aspects, operating on correct data, containing unspecified behavior shall be a correct program and act in accordance with 5.1.2.3.

The implementation shall not successfully translate a preprocessing translation unit containing a #error preprocessing directive unless it is part of a group skipped by conditional inclusion.

A strictly conforming program shall use only those features of the language and library specified in this International Standard.2) It shall not produce output dependent on any unspecified, undefined, or implementation- defined behavior, and shall not exceed any minimum implementation limit.

The two forms of conforming implementation are hosted and freestanding. A conforming hosted implementation shall accept any strictly conforming program. A conforming freestanding implementation shall accept any strictly conforming program that does not use complex types and in which the use of the features specified in the library clause (clause 7) is confined to the contents of the standard headers <float.h>, <iso646.h>, <limits.h>, <stdarg.h>, <stdbool.h>, <stddef.h>, and <stdint.h>. A conforming implementation may have extensions (including additional library functions), provided they do not alter

the behavior of any strictly conforming program.3)

A conforming program is one that is acceptable to a conforming implementation.4)

An implementation shall be accompanied by a document that defines all implementation-defined and locale-specific characteristics and all extensions.

Forward references: conditional inclusion (6.10.1), characteristics of floating types <float.h> (7.7), alternative spellings <iso646.h> (7.9), sizes of integer types <limits.h> (7.10), variable arguments <stdarg.h> (7.15), boolean type and values <stdbool.h> (7.16), common definitions <stddef.h> (7.17), integer types <stdint.h> (7.18).

5. Environment

An implementation translates C source files and executes C programs in two data-processing-system environments, which will be called the translation environment and the execution environment in this International Standard. Their characteristics define and constrain the results of executing conforming C programs constructed according to the syntactic and semantic rules for conforming implementations.

Forward references: In this clause, only a few of many possible forward references have been noted.

5.1 Conceptual models

5.1.1 Translation environment

5.1.1.1 Program structure

A C program need not all be translated at the same time. The text of the program is kept in units called source files, (or preprocessing files) in this International Standard. A source file together with all the headers and source files included via the preprocessing directive #include is known as a preprocessing translation unit. After preprocessing, a preprocessing translation unit is called a translation unit. Previously translated translation units may be preserved individually or in libraries. The separate translation units of a program communicate by (for example) calls to functions whose identifiers have external linkage, manipulation of objects whose identifiers have external linkage, or manipulation of data files. Translation units may be separately translated and then later linked to produce an executable program.

Forward references: conditional inclusion (6.10.1), linkages of identifiers (6.2.2), source file inclusion (6.10.2), external definitions (6.9), preprocessing directives (6.10).

5.1.1.2 Translation phases

The precedence among the syntax rules of translation is specified by the following phases.5)

1. Physical source file multibyte characters are mapped to the source character set (introducing new-line characters for end-of-line indicators) if necessary.

Trigraph sequences are replaced by corresponding single-character internal representations.

2. Each instance of a backslash character (\) immediately followed by a new-line character is deleted, splicing physical source lines to form logical source lines. If, as a result, a character sequence that matches the syntax of a universal character name is produced, the behavior is undefined. Only the last backslash on any physical source line shall be eligible for being part of such a splice. A source file that is not empty shall end in a new-line character, which shall not be immediately preceded by a backslash character before any such splicing takes place.

3. The source file is decomposed into preprocessing tokens6) and sequences of white-space characters (including comments). A source file shall not end in a partial preprocessing token or in a partial comment. Each comment is replaced by one space character. New- line characters are retained. Whether each nonempty sequence of white-space characters other than new-line is retained or replaced by one space character is implementation-defined.

4. Preprocessing directives are executed, macro invocations are expanded, and _Pragma unary operator expressions are executed. If a character sequence that matches the syntax of a universal character name is produced by token concatenation (6.10.3.3), the behavior is undefined. A #include preprocessing directive causes the named header or source file to be processed from phase 1 through phase 4, recursively. All preprocessing directives are then deleted.

5. Each source character set member and escape sequence in character constants and string literals is converted to the corresponding member of the execution character set; if there is no corresponding member, it is converted to an implementation-defined member.

6. Adjacent string literal tokens are concatenated.

7. White-space characters separating tokens are no longer significant. Each preprocessing token is converted into a token. The resulting tokens are syntactically and semantically analyzed and translated as a translation unit.

8. All external object and function references are resolved. Library components are linked to satisfy external references to functions and objects not defined in the current translation. All such translator output is collected into a program image which contains information needed for execution in its execution environment.

Forward references: universal character names (6.4.3), lexical elements (6.4), preprocessing directives (6.10), trigraph sequences (5.2.1.1), external definitions (6.9).

5.1.1.3 Diagnostics

A conforming implementation shall produce at least one diagnostic message (identified in an implementation-defined manner) if a preprocessing translation unit or translation unit contains a violation of any syntax rule or constraint, even if the behavior is also explicitly specified as undefined or implementation-defined. Diagnostic messages need not be produced in other circumstances.7)

EXAMPLE An implementation shall issue a diagnostic for the translation unit:

        char i;
        int i;

because in those cases where wording in this International Standard describes the behavior for a construct as being both a constraint error and resulting in undefined behavior, the constraint error shall be diagnosed.

5.1.2 Execution environments

Two execution environments are defined: freestanding and hosted. In both cases, program startup occurs when a designated C function is called by the execution environment. All objects in static storage shall be initialized (set to their initial values) before program startup. The manner and timing of such initialization are otherwise unspecified. Program termination returns control to the execution environment.

Forward references: initialization (6.7.8).

5.1.2.1 Freestanding environment

In a freestanding environment (in which C program execution may take place without any benefit of an operating system), the name and type of the function called at program startup are implementation-defined. Any library facilities available to a freestanding program, other than the minimal set required by clause 4, are implementation-defined.

The effect of program termination in a freestanding environment is implementation-defined.

5.1.2.2 Hosted environment

A hosted environment need not be provided, but shall conform to the following specifications if present.

5.1.2.2.1 Program startup

The function called at program startup is named main. The implementation declares no prototype for this function. It shall be defined with a return type of int and with no parameters:

        int main(void) { /* ... */ }

or with two parameters (referred to here as argc and argv, though any names may be used, as they are local to the function in which they are declared):

        int main(int argc, char *argv[]) { /* ... */ }

or equivalent;8) or in some other implementation-defined manner.

If they are declared, the parameters to the main function shall obey the following constraints:

-- The value of argc shall be nonnegative.

-- argv[argc] shall be a null pointer.

-- If the value of argc is greater than zero, the array members argv[0] through argv[argc-1] inclusive shall contain pointers to strings, which are given implementation-defined values by the host environment prior to program startup. The intent is to supply to

the program information determined prior to program startup from elsewhere in the hosted environment. If the host environment is not capable of supplying strings with letters in both uppercase and lowercase, the implementation shall ensure that the strings are received in lowercase.

-- If the value of argc is greater than zero, the string pointed to by argv[0] represents the program name; argv[0][0] shall be the null character if the program name is not available from the host environment. If the value of argc is greater than one, the strings pointed to by argv[1] through argv[argc-1] represent the program parameters.

-- The parameters argc and argv and the strings pointed to by the argv array shall be modifiable by the program, and retain their last-stored values between program startup and program termination.

5.1.2.2.2 Program execution

In a hosted environment, a program may use all the functions, macros, type definitions, and objects described in the library clause (clause 7).

5.1.2.2.3 Program termination

If the return type of the main function is a type compatible with int, a return from the initial call to the main function is equivalent to calling the exit function with the value returned by the main function as its argument;9) reaching the } that terminates the main function returns a value of 0. If the return type is not compatible with int, the termination status returned to the host environment is unspecified.

Forward references: definition of terms (7.1.1), the exit function (7.20.4.3).

5.1.2.3 Program execution

The semantic descriptions in this International Standard describe the behavior of an abstract machine in which issues of optimization are irrelevant.

Accessing a volatile object, modifying an object, modifying a file, or calling a function that does any of

those operations are all side effects,10) which are changes in the state of the execution environment. Evaluation of an expression may produce side effects. At certain specified points in the execution sequence called sequence points, all side effects of previous evaluations shall be complete and no side effects of subsequent evaluations shall have taken place. (A summary of the sequence points is given in annex C.)

In the abstract machine, all expressions are evaluated as specified by the semantics. An actual implementation need not evaluate part of an expression if it can deduce that its value is not used and that no needed side effects are produced (including any caused by calling a function or accessing a volatile object).

When the processing of the abstract machine is interrupted by receipt of a signal, only the values of objects as of the previous sequence point may be relied on. Objects that may be modified between the previous sequence point and the next sequence point need not have received their correct values yet.

An instance of each object with automatic storage duration is associated with each entry into its block. Such an object exists and retains its last-stored value during the execution of the block and while the block is suspended (by a call of a function or receipt of a signal).

The least requirements on a conforming implementation are:

-- At sequence points, volatile objects are stable in the sense that previous accesses are complete and subsequent accesses have not yet occurred.

-- At program termination, all data written into files shall be identical to the result that execution of the program according to the abstract semantics would have produced.

-- The input and output dynamics of interactive devices shall take place as specified in 7.19.3. The intent of these requirements is that unbuffered or line-buffered output appear as soon as possible, to ensure that prompting messages actually appear prior to a program waiting for input.

What constitutes an interactive device is implementation-defined.

More stringent correspondences between abstract and actual semantics may be defined by each implementation.

EXAMPLE 1 An implementation might define a one-to-one correspondence between abstract and actual semantics: at every sequence point, the values of the actual objects would agree with those specified by the abstract semantics. The keyword volatile would then be redundant.

#10

Alternatively, an implementation might perform various optimizations within each translation unit, such that the actual semantics would agree with the abstract semantics only when making function calls across translation unit boundaries. In such an implementation, at the time of each function entry and function return where the calling function and the called function are in different translation units, the values of all externally linked objects and of all objects accessible via pointers therein would agree with the abstract semantics. Furthermore, at the time of each such function entry the values of the parameters of the called function and of all objects accessible via pointers therein would agree with the abstract semantics. In this type of implementation, objects referred to by interrupt service routines activated by the signal function would require explicit specification of volatile storage, as well as other implementation-defined restrictions.

#11

EXAMPLE 2 In executing the fragment

        char c1, c2;
        /* ... */
        c1 = c1 + c2;

the ``integer promotions'' require that the abstract machine promote the value of each variable to int size and then add the two ints and truncate the sum. Provided the addition of two chars can be done without overflow, or with overflow wrapping silently to produce the correct result, the actual execution need only produce the same result, possibly omitting the promotions.

#12

EXAMPLE 3 Similarly, in the fragment

        float f1, f2;
        double d;
        /* ... */
        f1 = f2 * d;

the multiplication may be executed using single-precision arithmetic if the implementation can ascertain that the result would be the same as if it were executed using double-precision arithmetic (for example, if d were replaced by the constant 2.0, which has type double).

#13

EXAMPLE 4 Implementations employing wide registers have to take care to honor appropriate semantics. Values are independent of whether they are represented in a register or in memory. For example, an implicit spilling of a register is not permitted to alter the value. Also, an explicit store and load is required to round to the precision of the storage type. In particular, casts and assignments are required to perform their specified conversion. For the fragment

        double d1, d2;
        float f;
        d1 = f = expression;
        d2 = (float) expressions;

the values assigned to d1 and d2 are required to have been converted to float.

#14

EXAMPLE 5 Rearrangement for floating-point expressions is often restricted because of limitations in precision as well as range. The implementation cannot generally apply the mathematical associative rules for addition or multiplication, nor the distributive rule, because of roundoff error, even in the absence of overflow and underflow. Likewise, implementations cannot generally replace decimal constants in order to rearrange expressions. In the following fragment, rearrangements suggested by mathematical rules for real numbers are often not valid (see F.8).

        double x, y, z;
        /* ... */
        x = (x * y) * z;  // not equivalent to x *= y * z;
        z = (x - y) + y ; // not equivalent to z = x;
        z = x + x * y;    // not equivalent to z = x * (1.0 + y);
        y = x / 5.0;      // not equivalent to y = x * 0.2;

#15

EXAMPLE 6 To illustrate the grouping behavior of expressions, in the following fragment

        int a, b;
        /* ... */
        a = a + 32760 + b + 5;

the expression statement behaves exactly the same as

        a = (((a + 32760) + b) + 5);

due to the associativity and precedence of these operators. Thus, the result of the sum (a + 32760) is next added to b, and that result is then added to 5 which results in the value assigned to a. On a machine in which overflows produce an explicit trap and in which the range of values representable by an int is [-32768, +32767], the implementation cannot rewrite this expression as

        a = ((a + b) + 32765);

since if the values for a and b were, respectively, -32754 and -15, the sum a + b would produce a trap while the original expression would not; nor can the expression be rewritten either as

        a = ((a + 32765) + b);

        a = (a + (b + 32765));

since the values for a and b might have been, respectively, 4 and -8 or -17 and 12. However, on a machine in which overflow silently generates some value and where positive and negative overflows cancel, the above expression statement can be rewritten by the implementation in any of the above ways because the same result will occur.

#16

EXAMPLE 7 The grouping of an expression does not completely determine its evaluation. In the following fragment

        #include <stdio.h>
        int sum;
        char *p;
        /* ... */
        sum = sum * 10 - '0' + (*p++ = getchar());

the expression statement is grouped as if it were written as

        sum = (((sum * 10) - '0') + ((*(p++)) = (getchar())));

but the actual increment of p can occur at any time between the previous sequence point and the next sequence point (the ;), and the call to getchar can occur at any point prior to the need of its returned value.

Forward references: compound statement, or block (6.8.2), expressions (6.5), files (7.19.3), sequence points (6.5, 6.8), the signal function (7.14), type qualifiers (6.7.3).

5.2 Environmental considerations

5.2.1 Character sets

Two sets of characters and their associated collating sequences shall be defined: the set in which source files are written, and the set interpreted in the execution environment. The values of the members of the execution character set are implementation-defined; any additional members beyond those required by this subclause are locale- specific.

In a character constant or string literal, members of the execution character set shall be represented by corresponding members of the source character set or by escape sequences consisting of the backslash \ followed by one or more characters. A byte with all bits set to 0, called the null character, shall exist in the basic execution character set; it is used to terminate a character string.

Both the basic source and basic execution character sets shall have at least the following members: the 26 uppercase letters of the Latin alphabet

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

the 26 lowercase letters of the Latin alphabet

a b c d e f g h i j k l m n o p q r s t u v w x y z

the 10 decimal digits

0 1 2 3 4 5 6 7 8 9

the following 29 graphic characters

        !  "  #  %  &  '  (  )  *  +  ,  -  .  /  :
        ;  <  =  >  ?  [  \  ]  ^  _  {  |  }  ~

the space character, and control characters representing horizontal tab, vertical tab, and form feed. The representation of each member of the source and execution basic character sets shall fit in a byte. In both the source and execution basic character sets, the value of each character after 0 in the above list of decimal digits shall be one greater than the value of the previous. In source files, there shall be some way of indicating the end of each line of text; this International Standard treats such an end-of-line indicator as if it were a single new-line character. In the execution character set, there shall be control characters representing alert, backspace, carriage return, and new line. If any other characters are encountered in a source file (except in an identifier, a character constant, a string literal, a header name, a comment, or a preprocessing token that is never converted to a token), the behavior is undefined.

The universal character name construct provides a way to name other characters.

Forward references: universal character names (6.4.3), character constants (6.4.4.4), preprocessing directives (6.10), string literals (6.4.5), comments (6.4.9), string (7.1.1).

5.2.1.1 Trigraph sequences

All occurrences in a source file of the following sequences of three characters (called trigraph sequences11)) are replaced with the corresponding single character.

         ??=     #         ??)     ]         ??!     |
         ??(     [         ??'     ^         ??>     }
         ??/     \         ??<     {         ??-     ~

No other trigraph sequences exist. Each ? that does not begin one of the trigraphs listed above is not changed.

EXAMPLE The following source line

        printf("Eh???/n");

becomes (after replacement of the trigraph sequence ??/)

        printf("Eh?\n");

5.2.1.2 Multibyte characters

The source character set may contain multibyte characters, used to represent members of the extended character set. The execution character set may also contain multibyte characters, which need not have the same encoding as for the source character set. For both character sets, the following shall hold:

-- The single-byte characters defined in 5.2.1 shall be present.

-- The presence, meaning, and representation of any additional members is locale-specific.

-- A multibyte character set may have a state-dependent encoding, wherein each sequence of multibyte characters begins in an initial shift state and enters other locale-specific shift states when specific multibyte characters are encountered in the sequence. While in the initial shift state, all single-byte characters retain their usual interpretation and do not alter the shift state. The interpretation for subsequent bytes in the sequence is a function of the current shift state.

-- A byte with all bits zero shall be interpreted as a null character independent of shift state.

-- A byte with all bits zero shall not occur in the second or subsequent bytes of a multibyte character.

For source files, the following shall hold:

-- An identifier, comment, string literal, character constant, or header name shall begin and end in the initial shift state.

-- An identifier, comment, string literal, character constant, or header name shall consist of a sequence of valid multibyte characters.

5.2.2 Character display semantics

The active position is that location on a display device where the next character output by the fputc or fputwc function would appear. The intent of writing a printing character (as defined by the isprint or iswprint function) to a display device is to display a graphic representation of that character at the active position and then advance the active position to the next position on the current line. The direction of writing is locale-specific. If the active position is at the final position of a line (if there is one), the behavior is unspecified.

Alphabetic escape sequences representing nongraphic characters in the execution character set are intended to produce actions on display devices as follows:

\a (alert) Produces an audible or visible alert. The active position shall not be changed.

\b (backspace) Moves the active position to the previous position on the current line. If the active position is at the initial position of a line, the behavior is unspecified. \f (form feed) Moves the active position to the initial position at the start of the next logical page.

\n (new line) Moves the active position to the initial position of the next line.

\r (carriage return) Moves the active position to the initial position of the current line.

\t (horizontal tab) Moves the active position to the next horizontal tabulation position on the current line. If the active position is at or past the last defined horizontal tabulation position, the behavior is unspecified.

\v (vertical tab) Moves the active position to the initial position of the next vertical tabulation position. If the active position is at or past the last defined vertical tabulation position, the behavior is unspecified.

Each of these escape sequences shall produce a unique implementation-defined value which can be stored in a single char object. The external representations in a text file need not be identical to the internal representations, and are outside the scope of this International Standard.

Forward references: the isprint function (7.4.1.7), the fputc function (7.19.7.3), the fputwc functions (7.24.3.3), the iswprint function (7.25.2.1.7).

5.2.3 Signals and interrupts

Functions shall be implemented such that they may be interrupted at any time by a signal, or may be called by a signal handler, or both, with no alteration to earlier, but still active, invocations' control flow (after the interruption), function return values, or objects with automatic storage duration. All such objects shall be maintained outside the function image (the instructions that compose the executable representation of a function) on a per-invocation basis.

5.2.4 Environmental limits

Both the translation and execution environments constrain the implementation of language translators and libraries. The following summarizes the language-related environmental limits on a conforming implementation; the library-related limits are discussed in clause 7.

5.2.4.1 Translation limits

The implementation shall be able to translate and execute at least one program that contains at least one instance of every one of the following limits:12)

-- 127 nesting levels of blocks

-- 63 nesting levels of conditional inclusion

-- 12 pointer, array, and function declarators (in any combinations) modifying an arithmetic, structure, union, or incomplete type in a declaration

-- 63 nesting levels of parenthesized declarators within a full declarator

-- 63 nesting levels of parenthesized expressions within a full expression

-- 63 significant initial characters in an internal identifier or a macro name (each universal character name or extended source character is considered a single character)

-- 31 significant initial characters in an external identifier (each universal character name specifying a character short identifier of 0000FFFF or less is considered 6 characters, each universal character name specifying a character short identifier of 00010000 or more is considered 10 characters, and each extended source character is considered the same number of characters as the corresponding universal character name, if any)

-- 4095 external identifiers in one translation unit

-- 511 identifiers with block scope declared in one block

-- 4095 macro identifiers simultaneously defined in one preprocessing translation unit

-- 127 parameters in one function definition

-- 127 arguments in one function call

-- 127 parameters in one macro definition

-- 127 arguments in one macro invocation

-- 4095 characters in a logical source line

-- 4095 characters in a character string literal or wide string literal (after concatenation)

-- 65535 bytes in an object (in a hosted environment only)

-- 15 nesting levels for #included files

-- 1023 case labels for a switch statement (excluding those for any nested switch statements)

-- 1023 members in a single structure or union

-- 1023 enumeration constants in a single enumeration

-- 63 levels of nested structure or union definitions in a single struct-declaration-list

5.2.4.2 Numerical limits

A conforming implementation shall document all the limits specified in this subclause, which are specified in the headers <limits.h> and <float.h>. Additional limits are specified in <stdint.h>.

5.2.4.2.1 Sizes of integer types <limits.h>

The values given below shall be replaced by constant expressions suitable for use in #if preprocessing directives. Moreover, except for CHAR_BIT and MB_LEN_MAX, the following shall be replaced by expressions that have the same type as would an expression that is an object of the corresponding type converted according to the integer promotions. Their implementation-defined values shall be equal or greater in magnitude (absolute value) to those shown, with the same sign.

-- number of bits for smallest object that is not a bit- field (byte) CHAR_BIT 8

-- minimum value for an object of type signed char SCHAR_MIN -127 // -(27-1)

-- maximum value for an object of type signed char SCHAR_MAX +127 // 27-1

-- maximum value for an object of type unsigned char UCHAR_MAX 255 // 28-1

-- minimum value for an object of type char CHAR_MIN see below -- maximum value for an object of type char CHAR_MAX see below

-- maximum number of bytes in a multibyte character, for any supported locale MB_LEN_MAX 1

-- minimum value for an object of type short int SHRT_MIN -32767 // -(215-1)

-- maximum value for an object of type short int SHRT_MAX +32767 // 215-1

-- maximum value for an object of type unsigned short int USHRT_MAX 65535 // 216-1

-- minimum value for an object of type int INT_MIN -32767 // -(215-1)

-- maximum value for an object of type int INT_MAX +32767 // 215-1

-- maximum value for an object of type unsigned int UINT_MAX 65535 // 216-1

-- minimum value for an object of type long int LONG_MIN -2147483647 // -(231-1)

-- maximum value for an object of type long int LONG_MAX +2147483647 // 231-1

-- maximum value for an object of type unsigned long int ULONG_MAX 4294967295 // 232-1

-- minimum value for an object of type long long int LLONG_MIN -9223372036854775807 // -(263-1)

-- maximum value for an object of type long long int LLONG_MAX +9223372036854775807 // 263-1

-- maximum value for an object of type unsigned long long int ULLONG_MAX 18446744073709551615 // 264-1

If the value of an object of type char is treated as a signed integer when used in an expression, the value of CHAR_MIN shall be the same as that of SCHAR_MIN and the value of CHAR_MAX shall be the same as that of SCHAR_MAX. Otherwise, the value of CHAR_MIN shall be 0 and the value of CHAR_MAX shall be the same as that of UCHAR_MAX.13) The value UCHAR_MAX+1 shall equal 2 raised to the power

CHAR_BIT.

5.2.4.2.2 Characteristics of floating types <float.h>

The characteristics of floating types are defined in terms of a model that describes a representation of floating-point numbers and values that provide information about an implementation's floating-point arithmetic.14) The following parameters are used to define the model for each floating-point type:

s sign (+-1) b base or radix of exponent representation (an integer > 1) e exponent (an integer between a minimum emin and a maximum emax) p precision (the number of base-b digits in the significand) fk nonnegative integers less than b (the significand digits)

A normalized floating-point number x (f1 > 0 if x != 0) is defined by the following model:

x=s|be|k=1fk|b-k,emin<=e<=emax

Floating types may include values that are not normalized floating-point numbers, for example subnormal floating-point numbers (x!=0,e=emin,f1=0), infinities, and NaNs.15) A NaN is an encoding signifying Not-a-Number. A quiet NaN propagates through almost every arithmetic operation without raising an exception; a signaling NaN generally raises an exception when occurring as an arithmetic operand.16)

The accuracy of the floating-point operations (+, -, *, /) and of the library functions in <math.h> and <complex.h> that return floating-point results is implementation defined. The implementation may state that the accuracy is unknown.

All integer values in the <float.h> header, except FLT_ROUNDS, shall be constant expressions suitable for use in #if preprocessing directives; all floating values shall

be constant expressions. All except DECIMAL_DIG, FLT_EVAL_METHOD, FLT_RADIX, and FLT_ROUNDS have separate names for all three floating-point types. The floating- point model representation is provided for all values except FLT_EVAL_METHOD and FLT_ROUNDS.

The rounding mode for floating-point addition is characterized by the value of FLT_ROUNDS:17)

-1 indeterminable 0 toward zero 1 to nearest 2 toward positive infinity 3 toward negative infinity

All other values for FLT_ROUNDS characterize implementation- defined rounding behavior.

The values of operations with floating operands and values subject to the usual arithmetic conversions and of floating constants are evaluated to a format whose range and precision may be greater than required by the type. The use of evaluation formats is characterized by the value of FLT_EVAL_METHOD:18)

-1 indeterminable;

0 evaluate all operations and constants just to the range and precision of the type;

1 evaluate operations and constants of type float and double to the range and precision of the double type, evaluate long double operations and constants to the range and precision of the long double type;

2 evaluate all operations and constants to the range and precision of the long double type. All other negative values for FLT_EVAL_METHOD characterize implementation-defined behavior.

The values given in the following list shall be

replaced by implementation-defined constant expressions with values that are greater or equal in magnitude (absolute value) to those shown, with the same sign:

-- radix of exponent representation, b FLT_RADIX 2

-- number of base-FLT_RADIX digits in the floating-point significand, p

FLT_MANT_DIG DBL_MANT_DIG LDBL_MANT_DIG

-- number of decimal digits, n, such that any floating- point number in the widest supported floating type with pmax radix b digits can be rounded to a floating-point number with n decimal digits and back again without changpmax|log10blueif b is a power of 10

         |1+pmax|log10b|otherwise

DECIMAL_DIG 10

-- number of decimal digits, q, such that any floating- point number with q decimal digits can be rounded into a floating-point number with p radix b digits and back again without change to the q decimal digits, p|log10b if b is a power of 10

         |(p-1)|log10b|otherwise

FLT_DIG 6 DBL_DIG 10 LDBL_DIG 10

-- minimum negative integer such that FLT_RADIX raised to one less than that power is a normalized floating-point number, emin

FLT_MIN_EXP DBL_MIN_EXP LDBL_MIN_EXP

-- minimum negative integer such that 10 raised to that power is in the range of normalized floating-point numbers, |log10bemin-1|

FLT_MIN_10_EXP -37 DBL_MIN_10_EXP -37 LDBL_MIN_10_EXP -37

-- maximum integer such that FLT_RADIX raised to one less than that power is a representable finite floating- point number, emax

FLT_MAX_EXP DBL_MAX_EXP LDBL_MAX_EXP

-- maximum integer such that 10 raised to that power is in the range of representable finite floating-point numbers, |log10((1-b-p)|bemax)| FLT_MAX_10_EXP +37 DBL_MAX_10_EXP +37 LDBL_MAX_10_EXP +37

The values given in the following list shall be replaced by implementation-defined constant expressions with values that are greater than or equal to those shown:

-- maximum representable finite floating-point number, (1-b-p)|bemax

FLT_MAX 1E+37 DBL_MAX 1E+37 LDBL_MAX 1E+37

#10

The values given in the following list shall be replaced by implementation-defined constant expressions with (positive) values that are less than or equal to those shown:

-- the difference between 1 and the least value greater than 1 that is representable in the given floating point type, b1-p

FLT_EPSILON 1E-5 DBL_EPSILON 1E-9 LDBL_EPSILON 1E-9

-- minimum normalized positive floating-point number, bemin-1

FLT_MIN 1E-37 DBL_MIN 1E-37 LDBL_MIN 1E-37

#11

EXAMPLE 1 The following describes an artificial floating-point representation that meets the minimum requirements of this International Standard, and the appropriate values in a <float.h> header for type float:

x=s|16e|k=1fk|16-k,-31<=e<=+32

FLT_RADIX 16 FLT_MANT_DIG 6 FLT_EPSILON 9.53674316E-07F FLT_DIG 6 FLT_MIN_EXP -31 FLT_MIN 2.93873588E-39F FLT_MIN_10_EXP -38 FLT_MAX_EXP +32 FLT_MAX 3.40282347E+38F FLT_MAX_10_EXP +38

#12

EXAMPLE 2 The following describes floating-point representations that also meet the requirements for single- precision and double-precision normalized numbers in IEC 60559,19) and the appropriate values in a <float.h> header for types float and double:

xf=s|2e|k=1fk|2-k,-125<=e<=+128

xd=s|2e|k=1fk|2-k,-1021<=e<=+1024

FLT_RADIX 2 DECIMAL_DIG 17 FLT_MANT_DIG 24 FLT_EPSILON 1.19209290E-07F // decimal constant FLT_EPSILON 0X1P-23F // hex constant FLT_DIG 6 FLT_MIN_EXP -125 FLT_MIN 1.17549435E-38F // decimal constant FLT_MIN 0X1P-126F // hex constant FLT_MIN_10_EXP -37 FLT_MAX_EXP +128 FLT_MAX 3.40282347E+38F // decimal constant FLT_MAX 0X1.fffffeP127F // hex constant FLT_MAX_10_EXP +38 DBL_MANT_DIG 53 DBL_EPSILON 2.2204460492503131E-16 // decimal constant DBL_EPSILON 0X1P-52 // hex constant DBL_DIG 15 DBL_MIN_EXP -1021 DBL_MIN 2.2250738585072014E-308 // decimal constant DBL_MIN 0X1P-1022 // hex constant DBL_MIN_10_EXP -307 DBL_MAX_EXP +1024 DBL_MAX 1.7976931348623157E+308 // decimal constant DBL_MAX 0X1.ffffffffffffeP1023 // hex constant DBL_MAX_10_EXP +308

If a type wider than double were supported, then DECIMAL_DIG would be greater than 17. For example, if the widest type were to use the minimal-width IEC 60559 double-extended format (64 bits of precision), then DECIMAL_DIG would be 21.

Forward references: conditional inclusion (6.10.1), complex arithmetic <complex.h> (7.3), mathematics <math.h> (7.12), integer types <stdint.h> (7.18).

6. Language

6.1 Notation

In the syntax notation used in this clause, syntactic categories (nonterminals) are indicated by italic type, and literal words and character set members (terminals) by bold type. A colon (:) following a nonterminal introduces its definition. Alternative definitions are listed on separate lines, except when prefaced by the words ``one of''. An optional symbol is indicated by the suffix ``-opt'', so that

        { expression-opt }

indicates an optional expression enclosed in braces.

A summary of the language syntax is given in annex A.

6.2 Concepts

6.2.1 Scopes of identifiers

An identifier can denote an object; a function; a tag or a member of a structure, union, or enumeration; a typedef name; a label name; a macro name; or a macro parameter. The same identifier can denote different entities at different points in the program. A member of an enumeration is called an enumeration constant. Macro names and macro parameters are not considered further here, because prior to the semantic phase of program translation any occurrences of macro names in the source file are replaced by the preprocessing token sequences that constitute their macro definitions.

For each different entity that an identifier designates, the identifier is visible (i.e., can be used) only within a region of program text called its scope. Different entities designated by the same identifier either have different scopes, or are in different name spaces. There are four kinds of scopes: function, file, block, and function prototype. (A function prototype is a declaration of a function that declares the types of its parameters.)

A label name is the only kind of identifier that has function scope. It can be used (in a goto statement) anywhere in the function in which it appears, and is declared implicitly by its syntactic appearance (followed by a : and a statement).

Every other identifier has scope determined by the placement of its declaration (in a declarator or type specifier). If the declarator or type specifier that declares the identifier appears outside of any block or list of parameters, the identifier has file scope, which terminates at the end of the translation unit. If the declarator or type specifier that declares the identifier appears inside a block or within the list of parameter declarations in a function definition, the identifier has block scope, which terminates at the end of the associated block. If the declarator or type specifier that declares the identifier appears within the list of parameter declarations in a function prototype (not part of a function definition), the identifier has function prototype scope, which terminates at the end of the function declarator. If an identifier designates two different entities in the same name space, the scopes might overlap. If so, the scope of one entity (the inner scope) will be a strict subset of the scope of the other entity (the outer scope). Within the inner scope, the identifier designates the entity declared in the inner scope; the entity declared in the outer scope is hidden (and not visible) within the inner scope.

Unless explicitly stated otherwise, where this International Standard uses the term identifier to refer to some entity (as opposed to the syntactic construct), it refers to the entity in the relevant name space whose declaration is visible at the point the identifier occurs.

Two identifiers have the same scope if and only if their scopes terminate at the same point.

Structure, union, and enumeration tags have scope that begins just after the appearance of the tag in a type specifier that declares the tag. Each enumeration constant has scope that begins just after the appearance of its defining enumerator in an enumerator list. Any other identifier has scope that begins just after the completion of its declarator.

Forward references: compound statement, or block (6.8.2), declarations (6.7), enumeration specifiers (6.7.2.2), function calls (6.5.2.2), function declarators (including prototypes) (6.7.5.3), function definitions (6.9.1), the goto statement (6.8.6.1), labeled statements (6.8.1), name spaces of identifiers (6.2.3), scope of macro definitions (6.10.3.5), source file inclusion (6.10.2), tags (6.7.2.3), type specifiers (6.7.2).

6.2.2 Linkages of identifiers

An identifier declared in different scopes or in the same scope more than once can be made to refer to the same object or function by a process called linkage. There are three kinds of linkage: external, internal, and none.

In the set of translation units and libraries that constitutes an entire program, each declaration of a particular identifier with external linkage denotes the same object or function. Within one translation unit, each declaration of an identifier with internal linkage denotes the same object or function. Each declaration of an identifier with no linkage denotes a unique entity.

If the declaration of a file scope identifier for an object or a function contains the storage-class specifier static, the identifier has internal linkage.20)

For an identifier declared with the storage-class specifier extern in a scope in which a prior declaration of that identifier is visible,21) if the prior declaration specifies internal or external linkage, the linkage of the identifier at the later declaration is the same as the linkage specified at the prior declaration. If no prior declaration is visible, or if the prior declaration specifies no linkage, then the identifier has external linkage.

If the declaration of an identifier for a function has no storage-class specifier, its linkage is determined exactly as if it were declared with the storage-class specifier extern. If the declaration of an identifier for an object has file scope and no storage-class specifier, its linkage is external.

The following identifiers have no linkage: an identifier declared to be anything other than an object or a function; an identifier declared to be a function parameter; a block scope identifier for an object declared without the storage-class specifier extern.

If, within a translation unit, the same identifier appears with both internal and external linkage, the behavior is undefined.

Forward references: compound statement, or block (6.8.2), declarations (6.7), expressions (6.5), external definitions (6.9).

6.2.3 Name spaces of identifiers

If more than one declaration of a particular identifier is visible at any point in a translation unit, the syntactic context disambiguates uses that refer to different entities. Thus, there are separate name spaces for various categories of identifiers, as follows:

-- label names (disambiguated by the syntax of the label declaration and use);

-- the tags of structures, unions, and enumerations (disambiguated by following any22) of the keywords struct, union, or enum);

-- the members of structures or unions; each structure or union has a separate name space for its members (disambiguated by the type of the expression used to access the member via the . or -> operator);

-- all other identifiers, called ordinary identifiers (declared in ordinary declarators or as enumeration constants).

Forward references: enumeration specifiers (6.7.2.2), labeled statements (6.8.1), structure and union specifiers (6.7.2.1), structure and union members (6.5.2.3), tags (6.7.2.3).

6.2.4 Storage durations of objects

An object has a storage duration that determines its lifetime. There are three storage durations: static, automatic, and allocated. Allocated storage is described in 7.20.3.

An object whose identifier is declared with external or internal linkage, or with the storage-class specifier static has static storage duration. For such an object, storage is reserved and its stored value is initialized only once, prior to program startup. The object exists, has a constant address, and retains its last-stored value throughout the execution of the entire program.23)

An object whose identifier is declared with no linkage and without the storage-class specifier static has automatic storage duration.

For such an object that does not have a variable length array type, storage is guaranteed to be reserved for a new

instance of the object on each entry into the block with which it is associated; the initial value of the object is indeterminate. If an initialization is specified for the object, it is performed each time the declaration is reached in the execution of the block; otherwise, the value becomes indeterminate each time the declaration is reached. Storage for the object is no longer guaranteed to be reserved when execution of the block ends in any way. (Entering an enclosed block or calling a function suspends, but does not end, execution of the current block.)

For such an object that does have a variable length array type, storage is guaranteed to be reserved for a new instance of the object each time the declaration is reached in the execution of the program. The initial value of the object is indeterminate. Storage for the object is no longer guaranteed to be reserved when the execution of the program leaves the scope of the declaration.24)

If an object is referred to when storage is not reserved for it, the behavior is undefined. The value of a pointer that referred to an object whose storage is no longer reserved is indeterminate. During the time that its storage is reserved, an object has a constant address.

Forward references: compound statement, or block (6.8.2), function calls (6.5.2.2), declarators (6.7.5), array declarators (6.7.5.2), initialization (6.7.8).

6.2.5 Types

The meaning of a value stored in an object or returned by a function is determined by the type of the expression used to access it. (An identifier declared to be an object is the simplest such expression; the type is specified in the declaration of the identifier.) Types are partitioned into object types (types that describe objects), function types (types that describe functions), and incomplete types (types that describe objects but lack information needed to determine their sizes).

An object declared as type _Bool is large enough to store the values 0 and 1.

An object declared as type char is large enough to store any member of the basic execution character set. If a member of the required source character set enumerated in 5.2.1 is stored in a char object, its value is guaranteed to be positive. If any other character is stored in a char object, the resulting value is implementation-defined but shall be within the range of values that can be represented in that type.

There are five standard signed integer types, designated as signed char, short int, int, long int, and long long int. (These and other types may be designated in several additional ways, as described in 6.7.2.) There may also be implementation-defined extended signed integer types.25) The standard and extended signed integer types are collectively called signed integer types.26)

An object declared as type signed char occupies the same amount of storage as a ``plain'' char object. A ``plain'' int object has the natural size suggested by the architecture of the execution environment (large enough to contain any value in the range INT_MIN to INT_MAX as defined in the header <limits.h>).

For each of the signed integer types, there is a corresponding (but different) unsigned integer type (designated with the keyword unsigned) that uses the same amount of storage (including sign information) and has the same alignment requirements. The type _Bool and the unsigned integer types that correspond to the standard signed integer types are the standard unsigned integer types. The unsigned integer types that correspond to the extended signed integer types are the extended unsigned integer types. The standard and extended unsigned integer types are collectively called unsigned integer types.27)

The standard signed integer types and standard unsigned integer types are collectively called the standard integer types, the extended signed integer types and extended

unsigned integer types are collectively called the extended integer types.

For any two types with the same signedness and different integer conversion rank (see 6.3.1.1), the range of values of the type with smaller integer conversion rank is a subrange of the values of the other type.

The range of nonnegative values of a signed integer type is a subrange of the corresponding unsigned integer type, and the representation of the same value in each type is the same.28) A computation involving unsigned operands can never overflow, because a result that cannot be represented by the resulting unsigned integer type is reduced modulo the number that is one greater than the largest value that can be represented by the resulting type.

#10

There are three real floating types, designated as float, double, and long double.29) The set of values of the type float is a subset of the set of values of the type double; the set of values of the type double is a subset of the set of values of the type long double.

#11

There are three complex types, designated as float _Complex, double _Complex, and long double _Complex.30) The real floating and complex types are collectively called the floating types.

#12

For each floating type there is a corresponding real type, which is always a real floating type. For real floating types, it is the same type. For complex types, it is the type given by deleting the keyword _Complex from the type name.

#13

Each complex type has the same representation and alignment requirements as an array type containing exactly two elements of the corresponding real type; the first element is equal to the real part, and the second element to the imaginary part, of the complex number.

#14

The type char, the signed and unsigned integer types, and the floating types are collectively called the basic types. Even if the implementation defines two or more basic

types to have the same representation, they are nevertheless different types.31)

#15

The three types char, signed char, and unsigned char are collectively called the character types. The implementation shall define char to have the same range, representation, and behavior as either signed char or unsigned char.32)

#16

An enumeration comprises a set of named integer constant values. Each distinct enumeration constitutes a different enumerated type.

#17

The type char, the signed and unsigned integer types, and the enumerated types are collectively called integer types. The integer and real floating types are collectively called real types.

#18

The void type comprises an empty set of values; it is an incomplete type that cannot be completed.

#19

Any number of derived types can be constructed from the object, function, and incomplete types, as follows:

-- An array type describes a contiguously allocated nonempty set of objects with a particular member object type, called the element type.33) Array types are characterized by their element type and by the number of elements in the array. An array type is said to be derived from its element type, and if its element type is T, the array type is sometimes called ``array of T''. The construction of an array type from an element type is called ``array type derivation''.

-- A structure type describes a sequentially allocated nonempty set of member objects (and, in certain circumstances, an incomplete array), each of which has

an optionally specified name and possibly distinct type.

-- A union type describes an overlapping nonempty set of member objects, each of which has an optionally specified name and possibly distinct type.

-- A function type describes a function with specified return type. A function type is characterized by its return type and the number and types of its parameters. A function type is said to be derived from its return type, and if its return type is T, the function type is sometimes called ``function returning T''. The construction of a function type from a return type is called ``function type derivation''.

-- A pointer type may be derived from a function type, an object type, or an incomplete type, called the referenced type. A pointer type describes an object whose value provides a reference to an entity of the referenced type. A pointer type derived from the referenced type T is sometimes called ``pointer to T''. The construction of a pointer type from a referenced type is called ``pointer type derivation''.

#20

These methods of constructing derived types can be applied recursively.

#21

Integer and floating types are collectively called arithmetic types. Arithmetic types and pointer types are collectively called scalar types. Array and structure types are collectively called aggregate types.34)

#22

Each arithmetic type belongs to one type domain. The real type domain comprises the real types. The complex type domain comprises the complex types.

#23

An array type of unknown size is an incomplete type. It is completed, for an identifier of that type, by specifying the size in a later declaration (with internal or external linkage). A structure or union type of unknown content (as described in 6.7.2.3) is an incomplete type. It is completed, for all declarations of that type, by declaring the same structure or union tag with its defining content later in the same scope. A structure type containing a flexible array member is an incomplete type that cannot be completed.

#24

Array, function, and pointer types are collectively

called derived declarator types. A declarator type derivation from a type T is the construction of a derived declarator type from T by the application of an array-type, a function-type, or a pointer-type derivation to T.

#25

A type is characterized by its type category, which is either the outermost derivation of a derived type (as noted above in the construction of derived types), or the type itself if the type consists of no derived types.

#26

Any type so far mentioned is an unqualified type. Each unqualified type has several qualified versions of its type,35) corresponding to the combinations of one, two, or all three of the const, volatile, and restrict qualifiers. The qualified or unqualified versions of a type are distinct types that belong to the same type category and have the same representation and alignment requirements.28) A derived type is not qualified by the qualifiers (if any) of the type from which it is derived.

#27

A pointer to void shall have the same representation and alignment requirements as a pointer to a character type. Similarly, pointers to qualified or unqualified versions of compatible types shall have the same representation and alignment requirements.28) All pointers to structure types shall have the same representation and alignment requirements as each other. All pointers to union types shall have the same representation and alignment requirements as each other. Pointers to other types need not have the same representation or alignment requirements.

#28

EXAMPLE 1 The type designated as ``float *'' has type ``pointer to float''. Its type category is pointer, not a floating type. The const-qualified version of this type is designated as ``float * const'' whereas the type designated as ``const float *'' is not a qualified type -- its type is ``pointer to const-qualified float'' and is a pointer to a qualified type.

#29

EXAMPLE 2 The type designated as ``struct tag (*[5])(float)'' has type ``array of pointer to function returning struct tag''. The array has length five and the function has a single parameter of type float. Its type category is array.

Forward references: character constants (6.4.4.4), compatible type and composite type (6.2.7), declarations (6.7), tags (6.7.2.3), type qualifiers (6.7.3).

6.2.6 Representations of types

6.2.6.1 General

The representations of all types are unspecified except as stated in this subclause.

Except for bit-fields, objects are composed of contiguous sequences of one or more bytes, the number, order, and encoding of which are either explicitly specified or implementation-defined.

Values stored in objects of type unsigned char shall be represented using a pure binary notation.36)

Values stored in objects of any other object type consist of n|CHAR_BIT bits, where n is the size of an object of that type, in bytes. The value may be copied into an object of type unsigned char [n] (e.g., by memcpy); the resulting set of bytes is called the object representation of the value. Two values (other than NaNs) with the same object representation compare equal, but values that compare equal may have different object representations.

Certain object representations need not represent a value of the object type. If the stored value of an object has such a representation and is accessed by an lvalue expression that does not have character type, the behavior is undefined. If such a representation is produced by a side effect that modifies all or any part of the object by an lvalue expression that does not have character type, the behavior is undefined.37) Such a representation is called a trap representation.

When a value is stored in an object of structure or union type, including in a member object, the bytes of the object representation that correspond to any padding bytes take unspecified values.38) The values of padding bytes

shall not affect whether the value of such an object is a trap representation. Those bits of a structure or union object that are in the same byte as a bit-field member, but are not part of that member, shall similarly not affect whether the value of such an object is a trap representation.

When a value is stored in a member of an object of union type, the bytes of the object representation that do not correspond to that member but do correspond to other members take unspecified values, but the value of the union object shall not thereby become a trap representation.

Where an operator is applied to a value which has more than one object representation, which object representation is used shall not affect the value of the result. Where a value is stored in an object using a type that has more than one object representation for that value, it is unspecified which representation is used, but a trap representation shall not be generated.

6.2.6.2 Integer types

For unsigned integer types other than unsigned char, the bits of the object representation shall be divided into two groups: value bits and padding bits (there need not be any of the latter). If there are N value bits, each bit shall represent a different power of 2 between 1 and 2N-1, so that objects of that type shall be capable of representing values from 0 to 2N-1 using a pure binary representation; this shall be known as the value representation. The values of any padding bits are unspecified.39)

For signed integer types, the bits of the object representation shall be divided into three groups: value bits, padding bits, and the sign bit. There need not be any padding bits; there shall be exactly one sign bit. Each bit that is a value bit shall have the same value as the same bit in the object representation of the corresponding unsigned type (if there are M value bits in the signed type

and N in the unsigned type, then M<=N). If the sign bit is zero, it shall not affect the resulting value. If the sign bit is one, then the value shall be modified in one of the following ways:

-- the corresponding value with sign bit 0 is negated;

-- the sign bit has the value -2N;

-- the sign bit has the value 1-2N.

The values of any padding bits are unspecified.39) A valid (non-trap) object representation of a signed integer type where the sign bit is zero is a valid object representation of the corresponding unsigned type, and shall represent the same value.

The precision of an integer type is the number of bits it uses to represent values, excluding any sign and padding bits. The width of an integer type is the same but including any sign bit; thus for unsigned integer types the two values are the same, while for signed integer types the width is one greater than the precision.

6.2.7 Compatible type and composite type

Two types have compatible type if their types are the same. Additional rules for determining whether two types are compatible are described in 6.7.2 for type specifiers, in 6.7.3 for type qualifiers, and in 6.7.5 for declarators.40) Moreover, two structure, union, or enumerated types declared in separate translation units are compatible if their tags and members satisfy the following requirements: If one is declared with a tag, the other shall be declared with the same tag. If both are completed types, then the following additional requirements apply: there shall be a one-to-one correspondence between their members such that each pair of corresponding members are declared with compatible types, and such that if one member of a corresponding pair is declared with a name, the other member is declared with the same name. For two structures, corresponding members shall be declared in the same order. For two structures or unions, corresponding bit-fields shall have the same widths. For two enumerations, corresponding members shall have the same values.

All declarations that refer to the same object or function shall have compatible type; otherwise, the behavior is undefined.

A composite type can be constructed from two types that

are compatible; it is a type that is compatible with both of the two types and satisfies the following conditions:

-- If one type is an array of known constant size, the composite type is an array of that size; otherwise, if one type is a variable length array, the composite type is that type.

-- If only one type is a function type with a parameter type list (a function prototype), the composite type is a function prototype with the parameter type list.

-- If both types are function types with parameter type lists, the type of each parameter in the composite parameter type list is the composite type of the corresponding parameters.

These rules apply recursively to the types from which the two types are derived.

For an identifier with internal or external linkage declared in a scope in which a prior declaration of that identifier is visible,41) if the prior declaration specifies internal or external linkage, the type of the identifier at the later declaration becomes the composite type.

EXAMPLE Given the following two file scope declarations:

        int f(int (*)(), double (*)[3]);
        int f(int (*)(char *), double (*)[]);

The resulting composite type for the function is:

        int f(int (*)(char *), double (*)[3]);

Forward references: declarators (6.7.5), enumeration specifiers (6.7.2.2), structure and union specifiers (6.7.2.1), type definitions (6.7.7), type qualifiers (6.7.3), type specifiers (6.7.2).

6.3 Conversions

Several operators convert operand values from one type to another automatically. This subclause specifies the result required from such an implicit conversion, as well as those that result from a cast operation (an explicit conversion). The list in 6.3.1.8 summarizes the conversions performed by most ordinary operators; it is supplemented as required by the discussion of each operator in 6.5.

Conversion of an operand value to a compatible type causes no change to the value or the representation.

Forward references: cast operators (6.5.4).

6.3.1 Arithmetic operands

6.3.1.1 Boolean, characters, and integers

Every integer type has an integer conversion rank defined as follows:

-- No two signed integer types shall have the same rank, even if they have the same representation.

-- The rank of a signed integer type shall be greater than the rank of any signed integer type with less precision.

-- The rank of long long int shall be greater than the rank of long int, which shall be greater than the rank of int, which shall be greater than the rank of short int, which shall be greater than the rank of signed char.

-- The rank of any unsigned integer type shall equal the rank of the corresponding signed integer type, if any.

-- The rank of any standard integer type shall be greater than the rank of any extended integer type with the same width.

-- The rank of char shall equal the rank of signed char and unsigned char.

-- The rank of _Bool shall be less than the rank of all other standard integer types.

-- The rank of any enumerated type shall equal the rank of the compatible integer type (see 6.7.2.2).

-- The rank of any extended signed integer type relative to another extended signed integer type with the same precision is implementation-defined, but still subject to the other rules for determining the integer conversion rank.

-- For all integer types T1, T2, and T3, if T1 has greater rank than T2 and T2 has greater rank than T3, then T1 has greater rank than T3.

The following may be used in an expression wherever an int or unsigned int may be used:

-- An object or expression with an integer type whose integer conversion rank is less than the rank of int and unsigned int.

-- A bit-field of type _Bool, int, signed int, or unsigned int.

If an int can represent all values of the original type, the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions.42) All other types are unchanged by the integer promotions.

The integer promotions preserve value including sign. As discussed earlier, whether a ``plain'' char is treated as signed is implementation-defined.

Forward references: enumeration specifiers (6.7.2.2), structure and union specifiers (6.7.2.1).

6.3.1.2 Boolean type

When any scalar value is converted to _Bool, the result is 0 if the value compares equal to 0; otherwise, the result is 1.

6.3.1.3 Signed and unsigned integers

When a value with integer type is converted to another integer type other than _Bool, if the value can be represented by the new type, it is unchanged.

Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or subtracting one more than the maximum value that can be represented in the new type until the value is in the range of the new type.

Otherwise, the new type is signed and the value cannot be represented in it; the result is implementation-defined.

6.3.1.4 Real floating and integer

When a finite value of real floating type is converted to an integer type other than _Bool, the fractional part is discarded (i.e., the value is truncated toward zero). If the value of the integral part cannot be represented by the integer type, the behavior is undefined.43)

When a value of integer type is converted to a real floating type, if the value being converted is in the range of values that can be represented but cannot be represented exactly, the result is either the nearest higher or nearest lower value, chosen in an implementation-defined manner. If the value being converted is outside the range of values that can be represented, the behavior is undefined.

6.3.1.5 Real floating types

When a float is promoted to double or long double, or a double is promoted to long double, its value is unchanged.

When a double is demoted to float, a long double is demoted to double or float, or a value being represented in greater precision and range than required by its semantic type (see 6.3.1.8) is explicitly converted to its semantic type, if the value being converted is outside the range of values that can be represented, the behavior is undefined. If the value being converted is in the range of values that can be represented but cannot be represented exactly, the result is either the nearest higher or nearest lower representable value, chosen in an implementation-defined manner.

6.3.1.6 Complex types

When a value of complex type is converted to another complex type, both the real and imaginary parts follow the conversion rules for the corresponding real types.

6.3.1.7 Real and complex

When a value of real type is converted to a complex type, the real part of the complex result value is determined by the rules of conversion to the corresponding real type and the imaginary part of the complex result value is a positive zero or an unsigned zero.

When a value of complex type is converted to a real type, the imaginary part of the complex value is discarded and the value of the real part is converted according to the conversion rules for the corresponding real type.

6.3.1.8 Usual arithmetic conversions

Many operators that expect operands of arithmetic type cause conversions and yield result types in a similar way. The purpose is to determine a common real type for the operands and result. For the specified operands, each operand is converted, without change of type domain, to a type whose corresponding real type is the common real type. Unless explicitly stated otherwise, the common real type is also the corresponding real type of the result, whose type domain is the type domain of the operands if they are the same, and complex otherwise. This pattern is called the usual arithmetic conversions:

First, if the corresponding real type of either operand is long double, the other operand is converted, without change of type domain, to a type whose corresponding real type is long double.

Otherwise, if the corresponding real type of either operand is double, the other operand is converted, without change of type domain, to a type whose corresponding real type is double.

Otherwise, if the corresponding real type of either operand is float, the other operand is converted, without change of type domain, to a type whose corresponding real type is float.44)

Otherwise, the integer promotions are performed on both operands. Then the following rules are applied to the promoted operands:

If both operands have the same type, then no further conversion is needed.

Otherwise, if both operands have signed integer types or both have unsigned integer types, the operand with the type of lesser integer conversion rank is converted to the type of the operand with greater rank.

Otherwise, if the operand that has unsigned integer type has rank greater or equal to the rank of the type of the other operand, then the operand with signed integer type is converted to the type of the operand with unsigned integer type.

Otherwise, if the type of the operand with signed integer type can represent all of the values of the type of the operand with unsigned integer type, then the operand with unsigned integer type is converted to the type of the operand with signed integer type.

Otherwise, both operands are converted to the unsigned integer type corresponding to the type of the operand with signed integer type.

The values of floating operands and of the results of floating expressions may be represented in greater precision and range than that required by the type; the types are not changed thereby.45)

6.3.2 Other operands

6.3.2.1 Lvalues and function designators

An lvalue is an expression with an object type or an incomplete type other than void;46) if an lvalue does not designate an object when it is evaluated, the behavior is undefined. When an object is said to have a particular

type, the type is specified by the lvalue used to designate the object. A modifiable lvalue is an lvalue that does not have array type, does not have an incomplete type, does not have a const-qualified type, and if it is a structure or union, does not have any member (including, recursively, any member or element of all contained aggregates or unions) with a const-qualified type.

Except when it is the operand of the sizeof operator, the unary & operator, the ++ operator, the -- operator, or the left operand of the . operator or an assignment operator, an lvalue that does not have array type is converted to the value stored in the designated object (and is no longer an lvalue). If the lvalue has qualified type, the value has the unqualified version of the type of the lvalue; otherwise, the value has the type of the lvalue. If the lvalue has an incomplete type and does not have array type, the behavior is undefined.

Except when it is the operand of the sizeof operator or the unary & operator, or is a string literal used to initialize an array, an expression that has type ``array of type'' is converted to an expression with type ``pointer to type'' that points to the initial element of the array object and is not an lvalue. If the array object has register storage class, the behavior is undefined.

A function designator is an expression that has function type. Except when it is the operand of the sizeof operator47) or the unary & operator, a function designator with type ``function returning type'' is converted to an expression that has type ``pointer to function returning type''.

Forward references: address and indirection operators (6.5.3.2), assignment operators (6.5.16), common definitions <stddef.h> (7.17), initialization (6.7.8), postfix increment and decrement operators (6.5.2.4), prefix increment and decrement operators (6.5.3.1), the sizeof operator (6.5.3.4), structure and union members (6.5.2.3).

6.3.2.2 void

The (nonexistent) value of a void expression (an expression that has type void) shall not be used in any way, and implicit or explicit conversions (except to void) shall not be applied to such an expression. If an expression of any other type is evaluated as a void expression, its value or designator is discarded. (A void expression is evaluated for its side effects.)

6.3.2.3 Pointers

A pointer to void may be converted to or from a pointer to any incomplete or object type. A pointer to any incomplete or object type may be converted to a pointer to void and back again; the result shall compare equal to the original pointer.

For any qualifier q, a pointer to a non-q-qualified type may be converted to a pointer to the q-qualified version of the type; the values stored in the original and converted pointers shall compare equal.

An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant.48) If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

Conversion of a null pointer to another pointer type yields a null pointer of that type. Any two null pointers shall compare equal.

An integer may be converted to any pointer type. Except as previously specified, the result is implementation-defined, might not be properly aligned, and might not point to an entity of the referenced type.49)

Any pointer type may be converted to an integer type. Except as previously specified, the result is implementation-defined. If the result cannot be represented in the integer type, the behavior is undefined. The result need not be in the range of values of any integer type.

A pointer to an object or incomplete type may be converted to a pointer to a different object or incomplete type. If the resulting pointer is not correctly aligned50) for the pointed-to type, the behavior is undefined. Otherwise, when converted back again, the result shall compare equal to the original pointer. When a pointer to an object is converted to a pointer to a character type, the result points to the lowest addressed byte of the object.

Successive increments of the result, up to the size of the object, yield pointers to the remaining bytes of the object.

A pointer to a function of one type may be converted to a pointer to a function of another type and back again; the result shall compare equal to the original pointer. If a converted pointer is used to call a function whose type is not compatible with the pointed-to type, the behavior is undefined.

Forward references: cast operators (6.5.4), equality operators (6.5.9), simple assignment (6.5.16.1).

6.4 Lexical elements

Syntax

each non-white-space character that cannot be one of the above

Constraints

Each preprocessing token that is converted to a token shall have the lexical form of a keyword, an identifier, a constant, a string literal, or a punctuator.

Semantics

A token is the minimal lexical element of the language in translation phases 7 and 8. The categories of tokens are: keywords, identifiers, constants, string literals, and punctuators. A preprocessing token is the minimal lexical element of the language in translation phases 3 through 6. The categories of preprocessing token are: header names, identifiers, preprocessing numbers, character constants, string literals, punctuators, and single non-white-space characters that do not lexically match the other preprocessing token categories.51) If a ' or a " character matches the last category, the behavior is undefined. Preprocessing tokens can be separated by white space; this consists of comments (described later), or white-space characters (space, horizontal tab, new-line, vertical tab, and form-feed), or both. As described in 6.10, in certain circumstances during translation phase 4, white space (or the absence thereof) serves as more than preprocessing token separation. White space may appear within a preprocessing token only as part of a header name or between the quotation

characters in a character constant or string literal.

If the input stream has been parsed into preprocessing tokens up to a given character, the next preprocessing token is the longest sequence of characters that could constitute a preprocessing token. There is one exception to this rule: a header name preprocessing token is only recognized within a #include preprocessing directive, and within such a directive, a sequence of characters that could be either a header name or a string literal is recognized as the former.

EXAMPLE 1 The program fragment 1Ex is parsed as a preprocessing number token (one that is not a valid floating or integer constant token), even though a parse as the pair of preprocessing tokens 1 and Ex might produce a valid expression (for example, if Ex were a macro defined as +1). Similarly, the program fragment 1E1 is parsed as a preprocessing number (one that is a valid floating constant token), whether or not E is a macro name.

EXAMPLE 2 The program fragment x+++++y is parsed as x+++++y, which violates a constraint on increment operators, even though the parse x+++++y might yield a correct expression.

Forward references: character constants (6.4.4.4), comments (6.4.9), expressions (6.5), floating constants (6.4.4.2), header names (6.4.7), macro replacement (6.10.3), postfix increment and decrement operators (6.5.2.4), prefix increment and decrement operators (6.5.3.1), preprocessing directives (6.10), preprocessing numbers (6.4.8), string literals (6.4.5).

6.4.1 Keywords

Syntax

keyword: one of

auto enum restrict unsigned

break extern return void

case float short volatile

char for signed while

const goto sizeof _Bool

continue if static _Complex

default inline struct _Imaginary

do int switch

double long typedef

else register union

Semantics

The above tokens (case sensitive) are reserved (in translation phases 7 and 8) for use as keywords, and shall not be used otherwise.

6.4.2 Identifiers

6.4.2.1 General

Syntax

identifier:

identifier-nondigit

identifier identifier-nondigit

identifier digit

identifier-nondigit:

nondigit

universal-character-name

other implementation-defined characters

nondigit: one of

_ a b c d e f g h i j k l m

n o p q r s t u v w x y z

A B C D E F G H I J K L M

N O P Q R S T U V W X Y Z

digit: one of

0 1 2 3 4 5 6 7 8 9

Semantics

An identifier is a sequence of nondigit characters (including the underscore _, the lowercase and uppercase Latin letters, and other characters) and digits, which designates one or more entities as described in 6.2.1. Lowercase and uppercase letters are distinct. There is no specific limit on the maximum length of an identifier.

Each universal character name in an identifier shall designate a character whose encoding in ISO/IEC 10646 falls into one of the ranges specified in annex D.52) The initial character shall not be a universal character name designating a digit. An implementation may allow multibyte characters that are not part of the required source

character set to appear in identifiers; which characters and their correspondence to universal character names is implementation defined.

When preprocessing tokens are converted to tokens during translation phase 7, if a preprocessing token could be converted to either a keyword or an identifier, it is converted to a keyword.

Implementation limits

As discussed in 5.2.4.1, an implementation may limit the number of significant initial characters in an identifier; the limit for an external name (an identifier that has external linkage) may be more restrictive than that for an internal name (a macro name or an identifier that does not have external linkage). The number of significant characters in an identifier is implementation-defined.

Any identifiers that differ in a significant character are different identifiers. If two identifiers differ only in nonsignificant characters, the behavior is undefined.

Forward references: universal character names (6.4.3), macro replacement (6.10.3).

6.4.2.2 Predefined identifiers

Semantics

The identifier __func__ shall be implicitly declared by the translator as if, immediately following the opening brace of each function definition, the declaration

        static const char __func__[] = "function-name";

appeared, where function-name is the name of the lexically- enclosing function.53)

This name is encoded as if the implicit declaration had been written in the source character set and then translated into the execution character set as indicated in translation phase 5.

EXAMPLE Consider the code fragment:

        #include <stdio.h>
        void myfunc(void)
        {
                printf("%s\n", __func__);
                /* ... */
        }

Each time the function is called, it will print to the standard output stream:

myfunc

Forward references: function definitions (6.9.1).

6.4.3 Universal character names

Syntax

universal-character-name:

\u hex-quad

\U hex-quad hex-quad

hex-quad:

hexadecimal-digit hexadecimal-digit

Constraints

A universal character name shall not specify a character short identifier in the range 00000000 through 00000020, 0000007F through 0000009F, or 0000D800 through 0000DFFF inclusive. A universal character name shall not designate a character in the required character set.

Description

Universal character names may be used in identifiers, character constants, and string literals to designate characters that are not in the required character set.

Semantics

The universal character name \Unnnnnnnn designates the character whose character short identifier (as specified by ISO/IEC 10646) is nnnnnnnn. Similarly, the universal character name \unnnn designates the character whose character short identifier is 0000nnnn.

6.4.4 Constants

Syntax

Constraints

The value of a constant shall be in the range of representable values for its type.

Semantics

Each constant has a type, determined by its form and value, as detailed later.

6.4.4.1 Integer constants

Syntax

integer-constant:

decimal-constant integer-suffix_opt

octal-constant integer-suffix_opt

hexadecimal-constant integer-suffix_opt

decimal-constant:

nonzero-digit

decimal-constant digit

octal-constant:

octal-constant octal-digit

hexadecimal-constant:

hexadecimal-prefix hexadecimal-digit

hexadecimal-constant hexadecimal-digit

hexadecimal-prefix: one of

0x 0X

nonzero-digit: one of

1 2 3 4 5 6 7 8 9

octal-digit: one of

0 1 2 3 4 5 6 7

hexadecimal-digit: one of

0 1 2 3 4 5 6 7 8 9

a b c d e f

A B C D E F

integer-suffix:

unsigned-suffix long-suffix_opt

unsigned-suffix long-long-suffix

long-suffix unsigned-suffix_opt

long-long-suffix unsigned-suffix_opt

unsigned-suffix: one of

u U

long-suffix: one of

l L

long-long-suffix: one of

ll LL

Description

An integer constant begins with a digit, but has no period or exponent part. It may have a prefix that specifies its base and a suffix that specifies its type.

A decimal constant begins with a nonzero digit and consists of a sequence of decimal digits. An octal constant consists of the prefix 0 optionally followed by a sequence of the digits 0 through 7 only. A hexadecimal constant consists of the prefix 0x or 0X followed by a sequence of the decimal digits and the letters a (or A) through f (or F) with values 10 through 15 respectively.

Semantics

The value of a decimal constant is computed base 10; that of an octal constant, base 8; that of a hexadecimal constant, base 16. The lexically first digit is the most significant.

The type of an integer constant is the first of the corresponding list in which its value can be represented.

                     ||                       |
                     ||                       |  Octal or Hexadecimal
        Suffix       ||   Decimal Constant    |        Constant
        -------------++-----------------------+------------------------
        none         ||int                    | int
                     ||long int               | unsigned int
                     ||long long int          | long int
                     ||                       | unsigned long int
                     ||                       | long long int
                     ||                       | unsigned long long int
        -------------++-----------------------+------------------------
        u or U       ||unsigned int           | unsigned int
                     ||unsigned long int      | unsigned long int
                     ||unsigned long long int | unsigned long long int
        -------------++-----------------------+------------------------
        l or L       ||long int               | long int
                     ||long long int          | unsigned long int
                     ||                       | long long int
                     ||                       | unsigned long long int
        -------------++-----------------------+------------------------
        Both u or U  ||unsigned long int      | unsigned long int
        and l or L   ||unsigned long long int | unsigned long long int
        -------------++-----------------------+------------------------
        ll or LL     ||long long int          | long long int
                     ||                       | unsigned long long int
        -------------++-----------------------+------------------------
        Both u or U  ||unsigned long long int | unsigned long long int
        and ll or LL ||                       |

If an integer constant cannot be represented by any type in its list, it may have an extended integer type, if the extended integer type can represent its value. If all of the types in the list for the constant are signed, the extended integer type shall be signed. If all of the types in the list for the constant are unsigned, the extended integer type shall be unsigned. If the list contains both signed and unsigned types, the extended integer type may be signed or unsigned.

6.4.4.2 Floating constants

Syntax

floating-constant:

decimal-floating-constant

hexadecimal-floating-constant

decimal-floating-constant:

fractional-constant exponent-part_opt floating-suffix_opt

digit-sequence exponent-part floating-suffix_opt

hexadecimal-floating-constant:

hexadecimal-prefix hexadecimal-fractional-constant

binary-exponent-part floating-suffix_opt

hexadecimal-prefix hexadecimal-digit-sequence

binary-exponent-part floating-suffix_opt

fractional-constant:

digit-sequence_opt . digit-sequence

digit-sequence .

exponent-part:

e sign_opt digit-sequence

E sign_opt digit-sequence

sign: one of

+ -

digit-sequence:

digit

digit-sequence digit

hexadecimal-fractional-constant:

hexadecimal-digit-sequence_opt .

hexadecimal-digit-sequence

hexadecimal-digit-sequence .

binary-exponent-part:

p sign_opt digit-sequence

P sign_opt digit-sequence

hexadecimal-digit-sequence:

hexadecimal-digit

hexadecimal-digit-sequence hexadecimal-digit

floating-suffix: one of

f l F L

Description

A floating constant has a significand part that may be followed by an exponent part and a suffix that specifies its type. The components of the significand part may include a digit sequence representing the whole-number part, followed by a period (.), followed by a digit sequence representing the fraction part. The components of the exponent part are an e, E, p, or P followed by an exponent consisting of an optionally signed digit sequence. Either the whole-number part or the fraction part has to be present; for decimal floating constants, either the period or the exponent part has to be present.

Semantics

The significand part is interpreted as a (decimal or hexadecimal) rational number; the digit sequence in the exponent part is interpreted as a decimal integer. For decimal floating constants, the exponent indicates the power of 10 by which the significand part is to be scaled. For hexadecimal floating constants, the exponent indicates the power of 2 by which the significand part is to be scaled. For decimal floating constants, and also for hexadecimal floating constants when FLT_RADIX is not a power of 2, the result is either the nearest representable value, or the larger or smaller representable value immediately adjacent to the nearest representable value, chosen in an implementation-defined manner. For hexadecimal floating constants when FLT_RADIX is a power of 2, the result is correctly rounded.

An unsuffixed floating constant has type double. If suffixed by the letter f or F, it has type float. If suffixed by the letter l or L, it has type long double.

Recommended practice

The implementation should produce a diagnostic message if a hexadecimal constant cannot be represented exactly in its evaluation format; the implementation should then proceed with the translation of the program.

The translation-time conversion of floating constants should match the execution-time conversion of character strings by library functions, such as strtod, given matching inputs suitable for both conversions, the same result format, and default execution-time rounding.54)

6.4.4.3 Enumeration constants

Syntax

enumeration-constant:

identifier

Semantics

An identifier declared as an enumeration constant has type int.

Forward references: enumeration specifiers (6.7.2.2).

6.4.4.4 Character constants

Syntax

character-constant:

' c-char-sequence '

L' c-char-sequence '

c-char-sequence:

c-char

c-char-sequence c-char

c-char:

any member of the source character set except the single-quote ', backslash \, or new-line character

escape-sequence

escape-sequence:

simple-escape-sequence

octal-escape-sequence

hexadecimal-escape-sequence

universal-character-name

simple-escape-sequence: one of

\' \" \? \\

\a \b \f \n \r \t \v

octal-escape-sequence:

\ octal-digit

\ octal-digit octal-digit

\ octal-digit octal-digit octal-digit

hexadecimal-escape-sequence:

\x hexadecimal-digit

hexadecimal-escape-sequence hexadecimal-digit

Description

An integer character constant is a sequence of one or more multibyte characters enclosed in single-quotes, as in 'x' or 'ab'. A wide character constant is the same, except prefixed by the letter L. With a few exceptions detailed later, the elements of the sequence are any members of the source character set; they are mapped in an implementation- defined manner to members of the execution character set.

The single-quote ', the double-quote ", the question- mark ?, the backslash \, and arbitrary integer values, are representable according to the following table of escape sequences: single quote ' \' double quote " \" question mark ? \? backslash \ \\ octal character \o ctal digits hexadecimal character \x hexadecimal digits

The double-quote " and question-mark ? are representable either by themselves or by the escape sequences \" and \?, respectively, but the single-quote ' and the backslash \ shall be represented, respectively, by the escape sequences \' and \\.

The octal digits that follow the backslash in an octal escape sequence are taken to be part of the construction of a single character for an integer character constant or of a single wide character for a wide character constant. The numerical value of the octal integer so formed specifies the value of the desired character or wide character.

The hexadecimal digits that follow the backslash and the letter x in a hexadecimal escape sequence are taken to be part of the construction of a single character for an integer character constant or of a single wide character for a wide character constant. The numerical value of the hexadecimal integer so formed specifies the value of the desired character or wide character.

Each octal or hexadecimal escape sequence is the longest sequence of characters that can constitute the escape sequence.

In addition, characters not in the required character set are representable by universal character names and certain nongraphic characters are representable by escape sequences consisting of the backslash \ followed by a lowercase letter: \a, \b, \f, \n, \r, \t, and \v.55)

Constraints

The value of an octal or hexadecimal escape sequence shall be in the range of representable values for the type unsigned char for an integer character constant, or the unsigned type corresponding to wchar_t for a wide character constant.

Semantics

#10

An integer character constant has type int. The value of an integer character constant containing a single character that maps to a member of the basic execution character set is the numerical value of the representation of the mapped character interpreted as an integer. The value of an integer character constant containing more than one character, or containing a character or escape sequence not represented in the basic execution character set, is implementation-defined. If an integer character constant contains a single character or escape sequence, its value is the one that results when an object with type char whose value is that of the single character or escape sequence is converted to type int.

#11

A wide character constant has type wchar_t, an integer type defined in the <stddef.h> header. The value of a wide character constant containing a single multibyte character that maps to a member of the extended execution character set is the wide character (code) corresponding to that multibyte character, as defined by the mbtowc function, with an implementation-defined current locale. The value of a wide character constant containing more than one multibyte character, or containing a multibyte character or escape sequence not represented in the extended execution character set, is implementation-defined.

#12

EXAMPLE 1 The construction '\0' is commonly used to represent the null character.

#13

EXAMPLE 2 Consider implementations that use two's- complement representation for integers and eight bits for objects that have type char. In an implementation in which type char has the same range of values as signed char, the integer character constant '\xFF' has the value -1; if type char has the same range of values as unsigned char, the character constant '\xFF' has the value +255 .

#14

EXAMPLE 3 Even if eight bits are used for objects that have type char, the construction '\x123' specifies an integer character constant containing only one character, since a hexadecimal escape sequence is terminated only by a non-hexadecimal character. To specify an integer character constant containing the two characters whose values are '\x12' and '3', the construction '\0223' may be used, since an octal escape sequence is terminated after three octal digits. (The value of this two-character integer character constant is implementation-defined.)

#15

EXAMPLE 4 Even if 12 or more bits are used for objects that have type wchar_t, the construction L'\1234' specifies the implementation-defined value that results from the combination of the values 0123 and '4'.

Forward references: common definitions <stddef.h> (7.17), the mbtowc function (7.20.7.2).

6.4.5 String literals

Syntax

string-literal:

" s-char-sequence_opt "

L" s-char-sequence_opt "

s-char-sequence:

s-char

s-char-sequence s-char

s-char:

any member of the source character set except the double-quote ", backslash \, or new-line character

escape-sequence

Description

A character string literal is a sequence of zero or more multibyte characters enclosed in double-quotes, as in "xyz". A wide string literal is the same, except prefixed by the letter L.

The same considerations apply to each element of the sequence in a character string literal or a wide string literal as if it were in an integer character constant or a wide character constant, except that the single-quote ' is representable either by itself or by the escape sequence \', but the double-quote " shall be represented by the escape sequence \".

Semantics

In translation phase 6, the multibyte character sequences specified by any sequence of adjacent character and wide string literal tokens are concatenated into a single multibyte character sequence. If any of the tokens are wide string literal tokens, the resulting multibyte character sequence is treated as a wide string literal; otherwise, it is treated as a character string literal.

In translation phase 7, a byte or code of value zero is appended to each multibyte character sequence that results from a string literal or literals.56) The multibyte character sequence is then used to initialize an array of static storage duration and length just sufficient to contain the sequence. For character string literals, the array elements have type char, and are initialized with the individual bytes of the multibyte character sequence; for wide string literals, the array elements have type wchar_t, and are initialized with the sequence of wide characters corresponding to the multibyte character sequence, as defined by the mbstowcs function with an implementation- defined current locale. The value of a string literal containing a multibyte character or escape sequence not represented in the execution character set is implementation-defined.

It is unspecified whether these arrays are distinct provided their elements have the appropriate values. If the program attempts to modify such an array, the behavior is undefined.

EXAMPLE This pair of adjacent character string literals

"\x12" "3"

produces a single character string literal containing the two characters whose values are '\x12' and '3', because escape sequences are converted into single members of the execution character set just prior to adjacent string literal concatenation.

Forward references: common definitions <stddef.h> (7.17).

6.4.6 Punctuators

Syntax

punctuator: one of

[ ] ( ) { } . ->

++ -- & * + - ~ !

/ % << >> < > <= >= == != ^ | && |

? : ; ...

= *= /= %= += -= <<= >>= &= ^= |=

, # ##

<: :> <% %> %: %:%:

Semantics

A punctuator is a symbol that has independent syntactic and semantic significance. Depending on context, it may specify an operation to be performed (which in turn may yield a value or a function designator, produce a side

effect, or some combination thereof) in which case it is known as an operator (other forms of operator also exist in some contexts). An operand is an entity on which an operator acts.

In all aspects of the language, these six tokens

        <:  :>  <%  %>  %:  %:%:

behave, respectively, the same as these six tokens

        [   ]   {   }   #   ##

except for their spelling.57)

Forward references: expressions (6.5), declarations (6.7), preprocessing directives (6.10), statements (6.8).

6.4.7 Header names

Syntax

header-name:

< h-char-sequence >

" q-char-sequence "

h-char-sequence:

h-char

h-char-sequence h-char

h-char:

any member of the source character set except the new-line character and >

q-char-sequence:

q-char

q-char-sequence q-char

q-char:

any member of the source character set except the new-line character and "

Semantics

The sequences in both forms of header names are mapped in an implementation-defined manner to headers or external source file names as specified in 6.10.2.

If the characters ', \, ", //, or /* occur in the sequence between the < and > delimiters, the behavior is undefined. Similarly, if the characters ', \, //, or /* occur in the sequence between the " delimiters, the behavior is undefined.58) A header name preprocessing token is recognized only within a #include preprocessing directive.

EXAMPLE The following sequence of characters:

        0x3<1/a.h>1e2
        #include <1/a.h>
        #define const.member@$

forms the following sequence of preprocessing tokens (with each individual preprocessing token delimited by a { on the left and a } on the right).

        {0x3}{<}{1}{/}{a}{.}{h}{>}{1e2}
        {#}{include} {<1/a.h>}
        {#}{define} {const}{.}{member}{@}{$}

Forward references: source file inclusion (6.10.2).

6.4.8 Preprocessing numbers

Syntax

pp-number:

digit

. digit

pp-number digit

pp-number identifier-nondigit

Description

A preprocessing number begins with a digit optionally preceded by a period (.) and may be followed by letters, underscores, digits, periods, and e+, e-, E+, E-, p+, p-, P+, or P- character sequences.

Preprocessing number tokens lexically include all floating and integer constant tokens.

Semantics

A preprocessing number does not have type or a value; it acquires both after a successful conversion (as part of translation phase 7) to a floating constant token or an integer constant token.

6.4.9 Comments

Except within a character constant, a string literal, or a comment, the characters /* introduce a comment. The contents of a comment are examined only to identify multibyte characters and to find the characters */ that terminate it.59)

Except within a character constant, a string literal, or a comment, the characters // introduce a comment that includes all multibyte characters up to, but not including, the next new-line character. The contents of such a comment are examined only to identify multibyte characters and to find the terminating new-line character.

EXAMPLE 1

        "a//b"                   // four-character string literal
        #include "//e"           // undefined behavior
        // */                    // comment, not syntax error
        f = g/**//h;             // equivalent to f = g / h;
        //\
        i();                     // part of a two-line comment
        /\
        / j();                   // part of a two-line comment
        #define glue(x,y) x##y
        glue(/,/) k();           // syntax error, not comment
        /*//*/ l();              // equivalent to l();
        m = n//**/o
          + p;                   // equivalent to m = n + p;

6.5 Expressions

An expression is a sequence of operators and operands that specifies computation of a value, or that designates an object or a function, or that generates side effects, or that performs a combination thereof.

Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be accessed only to determine the value to be stored.60)

The grouping of operators and operands is indicated by the syntax.61) Except as specified later (for the function- call (), &&, ||, ?:, and comma operators), the order of evaluation of subexpressions and the order in which side effects take place are both unspecified.

Some operators (the unary operator ~, and the binary operators <<, >>, &, ^, and |, collectively described as bitwise operators) are required to have operands that have integer type. These operators return values that depend on the internal representations of integers, and have implementation-defined and undefined aspects for signed types.

If an exception occurs during the evaluation of an expression (that is, if the result is not mathematically defined or not in the range of representable values for its type), the behavior is undefined.

The effective type of an object for an access to its stored value is the declared type of the object, if any.62) If a value is stored into an object having no declared type through an lvalue having a type that is not a character type, then the type of the lvalue becomes the effective type of the object for that access and for subsequent accesses that do not modify the stored value. If a value is copied into an object having no declared type using memcpy or memmove, or is copied as an array of character type, then the effective type of the modified object for that access and for subsequent accesses that do not modify the value is the effective type of the object from which the value is copied, if it has one. For all other accesses to an object having no declared type, the effective type of the object is simply the type of the lvalue used for the access.

An object shall have its stored value accessed only by an lvalue expression that has one of the following types:63)

-- a type compatible with the effective type of the object,

-- a qualified version of a type compatible with the effective type of the object,

-- a type that is the signed or unsigned type corresponding to the effective type of the object,

-- a type that is the signed or unsigned type corresponding to a qualified version of the effective type of the object,

-- an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union), or

-- a character type.

A floating expression may be contracted, that is, evaluated as though it were an atomic operation, thereby omitting rounding errors implied by the source code and the expression evaluation method.64) The FP_CONTRACT pragma in

<math.h> provides a way to disallow contracted expressions. Otherwise, whether and how expressions are contracted is implementation-defined.65)

6.5.1 Primary expressions

Syntax

( expression )

Semantics

An identifier is a primary expression, provided it has been declared as designating an object (in which case it is an lvalue) or a function (in which case it is a function designator).66)

A constant is a primary expression. Its type depends on its form and value, as detailed in 6.4.4.

A string literal is a primary expression. It is an lvalue with type as detailed in 6.4.5.

A parenthesized expression is a primary expression. Its type and value are identical to those of the unparenthesized expression. It is an lvalue, a function designator, or a void expression if the unparenthesized expression is, respectively, an lvalue, a function designator, or a void expression.

Forward references: declarations (6.7).

6.5.2 Postfix operators

Syntax

postfix-expr:

primary-expr

postfix-expr [ expression ]

postfix-expr ( argument-expr-list_opt )

postfix-expr . identifier

postfix-expr -> identifier

postfix-expr ++

postfix-expr --

( type-name ) { initializer-list }

( type-name ) { initializer-list , }

argument-expr-list:

assignment-expr

argument-expr-list , assignment-expr

6.5.2.1 Array subscripting

Constraints

One of the expressions shall have type ``pointer to object type'', the other expression shall have integer type, and the result has type ``type''.

Semantics

A postfix expression followed by an expression in square brackets [] is a subscripted designation of an element of an array object. The definition of the subscript operator [] is that E1[E2] is identical to (*((E1)+(E2))). Because of the conversion rules that apply to the binary + operator, if E1 is an array object (equivalently, a pointer to the initial element of an array object) and E2 is an integer, E1[E2] designates the E2-th element of E1 (counting from zero).

Successive subscript operators designate an element of a multidimensional array object. If E is an n-dimensional array (n>=2) with dimensions i|j| ... |k, then E (used as other than an lvalue) is converted to a pointer to an (n-1)-dimensional array with dimensions j| ... |k. If the unary * operator is applied to this pointer explicitly, or implicitly as a result of subscripting, the result is the pointed-to (n-1)-dimensional array, which itself is converted into a pointer if used as other than an lvalue. It follows from this that arrays are stored in row-major order (last subscript varies fastest).

EXAMPLE Consider the array object defined by the declaration

        int x[3][5];

Here x is a 3|5 array of ints; more precisely, x is an array of three element objects, each of which is an array of five ints. In the expression x[i], which is equivalent to (*((x)+(i))), x is first converted to a pointer to the initial array of five ints. Then i is adjusted according to the type of x, which conceptually entails multiplying i by the size of the object to which the pointer points, namely an array of five int objects. The results are added and indirection is applied to yield an array of five ints. When used in the expression x[i][j], that array is in turn converted to a pointer to the first of the ints, so x[i][j] yields an int.

Forward references: additive operators (6.5.6), address and indirection operators (6.5.3.2), array declarators (6.7.5.2).

6.5.2.2 Function calls

Constraints

The expression that denotes the called function67) shall have type pointer to function returning void or returning an object type other than an array type.

If the expression that denotes the called function has a type that includes a prototype, the number of arguments shall agree with the number of parameters. Each argument shall have a type such that its value may be assigned to an object with the unqualified version of the type of its corresponding parameter.

Semantics

A postfix expression followed by parentheses () containing a possibly empty, comma-separated list of expressions is a function call. The postfix expression denotes the called function. The list of expressions specifies the arguments to the function.

An argument may be an expression of any object type. In preparing for the call to a function, the arguments are evaluated, and each parameter is assigned the value of the corresponding argument.68)

If the expression that denotes the called function has

type pointer to function returning an object type, the function call expression has the same type as that object type, and has the value determined as specified in 6.8.6.4. Otherwise, the function call has type void. If an attempt is made to modify the result of a function call or to access it after the next sequence point, the behavior is undefined.

If the expression that denotes the called function has a type that does not include a prototype, the integer promotions are performed on each argument, and arguments that have type float are promoted to double. These are called the default argument promotions. If the number of arguments does not agree with the number of parameters, the behavior is undefined. If the function is defined with a type that includes a prototype, and either the prototype ends with an ellipsis (, ...) or the types of the arguments after promotion are not compatible with the types of the parameters, the behavior is undefined. If the function is defined with a type that does not include a prototype, and the types of the arguments after promotion are not compatible with those of the parameters after promotion, the behavior is undefined, except for the following cases:

-- one promoted type is a signed integer type, the other promoted type is the corresponding unsigned integer type, and the value is representable in both types;

-- one type is pointer to void and the other is a pointer to a character type.

If the expression that denotes the called function has a type that does include a prototype, the arguments are implicitly converted, as if by assignment, to the types of the corresponding parameters, taking the type of each parameter to be the unqualified version of its declared type. The ellipsis notation in a function prototype declarator causes argument type conversion to stop after the last declared parameter. The default argument promotions are performed on trailing arguments.

No other conversions are performed implicitly; in particular, the number and types of arguments are not compared with those of the parameters in a function definition that does not include a function prototype declarator.

If the function is defined with a type that is not compatible with the type (of the expression) pointed to by the expression that denotes the called function, the behavior is undefined.

#10

The order of evaluation of the function designator, the actual arguments, and subexpressions within the actual arguments is unspecified, but there is a sequence point before the actual call.

#11

Recursive function calls shall be permitted, both directly and indirectly through any chain of other functions.

#12

EXAMPLE In the function call

        (*pf[f1()]) (f2(), f3() + f4())

the functions f1, f2, f3, and f4 may be called in any order. All side effects have to be completed before the function pointed to by pf[f1()] is called.

Forward references: function declarators (including prototypes) (6.7.5.3), function definitions (6.9.1), the return statement (6.8.6.4), simple assignment (6.5.16.1).

6.5.2.3 Structure and union members

Constraints

The first operand of the . operator shall have a qualified or unqualified structure or union type, and the second operand shall name a member of that type.

The first operand of the -> operator shall have type ``pointer to qualified or unqualified structure'' or ``pointer to qualified or unqualified union'', and the second operand shall name a member of the type pointed to.

Semantics

A postfix expression followed by the . operator and an identifier designates a member of a structure or union object. The value is that of the named member, and is an lvalue if the first expression is an lvalue. If the first expression has qualified type, the result has the so- qualified version of the type of the designated member.

A postfix expression followed by the -> operator and an identifier designates a member of a structure or union object. The value is that of the named member of the object to which the first expression points, and is an lvalue.69) If the first expression is a pointer to a qualified type, the result has the so-qualified version of the type of the designated member.

With one exception, if the value of a member of a union object is used when the most recent store to the object was to a different member, the behavior is implementation-defined.70) One special guarantee is made in order to simplify the use of unions: If a union contains several structures that share a common initial sequence (see below), and if the union object currently contains one of these structures, it is permitted to inspect the common initial part of any of them anywhere that a declaration of the completed type of the union is visible. Two structures share a common initial sequence if corresponding members have compatible types (and, for bit-fields, the same widths) for a sequence of one or more initial members.

EXAMPLE 1 If f is a function returning a structure or union, and x is a member of that structure or union, f().x is a valid postfix expression but is not an lvalue.

EXAMPLE 2 In:

        struct s { int i; const int ci; };
        struct s s;
        const struct s cs;
        volatile struct s vs;

the various members have the types:

        s.i     int
        s.ci    const int
        cs.i    const int
        cs.ci   const int
        vs.i    volatile int
        vs.ci   volatile const int

EXAMPLE 3 The following is a valid fragment:

        union {
                struct {
                        int     alltypes;
                } n;
                struct {
                        int     type;
                        int     intnode;
                } ni;
                struct {
                        int     type;
                        double  doublenode;
                } nf;
        } u;
        u.nf.type = 1;
        u.nf.doublenode = 3.14;
        /* ... */
        if (u.n.alltypes == 1)
                if (sin(u.nf.doublenode) == 0.0)
                        /* ... */

The following is not a valid fragment (because the union type is not visible within function f):

        struct t1 { int m; };
        struct t2 { int m; };
        int f(struct t1 * p1, struct t2 * p2)
        {
                if (p1->m < 0)
                        p2->m = -p2->m;
                return p1->m;
        }
        int g()
        {
                union {
                        struct t1 s1;
                        struct t2 s2;
                } u;
                /* ... */
                return f(&u.s1, &u.s2);
        }

Forward references: address and indirection operators (6.5.3.2), structure and union specifiers (6.7.2.1).

6.5.2.4 Postfix increment and decrement operators

Constraints

The operand of the postfix increment or decrement operator shall have qualified or unqualified real or pointer type and shall be a modifiable lvalue.

Semantics

The result of the postfix ++ operator is the value of the operand. After the result is obtained, the value of the operand is incremented. (That is, the value 1 of the appropriate type is added to it.) See the discussions of additive operators and compound assignment for information on constraints, types, and conversions and the effects of operations on pointers. The side effect of updating the stored value of the operand shall occur between the previous and the next sequence point.

The postfix -- operator is analogous to the postfix ++ operator, except that the value of the operand is decremented (that is, the value 1 of the appropriate type is subtracted from it).

Forward references: additive operators (6.5.6), compound assignment (6.5.16.2).

6.5.2.5 Compound literals

Constraints

The type name shall specify an object type or an array of unknown size, but not a variable length array type.

No initializer shall attempt to provide a value for an object not contained within the entire unnamed object specified by the compound literal.

If the compound literal occurs outside the body of a function, the initializer list shall consist of constant expressions.

Semantics

A postfix expression that consists of a parenthesized type name followed by a brace-enclosed list of initializers is a compound literal. It provides an unnamed object whose value is given by the initializer list.71)

If the type name specifies an array of unknown size, the size is determined by the initializer list as specified in 6.7.7, and the type of the compound literal is that of the completed array type. Otherwise (when the type name specifies an object type), the type of the compound literal is that specified by the type name. In either case, the

result is an lvalue.

The value of the compound literal is that of an unnamed object initialized by the initializer list. If the compound literal occurs outside the body of a function, the object has static storage duration; otherwise, it has automatic storage duration associated with the enclosing block.

All the semantic rules and constraints for initializer lists in 6.7.8 are applicable to compound literals.72)

String literals, and compound literals with const- qualified types, need not designate distinct objects.73)

EXAMPLE 1 The file scope definition

        int *p = (int []){2, 4};

initializes p to point to the first element of an array of two ints, the first having the value two and the second, four. The expressions in this compound literal are required to be constant. The unnamed object has static storage duration.

#10

EXAMPLE 2 In contrast, in

        void f(void)
        {
                int *p;
                /*...*/
                p = (int [2]){*p};
                /*...*/
        }

p is assigned the address of the first element of an array of two ints, the first having the value previously pointed to by p and the second, zero. The expressions in this compound literal need not be constant. The unnamed object has automatic storage duration.

#11

EXAMPLE 3 Initializers with designations can be combined with compound literals. Structure objects created using compound literals can be passed to functions without depending on member order:

        drawline((struct point){.x=1, .y=1},
                (struct point){.x=3, .y=4});

Or, if drawline instead expected pointers to struct point:

        drawline(&(struct point){.x=1, .y=1},
                &(struct point){.x=3, .y=4});

#12

EXAMPLE 4 A read-only compound literal can be specified through constructions like:

        (const float []){1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6}

#13

EXAMPLE 5 The following three expressions have different meanings:

        "/tmp/fileXXXXXX"
        (char []){"/tmp/fileXXXXXX"}
        (const char []){"/tmp/fileXXXXXX"}

The first always has static storage duration and has type array of char, but need not be modifiable; the last two have automatic storage duration when they occur within the body of a function, and the first of these two is modifiable.

#14

EXAMPLE 6 Like string literals, const-qualified compound literals can be placed into read-only memory and can even be shared. For example,

        (const char []){"abc"} == "abc"

might yield 1 if the literals' storage is shared.

#15

EXAMPLE 7 Since compound literals are unnamed, a single compound literal cannot specify a circularly linked object. For example, there is no way to write a self- referential compound literal that could be used as the function argument in place of the named object endless_zeros below:

        struct int_list { int car; struct int_list *cdr; };
        struct int_list endless_zeros = {0, &endless_zeros};
        eval(endless_zeros);

#16

EXAMPLE 8 Each compound literal creates only a single object in a given scope: struct s { int i; };

        int f (void)
        {
                struct s *p = 0, *q;
                int j = 0;

        again:                                               
                q = p, p = &((struct s){ j++ });             
                if (j < 2) goto again;                       

                return p == q && q->i == 1;
        }

The function f() always returns the value 1.

#17

Note that if an iteration statement were used instead of an explicit goto and a labeled statement, the lifetime of the unnamed object would be the body of the loop only, and on entry next time around p would be pointing to an object which is no longer guaranteed to exist, which would result in undefined behavior.

6.5.3 Unary operators

Syntax

unary-expr:

postfix-expr

++ unary-expr

-- unary-expr

unary-operator cast-expr

sizeof unary-expr

sizeof ( type-name )

unary-operator: one of

& * + - ~ !

6.5.3.1 Prefix increment and decrement operators

Constraints

The operand of the prefix increment or decrement operator shall have qualified or unqualified real or pointer type and shall be a modifiable lvalue.

Semantics

The value of the operand of the prefix ++ operator is incremented. The result is the new value of the operand after incrementation. The expression ++E is equivalent to (E+=1). See the discussions of additive operators and compound assignment for information on constraints, types, side effects, and conversions and the effects of operations on pointers.

The prefix -- operator is analogous to the prefix ++ operator, except that the value of the operand is decremented.

Forward references: additive operators (6.5.6), compound assignment (6.5.16.2).

6.5.3.2 Address and indirection operators

Constraints

The operand of the unary & operator shall be either a function designator, the result of a [] or unary * operator, or an lvalue that designates an object that is not a bit- field and is not declared with the register storage-class specifier.

The operand of the unary * operator shall have pointer type.

Semantics

The unary & operator returns the address of its operand. If the operand has type ``type'', the result has type ``pointer to type''. If the operand is the result of a unary * operator, neither that operator nor the & operator is evaluated and the result is as if both were omitted, except that the constraints on the operators still apply and the result is not an lvalue. Similarly, if the operand is the result of a [] operator, neither the & operator nor the unary * that is implied by the [] is evaluated and the result is as if the & operator were removed and the [] operator were changed to a + operator. Otherwise, the result is a pointer to the object or function designated by its operand.

The unary * operator denotes indirection. If the operand points to a function, the result is a function designator; if it points to an object, the result is an lvalue designating the object. If the operand has type ``pointer to type'', the result has type ``type''. If an invalid value has been assigned to the pointer, the behavior of the unary * operator is undefined.74)

Forward references: storage-class specifiers (6.7.1), structure and union specifiers (6.7.2.1).

6.5.3.3 Unary arithmetic operators

Constraints

The operand of the unary + or - operator shall have arithmetic type; of the ~ operator, integer type; of the ! operator, scalar type.

Semantics

The result of the unary + operator is the value of its (promoted) operand. The integer promotions are performed on the operand, and the result has the promoted type.

The result of the unary - operator is the negative of its (promoted) operand. The integer promotions are performed on the operand, and the result has the promoted type.

The result of the ~ operator is the bitwise complement of its (promoted) operand (that is, each bit in the result is set if and only if the corresponding bit in the converted operand is not set). The integer promotions are performed on the operand, and the result has the promoted type. If the promoted type is an unsigned type, the expression ~E is equivalent to the maximum value representable in that type minus E.

The result of the logical negation operator ! is 0 if the value of its operand compares unequal to 0, 1 if the value of its operand compares equal to 0. The result has type int. The expression !E is equivalent to (0==E).

Forward references: characteristics of floating types <float.h> (7.7), sizes of integer types <limits.h> (7.10).

6.5.3.4 The sizeof operator

Constraints

The sizeof operator shall not be applied to an expression that has function type or an incomplete type, to the parenthesized name of such a type, or to an expression that designates a bit-field member.

Semantics

The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand. The result is an integer. If the type of the operand is a variable length array type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an integer constant.

When applied to an operand that has type char, unsigned char, or signed char, (or a qualified version thereof) the result is 1. When applied to an operand that has array type, the result is the total number of bytes in the array.75) When applied to an operand that has structure or union type, the result is the total number of bytes in such an object, including internal and trailing padding.

The value of the result is implementation-defined, and its type (an unsigned integer type) is size_t, defined in the <stddef.h> header.

EXAMPLE 1 A principal use of the sizeof operator is in communication with routines such as storage allocators and I/O systems. A storage-allocation function might accept a size (in bytes) of an object to allocate and return a pointer to void. For example:

        extern void *alloc(size_t);
        double *dp = alloc(sizeof *dp);

The implementation of the alloc function should ensure that its return value is aligned suitably for conversion to a pointer to double.

EXAMPLE 2 Another use of the sizeof operator is to compute the number of elements in an array:

        sizeof array / sizeof array[0]

EXAMPLE 3 In this example, the size of a variable- length array is computed and returned from a function:

        size_t fsize3 (int n)
        {
                char b[n+3];       // Variable length array.
                return sizeof b;   // Execution time sizeof.
        }
        int main()
        {
                size_t size;
                size = fsize3(10); // fsize3 returns 13.
                return 0;
        }

Forward references: common definitions <stddef.h> (7.17), declarations (6.7), structure and union specifiers (6.7.2.1), type names (6.7.6), array declarators (6.7.5.2).

6.5.4 Cast operators

Syntax

cast-expr:

unary-expr

( type-name ) cast-expr

Constraints

Unless the type name specifies a void type, the type name shall specify qualified or unqualified scalar type and the operand shall have scalar type.

Conversions that involve pointers, other than where permitted by the constraints of 6.5.16.1, shall be specified by means of an explicit cast.

Semantics

Preceding an expression by a parenthesized type name converts the value of the expression to the named type. This construction is called a cast.76) A cast that specifies no conversion has no effect on the type or value of an expression.77)

Forward references: equality operators (6.5.9), function declarators (including prototypes) (6.7.5.3), simple assignment (6.5.16.1), type names (6.7.6).

6.5.5 Multiplicative operators

Syntax

multiplicative-expr:

cast-expr

multiplicative-expr * cast-expr

multiplicative-expr / cast-expr

multiplicative-expr % cast-expr

Constraints

Each of the operands shall have arithmetic type. The operands of the % operator shall have integer type.

Semantics

The usual arithmetic conversions are performed on the operands.

The result of the binary * operator is the product of the operands.

The result of the / operator is the quotient from the division of the first operand by the second; the result of the % operator is the remainder. In both operations, if the value of the second operand is zero, the behavior is undefined.

When integers are divided, the result of the / operator is the algebraic quotient with any fractional part discarded.78) If the quotient a/b is representable, the expression (a/b)*b + a%b shall equal a.

6.5.6 Additive operators

Syntax

additive-expr:

multiplicative-expr

additive-expr + multiplicative-expr

additive-expr - multiplicative-expr

Constraints

For addition, either both operands shall have arithmetic type, or one operand shall be a pointer to an object type and the other shall have integer type. (Incrementing is equivalent to adding 1.)

For subtraction, one of the following shall hold:

-- both operands have arithmetic type;

-- both operands are pointers to qualified or unqualified versions of compatible object types; or

-- the left operand is a pointer to an object type and the right operand has integer type.

(Decrementing is equivalent to subtracting 1.)

Semantics

If both operands have arithmetic type, the usual arithmetic conversions are performed on them.

The result of the binary + operator is the sum of the operands.

The result of the binary - operator is the difference resulting from the subtraction of the second operand from the first.

For the purposes of these operators, a pointer to a nonarray object behaves the same as a pointer to the first element of an array of length one with the type of the object as its element type.

When an expression that has integer type is added to or subtracted from a pointer, the result has the type of the pointer operand. If the pointer operand points to an element of an array object, and the array is large enough, the result points to an element offset from the original element such that the difference of the subscripts of the resulting and original array elements equals the integer expression. In other words, if the expression P points to the i-th element of an array object, the expressions (P)+N (equivalently, N+(P)) and (P)-N (where N has the value n) point to, respectively, the i+n-th and i-n-th elements of the array object, provided they exist. Moreover, if the expression P points to the last element of an array object, the expression (P)+1 points one past the last element of the array object, and if the expression Q points one past the last element of an array object, the expression (Q)-1 points to the last element of the array object. If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined. If the result points one past the last element of the array object, it shall not be used as the operand of a unary * operator that is evaluated.

When two pointers are subtracted, both shall point to elements of the same array object, or one past the last element of the array object; the result is the difference of the subscripts of the two array elements. The size of the result is implementation-defined, and its type (a signed integer type) is ptrdiff_t defined in the <stddef.h> header. If the result is not representable in an object of that type, the behavior is undefined. In other words, if the expressions P and Q point to, respectively, the i-th and j- th elements of an array object, the expression (P)-(Q) has the value i-j provided the value fits in an object of type ptrdiff_t. Moreover, if the expression P points either to an element of an array object or one past the last element of an array object, and the expression Q points to the last element of the same array object, the expression ((Q)+1)-(P) has the same value as ((Q)-(P))+1 and as -((P)-((Q)+1)), and has the value zero if the expression P points one past the last element of the array object, even though the expression (Q)+1 does not point to an element of the array object.79)

#10

EXAMPLE Pointer arithmetic is well defined with pointers to variable length array types.

        {
                int n = 4, m = 3;
                int a[n][m];
                int (*p)[m] = a;        // p == &a[0]
                p += 1;                 // p == &a[1]
                (*p)[2] = 99;           // a[1][2] == 99
                n = p - a;              // n == 1
        }

#11

If array a in the above example were declared to be an array of known constant size, and pointer p were declared to be a pointer to an array of the same known constant size (pointing to a), the results would be the same.

Forward references: array declarators (6.7.5.2), common definitions <stddef.h> (7.17).

6.5.7 Bitwise shift operators

Syntax

shift-expr:

additive-expr

shift-expr << additive-expr

shift-expr >> additive-expr

Constraints

Each of the operands shall have integer type.

Semantics

The integer promotions are performed on each of the operands. The type of the result is that of the promoted left operand. If the value of the right operand is negative or is greater than or equal to the width of the promoted left operand, the behavior is undefined.

The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with zeros. If E1 has an unsigned type, the value of the result is E1|2E2, reduced modulo one more than the maximum value representable in the result type. If E1 has a signed type and nonnegative value, and E1|2E2 is representable in the result type, then that is the resulting value; otherwise, the behavior is undefined.

The result of E1 >> E2 is E1 right-shifted E2 bit positions. If E1 has an unsigned type or if E1 has a signed type and a nonnegative value, the value of the result is the integral part of the quotient of E1 divided by the quantity, 2 raised to the power E2. If E1 has a signed type and a negative value, the resulting value is implementation- defined.

6.5.8 Relational operators

Syntax

relational-expr:

shift-expr

relational-expr < shift-expr

relational-expr > shift-expr

relational-expr <= shift-expr

relational-expr >= shift-expr

Constraints

One of the following shall hold:

-- both operands have real type;

-- both operands are pointers to qualified or unqualified versions of compatible object types; or

-- both operands are pointers to qualified or unqualified versions of compatible incomplete types.

Semantics

If both of the operands have arithmetic type, the usual arithmetic conversions are performed.

For the purposes of these operators, a pointer to a nonarray object behaves the same as a pointer to the first element of an array of length one with the type of the object as its element type.

When two pointers are compared, the result depends on the relative locations in the address space of the objects pointed to. If two pointers to object or incomplete types both point to the same object, or both point one past the last element of the same array object, they compare equal. If the objects pointed to are members of the same aggregate object, pointers to structure members declared later compare greater than pointers to members declared earlier in the structure, and pointers to array elements with larger subscript values compare greater than pointers to elements of the same array with lower subscript values. All pointers to members of the same union object compare equal. If the expression P points to an element of an array object and the expression Q points to the last element of the same array object, the pointer expression Q+1 compares greater than P. In all other cases, the behavior is undefined.

Each of the operators < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to) shall yield 1 if the specified relation is true and 0 if it is false.80) The result has type int.

6.5.9 Equality operators

Syntax

equality-expr:

relational-expr

equality-expr == relational-expr

equality-expr != relational-expr

Constraints

One of the following shall hold:

-- both operands have arithmetic type;

-- both operands are pointers to qualified or unqualified versions of compatible types;

-- one operand is a pointer to an object or incomplete type and the other is a pointer to a qualified or unqualified version of void; or

-- one operand is a pointer and the other is a null pointer constant.

Semantics

The == (equal to) and != (not equal to) operators are analogous to the relational operators except for their lower precedence.81) Each of the operators yields 1 if the specified relation is true and 0 if it is false. The result has type int. For any pair of operands, exactly one of the relations is true.

The first operand shall have scalar type.

One of the following shall hold for the second and third operands:

-- both operands have arithmetic type;

-- both operands have compatible structure or union types;

-- both operands have void type;

-- both operands are pointers to qualified or unqualified versions of compatible types;

-- one operand is a pointer and the other is a null pointer constant; or

-- one operand is a pointer to an object or incomplete type and the other is a pointer to a qualified or unqualified version of void.

Semantics

The first operand is evaluated; there is a sequence point after its evaluation. The second operand is evaluated only if the first compares unequal to 0; the third operand is evaluated only if the first compares equal to 0; the result is the value of the second or third operand (whichever is evaluated), converted to the type described below.83) If an attempt is made to modify the result of a conditional operator or to access it after the next sequence point, the behavior is undefined.

If both the second and third operands have arithmetic type, the result type that would be determined by the usual arithmetic conversions, were they applied to those two operands, is the type of the result. If both the operands have structure or union type, the result has that type. If both operands have void type, the result has void type.

If both the second and third operands are pointers or one is a null pointer constant and the other is a pointer, the result type is a pointer to a type qualified with all the type qualifiers of the types pointed-to by both operands. Furthermore, if both operands are pointers to compatible types or to differently qualified versions of compatible types, the result type is a pointer to an appropriately qualified version of the composite type; if one operand is a null pointer constant, the result has the type of the other operand; otherwise, one operand is a pointer to void or a qualified version of void, in which case the result type is a pointer to an appropriately qualified version of void.

EXAMPLE The common type that results when the second and third operands are pointers is determined in two independent stages. The appropriate qualifiers, for example, do not depend on whether the two pointers have compatible types.

Given the declarations

        const void *c_vp;
        void *vp;
        const int *c_ip;
        volatile int *v_ip;
        int *ip;
        const char *c_cp;

the third column in the following table is the common type that is the result of a conditional expression in which the first two columns are the second and third operands (in either order):

        c_vp    c_ip            const void *
        v_ip    0               volatile int *
        c_ip    v_ip            const volatile int *
        vp      c_cp            const void *
        ip      c_ip            const int *
        vp      ip              void *

6.5.16 Assignment operators

Syntax

assignment-expr:

conditional-expr

unary-expr assignment-operator assignment-expr

assignment-operator: one of

= *= /= %= += -= <<= >>= &= ^= |=

Constraints

An assignment operator shall have a modifiable lvalue as its left operand.

Semantics

An assignment operator stores a value in the object designated by the left operand. An assignment expression has the value of the left operand after the assignment, but is not an lvalue. The type of an assignment expression is the type of the left operand unless the left operand has qualified type, in which case it is the unqualified version of the type of the left operand. The side effect of updating the stored value of the left operand shall occur between the previous and the next sequence point.

The order of evaluation of the operands is unspecified. If an attempt is made to modify the result of an assignment operator or to access it after the next sequence point, the behavior is undefined.

6.5.16.1 Simple assignment

Constraints

One of the following shall hold:84)

-- the left operand has qualified or unqualified arithmetic type and the right has arithmetic type;

-- the left operand has a qualified or unqualified version of a structure or union type compatible with the type of the right;

-- both operands are pointers to qualified or unqualified versions of compatible types, and the type pointed to by the left has all the qualifiers of the type pointed to by the right;

-- one operand is a pointer to an object or incomplete type and the other is a pointer to a qualified or unqualified version of void, and the type pointed to by the left has all the qualifiers of the type pointed to by the right; or

-- the left operand is a pointer and the right is a null pointer constant.

-- the left operand has type _Bool and the right is a pointer.

Semantics

In simple assignment (=), the value of the right operand is converted to the type of the assignment expression and replaces the value stored in the object designated by the left operand.

If the value being stored in an object is accessed from another object that overlaps in any way the storage of the first object, then the overlap shall be exact and the two objects shall have qualified or unqualified versions of a compatible type; otherwise, the behavior is undefined.

EXAMPLE 1 In the program fragment

        int f(void);
        char c;
        /* ... */
        if ((c = f()) == -1)
                /* ... */

the int value returned by the function may be truncated when stored in the char, and then converted back to int width prior to the comparison. In an implementation in which ``plain'' char has the same range of values as unsigned char (and char is narrower than int), the result of the conversion cannot be negative, so the operands of the comparison can never compare equal. Therefore, for full portability, the variable c should be declared as int.

EXAMPLE 2 In the fragment:

        char c;
        int i;
        long l;

        l = (c = i);

the value of i is converted to the type of the assignment expression c = i, that is, char type. The value of the expression enclosed in parentheses is then converted to the type of the outer assignment expression, that is, long int type.

EXAMPLE 3 Consider the fragment: const char **cpp; char *p; const char c = 'A';

        cpp = &p;       // constraint violation
        *cpp = &c;      // valid
        *p = 0;         // valid

The first assignment is unsafe because it would allow the following valid code to attempt to change the value of the const object c.

6.5.16.2 Compound assignment

Constraints

For the operators += and -= only, either the left operand shall be a pointer to an object type and the right shall have integer type, or the left operand shall have qualified or unqualified arithmetic type and the right shall have arithmetic type.

For the other operators, each operand shall have arithmetic type consistent with those allowed by the corresponding binary operator.

Semantics

A compound assignment of the form E1 op= E2 differs from the simple assignment expression E1 = E1 op (E2) only in that the lvalue E1 is evaluated only once.

6.5.17 Comma operator

Syntax

expression:

assignment-expr

expression , assignment-expr

Semantics

The left operand of a comma operator is evaluated as a void expression; there is a sequence point after its evaluation. Then the right operand is evaluated; the result has its type and value.85) If an attempt is made to modify the result of a comma operator or to access it after the next sequence point, the behavior is undefined.

EXAMPLE As indicated by the syntax, the comma operator (as described in this subclause) cannot appear in contexts where a comma is used to separate items in a list (such as arguments to functions or lists of initializers). On the other hand, it can be used within a parenthesized expression or within the second expression of a conditional operator in such contexts. In the function call

        f(a, (t=3, t+2), c)

the function has three arguments, the second of which has the value 5.

Forward references: initialization (6.7.8).

6.6 Constant expressions

Syntax

constant-expr:

conditional-expr

Description

A constant expression can be evaluated during translation rather than runtime, and accordingly may be used in any place that a constant may be.

Constraints

Constant expressions shall not contain assignment, increment, decrement, function-call, or comma operators, except when they are contained within a subexpression that is not evaluated.86)

Each constant expression shall evaluate to a constant that is in the range of representable values for its type.

Semantics

An expression that evaluates to a constant is required in several contexts. If a floating expression is evaluated in the translation environment, the arithmetic precision and range shall be at least as great as if the expression were being evaluated in the execution environment.

An integer constant expression87) shall have integer type and shall only have operands that are integer constants, enumeration constants, character constants, sizeof expressions whose results are integer constants, and floating constants that are the immediate operands of casts. Cast operators in an integer constant expression shall only convert arithmetic types to integer types, except as part of an operand to the sizeof operator.

More latitude is permitted for constant expressions in initializers. Such a constant expression shall be, or evaluate to, one of the following:

-- an arithmetic constant expression,

-- a null pointer constant,

-- an address constant, or

-- an address constant for an object type plus or minus an integer constant expression.

An arithmetic constant expression shall have arithmetic type and shall only have operands that are integer constants, floating constants, enumeration constants, character constants, and sizeof expressions. Cast operators in an arithmetic constant expression shall only convert arithmetic types to arithmetic types, except as part of an operand to the sizeof operator.

An address constant is a null pointer, a pointer to an lvalue designating an object of static storage duration, or to a function designator; it shall be created explicitly using the unary & operator or an integer constant cast to pointer type, or implicitly by the use of an expression of array or function type. The array-subscript [] and member- access . and -> operators, the address & and indirection * unary operators, and pointer casts may be used in the creation of an address constant, but the value of an object shall not be accessed by use of these operators.

#10

An implementation may accept other forms of constant expressions.

#11

The semantic rules for the evaluation of a constant expression are the same as for nonconstant expressions.88)

Forward references: array declarators (6.7.5.2), initialization (6.7.8).

6.7 Declarations

Syntax

declaration:

declaration-specifiers init-declarator-list_opt ;

declaration-specifiers:

storage-class-specifier declaration-specifiers_opt

type-specifier declaration-specifiers_opt

type-qualifier declaration-specifiers_opt

function-specifier declaration-specifiers_opt

init-declarator-list:

init-declarator

init-declarator-list , init-declarator

init-declarator:

declarator

declarator = initializer

Constraints

A declaration shall declare at least a declarator (other than the parameters of a function or the members of a structure or union), a tag, or the members of an enumeration.

If an identifier has no linkage, there shall be no more than one declaration of the identifier (in a declarator or type specifier) with the same scope and in the same name space, except for tags as specified in 6.7.2.3.

All declarations in the same scope that refer to the same object or function shall specify compatible types.

Semantics

A declaration specifies the interpretation and attributes of a set of identifiers. A definition of an identifier is a declaration for that identifier that:

-- for an object, causes storage to be reserved for that object;

-- for a function, includes the function body;89)

-- for an enumeration constant or typedef name, is the (only) declaration of the identifier.

The declaration specifiers consist of a sequence of specifiers that indicate the linkage, storage duration, and part of the type of the entities that the declarators denote. The init-declarator-list is a comma-separated sequence of declarators, each of which may have additional type information, or an initializer, or both. The declarators contain the identifiers (if any) being declared.

If an identifier for an object is declared with no linkage, the type for the object shall be complete by the end of its declarator, or by the end of its init-declarator if it has an initializer.

Forward references: declarators (6.7.5), enumeration specifiers (6.7.2.2), initialization (6.7.8), tags (6.7.2.3).

6.7.1 Storage-class specifiers

Syntax

storage-class-specifier:

typedef

extern

static

auto

Constraints

At most, one storage-class specifier may be given in the declaration specifiers in a declaration.90)

Semantics

The typedef specifier is called a ``storage-class specifier'' for syntactic convenience only; it is discussed in 6.7.7. The meanings of the various linkages and storage durations were discussed in 6.2.2 and 6.2.4.

A declaration of an identifier for an object with storage-class specifier register suggests that access to the object be as fast as possible. The extent to which such suggestions are effective is implementation-defined.91)

The declaration of an identifier for a function that

has block scope shall have no explicit storage-class specifier other than extern.

If an aggregate or union object is declared with a storage-class specifier other than typedef, the properties resulting from the storage-class specifier, except with respect to linkage, also apply to the members of the object, and so on recursively for any aggregate or union member objects.

Forward references: type definitions (6.7.7).

6.7.2 Type specifiers

Syntax

type-specifier:

void

char

short

int

long

float

double

signed

unsigned

_Bool

_Complex

_Imaginary

struct-or-union-specifier

enum-specifier

typedef-name

Constraints

At least one type specifier shall be given in the declaration specifiers in each declaration, and in the specifier-qualifier list in each struct declaration and type name. Each list of type specifiers shall be one of the following sets (delimited by commas, when there is more than

one set on a line); the type specifiers may occur in any order, possibly intermixed with the other declaration specifiers.

-- void

-- char

-- signed char

-- unsigned char

-- short, signed short, short int, or signed short int

-- unsigned short, or unsigned short int

-- int, signed, or signed int

-- unsigned, or unsigned int

-- long, signed long, long int, or signed long int

-- unsigned long, or unsigned long int

-- long long, signed long long, long long int, or signed long long int

-- unsigned long long, or unsigned long long int

-- float

-- double

-- long double

-- _Bool

-- float _Complex

-- double _Complex

-- long double _Complex

-- float _Imaginary

-- double _Imaginary

-- long double _Imaginary

-- struct or union specifier

-- enum specifier -- typedef name

The type specifiers _Complex and _Imaginary shall not be used if the implementation does not provide those types.92)

Semantics

Specifiers for structures, unions, and enumerations are discussed in 6.7.2.1 through 6.7.2.3. Declarations of typedef names are discussed in 6.7.7. The characteristics of the other types are discussed in 6.2.5.

Each of the comma-separated sets designates the same type, except that for bit-fields, it is implementation- defined whether the specifier int designates the same type as signed int or the same type as unsigned int.

Forward references: enumeration specifiers (6.7.2.2), structure and union specifiers (6.7.2.1), tags (6.7.2.3), type definitions (6.7.7).

6.7.2.1 Structure and union specifiers

Syntax

struct-or-union-specifier:

struct-or-union identifier_opt { struct-declaration-list }

struct-or-union identifier

struct-or-union:

struct

union

struct-declaration-list:

struct-declaration

struct-declaration-list struct-declaration

struct-declaration:

specifier-qualifier-list struct-declarator-list ;

specifier-qualifier-list:

type-specifier specifier-qualifier-list_opt

type-qualifier specifier-qualifier-list_opt

struct-declarator-list:

struct-declarator

struct-declarator-list , struct-declarator

struct-declarator:

declarator

declarator_opt : constant-expr

Constraints

A structure or union shall not contain a member with incomplete or function type (hence, a structure shall not contain an instance of itself, but may contain a pointer to an instance of itself), except that the last member of a structure with more than one named member may have incomplete array type; such a structure (and any union containing, possibly recursively, a member that is such a structure) shall not be a member of a structure or an element of an array.

The expression that specifies the width of a bit-field shall be an integer constant expression that has nonnegative value that shall not exceed the number of bits in an object of the type that is specified if the colon and expression are omitted. If the value is zero, the declaration shall have no declarator.

Semantics

As discussed in 6.2.5, a structure is a type consisting of a sequence of members, whose storage is allocated in an ordered sequence, and a union is a type consisting of a sequence of members whose storage overlap.

Structure and union specifiers have the same form.

The presence of a struct-declaration-list in a struct- or-union-specifier declares a new type, within a translation unit. The struct-declaration-list is a sequence of declarations for the members of the structure or union. If the struct-declaration-list contains no named members, the behavior is undefined. The type is incomplete until after the } that terminates the list.

A member of a structure or union may have any object type other than a variably modified type.93) In addition, a member may be declared to consist of a specified number of bits (including a sign bit, if any). Such a member is called a bit-field;94) its width is preceded by a colon.

A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed int, or unsigned int. A bit-field is interpreted as a signed or unsigned integer type consisting of the specified number of bits.95) If the value 0 or 1 is stored into a nonzero-width bit-field of type _Bool, the value of the bit-field shall compare equal to the value stored.

An implementation may allocate any addressable storage unit large enough to hold a bit-field. If enough space remains, a bit-field that immediately follows another bit- field in a structure shall be packed into adjacent bits of the same unit. If insufficient space remains, whether a bit-field that does not fit is put into the next unit or overlaps adjacent units is implementation-defined. The order of allocation of bit-fields within a unit (high-order to low-order or low-order to high-order) is implementation- defined. The alignment of the addressable storage unit is unspecified.

#10

A bit-field declaration with no declarator, but only a colon and a width, indicates an unnamed bit-field.96) As a special case, a bit-field structure member with a width of 0 indicates that no further bit-field is to be packed into the unit in which the previous bit-field, if any, was placed.

#11

Each non-bit-field member of a structure or union object is aligned in an implementation-defined manner appropriate to its type.

#12

Within a structure object, the non-bit-field members and the units in which bit-fields reside have addresses that increase in the order in which they are declared. A pointer to a structure object, suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in which it resides), and vice versa. There may be unnamed padding within a structure object, but not at its beginning.

#13

The size of a union is sufficient to contain the largest of its members. The value of at most one of the

members can be stored in a union object at any time. A pointer to a union object, suitably converted, points to each of its members (or if a member is a bit-field, then to the unit in which it resides), and vice versa.

#14

There may be unnamed padding at the end of a structure or union.

#15

As a special case, the last element of a structure with more than one named member may have an incomplete array type. This is called a flexible array member, and the size of the structure shall be equal to the offset of the last element of an otherwise identical structure that replaces the flexible array member with an array of unspecified length.97) When an lvalue whose type is a structure with a flexible array member is used to access an object, it behaves as if that member were replaced with the longest array, with the same element type, that would not make the structure larger than the object being accessed; the offset of the array shall remain that of the flexible array member, even if this would differ from that of the replacement array. If this array would have no elements, then it behaves as if it had one element, but the behavior is undefined if any attempt is made to access that element or to generate a pointer one past it.

#16

EXAMPLE Assuming that all array members are aligned the same, after the declarations:

        struct s { int n; double d[]; };
        struct ss { int n; double d[1]; };

the three expressions:

        sizeof (struct s)
        offsetof(struct s, d)
        offsetof(struct ss, d)

have the same value. The structure struct s has a flexible array member d.

#17

If sizeof (double) is 8, then after the following code is executed:

        struct s *s1;
        struct s *s2;
        s1 = malloc(sizeof (struct s) + 64);
        s2 = malloc(sizeof (struct s) + 46);

and assuming that the calls to malloc succeed, the objects pointed to by s1 and s2 behave as if the identifiers had been declared as:

        struct { int n; double d[8]; } *s1;
        struct { int n; double d[5]; } *s2;

#18

Following the further successful assignments:

        s1 = malloc(sizeof (struct s) + 10);
        s2 = malloc(sizeof (struct s) +  6);

they then behave as if the declarations were:

        struct { int n; double d[1]; } *s1, *s2;

and:

        double *dp;
        dp = &(s1->d[0]);       // Permitted
        *dp = 42;               // Permitted
        dp = &(s2->d[0]);       // Permitted
        *dp = 42;               // Undefined behavior

Forward references: tags (6.7.2.3).

6.7.2.2 Enumeration specifiers

Syntax

enum-specifier:

enum identifier_opt { enumerator-list }

enum identifier_opt { enumerator-list , }

enum identifier

enumerator-list:

enumerator

enumerator-list , enumerator

enumerator:

enumeration-constant

enumeration-constant = constant-expr

Constraints

The expression that defines the value of an enumeration constant shall be an integer constant expression that has a value representable as an int.

Semantics

The identifiers in an enumerator list are declared as constants that have type int and may appear wherever such are permitted.98) An enumerator with = defines its enumeration constant as the value of the constant expression. If the first enumerator has no =, the value of its enumeration constant is 0. Each subsequent enumerator with no = defines its enumeration constant as the value of the constant expression obtained by adding 1 to the value of the previous enumeration constant. (The use of enumerators with = may produce enumeration constants with values that duplicate other values in the same enumeration.) The enumerators of an enumeration are also known as its members.

Each enumerated type shall be compatible with an integer type. The choice of type is implementation-defined,99) but shall be capable of representing the values of all the members of the enumeration. The enumerated type is incomplete until after the } that terminates the list of enumerator declarations.

EXAMPLE The following fragment:

        enum hue { chartreuse, burgundy, claret=20, winedark };
        enum hue col, *cp;
        col = claret;
        cp = &col;
        if (*cp != burgundy)
                /* ... */

makes hue the tag of an enumeration, and then declares col as an object that has that type and cp as a pointer to an object that has that type. The enumerated values are in the set {0, 1, 20, 21}.

Forward references: tags (6.7.2.3).

6.7.2.3 Tags

Constraints

A specific type shall have its content defined at most once.

A type specifier of the form

enum identifier

without an enumerator list shall only appear after the type it specifies is completed.

Semantics

All declarations of structure, union, or enumerated types that have the same scope and use the same tag declare the same type. The type is incomplete100) until the closing brace of the list defining the content, and complete thereafter.

Two declarations of structure, union, or enumerated types which are in different scopes or use different tags declare distinct types. Each declaration of a structure, union, or enumerated type which does not include a tag declares a distinct type.

A type specifier of the form

        struct-or-union identifier-opt { struct-declaration-list }

        enum identifier { enumerator-list }

        enum identifier { enumerator-list , }

declares a structure, union, or enumerated type. The list defines the structure content, union content, or enumeration content. If an identifier is provided,101) the type specifier also declares the identifier to be the tag of that type.

A declaration of the form

        struct-or-union identifier ;

use of that typedef name to declare objects having the specified structure, union, or enumerated type. specifies a structure or union type and declares the identifier as a tag of that type.102)

If a type specifier of the form

struct-or-union identifier

occurs other than as part of one of the above forms, and no other declaration of the identifier as a tag is visible, then it declares an incomplete structure or union type, and declares the identifier as the tag of that type.102)

If a type specifier of the form

struct-or-union identifier

enum identifier

occurs other than as part of one of the above forms, and a declaration of the identifier as a tag is visible, then it specifies the same type as that other declaration, and does not redeclare the tag.

EXAMPLE 1 This mechanism allows declaration of a self- referential structure.

        struct tnode {
                int count;
                struct tnode *left, *right;
        };

specifies a structure that contains an integer and two pointers to objects of the same type. Once this declaration has been given, the declaration

        struct tnode s, *sp;

declares s to be an object of the given type and sp to be a pointer to an object of the given type. With these declarations, the expression sp->left refers to the left struct tnode pointer of the object to which sp points; the expression s.right->count designates the count member of the right struct tnode pointed to from s.

#10

The following alternative formulation uses the typedef mechanism:

        typedef struct tnode TNODE;
        struct tnode {
                int count;
                TNODE *left, *right;
        };
        TNODE s, *sp;

#11

EXAMPLE 2 To illustrate the use of prior declaration of a tag to specify a pair of mutually referential structures, the declarations

        struct s1 { struct s2 *s2p; /* ... */ }; // D1
        struct s2 { struct s1 *s1p; /* ... */ }; // D2

specify a pair of structures that contain pointers to each other. Note, however, that if s2 were already declared as a tag in an enclosing scope, the declaration D1 would refer to it, not to the tag s2 declared in D2. To eliminate this context sensitivity, the declaration

        struct s2;

may be inserted ahead of D1. This declares a new tag s2 in the inner scope; the declaration D2 then completes the specification of the new type.

Forward references: declarators (6.7.5), array declarators (6.7.5.2), type definitions (6.7.7).

6.7.3 Type qualifiers

Syntax

type-qualifier:

const

restrict

volatile

Constraints

Types other than pointer types derived from object or incomplete types shall not be restrict-qualified.

Semantics

The properties associated with qualified types are meaningful only for expressions that are lvalues.103)

If the same qualifier appears more than once in the same specifier-qualifier-list, either directly or via one or more typedefs, the behavior is the same as if it appeared only once.

If an attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non- const-qualified type, the behavior is undefined. If an attempt is made to refer to an object defined with a volatile-qualified type through use of an lvalue with non- volatile-qualified type, the behavior is undefined.104)

An object that has volatile-qualified type may be modified in ways unknown to the implementation or have other unknown side effects. Therefore any expression referring to such an object shall be evaluated strictly according to the rules of the abstract machine, as described in 5.1.2.3. Furthermore, at every sequence point the value last stored in the object shall agree with that prescribed by the abstract machine, except as modified by the unknown factors mentioned previously.105) What constitutes an access to an object that has volatile-qualified type is implementation- defined.

An object that is accessed through a restrict-qualified pointer has a special association with that pointer. This association, defined in 6.7.3.1 below, requires that all accesses to that object use, directly or indirectly, the value of that particular pointer.106) The intended use of the restrict qualifier (like the register storage class) is to promote optimization, and deleting all instances of the qualifier from a conforming program does not change its meaning (i.e., observable behavior).

If the specification of an array type includes any type qualifiers, the element type is so-qualified, not the array

105A volatile declaration may be used to describe an object corresponding to a memory-mapped input/output port or an object accessed by an asynchronously interrupting function. Actions on objects so declared shall not be ``optimized out'' by an implementation or reordered except as permitted by the rules for evaluating expressions.

106For example, a statement that assigns a value returned by malloc to a single pointer establishes this association between the allocated object and the pointer. type. If the specification of a function type includes any type qualifiers, the behavior is undefined.107)

For two qualified types to be compatible, both shall have the identically qualified version of a compatible type; the order of type qualifiers within a list of specifiers or qualifiers does not affect the specified type.

#10

EXAMPLE 1 An object declared

        extern const volatile int real_time_clock;

may be modifiable by hardware, but cannot be assigned to, incremented, or decremented.

#11

EXAMPLE 2 The following declarations and expressions illustrate the behavior when type qualifiers modify an aggregate type:

          const struct s { int mem; } cs = { 1 };
          struct s ncs;  // the object ncs is modifiable
          typedef int A[2][3];
          const A a = {{4, 5, 6}, {7, 8, 9}}; // array of array of
                                              // const int
          int *pi;
          const int *pci;

          ncs = cs;      // valid
          cs = ncs;      // violates modifiable lvalue constraint for =
          pi = &ncs.mem; // valid
          pi = &cs.mem;  // violates type constraints for =
          pci = &cs.mem; // valid
          pi = a[0];     // invalid: a[0] has type ``const int *''

6.7.3.1 Formal definition of restrict

Let D be a declaration of an ordinary identifier that provides a means of designating an object P as a restrict- qualified pointer.

If D appears inside a block and does not have storage class extern, let B denote the block. If D appears in the list of parameter declarations of a function definition, let B denote the associated block. Otherwise, let B denote the block of main (or the block of whatever function is called at program startup in a freestanding environment).

In what follows, a pointer expression E is said to be based on object P if (at some sequence point in the execution of B prior to the evaluation of E) modifying P to point to a copy of the array object into which it formerly

pointed would change the value of E.108) Note that ``based'' is defined only for expressions with pointer types.

During each execution of B, let A be the array object that is determined dynamically by all accesses through pointer expressions based on P. Then all accesses to values of A shall be through pointer expressions based on P. Furthermore, if P is assigned the value of a pointer expression E that is based on another restricted pointer object P2, associated with block B2, then either the execution of B2 shall begin before the execution of B, or the execution of B2 shall end prior to the assignment. If these requirements are not met, then the behavior is undefined.

Here an execution of B means that portion of the execution of the program during which storage is guaranteed to be reserved for an instance of an object that is associated with B and that has automatic storage duration. An access to a value means either fetching it or modifying it; expressions that are not evaluated do not access values.

A translator is free to ignore any or all aliasing implications of uses of restrict.

EXAMPLE 1 The file scope declarations

            int * restrict a;
            int * restrict b;
            extern int c[];

assert that if an object is accessed using the value of one of a, b, or c, then it is never accessed using the value of either of the other two.

EXAMPLE 2 The function parameter declarations in the following example

        void f(int n, int * restrict p, int * restrict q)
        {
                while (n-- > 0)
                        *p++ = *q++;
        }

assert that, during each execution of the function, if an

object is accessed through one of the pointer parameters, then it is not also accessed through the other.

The benefit of the restrict qualifiers is that they enable a translator to make an effective dependence analysis of function f without examining any of the calls of f in the program. The cost is that the programmer has to examine all of those calls to ensure that none give undefined behavior. For example, the second call of f in g has undefined behavior because each of d[1] through d[49] is accessed through both p and q.

        void g(void)
        {
                extern int d[100];
                f(50, d + 50, d); // ok
                f(50, d +  1, d); // undefined behavior
        }

#10

EXAMPLE 3 The function parameter declarations

        void h(int n, int * const restrict p,
                int * const q, int * const r)
        {
                int i;
                for (i = 0; i < n; i++)
                        p[i] = q[i] + r[i];
        }

show how const can be used in conjunction with restrict. The const qualifiers imply, without the need to examine the body of h, that q and r cannot become based on p. The fact that p is restrict-qualified therefore implies that an object accessed through p is never accessed through either of q or r. This is the precise assertion required to optimize the loop. Note that a call of the form h(100, a, b, b) has defined behavior, which would not be true if all three of p, q, and r were restrict-qualified.

#11

EXAMPLE 4 The rule limiting assignments between restricted pointers does not distinguish between a function call and an equivalent nested block. With one exception, only ``outer-to-inner'' assignments between restricted pointers declared in nested blocks have defined behavior.

            {
                int * restrict p1;
                int * restrict q1;
                p1 = q1; // undefined behavior
                {
                        int * restrict p2 = p1; // ok
                        int * restrict q2 = q1; // ok
                        p1 = q2; // undefined behavior
                        p2 = q2; // undefined behavior
                }
            }

The exception allows the value of a restricted pointer to be carried out of the block in which it (or, more precisely, the ordinary identifier used to designate it) is declared when that block finishes execution. For example, this permits new_vector to return a vector.

        typedef struct { int n; float * restrict v; } vector;
        vector new_vector(int n)
        {
                vector t;
                t.n = n;
                t.v = malloc(n * sizeof (float));
                return t;
        }

6.7.4 Function specifiers

Syntax

function-specifier:

inline

Constraints

Function specifiers shall be used only in the declaration of an identifier for a function.

An inline definition of a function with external linkage shall not contain a definition of a modifiable object with static storage duration, and shall not contain a reference to an identifier with internal linkage.

The inline function specifier shall not appear in a declaration of main.

Semantics

A function declared with an inline function specifier is an inline function. The function specifier may appear more than once; the behavior is the same as if it appeared only once. Making a function an inline function suggests that calls to the function be as fast as possible.109) The extent to which such suggestions are effective is implementation-defined.110)

Any function with internal linkage can be an inline function. For a function with external linkage, the following restrictions apply: If a function is declared with an inline function specifier, then it shall also be defined in the same translation unit. If all of the file scope declarations for a function in a translation unit include the inline function specifier without extern, then the definition in that translation unit is an inline definition. An inline definition does not provide an external definition for the function, and does not forbid an external definition in another translation unit. An inline definition provides an alternative to an external definition, which a translator may use to implement any call to the function in the same translation unit. It is unspecified whether a call to the function uses the inline definition or the external definition.111)

EXAMPLE The declaration of an inline function with external linkage can result in either an external definition, or a definition available for use only within the translation unit. A file scope declaration with extern creates an external definition. The following example shows an entire translation unit.

111Since an inline definition is distinct from the corresponding external definition and from any other corresponding inline definitions in other translation units, all corresponding objects with static storage duration are also distinct in each of the definitions.

        inline double fahr(double t)
        {
                return (9.0 * t) / 5.0 + 32.0;
        }

        inline double cels(double t)
        {
                return (5.0 * (t - 32.0)) / 9.0;
        }

        extern double fahr(double);     // creates an external definition

        double convert(int is_fahr, double temp)
        {
                /* A translator may perform inline substitutions. */
                return is_fahr ? cels(temp) : fahr(temp);
        }

Note that the definition of fahr is an external definition because fahr is also declared with extern, but the definition of cels is an inline definition. Because cels has external linkage and is referenced, an external definition has to appear in another translation unit (see 6.9); the inline definition and the external definition are distinct and either may be used for the call.

6.7.5 Declarators

Syntax

declarator:

pointer_opt direct-declarator

direct-declarator:

identifier

( declarator )

direct-declarator [ assignment-expr_opt ]

direct-declarator [ * ]

direct-declarator ( parameter-type-list )

direct-declarator ( identifier-list_opt )

pointer:

* type-qualifier-list_opt

* type-qualifier-list_opt pointer

type-qualifier-list:

type-qualifier

type-qualifier-list type-qualifier

parameter-type-list:

parameter-list

parameter-list , ...

parameter-list:

parameter-declaration

parameter-list , parameter-declaration

parameter-declaration:

declaration-specifiers declarator

declaration-specifiers abstract-declarator_opt

identifier-list:

identifier

identifier-list , identifier

Semantics

Each declarator declares one identifier, and asserts that when an operand of the same form as the declarator appears in an expression, it designates a function or object with the scope, storage duration, and type indicated by the declaration specifiers.

A full declarator is a declarator that is not part of another declarator. The end of a full declarator is a sequence point. If the nested sequence of declarators in a full declarator contains a variable length array type, the type specified by the full declarator is said to be variably modified.

In the following subclauses, consider a declaration

T D1

where T contains the declaration specifiers that specify a type T (such as int) and D1 is a declarator that contains an identifier ident. The type specified for the identifier ident in the various forms of declarator is described inductively using this notation.

If, in the declaration ``T D1'', D1 has the form

identifier

then the type specified for ident is T.

If, in the declaration ``T D1'', D1 has the form

        ( D )

then ident has the type specified by the declaration ``T D''. Thus, a declarator in parentheses is identical to the unparenthesized declarator, but the binding of complicated declarators may be altered by parentheses.

Implementation limits

As discussed in 5.2.4.1, an implementation may limit the number of pointer, array, and function declarators that modify an arithmetic, structure, union, or incomplete type, either directly or via one or more typedefs.

Forward references: array declarators (6.7.5.2), type definitions (6.7.7).

6.7.5.1 Pointer declarators

Semantics

If, in the declaration ``T D1'', D1 has the form

* type-qualifier-list-opt D

and the type specified for ident in the declaration ``T D'' is ``derived-declarator-type-list T'', then the type specified for ident is ``derived-declarator-type-list type- qualifier-list pointer to T''. For each type qualifier in the list, ident is a so-qualified pointer.

For two pointer types to be compatible, both shall be identically qualified and both shall be pointers to compatible types.

EXAMPLE The following pair of declarations demonstrates the difference between a ``variable pointer to a constant value'' and a ``constant pointer to a variable value''.

        const int *ptr_to_constant;
        int *const constant_ptr;

The contents of any object pointed to by ptr_to_constant shall not be modified through that pointer, but ptr_to_constant itself may be changed to point to another object. Similarly, the contents of the int pointed to by constant_ptr may be modified, but constant_ptr itself shall always point to the same location.

The declaration of the constant pointer constant_ptr may be clarified by including a definition for the type ``pointer to int''.

        typedef int *int_ptr;
        const int_ptr constant_ptr;

declares constant_ptr as an object that has type ``const- qualified pointer to int''.

6.7.5.2 Array declarators

Constraints

The [ and ] may delimit an expression or *. If [ and ] delimit an expression (which specifies the size of an array), it shall have an integer type. If the expression is a constant expression then it shall have a value greater than zero. The element type shall not be an incomplete or function type.

Only ordinary identifiers (as defined in 6.2.3) with both block scope or function prototype scope and no linkage shall have a variably modified type. If an identifier is declared to be an object with static storage duration, it shall not have a variable length array type.

Semantics

If, in the declaration ``T D1'', D1 has the form

        D[assignment-expr-opt]

        D[*]

and the type specified for ident in the declaration ``T D'' is ``derived-declarator-type-list T'', then the type specified for ident is ``derived-declarator-type-list array of T''.112) If the size is not present, the array type is an incomplete type. If * is used instead of a size expression, the array type is a variable length array type of unspecified size, which can only be used in declarations with function prototype scope.113) If the size expression is an integer constant expression and the element type has a known constant size, the array type is not a variable length array type; otherwise, the array type is a variable length array type. If the size expression is not a constant expression, and it is evaluated at program execution time, it shall evaluate to a value greater than zero. It is unspecified whether side effects are produced when the size expression is evaluated. The size of each instance of a variable length array type does not change during its lifetime.

compatible element types, and if both size specifiers are present, and are integer constant expressions, then both size specifiers shall have the same constant value. If the two array types are used in a context which requires them to be compatible, it is undefined behavior if the two size specifiers evaluate to unequal values.

EXAMPLE 1

        float fa[11], *afp[17];

declares an array of float numbers and an array of pointers to float numbers.

EXAMPLE 2 Note the distinction between the declarations

        extern int *x;
        extern int y[];

The first declares x to be a pointer to int; the second declares y to be an array of int of unspecified size (an incomplete type), the storage for which is defined elsewhere.

EXAMPLE 3 The following declarations demonstrate the compatibility rules for variably modified types.

        extern int n;
        extern int m;
        void fcompat(void)
        {
                int a[n][6][m];
                int (*p)[4][n+1];
                int c[n][n][6][m];
                int (*r)[n][n][n+1];
                p = a; // Error - not compatible because 4 != 6.
                r = c; // Compatible, but defined behavior
                       // only if n == 6 and m == n+1.
        }

EXAMPLE 4 All declarations of variably modified (VM) types have to be at either block scope or function prototype scope. Array objects declared with the static or extern storage class specifier cannot have a variable length array (VLA) type. However, an object declared with the static storage class specifier can have a VM type (that is, a pointer to a VLA type). Finally, all identifiers declared with a VM type have to be ordinary identifiers and cannot, therefore, be members of structures or unions.

        extern int n;
        int A[n];                                       // Error - file scope VLA.
        extern int (*p2)[n];            // Error - file scope VM.
        int B[100];                             // OK - file scope but not VM.

        void fvla(int m, int C[m][m])   // OK - VLA with prototype scope.
        {
                typedef int VLA[m][m]   // OK - block scope typedef VLA.

                struct tag {
                        int (*y)[n];            // Error - y not ordinary identifier.
                        int z[n];                       // Error - z not ordinary identifier.
                };
                int D[m];                               // OK - auto VLA.
                static int E[m];                // Error - static block scope VLA.
                extern int F[m];                // Error - F has linkage and is VLA.
                int (*s)[m];                    // OK - auto pointer to VLA.
                extern int (*r)[m];             // Error - r had linkage and is
                                                        // a pointer to VLA.
                static int (*q)[m] = &B; // OK - q is a static block
                                                // pointer to VLA.
        }

Forward references: function declarators (6.7.5.3), function definitions (6.9.1), initialization (6.7.8).

6.7.5.3 Function declarators (including prototypes)

Constraints

A function declarator shall not specify a return type that is a function type or an array type.

The only storage-class specifier that shall occur in a parameter declaration is register.

An identifier list in a function declarator that is not part of a definition of that function shall be empty.

After adjustment, the parameters in a parameter type list in a function declarator that is part of a definition of that function shall not have incomplete type.

Semantics

If, in the declaration ``T D1'', D1 has the form

        D(parameter-type-list)

        D(identifier-list-opt)

and the type specified for ident in the declaration ``T D'' is ``derived-declarator-type-list T'', then the type specified for ident is ``derived-declarator-type-list function returning T''.

A parameter type list specifies the types of, and may declare identifiers for, the parameters of the function. A declaration of a parameter as ``array of type'' shall be adjusted to ``pointer to type'', and a declaration of a parameter as ``function returning type'' shall be adjusted to ``pointer to function returning type'', as in 6.3.2.1. If the list terminates with an ellipsis (, ...), no information about the number or types of the parameters after the comma is supplied.114) The special case of an unnamed parameter of type void as the only item in the list specifies that the function has no parameters.

In a parameter declaration, a single typedef name in parentheses is taken to be an abstract declarator that specifies a function with a single parameter, not as redundant parentheses around the identifier for a declarator.

If the function declarator is not part of a definition of that function, parameters may have incomplete type and may use the [*] notation in their sequences of declarator specifiers to specify variable length array types.

The storage-class specifier in the declaration specifiers for a parameter declaration, if present, is ignored unless the declared parameter is one of the members of the parameter type list for a function definition.

#10

An identifier list declares only the identifiers of the parameters of the function. An empty list in a function declarator that is part of a definition of that function specifies that the function has no parameters. The empty list in a function declarator that is not part of a definition of that function specifies that no information about the number or types of the parameters is supplied.115)

#11

For two function types to be compatible, both shall specify compatible return types.116) Moreover, the parameter type lists, if both are present, shall agree in the number of parameters and in use of the ellipsis terminator; corresponding parameters shall have compatible types. If one type has a parameter type list and the other

type is specified by a function declarator that is not part of a function definition and that contains an empty identifier list, the parameter list shall not have an ellipsis terminator and the type of each parameter shall be compatible with the type that results from the application of the default argument promotions. If one type has a parameter type list and the other type is specified by a function definition that contains a (possibly empty) identifier list, both shall agree in the number of parameters, and the type of each prototype parameter shall be compatible with the type that results from the application of the default argument promotions to the type of the corresponding identifier. (In the determination of type compatibility and of a composite type, each parameter declared with function or array type is taken as having the adjusted type and each parameter declared with qualified type is taken as having the unqualified version of its declared type.)

#12

EXAMPLE 1 The declaration

        int f(void), *fip(), (*pfi)();

declares a function f with no parameters returning an int, a function fip with no parameter specification returning a pointer to an int, and a pointer pfi to a function with no parameter specification returning an int. It is especially useful to compare the last two. The binding of *fip() is *(fip()), so that the declaration suggests, and the same construction in an expression requires, the calling of a function fip, and then using indirection through the pointer result to yield an int. In the declarator (*pfi)(), the extra parentheses are necessary to indicate that indirection through a pointer to a function yields a function designator, which is then used to call the function; it returns an int.

#13

If the declaration occurs outside of any function, the identifiers have file scope and external linkage. If the declaration occurs inside a function, the identifiers of the functions f and fip have block scope and either internal or external linkage (depending on what file scope declarations for these identifiers are visible), and the identifier of the pointer pfi has block scope and no linkage.

#14

EXAMPLE 2 The declaration

        int (*apfi[3])(int *x, int *y);

declares an array apfi of three pointers to functions returning int. Each of these functions has two parameters that are pointers to int. The identifiers x and y are declared for descriptive purposes only and go out of scope at the end of the declaration of apfi.

#15

EXAMPLE 3 The declaration

        int (*fpfi(int (*)(long), int))(int, ...);

declares a function fpfi that returns a pointer to a function returning an int. The function fpfi has two parameters: a pointer to a function returning an int (with one parameter of type long int), and an int. The pointer returned by fpfi points to a function that has one int parameter and accepts zero or more additional arguments of any type.

#16

EXAMPLE 4 The following prototype has a variably modified parameter.

        void addscalar(int n, int m,
                double a[n][n*m+300], double x);

        int main()
        {
                double b[4][308];
                addscalar(4, 2, b, 2.17);
                return 0;
        }

        void addscalar(int n, int m,
                double a[n][n*m+300], double x)
        {
                for (int i = 0; i < n; i++)
                        for (int j = 0, k = n*m+300; j < k; j++)
                                // a is a pointer to a VLA
                                // with n*m+300 elements
                                a[i][j] += x;
        }

#17

EXAMPLE 5 The following are all compatible function prototype declarators.

        double maximum(int n, int m, double a[n][m]);
        double maximum(int n, int m, double a[*][*]);
        double maximum(int n, int m, double a[ ][*]);
        double maximum(int n, int m, double a[ ][m]);

Forward references: function definitions (6.9.1), type names (6.7.6).

6.7.6 Type names

Syntax

type-name:

specifier-qualifier-list abstract-declarator_opt

abstract-declarator:

pointer

pointer_opt direct-abstract-declarator

direct-abstract-declarator:

( abstract-declarator )

direct-abstract-declarator_opt [ assignment-expr_opt ]

direct-abstract-declarator_opt [ * ]

direct-abstract-declarator_opt ( parameter-type-list_opt )

Semantics

In several contexts, it is necessary to specify a type. This is accomplished using a type name, which is syntactically a declaration for a function or an object of that type that omits the identifier.117)

EXAMPLE The constructions

        (a)     int
        (b)     int *
        (c)     int *[3]
        (d)     int (*)[3]
        (e)     int (*)[*]
        (f)     int *()
        (g)     int (*)(void)
        (h)     int (*const [])(unsigned int, ...)

name respectively the types (a) int, (b) pointer to int, (c) array of three pointers to int, (d) pointer to an array of three ints, (e) pointer to a variable length array of an unspecified number of ints, (f) function with no parameter specification returning a pointer to int, (g) pointer to function with no parameters returning an int, and (h) array of an unspecified number of constant pointers to functions, each with one parameter that has type unsigned int and an unspecified number of other parameters, returning an int.

6.7.7 Type definitions

Syntax

typedef-name:

identifier

Constraints

If a typedef name specifies a variably modified type then it shall have block scope.

Semantics

In a declaration whose storage-class specifier is typedef, each declarator defines an identifier to be a typedef name that denotes the type specified for the identifier in the way described in 6.7.5. Any array size expressions associated with variable length array declarators are evaluated each time the declaration of the typedef name is reached in the order of execution. A typedef declaration does not introduce a new type, only a synonym for the type so specified. That is, in the following declarations:

        typedef T type_ident;
        type_ident D;

type_ident is defined as a typedef name with the type specified by the declaration specifiers in T (known as T), and the identifier in D has the type ``derived-declarator- type-list T'' where the derived-declarator-type-list is specified by the declarators of D. A typedef name shares the same name space as other identifiers declared in ordinary declarators.

EXAMPLE 1 After

        typedef int MILES, KLICKSP();
        typedef struct { double hi, lo; } range;

the constructions

        MILES distance;
        extern KLICKSP *metricp;
        range x;
        range z, *zp;

are all valid declarations. The type of distance is int, that of metricp is ``pointer to function with no parameter specification returning int'', and that of x and z is the specified structure; zp is a pointer to such a structure. The object distance has a type compatible with any other int object.

EXAMPLE 2 After the declarations

        typedef struct s1 { int x; } t1, *tp1;
        typedef struct s2 { int x; } t2, *tp2;

type t1 and the type pointed to by tp1 are compatible. Type t1 is also compatible with type struct s1, but not compatible with the types struct s2, t2, the type pointed to by tp2, or int.

EXAMPLE 3 The following obscure constructions

        typedef signed int t;
        typedef int plain;
        struct tag {
                unsigned t:4;
                const t:5;
                plain r:5;
        };

declare a typedef name t with type signed int, a typedef name plain with type int, and a structure with three bit- field members, one named t that contains values in the range [0, 15], an unnamed const-qualified bit-field which (if it could be accessed) would contain values in at least the range [-15, +15], and one named r that contains values in the range [0, 31] or values in at least the range [-15, +15]. (The choice of range is implementation-defined.) The first two bit-field declarations differ in that unsigned is a type specifier (which forces t to be the name of a structure member), while const is a type qualifier (which modifies t which is still visible as a typedef name). If these declarations are followed in an inner scope by

        t f(t (t));
        long t;

then a function f is declared with type ``function returning signed int with one unnamed parameter with type pointer to function returning signed int with one unnamed parameter with type signed int'', and an identifier t with type long int.

EXAMPLE 4 On the other hand, typedef names can be used to improve code readability. All three of the following declarations of the signal function specify exactly the same type, the first without making use of any typedef names.

        typedef void fv(int), (*pfv)(int);
        void (*signal(int, void (*)(int)))(int);
        fv *signal(int, fv *);
        pfv signal(int, pfv);

EXAMPLE 5 If a typedef name denotes a variable length array type, the length of the array is fixed at the time the typedef name is defined, not each time it is used:

        void copyt(int n)
        {
                typedef int B[n];   // B is n ints, n evaluated now.
                n += 1;
                B a;                // a is n ints, n without += 1.
                int b[n];           // a and b are different sizes
                for (int i = 1; i < n; i++)
                        a[i-1] = b[i];
        }

Forward references: the signal function (7.14.1.1).

6.7.8 Initialization

Syntax

initializer:

assignment-expr

{ initializer-list }

{ initializer-list , }

initializer-list:

designation_opt initializer

initializer-list , designation_opt initializer

designator-list designator

designator:

[ constant-expr ]

. identifier

Constraints

No initializer shall attempt to provide a value for an object not contained within the entity being initialized.

The type of the entity to be initialized shall be an array of unknown size or an object type that is not a variable length array type.

All the expressions in an initializer for an object that has static storage duration shall be constant expressions or string literals.

If the declaration of an identifier has block scope, and the identifier has external or internal linkage, the declaration shall have no initializer for the identifier.

If a designator has the form

        [ constant-expr ]

then the current object (defined below) shall have array type and the expression shall be an integer constant expression. If the array is of unknown size, any nonnegative value is valid.

If a designator has the form

. identifier

then the current object (defined below) shall have structure or union type and the identifier shall be the name of a member of that type.

Semantics

An initializer specifies the initial value stored in an object.

Except where explicitly stated otherwise, for the purposes of this subclause unnamed members of objects of structure and union type do not participate in initialization. Unnamed members of structure objects have indeterminate value even after initialization.

#10

If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. If an object that has static storage duration is not initialized explicitly, then:

-- if it has pointer type, it is initialized to a null pointer;

-- if it has arithmetic type, it is initialized to (positive or unsigned) zero;

-- if it is an aggregate, every member is initialized (recursively) according to these rules;

-- if it is a union, the first named member is initialized (recursively) according to these rules.

#11

The initializer for a scalar shall be a single expression, optionally enclosed in braces. The initial value of the object is that of the expression (after conversion); the same type constraints and conversions as for simple assignment apply, taking the type of the scalar to be the unqualified version of its declared type.

#12

The rest of this subclause deals with initializers for objects that have aggregate or union type.

#13

The initializer for a structure or union object that has automatic storage duration shall be either an initializer list as described below, or a single expression that has compatible structure or union type. In the latter case, the initial value of the object, including unnamed members, is that of the expression.

#14

An array of character type may be initialized by a character string literal, optionally enclosed in braces. Successive characters of the character string literal (including the terminating null character if there is room or if the array is of unknown size) initialize the elements of the array.

#15

An array with element type compatible with wchar_t may be initialized by a wide string literal, optionally enclosed in braces. Successive wide characters of the wide string literal (including the terminating null wide character if there is room or if the array is of unknown size) initialize the elements of the array.

#16

Otherwise, the initializer for an object that has aggregate or union type shall be a brace-enclosed list of initializers for the elements or named members.

#17

Each brace-enclosed initializer list has an associated current object. When no designations are present, subobjects of the current object are initialized in order according to the type of the current object: array elements in increasing subscript order, structure members in declaration order, and the first named member of a union.118) In contrast, a designation causes the following initializer to begin initialization of the subobject described by the designator. Initialization then continues forward in order, beginning with the next subobject after that described by the designator.119)

#18

Each designator list begins its description with the current object associated with the closest surrounding brace pair. Each item in the designator list (in order) specifies a particular member of its current object and changes the current object for the next designator (if any) to be that member.120) The current object that results at the end of the designator list is the subobject to be initialized by the following initializer.

#19

The initialization shall occur in initializer list order, each initializer provided for a particular subobject overriding any previously listed initializer for the same subobject; all subobjects that are not initialized explicitly shall be initialized implicitly the same as objects that have static storage duration.

#20

If the aggregate or union contains elements or members that are aggregates or unions, these rules apply recursively to the subaggregates or contained unions. If the initializer of a subaggregate or contained union begins with a left brace, the initializers enclosed by that brace and its matching right brace initialize the elements or members of the subaggregate or the contained union. Otherwise, only enough initializers from the list are taken to account for the elements or members of the subaggregate or the first member of the contained union; any remaining initializers are left to initialize the next element or member of the aggregate of which the current subaggregate or contained union is a part.

#21

If there are fewer initializers in a brace-enclosed list than there are elements or members of an aggregate, or fewer characters in a string literal used to initialize an array of known size than there are elements in the array, the remainder of the aggregate shall be initialized implicitly the same as objects that have static storage duration.

#22

If an array of unknown size is initialized, its size is determined by the largest indexed element with an explicit initializer. At the end of its initializer list, the array no longer has incomplete type.

#23

The order in which any side effects occur among the

initialization list expressions is unspecified.121)

#24

EXAMPLE 1 Provided that <complex.h> has been #included, the declarations

        int i = 3.5;
        complex c = 5 + 3 * I;

define and initialize i with the value 3 and c with the value 5.0+3.0i.

#25

EXAMPLE 2 The declaration

        int x[] = { 1, 3, 5 };

defines and initializes x as a one-dimensional array object that has three elements, as no size was specified and there are three initializers.

#26

EXAMPLE 3 The declaration

        int y[4][3] = {
                { 1, 3, 5 },
                { 2, 4, 6 },
                { 3, 5, 7 },
        };

is a definition with a fully bracketed initialization: 1, 3, and 5 initialize the first row of y (the array object y[0]), namely y[0][0], y[0][1], and y[0][2]. Likewise the next two lines initialize y[1] and y[2]. The initializer ends early, so y[3] is initialized with zeros. Precisely the same effect could have been achieved by

        int y[4][3] = {
                1, 3, 5, 2, 4, 6, 3, 5, 7
        };

The initializer for y[0] does not begin with a left brace, so three items from the list are used. Likewise the next three are taken successively for y[1] and y[2].

#27

EXAMPLE 4 The declaration

        int z[4][3] = {
                { 1 }, { 2 }, { 3 }, { 4 }
        };

initializes the first column of z as specified and initializes the rest with zeros.

#28

EXAMPLE 5 The declaration

        struct { int a[3], b; } w[] = { { 1 }, 2 };

is a definition with an inconsistently bracketed initialization. It defines an array with two element structures: w[0].a[0] is 1 and w[1].a[0] is 2; all the other elements are zero.

#29

EXAMPLE 6 The declaration

        short q[4][3][2] = {
                { 1 },
                { 2, 3 },
                { 4, 5, 6 }
        };

contains an incompletely but consistently bracketed initialization. It defines a three-dimensional array object: q[0][0][0] is 1, q[1][0][0] is 2, q[1][0][1] is 3, and 4, 5, and 6 initialize q[2][0][0], q[2][0][1], and q[2][1][0], respectively; all the rest are zero. The initializer for q[0][0] does not begin with a left brace, so up to six items from the current list may be used. There is only one, so the values for the remaining five elements are initialized with zero. Likewise, the initializers for q[1][0] and q[2][0] do not begin with a left brace, so each uses up to six items, initializing their respective two- dimensional subaggregates. If there had been more than six items in any of the lists, a diagnostic message would have been issued. The same initialization result could have been achieved by:

        short q[4][3][2] = {
                1, 0, 0, 0, 0, 0,
                2, 3, 0, 0, 0, 0,
                4, 5, 6
        };

or by:

        short q[4][3][2] = {
                {
                        { 1 },
                },
                {
                        { 2, 3 },
                },
                {
                        { 4, 5 },
                        { 6 },
                }
        };
in a fully bracketed form.

#30

Note that the fully bracketed and minimally bracketed forms of initialization are, in general, less likely to cause confusion.

#31

EXAMPLE 7 One form of initialization that completes array types involves typedef names. Given the declaration

        typedef int A[];        // OK - declared with block scope

the declaration

        A a = { 1, 2 }, b = { 3, 4, 5 };

is identical to

        int a[] = { 1, 2 }, b[] = { 3, 4, 5 };

due to the rules for incomplete types.

#32

EXAMPLE 8 The declaration

        char s[] = "abc", t[3] = "abc";

defines ``plain'' char array objects s and t whose elements are initialized with character string literals. This declaration is identical to

        char s[] = { 'a', 'b', 'c', '\0' },
             t[] = { 'a', 'b', 'c' };

The contents of the arrays are modifiable. On the other hand, the declaration

        char *p = "abc";

defines p with type ``pointer to char'' and initializes it to point to an object with type ``array of char'' with length 4 whose elements are initialized with a character string literal. If an attempt is made to use p to modify the contents of the array, the behavior is undefined.

#33

EXAMPLE 9 Arrays can be initialized to correspond to the elements of an enumeration by using designators:

        enum { member_one, member_two };
        const char *nm[] = {
                [member_two] = "member two",
                [member_one] = "member one",
        };

#34

EXAMPLE 10 Structure members can be initialized to nonzero values without depending on their order:

        div_t answer = { .quot = 2, .rem = -1 };

#35

EXAMPLE 11 Designators can be used to provide explicit initialization when unadorned initializer lists might be misunderstood:

        struct { int a[3], b; } w[] =
                { [0].a = {1}, [1].a[0] = 2 };

#36

EXAMPLE 12 Space can be ``allocated'' from both ends of an array by using a single designator:

        int a[MAX] = {
                1, 3, 5, 7, 9, [MAX-5] = 8, 6, 4, 2, 0
        };

#37

In the above, if MAX is greater than ten, there will be some zero-valued elements in the middle; if it is less than ten, some of the values provided by the first five initializers will be overridden by the second five.

#38

EXAMPLE 13 Any member of a union can be initialized:

        union { /* ... */ } u = { .any_member = 42 };

Forward references: common definitions <stddef.h> (7.17).

6.8 Statements

Syntax

Semantics

A statement specifies an action to be performed. Except as indicated, statements are executed in sequence.

A block allows a set of declarations and statements to be grouped into one syntactic unit. The initializers of objects that have automatic storage duration, and the variable length array declarators of ordinary identifiers with block scope, are evaluated and the values are stored in the objects (including storing an indeterminate value in objects without an initializer) each time the declaration is reached in the order of execution, as if it were a statement, and within each declaration in the order that declarators appear.

A full expression is an expression that is not part of another expression or declarator. Each of the following is a full expression: an initializer; the expression in an expression statement; the controlling expression of a selection statement (if or switch); the controlling expression of a while or do statement; each of the (optional) expressions of a for statement; the (optional) expression in a return statement. The end of a full expression is a sequence point.

Forward references: expression and null statements (6.8.3), selection statements (6.8.4), iteration statements (6.8.5), the return statement (6.8.6.4).

6.8.1 Labeled statements

Syntax

labeled-statement:

identifier : statement

case constant-expr : statement

default : statement

Constraints

A case or default label shall appear only in a switch statement. Further constraints on such labels are discussed under the switch statement.

Label names shall be unique within a function.

Semantics

Any statement may be preceded by a prefix that declares an identifier as a label name. Labels in themselves do not alter the flow of control, which continues unimpeded across them.

Forward references: the goto statement (6.8.6.1), the switch statement (6.8.4.2).

6.8.2 Compound statement, or block

Syntax

compound-statement:

{ block-item-list_opt }

block-item-list:

block-item

block-item-list block-item

block-item:

declaration

statement

Semantics

A compound statement is a block.

6.8.3 Expression and null statements

Syntax

expression-statement:

expression_opt ;

Semantics

The expression in an expression statement is evaluated as a void expression for its side effects.122)

A null statement (consisting of just a semicolon) performs no operations.

EXAMPLE 1 If a function call is evaluated as an expression statement for its side effects only, the discarding of its value may be made explicit by converting the expression to a void expression by means of a cast:

        int p(int);
        /* ... */
        (void)p(0);

EXAMPLE 2 In the program fragment

        char *s;
        /* ... */
        while (*s++ != '\0')
                ;

a null statement is used to supply an empty loop body to the iteration statement.

EXAMPLE 3 A null statement may also be used to carry a label just before the closing } of a compound statement.

        while (loop1) {
                /* ... */
                while (loop2) {
                        /* ... */
                        if (want_out)
                                goto end_loop1;
                        /* ... */
                }
                /* ... */
        end_loop1: ;
        }

Forward references: iteration statements (6.8.5).

6.8.4 Selection statements

Syntax

selection-statement:

if ( expression ) statement

if ( expression ) statement else statement

switch ( expression ) statement

Semantics

A selection statement selects among a set of statements depending on the value of a controlling expression.

A selection statement is a block whose scope is a strict subset of the scope of its enclosing block. Each associated substatement is also a block whose scope is a strict subset of the scope of the selection statement.

6.8.4.1 The if statement

Constraints

The controlling expression of an if statement shall have scalar type.

Semantics

In both forms, the first substatement is executed if the expression compares unequal to 0. In the else form, the second substatement is executed if the expression compares equal to 0. If the first substatement is reached via a label, the second substatement is not executed.

An else is associated with the lexically nearest preceding if that is allowed by the syntax.

6.8.4.2 The switch statement

Constraints

The controlling expression of a switch statement shall have integer type.

If a switch statement has an accessible case or default label within the scope of an identifier with a variably modified type, the entire switch statement shall be within the scope of that identifier.123)

The expression of each case label shall be an integer constant expression and no two of the case constant expressions in the same switch statement shall have the same value after conversion. There may be at most one default label in a switch statement. (Any enclosed switch statement may have a default label or case constant expressions with values that duplicate case constant expressions in the enclosing switch statement.)

Semantics

A switch statement causes control to jump to, into, or past the statement that is the switch body, depending on the value of a controlling expression, and on the presence of a default label and the values of any case labels on or in the switch body. A case or default label is accessible only within the closest enclosing switch statement.

The integer promotions are performed on the controlling expression. The constant expression in each case label is converted to the promoted type of the controlling expression. If a converted value matches that of the promoted controlling expression, control jumps to the statement following the matched case label. Otherwise, if there is a default label, control jumps to the labeled statement. If no converted case constant expression matches and there is no default label, no part of the switch body is executed.

Implementation limits

As discussed in 5.2.4.1, the implementation may limit the number of case values in a switch statement.

EXAMPLE In the artificial program fragment

        switch (expr)
        {
                int i = 4;
                f(i);
        case 0:
                i = 17;
                /* falls through into default code */
        default:
                printf("%d\n", i);
        }

the object whose identifier is i exists with automatic storage duration (within the block) but is never initialized, and thus if the controlling expression has a nonzero value, the call to the printf function will access

an indeterminate value. Similarly, the call to the function f cannot be reached.

6.8.5 Iteration statements

Syntax

iteration-statement:

while ( expression ) statement

do statement while ( expression ) ;

for ( expr_opt ; expr_opt ; expr_opt ) statement

for ( declaration expr_opt ; expr_opt ) statement

Constraints

The controlling expression of an iteration statement shall have scalar type.

The declaration part of a for statement shall only declare identifiers for objects having storage class auto or register.

Semantics

An iteration statement causes a statement called the loop body to be executed repeatedly until the controlling expression compares equal to 0.

An iteration statement is a block whose scope is a strict subset of the scope of its enclosing block. The loop body is also a block whose scope is a strict subset of the scope of the iteration statement.

6.8.5.1 The while statement

The evaluation of the controlling expression takes place before each execution of the loop body.

6.8.5.2 The do statement

The evaluation of the controlling expression takes place after each execution of the loop body.

6.8.5.3 The for statement

The statement

        for ( clause-1 ; expr-2 ; expr-3 ) statement

behaves as follows: The expression expr-2 is the controlling expression that is evaluated before each execution of the loop body. The expression expr-3 is evaluated as a void expression after each execution of the loop body. If clause-1 is an expression, it is evaluated as a void expression before the first evaluation of the controlling expression.124)

Both clause-1 and expr-3 can be omitted. An omitted expr-2 is replaced by a nonzero constant.

6.8.6 Jump statements

Syntax

jump-statement:

goto identifier ;

continue ;

break ;

return expression_opt ;

Semantics

A jump statement causes an unconditional jump to another place.

6.8.6.1 The goto statement

Constraints

The identifier in a goto statement shall name a label located somewhere in the enclosing function. A goto statement shall not jump from outside the scope of an identifier having a variably modified type to inside the scope of that identifier.

Semantics

A goto statement causes an unconditional jump to the statement prefixed by the named label in the enclosing function.

EXAMPLE 1 It is sometimes convenient to jump into the middle of a complicated set of statements. The following outline presents one possible approach to a problem based on these three assumptions:

1. The general initialization code accesses objects only visible to the current function.

2. The general initialization code is too large to warrant duplication.

3. The code to determine the next operation is at the head of the loop. (To allow it to be reached by continue statements, for example.)

/* ... */ goto first_time; for (;;) { // determine next operation /* ... */ if (need to reinitialize) { // reinitialize-only code /* ... */ first_time: // general initialization code /* ... */ continue; } // handle other operations /* ... */ }

EXAMPLE 2 A goto statement is not allowed to jump past any declarations of objects with variably modified types. A jump within the scope, however, is permitted. goto lab3; // Error: going INTO scope of VLA. { double a[n]; a[j] = 4.4; lab3: a[j] = 3.3; goto lab4; // OK, going WITHIN scope of VLA. a[j] = 5.5; lab4: a[j] = 6.6; } goto lab4; // Error: going INTO scope of VLA.

6.8.6.2 The continue statement

Constraints

A continue statement shall appear only in or as a loop body.

Semantics

A continue statement causes a jump to the loop- continuation portion of the smallest enclosing iteration statement; that is, to the end of the loop body. More precisely, in each of the statements

        while (/* ... */) {  do {                  for (/* ... */) {
           /* ... */            /* ... */             /* ... */
           continue;            continue;             continue;
           /* ... */            /* ... */             /* ... */
        contin: ;            contin: ;             contin: ;
        }                    } while (/* ... */);  }

unless the continue statement shown is in an enclosed iteration statement (in which case it is interpreted within that statement), it is equivalent to goto contin;.125)

6.8.6.3 The break statement

Constraints

A break statement shall appear only in or as a switch body or loop body.

Semantics

A break statement terminates execution of the smallest enclosing switch or iteration statement.

6.8.6.4 The return statement

Constraints

A return statement with an expression shall not appear in a function whose return type is void. A return statement without an expression shall only appear in a function whose return type is void.

Semantics

A return statement terminates execution of the current function and returns control to its caller. A function may have any number of return statements.

If a return statement with an expression is executed, the value of the expression is returned to the caller as the value of the function call expression. If the expression has a type different from the return type of the function in which it appears, the value is converted as if by assignment to an object having the return type of the function.126)

EXAMPLE In:

        struct s { double i; } f(void);
        union {
                struct {
                        int f1;
                        struct s f2;
                } u1;
                struct {
                        struct s f3;
                        int f4;
                } u2;
        } g;

        struct s f(void)
        {
                return g.u1.f2;
        }

        /* ... */
        g.u2.f3 = f();

there is no undefined behavior, although there would be if the assignment were done directly (without using a function call to fetch the value).

6.9 External definitions

Syntax

translation-unit:

external-declaration

translation-unit external-declaration

external-declaration:

function-definition

declaration

Constraints

The storage-class specifiers auto and register shall not appear in the declaration specifiers in an external declaration.

There shall be no more than one external definition for each identifier declared with internal linkage in a translation unit. Moreover, if an identifier declared with internal linkage is used in an expression (other than as a part of the operand of a sizeof operator), there shall be exactly one external definition for the identifier in the translation unit.

Semantics

As discussed in 5.1.1.1, the unit of program text after preprocessing is a translation unit, which consists of a sequence of external declarations. These are described as ``external'' because they appear outside any function (and hence have file scope). As discussed in 6.7, a declaration that also causes storage to be reserved for an object or a function named by the identifier is a definition.

An external definition is an external declaration that is also a definition of a function or an object. If an identifier declared with external linkage is used in an expression (other than as part of the operand of a sizeof operator), somewhere in the entire program there shall be exactly one external definition for the identifier; otherwise, there shall be no more than one.127)

6.9.1 Function definitions

Syntax

function-definition:

declaration-specifiers declarator declaration-list_opt compound-statement

declaration-list:

declaration

declaration-list declaration

Constraints

The identifier declared in a function definition (which is the name of the function) shall have a function type, as specified by the declarator portion of the function definition.128)

The return type of a function shall be void or an object type other than array type.

The storage-class specifier, if any, in the declaration specifiers shall be either extern or static.

If the declarator includes a parameter type list, the declaration of each parameter shall include an identifier, except for the special case of a parameter list consisting of a single parameter of type void, in which case there shall not be an identifier. No declaration list shall follow.

If the declarator includes an identifier list, each declaration in the declaration list shall have at least one declarator, those declarators shall declare only identifiers from the identifier list, and every identifier in the identifier list shall be declared. An identifier declared as a typedef name shall not be redeclared as a parameter. The declarations in the declaration list shall contain no

storage-class specifier other than register and no initializations.

Semantics

The declarator in a function definition specifies the name of the function being defined and the identifiers of its parameters. If the declarator includes a parameter type list, the list also specifies the types of all the parameters; such a declarator also serves as a function prototype for later calls to the same function in the same translation unit. If the declarator includes an identifier list,129) the types of the parameters shall be declared in a following declaration list. In either case, the type of each parameter is adjusted as described in 6.7.5.3 for a parameter type list; the resulting type shall be an object type.

If a function that accepts a variable number of arguments is defined without a parameter type list that ends with the ellipsis notation, the behavior is undefined.

Each parameter has automatic storage duration. Its identifier is an lvalue, which is in effect declared at the head of the compound statement that constitutes the function body (and therefore cannot be redeclared in the function body except in an enclosed block). The layout of the storage for parameters is unspecified.

#10

On entry to the function, the size expressions of each variably modified parameter are evaluated and the value of each argument expression is converted to the type of the corresponding parameter as if by assignment. (Array expressions and function designators as arguments were converted to pointers before the call.)

#11

After all parameters have been assigned, the compound statement that constitutes the body of the function definition is executed.

#12

If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined.

#13

EXAMPLE 1 In the following:

        extern int max(int a, int b)
        {
                return a > b ? a : b;
        }

extern is the storage-class specifier and int is the type specifier; max(int a, int b) is the function declarator; and

        { return a > b ? a : b; }

is the function body. The following similar definition uses the identifier-list form for the parameter declarations:

        extern int max(a, b)
        int a, b;
        {
                return a > b ? a : b;
        }

Here int a, b; is the declaration list for the parameters. The difference between these two definitions is that the first form acts as a prototype declaration that forces conversion of the arguments of subsequent calls to the function, whereas the second form does not.

#14

EXAMPLE 2 To pass one function to another, one might say

                int f(void);
                /* ... */
                g(f);

Then the definition of g might read

        void g(int (*funcp)(void))
        {
                /* ... */ (*funcp)() /* or funcp()  ...  */
        }

or, equivalently,

        void g(int func(void))
        {
                /* ... */ func() /* or (*func)()  ...  */
        }

6.9.2 External object definitions

Semantics

If the declaration of an identifier for an object has file scope and an initializer, the declaration is an external definition for the identifier.

A declaration of an identifier for an object that has file scope without an initializer, and without a storage- class specifier or with the storage-class specifier static, constitutes a tentative definition. If a translation unit contains one or more tentative definitions for an identifier, and the translation unit contains no external definition for that identifier, then the behavior is exactly as if the translation unit contains a file scope declaration of that identifier, with the composite type as of the end of the translation unit, with an initializer equal to 0.

If the declaration of an identifier for an object is a tentative definition and has internal linkage, the declared type shall not be an incomplete type.

EXAMPLE 1

        int i1 = 1;         // definition, external linkage
        static int i2 = 2;  // definition, internal linkage
        extern int i3 = 3;  // definition, external linkage
        int i4;             // tentative definition, external linkage
        static int i5;      // tentative definition, internal linkage

        int i1;             // valid tentative definition, refers to previous
        int i2;             // 6.2.2 renders undefined, linkage disagreement
        int i3;             // valid tentative definition, refers to previous
        int i4;             // valid tentative definition, refers to previous
        int i5;             // 6.2.2 renders undefined, linkage disagreement

        extern int i1;      // refers to previous, whose linkage is external
        extern int i2;      // refers to previous, whose linkage is internal
        extern int i3;      // refers to previous, whose linkage is external
        extern int i4;      // refers to previous, whose linkage is external
        extern int i5;      // refers to previous, whose linkage is internal

EXAMPLE 2 If at the end of the translation unit containing

            int i[];

the array i still has incomplete type, the implicit initializer causes it to have one element, which is set to zero on program startup.

6.10 Preprocessing directives

Syntax

group_opt

pp-tokens_opt new-line

if-section

control-line

if-section:

if-group elif-groups_opt else-group_opt endif-line

if-group:

# if constant-expr new-line group_opt

# ifdef identifier new-line group_opt

# ifndef identifier new-line group_opt

elif-groups:

elif-group

elif-groups elif-group

elif-group:

# elif constant-expr new-line group_opt

else-group:

# else new-line group_opt

endif-line:

# endif new-line

control-line:

# include pp-tokens new-line

# define identifier replacement-list new-line

# define identifier lparen identifier-list_opt ) replacement-list new-line

# define identifier lparen ... ) replacement-list new-line

# define identifier lparen identifier-list , ... ) replacement-list new-line

# undef identifier new-line

# line pp-tokens new-line

# error pp-tokens_opt new-line

# pragma pp-tokens_opt new-line

# new-line

lparen:

a ( character not immediately preceded by white-space

replacement-list:

pp-tokens_opt

pp-tokens:

preprocessing-token

pp-tokens preprocessing-token

new-line:

the new-line character

Description

A preprocessing directive consists of a sequence of preprocessing tokens that begins with a # preprocessing token that (at the start of translation phase 4) is either the first character in the source file (optionally after white space containing no new-line characters) or that follows white space containing at least one new-line character, and is ended by the next new-line character.130) A new-line character ends the preprocessing directive even if it occurs within what would otherwise be an invocation of a function-like macro.

Constraints

The only white-space characters that shall appear between preprocessing tokens within a preprocessing directive (from just after the introducing # preprocessing token through just before the terminating new-line character) are space and horizontal-tab (including spaces that have replaced comments or possibly other white-space characters in translation phase 3).

Semantics

The implementation can process and skip sections of source files conditionally, include other source files, and replace macros. These capabilities are called preprocessing, because conceptually they occur before translation of the resulting translation unit.

The preprocessing tokens within a preprocessing directive are not subject to macro expansion unless

otherwise stated.

EXAMPLE In:

          #define EMPTY
          EMPTY # include <file.h>

the sequence of preprocessing tokens on the second line is not a preprocessing directive, because it does not begin with a # at the start of translation phase 4, even though it will do so after the macro EMPTY has been replaced.

6.10.1 Conditional inclusion

Constraints

The expression that controls conditional inclusion shall be an integer constant expression except that: it shall not contain a cast; identifiers (including those lexically identical to keywords) are interpreted as described below;131) and it may contain unary operator expressions of the form

defined identifier

          defined ( identifier )

which evaluate to 1 if the identifier is currently defined as a macro name (that is, if it is predefined or if it has been the subject of a #define preprocessing directive without an intervening #undef directive with the same subject identifier), 0 if it is not.

Semantics

Preprocessing directives of the forms

# if constant-expr new-line group-opt # elif constant-expr new-line group-opt

check whether the controlling constant expression evaluates to nonzero.

Prior to evaluation, macro invocations in the list of preprocessing tokens that will become the controlling constant expression are replaced (except for those macro names modified by the defined unary operator), just as in normal text. If the token defined is generated as a result

of this replacement process or use of the defined unary operator does not match one of the two specified forms prior to macro replacement, the behavior is undefined. After all replacements due to macro expansion and the defined unary operator have been performed, all remaining identifiers are replaced with the pp-number 0, and then each preprocessing token is converted into a token. The resulting tokens compose the controlling constant expression which is evaluated according to the rules of 6.6, except that all signed integer types and all unsigned integer types act as if they have the same representation as, respectively, the types intmax_t and uintmax_t defined in the header <stdint.h>. This includes interpreting character constants, which may involve converting escape sequences into execution character set members. Whether the numeric value for these character constants matches the value obtained when an identical character constant occurs in an expression (other than within a #if or #elif directive) is implementation-defined.132) Also, whether a single- character character constant may have a negative value is implementation-defined.

Preprocessing directives of the forms

# ifdef identifier new-line group-opt # ifndef identifier new-line group-opt

check whether the identifier is or is not currently defined as a macro name. Their conditions are equivalent to #if defined identifier and #if !defined identifier respectively.

Each directive's condition is checked in order. If it evaluates to false (zero), the group that it controls is skipped: directives are processed only through the name that determines the directive in order to keep track of the level of nested conditionals; the rest of the directives' preprocessing tokens are ignored, as are the other preprocessing tokens in the group. Only the first group whose control condition evaluates to true (nonzero) is processed. If none of the conditions evaluates to true, and there is a #else directive, the group controlled by the #else is processed; lacking a #else directive, all the groups until the #endif are skipped.133)

Forward references: macro replacement (6.10.3), source file inclusion (6.10.2), largest integer types (7.18.1.5).

6.10.2 Source file inclusion

Constraints

A #include directive shall identify a header or source file that can be processed by the implementation.

Semantics

A preprocessing directive of the form

          # include <h-char-sequence> new-line

searches a sequence of implementation-defined places for a header identified uniquely by the specified sequence between the < and > delimiters, and causes the replacement of that directive by the entire contents of the header. How the places are specified or the header identified is implementation-defined.

A preprocessing directive of the form

# include "q-char-sequence" new-line

causes the replacement of that directive by the entire contents of the source file identified by the specified sequence between the " delimiters. The named source file is searched for in an implementation-defined manner. If this search is not supported, or if the search fails, the directive is reprocessed as if it read

          # include <h-char-sequence> new-line

with the identical contained sequence (including > characters, if any) from the original directive.

A preprocessing directive of the form

# include pp-tokens new-line

(that does not match one of the two previous forms) is permitted. The preprocessing tokens after include in the directive are processed just as in normal text. (Each identifier currently defined as a macro name is replaced by its replacement list of preprocessing tokens.) The directive resulting after all replacements shall match one of the two previous forms.134) The method by which a

sequence of preprocessing tokens between a < and a > preprocessing token pair or a pair of " characters is combined into a single header name preprocessing token is implementation-defined.

The implementation shall provide unique mappings for sequences consisting of one or more letters or digits (as defined in 5.2.1) followed by a period (.) and a single letter. The first character shall be a letter. The implementation may ignore the distinctions of alphabetical case and restrict the mapping to eight significant characters before the period.

A #include preprocessing directive may appear in a source file that has been read because of a #include directive in another file, up to an implementation-defined nesting limit (see 5.2.4.1).

EXAMPLE 1 The most common uses of #include preprocessing directives are as in the following:

          #include <stdio.h>
          #include "myprog.h"

EXAMPLE 2 This illustrates macro-replaced #include directives:

#if VERSION == 1 #define INCFILE "vers1.h" #elif VERSION == 2 #define INCFILE "vers2.h" // and so on #else #define INCFILE "versN.h" #endif #include INCFILE

Forward references: macro replacement (6.10.3).

6.10.3 Macro replacement

Constraints

Two replacement lists are identical if and only if the preprocessing tokens in both have the same number, ordering, spelling, and white-space separation, where all white-space separations are considered identical.

An identifier currently defined as a macro without use

of lparen (an object-like macro) shall not be redefined by another #define preprocessing directive unless the second definition is an object-like macro definition and the two replacement lists are identical.

An identifier currently defined as a macro using lparen (a function-like macro) shall not be redefined by another #define preprocessing directive unless the second definition is a function-like macro definition that has the same number and spelling of parameters, and the two replacement lists are identical.

If the identifier-list in the macro definition does not end with an ellipsis, the number of arguments, including those arguments consisting of no preprocessing tokens, in an invocation of a function-like macro shall agree with the number of parameters in the macro definition. Otherwise, there shall be more arguments in the invocation than there are parameters in the macro definition (excluding the ...). There shall exist a ) preprocessing token that terminates the invocation.

The identifier __VA_ARGS__ shall only occur in the replacement-list of a #define preprocessing directive using the ellipsis notation in the arguments.

A parameter identifier in a function-like macro shall be uniquely declared within its scope.

Semantics

The identifier immediately following the define is called the macro name. There is one name space for macro names. Any white-space characters preceding or following the replacement list of preprocessing tokens are not considered part of the replacement list for either form of macro.

If a # preprocessing token, followed by an identifier, occurs lexically at the point at which a preprocessing directive could begin, the identifier is not subject to macro replacement.

A preprocessing directive of the form

# define identifier replacement-list new-line

defines an object-like macro that causes each subsequent instance of the macro name135) to be replaced by the replacement list of preprocessing tokens that constitute the remainder of the directive.

#10

A preprocessing directive of the form # define identifier lparen identifier-list-opt ) replacement-list new-line # define identifier lparen ... ) replacement-list new-line # define identifier lparen identifier-list , ... ) replacement-list new-line

defines a function-like macro with arguments, similar syntactically to a function call. The parameters are specified by the optional list of identifiers, whose scope extends from their declaration in the identifier list until the new-line character that terminates the #define preprocessing directive. Each subsequent instance of the function-like macro name followed by a ( as the next preprocessing token introduces the sequence of preprocessing tokens that is replaced by the replacement list in the definition (an invocation of the macro). The replaced sequence of preprocessing tokens is terminated by the matching ) preprocessing token, skipping intervening matched pairs of left and right parenthesis preprocessing tokens. Within the sequence of preprocessing tokens making up an invocation of a function-like macro, new-line is considered a normal white-space character.

#11

The sequence of preprocessing tokens bounded by the outside-most matching parentheses forms the list of arguments for the function-like macro. The individual arguments within the list are separated by comma preprocessing tokens, but comma preprocessing tokens between matching inner parentheses do not separate arguments. If there are sequences of preprocessing tokens within the list of arguments that would otherwise act as preprocessing directives, the behavior is undefined.

#12

If there is a ... in the identifier-list in the macro definition, then the trailing arguments, including any separating comma preprocessing tokens, are merged to form a single item: the variable arguments. The number of arguments so combined is such that, following merger, the number of arguments is one more than the number of parameters in the macro definition (excluding the ...).

6.10.3.1 Argument substitution

After the arguments for the invocation of a function- like macro have been identified, argument substitution takes place. A parameter in the replacement list, unless preceded by a # or ## preprocessing token or followed by a ## preprocessing token (see below), is replaced by the corresponding argument after all macros contained therein

have been expanded. Before being substituted, each argument's preprocessing tokens are completely macro replaced as if they formed the rest of the preprocessing file; no other preprocessing tokens are available.

An identifier __VA_ARGS__ that occurs in the replacement list shall be treated as if it were a parameter, and the variable arguments shall form the preprocessing tokens used to replace it.

6.10.3.2 The # operator

Constraints

Each # preprocessing token in the replacement list for a function-like macro shall be followed by a parameter as the next preprocessing token in the replacement list.

Semantics

If, in the replacement list, a parameter is immediately preceded by a # preprocessing token, both are replaced by a single character string literal preprocessing token that contains the spelling of the preprocessing token sequence for the corresponding argument. Each occurrence of white space between the argument's preprocessing tokens becomes a single space character in the character string literal. White space before the first preprocessing token and after the last preprocessing token composing the argument is deleted. Otherwise, the original spelling of each preprocessing token in the argument is retained in the character string literal, except for special handling for producing the spelling of string literals and character constants: a \ character is inserted before each " and \ character of a character constant or string literal (including the delimiting " characters), except that it is unspecified whether a \ character is inserted before the \ character beginning a universal character name. If the replacement that results is not a valid character string literal, the behavior is undefined. The character string literal corresponding to an empty argument is "". The order of evaluation of # and ## operators is unspecified.

6.10.3.3 The ## operator

Constraints

A ## preprocessing token shall not occur at the beginning or at the end of a replacement list for either form of macro definition.

Semantics

If, in the replacement list of a function-like macro, a parameter is immediately preceded or followed by a ## preprocessing token, the parameter is replaced by the corresponding argument's preprocessing token sequence; however, if an argument consists of no preprocessing tokens, the parameter is replaced by a placemarker preprocessing token instead.

For both object-like and function-like macro invocations, before the replacement list is reexamined for more macro names to replace, each instance of a ## preprocessing token in the replacement list (not from an argument) is deleted and the preceding preprocessing token is concatenated with the following preprocessing token. Placemarker preprocessing tokens are handled specially: concatenation of two placemarkers results in a single placemarker preprocessing token, and concatenation of a placemarker with a non-placemarker preprocessing token results in the non-placemarker preprocessing token. If the result is not a valid preprocessing token, the behavior is undefined. The resulting token is available for further macro replacement. The order of evaluation of ## operators is unspecified.

EXAMPLE In the following fragment:

          #define hash_hash # ## #
          #define mkstr(a) # a
          #define in_between(a) mkstr(a)
          #define join(c, d) in_between(c hash_hash d)

          char p[] = join(x, y); // equivalent to
                                 // char p[] = "x ## y";

The expansion produces, at various stages: join(x, y)

          in_between(x hash_hash y)

          in_between(x ## y)

          mkstr(x ## y)

          "x ## y"

In other words, expanding hash_hash produces a new token, consisting of two adjacent sharp signs, but this new token is not the ## operator.

6.10.3.4 Rescanning and further replacement

After all parameters in the replacement list have been substituted and # and ## processing has taken place, all placemarker preprocessing tokens are removed. Then, the resulting preprocessing token sequence is rescanned, along with all subsequent preprocessing tokens of the source file, for more macro names to replace.

If the name of the macro being replaced is found during this scan of the replacement list (not including the rest of the source file's preprocessing tokens), it is not replaced. Further, if any nested replacements encounter the name of the macro being replaced, it is not replaced. These nonreplaced macro name preprocessing tokens are no longer available for further replacement even if they are later (re)examined in contexts in which that macro name preprocessing token would otherwise have been replaced.

The resulting completely macro-replaced preprocessing token sequence is not processed as a preprocessing directive even if it resembles one, but all pragma unary operator expressions within it are then processed as specified in 6.10.9 below.

6.10.3.5 Scope of macro definitions

A macro definition lasts (independent of block structure) until a corresponding #undef directive is encountered or (if none is encountered) until the end of the preprocessing translation unit. Macro definitions have no significance after translation phase 4.

A preprocessing directive of the form

# undef identifier new-line

causes the specified identifier no longer to be defined as a macro name. It is ignored if the specified identifier is not currently defined as a macro name.

EXAMPLE 1 The simplest use of this facility is to define a ``manifest constant'', as in

          #define TABSIZE 100

          int table[TABSIZE];

EXAMPLE 2 The following defines a function-like macro whose value is the maximum of its arguments. It has the advantages of working for any compatible types of the arguments and of generating in-line code without the overhead of function calling. It has the disadvantages of evaluating one or the other of its arguments a second time (including side effects) and generating more code than a function if invoked several times. It also cannot have its address taken, as it has none.

          #define max(a, b) ((a) > (b) ? (a) : (b))

The parentheses ensure that the arguments and the resulting expression are bound properly.

EXAMPLE 3 To illustrate the rules for redefinition and reexamination, the sequence

          #define x      3
          #define f(a)   f(x * (a))
          #undef  x
          #define x      2
          #define g      f
          #define z      z[0]
          #define h      g(~
          #define m(a)   a(w)
          #define w      0,1
          #define t(a)   a
          #define p()    int
          #define q(x)   x
          #define r(x,y) x ## y
          #define str(x) # x

          f(y+1) + f(f(z)) % t(t(g)(0) + t)(1);
          g(x+(3,4)-w) | h 5) & m
                (f)^m(m);
          p() i[q()] = { q(1), r(2,3), r(4,), r(,5), r(,) };
          char c[2][6] = { str(hello), str() };

results in

          f(2 * (y+1)) + f(2 * (f(2 * (z[0])))) % f(2 * (0)) + t(1);
          f(2 * (2+(3,4)-0,1)) | f(2 * (~ 5)) & f(2 * (0,1))^m(0,1);
          int i[] = { 1, 23, 4, 5,  };
          char c[2][6] = { "hello", "" };

EXAMPLE 4 To illustrate the rules for creating character string literals and concatenating tokens, the sequence

          #define str(s)      # s
          #define xstr(s)     str(s)
          #define debug(s, t) printf("x" # s "= %d, x" # t "= %s", \
                                  x ## s, x ## t)
          #define INCFILE(n)  vers ## n // from previous #include example
          #define glue(a, b)  a ## b
          #define xglue(a, b) glue(a, b)
          #define HIGHLOW     "hello"
          #define LOW         LOW ", world"

          debug(1, 2);
          fputs(str(strncmp("abc\0d", "abc", '\4') // this goes away
                == 0) str(: @\n), s);
          #include xstr(INCFILE(2).h)
          glue(HIGH, LOW);
          xglue(HIGH, LOW)

results in

          printf("x" "1" "= %d, x" "2" "= %s", x1, x2);
          fputs(
            "strncmp(\"abc\\0d\", \"abc\", '\\4') == 0" ": @\n",
            s);
          #include "vers2.h"    (after macro replacement, before file access)
          "hello";
          "hello" ", world"

or, after concatenation of the character string literals,

          printf("x1= %d, x2= %s", x1, x2);
          fputs(
            "strncmp(\"abc\\0d\", \"abc\", '\\4') == 0: @\n",
            s);
          #include "vers2.h"    (after macro replacement, before file access)
          "hello";
          "hello, world"

Space around the # and ## tokens in the macro definition is optional.

EXAMPLE 5 To illustrate the rules for

placemarker ## placemarker

the sequence

          #define t(x,y,z) x ## y ## z
          int j[] = { t(1,2,3), t(,4,5), t(6,,7), t(8,9,),
                     t(10,,), t(,11,), t(,,12), t(,,) };

results in int j[] = { 123, 45, 67, 89, 10, 11, 12, };

EXAMPLE 6 To demonstrate the redefinition rules, the following sequence is valid.

          #define OBJ_LIKE      (1-1)
          #define OBJ_LIKE      /* white space */ (1-1) /* other */
          #define FUNC_LIKE(a)   ( a )
          #define FUNC_LIKE( a )(       /* note the white space */ \
                                 a /* other stuff on this line
                                   */ )

But the following redefinitions are invalid:

          #define OBJ_LIKE    (0)     /* different token sequence */
          #define OBJ_LIKE    (1 - 1) /* different white space */
          #define FUNC_LIKE(b) ( a )  /* different parameter usage */
          #define FUNC_LIKE(b) ( b )  /* different parameter spelling */

EXAMPLE 7 Finally, to show the variable argument list macro facilities:

          #define debug(...)    fprintf(stderr, __VA_ARGS__)
          #define showlist(...) puts(#__VA_ARGS__)
          #define report(test, ...) ((test)?puts(#test):\
                        printf(__VA_ARGS__))
          debug("Flag");
          debug("X = %d\n", x);
          showlist(The first, second, and third items.);
          report(x>y, "x is %d but y is %d", x, y);

results in

          fprintf(stderr,  "Flag" );
          fprintf(stderr,  "X = %d\n", x );
          puts( "The first, second, and third items." );
          ((x>y)?puts("x>y"):
                  printf("x is %d but y is %d", x, y));

6.10.4 Line control

Constraints

The string literal of a #line directive, if present, shall be a character string literal.

Semantics

The line number of the current source line is one greater than the number of new-line characters read or introduced in translation phase 1 (5.1.1.2) while processing the source file to the current token.

A preprocessing directive of the form

# line digit-sequence new-line

causes the implementation to behave as if the following sequence of source lines begins with a source line that has a line number as specified by the digit sequence (interpreted as a decimal integer). The digit sequence shall not specify zero, nor a number greater than 2147483647.

A preprocessing directive of the form

# line digit-sequence "s-char-sequence-opt" new-line

sets the presumed line number similarly and changes the presumed name of the source file to be the contents of the character string literal.

A preprocessing directive of the form

# line pp-tokens new-line

(that does not match one of the two previous forms) is permitted. The preprocessing tokens after line on the directive are processed just as in normal text (each identifier currently defined as a macro name is replaced by its replacement list of preprocessing tokens). The directive resulting after all replacements shall match one of the two previous forms and is then processed as appropriate.

6.10.5 Error directive

Semantics

A preprocessing directive of the form

# error pp-tokens-opt new-line

causes the implementation to produce a diagnostic message that includes the specified sequence of preprocessing tokens.

6.10.6 Pragma directive

Semantics

A preprocessing directive of the form

# pragma pp-tokens-opt new-line

where the preprocessing token STDC does not immediately follow pragma in the directive (prior to any macro replacement)136) causes the implementation to behave in an implementation-defined manner. The behavior might cause translation to fail or cause the translator or the resulting program to behave in a non-conforming manner. Any such pragma that is not recognized by the implementation is ignored.

If the preprocessing token STDC does immediately follow pragma in the directive (prior to any macro replacement), then no macro replacement is performed on the directive, and the directive shall have one of the following forms whose meanings are described elsewhere:

#pragma STDC FP_CONTRACT on-off-switch #pragma STDC FENV_ACCESS on-off-switch #pragma STDC CX_LIMITED_RANGE on-off-switch on-off-switch: one of ON OFF DEFAULT

Forward references: the FP_CONTRACT pragma (7.12.2), the FENV_ACCESS pragma (7.6.1), the CX_LIMITED_RANGE pragma (7.3.4).

6.10.7 Null directive

Semantics

A preprocessing directive of the form

# new-line

has no effect.

6.10.8 Predefined macro names

The following macro names shall be defined by the implementation:

__LINE__ The presumed line number (within the current source file) of the current source line (a decimal constant).137)

__FILE__ The presumed name of the current source file (a character string literal).137)

__DATE__ The date of translation of the source file: a character string literal of the form "Mmm dd yyyy", where the names of the months are the same as those generated by the asctime function, and the first character of dd is a space character if the value is less than 10. If the date of translation is not available, an implementation-defined valid date shall be supplied.

__TIME__ The time of translation of the source file: a character string literal of the form "hh:mm:ss" as in the time generated by the asctime function. If the time of translation is not available, an implementation-defined valid time shall be supplied.

__STDC__ The decimal constant 1, intended to indicate a conforming implementation.

__STDC_VERSION__ The decimal constant 199901L.138)

The following macro names are conditionally defined by the implementation:

__STDC_ISO_10646__ A decimal constant of the form yyyymmL (for example, 199712L), intended to indicate that values of type wchar_t are the coded representations of the characters defined by ISO/IEC 10646, along with all amendments and technical corrigenda as of the specified year and month.

__STDC_IEC_559__ The decimal constant 1, intended to indicate conformance to the specifications in annex F (IEC 60559 floating-point arithmetic).

__STDC_IEC_559_COMPLEX__ The decimal constant 1, intended to indicate adherence to the specifications in informative annex G (IEC 60559 compatible complex arithmetic).

__LINE__ and __FILE__) remain constant throughout the translation unit.

None of these macro names, nor the identifier defined, shall be the subject of a #define or a #undef preprocessing directive. Any other predefined macro names shall begin with a leading underscore followed by an uppercase letter or a second underscore.

Forward references: the asctime function (7.23.3.1).

6.10.9 Pragma operator

Semantics

A unary operator expression of the form:

            _Pragma ( string-literal  )

is processed as follows: The string literal is destringized by deleting the L prefix, if present, deleting the leading and trailing double-quotes, replacing each escape sequence \" by a double-quote, and replacing each escape sequence \\ by a single backslash. The resulting sequence of characters is processed through translation phase 3 to produce preprocessing tokens that are executed as if they were the pp-tokens in a pragma directive. The original four preprocessing tokens in the unary operator expression are removed.

EXAMPLE A directive of the form:

#pragma listing on "..\listing.dir"

can also be expressed as:

            _Pragma ( "listing on \"..\\listing.dir\"" )

The latter form is processed in the same way whether it appears literally as shown, or results from macro replacement, as in:

            #define LISTING(x) PRAGMA(listing on #x)
            #define PRAGMA(x)  _Pragma(#x)

            LISTING ( ..\listing.dir )

6.11 Future language directions

6.11.1 Floating Types

Future standardization may include additional floating- point types, including those with greater range, precision, or both than long double.

6.11.2 Character escape sequences

Lowercase letters as escape sequences are reserved for future standardization. Other characters may be used in extensions.

6.11.3 Storage-class specifiers

The placement of a storage-class specifier other than at the beginning of the declaration specifiers in a declaration is an obsolescent feature.

6.11.4 Function declarators

The use of function declarators with empty parentheses (not prototype-format parameter type declarators) is an obsolescent feature.

6.11.5 Function definitions

The use of function definitions with separate parameter identifier and declaration lists (not prototype-format parameter type and identifier declarators) is an obsolescent feature.

6.11.6 Pragma directives

Pragmas whose first pp-token is STDC are reserved for future standardization.

7. Library

7.1 Introduction

7.1.1 Definitions of terms

A string is a contiguous sequence of characters terminated by and including the first null character. The term multibyte string is sometimes used instead to emphasize special processing given to multibyte characters contained in the string or to avoid confusion with a wide string. A pointer to a string is a pointer to its initial (lowest addressed) character. The length of a string is the number of characters preceding the null character and the value of a string is the sequence of the values of the contained characters, in order.

A letter is a printing character in the execution character set corresponding to any of the 52 required lowercase and uppercase letters in the source character set, listed in 5.2.1.

The decimal-point character is the character used by functions that convert floating-point numbers to or from character sequences to denote the beginning of the fractional part of such character sequences.139) It is represented in the text and examples by a period, but may be changed by the setlocale function.

A wide character is a code value (a binary encoded integer) of an object of type wchar_t that corresponds to a member of the extended character set.140)

A null wide character is a wide character with code value zero.

A wide string is a contiguous sequence of wide characters terminated by and including the first null wide character. A pointer to a wide string is a pointer to its initial (lowest addressed) wide character. The length of a wide string is the number of wide characters preceding the order.

A shift sequence is a contiguous sequence of bytes within a multibyte string that (potentially) causes a change in shift state (see 5.2.1.2). A shift sequence shall not have a corresponding wide character; it is instead taken to be an adjunct to an adjacent multibyte character.141)

Forward references: character handling (7.4), the setlocale function (7.11.1.1).

7.1.2 Standard headers

Each library function is declared, with a type that includes a prototype, in a header,142) whose contents are made available by the #include preprocessing directive. The header declares a set of related functions, plus any necessary types and additional macros needed to facilitate their use. Declarations of types described in this clause shall not include type qualifiers, unless explicitly stated otherwise.

The standard headers are

<assert.h> <inttypes.h> <signal.h> <stdlib.h> <complex.h> <iso646.h> <stdarg.h> <string.h> <ctype.h> <limits.h> <stdbool.h> <tgmath.h> <errno.h> <locale.h> <stddef.h> <time.h> <fenv.h> <math.h> <stdint.h> <wchar.h> <float.h> <setjmp.h> <stdio.h> <wctype.h>

If a file with the same name as one of the above < and > delimited sequences, not provided as part of the implementation, is placed in any of the standard places that are searched for included source files, the behavior is undefined.

Standard headers may be included in any order; each may be included more than once in a given scope, with no effect different from being included only once, except that the effect of including <assert.h> depends on the definition of NDEBUG (see 7.2). If used, a header shall be included outside of any external declaration or definition, and it shall first be included before the first reference to any of the functions or objects it declares, or to any of the types or macros it defines. However, if an identifier is declared or defined in more than one header, the second and subsequent associated headers may be included after the initial reference to the identifier. The program shall not have any macros with names lexically identical to keywords currently defined prior to the inclusion.

Any definition of an object-like macro described in this clause shall expand to code that is fully protected by parentheses where necessary, so that it groups in an arbitrary expression as if it were a single identifier.

Any declaration of a library function shall have external linkage.

A summary of the contents of the standard headers is given in annex B.

Forward references: diagnostics (7.2).

7.1.3 Reserved identifiers

Each header declares or defines all identifiers listed in its associated subclause, and optionally declares or defines identifiers listed in its associated future library directions subclause and identifiers which are always reserved either for any use or for use as file scope identifiers.

-- All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use.

-- All identifiers that begin with an underscore are always reserved for use as identifiers with file scope in both the ordinary and tag name spaces.

-- Each macro name in any of the following subclauses (including the future library directions) is reserved for use as specified if any of its associated headers is included; unless explicitly stated otherwise (see 7.1.4).

-- All identifiers with external linkage in any of the following subclauses (including the future library directions) are always reserved for use as identifiers with external linkage.143)

-- Each identifier with file scope listed in any of the following subclauses (including the future library directions) is reserved for use as macro and as an identifier with file scope in the same name space if any of its associated headers is included.

No other identifiers are reserved. If the program declares or defines an identifier in a context in which it is reserved (other than as allowed by 7.1.4), or defines a reserved identifier as a macro name, the behavior is undefined.

If the program removes (with #undef) any macro definition of an identifier in the first group listed above, the behavior is undefined.

7.1.4 Use of library functions

Each of the following statements applies unless explicitly stated otherwise in the detailed descriptions that follow: If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer outside the address space of the program, or a null pointer) or a type (after promotion) not expected by a function with variable number of arguments, the behavior is undefined. If a function argument is described as being an array, the pointer actually passed to the function shall have a value such that all address computations and accesses to objects (that would be valid if the pointer did point to the first element of such an array) are in fact valid. Any function declared in a header may be additionally implemented as a function-like macro defined in the header, so if a library function is declared explicitly when its header is included, one of the techniques shown below can be used to ensure the declaration is not affected by such a macro. Any macro definition of a function can be suppressed locally by enclosing the name of the function in parentheses, because the name is then not followed by the left parenthesis that indicates expansion of a macro function name. For the same syntactic reason, it is permitted to take the address of a library function even if it is also defined as a macro.144) The use of #undef to remove any macro definition will also ensure that an actual function is referred to. Any invocation of a library function that is implemented as a macro shall expand to code that evaluates each of its arguments exactly once, fully protected by parentheses where necessary, so it is generally safe to use arbitrary expressions as arguments.145) Likewise, those function-like macros described in the

following subclauses may be invoked in an expression anywhere a function with a compatible return type could be called.146) All object-like macros listed as expanding to integer constant expressions shall additionally be suitable for use in #if preprocessing directives.

Provided that a library function can be declared without reference to any type defined in a header, it is also permissible to declare the function and use it without including its associated header.

There is a sequence point immediately before a library function returns.

The functions in the standard library are not guaranteed to be reentrant and may modify objects with static storage duration.147)

EXAMPLE The function atoi may be used in any of several ways:

-- by use of its associated header (possibly generating a macro expansion)

             #include <stdlib.h>
             const char *str;
             /* ... */
             i = atoi(str);

used to indicate generation of in-line code for the abs function. Thus, the appropriate header could specify #define abs(x) _BUILTIN_abs(x) for a compiler whose code generator will accept it.

In this manner, a user desiring to guarantee that a given library function such as abs will be a genuine function may write #undef abs whether the implementation's header provides a macro implementation of abs or a built-in implementation. The prototype for the function, which precedes and is hidden by any macro definition, is thereby revealed also.

147Thus, a signal handler cannot, in general, call standard library functions. -- by use of its associated header (assuredly generating a true function reference)

             #include <stdlib.h>
             #undef atoi
             const char *str;
             /* ... */
             i = atoi(str);
     or
             #include <stdlib.h>
             const char *str;
             /* ... */
             i = (atoi)(str);

-- by explicit declaration

             extern int atoi(const char *);
             const char *str;
             /* ... */
             i = atoi(str);

7.2 Diagnostics <assert.h>

The header <assert.h> defines the assert macro and refers to another macro,

NDEBUG

which is not defined by <assert.h>. If NDEBUG is defined as a macro name at the point in the source file where <assert.h> is included, the assert macro is defined simply as

        #define assert(ignore) ((void)0)

The assert macro is redefined according to the current state of NDEBUG each time that <assert.h> is included.

The assert macro shall be implemented as a macro, not as an actual function. If the macro definition is suppressed in order to access an actual function, the behavior is undefined.

7.2.1 Program diagnostics

7.2.1.1 The assert macro

Synopsis

        #include <assert.h>
        void assert(scalar expression);

Description

The assert macro puts diagnostic tests into programs; it expands to a void expression. When it is executed, if expression (which shall have a scalar type) is false (that is, compares equal to 0), the assert macro writes information about the particular call that failed (including the text of the argument, the name of the source file, the source line number, and the name of the enclosing function -- the latter are respectively the values of the preprocessing macros __FILE__ and __LINE__ and of the identifier __func__) on the standard error file in an implementation-defined format.148) It then calls the abort function.

Returns

The assert macro returns no value.

Forward references: the abort function (7.20.4.1).

7.3 Complex arithmetic <complex.h>

7.3.1 Introduction

The header <complex.h> defines macros and declares functions that support complex arithmetic.149) Each synopsis specifies a family of functions consisting of a principal function with one or more double complex parameters and a double complex or double return value; and other functions with the same name but with f and l suffixes which are corresponding functions with float and long double parameters and return values.

The macro

complex

expands to _Complex; the macro

_Complex_I

expands to a constant expression of type const float _Complex, with the value of the imaginary unit.150)

The macros

imaginary

and

_Imaginary_I

are defined if and only if the implementation supports imaginary types;151) if defined, they expand to _Imaginary and a constant expression of type const float _Imaginary with the value of the imaginary unit.

The macro

expands to either _Imaginary_I or _Complex_I. If _Imaginary_I is not defined, I shall expand to _Complex_I.

Notwithstanding the provisions of 7.1.3, a program is permitted to undefine and perhaps then redefine the macros

complex, imaginary, and I.

Forward references: IEC 60559-compatible complex arithmetic (annex G).

7.3.2 Conventions

Values are interpreted as radians, not degrees. An implementation may set errno but is not required to.

7.3.3 Branch cuts

Some of the functions below have branch cuts, across which the function is discontinuous. For implementations with a signed zero (including all IEC 60559 implementations) that follow the specification of annex G, the sign of zero distinguishes one side of a cut from another so the function is continuous (except for format limitations) as the cut is approached from either side. For example, for the square root function, which has a branch cut along the negative real axis, the top of the cut, with imaginary part +0, maps to the positive imaginary axis, and the bottom of the cut, with imaginary part -0, maps to the negative imaginary axis.

Implementations that do not support a signed zero (see annex F) cannot distinguish the sides of branch cuts. These implementations shall map a cut so the function is continuous as the cut is approached coming around the finite endpoint of the cut in a counter clockwise direction. (Branch cuts for the functions specified here have just one finite endpoint.) For example, for the square root function, coming counter clockwise around the finite endpoint of the cut along the negative real axis approaches the cut from above, so the cut maps to the positive imaginary axis.

7.3.4 The CX_LIMITED_RANGE pragma

Synopsis

#include <complex.h> #pragma STDC CX_LIMITED_RANGE on-off-switch

Description

The usual mathematical formula for complex multiply, divide, and absolute value are problematic because of their treatment of infinities and because of undue overflow and underflow. The CX_LIMITED_RANGE pragma can be used to inform the implementation that (where the state is on) the usual mathematical formulas are acceptable.152) The pragma can occur either outside external declarations or preceding all explicit declarations and statements inside a compound statement. When outside external declarations, the pragma takes effect from its occurrence until another CX_LIMITED_RANGE pragma is encountered, or until the end of the translation unit. When inside a compound statement, the pragma takes effect from its occurrence until another CX_LIMITED_RANGE pragma is encountered (including within a nested compound statement), or until the end of the compound statement; at the end of a compound statement the state for the pragma is restored to its condition just before the compound statement. If this pragma is used in any other context, the behavior is undefined. The default state for the pragma is off.

7.3.5 Trigonometric functions

7.3.5.1 The cacos functions

Synopsis

        #include <complex.h>
        double complex cacos(double complex z);
        float complex cacosf(float complex z);
        long double complex cacosl(long double complex z);

Description

The cacos functions compute the complex arc cosine of z, with branch cuts outside the interval [-1, 1] along the real axis.

Returns

The cacos functions return the complex arc cosine value, in the range of a strip mathematically unbounded along the imaginary axis and in the interval [0, pi] along the real axis.

7.3.5.2 The casin functions

Synopsis

        #include <complex.h>
        double complex casin(double complex z);
        float complex casinf(float complex z);
        long double complex casinl(long double complex z);

Description

The casin functions compute the complex arc sine of z, with branch cuts outside the interval [-1, 1] along the real axis.

Returns

The casin functions return the complex arc sine value, in the range of a strip mathematically unbounded along the imaginary axis and in the interval [-pi/2, pi/2] along the real axis.

7.3.5.3 The catan functions

Synopsis

        #include <complex.h>
        double complex catan(double complex z);
        float complex catanf(float complex z);
        long double complex catanl(long double complex z);

Description

The catan functions compute the complex arc tangent of z, with branch cuts outside the interval [-i, i] along the imaginary axis.

Returns

The catan functions return the complex arc tangent value, in the range of a strip mathematically unbounded along the imaginary axis and in the interval [-pi/2, pi/2] along the real axis.

7.3.5.4 The ccos functions

Synopsis

        #include <complex.h>
        double complex ccos(double complex z);
        float complex ccosf(float complex z);
        long double complex ccosl(long double complex z);

Description

The ccos functions compute the complex cosine of z.

Returns

The ccos functions return the complex cosine value.

7.3.5.5 The csin functions

Synopsis

        #include <complex.h>
        double complex csin(double complex z);
        float complex csinf(float complex z);
        long double complex csinl(long double complex z);

Description

The csin functions compute the complex sine of z.

Returns

The csin functions return the complex sine value.

7.3.5.6 The ctan functions

Synopsis

        #include <complex.h>
        double complex ctan(double complex z);
        float complex ctanf(float complex z);
        long double complex ctanl(long double complex z);

Description

The ctan functions compute the complex tangent of z.

Returns

The ctan functions return the complex tangent value.

7.3.6 Hyperbolic functions

7.3.6.1 The cacosh functions

Synopsis

        #include <complex.h>
        double complex cacosh(double complex z);
        float complex cacoshf(float complex z);
        long double complex cacoshl(long double complex z);

Description

The cacosh functions compute the complex arc hyperbolic cosine of z, with a branch cut at values less than 1 along the real axis.

Returns

The cacosh functions return the complex arc hyperbolic cosine value, in the range of a half-strip of non-negative values along the real axis and in the interval [-ipi, ipi] along the imaginary axis.

7.3.6.2 The casinh functions

Synopsis

        #include <complex.h>
        double complex casinh(double complex z);
        float complex casinhf(float complex z);
        long double complex casinhl(long double complex z);

Description

The casinh functions compute the complex arc hyperbolic sine of z, with branch cuts outside the interval [-i, i] along the imaginary axis.

Returns

The casinh functions return the complex arc hyperbolic sine value, in the range of a strip mathematically unbounded along the real axis and in the interval [-ipi/2, ipi/2] along the imaginary axis.

7.3.6.3 The catanh functions

Synopsis

        #include <complex.h>
        double complex catanh(double complex z);
        float complex catanhf(float complex z);
        long double complex catanhl(long double complex z);

Description

The catanh functions compute the complex arc hyperbolic tangent of z, with branch cuts outside the interval [-1, 1] along the real axis.

Returns

The catanh functions return the complex arc hyperbolic tangent value, in the range of a strip mathematically unbounded along the real axis and in the interval [-ipi/2, ipi/2] along the imaginary axis.

7.3.6.4 The ccosh functions

Synopsis

        #include <complex.h>
        double complex ccosh(double complex z);
        float complex ccoshf(float complex z);
        long double complex ccoshl(long double complex z);

Description

The ccosh functions compute the complex hyperbolic cosine of z.

Returns

The ccosh functions return the complex hyperbolic cosine value.

7.3.6.5 The csinh functions

Synopsis

        #include <complex.h>
        double complex csinh(double complex z);
        float complex csinhf(float complex z);
        long double complex csinhl(long double complex z);

Description

The csinh functions compute the complex hyperbolic sine of z.

Returns

The csinh functions return the complex hyperbolic sine value.

7.3.6.6 The ctanh functions

Synopsis

        #include <complex.h>
        double complex ctanh(double complex z);
        float complex ctanhf(float complex z);
        long double complex ctanhl(long double complex z);

Description

The ctanh functions compute the complex hyperbolic tangent of z.

Returns

The ctanh functions return the complex hyperbolic tangent value.

7.3.7 Exponential and logarithmic functions

7.3.7.1 The cexp functions

Synopsis

        #include <complex.h>
        double complex cexp(double complex z);
        float complex cexpf(float complex z);
        long double complex cexpl(long double complex z);

Description

The cexp functions compute the complex base-e exponential of z.

Returns

The cexp functions return the complex base-e exponential value.

7.3.7.2 The clog functions

Synopsis

        #include <complex.h>
        double complex clog(double complex z);
        float complex clogf(float complex z);
        long double complex clogl(long double complex z);

Description

The clog functions compute the complex natural (base-e) logarithm of z, with a branch cut along the negative real axis.

Returns

The clog functions return the complex natural logarithm value, in the range of a strip mathematically unbounded along the real axis and in the interval [-ipi, ipi] along the imaginary axis.

7.3.8 Power and absolute-value functions

7.3.8.1 The cabs functions

Synopsis

        #include <complex.h>
        double cabs(double complex z);
        float cabsf(float complex z);
        long double cabsl(long double complex z);

Description

The cabs functions compute the complex absolute value (also called norm, modulus, or magnitude) of z.

Returns

The cabs functions return the complex absolute value.

7.3.8.2 The cpow functions

Synopsis

        #include <complex.h>
        double complex cpow(double complex x, double complex y);
        float complex cpowf(float complex x, float complex y);
        long double complex cpowl(long double complex x,
                long double complex y);

Description

The cpow functions compute the complex power function xy, with a branch cut for the first parameter along the negative real axis.

Returns

The cpow functions return the complex power function value.

7.3.8.3 The csqrt functions

Synopsis

        #include <complex.h>
        double complex csqrt(double complex z);
        float complex csqrtf(float complex z);
        long double complex csqrtl(long double complex z);

Description

The csqrt functions compute the complex square root of z, with a branch cut along the negative real axis.

Returns

The csqrt functions return the complex square root value, in the range of the right half-plane (including the imaginary axis).

7.3.9 Manipulation functions

7.3.9.1 The carg functions

Synopsis

        #include <complex.h>
        double carg(double complex z);
        float cargf(float complex z);
        long double cargl(long double complex z);

Description

The carg functions compute the argument (also called phase angle) of z, with a branch cut along the negative real axis.

Returns

The carg functions return the value of the argument in the range [-pi, pi].

7.3.9.2 The cimag functions

Synopsis

        #include <complex.h>
        double cimag(double complex z);
        float cimagf(float complex z);
        long double cimagl(long double complex z);

Description

The cimag functions compute the imaginary part of z.153)

Returns

The cimag functions return the imaginary part value (as a real).

7.3.9.3 The conj functions

Synopsis

        #include <complex.h>
        double complex conj(double complex z);
        float complex conjf(float complex z);
        long double complex conjl(long double complex z);

Description

The conj functions compute the complex conjugate of z, by reversing the sign of its imaginary part.

Returns

The conj functions return the complex conjugate value.

7.3.9.4 The cproj functions

Synopsis

        #include <complex.h>
        double complex cproj(double complex z);
        float complex cprojf(float complex z);
        long double complex cprojl(long double complex z);

Description

The cproj functions compute a projection of z onto the Riemann sphere: z projects to z except that all complex infinities (even those with one infinite part and one NaN part) project to positive infinity on the real axis. If z has an infinite part, then cproj(z) is equivalent to

        INFINITY + I * copysign(0.0, cimag(z))

Returns

The cproj functions return the value of the projection onto the Riemann sphere.

7.3.9.5 The creal functions

Synopsis

        #include <complex.h>
        double creal(double complex z);
        float crealf(float complex z);
        long double creall(long double complex z);

Description

The creal functions compute the real part of z.154)

Returns

The creal functions return the real part value.

7.4 Character handling <ctype.h>

The header <ctype.h> declares several functions useful for testing and mapping characters.155) In all cases the argument is an int, the value of which shall be representable as an unsigned char or shall equal the value of the macro EOF. If the argument has any other value, the behavior is undefined.

The behavior of these functions is affected by the current locale. Those functions that have locale-specific aspects only when not in the "C" locale are noted below.

The term printing character refers to a member of a locale-specific set of characters, each of which occupies one printing position on a display device; the term control character refers to a member of a locale-specific set of characters that are not printing characters.156)

Forward references: EOF (7.19.1), localization (7.11).

7.4.1 Character testing functions

The functions in this subclause return nonzero (true) if and only if the value of the argument c conforms to that in the description of the function.

7.4.1.1 The isalnum function

Synopsis

        #include <ctype.h>
        int isalnum(int c);

Description

The isalnum function tests for any character for which isalpha or isdigit is true.

(NUL) through 0x1F (US), and the character 0x7F (DEL).

7.4.1.2 The isalpha function

Synopsis

        #include <ctype.h>
        int isalpha(int c);

Description

The isalpha function tests for any character for which isupper or islower is true, or any character that is one of a locale-specific set of alphabetic characters for which none of iscntrl, isdigit, ispunct, or isspace is true.157) In the "C" locale, isalpha returns true only for the characters for which isupper or islower is true.

7.4.1.3 The iscntrl function

Synopsis

        #include <ctype.h>
        int iscntrl(int c);

Description

The iscntrl function tests for any control character.

7.4.1.4 The isdigit function

Synopsis

        #include <ctype.h>
        int isdigit(int c);

Description

The isdigit function tests for any decimal-digit character (as defined in 5.2.1).

7.4.1.5 The isgraph function

Synopsis

        #include <ctype.h>
        int isgraph(int c);

Description

The isgraph function tests for any printing character except space (' ').

7.4.1.6 The islower function

Synopsis

        #include <ctype.h>
        int islower(int c);

Description

The islower function tests for any character that is a lowercase letter or is one of a locale-specific set of characters for which none of iscntrl, isdigit, ispunct, or isspace is true. In the "C" locale, islower returns true only for the characters defined as lowercase letters (as defined in 5.2.1).

7.4.1.7 The isprint function

Synopsis

        #include <ctype.h>
        int isprint(int c);

Description

The isprint function tests for any printing character including space (' ').

7.4.1.8 The ispunct function

Synopsis

        #include <ctype.h>
        int ispunct(int c);

Description

The ispunct function tests for any printing character that is one of a locale-specific set of punctuation characters for which neither isspace nor isalnum is true. In the "C" locale, ispunct returns true for every printing character for which neither isspace nor isalnum is true.

7.4.1.9 The isspace function

Synopsis

        #include <ctype.h>
        int isspace(int c);

Description

The isspace function tests for any character that is a standard white-space character or is one of a locale- specific set of characters for which isalnum is false. The standard white-space characters are the following: space (' '), form feed ('\f'), new-line ('\n'), carriage return ('\r'), horizontal tab ('\t'), and vertical tab ('\v'). In the "C" locale, isspace returns true only for the standard white-space characters.

7.4.1.10 The isupper function

Synopsis

        #include <ctype.h>
        int isupper(int c);

Description

The isupper function tests for any character that is an uppercase letter or is one of a locale-specific set of characters for which none of iscntrl, isdigit, ispunct, or isspace is true. In the "C" locale, isupper returns true only for the characters defined as uppercase letters (as defined in 5.2.1).

7.4.1.11 The isxdigit function

Synopsis

        #include <ctype.h>
        int isxdigit(int c);

Description

The isxdigit function tests for any hexadecimal-digit character (as defined in 6.4.4.2).

7.4.2 Character case mapping functions

7.4.2.1 The tolower function

Synopsis

        #include <ctype.h>
        int tolower(int c);

Description

The tolower function converts an uppercase letter to a corresponding lowercase letter.

Returns

If the argument is a character for which isupper is true and there are one or more corresponding characters, as specified by the current locale, for which islower is true, the tolower function returns one of the corresponding characters (always the same one for any given locale); otherwise, the argument is returned unchanged.

7.4.2.2 The toupper function

Synopsis

        #include <ctype.h>
        int toupper(int c);

Description

The toupper function converts a lowercase letter to a corresponding uppercase letter.

Returns

If the argument is a character for which islower is true and there are one or more corresponding characters, as specified by the current locale, for which isupper is true, the toupper function returns one of the corresponding characters (always the same one for any given locale); otherwise, the argument is returned unchanged.

7.5 Errors <errno.h>

The header <errno.h> defines several macros, all relating to the reporting of error conditions.

The macros are

EDOM EILSEQ ERANGE

which expand to integer constant expressions with type int, distinct positive values, and which are suitable for use in #if preprocessing directives; and

errno

which expands to a modifiable lvalue158) that has type int, the value of which is set to a positive error number by several library functions. It is unspecified whether errno is a macro or an identifier declared with external linkage. If a macro definition is suppressed in order to access an actual object, or a program defines an identifier with the name errno, the behavior is undefined.

The value of errno is zero at program startup, but is never set to zero by any library function.159) The value of errno may be set to nonzero by a library function call whether or not there is an error, provided the use of errno is not documented in the description of the function in this International Standard.

Additional macro definitions, beginning with E and a digit or E and an uppercase letter,160) may also be specified by the implementation.

entry and then set it to zero, as long as the original value is restored if errno's value is still zero just before the return.

160See ``future library directions'' (7.26.3).

7.6 Floating-point environment <fenv.h>

The header <fenv.h> declares two types and several macros and functions to provide access to the floating-point environment. The floating-point environment refers collectively to any floating-point status flags and control modes supported by the implementation.161) A floating-point status flag is a system variable whose value is set (but never cleared) as a side effect of floating-point arithmetic to provide auxiliary information. A floating-point control mode is a system variable whose value may be set by the user to affect the subsequent behavior of floating-point arithmetic.

Certain programming conventions support the intended model of use for the floating-point environment:162)

-- a function call does not alter its caller's modes, clear its caller's flags, nor depend on the state of its caller's flags unless the function is so documented;

-- a function call is assumed to require default modes, unless its documentation promises otherwise or unless the function is known not to use floating-point;

-- a function call is assumed to have the potential for raising floating-point exceptions, unless its documentation promises otherwise, or unless the function is known not to use floating-point.

The type

fenv_t

represents the entire floating-point environment.

The type

fexcept_t

does so explicitly. represents the floating-point exception flags collectively, including any status the implementation associates with the flags.

Each of the macros

FE_DIVBYZERO FE_INEXACT FE_INVALID FE_OVERFLOW FE_UNDERFLOW

is defined if and only if the implementation supports the exception by means of the functions in 7.6.2. Additional floating-point exceptions, with macro definitions beginning with FE_ and an uppercase letter, may also be specified by the implementation. The defined macros expand to integer constant expressions with values such that bitwise ORs of all combinations of the macros result in distinct values.

The macro

FE_ALL_EXCEPT

is simply the bitwise OR of all exception macros defined by the implementation.

Each of the macros

FE_DOWNWARD FE_TONEAREST FE_TOWARDZERO FE_UPWARD

is defined if and only if the implementation supports getting and setting the represented rounding direction by means of the fegetround and fesetround functions. Additional rounding directions, with macro definitions beginning with FE_ and an uppercase letter, may also be specified by the implementation. The defined macros expand to integer constant expressions whose values are distinct nonnegative values.163)

The macro

FE_DFL_ENV

represents the default floating-point environment -- the one installed at program startup -- and has type pointer

to const-qualified fenv_t. It can be used as an argument to <fenv.h> functions that manage the floating-point environment.

Additional macro definitions, beginning with FE_ and having type pointer to const-qualified fenv_t, may also be specified by the implementation.

7.6.1 The FENV_ACCESS pragma

Synopsis

        #include <fenv.h>
        #pragma STDC FENV_ACCESS on-off-switch

Description

The FENV_ACCESS pragma provides a means to inform the implementation when a program might access the floating- point environment to test flags or run under non-default modes.164) The pragma shall occur either outside external declarations or preceding all explicit declarations and statements inside a compound statement. When outside external declarations, the pragma takes effect from its occurrence until another FENV_ACCESS pragma is encountered, or until the end of the translation unit. When inside a compound statement, the pragma takes effect from its occurrence until another FENV_ACCESS pragma is encountered (including within a nested compound statement), or until the end of the compound statement; at the end of a compound statement the state for the pragma is restored to its condition just before the compound statement. If this pragma is used in any other context, the behavior is undefined. If part of a program tests flags or runs under non-default mode settings, but was translated with the state for the FENV_ACCESS pragma off, then the behavior is undefined. The default state (on or off) for the pragma is implementation-defined.

EXAMPLE

        #include <fenv.h>
        void f(double x)
        {
                #pragma STDC FENV_ACCESS ON
                void g(double);
                void h(double);
                /* ... */
                g(x + 1);
                h(x + 1);
                /* ... */
        }

If the function g might depend on status flags set as a side effect of the first x + 1, or if the second x + 1 might depend on control modes set as a side effect of the call to function g, then the program shall contain an appropriately placed invocation of #pragma STDC FENV_ACCESS ON.165)

7.6.2 Exceptions

The following functions provide access to the exception flags.166) The int input argument for the functions represents a subset of floating-point exceptions, and can be zero or the bitwise OR of one or more exception macros, for example FE_OVERFLOW | FE_INEXACT. For other argument values the behavior of these functions is undefined.

the address of the code which first raised the exception; the functions fegetexceptflag and fesetexceptflag deal with the full content of flags.

7.6.2.1 The feclearexcept function

Synopsis

        #include <fenv.h>
        void feclearexcept(int excepts);

Description

The feclearexcept function clears the supported exceptions represented by its argument.

7.6.2.2 The fegetexceptflag function

Synopsis

        #include <fenv.h>
        void fegetexceptflag(fexcept_t *flagp,
                int excepts);

Description

The fegetexceptflag function stores an implementation- defined representation of the exception flags indicated by the argument excepts in the object pointed to by the argument flagp.

7.6.2.3 The feraiseexcept function

Synopsis

        #include <fenv.h>
        void feraiseexcept(int excepts);

Description

The feraiseexcept function raises the supported exceptions represented by its argument.167) The order in which these exceptions are raised is unspecified, except as stated in F.7.6. Whether the feraiseexcept function additionally raises the inexact exception whenever it raises the overflow or underflow exception is implementation-

defined.

7.6.2.4 The fesetexceptflag function

Synopsis

        #include <fenv.h>
        void fesetexceptflag(const fexcept_t *flagp,
                int excepts);

Description

The fesetexceptflag function sets the complete status for those exception flags indicated by the argument excepts, according to the representation in the object pointed to by flagp. The value of *flagp shall have been set by a previous call to fegetexceptflag whose second argument represented at least those exceptions represented by the argument excepts. This function does not raise exceptions, but only sets the state of the flags.

7.6.2.5 The fetestexcept function

Synopsis

        #include <fenv.h>
        int fetestexcept(int excepts);

Description

The fetestexcept function determines which of a specified subset of the exception flags are currently set. The excepts argument specifies the exception flags to be queried.168)

Returns

The fetestexcept function returns the value of the bitwise OR of the exception macros corresponding to the currently set exceptions included in excepts.

EXAMPLE Call f if invalid is set, then g if overflow is set:

        #include <fenv.h>
        /* ... */
        {
                #pragma STDC FENV_ACCESS ON
                int set_excepts;
                feclearexcept(FE_INVALID | FE_OVERFLOW);     
                // maybe raise exceptions
                set_excepts =
                        fetestexcept(FE_INVALID | FE_OVERFLOW);
                if (set_excepts & FE_INVALID) f();
                if (set_excepts & FE_OVERFLOW) g();
                /* ... */
        }

7.6.3 Rounding

The fegetround and fesetround functions provide control of rounding direction modes.

7.6.3.1 The fegetround function

Synopsis

        #include <fenv.h>
        int fegetround(void);

Description

The fegetround function gets the current rounding direction.

Returns

The fegetround function returns the value of the rounding direction macro representing the current rounding direction.

7.6.3.2 The fesetround function

Synopsis

        #include <fenv.h>
        int fesetround(int round);

Description

The fesetround function establishes the rounding direction represented by its argument round. If the argument is not equal to the value of a rounding direction macro, the rounding direction is not changed.

Returns

The fesetround function returns a zero value if and only if the argument is equal to a rounding direction macro (that is, if and only if the requested rounding direction was established).

EXAMPLE 1 Save, set, and restore the rounding direction. Report an error and abort if setting the rounding direction fails.

        #include <fenv.h>
        #include <assert.h>
        /* ... */
        {
                #pragma STDC FENV_ACCESS ON
                int save_round;
                int setround_ok;
                save_round = fegetround();
                setround_ok = fesetround(FE_UPWARD);
                assert(setround_ok == 0);                    
                /* ... */
                fesetround(save_round);
                /* ... */
        }

7.6.4 Environment

The functions in this section manage the floating-point environment -- status flags and control modes -- as one entity.

7.6.4.1 The fegetenv function

Synopsis

        #include <fenv.h>
        void fegetenv(fenv_t *envp);

Description

The fegetenv function stores the current floating-point environment in the object pointed to by envp.

7.6.4.2 The feholdexcept function

Synopsis

        #include <fenv.h>
        int feholdexcept(fenv_t *envp);

Description

The feholdexcept function saves the current floating- point environment in the object pointed to by envp, clears the exception flags, and then installs a non-stop (continue on exceptions) mode, if available, for all exceptions.169)

Returns

The feholdexcept function returns zero if and only if non-stop exception handling was successfully installed.

7.6.4.3 The fesetenv function

Synopsis

        #include <fenv.h>
        void fesetenv(const fenv_t *envp);

Description

The fesetenv function establishes the floating-point environment represented by the object pointed to by envp. The argument envp shall point to an object set by a call to fegetenv or feholdexcept, or equal the macro FE_DFL_ENV or an implementation-defined environment macro. Note that fesetenv merely installs the state of the exception flags represented through its argument, and does not raise these exceptions.

7.6.4.4 The feupdateenv function

Synopsis

        #include <fenv.h>
        void feupdateenv(const fenv_t *envp);

Description

The feupdateenv function saves the currently raised exceptions in its automatic storage, installs the floating- point environment represented by the object pointed to by envp, and then raises the saved exceptions. The argument envp shall point to an object set by a call to feholdexcept or fegetenv, or equal the macro FE_DFL_ENV or an implementation-defined environment macro.

EXAMPLE 1 Hide spurious underflow exceptions:

        #include <fenv.h>
        double f(double x)
        {
                #pragma STDC FENV_ACCESS ON
                double result;
                fenv_t save_env;
                feholdexcept(&save_env);
                // compute result
                if (/* test spurious underflow */)
                        feclearexcept(FE_UNDERFLOW);
                feupdateenv(&save_env);
                return result;
        }

7.7 Characteristics of floating types <float.h>

The header <float.h> defines several macros that expand to various limits and parameters of the standard floating- point types.

The macros, their meanings, and the constraints (or restrictions) on their values are listed in 5.2.4.2.2.

7.8 Format conversion of integer types <inttypes.h>

The header <inttypes.h> includes the header <stdint.h> and extends it with additional facilities provided by hosted implementations.

It declares four functions for converting numeric character strings to greatest-width integers and, for each type declared in <stdint.h>, it defines corresponding macros for conversion specifiers for use with the formatted input/output functions.170)

Forward references: integer types <stdint.h> (7.18).

7.8.1 Macros for format specifiers

Each of the following object-like macros171) expands to a character string literal containing a conversion specifier, possibly modified by a length modifier, suitable for use within the format argument of a formatted input/output function when converting the corresponding integer type. These macro names have the general form of PRI (character string literals for the fprintf family) or SCN (character string literals for the fscanf family),172) followed by the conversion specifier, followed by a name corresponding to a similar type name in 7.18.1. In these names, N represents the width of the type as described in 7.18.1. For example, PRIdFAST32 can be used in a format string to print the value of an integer of type int_fast32_t.

The fprintf macros for signed integers are:

PRIdN PRIdLEASTN PRIdFASTN PRIdMAX PRIdPTR PRIiN PRIiLEASTN PRIiFASTN PRIiMAX PRIiPTR

The fprintf macros for unsigned integers are:

172Separate macros are given for use with fprintf and fscanf functions because, in the general case, different format specifiers may be required for fprintf and fscanf, even when the type is the same. PRIoN PRIoLEASTN PRIoFASTN PRIoMAX PRIoPTR PRIuN PRIuLEASTN PRIuFASTN PRIuMAX PRIuPTR PRIxN PRIxLEASTN PRIxFASTN PRIxMAX PRIxPTR PRIXN PRIXLEASTN PRIXFASTN PRIXMAX PRIXPTR

The fscanf macros for signed integers are:

SCNdN SCNdLEASTN SCNdFASTN SCNdMAX SCNdPTR SCNiN SCNiLEASTN SCNiFASTN SCNiMAX SCNiPTR

The fscanf macros for unsigned integers are:

SCNoN SCNoLEASTN SCNoFASTN SCNoMAX SCNoPTR SCNuN SCNuLEASTN SCNuFASTN SCNuMAX SCNuPTR SCNxN SCNxLEASTN SCNxFASTN SCNxMAX SCNxPTR

For each type that the implementation provides in <stdint.h>, the corresponding fprintf macros shall be defined and the corresponding fscanf macros shall be defined unless the implementation does not have a suitable fscanf length modifier for the type.

EXAMPLE

        #include <inttypes.h>
        #include <wchar.h>
        int main(void)
        {
                uintmax_t i = UINTMAX_MAX;    // this type always exists
                wprintf(L"The largest integer value is %020"
                        PRIxMAX "\n", i);
                return 0;
        }

7.8.2 Conversion functions for greatest-width integer types

7.8.2.1 The strtoimax and strtoumax functions

Synopsis

        #include <inttypes.h>
        intmax_t strtoimax(const char * restrict nptr,
                char ** restrict endptr, int base);
        uintmax_t strtoumax(const char * restrict nptr,
                char ** restrict endptr, int base);

Description

The strtoimax and strtoumax functions are equivalent to the strtol, strtoll, strtoul, and strtoull functions, except that the initial portion of the string is converted to intmax_t and uintmax_t representation, respectively.

Returns

The strtoimax and strtoumax functions return the converted value, if any. If no conversion could be performed, zero is returned. If the correct value is outside the range of representable values, INTMAX_MAX, INTMAX_MIN, or UINTMAX_MAX is returned (according to the return type and sign of the value, if any), and the value of the macro ERANGE is stored in errno.

7.8.2.2 The wcstoimax and wcstoumax functions

Synopsis

        #include <stddef.h>           // for wchar_t
        #include <inttypes.h>
        intmax_t wcstoimax(const wchar_t * restrict nptr,
                wchar_t ** restrict endptr, int base);
        uintmax_t wcstoumax(const wchar_t * restrict nptr,
                wchar_t ** restrict endptr, int base);

Description

The wcstoimax and wcstoumax functions are equivalent to the wcstol, wcstoll, wcstoul, and wcstoull functions except that the initial portion of the wide string is converted to intmax_t and uintmax_t representation, respectively.

Returns

The wcstoimax function returns the converted value, if any. If no conversion could be performed, zero is returned. If the correct value is outside the range of representable values, INTMAX_MAX, INTMAX_MIN, or UINTMAX_MAX is returned (according to the return type and sign of the value, if any), and the value of the macro ERANGE is stored in errno.

7.9 Alternative spellings <iso646.h>

The header <iso646.h> defines the following eleven macros (on the left) that expand to the corresponding tokens (on the right):

        and      &&
        and_eq   &=
        bitand   &
        bitor    |
        compl    ~
        not      !
        not_eq   !=
        or       ||
        or_eq    |=
        xor      ^
        xor_eq   ^=

7.10 Sizes of integer types <limits.h>

The header <limits.h> defines several macros that expand to various limits and parameters of the standard integer types.

The macros, their meanings, and the constraints (or restrictions) on their values are listed in 5.2.4.2.1.

7.11 Localization <locale.h>

The header <locale.h> declares two functions, one type, and defines several macros.

The type is

        struct lconv

which contains members related to the formatting of numeric values. The structure shall contain at least the following members, in any order. The semantics of the members and their normal ranges are explained in 7.11.2.1. In the "C" locale, the members shall have the values specified in the comments.

char *decimal_point; // "." char *thousands_sep; // "" char *grouping; // "" char *mon_decimal_point; // "" char *mon_thousands_sep; // "" char *mon_grouping; // "" char *positive_sign; // "" char *negative_sign; // "" char *currency_symbol; // "" char frac_digits; // CHAR_MAX char p_cs_precedes; // CHAR_MAX char n_cs_precedes; // CHAR_MAX char p_sep_by_space; // CHAR_MAX char n_sep_by_space; // CHAR_MAX char p_sign_posn; // CHAR_MAX char n_sign_posn; // CHAR_MAX char *int_curr_symbol; // "" char int_frac_digits; // CHAR_MAX char int_p_cs_precedes; // CHAR_MAX char int_n_cs_precedes; // CHAR_MAX char int_p_sep_by_space; // CHAR_MAX char int_n_sep_by_space; // CHAR_MAX char int_p_sign_posn; // CHAR_MAX char int_n_sign_posn; // CHAR_MAX

The macros defined are NULL (described in 7.17); and

LC_ALL LC_COLLATE LC_CTYPE LC_MONETARY LC_NUMERIC LC_TIME

which expand to integer constant expressions with distinct values, suitable for use as the first argument to the setlocale function.173) Additional macro definitions, beginning with the characters LC_ and an uppercase letter,174) may also be specified by the implementation.

7.11.1 Locale control

7.11.1.1 The setlocale function

Synopsis

        #include <locale.h>
        char *setlocale(int category, const char *locale);

Description

The setlocale function selects the appropriate portion of the program's locale as specified by the category and locale arguments. The setlocale function may be used to change or query the program's entire current locale or portions thereof. The value LC_ALL for category names the program's entire locale; the other values for category name only a portion of the program's locale. LC_COLLATE affects the behavior of the strcoll and strxfrm functions. LC_CTYPE affects the behavior of the character handling functions175) and the multibyte and wide-character functions. LC_MONETARY affects the monetary formatting information returned by the localeconv function. LC_NUMERIC affects the decimal-point character for the formatted input/output functions and the string conversion functions, as well as the nonmonetary formatting information returned by the localeconv function. LC_TIME affects the behavior of the strftime function.

A value of "C" for locale specifies the minimal environment for C translation; a value of "" for locale specifies the locale-specific native environment. Other implementation-defined strings may be passed as the second argument to setlocale.

At program startup, the equivalent of

        setlocale(LC_ALL, "C");

is executed.

The implementation shall behave as if no library

function calls the setlocale function.

Returns

If a pointer to a string is given for locale and the selection can be honored, the setlocale function returns a pointer to the string associated with the specified category for the new locale. If the selection cannot be honored, the setlocale function returns a null pointer and the program's locale is not changed.

A null pointer for locale causes the setlocale function to return a pointer to the string associated with the category for the program's current locale; the program's locale is not changed.176)

The pointer to string returned by the setlocale function is such that a subsequent call with that string value and its associated category will restore that part of the program's locale. The string pointed to shall not be modified by the program, but may be overwritten by a subsequent call to the setlocale function.

Forward references: formatted input/output functions (7.19.6), the multibyte character functions (7.20.7), the multibyte string functions (7.20.8), string conversion functions (7.20.1), the strcoll function (7.21.4.3), the strftime function (7.23.3.5), the strxfrm function (7.21.4.5).

7.11.2 Numeric formatting convention inquiry

7.11.2.1 The localeconv function

Synopsis

        #include <locale.h>
        struct lconv *localeconv(void);

Description

The localeconv function sets the components of an object with type struct lconv with values appropriate for the formatting of numeric quantities (monetary and otherwise) according to the rules of the current locale.

The members of the structure with type char * are

pointers to strings, any of which (except decimal_point) can point to "", to indicate that the value is not available in the current locale or is of zero length. Apart from grouping and mon_grouping, the strings shall start and end in the initial shift state. The members with type char are nonnegative numbers, any of which can be CHAR_MAX to indicate that the value is not available in the current locale. The members include the following:

char *decimal_point The decimal-point character used to format nonmonetary quantities.

char *thousands_sep The character used to separate groups of digits before the decimal-point character in formatted nonmonetary quantities.

char *grouping A string whose elements indicate the size of each group of digits in formatted nonmonetary quantities.

char *mon_decimal_point The decimal-point used to format monetary quantities.

char *mon_thousands_sep The separator for groups of digits before the decimal-point in formatted monetary quantities.

char *mon_grouping A string whose elements indicate the size of each group of digits in formatted monetary quantities.

char *positive_sign The string used to indicate a nonnegative-valued formatted monetary quantity.

char *negative_sign The string used to indicate a negative-valued formatted monetary quantity.

char *currency_symbol The local currency symbol applicable to the current locale.

char frac_digits The number of fractional digits (those after the decimal-point) to be displayed in a locally formatted monetary quantity.

char p_cs_precedes Set to 1 or 0 if the currency_symbol respectively precedes or succeeds the value for a nonnegative locally formatted monetary quantity. char n_cs_precedes Set to 1 or 0 if the currency_symbol respectively precedes or succeeds the value for a negative locally formatted monetary quantity.

char p_sep_by_space Set to a value indicating the separation of the currency_symbol, the sign string, and the value for a nonnegative locally formatted monetary quantity.

char n_sep_by_space Set to a value indicating the separation of the currency_symbol, the sign string, and the value for a negative locally formatted monetary quantity.

char p_sign_posn Set to a value indicating the positioning of the positive_sign for a nonnegative locally formatted monetary quantity.

char n_sign_posn Set to a value indicating the positioning of the negative_sign for a negative locally formatted monetary quantity.

char *int_curr_symbol The international currency symbol applicable to the current locale. The first three characters contain the alphabetic international currency symbol in accordance with those specified in ISO 4217:1995. The fourth character (immediately preceding the null character) is the character used to separate the international currency symbol from the monetary quantity.

char int_frac_digits The number of fractional digits (those after the decimal-point) to be displayed in an internationally formatted monetary quantity.

char int_p_cs_precedes Set to 1 or 0 if the int_currency_symbol respectively precedes or succeeds the value for a nonnegative internationally formatted monetary quantity.

char int_n_cs_precedes Set to 1 or 0 if the int_currency_symbol respectively precedes or succeeds the value for a negative internationally formatted monetary quantity.

char int_p_sep_by_space Set to a value indicating the separation of the int_currency_symbol, the sign string, and the value for a nonnegative internationally formatted monetary quantity.

char int_n_sep_by_space Set to a value indicating the separation of the int_currency_symbol, the sign string, and the value for a negative internationally formatted monetary quantity.

char int_p_sign_posn Set to a value indicating the positioning of the positive_sign for a nonnegative internationally formatted monetary quantity.

char int_n_sign_posn Set to a value indicating the positioning of the negative_sign for a negative internationally formatted monetary quantity.

The elements of grouping and mon_grouping are interpreted according to the following:

CHAR_MAX No further grouping is to be performed.

0 The previous element is to be repeatedly used for the remainder of the digits.

other The integer value is the number of digits that compose the current group. The next element is examined to determine the size of the next group of digits before the current group.

The values of p_sep_by_space, n_sep_by_space, int_p_sep_by_space, and int_n_sep_by_space are interpreted according to the following:

0 No space separates the currency symbol and value.

1 If the currency symbol and sign string are adjacent, a space separates them from the value; otherwise, a space separates the currency symbol from the value.

2 If the currency symbol and sign string are adjacent, a space separates them; otherwise, a space separates the sign string from the value.

The values of p_sign_posn, n_sign_posn, int_p_sign_posn, and int_n_sign_posn are interpreted according to the following:

0 Parentheses surround the quantity and currency symbol.

1 The sign string precedes the quantity and currency symbol. 2 The sign string succeeds the quantity and currency symbol.

3 The sign string immediately precedes the currency symbol.

4 The sign string immediately succeeds the currency symbol.

The implementation shall behave as if no library function calls the localeconv function.

Returns

The localeconv function returns a pointer to the filled-in object. The structure pointed to by the return value shall not be modified by the program, but may be overwritten by a subsequent call to the localeconv function. In addition, calls to the setlocale function with categories LC_ALL, LC_MONETARY, or LC_NUMERIC may overwrite the contents of the structure.

EXAMPLE 1 The following table illustrates the rules which may well be used by four countries to format monetary quantities.

| | | Local format | International format +------------+-------------+-----------+------------- Country | Positive | Negative | Positive | Negative -----------+------------+-------------+-----------+------------- Finland 1.234,56 mk -1.234,56 mk FIM 1.234,56FIM -1.234,56 Italy L.1.234 -L.1.234 ITL 1.234 -ITL 1.234 Netherlandsf 1.234,56 f -1.234,56 NLG 1.234,56NLG -1.234,56 SwitzerlandSFrs.1,234.56SFrs.1,234.56CCHF 1,234.56CHF 1,234.56C

#10

For these four countries, the respective values for the monetary members of the structure returned by localeconv are:

                          |          |          |          |
                          | Finland  |  Italy   NetherlandsSwitzerland
        ------------------+----------+----------+----------+-----------
        mon_decimal_point | ","      | ""       |","       | "."
        mon_thousands_sep | "."      | "."      |"."       | ","
        mon_grouping      | "\3"     | "\3"     |"\3"      | "\3"
        positive_sign     | ""       | ""       |""        | ""
        negative_sign     | "-"      | "-"      |"-"       | "C"
        currency_symbol   | "mk"     | "L."     |"\u0192"  | "SFrs." 
        frac_digits       | 2        | 0        |2         | 2
        p_cs_precedes     | 0        | 1        |1         | 1
        n_cs_precedes     | 0        | 1        |1         | 1
        p_sep_by_space    | 1        | 0        |1         | 0
        n_sep_by_space    | 1        | 0        |1         | 0
        p_sign_posn       | 1        | 1        |1         | 1
        n_sign_posn       | 1        | 1        |4         | 2
        int_curr_symbol   | "FIM "   | "ITL "   |"NLG "    | "CHF "
        int_frac_digits   | 2        | 0        |2         | 2
        int_p_cs_precedes | 1        | 1        |1         | 1
        int_n_cs_precedes | 1        | 1        |1         | 1
        int_p_sep_by_space| 0        | 0        |0         | 0       
        int_n_sep_by_space| 0        | 0        |0         | 0       
        int_p_sign_posn   | 1        | 1        |1         | 1
        int_n_sign_posn   | 4        | 1        |4         | 2

#11

EXAMPLE 2 The following table illustrates how the cs_precedes, sep_by_space, and sign_posn members affect the formatted value.

                      |           |
                      |           |         p_sep_by_space
                      |           +-----------+----------+-----------
        p_cs_precedes p_sign_posn |    0      |   1      |    2
        --------------+-----------+-----------+----------+-----------
                    0 |         0 | (1.25$)   |(1.25 $)  | (1.25$)
                      |         1 | +1.25$    |+1.25 $   | + 1.25$
                      |         2 | 1.25$+    |1.25 $+   | 1.25$ +
                      |         3 | 1.25+$    |1.25 +$   | 1.25+ $
                      |         4 | 1.25$+    |1.25 $+   | 1.25$ +
        --------------+-----------+-----------+----------+-----------
                    1 |         0 | ($1.25)   |($ 1.25)  | ($1.25)
                      |         1 | +$1.25    |+$ 1.25   | + $1.25
                      |         2 | $1.25+    |$ 1.25+   | $1.25 +
                      |         3 | +$1.25    |+$ 1.25   | + $1.25
                      |         4 | $+1.25    |$+ 1.25   | $ +1.25

7.12 Mathematics <math.h>

The header <math.h> declares two types and several mathematical functions and defines several macros. Most synopses specify a family of functions consisting of a principal function with one or more double parameters, a double return value, or both; and other functions with the same name but with f and l suffixes which are corresponding functions with float and long double parameters, return values, or both.177) Integer arithmetic functions and conversion functions are discussed later.

The types

float_t double_t

are floating types at least as wide as float and double, respectively, and such that double_t is at least as wide as float_t. If FLT_EVAL_METHOD equals 0, float_t and double_t are float and double, respectively; if FLT_EVAL_METHOD equals 1, they are both double; if FLT_EVAL_METHOD equals 2, they are both long double; and for other values of FLT_EVAL_METHOD, they are otherwise implementation-defined.178)

The macro

HUGE_VAL

expands to a positive double constant expression, not necessarily representable as a float. The macros

HUGE_VALF HUGE_VALL

are respectively float and long double analogs of HUGE_VAL.179)

used by the implementation to evaluate floating expressions.

179HUGE_VAL, HUGE_VALF, and HUGE_VALL can be positive infinities in an implementation that supports infinities.

The macro

INFINITY

expands to a constant expression of type float representing positive or unsigned infinity, if available; else to a positive constant of type float that overflows at translation time.180)

The macro

NAN

is defined if and only if the implementation supports quiet NaNs for the float type. It expands to a constant expression of type float representing a quiet NaN.

The macros

FP_INFINITE FP_NAN FP_NORMAL FP_SUBNORMAL FP_ZERO

are for number classification. They represent the mutually exclusive kinds of floating-point values. They expand to integer constant expressions with distinct values.

The macro

FP_FAST_FMA

is optionally defined. If defined, it indicates that the fma function generally executes about as fast as, or faster than, a multiply and an add of double operands.181) The macros

FP_FAST_FMAF FP_FAST_FMAL

are, respectively, float and long double analogs of FP_FAST_FMA.

The macros

FP_ILOGB0 FP_ILOGBNAN

expand to integer constant expressions whose values are returned by ilogb(x) if x is zero or NaN, respectively. The value of FP_ILOGB0 shall be either INT_MIN or -INT_MAX. The value of FP_ILOGBNAN shall be either INT_MAX or INT_MIN.

Recommended practice

Conversion from (at least) double to decimal with DECIMAL_DIG digits and back should be the identity function (which assures that conversion from the widest supported IEC 60559 format to decimal with DECIMAL_DIG digits and back is the identity function).

7.12.1 Treatment of error conditions

The behavior of each of the functions in <math.h> is specified for all representable values of its input arguments, except where stated otherwise.

For all functions, a domain error occurs if an input argument is outside the domain over which the mathematical function is defined. The description of each function lists any required domain errors; an implementation may define additional domain errors, provided that such errors are consistent with the mathematical definition of the function.182) On a domain error, the function returns an implementation-defined value; whether the integer expression errno acquires the value EDOM is implementation-defined.

Similarly, a range error occurs if the mathematical result of the function cannot be represented in an object of the specified type, due to extreme magnitude. A floating result overflows if the magnitude of the mathematical result is finite but so large that the mathematical result cannot be represented, without extraordinary roundoff error, in an object of the specified type. If a floating result overflows and default rounding is in effect, or if the mathematical result is an exact infinity (for example log(0.0)), then the function returns the value of the macro HUGE_VAL, HUGE_VALF, or HUGE_VALL according to the return type, with the same sign as the correct value of the function; whether errno acquires the value ERANGE when a range error occurs is implementation-defined. The result underflows if the magnitude of the mathematical result is so small that the mathematical result cannot be represented,

without extraordinary roundoff error, in an object of the specified type.183) If the result underflows, the function returns a value whose magnitude is no greater than the smallest normalized positive number in the specified type and is otherwise implementation-defined; whether errno acquires the value ERANGE is implementation-defined.

7.12.2 The FP_CONTRACT pragma

Synopsis

        #include <math.h>
        #pragma STDC FP_CONTRACT on-off-switch

Description

The FP_CONTRACT pragma can be used to allow (if the state is on) or disallow (if the state is off) the implementation to contract expressions (6.5). Each pragma can occur either outside external declarations or preceding all explicit declarations and statements inside a compound statement. When outside external declarations, the pragma takes effect from its occurrence until another FP_CONTRACT pragma is encountered, or until the end of the translation unit. When inside a compound statement, the pragma takes effect from its occurrence until another FP_CONTRACT pragma is encountered (including within a nested compound statement), or until the end of the compound statement; at the end of a compound statement the state for the pragma is restored to its condition just before the compound statement. If this pragma is used in any other context, the behavior is undefined. The default state (on or off) for the pragma is implementation-defined.

7.12.3 Classification macros

In the synopses in this subclause, real-floating indicates that the argument shall be an expression of real floating type.

7.12.3.1 The fpclassify macro

Synopsis

        #include <math.h>
        int fpclassify(real-floating x);

Description

The fpclassify macro classifies its argument value as NaN, infinite, normal, subnormal, or zero. First, an argument represented in a format wider than its semantic type is converted to its semantic type. Then classification is based on the type of the argument.184)

Returns

The fpclassify macro returns the value of the number classification macro appropriate to the value of its argument.

EXAMPLE The fpclassify macro might be implemented in terms of ordinary functions as

        #define fpclassify(x) \
                ((sizeof (x) == sizeof (float)) ? \
                        __fpclassifyf(x) \
                : (sizeof (x) == sizeof (double)) ? \
                        __fpclassifyd(x) \
                :       __fpclassifyl(x))

7.12.3.2 The isfinite macro

Synopsis

        #include <math.h>
        int isfinite(real-floating x);

Description

The isfinite macro determines whether its argument has a finite value (zero, subnormal, or normal, and not infinite or NaN). First, an argument represented in a format wider than its semantic type is converted to its semantic type. Then determination is based on the type of the argument.

Returns

The isfinite macro returns a nonzero value if and only if its argument has a finite value.

7.12.3.3 The isinf macro

Synopsis

        #include <math.h>
        int isinf(real-floating x);

Description

The isinf macro determines whether its argument value is an infinity (positive or negative). First, an argument represented in a format wider than its semantic type is converted to its semantic type. Then determination is based on the type of the argument.

Returns

The isinf macro returns a nonzero value if and only if its argument has an infinite value.

7.12.3.4 The isnan macro

Synopsis

        #include <math.h>
        int isnan(real-floating x);

Description

The isnan macro determines whether its argument value is a NaN. First, an argument represented in a format wider than its semantic type is converted to its semantic type. Then determination is based on the type of the argument.185)

Returns

The isnan macro returns a nonzero value if and only if its argument has a NaN value.

7.12.3.5 The isnormal macro

Synopsis

        #include <math.h>
        int isnormal(real-floating x);

Description

The isnormal macro determines whether its argument value is normal (neither zero, subnormal, infinite, nor NaN). First, an argument represented in a format wider than its semantic type is converted to its semantic type. Then determination is based on the type of the argument.

Returns

The isnormal macro returns a nonzero value if and only if its argument has a normal value.

7.12.3.6 The signbit macro

Synopsis

        #include <math.h>
        int signbit(real-floating x);

Description

The signbit macro determines whether the sign of its argument value is negative.186)

Returns

The signbit macro returns a nonzero value if and only if the sign of its argument value is negative.

7.12.4 Trigonometric functions

7.12.4.1 The acos functions

Synopsis

        #include <math.h>
        double acos(double x);
        float acosf(float x);
        long double acosl(long double x);

Description

The acos functions compute the principal value of the arc cosine of x. A domain error occurs for arguments not in the range [-1, +1].

Returns

The acos functions return the arc cosine in the range [0, pi] radians.

7.12.4.2 The asin functions

Synopsis

        #include <math.h>
        double asin(double x);
        float asinf(float x);
        long double asinl(long double x);

Description

The asin functions compute the principal value of the arc sine of x. A domain error occurs for arguments not in the range [-1, +1].

Returns

The asin functions return the arc sine in the range [-pi/2, +pi/2] radians.

7.12.4.3 The atan functions

Synopsis

        #include <math.h>
        double atan(double x);
        float atanf(float x);
        long double atanl(long double x);

Description

The atan functions compute the principal value of the arc tangent of x.

Returns

The atan functions return the arc tangent in the range [-pi/2, +pi/2] radians.

7.12.4.4 The atan2 functions

Synopsis

        #include <math.h>
        double atan2(double y, double x);
        float atan2f(float y, float x);
        long double atan2l(long double y, long double x);

Description

The atan2 functions compute the principal value of the arc tangent of y/x, using the signs of both arguments to determine the quadrant of the return value. A domain error may occur if both arguments are zero.

Returns

The atan2 functions return the arc tangent of y/x, in the range [-pi, +pi] radians.

7.12.4.5 The cos functions

Synopsis

        #include <math.h>
        double cos(double x);
        float cosf(float x);
        long double cosl(long double x);

Description

The cos functions compute the cosine of x (measured in radians).

Returns

The cos functions return the cosine value.

7.12.4.6 The sin functions

Synopsis

        #include <math.h>
        double sin(double x);
        float sinf(float x);
        long double sinl(long double x);

Description

The sin functions compute the sine of x (measured in radians).

Returns

The sin functions return the sine value.

7.12.4.7 The tan functions

Synopsis

        #include <math.h>
        double tan(double x);
        float tanf(float x);
        long double tanl(long double x);

Description

The tan functions return the tangent of x (measured in radians).

Returns

The tan functions return the tangent value.

7.12.5 Hyperbolic functions

7.12.5.1 The acosh functions

Synopsis

        #include <math.h>
        double acosh(double x);
        float acoshf(float x);
        long double acoshl(long double x);

Description

The acosh functions compute the (nonnegative) arc hyperbolic cosine of x. A domain error occurs for arguments less than 1.

Returns

The acosh functions return the arc hyperbolic cosine in the range [0, +].

7.12.5.2 The asinh functions

Synopsis

        #include <math.h>
        double asinh(double x);
        float asinhf(float x);
        long double asinhl(long double x);

Description

The asinh functions compute the arc hyperbolic sine of x.

Returns

The asinh functions return the arc hyperbolic sine value.

7.12.5.3 The atanh functions

Synopsis

        #include <math.h>
        double atanh(double x);
        float atanhf(float x);
        long double atanhl(long double x);

Description

The atanh functions compute the arc hyperbolic tangent of x. A domain error occurs for arguments not in the range [-1, +1]. A range error may occur if the argument equals -1 or +1.

Returns

The atanh functions return the arc hyperbolic tangent value.

7.12.5.4 The cosh functions

Synopsis

        #include <math.h>
        double cosh(double x);
        float coshf(float x);
        long double coshl(long double x);

Description

The cosh functions compute the hyperbolic cosine of x. A range error occurs if the magnitude of x is too large.

Returns

The cosh functions return the hyperbolic cosine value.

7.12.5.5 The sinh functions

Synopsis

        #include <math.h>
        double sinh(double x);
        float sinhf(float x);
        long double sinhl(long double x);

Description

The sinh functions compute the hyperbolic sine of x. A range error occurs if the magnitude of x is too large.

Returns

The sinh functions return the hyperbolic sine value.

7.12.5.6 The tanh functions

Synopsis

        #include <math.h>
        double tanh(double x);
        float tanhf(float x);
        long double tanhl(long double x);

Description

The tanh functions compute the hyperbolic tangent of x.

Returns

The tanh functions return the hyperbolic tangent value.

7.12.6 Exponential and logarithmic functions

7.12.6.1 The exp functions

Synopsis

        #include <math.h>
        double exp(double x);
        float expf(float x);
        long double expl(long double x);

Description

The exp functions compute the base-e exponential of x: ex. A range error occurs if the magnitude of x is too large.

Returns

The exp functions return the exponential value.

7.12.6.2 The exp2 functions

Synopsis

        #include <math.h>
        double exp2(double x);
        float exp2f(float x);
        long double exp2l(long double x);

Description

The exp2 functions compute the base-2 exponential of x: 2x. A range error occurs if the magnitude of x is too large.

Returns

The exp2 functions return the base-2 exponential value.

7.12.6.3 The expm1 functions

Synopsis

        #include <math.h>
        double expm1(double x);
        float expm1f(float x);
        long double expm1l(long double x);

Description

The expm1 functions compute the base-e exponential of the argument, minus 1: ex-1. A range error occurs if x is too large.187)

Returns

The expm1 functions return the value of ex-1.

7.12.6.4 The frexp functions

Synopsis

        #include <math.h>
        double frexp(double value, int *exp);
        float frexpf(float value, int *exp);
        long double frexpl(long double value, int *exp);

Description

The frexp functions break a floating-point number into a normalized fraction and an integral power of 2. They store the integer in the int object pointed to by exp.

Returns

The frexp functions return the value x, such that x has a magnitude in the interval [1/2, 1) or zero, and value equals x|2*exp. If value is zero, both parts of the result are zero.

7.12.6.5 The ilogb functions

Synopsis

        #include <math.h>
        int ilogb(double x);
        int ilogbf(float x);
        int ilogbl(long double x);

Description

The ilogb functions extract the exponent of x as a signed int value. If x is zero they compute the value FP_ILOGB0; if x is infinite they compute the value INT_MAX; if x is a NaN they compute the value FP_ILOGBNAN; otherwise, they are equivalent to calling the corresponding logb function and casting the returned value to type int. A range error may occur if x is 0.

Returns

The ilogb functions return the exponent of x as a signed int value.

Forward references: the logb functions (7.12.6.11).

7.12.6.6 The ldexp functions

Synopsis

        #include <math.h>
        double ldexp(double x, int exp);
        float ldexpf(float x, int exp);
        long double ldexpl(long double x, int exp);

Description

The ldexp functions multiply a floating-point number by an integral power of 2. A range error may occur.

Returns

The ldexp functions return the value of x|2exp.

7.12.6.7 The log functions

Synopsis

        #include <math.h>
        double log(double x);
        float logf(float x);
        long double logl(long double x);

Description

The log functions compute the base-e (natural) logarithm of x. A domain error occurs if the argument is negative. A range error may occur if the argument is zero.

Returns

The log functions return the base-e logarithm value.

7.12.6.8 The log10 functions

Synopsis

        #include <math.h>
        double log10(double x);
        float log10f(float x);
        long double log10l(long double x);

Description

The log10 functions compute the base-10 (common) logarithm of x. A domain error occurs if the argument is negative. A range error may occur if the argument is zero.

Returns

The log10 functions return the base-10 logarithm value.

7.12.6.9 The log1p functions

Synopsis

        #include <math.h>
        double log1p(double x);
        float log1pf(float x);
        long double log1pl(long double x);

Description

The log1p functions compute the base-e (natural) logarithm of 1 plus the argument.188) A domain error occurs if the argument is less than -1. A range error may occur if the argument equals -1.

Returns

The log1p functions return the value of the base-e logarithm of 1 plus the argument.

7.12.6.10 The log2 functions

Synopsis

        #include <math.h>
        double log2(double x);
        float log2f(float x);
        long double log2l(long double x);

Description

The log2 functions compute the base-2 logarithm of x. A domain error occurs if the argument is less than zero. A range error may occur if the argument is zero.

Returns

The log2 functions return the base-2 logarithm value.

7.12.6.11 The logb functions

Synopsis

        #include <math.h>
        double logb(double x);
        float logbf(float x);
        long double logbl(long double x);

Description

The logb functions extract the exponent of x, as a signed integer value in floating-point format. If x is subnormal it is treated as though it were normalized; thus, for positive finite x,

1<=x|FLT_RADIX-logb(x)<FLT_RADIX

A domain error may occur if the argument is zero.

Returns

The logb functions return the signed exponent of x.

7.12.6.12 The modf functions

Synopsis

        #include <math.h>
        double modf(double value, double *iptr);
        float modff(float value, float *iptr);
        long double modfl(long double value, long double *iptr);

Description

The modf functions break the argument value into integral and fractional parts, each of which has the same type and sign as the argument. They store the integral part (in floating-point format) in the object pointed to by iptr.

Returns

The modf functions return the value of the signed fractional part of value.

7.12.6.13 The scalbn and scalbln functions

Synopsis

        #include <math.h>
        double scalbn(double x, int n);
        float scalbnf(float x, int n);
        long double scalbnl(long double x, int n);
        double scalbln(double x, long int n);
        float scalblnf(float x, long int n);
        long double scalblnl(long double x, long int n);

Description

The scalbn and scalbln functions compute x|FLT_RADIXn efficiently, not normally by computing FLT_RADIXn explicitly. A range error may occur.

Returns

The scalbn and scalbln functions return the value of x|FLT_RADIXn.

7.12.7 Power and absolute-value functions

7.12.7.1 The cbrt functions

Synopsis

        #include <math.h>
        double cbrt(double x);
        float cbrtf(float x);
        long double cbrtl(long double x);

Description

The cbrt functions compute the real cube root of x.

Returns

The cbrt functions return the value of the cube root.

7.12.7.2 The fabs functions

Synopsis

        #include <math.h>
        double fabs(double x);
        float fabsf(float x);
        long double fabsl(long double x);

Description

The fabs functions compute the absolute value of a floating-point number x.

Returns

The fabs functions return the absolute value of x.

7.12.7.3 The hypot functions

Synopsis

        #include <math.h>
        double hypot(double x, double y);
        float hypotf(float x, float y);
        long double hypotl(long double x, long double y);

Description

The hypot functions compute the square root of the sum of the squares of x and y, without undue overflow or underflow. A range error may occur.

Returns

The hypot functions return the value of the square root of the sum of the squares.

7.12.7.4 The pow functions

Synopsis

        #include <math.h>
        double pow(double x, double y);
        float powf(float x, float y);
        long double powl(long double x, long double y);

Description

The pow functions compute x raised to the power y. A domain error occurs if x is negative and y is finite and not an integer value. A domain error occurs if the result cannot be represented when x is zero and y is less than or equal to zero. A range error may occur.

Returns

The pow functions return the value of x raised to the power y.

7.12.7.5 The sqrt functions

Synopsis

        #include <math.h>
        double sqrt(double x);
        float sqrtf(float x);
        long double sqrtl(long double x);

Description

The sqrt functions compute the nonnegative square root of x. A domain error occurs if the argument is less than zero.

Returns

The sqrt functions return the value of the square root.

7.12.8 Error and gamma functions

7.12.8.1 The erf functions

Synopsis

        #include <math.h>
        double erf(double x);
        float erff(float x);
        long double erfl(long double x);

Description

The erf functions compute the error function of x: _0e-t2dt.

Returns

The erf functions return the error function value.

7.12.8.2 The erfc functions

Synopsis

        #include <math.h>
        double erfc(double x);
        float erfcf(float x);
        long double erfcl(long double x);

Description

The erfc functions compute the complementary error function of x: _xe-t2dt. A range error occurs if x is too large.

Returns

The erfc functions return the complementary error function value.

7.12.8.3 The lgamma functions

Synopsis

        #include <math.h>
        double lgamma(double x);
        float lgammaf(float x);
        long double lgammal(long double x);

Description

The lgamma functions compute the natural logarithm of the absolute value of gamma of x: loge|(x)|. A range error occurs if x is too large or if x is a negative integer or zero.

Returns

The lgamma functions return the value of the natural logarithm of the absolute value of gamma of x.

7.12.8.4 The tgamma functions

Synopsis

        #include <math.h>
        double tgamma(double x);
        float tgammaf(float x);
        long double tgammal(long double x);

Description

The tgamma functions compute the gamma function of x: (x). A domain error occurs if x is a negative integer or zero. A range error may occur if the magnitude of x is too large or too small.

Returns

The tgamma functions return the gamma function value.

7.12.9 Nearest integer functions

7.12.9.1 The ceil functions

Synopsis

        #include <math.h>
        double ceil(double x);
        float ceilf(float x);
        long double ceill(long double x);

Description

The ceil functions compute the smallest integer value not less than x: |x|.

Returns

The ceil functions return the smallest integer value not less than x, expressed as a floating-point number.

7.12.9.2 The floor functions

Synopsis

        #include <math.h>
        double floor(double x);
        float floorf(float x);
        long double floorl(long double x);

Description

The floor functions compute the largest integer value not greater than x: |x|.

Returns

The floor functions return the largest integer value not greater than x, expressed as a floating-point number.

7.12.9.3 The nearbyint functions

Synopsis

        #include <math.h>
        double nearbyint(double x);
        float nearbyintf(float x);
        long double nearbyintl(long double x);

Description

The nearbyint functions round their argument to an integer value in floating-point format, using the current rounding direction and without raising the inexact exception.

Returns

The nearbyint functions return the rounded integer value.

7.12.9.4 The rint functions

Synopsis

        #include <math.h>
        double rint(double x);
        float rintf(float x);
        long double rintl(long double x);

Description

The rint functions differ from the nearbyint functions (7.12.9.3) only in that the rint functions may raise the inexact exception if the result differs in value from the argument (see F.9.6.3 and F.9.6.4).

Returns

The rint functions return the rounded integer value.

7.12.9.5 The lrint and llrint functions

Synopsis

        #include <math.h>
        long int lrint(double x);
        long int lrintf(float x);
        long int lrintl(long double x);
        long long int llrint(double x);
        long long int llrintf(float x);
        long long int llrintl(long double x);

Description

The lrint and llrint functions round their argument to the nearest integer value, rounding according to the current rounding direction. If the rounded value is outside the range of the return type, the numeric result is unspecified. A range error may occur if the magnitude of x is too large.

Returns

The lrint and llrint functions return the rounded integer value.

7.12.9.6 The round functions

Synopsis

        #include <math.h>
        double round(double x);
        float roundf(float x);
        long double roundl(long double x);

Description

The round functions round their argument to the nearest integer value in floating-point format, rounding halfway cases away from zero, regardless of the current rounding direction.

Returns

The round functions return the rounded integer value.

7.12.9.7 The lround and llround functions

Synopsis

        #include <math.h>
        long int lround(double x);
        long int lroundf(float x);
        long int lroundl(long double x);
        long long int llround(double x);
        long long int llroundf(float x);
        long long int llroundl(long double x);

Description

The lround and llround functions round their argument to the nearest integer value, rounding halfway cases away from zero, regardless of the current rounding direction. If the rounded value is outside the range of the return type, the numeric result is unspecified. A range error may occur if the magnitude of x is too large.

Returns

The lround and llround functions return the rounded integer value.

7.12.9.8 The trunc functions

Synopsis

        #include <math.h>
        double trunc(double x);
        float truncf(float x);
        long double truncl(long double x);

Description

The trunc functions round their argument to the integer value, in floating format, nearest to but no larger in magnitude than the argument.

Returns

The trunc functions return the truncated integer value.

7.12.10 Remainder functions

7.12.10.1 The fmod functions

Synopsis

        #include <math.h>
        double fmod(double x, double y);
        float fmodf(float x, float y);
        long double fmodl(long double x, long double y);

Description

The fmod functions compute the floating-point remainder of x/y.

Returns

The fmod functions return the value x-ny, for some integer n such that, if y is nonzero, the result has the same sign as x and magnitude less than the magnitude of y. If y is zero, whether a domain error occurs or the fmod functions return zero is implementation-defined.

7.12.10.2 The remainder functions

Synopsis

        #include <math.h>
        double remainder(double x, double y);
        float remainderf(float x, float y);
        long double remainderl(long double x, long double y);

Description

The remainder functions compute the remainder x REM y required by IEC 60559.189)

Returns

The remainder functions return the value of x REM y.

7.12.10.3 The remquo functions

Synopsis

        #include <math.h>
        double remquo(double x, double y, int *quo);
        float remquof(float x, float y, int *quo);
        long double remquol(long double x, long double y,
                int *quo);

Description

The remquo functions compute the same remainder as the remainder functions. In the object pointed to by quo they store a value whose sign is the sign of x/y and whose magnitude is congruent modulo 2n to the magnitude of the integral quotient of x/y, where n is an implementation- defined integer greater than or equal to 3.

Returns

The remquo functions return the value of x REM y.

7.12.11 Manipulation functions

7.12.11.1 The copysign functions

Synopsis

        #include <math.h>
        double copysign(double x, double y);
        float copysignf(float x, float y);
        long double copysignl(long double x, long double y);

Description

The copysign functions produce a value with the magnitude of x and the sign of y. They produce a NaN (with the sign of y) if x is a NaN. On implementations that represent a signed zero but do not treat negative zero consistently in arithmetic operations, the copysign functions regard the sign of zero as positive.

Returns

The copysign functions return a value with the magnitude of x and the sign of y.

7.12.11.2 The nan functions

Synopsis

        #include <math.h>
        double nan(const char *tagp);
        float nanf(const char *tagp);
        long double nanl(const char *tagp);

Description

The call nan("n-char-sequence") is equivalent to strtod("NAN(n-char-sequence)", (char**) NULL); the call nan("") is equivalent to strtod("NAN()", (char**) NULL). If tagp does not point to an n-char sequence or an empty string, the call is equivalent to strtod("NAN", (char**) NULL). Calls to nanf and nanl are equivalent to the corresponding calls to strtof and strtold.

Returns

The nan functions return a quiet NaN, if available, with content indicated through tagp. If the implementation does not support quiet NaNs, the functions return zero.

Forward references: the strtod, strtof, and strtold functions (7.20.1.3).

7.12.11.3 The nextafter functions

Synopsis

        #include <math.h>
        double nextafter(double x, double y);
        float nextafterf(float x, float y);
        long double nextafterl(long double x, long double y);

Description

The nextafter functions determine the next representable value, in the type of the function, after x in the direction of y, where x and y are first converted to the type of the function.190) The nextafter functions return y if x equals y. A range error may occur if the magnitude of x is the largest finite value representable in the type and the result is infinite or not representable in the type.

Returns

The nextafter functions return the next representable value in the specified format after x in the direction of y.

7.12.11.4 The nexttoward functions

Synopsis

        #include <math.h>
        double nexttoward(double x, long double y);          
        float nexttowardf(float x, long double y);           
        long double nexttowardl(long double x, long double y);

Description

The nexttoward functions are equivalent to the nextafter functions except that the second parameter has type long double.191)

7.12.12 Maximum, minimum, and positive difference functions

7.12.12.1 The fdim functions

Synopsis

        #include <math.h>
        double fdim(double x, double y);
        float fdimf(float x, float y);
        long double fdiml(long double x, long double y);

Description

The fdim functions determine the positive difference between their arguments:

x-yif x>y

+0 if x<=y

A range error may occur.

Returns

The fdim functions return the positive difference value.

7.12.12.2 The fmax functions

Synopsis

        #include <math.h>
        double fmax(double x, double y);
        float fmaxf(float x, float y);
        long double fmaxl(long double x, long double y);

Description

The fmax functions determine the maximum numeric value of their arguments.192)

Returns

The fmax functions return the maximum numeric value of their arguments.

7.12.12.3 The fmin functions

Synopsis

        #include <math.h>
        double fmin(double x, double y);
        float fminf(float x, float y);
        long double fminl(long double x, long double y);

Description

The fmin functions determine the minimum numeric value of their arguments.193)

Returns

The fmin functions return the minimum numeric value of their arguments.

7.12.13 Floating multiply-add

7.12.13.1 The fma functions

Synopsis

        #include <math.h>
        double fma(double x, double y, double z);
        float fmaf(float x, float y, float z);
        long double fmal(long double x, long double y,
                long double z);

The header <setjmp.h> defines the macro setjmp, and declares one function and one type, for bypassing the normal function call and return discipline.195)

The type declared is

jmp_buf

which is an array type suitable for holding the information needed to restore a calling environment.

It is unspecified whether setjmp is a macro or an identifier declared with external linkage. If a macro definition is suppressed in order to access an actual function, or a program defines an external identifier with the name setjmp, the behavior is undefined.

7.13.1 Save calling environment

7.13.1.1 The setjmp macro

Synopsis

        #include <setjmp.h>
        int setjmp(jmp_buf env);

Description

The setjmp macro saves its calling environment in its jmp_buf argument for later use by the longjmp function.

Returns

If the return is from a direct invocation, the setjmp macro returns the value zero. If the return is from a call to the longjmp function, the setjmp macro returns a nonzero value.

Environmental limits

An invocation of the setjmp macro shall appear only in one of the following contexts:

-- the entire controlling expression of a selection or iteration statement;

-- one operand of a relational or equality operator with the other operand an integer constant expression, with the resulting expression being the entire controlling expression of a selection or iteration statement;

-- the operand of a unary ! operator with the resulting expression being the entire controlling expression of a selection or iteration statement; or

-- the entire expression of an expression statement (possibly cast to void).

If the invocation appears in any other context, the behavior is undefined.

7.13.2 Restore calling environment

7.13.2.1 The longjmp function

Synopsis

        #include <setjmp.h>
        void longjmp(jmp_buf env, int val);

Description

The longjmp function restores the environment saved by the most recent invocation of the setjmp macro in the same invocation of the program with the corresponding jmp_buf argument. If there has been no such invocation, or if the function containing the invocation of the setjmp macro has terminated execution196) in the interim, or if the invocation of the setjmp macro was within the scope of an identifier with variably modified type and execution has left that scope in the interim, the behavior is undefined.

All accessible objects have values as of the time longjmp was called, except that the values of objects of automatic storage duration that are local to the function containing the invocation of the corresponding setjmp macro that do not have volatile-qualified type and have been changed between the setjmp invocation and longjmp call are indeterminate.

Returns

After longjmp is completed, program execution continues as if the corresponding invocation of the setjmp macro had just returned the value specified by val. The longjmp function cannot cause the setjmp macro to return the value 0; if val is 0, the setjmp macro returns the value 1.

EXAMPLE The longjmp function that returns control back to the point of the setjmp invocation might cause memory associated with a variable length array object to be squandered.

        #include <setjmp.h>
        jmp_buf buf;
        void g(int n);
        void h(int n);
        int n = 6;

        void f(void)
        {
                int x[n];        // OK, f is not terminated.
                setjmp(buf);
                g(n);
        }

        void g(int n)
        {
                int a[n];        // a may remain allocated.
                h(n);
        }

        void h(int n)
        {
                int b[n];        // b may remain allocated.
                longjmp(buf,2);  // might cause memory loss.
        }

7.14 Signal handling <signal.h>

The header <signal.h> declares a type and two functions and defines several macros, for handling various signals (conditions that may be reported during program execution).

The type defined is

sig_atomic_t

which is the (possibly volatile-qualified) integer type of an object that can be accessed as an atomic entity, even in the presence of asynchronous interrupts.

The macros defined are

SIG_DFL SIG_ERR SIG_IGN

which expand to constant expressions with distinct values that have type compatible with the second argument to, and the return value of, the signal function, and whose values compare unequal to the address of any declarable function; and the following, which expand to positive integer constant expressions with type int and distinct values that are the signal numbers, each corresponding to the specified condition:

        SIGABRT abnormal termination, such as  is  initiated
                by the abort function

        SIGFPE  an  erroneous  arithmetic operation, such as
                zero divide or  an  operation  resulting  in
                overflow

        SIGILL  detection of an invalid function image, such
                as an invalid instruction

        SIGINT  receipt of an interactive attention signal

        SIGSEGV an invalid access to storage

        SIGTERM a termination request sent to the program

An implementation need not generate any of these signals, except as a result of explicit calls to the raise function. Additional signals and pointers to undeclarable functions, with macro definitions beginning, respectively, with the letters SIG and an uppercase letter or with SIG_ and an uppercase letter,197) may also be specified by the implementation. The complete set of signals, their semantics, and their default handling is implementation- defined; all signal numbers shall be positive.

7.14.1 Specify signal handling

7.14.1.1 The signal function

Synopsis

        #include <signal.h>
        void (*signal(int sig, void (*func)(int)))(int);

Description

The signal function chooses one of three ways in which receipt of the signal number sig is to be subsequently handled. If the value of func is SIG_DFL, default handling for that signal will occur. If the value of func is SIG_IGN, the signal will be ignored. Otherwise, func shall point to a function to be called when that signal occurs. An invocation of such a function because of a signal, or (recursively) of any further functions called by that invocation (other than functions in the standard library), is called a signal handler.

When a signal occurs and func points to a function, it is implementation-defined whether the equivalent of signal(sig, SIG_DFL); is executed or the implementation prevents some implementation-defined set of signals (at least including sig) from occurring until the current signal handling has completed; in the case of SIGILL, the implementation may alternatively define that no action is taken. Then the equivalent of (*func)(sig); is executed. If and when the function returns, if the value of sig is SIGFPE, SIGILL, SIGSEGV, or any other implementation-defined value corresponding to a computational exception, the behavior is undefined; otherwise the program will resume execution at the point it was interrupted.

If the signal occurs as the result of calling the abort or raise function, the signal handler shall not call the raise function.

If the signal occurs other than as the result of calling the abort or raise function, the behavior is undefined if the signal handler refers to any object with static storage duration other than by assigning a value to an object declared as volatile sig_atomic_t, or the signal

handler calls any function in the standard library other than the abort function or the signal function with the first argument equal to the signal number corresponding to the signal that caused the invocation of the handler. Furthermore, if such a call to the signal function results in a SIG_ERR return, the value of errno is indeterminate.198)

At program startup, the equivalent of

        signal(sig, SIG_IGN);

may be executed for some signals selected in an implementation-defined manner; the equivalent of

        signal(sig, SIG_DFL);

is executed for all other signals defined by the implementation.

The implementation shall behave as if no library function calls the signal function.

Returns

If the request can be honored, the signal function returns the value of func for the most recent successful call to signal for the specified signal sig. Otherwise, a value of SIG_ERR is returned and a positive value is stored in errno.

Forward references: the abort function (7.20.4.1), the exit function (7.20.4.3).

7.14.2 Send signal

7.14.2.1 The raise function

Synopsis

        #include <signal.h>
        int raise(int sig);

Description

The raise function carries out the actions described in 7.14.1.1 for the signal sig. If a signal handler is called, the raise function shall not return until after the signal handler does.

Returns

The raise function returns zero if successful, nonzero if unsuccessful.

7.15 Variable arguments <stdarg.h>

The header <stdarg.h> declares a type and defines four macros, for advancing through a list of arguments whose number and types are not known to the called function when it is translated.

A function may be called with a variable number of arguments of varying types. As described in 6.9.1, its parameter list contains one or more parameters. The rightmost parameter plays a special role in the access mechanism, and will be designated parmN in this description.

The type declared is

va_list

which is an object type suitable for holding information needed by the macros va_start, va_arg, va_end, and va_copy. If access to the varying arguments is desired, the called function shall declare an object (referred to as ap in this subclause) having type va_list. The object ap may be passed as an argument to another function; if that function invokes the va_arg macro with parameter ap, the value of ap in the calling function is indeterminate and shall be passed to the va_end macro prior to any further reference to ap.199)

7.15.1 Variable argument list access macros

The va_start, va_arg, and va_copy macros described in this subclause shall be implemented as macros, not functions. It is unspecified whether va_end is a macro or an identifier declared with external linkage. If a macro definition is suppressed in order to access an actual function, or a program defines an external identifier with the name va_end, the behavior is undefined. Each invocation of the va_start or va_copy macros shall be matched by a corresponding invocation of the va_end macro in the function accepting a varying number of arguments.

7.15.1.1 The va_arg macro

Synopsis

        #include <stdarg.h>
        type va_arg(va_list ap, type);

Description

The va_arg macro expands to an expression that has the specified type and the value of the next argument in the call. The parameter ap shall be the same as the va_list ap initialized by va_start. Each invocation of va_arg modifies ap so that the values of successive arguments are returned in turn. The parameter type shall be a type name specified such that the type of a pointer to an object that has the specified type can be obtained simply by postfixing a * to type. If there is no actual next argument, or if type is not compatible with the type of the actual next argument (as promoted according to the default argument promotions), the behavior is undefined, except for the following cases:

-- one type is a signed integer type, the other type is the corresponding unsigned integer type, and the value is representable in both types;

-- one type is pointer to void and the other is a pointer to a character type.

Returns

The first invocation of the va_arg macro after that of the va_start macro returns the value of the argument after that specified by parmN. Successive invocations return the values of the remaining arguments in succession.

7.15.1.2 The va_copy macro

Synopsis

        #include <stdarg.h>
        void va_copy(va_list dest, va_list src);

Description

The va_copy macro makes the va_list dest be a copy of the va_list src, as if the va_start macro had been applied to it followed by the same sequence of uses of the va_arg macro as had previously been used to reach the present state of src.

Returns

The va_copy macro returns no value.

7.15.1.3 The va_end macro

Synopsis

        #include <stdarg.h>
        void va_end(va_list ap);

Description

The va_end macro facilitates a normal return from the function whose variable argument list was referred to by the expansion of va_start that initialized the va_list ap. The va_end macro may modify ap so that it is no longer usable (without an intervening invocation of va_start). If there is no corresponding invocation of the va_start macro, or if the va_end macro is not invoked before the return, the behavior is undefined.

Returns

The va_end macro returns no value.

7.15.1.4 The va_start macro

Synopsis

        #include <stdarg.h>
        void va_start(va_list ap, parmN);

Description

The va_start macro shall be invoked before any access to the unnamed arguments.

The va_start macro initializes ap for subsequent use by va_arg and va_end. va_start shall not be invoked again for the same ap without an intervening invocation of va_end for the same ap.

The parameter parmN is the identifier of the rightmost parameter in the variable parameter list in the function definition (the one just before the , ...). If the parameter parmN is declared with the register storage class, with a function or array type, or with a type that is not compatible with the type that results after application of the default argument promotions, the behavior is undefined.

Returns

The va_start macro returns no value.

EXAMPLE The function f1 gathers into an array a list of arguments that are pointers to strings (but not more than MAXARGS arguments), then passes the array as a single argument to function f2. The number of pointers is specified by the first argument to f1.

        #include <stdarg.h>
        #define MAXARGS 31

        void f1(int n_ptrs, ...)
        {
                va_list ap;
                char *array[MAXARGS];
                int ptr_no = 0;

                if (n_ptrs > MAXARGS)
                        n_ptrs = MAXARGS;
                va_start(ap, n_ptrs);
                while (ptr_no < n_ptrs)
                        array[ptr_no++] = va_arg(ap, char *);
                va_end(ap);
                f2(n_ptrs, array);
        }

Each call to f1 shall have visible the definition of the function or a declaration such as

        void f1(int, ...);

The function f3 is similar, but saves the status of the variable argument list after the indicated number of arguments; after f2 has been called once with the whole list, the trailing part of the list is gathered again and passed to function f4. #include <stdarg.h> #define MAXARGS 31

        void f3(int n_ptrs, int f4_after, ...)
        {
                va_list ap, ap_save;
                char *array[MAXARGS];
                int ptr_no = 0;
                if (n_ptrs > MAXARGS)
                        n_ptrs = MAXARGS;
                va_start(ap, n_ptrs);
                while (ptr_no < n_ptrs) {
                        array[ptr_no++] = va_arg(ap, char *);
                        if (ptr_no == f4_after)
                                va_copy(ap_save, ap);
                }
                va_end(ap);
                f2(n_ptrs, array);

                // Now process the saved copy.

                n_ptrs -= f4_after;
                ptr_no = 0;
                while (ptr_no < n_ptrs)
                        array[ptr_no++] = va_arg(ap_save, char *);
                va_end(ap_save);
                f4(n_ptrs, array);
        }

7.16 Boolean type and values <stdbool.h>

The header <stdbool.h> defines four macros.

The macro

bool

expands to _Bool.

The remaining three macros are suitable for use in #if preprocessing directives. They are

true

which expands to the integer constant 1,

false

which expands to the integer constant 0, and

__bool_true_false_are_defined

which expands to the decimal constant 1.

Notwithstanding the provisions of 7.1.3, a program is permitted to undefine and perhaps then redefine the macros bool, true, and false.200)

7.17 Common definitions <stddef.h>

The following types and macros are defined in the standard header <stddef.h>. Some are also defined in other headers, as noted in their respective subclauses.

The types are

ptrdiff_t

which is the signed integer type of the result of subtracting two pointers;

size_t

which is the unsigned integer type of the result of the sizeof operator; and

wchar_t

which is an integer type whose range of values can represent distinct codes for all members of the largest extended character set specified among the supported locales; the null character shall have the code value zero and each member of the basic character set defined in 5.2.1 shall have a code value equal to its value when used as the lone character in an integer character constant.

The macros are

NULL

which expands to an implementation-defined null pointer constant; and

        offsetof(type, member-designator)

which expands to an integer constant expression that has type size_t, the value of which is the offset in bytes, to the structure member (designated by member-designator), from the beginning of its structure (designated by type). The type and member designator shall be such that given

        static type t;

then the expression &(t.member-designator) evaluates to an address constant. (If the specified member is a bit-field, the behavior is undefined.)

Forward references: localization (7.11).

7.18 Integer types <stdint.h>

The header <stdint.h> declares sets of integer types having specified widths, and defines corresponding sets of macros.201) It also defines macros that specify limits of integer types corresponding to types defined in other standard headers.

Types are defined in the following categories:

-- integer types having certain exact widths;

-- integer types having at least certain specified widths;

-- fastest integer types having at least certain specified widths;

-- integer types wide enough to hold pointers to objects;

-- integer types having greatest width.

(Some of these types may denote the same type.)

Corresponding macros specify limits of the declared types and construct suitable constants.

For each type described herein that the implementation provides,202) <stdint.h> shall declare that typedef name and define the associated macros. Conversely, for each type described herein that the implementation does not provide, <stdint.h> shall not declare that typedef name nor shall it define the associated macros. An implementation shall provide those types described as ``required'', but need not provide any of the others (described as ``optional'').

7.18.1 Integer types

When typedef names differing only in the absence or presence of the initial u are defined, they shall denote corresponding signed and unsigned types as described in 6.2.5; an implementation shall not provide a type without also providing its corresponding type.

In the following descriptions, the symbol N represents an unsigned decimal integer with no leading zeros (e.g., 8

7.18.1.1 Exact-width integer types

The typedef name intN_t designates a signed integer type with width N. Thus, int8_t denotes a signed integer type with a width of exactly 8 bits.

The typedef name uintN_t designates an unsigned integer type with width N. Thus, uint24_t denotes an unsigned integer type with a width of exactly 24 bits.

These types are optional. However, if an implementation provides integer types with widths of 8, 16, 32, or 64 bits, it shall define the corresponding typedef names.

7.18.1.2 Minimum-width integer types

The typedef name int_leastN_t designates a signed integer type with a width of at least N, such that no signed integer type with lesser size has at least the specified width. Thus, int_least32_t denotes a signed integer type with a width of at least 32 bits.

The typedef name uint_leastN_t designates an unsigned integer type with a width of at least N, such that no unsigned integer type with lesser size has at least the specified width. Thus, uint_least16_t denotes an unsigned integer type with a width of at least 16 bits.

The following types are required:

int_least8_t uint_least8_t * int_least16_t uint_least16_t int_least32_t uint_least32_t int_least64_t uint_least64_t

All other types of this form are optional.

7.18.1.3 Fastest minimum-width integer types

Each of the following types designates an integer type that is usually fastest203) to operate with among all integer types that have at least the specified width.

The typedef name int_fastN_t designates the fastest signed integer type with a width of at least N. The typedef name uint_fastN_t designates the fastest unsigned integer

type with a width of at least N.

The following types are required:

int_fast8_t uint_fast8_t * int_fast16_t uint_fast16_t int_fast32_t uint_fast32_t int_fast64_t uint_fast64_t

All other types of this form are optional.

7.18.1.4 Integer types capable of holding object pointers

The following type designates a signed integer type with the property that any valid pointer to void can be converted to this type, then converted back to pointer to void, and the result will compare equal to the original pointer:

intptr_t

The following type designates an unsigned integer type with the property that any valid pointer to void can be converted to this type, then converted back to pointer to void, and the result will compare equal to the original pointer:

uintptr_t

These types are optional.

7.18.1.5 Greatest-width integer types

The following type designates a signed integer type capable of representing any value of any signed integer type:

intmax_t

The following type designates an unsigned integer type capable of representing any value of any unsigned integer type:

uintmax_t

These types are required.

7.18.2 Limits of specified-width integer types

The following object-like macros204) specify the minimum and maximum limits of the types declared in <stdint.h>. Each macro name corresponds to a similar type name in 7.18.1.

Each instance of any defined macro shall be replaced by a constant expression suitable for use in #if preprocessing directives, and this expression shall have the same type as would an expression that is an object of the corresponding type converted according to the integer promotions. Its implementation-defined value shall be equal to or greater in magnitude (absolute value) than the corresponding value given below, with the same sign, except where stated to be exactly the given value.

7.18.2.1 Limits of exact-width integer types

-- minimum values of exact-width signed integer types

INTN_MIN exactly either 1-2N-1 or -2N-1

-- maximum values of exact-width signed integer types

INTN_MAX exactly 2N-1-1

-- maximum values of exact-width unsigned integer types

UINTN_MAX exactly 2N-1

7.18.2.2 Limits of minimum-width integer types

-- minimum values of minimum-width signed integer types

INT_LEASTN_MIN 1-2N-1

-- maximum values of minimum-width signed integer types

INT_LEASTN_MAX 2N-1-1

-- maximum values of minimum-width unsigned integer types

UINT_LEASTN_MAX 2N-1

7.18.2.3 Limits of fastest minimum-width integer types

-- minimum values of fastest minimum-width signed integer types

INT_FASTN_MIN 1-2N-1

-- maximum values of fastest minimum-width signed integer types

INT_FASTN_MAX 2N-1-1

-- maximum values of fastest minimum-width unsigned integer types

UINT_FASTN_MAX 2N-1

7.18.2.4 Limits of integer types capable of holding object

pointers

-- minimum value of pointer-holding signed integer type

INTPTR_MIN 1-215

-- maximum value of pointer-holding signed integer type

INTPTR_MAX 215-1

-- maximum value of pointer-holding unsigned integer type

UINTPTR_MAX 216-1

7.18.2.5 Limits of greatest-width integer types

-- minimum value of greatest-width signed integer type

INTMAX_MIN 1-263

-- maximum value of greatest-width signed integer type

INTMAX_MAX 263-1

-- maximum value of greatest-width unsigned integer type

UINTMAX_MAX 264-1

7.18.3 Limits of other integer types

The following object-like macros205) specify the minimum and maximum limits of integer types corresponding to types defined in other standard headers.

Each instance of these macros shall be replaced by a constant expression suitable for use in #if preprocessing directives, and this expression shall have the same type as would an expression that is an object of the corresponding type converted according to the integer promotions. Its implementation-defined value shall be equal to or greater in magnitude (absolute value) than the corresponding value given below, with the same sign.

-- limits of ptrdiff_t

PTRDIFF_MIN -65535 PTRDIFF_MAX +65535

-- limits of sig_atomic_t

SIG_ATOMIC_MIN see below SIG_ATOMIC_MAX see below

-- limit of size_t

SIZE_MAX 65535

-- limits of wchar_t

WCHAR_MIN see below WCHAR_MAX see below

-- limits of wint_t

WINT_MIN see below WINT_MAX see below

If sig_atomic_t (see 7.14) is defined as a signed integer type, the value of SIG_ATOMIC_MIN shall be no greater than -127 and the value of SIG_ATOMIC_MAX shall be no less than 127; otherwise, sig_atomic_t is defined as an unsigned integer type, and the value of SIG_ATOMIC_MIN shall be 0 and the value of SIG_ATOMIC_MAX shall be no less than 255.

If wchar_t is defined as a signed integer type, the value of WCHAR_MIN shall be no greater than -127 and the

value of WCHAR_MAX shall be no less than 127; otherwise, wchar_t is defined as an unsigned integer type, and the value of WCHAR_MIN shall be 0 and the value of WCHAR_MAX shall be no less than 255.

If wint_t (see 7.25) is defined as a signed integer type, the value of WINT_MIN shall be no greater than -32767 and the value of WINT_MAX shall be no less than 32767; otherwise, wint_t is defined as an unsigned integer type, and the value of WINT_MIN shall be 0 and the value of WINT_MAX shall be no less than 65535.

7.18.4 Macros for integer constants

The following function-like macros206) expand to integer constants suitable for initializing objects that have integer types corresponding to types defined in <stdint.h>. Each macro name corresponds to a similar type name in 7.18.1.2 or 7.18.1.5.

The argument in any instance of these macros shall be a decimal, octal, or hexadecimal constant (as defined in

6.4.4.1 ) with a value that does not exceed the limits for

the corresponding type.

7.18.4.1 Macros for minimum-width integer constants

Each of the following macros expands to an integer constant having the value specified by its argument and a type with at least the specified width.207)

The macro INTN_C(value) shall expand to a signed integer constant with the specified value and type int_leastN_t. The macro UINTN_C(value) shall expand to an unsigned integer constant with the specified value and type uint_leastN_t. For example, if uint_least64_t is a name for the type unsigned long long int, then UINT64_C(0x123) might expand to the integer constant 0x123ULL.

7.18.4.2 Macros for greatest-width integer constants

The following macro expands to an integer constant having the value specified by its argument and the type intmax_t: INTMAX_C(value)

The following macro expands to an integer constant having the value specified by its argument and the type uintmax_t:

        UINTMAX_C(value)

7.19 Input/output <stdio.h>

7.19.1 Introduction

The header <stdio.h> declares three types, several macros, and many functions for performing input and output.

The types declared are size_t (described in 7.17);

FILE

which is an object type capable of recording all the information needed to control a stream, including its file position indicator, a pointer to its associated buffer (if any), an error indicator that records whether a read/write error has occurred, and an end-of-file indicator that records whether the end of the file has been reached; and

fpos_t

which is an object type other than an array type capable of recording all the information needed to specify uniquely every position within a file.

The macros are NULL (described in 7.17);

_IOFBF _IOLBF _IONBF

which expand to integer constant expressions with distinct values, suitable for use as the third argument to the setvbuf function;

BUFSIZ

which expands to an integer constant expression, which is the size of the buffer used by the setbuf function;

EOF

which expands to an integer constant expression, with type int and a negative value, that is returned by several functions to indicate end-of-file, that is, no more input from a stream;

FOPEN_MAX

which expands to an integer constant expression that is the minimum number of files that the implementation guarantees can be open simultaneously;

FILENAME_MAX

which expands to an integer constant expression that is the size needed for an array of char large enough to hold the longest file name string that the implementation guarantees can be opened;208)

L_tmpnam

which expands to an integer constant expression that is the size needed for an array of char large enough to hold a temporary file name string generated by the tmpnam function;

SEEK_CUR SEEK_END SEEK_SET

which expand to integer constant expressions with distinct values, suitable for use as the third argument to the fseek function;

TMP_MAX

which expands to an integer constant expression that is the minimum number of unique file names that can be generated by the tmpnam function;

stderr stdin stdout

which are expressions of type ``pointer to FILE'' that point to the FILE objects associated, respectively, with the standard error, input, and output streams.

The header <wchar.h> declares a number of functions useful for wide-character input and output. The wide- character input/output functions described in that subclause provide operations analogous to most of those described here, except that the fundamental units internal to the program are wide characters. The external representation (in the file) is a sequence of ``generalized'' multibyte characters, as described further in 7.19.3.

The input/output functions are given the following collective terms:

-- The wide-character input functions -- those functions described in 7.24 that perform input into wide characters and wide strings: fgetwc, fgetws, getwc, getwchar, fwscanf, wscanf, vfwscanf, and vwscanf.

-- The wide-character output functions -- those functions described in 7.24 that perform output from wide characters and wide strings: fputwc, fputws, putwc, putwchar, fwprintf, wprintf, vfwprintf, and vwprintf.

-- The wide-character input/output functions -- the union of the ungetwc function, the wide- character input functions, and the wide-character output functions.

-- The byte input/output functions -- those functions described in this subclause that perform input/output: fgetc, fgets, fprintf, fputc, fputs, fread, fscanf, fwrite, getc, getchar, gets, printf, putc, putchar, puts, scanf, ungetc, vfprintf, vfscanf, vprintf, and vscanf.

Forward references: files (7.19.3), the fseek function (7.19.9.2), streams (7.19.2), the tmpnam function (7.19.4.4), <wchar.h> (7.24).

7.19.2 Streams

Input and output, whether to or from physical devices such as terminals and tape drives, or whether to or from files supported on structured storage devices, are mapped into logical data streams, whose properties are more uniform than their various inputs and outputs. Two forms of mapping are supported, for text streams and for binary streams.209)

A text stream is an ordered sequence of characters composed into lines, each line consisting of zero or more characters plus a terminating new-line character. Whether the last line requires a terminating new-line character is implementation-defined. Characters may have to be added, altered, or deleted on input and output to conform to differing conventions for representing text in the host environment. Thus, there need not be a one-to-one correspondence between the characters in a stream and those in the external representation. Data read in from a text stream will necessarily compare equal to the data that were

earlier written out to that stream only if: the data consist only of printing characters and the control characters horizontal tab and new-line; no new-line character is immediately preceded by space characters; and the last character is a new-line character. Whether space characters that are written out immediately before a new-line character appear when read in is implementation-defined.

A binary stream is an ordered sequence of characters that can transparently record internal data. Data read in from a binary stream shall compare equal to the data that were earlier written out to that stream, under the same implementation. Such a stream may, however, have an implementation-defined number of null characters appended to the end of the stream.

Each stream has an orientation. After a stream is associated with an external file, but before any operations are performed on it, the stream is without orientation. Once a wide-character input/output function has been applied to a stream without orientation, the stream becomes a wide- oriented stream. Similarly, once a byte input/output function has been applied to a stream without orientation, the stream becomes a byte-oriented stream. Only a call to the freopen function or the fwide function can otherwise alter the orientation of a stream. (A successful call to freopen removes any orientation.)210)

Byte input/output functions shall not be applied to a wide-oriented stream and wide-character input/output functions shall not be applied to a byte-oriented stream. The remaining stream operations do not affect, and are not affected by, a stream's orientation, except for the following additional restrictions:

-- Binary wide-oriented streams have the file-positioning restrictions ascribed to both text and binary streams.

-- For wide-oriented streams, after a successful call to a file-positioning function that leaves the file position indicator prior to the end-of-file, a wide-character output function can overwrite a partial multibyte character; any file contents beyond the byte(s) written are henceforth indeterminate.

Each wide-oriented stream has an associated mbstate_t object that stores the current parse state of the stream. A successful call to fgetpos stores a representation of the value of this mbstate_t object as part of the value of the fpos_t object. A later successful call to fsetpos using the

same stored fpos_t value restores the value of the associated mbstate_t object as well as the position within the controlled stream.

Environmental limits

An implementation shall support text files with lines containing at least 254 characters, including the terminating new-line character. The value of the macro BUFSIZ shall be at least 256.

Forward references: the freopen function (7.19.5.4), the fwide function (7.24.3.5), mbstate_t (7.25.1), the fgetpos function (7.19.9.1), the fsetpos function (7.19.9.3).

7.19.3 Files

A stream is associated with an external file (which may be a physical device) by opening a file, which may involve creating a new file. Creating an existing file causes its former contents to be discarded, if necessary. If a file can support positioning requests (such as a disk file, as opposed to a terminal), then a file position indicator associated with the stream is positioned at the start (character number zero) of the file, unless the file is opened with append mode in which case it is implementation- defined whether the file position indicator is initially positioned at the beginning or the end of the file. The file position indicator is maintained by subsequent reads, writes, and positioning requests, to facilitate an orderly progression through the file.

Binary files are not truncated, except as defined in 7.19.5.3. Whether a write on a text stream causes the associated file to be truncated beyond that point is implementation-defined.

When a stream is unbuffered, characters are intended to appear from the source or at the destination as soon as possible. Otherwise characters may be accumulated and transmitted to or from the host environment as a block. When a stream is fully buffered, characters are intended to be transmitted to or from the host environment as a block when a buffer is filled. When a stream is line buffered, characters are intended to be transmitted to or from the host environment as a block when a new-line character is encountered. Furthermore, characters are intended to be transmitted as a block to the host environment when a buffer is filled, when input is requested on an unbuffered stream, or when input is requested on a line buffered stream that requires the transmission of characters from the host environment. Support for these characteristics is implementation-defined, and may be affected via the setbuf and setvbuf functions.

A file may be disassociated from a controlling stream by closing the file. Output streams are flushed (any unwritten buffer contents are transmitted to the host environment) before the stream is disassociated from the file. The value of a pointer to a FILE object is indeterminate after the associated file is closed (including the standard text streams). Whether a file of zero length (on which no characters have been written by an output stream) actually exists is implementation-defined.

The file may be subsequently reopened, by the same or another program execution, and its contents reclaimed or modified (if it can be repositioned at its start). If the main function returns to its original caller, or if the exit function is called, all open files are closed (hence all output streams are flushed) before program termination. Other paths to program termination, such as calling the abort function, need not close all files properly.

The address of the FILE object used to control a stream may be significant; a copy of a FILE object need not serve in place of the original.

At program startup, three text streams are predefined and need not be opened explicitly -- standard input (for reading conventional input), standard output (for writing conventional output), and standard error (for writing diagnostic output). As initially opened, the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device.

Functions that open additional (nontemporary) files require a file name, which is a string. The rules for composing valid file names are implementation-defined. Whether the same file can be simultaneously open multiple times is also implementation-defined.

Although both text and binary wide-oriented streams are conceptually sequences of wide characters, the external file associated with a wide-oriented stream is a sequence of multibyte characters, generalized as follows:

-- Multibyte encodings within files may contain embedded null bytes (unlike multibyte encodings valid for use internal to the program).

-- A file need not begin nor end in the initial shift state.211)

#10

Moreover, the encodings used for multibyte characters may differ among files. Both the nature and choice of such encodings are implementation-defined.

#11

The wide-character input functions read multibyte characters from the stream and convert them to wide characters as if they were read by successive calls to the fgetwc function. Each conversion occurs as if by a call to the mbrtowc function, with the conversion state described by the stream's own mbstate_t object. The byte input functions read characters from the stream as if by successive calls to the fgetc function.

#12

The wide-character output functions convert wide characters to multibyte characters and write them to the stream as if they were written by successive calls to the fputwc function. Each conversion occurs as if by a call to the wcrtomb function, with the conversion state described by the stream's own mbstate_t object. The byte output functions write characters to the stream as if by successive calls to the fputc function.

#13

In some cases, some of the byte input/output functions also perform conversions between multibyte characters and wide characters. These conversions also occur as if by calls to the mbrtowc and wcrtomb functions.

#14

An encoding error occurs if the character sequence presented to the underlying mbrtowc function does not form a valid (generalized) multibyte character, or if the code value passed to the underlying wcrtomb does not correspond to a valid (generalized) multibyte character. The wide- character input/output functions and the byte input/output functions store the value of the macro EILSEQ in errno if and only if an encoding error occurs.

Environmental limits

#15

The value of FOPEN_MAX shall be at least eight, including the three standard text streams.

Forward references: the exit function (7.20.4.3), the fgetc function (7.19.7.1), the fopen function (7.19.5.3), the fputc function (7.19.7.3), the setbuf function (7.19.5.5), the setvbuf function (7.19.5.6), the fgetwc function (7.24.3.1), the fputwc function (7.24.3.3), conversion state (7.24.6), the mbrtowc function (7.24.6.3.2), the wcrtomb function (7.24.6.3.3).

7.19.4 Operations on files

7.19.4.1 The remove function

Synopsis

        #include <stdio.h>
        int remove(const char *filename);

Description

The remove function causes the file whose name is the string pointed to by filename to be no longer accessible by that name. A subsequent attempt to open that file using that name will fail, unless it is created anew. If the file is open, the behavior of the remove function is implementation-defined.

Returns

The remove function returns zero if the operation succeeds, nonzero if it fails.

7.19.4.2 The rename function

Synopsis

        #include <stdio.h>
        int rename(const char *old, const char *new);

Description

The rename function causes the file whose name is the string pointed to by old to be henceforth known by the name given by the string pointed to by new. The file named old is no longer accessible by that name. If a file named by the string pointed to by new exists prior to the call to the rename function, the behavior is implementation-defined.

Returns

The rename function returns zero if the operation succeeds, nonzero if it fails,212) in which case if the file existed previously it is still known by its original name.

7.19.4.3 The tmpfile function

Synopsis

        #include <stdio.h>
        FILE *tmpfile(void);

Description

The tmpfile function creates a temporary binary file that will automatically be removed when it is closed or at program termination. If the program terminates abnormally, whether an open temporary file is removed is implementation- defined. The file is opened for update with "wb+" mode.

Returns

The tmpfile function returns a pointer to the stream of the file that it created. If the file cannot be created, the tmpfile function returns a null pointer.

Forward references: the fopen function (7.19.5.3).

7.19.4.4 The tmpnam function

Synopsis

        #include <stdio.h>
        char *tmpnam(char *s);

Description

The tmpnam function generates a string that is a valid file name and that is not the same as the name of an existing file.213)

The tmpnam function generates a different string each time it is called, up to TMP_MAX times. If it is called more than TMP_MAX times, the behavior is implementation- defined.

The implementation shall behave as if no library

function calls the tmpnam function.

Returns

If the argument is a null pointer, the tmpnam function leaves its result in an internal static object and returns a pointer to that object. Subsequent calls to the tmpnam function may modify the same object. If the argument is not a null pointer, it is assumed to point to an array of at least L_tmpnam chars; the tmpnam function writes its result in that array and returns the argument as its value.

Environmental limits

The value of the macro TMP_MAX shall be at least 25.

7.19.5 File access functions

7.19.5.1 The fclose function

Synopsis

        #include <stdio.h>
        int fclose(FILE *stream);

Description

The fclose function causes the stream pointed to by stream to be flushed and the associated file to be closed. Any unwritten buffered data for the stream are delivered to the host environment to be written to the file; any unread buffered data are discarded. The stream is disassociated from the file. If the associated buffer was automatically allocated, it is deallocated.

Returns

The fclose function returns zero if the stream was successfully closed, or EOF if any errors were detected.

7.19.5.2 The fflush function

Synopsis

        #include <stdio.h>
        int fflush(FILE *stream);

Description

If stream points to an output stream or an update stream in which the most recent operation was not input, the fflush function causes any unwritten data for that stream to be delivered to the host environment to be written to the file; otherwise, the behavior is undefined.

If stream is a null pointer, the fflush function performs this flushing action on all streams for which the behavior is defined above.

Returns

The fflush function sets the error indicator for the stream and returns EOF if a write error occurs, otherwise it returns zero.

Forward references: the fopen function (7.19.5.3).

7.19.5.3 The fopen function

Synopsis

        #include <stdio.h>
        FILE *fopen(const char * filename,
                const char * mode);

Description

The fopen function opens the file whose name is the string pointed to by filename, and associates a stream with it.

The argument mode points to a string. If the string is one of the following, the file is open in the indicated mode. Otherwise, the behavior is undefined.214) r open text file for reading w truncate to zero length or create text file for writing a append; open or create text file for writing at end-of-file rb open binary file for reading wb truncate to zero length or create binary file for writing ab append; open or create binary file for writing at end-of-file r+ open text file for update (reading and writing) w+ truncate to zero length or create text file for update a+ append; open or create text file for update, writing at end-of-file r+b or rb+ open binary file for update (reading and writing) w+b or wb+ truncate to zero length or create binary file for update a+b or ab+ append; open or create binary file for update, writing at end-of-file

Opening a file with read mode ('r' as the first character in the mode argument) fails if the file does not exist or cannot be read.

Opening a file with append mode ('a' as the first character in the mode argument) causes all subsequent writes to the file to be forced to the then current end-of-file, regardless of intervening calls to the fseek function. In some implementations, opening a binary file with append mode ('b' as the second or third character in the above list of mode argument values) may initially position the file position indicator for the stream beyond the last data written, because of null character padding.

When a file is opened with update mode ('+' as the second or third character in the above list of mode argument values), both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file. Opening (or creating) a text file with update mode may instead open (or create) a binary stream in some implementations.

When opened, a stream is fully buffered if and only if it can be determined not to refer to an interactive device. The error and end-of-file indicators for the stream are cleared.

Returns

The fopen function returns a pointer to the object controlling the stream. If the open operation fails, fopen returns a null pointer.

Forward references: file positioning functions (7.19.9).

7.19.5.4 The freopen function

Synopsis

        #include <stdio.h>
        FILE *freopen(const char * filename,
                const char * mode,
                FILE * restrict stream);

Description

The freopen function opens the file whose name is the string pointed to by filename and associates the stream pointed to by stream with it. The mode argument is used just as in the fopen function.215)

If filename is a null pointer, the freopen function attempts to change the mode of the stream to that specified by mode, as if the name of the file currently associated with the stream had been used. It is implementation-defined which changes of mode are permitted (if any), and under what circumstances.

The freopen function first attempts to close any file that is associated with the specified stream. Failure to close the file is ignored. The error and end-of-file indicators for the stream are cleared.

Returns

The freopen function returns a null pointer if the open operation fails. Otherwise, freopen returns the value of stream.

7.19.5.5 The setbuf function

Synopsis

        #include <stdio.h>
        void setbuf(FILE * restrict stream,
                char * restrict buf);

Description

Except that it returns no value, the setbuf function is equivalent to the setvbuf function invoked with the values _IOFBF for mode and BUFSIZ for size, or (if buf is a null pointer), with the value _IONBF for mode.

Returns

The setbuf function returns no value.

Forward references: the setvbuf function (7.19.5.6).

7.19.5.6 The setvbuf function

Synopsis

        #include <stdio.h>
        int setvbuf(FILE * restrict stream,
                char * restrict buf,
                int mode, size_t size);

Description

The setvbuf function may be used only after the stream pointed to by stream has been associated with an open file and before any other operation (other than an unsuccessful call to setvbuf) is performed on the stream. The argument mode determines how stream will be buffered, as follows: _IOFBF causes input/output to be fully buffered; _IOLBF causes input/output to be line buffered; _IONBF causes input/output to be unbuffered. If buf is not a null pointer, the array it points to may be used instead of a buffer allocated by the setvbuf function216) and the argument size specifies the size of the array; otherwise, size may determine the size of a buffer allocated by the

setvbuf function. The contents of the array at any time are indeterminate.

Returns

The setvbuf function returns zero on success, or nonzero if an invalid value is given for mode or if the request cannot be honored.

7.19.6 Formatted input/output functions

The formatted input/output functions217) shall behave as if there is a sequence point after the actions associated with each specifier.

7.19.6.1 The fprintf function

Synopsis

        #include <stdio.h>
        int fprintf(FILE * restrict stream,
                const char * restrict format, ...);

Description

The fprintf function writes output to the stream pointed to by stream, under control of the string pointed to by format that specifies how subsequent arguments are converted for output. If there are insufficient arguments for the format, the behavior is undefined. If the format is exhausted while arguments remain, the excess arguments are evaluated (as always) but are otherwise ignored. The fprintf function returns when the end of the format string is encountered.

The format shall be a multibyte character sequence, beginning and ending in its initial shift state. The format is composed of zero or more directives: ordinary multibyte characters (not %), which are copied unchanged to the output stream; and conversion specifications, each of which results in fetching zero or more subsequent arguments, converting them, if applicable, according to the corresponding conversion specifier, and then writing the result to the output stream.

Each conversion specification is introduced by the character %. After the %, the following appear in sequence:

-- Zero or more flags (in any order) that modify the meaning of the conversion specification.

-- An optional minimum field width. If the converted value has fewer characters than the field width, it is padded with spaces (by default) on the left (or right, if the left adjustment flag, described later, has been given) to the field width. The field width takes the form of an asterisk * (described later) or a decimal integer.218)

-- An optional precision that gives the minimum number of digits to appear for the d, i, o, u, x, and X conversions, the number of digits to appear after the decimal-point character for a, A, e, E, f, and F conversions, the maximum number of significant digits for the g and G conversions, or the maximum number of characters to be written from a string in s conversions. The precision takes the form of a period (.) followed either by an asterisk * (described later) or by an optional decimal integer; if only the period is specified, the precision is taken as zero. If a precision appears with any other conversion specifier, the behavior is undefined.

-- An optional length modifier that specifies the size of the argument.

-- A conversion specifier character that specifies the type of conversion to be applied.

As noted above, a field width, or precision, or both, may be indicated by an asterisk. In this case, an int argument supplies the field width or precision. The arguments specifying field width, or precision, or both, shall appear (in that order) before the argument (if any) to be converted. A negative field width argument is taken as a - flag followed by a positive field width. A negative precision argument is taken as if the precision were omitted.

The flag characters and their meanings are:

- The result of the conversion is left-justified within the field. (It is right-justified if this flag is not specified.)

+ The result of a signed conversion always begins with a plus or minus sign. (It begins with a sign only when a negative value is converted if this flag is not

specified.)219)

space If the first character of a signed conversion is not a sign, or if a signed conversion results in no characters, a space is prefixed to the result. If the space and + flags both appear, the space flag is ignored.

# The result is converted to an ``alternative form''. For o conversion, it increases the precision, if and only if necessary, to force the first digit of the result to be a zero (if the value and precision are both 0, a single 0 is printed). For x (or X) conversion, a nonzero result has 0x (or 0X) prefixed to it. For a, A, e, E, f, F, g, and G conversions, the result of converting a floating-point number always contains a decimal-point character, even if no digits follow it. (Normally, a decimal-point character appears in the result of these conversions only if a digit follows it.) For g and G conversions, trailing zeros are not removed from the result. For other conversions, the behavior is undefined.

0 For d, i, o, u, x, X, a, A, e, E, f, F, g, and G conversions, leading zeros (following any indication of sign or base) are used to pad to the field width rather than performing space padding, except when converting an infinity or NaN. If the 0 and - flags both appear, the 0 flag is ignored. For d, i, o, u, x, and X conversions, if a precision is specified, the 0 flag is ignored. For other conversions, the behavior is undefined.

The length modifiers and their meanings are:

hh Specifies that a following d, i, o, u, x, or X conversion specifier applies to a signed char or unsigned char argument (the argument will have been promoted according to the integer promotions, but its value shall be converted to signed char or unsigned char before printing); or that a following n conversion specifier applies to a pointer to a signed char argument.

h Specifies that a following d, i, o, u, x, or X conversion specifier applies to a short int or unsigned short int argument (the argument will have been promoted according to the integer promotions, but its value shall be converted to short int or unsigned short int before printing); or that a following n conversion specifier applies to a pointer to a short int argument.

l (ell) Specifies that a following d, i, o, u, x, or X conversion specifier applies to a long int or unsigned long int argument; that a following n conversion specifier applies to a pointer to a long int argument; that a following c conversion specifier applies to a wint_t argument; that a following s conversion specifier applies to a pointer to a wchar_t argument; or has no effect on a following a, A, e, E, f, F, g, or G conversion specifier.

ll (ell-ell) Specifies that a following d, i, o, u, x, or X conversion specifier applies to a long long int or unsigned long long int argument; or that a following n conversion specifier applies to a pointer to a long long int argument.

j Specifies that a following d, i, o, u, x, or X conversion specifier applies to an intmax_t or uintmax_t argument; or that a following n conversion specifier applies to a pointer to an intmax_t argument.

z Specifies that a following d, i, o, u, x, or X conversion specifier applies to a size_t or the corresponding signed integer type argument; or that a following n conversion specifier applies to a pointer to a signed integer type corresponding to size_t argument.

t Specifies that a following d, i, o, u, x, or X conversion specifier applies to a ptrdiff_t or the corresponding unsigned integer type argument; or that a following n conversion specifier applies to a pointer to a ptrdiff_t argument.

L Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to a long double argument.

If a length modifier appears with any conversion specifier other than as specified above, the behavior is undefined.

The conversion specifiers and their meanings are:

d,i The int argument is converted to signed decimal in the style [-]dddd. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it is expanded with leading zeros. The default precision is 1. The result of converting a zero value with a precision of zero is no characters.

o,u,x,X The unsigned int argument is converted to unsigned octal (o), unsigned decimal (u), or unsigned hexadecimal notation (x or X) in the style dddd; the letters abcdef are used for x conversion and the letters ABCDEF for X conversion. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it is expanded with leading zeros. The default precision is 1. The result of converting a zero value with a precision of zero is no characters.

f,F A double argument representing a floating-point number is converted to decimal notation in the style [-]ddd.ddd, where the number of digits after the decimal-point character is equal to the precision specification. If the precision is missing, it is taken as 6; if the precision is zero and the # flag is not specified, no decimal-point character appears. If a decimal-point character appears, at least one digit appears before it. The value is rounded to the appropriate number of digits.

A double argument representing an infinity is converted in one of the styles [-]inf or [-]infinity -- which style is implementation-defined. A double argument representing a NaN is converted in one of the styles [-]nan or [-]nan(n-char-sequence) -- which style, and the meaning of any n-char- sequence, is implementation-defined. The F conversion specifier produces INF, INFINITY, or NAN instead of inf, infinity, or nan, respectively.220)

e,E A double argument representing a floating-point number is converted in the style [-]d.ddde+-dd, where there is one digit (which is nonzero if the argument is nonzero) before the decimal-point character and the number of digits after it is equal to the precision; if the precision is missing, it is taken as 6; if the precision is zero and the # flag is not specified, no decimal-point character appears. The value is rounded to the appropriate number of digits. The E conversion specifier produces a number with E instead of e introducing the exponent.

The exponent always contains at least two digits, and only as many more digits as necessary to represent the exponent. If the value is zero, the exponent is zero.

A double argument representing an infinity or NaN is converted in the style of an f or F conversion specifier.

g,G A double argument representing a floating-point number is converted in style f or e (or in style F or E in the case of a G conversion specifier), with the precision specifying the number of significant digits. If the precision is zero, it is taken as 1. The style used depends on the value converted; style e (or E) is used only if the exponent resulting from such a conversion is less than -4 or greater than or equal to the precision. Trailing zeros are removed from the fractional portion of the result unless the # flag is specified; a decimal-point character appears only if it is followed by a digit.

A double argument representing an infinity or NaN is converted in the style of an f or F conversion specifier.

a,A A double argument representing a floating-point number is converted in the style [-]0xh.hhhhp+-d, where there is one hexadecimal digit (which is nonzero if the argument is a normalized floating- point number and is otherwise unspecified) before the decimal-point character221) and the number of hexadecimal digits after it is equal to the precision; if the precision is missing and FLT_RADIX is a power of 2, then the precision is sufficient for an exact representation of the value; if the precision is missing and FLT_RADIX is not a power of 2, then the precision is sufficient to distinguish222) values of type double, except that trailing zeros may be omitted; if the precision is zero and the # flag is not specified, no decimal- point character appears. The letters abcdef are

implementation's scheme for determining the digit to the left of the decimal-point character. used for a conversion and the letters ABCDEF for A conversion. The A conversion specifier produces a number with X and P instead of x and p. The exponent always contains at least one digit, and only as many more digits as necessary to represent the decimal exponent of 2. If the value is zero, the exponent is zero.

A double argument representing an infinity or NaN is converted in the style of an f or F conversion specifier.

c If no l length modifier is present, the int argument is converted to an unsigned char, and the resulting character is written.

If an l length modifier is present, the wint_t argument is converted as if by an ls conversion specification with no precision and an argument that points to the initial element of a two-element array of wchar_t, the first element containing the wint_t argument to the lc conversion specification and the second a null wide character.

s If no l length modifier is present, the argument shall be a pointer to the initial element of an array of character type.223) Characters from the array are written up to (but not including) the terminating null character. If the precision is specified, no more than that many characters are written. If the precision is not specified or is greater than the size of the array, the array shall contain a null character.

If an l length modifier is present, the argument shall be a pointer to the initial element of an array of wchar_t type. Wide characters from the array are converted to multibyte characters (each as if by a call to the wcrtomb function, with the conversion state described by an mbstate_t object initialized to zero before the first wide character is converted) up to and including a terminating null wide character. The resulting multibyte characters are written up to (but not including) the terminating null character (byte). If no precision is specified, the array shall contain a null wide character. If a precision is specified, no more than that many characters (bytes) are written (including shift sequences, if any), and the array shall contain a null wide character if, to equal the multibyte character sequence length given by the precision, the function would need to access a wide character one past the end of the array. In no case is a partial multibyte character written.224)

p The argument shall be a pointer to void. The value of the pointer is converted to a sequence of printing characters, in an implementation-defined manner.

n The argument shall be a pointer to signed integer into which is written the number of characters written to the output stream so far by this call to fprintf. No argument is converted, but one is consumed. If the conversion specification includes any flags, a field width, or a precision, the behavior is undefined.

% A % character is written. No argument is converted. The complete conversion specification shall be %%.

If a conversion specification is invalid, the behavior is undefined.225) If any argument is not the correct type for the corresponding coversion specification, the behavior is undefined.

#10

In no case does a nonexistent or small field width cause truncation of a field; if the result of a conversion is wider than the field width, the field is expanded to contain the conversion result.

#11

For a and A conversions, if FLT_RADIX is a power of 2, the value is correctly rounded to a hexadecimal floating number with the given precision.

Recommended practice

#12

If FLT_RADIX is not a power of 2, the result should be one of the two adjacent numbers in hexadecimal floating style with the given precision, with the extra stipulation that the error should have a correct sign for the current rounding direction.

#13

For e, E, f, F, g, and G conversions, if the number of significant decimal digits is at most DECIMAL_DIG, then the result should be correctly rounded.226) If the number of significant decimal digits is more than DECIMAL_DIG but the with trailing zeros. Otherwise, the source value is bounded by two adjacent decimal strings L < U, both having DECIMAL_DIG significant digits; the value of the resultant decimal string D should satisfy L <= D <= U, with the extra stipulation that the error should have a correct sign for the current rounding direction.

Returns

#14

The fprintf function returns the number of characters transmitted, or a negative value if an output or encoding error occurred.

Environmental limits

#15

The number of characters that can be produced by any single conversion shall be at least 4095.

#16

EXAMPLE 1 To print a date and time in the form ``Sunday, July 3, 10:02'' followed by pi to five decimal places:

        #include <math.h>
        #include <stdio.h>
        /* ... */
        char *weekday, *month;    // pointers to strings
        int day, hour, min;
        fprintf(stdout, "%s, %s %d, %.2d:%.2d\n",
                weekday, month, day, hour, min);
        fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));

#17

EXAMPLE 2 In this example, multibyte characters do not have a state-dependent encoding, and the multibyte members of the extended character set each consist of two bytes, the first of which is denoted here by a [] and the second by an uppercase letter.

#18

Given the following wide string with length seven,

        static wchar_t wstr[] = L"[]X[]Yabc[]Z[]W";

the seven calls

        fprintf(stdout, "|1234567890123|\n");
        fprintf(stdout, "|%13ls|\n", wstr);
        fprintf(stdout, "|%-13.9ls|\n", wstr);
        fprintf(stdout, "|%13.10ls|\n", wstr);
        fprintf(stdout, "|%13.11ls|\n", wstr);
        fprintf(stdout, "|%13.15ls|\n", &wstr[2]);
        fprintf(stdout, "|%13lc|\n", wstr[5]);

will print the following seven lines:

        |1234567890123|
        |  []X[]Yabc[]Z[]W|
        |[]X[]Yabc[]Z    |
        |    []X[]Yabc[]Z|
        |  []X[]Yabc[]Z[]W|
        |      abc[]Z[]W|
        |           []Z|

Forward references: conversion state (7.24.6), the wcrtomb function (7.24.6.3.3).

7.19.6.2 The fscanf function

Synopsis

        #include <stdio.h>
        int fscanf(FILE * restrict stream,
                const char * restrict format, ...);

Description

The fscanf function reads input from the stream pointed to by stream, under control of the string pointed to by format that specifies the admissible input sequences and how they are to be converted for assignment, using subsequent arguments as pointers to the objects to receive the converted input. If there are insufficient arguments for the format, the behavior is undefined. If the format is exhausted while arguments remain, the excess arguments are evaluated (as always) but are otherwise ignored.

The format shall be a multibyte character sequence, beginning and ending in its initial shift state. The format is composed of zero or more directives: one or more white- space characters, an ordinary multibyte character (neither % nor a white-space character), or a conversion specification. Each conversion specification is introduced by the character %. After the %, the following appear in sequence:

-- An optional assignment-suppressing character *. -- An optional nonzero decimal integer that specifies the maximum field width (in characters).

-- An optional length modifier that specifies the size of the receiving object.

-- A conversion specifier character that specifies the type of conversion to be applied.

The fscanf function executes each directive of the format in turn. If a directive fails, as detailed below, the function returns. Failures are described as input failures (due to the occurrence of an encoding error or the unavailability of input characters), or matching failures (due to inappropriate input).

A directive composed of white-space character(s) is executed by reading input up to the first non-white-space character (which remains unread), or until no more characters can be read.

A directive that is an ordinary multibyte character is executed by reading the next characters of the stream. If any of those characters differ from the ones composing the directive, the directive fails and the differing and subsequent characters remain unread.

A directive that is a conversion specification defines a set of matching input sequences, as described below for each specifier. A conversion specification is executed in the following steps:

Input white-space characters (as specified by the isspace function) are skipped, unless the specification includes a [, c, or n specifier.227)

An input item is read from the stream, unless the specification includes an n specifier. An input item is defined as the longest sequence of input characters which does not exceed any specified field width and which is, or is a prefix of, a matching input sequence. The first character, if any, after the input item remains unread. If the length of the input item is zero, the execution of the directive fails; this condition is a matching failure unless end-of-file, an encoding error, or a read error prevented input from the stream, in which case it is an input failure.

#10

Except in the case of a % specifier, the input item (or, in the case of a %n directive, the count of input characters) is converted to a type appropriate to the

conversion specifier. If the input item is not a matching sequence, the execution of the directive fails: this condition is a matching failure. Unless assignment suppression was indicated by a *, the result of the conversion is placed in the object pointed to by the first argument following the format argument that has not already received a conversion result. If this object does not have an appropriate type, or if the result of the conversion cannot be represented in the object, the behavior is undefined.

#11

The length modifiers and their meanings are:

hh Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to signed char or unsigned char.

h Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to short int or unsigned short int.

l (ell) Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to long int or unsigned long int; that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument with type pointer to double; or that a following c, s, or [ conversion specifier applies to an argument with type pointer to wchar_t.

ll (ell-ell) Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to long long int or unsigned long long int.

j Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to intmax_t or uintmax_t.

z Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to size_t or the corresponding signed integer type.

t Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to ptrdiff_t or the corresponding unsigned integer type.

L Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument with type pointer to long double.

If a length modifier appears with any conversion specifier other than as specified above, the behavior is undefined.

#12

The conversion specifiers and their meanings are:

d Matches an optionally signed decimal integer, whose format is the same as expected for the subject sequence of the strtol function with the value 10 for the base argument. The corresponding argument shall be a pointer to signed integer.

i Matches an optionally signed integer, whose format is the same as expected for the subject sequence of the strtol function with the value 0 for the base argument. The corresponding argument shall be a pointer to signed integer.

o Matches an optionally signed octal integer, whose format is the same as expected for the subject sequence of the strtoul function with the value 8 for the base argument. The corresponding argument shall be a pointer to unsigned integer.

u Matches an optionally signed decimal integer, whose format is the same as expected for the subject sequence of the strtoul function with the value 10 for the base argument. The corresponding argument shall be a pointer to unsigned integer.

x Matches an optionally signed hexadecimal integer, whose format is the same as expected for the subject sequence of the strtoul function with the value 16 for the base argument. The corresponding argument shall be a pointer to unsigned integer.

a,e,f,g Matches an optionally signed floating-point number, infinity, or NaN, whose format is the same as expected for the subject sequence of the strtod function. The corresponding argument shall be a pointer to floating.

c Matches a sequence of characters of exactly the number specified by the field width (1 if no field width is present in the directive).228)

If no l length modifier is present, the corresponding argument shall be a pointer to the initial element of a character array large enough to accept the sequence. No null character is added.

If an l length modifier is present, the input shall be a sequence of multibyte characters that begins in the initial shift state. Each multibyte character in the sequence is converted to a wide character as if by a call to the mbrtowc function, with the conversion state described by an mbstate_t object initialized to zero before the first multibyte character is converted. The corresponding argument shall be a pointer to the initial element of an array of wchar_t large enough to accept the resulting sequence of wide characters. No null wide character is added.

s Matches a sequence of non-white-space characters.228)

If no l length modifier is present, the corresponding argument shall be a pointer to the initial element of a character array large enough to accept the sequence and a terminating null character, which will be added automatically.

If an l length modifier is present, the input shall be a sequence of multibyte characters that begins in the initial shift state. Each multibyte character is converted to a wide character as if by a call to the mbrtowc function, with the conversion state described by an mbstate_t object initialized to zero before the first multibyte character is converted. The corresponding argument shall be a pointer to the initial element of an array of wchar_t large enough to accept the sequence and the terminating null wide character, which will be added automatically.

[ Matches a nonempty sequence of characters from a set of expected characters (the scanset).228)

the mbrtowc function, with the conversion state described by an mbstate_t object initialized to zero before the first multibyte character is converted. The corresponding argument shall be a pointer to the initial element of an array of wchar_t large enough to accept the sequence and the terminating null wide character, which will be added automatically.

The conversion specifier includes all subsequent characters in the format string, up to and including the matching right bracket (]). The characters between the brackets (the scanlist) compose the scanset, unless the character after the left bracket is a circumflex (^), in which case the scanset contains all characters that do not appear in the scanlist between the circumflex and the right bracket. If the conversion specifier begins with [] or [^], the right bracket character is in the scanlist and the next following right bracket character is the matching right bracket that ends the specification; otherwise the first following right bracket character is the one that ends the specification. If a - character is in the scanlist and is not the first, nor the second where the first character is a ^, nor the last character, the behavior is implementation-defined.

p Matches an implementation-defined set of sequences, which should be the same as the set of sequences that may be produced by the %p conversion of the fprintf function. The corresponding argument shall be a pointer to a pointer to void. The input item is converted to a pointer value in an implementation-defined manner. If the input item is a value converted earlier during the same program execution, the pointer that results shall compare equal to that value; otherwise the behavior of the %p conversion is undefined.

n No input is consumed. The corresponding argument shall be a pointer to signed integer into which is to be written the number of characters read from the input stream so far by this call to the fscanf function. Execution of a %n directive does not increment the assignment count returned at the completion of execution of the fscanf function. No argument is converted, but one is consumed. If the conversion specification includes an assignment- suppressing character or a field width, the behavior is undefined.

% Matches a single % character; no conversion or assignment occurs. The complete conversion specification shall be %%.

#13

If a conversion specification is invalid, the behavior is undefined.229)

#14

The conversion specifiers A, E, F, G, and X are also valid and behave the same as, respectively, a, e, f, g, and x.

#15

If end-of-file is encountered during input, conversion is terminated. If end-of-file occurs before any characters matching the current directive have been read (other than leading white space, where permitted), execution of the current directive terminates with an input failure; otherwise, unless execution of the current directive is terminated with a matching failure, execution of the following directive (other than %n, if any) is terminated with an input failure.

#16

Trailing white space (including new-line characters) is left unread unless matched by a directive. The success of literal matches and suppressed assignments is not directly determinable other than via the %n directive.

#17

If conversion terminates on a conflicting input character, the offending input character is left unread in the input stream.230)

Returns

#18

The fscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

#19

EXAMPLE 1 The call:

        #include <stdio.h>
        /* ... */
        int n, i; float x; char name[50];
        n = fscanf(stdin, "%d%f%s", &i, &x, name);

with the input line:

25 54.32E-1 thompson

will assign to n the value 3, to i the value 25, to x the

value 5.432, and to name the sequence thompson\0.

#20

EXAMPLE 2 The call:

        #include <stdio.h>
        /* ... */
        int i; float x; char name[50];
        fscanf(stdin, "%2d%f%*d %[0123456789]", &i, &x, name);

with input:

56789 0123 56a72

will assign to i the value 56 and to x the value 789.0, will skip 0123, and will assign to name the sequence 56\0. The next character read from the input stream will be a.

#21

EXAMPLE 3 To accept repeatedly from stdin a quantity, a unit of measure, and an item name:

        #include <stdio.h>
        /* ... */
        int count; float quant; char units[21], item[21];
        do {
                count = fscanf(stdin, "%f%20s of %20s", &quant, units, item);
                fscanf(stdin,"%*[^\n]");
        } while (!feof(stdin) && !ferror(stdin));

#22

If the stdin stream contains the following lines:

2 quarts of oil -12.8degrees Celsius lots of luck 10.0LBS of dirt 100ergs of energy

the execution of the above example will be analogous to the following assignments:

        quant = 2; strcpy(units, "quarts"); strcpy(item, "oil");
        count = 3;
        quant = -12.8; strcpy(units, "degrees");
        count = 2; // "C" fails to match "o"
        count = 0; // "l" fails to match "%f"
        quant = 10.0; strcpy(units, "LBS"); strcpy(item, "dirt");
        count = 3;
        count = 0; // "100e" fails to match "%f"
        count = EOF;

#23

EXAMPLE 4 In: #include <stdio.h> /* ... */ int d1, d2, n1, n2, i; i = sscanf("123", "%d%n%n%d", &d1, &n1, &n2, &d2);

the value 123 is assigned to d1 and the value 3 to n1. Because %n can never get an input failure the value of 3 is also assigned to n2. The value of d2 is not affected. The value 1 is assigned to i.

#24

EXAMPLE 5 In these examples, multibyte characters do have a state-dependent encoding, and multibyte members of the extended character set consist of two bytes, the first of which is denoted here by a [] and the second by an uppercase letter, but are only recognized as such when in the alternate shift state. The shift sequences are denoted by and , in which the first causes entry into the alternate shift state.

#25

After the call:

        #include <stdio.h>
        /* ... */
        char str[50];
        fscanf(stdin, "a%s", str);

with the input line:

        a[]X[]Y bc

str will contain []X[]Y\0 assuming that none of the bytes of the shift sequences (or of the multibyte characters, in the more general case) appears to be a single-byte white-space character.

#26

In contrast, after the call:

        #include <stdio.h>
        #include <stddef.h>
        /* ... */
        wchar_t wstr[50];
        fscanf(stdin, "a%ls", wstr);

with the same input line, wstr will contain the two wide characters that correspond to []X and []Y and a terminating null wide character.

#27

However, the call: #include <stdio.h> #include <stddef.h> /* ... */ wchar_t wstr[50]; fscanf(stdin, "a[]X%ls", wstr);

with the same input line will return zero due to a matching failure against the sequence in the format string.

#28

Assuming that the first byte of the multibyte character []X is the same as the first byte of the multibyte character []Y, after the call:

        #include <stdio.h>
        #include <stddef.h>
        /* ... */
        wchar_t wstr[50];
        fscanf(stdin, "a[]Y%ls", wstr);

with the same input line, zero will again be returned, but stdin will be left with a partially consumed multibyte character.

Forward references: the strtod, strtof, and strtold functions (7.20.1.3), the strtol, strtoll, strtoul, and strtoull functions (7.20.1.4), conversion state (7.24.6), the wcrtomb function (7.24.6.3.3).

7.19.6.3 The printf function

Synopsis

        #include <stdio.h>
        int printf(const char * restrict format, ...);

Description

The printf function is equivalent to fprintf with the argument stdout interposed before the arguments to printf.

Returns

The printf function returns the number of characters transmitted, or a negative value if an output or encoding error occurred.

7.19.6.4 The scanf function

Synopsis

        #include <stdio.h>
        int scanf(const char * restrict format, ...);

Description

The scanf function is equivalent to fscanf with the argument stdin interposed before the arguments to scanf.

Returns

The scanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the scanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

7.19.6.5 The snprintf function

Synopsis

        #include <stdio.h>
        int snprintf(char * restrict s, size_t n,
                const char * restrict format, ...);

Description

The snprintf function is equivalent to fprintf, except that the output is written into an array (specified by argument s) rather than to a stream. If n is zero, nothing is written, and s may be a null pointer. Otherwise, output characters beyond the n-1st are discarded rather than being written to the array, and a null character is written at the end of the characters actually written into the array. If copying takes place between objects that overlap, the behavior is undefined.

Returns

The snprintf function returns the number of characters that would have been written had n been sufficiently large, not counting the terminating null character, or a negative value if an encoding error occurred. Thus, the null- terminated output has been completely written if and only if the returned value is nonnegative and less than n.

7.19.6.6 The sprintf function

Synopsis

        #include <stdio.h>
        int sprintf(char * restrict s,
                const char * restrict format, ...);

Description

The sprintf function is equivalent to fprintf, except that the output is written into an array (specified by the argument s) rather than to a stream. A null character is written at the end of the characters written; it is not counted as part of the returned value. If copying takes place between objects that overlap, the behavior is undefined.

Returns

The sprintf function returns the number of characters written in the array, not counting the terminating null character, or a negative value if an encoding error occurred.

7.19.6.7 The sscanf function

Synopsis

        #include <stdio.h>
        int sscanf(const char * restrict s,
                const char * restrict format, ...);

Description

The sscanf function is equivalent to fscanf, except that input is obtained from a string (specified by the argument s) rather than from a stream. Reaching the end of the string is equivalent to encountering end-of-file for the fscanf function. If copying takes place between objects that overlap, the behavior is undefined.

Returns

The sscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the sscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

7.19.6.8 The vfprintf function

Synopsis

        #include <stdarg.h>
        #include <stdio.h>
        int vfprintf(FILE * restrict stream,
                const char * restrict format,
                va_list arg);

Description

The vfprintf function is equivalent to fprintf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vfprintf function does not invoke the va_end macro.231)

Returns

The vfprintf function returns the number of characters transmitted, or a negative value if an output or encoding error occurred.

EXAMPLE The following shows the use of the vfprintf function in a general error-reporting routine.

        #include <stdarg.h>
        #include <stdio.h>

        void error(char *function_name, char *format, ...)
        {
                va_list args;

                va_start(args, format);
                // print out name of function causing error
                fprintf(stderr, "ERROR in %s: ", function_name);
                // print out remainder of message
                vfprintf(stderr, format, args);
                va_end(args);
        }

7.19.6.9 The vfscanf function

Synopsis

        #include <stdarg.h>
        #include <stdio.h>
        int vfscanf(FILE * restrict stream,
                const char * restrict format,
                va_list arg);

Description

The vfscanf function is equivalent to fscanf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vfscanf function does not invoke the va_end macro.231)

Returns

The vfscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the vfscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

7.19.6.10 The vprintf function

Synopsis

        #include <stdarg.h>
        #include <stdio.h>
        int vprintf(const char * restrict format,
                va_list arg);

Description

The vprintf function is equivalent to printf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vprintf function does not invoke the va_end macro.231)

Returns

The vprintf function returns the number of characters transmitted, or a negative value if an output or encoding error occurred.

7.19.6.11 The vscanf function

Synopsis

        #include <stdarg.h>
        #include <stdio.h>
        int vscanf(const char * restrict format,
                va_list arg);

Description

The vscanf function is equivalent to scanf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vscanf function does not invoke the va_end macro.231)

Returns

The vscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the vscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

7.19.6.12 The vsnprintf function

Synopsis

        #include <stdarg.h>
        #include <stdio.h>
        int vsprintf(char * restrict s, size_t n,
                const char * restrict format,
                va_list arg);

Description

The vsnprintf function is equivalent to snprintf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vsnprintf function does not invoke the va_end macro.231) If copying takes place between objects that overlap, the behavior is undefined.

Returns

The vsnprintf function returns the number of characters that would have been written had n been sufficiently large, not counting the terminating null character, or a negative value if an encoding error occurred. Thus, the null- terminated output has been completely written if and only if the returned value is nonnegative and less than n.

7.19.6.13 The vsprintf function

Synopsis

        #include <stdarg.h>
        #include <stdio.h>
        int vsprintf(char * restrict s,
                const char * restrict format,
                va_list arg);

Description

The vsprintf function is equivalent to sprintf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vsprintf function does not invoke the va_end macro.231) If copying takes place between objects that overlap, the behavior is undefined.

Returns

The vsprintf function returns the number of characters written in the array, not counting the terminating null character, or a negative value if an encoding error occurred.

7.19.6.14 The vsscanf function

Synopsis

        #include <stdarg.h>
        #include <stdio.h>
        int vsscanf(const char * restrict s,
                const char * restrict format,
                va_list arg);

Description

The vsscanf function is equivalent to sscanf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vsscanf function does not invoke the va_end macro.231)

Returns

The vsscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the vscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

7.19.7 Character input/output functions

7.19.7.1 The fgetc function

Synopsis

        #include <stdio.h>
        int fgetc(FILE *stream);

Description

If the end-of-file indicator for the input stream pointed to by stream is not set and a next character is present, the fgetc function obtains that character as an unsigned char converted to an int and advances the associated file position indicator for the stream (if defined).

Returns

If the end-of-file indicator for the stream is set, or if the stream is at end-of-file, the end-of-file indicator for the stream is set and fgetc returns EOF. Otherwise, the fgetc function returns the next character from the input stream pointed to by stream. If a read error occurs, the error indicator for the stream is set and fgetc returns EOF.232)

7.19.7.2 The fgets function

Synopsis

        #include <stdio.h>
        char *fgets(char * restrict s, int n,
                FILE * restrict stream);

Description

The fgets function reads at most one less than the number of characters specified by n from the stream pointed to by stream into the array pointed to by s. No additional

characters are read after a new-line character (which is retained) or after end-of-file. A null character is written immediately after the last character read into the array.

Returns

The fgets function returns s if successful. If end-of- file is encountered and no characters have been read into the array, the contents of the array remain unchanged and a null pointer is returned. If a read error occurs during the operation, the array contents are indeterminate and a null pointer is returned.

7.19.7.3 The fputc function

Synopsis

        #include <stdio.h>
        int fputc(int c, FILE *stream);

Description

The fputc function writes the character specified by c (converted to an unsigned char) to the output stream pointed to by stream, at the position indicated by the associated file position indicator for the stream (if defined), and advances the indicator appropriately. If the file cannot support positioning requests, or if the stream was opened with append mode, the character is appended to the output stream.

Returns

The fputc function returns the character written. If a write error occurs, the error indicator for the stream is set and fputc returns EOF.

7.19.7.4 The fputs function

Synopsis

        #include <stdio.h>
        int fputs(const char * restrict s,
                FILE * restrict stream);

Description

The fputs function writes the string pointed to by s to the stream pointed to by stream. The terminating null character is not written.

Returns

The fputs function returns EOF if a write error occurs; otherwise it returns a nonnegative value.

7.19.7.5 The getc function

Synopsis

        #include <stdio.h>
        int getc(FILE *stream);

Description

The getc function is equivalent to fgetc, except that if it is implemented as a macro, it may evaluate stream more than once, so the argument should never be an expression with side effects.

Returns

The getc function returns the next character from the input stream pointed to by stream. If the stream is at end- of-file, the end-of-file indicator for the stream is set and getc returns EOF. If a read error occurs, the error indicator for the stream is set and getc returns EOF.

7.19.7.6 The getchar function

Synopsis

        #include <stdio.h>
        int getchar(void);

Description

The getchar function is equivalent to getc with the argument stdin.

Returns

The getchar function returns the next character from the input stream pointed to by stdin. If the stream is at end-of-file, the end-of-file indicator for the stream is set and getchar returns EOF. If a read error occurs, the error indicator for the stream is set and getchar returns EOF.

7.19.7.7 The gets function

Synopsis

        #include <stdio.h>
        char *gets(char *s);

Description

The gets function reads characters from the input stream pointed to by stdin, into the array pointed to by s, until end-of-file is encountered or a new-line character is read. Any new-line character is discarded, and a null character is written immediately after the last character read into the array.

Returns

The gets function returns s if successful. If end-of- file is encountered and no characters have been read into the array, the contents of the array remain unchanged and a null pointer is returned. If a read error occurs during the operation, the array contents are indeterminate and a null pointer is returned.

7.19.7.8 The putc function

Synopsis

        #include <stdio.h>
        int putc(int c, FILE *stream);

Description

The putc function is equivalent to fputc, except that if it is implemented as a macro, it may evaluate stream more than once, so that argument should never be an expression with side effects.

Returns

The putc function returns the character written. If a write error occurs, the error indicator for the stream is set and putc returns EOF.

7.19.7.9 The putchar function

Synopsis

        #include <stdio.h>
        int putchar(int c);

Description

The putchar function is equivalent to putc with the second argument stdout.

Returns

The putchar function returns the character written. If a write error occurs, the error indicator for the stream is set and putchar returns EOF.

7.19.7.10 The puts function

Synopsis

        #include <stdio.h>
        int puts(const char *s);

Description

The puts function writes the string pointed to by s to the stream pointed to by stdout, and appends a new-line character to the output. The terminating null character is not written.

Returns

The puts function returns EOF if a write error occurs; otherwise it returns a nonnegative value.

7.19.7.11 The ungetc function

Synopsis

        #include <stdio.h>
        int ungetc(int c, FILE *stream);

Description

The ungetc function pushes the character specified by c (converted to an unsigned char) back onto the input stream pointed to by stream. Pushed-back characters will be returned by subsequent reads on that stream in the reverse order of their pushing. A successful intervening call (with the stream pointed to by stream) to a file positioning function (fseek, fsetpos, or rewind) discards any pushed- back characters for the stream. The external storage corresponding to the stream is unchanged.

One character of pushback is guaranteed. If the ungetc function is called too many times on the same stream without an intervening read or file positioning operation on that stream, the operation may fail.

If the value of c equals that of the macro EOF, the operation fails and the input stream is unchanged.

A successful call to the ungetc function clears the end-of-file indicator for the stream. The value of the file position indicator for the stream after reading or discarding all pushed-back characters shall be the same as it was before the characters were pushed back. For a text stream, the value of its file position indicator after a successful call to the ungetc function is unspecified until all pushed-back characters are read or discarded. For a binary stream, its file position indicator is decremented by each successful call to the ungetc function; if its value was zero before a call, it is indeterminate after the call.233)

Returns

The ungetc function returns the character pushed back after conversion, or EOF if the operation fails.

Forward references: file positioning functions (7.19.9).

7.19.8 Direct input/output functions

7.19.8.1 The fread function

Synopsis

        #include <stdio.h>
        size_t fread(void * restrict ptr,
                size_t size, size_t nmemb,
                FILE * restrict stream);

Description

The fread function reads, into the array pointed to by ptr, up to nmemb elements whose size is specified by size, from the stream pointed to by stream. The file position indicator for the stream (if defined) is advanced by the number of characters successfully read. If an error occurs, the resulting value of the file position indicator for the stream is indeterminate. If a partial element is read, its value is indeterminate.

Returns

The fread function returns the number of elements successfully read, which may be less than nmemb if a read error or end-of-file is encountered. If size or nmemb is zero, fread returns zero and the contents of the array and the state of the stream remain unchanged.

7.19.8.2 The fwrite function

Synopsis

        #include <stdio.h>
        size_t fwrite(const void * restrict ptr,
                size_t size, size_t nmemb,
                FILE * restrict stream);

Description

The fwrite function writes, from the array pointed to by ptr, up to nmemb elements whose size is specified by size, to the stream pointed to by stream. The file position indicator for the stream (if defined) is advanced by the number of characters successfully written. If an error occurs, the resulting value of the file position indicator for the stream is indeterminate.

Returns

The fwrite function returns the number of elements successfully written, which will be less than nmemb only if a write error is encountered.

7.19.9 File positioning functions

7.19.9.1 The fgetpos function

Synopsis

        #include <stdio.h>
        int fgetpos(FILE * restrict stream,
                fpos_t * restrict pos);

Description

The fgetpos function stores the current values of the parse state (if any) and file position indicator for the stream pointed to by stream in the object pointed to by pos. The values stored contain unspecified information usable by the fsetpos function for repositioning the stream to its position at the time of the call to the fgetpos function.

Returns

If successful, the fgetpos function returns zero; on failure, the fgetpos function returns nonzero and stores an implementation-defined positive value in errno.

Forward references: the fsetpos function (7.19.9.3).

7.19.9.2 The fseek function

Synopsis

        #include <stdio.h>
        int fseek(FILE *stream, long int offset, int whence);

Description

The fseek function sets the file position indicator for the stream pointed to by stream. If a read or write error occurs, the error indicator for the stream is set and fseek fails.

For a binary stream, the new position, measured in characters from the beginning of the file, is obtained by adding offset to the position specified by whence. The specified position is the beginning of the file if whence is SEEK_SET, the current value of the file position indicator if SEEK_CUR, or end-of-file if SEEK_END. A binary stream need not meaningfully support fseek calls with a whence value of SEEK_END.

For a text stream, either offset shall be zero, or offset shall be a value returned by an earlier successful call to the ftell function on a stream associated with the same file and whence shall be SEEK_SET.

After determining the new position, a successful call to the fseek function undoes any effects of the ungetc function on the stream, clears the end-of-file indicator for the stream, and then establishes the new position. After a successful fseek call, the next operation on an update stream may be either input or output.

Returns

The fseek function returns nonzero only for a request that cannot be satisfied.

Forward references: the ftell function (7.19.9.4).

7.19.9.3 The fsetpos function

Synopsis

        #include <stdio.h>
        int fsetpos(FILE *stream, const fpos_t *pos);

Description

The fsetpos function sets the mbstate_t object (if any) and file position indicator for the stream pointed to by stream according to the value of the object pointed to by pos, which shall be a value obtained from an earlier successful call to the fgetpos function on a stream associated with the same file. If a read or write error occurs, the error indicator for the stream is set and fsetpos fails.

A successful call to the fsetpos function undoes any effects of the ungetc function on the stream, clears the end-of-file indicator for the stream, and then establishes the new parse state and position. After a successful fsetpos call, the next operation on an update stream may be either input or output.

Returns

If successful, the fsetpos function returns zero; on failure, the fsetpos function returns nonzero and stores an implementation-defined positive value in errno.

7.19.9.4 The ftell function

Synopsis

        #include <stdio.h>
        long int ftell(FILE *stream);

Description

The ftell function obtains the current value of the file position indicator for the stream pointed to by stream. For a binary stream, the value is the number of characters from the beginning of the file. For a text stream, its file position indicator contains unspecified information, usable by the fseek function for returning the file position indicator for the stream to its position at the time of the ftell call; the difference between two such return values is not necessarily a meaningful measure of the number of characters written or read.

Returns

If successful, the ftell function returns the current value of the file position indicator for the stream. On failure, the ftell function returns -1L and stores an implementation-defined positive value in errno.

7.19.9.5 The rewind function

Synopsis

        #include <stdio.h>
        void rewind(FILE *stream);

Description

The rewind function sets the file position indicator for the stream pointed to by stream to the beginning of the file. It is equivalent to

        (void)fseek(stream, 0L, SEEK_SET)

except that the error indicator for the stream is also cleared.

Returns

The rewind function returns no value.

7.19.10 Error-handling functions

7.19.10.1 The clearerr function

Synopsis

        #include <stdio.h>
        void clearerr(FILE *stream);

Description

The clearerr function clears the end-of-file and error indicators for the stream pointed to by stream.

Returns

The clearerr function returns no value.

7.19.10.2 The feof function

Synopsis

        #include <stdio.h>
        int feof(FILE *stream);

Description

The feof function tests the end-of-file indicator for the stream pointed to by stream.

Returns

The feof function returns nonzero if and only if the end-of-file indicator is set for stream.

7.19.10.3 The ferror function

Synopsis

        #include <stdio.h>
        int ferror(FILE *stream);

Description

The ferror function tests the error indicator for the stream pointed to by stream.

Returns

The ferror function returns nonzero if and only if the error indicator is set for stream.

7.19.10.4 The perror function

Synopsis

        #include <stdio.h>
        void perror(const char *s);

Description

The perror function maps the error number in the integer expression errno to an error message. It writes a sequence of characters to the standard error stream thus: first (if s is not a null pointer and the character pointed to by s is not the null character), the string pointed to by s followed by a colon (:) and a space; then an appropriate error message string followed by a new-line character. The contents of the error message strings are the same as those returned by the strerror function with argument errno.

Returns

The perror function returns no value.

Forward references: the strerror function (7.21.6.2).

7.20 General utilities <stdlib.h>

The header <stdlib.h> declares five types and several functions of general utility, and defines several macros.234)

The types declared are size_t and wchar_t (both described in 7.17),

div_t

which is a structure type that is the type of the value returned by the div function,

ldiv_t

which is a structure type that is the type of the value returned by the ldiv function, and

lldiv_t

which is a structure type that is the type of the value returned by the lldiv function.

The macros defined are NULL (described in 7.17);

EXIT_FAILURE

and

EXIT_SUCCESS

which expand to integer constant expressions that may be used as the argument to the exit function to return unsuccessful or successful termination status, respectively, to the host environment;

RAND_MAX

which expands to an integer constant expression, the value of which is the maximum value returned by the rand function; and

MB_CUR_MAX

which expands to a positive integer expression with type size_t whose value is the maximum number of bytes in a multibyte character for the extended character set specified by the current locale (category LC_CTYPE), and whose value is never greater than MB_LEN_MAX.

7.20.1 String conversion functions

The functions atof, atoi, atol, and atoll need not affect the value of the integer expression errno on an error. If the value of the result cannot be represented, the behavior is undefined.

7.20.1.1 The atof function

Synopsis

        #include <stdlib.h>
        double atof(const char *nptr);

Description

The atof function converts the initial portion of the string pointed to by nptr to double representation. Except for the behavior on error, it is equivalent to

        strtod(nptr, (char **)NULL)

Returns

The atof function returns the converted value.

Forward references: the strtod, strtof, and strtold functions (7.20.1.3).

7.20.1.2 The atoi, atol, and atoll functions

Synopsis

        #include <stdlib.h>
        int atoi(const char *nptr);
        long int atol(const char *nptr);
        long long int atoll(const char *nptr);

Description

The atoi, atol, and atoll functions convert the initial portion of the string pointed to by nptr to int, long int, and long long int representation, respectively. Except for the behavior on error, they are equivalent to

        atoi:  (int)strtol(nptr, (char **)NULL, 10)
        atol:  strtol(nptr, (char **)NULL, 10)
        atoll: strtoll(nptr, (char **)NULL, 10)

Returns

The atoi, atol, and atoll functions return the converted value.

Forward references: the strtol, strtoll, strtoul, and strtoull functions (7.20.1.4).

7.20.1.3 The strtod, strtof, and strtold functions

Synopsis

        #include <stdlib.h>
        double strtod(const char * restrict nptr,
                char ** restrict endptr);
        float strtof(const char * restrict nptr,
                char ** restrict endptr);
        long double strtold(const char * restrict nptr,
                char ** restrict endptr);

Description

The strtod, strtof, and strtold functions convert the initial portion of the string pointed to by nptr to double, float, and long double representation, respectively. First, they decompose the input string into three parts: an initial, possibly empty, sequence of white-space characters (as specified by the isspace function), a subject sequence resembling a floating-point constant or representing an infinity or NaN; and a final string of one or more unrecognized characters, including the terminating null character of the input string. Then, they attempt to convert the subject sequence to a floating-point number, and return the result.

The expected form of the subject sequence is an optional plus or minus sign, then one of the following:

-- a nonempty sequence of decimal digits optionally containing a decimal-point character, then an optional exponent part as defined in 6.4.4.2;

-- a 0x or 0X, then a nonempty sequence of hexadecimal digits optionally containing a decimal-point character, then an optional binary exponent part as defined in 6.4.4.2;

-- one of INF or INFINITY, ignoring case

-- one of NAN or NAN(n-char-sequence-opt), ignoring case in the NAN part, where: n-char-sequence: digit nondigit n-char-sequence digit n-char-sequence nondigit

If the subject sequence has the expected form for a floating-point number, the sequence of characters starting with the first digit or the decimal-point character (whichever occurs first) is interpreted as a floating constant according to the rules of 6.4.4.2, except that the decimal-point character is used in place of a period, and that if neither an exponent part nor a decimal-point character appears in a decimal floating point number, or if a binary exponent part does not appear in a binary floating point number, an exponent part of the appropriate type with value zero is assumed to follow the last digit in the string. If the subject sequence begins with a minus sign, the sequence is interpreted as negated.235) A character sequence INF or INFINITY is interpreted as an infinity, if representable in the return type, else like a floating constant that is too large for the range of the return type. A character sequence NAN or NAN(n-char-sequence-opt), is interpreted as a quiet NaN, if supported in the return type, else like a subject sequence part that does not have the expected form; the meaning of the n-char sequences is implementation-defined.236) A pointer to the final string is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

If the subject sequence has the hexadecimal form and FLT_RADIX is a power of 2, the value resulting from the conversion is correctly rounded.

In other than the "C" locale, additional locale- specific subject sequence forms may be accepted.

If the subject sequence is empty or does not have the expected form, no conversion is performed; the value of nptr is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

Recommended practice

If the subject sequence has the hexadecimal form and FLT_RADIX is not a power of 2, the result should be one of the two numbers in the appropriate internal format that are adjacent to the hexadecimal floating source value, with the extra stipulation that the error should have a correct sign for the current rounding direction.

If the subject sequence has the decimal form and at most DECIMAL_DIG (defined in <float.h>) significant digits, the result should be correctly rounded. If the subject sequence D has the decimal form and more than DECIMAL_DIG significant digits, consider the two bounding, adjacent decimal strings L and U, both having DECIMAL_DIG significant digits, such that the values of L, D, and U satisfy L <= D <= U. The result should be one of the (equal or adjacent) values that would be obtained by correctly rounding L and U according to the current rounding direction, with the extra stipulation that the error with respect to D should have a correct sign for the current rounding direction.237)

Returns

#10

The functions return the converted value, if any. If no conversion could be performed, zero is returned. If the correct value is outside the range of representable values, plus or minus HUGE_VAL, HUGE_VALF, or HUGE_VALL is returned (according to the return type and sign of the value), and the value of the macro ERANGE is stored in errno. If the result underflows (7.12.1), the functions return a value whose magnitude is no greater than the smallest normalized positive number in the return type; whether errno acquires the value ERANGE is implementation-defined.

7.20.1.4 The strtol, strtoll, strtoul, and strtoull

functions

Synopsis

        #include <stdlib.h>
        long int strtol(
                const char * restrict nptr,
                char ** restrict endptr,
                int base);
        long long int strtoll(
                const char * restrict nptr,
                char ** restrict endptr,
                int base);
        unsigned long int strtoul(
                const char * restrict nptr,
                char ** restrict endptr,
                int base);
        unsigned long long int strtoull(
                const char * restrict nptr,
                char ** restrict endptr,
                int base);

Description

The strtol, strtoll, strtoul, and strtoull functions convert the initial portion of the string pointed to by nptr to long int, long long int, unsigned long int, and unsigned long long int representation, respectively. First, they decompose the input string into three parts: an initial, possibly empty, sequence of white-space characters (as specified by the isspace function), a subject sequence resembling an integer represented in some radix determined by the value of base, and a final string of one or more unrecognized characters, including the terminating null character of the input string. Then, they attempt to convert the subject sequence to an integer, and return the result.

If the value of base is zero, the expected form of the subject sequence is that of an integer constant as described in 6.4.4.1, optionally preceded by a plus or minus sign, but not including an integer suffix. If the value of base is between 2 and 36 (inclusive), the expected form of the subject sequence is a sequence of letters and digits representing an integer with the radix specified by base, optionally preceded by a plus or minus sign, but not including an integer suffix. The letters from a (or A) through z (or Z) are ascribed the values 10 through 35; only letters and digits whose ascribed values are less than that of base are permitted. If the value of base is 16, the characters 0x or 0X may optionally precede the sequence of letters and digits, following the sign if present.

The subject sequence is defined as the longest initial subsequence of the input string, starting with the first non-white-space character, that is of the expected form. The subject sequence contains no characters if the input string is empty or consists entirely of white space, or if the first non-white-space character is other than a sign or a permissible letter or digit.

If the subject sequence has the expected form and the value of base is zero, the sequence of characters starting with the first digit is interpreted as an integer constant according to the rules of 6.4.4.1. If the subject sequence has the expected form and the value of base is between 2 and 36, it is used as the base for conversion, ascribing to each letter its value as given above. If the subject sequence begins with a minus sign, the value resulting from the conversion is negated (in the return type). A pointer to the final string is stored in the object pointed to by endptr, provided that endptr is not a null pointer.

In other than the "C" locale, additional locale- specific subject sequence forms may be accepted.

Returns

The strtol, strtoll, strtoul, and strtoull functions return the converted value, if any. If no conversion could be performed, zero is returned. If the correct value is outside the range of representable values, LONG_MIN, LONG_MAX, LLONG_MIN, LLONG_MAX, ULONG_MAX, or ULLONG_MAX is returned (according to the return type and sign of the value, if any), and the value of the macro ERANGE is stored in errno.

7.20.2 Pseudo-random sequence generation functions

7.20.2.1 The rand function

Synopsis

        #include <stdlib.h>
        int rand(void);

Description

The rand function computes a sequence of pseudo-random integers in the range 0 to RAND_MAX.

The implementation shall behave as if no library function calls the rand function.

Returns

The rand function returns a pseudo-random integer.

Environmental limits

The value of the RAND_MAX macro shall be at least 32767.

7.20.2.2 The srand function

Synopsis

        #include <stdlib.h>
        void srand(unsigned int seed);

Description

The srand function uses the argument as a seed for a new sequence of pseudo-random numbers to be returned by subsequent calls to rand. If srand is then called with the same seed value, the sequence of pseudo-random numbers shall be repeated. If rand is called before any calls to srand have been made, the same sequence shall be generated as when srand is first called with a seed value of 1.

The implementation shall behave as if no library function calls the srand function.

Returns

The srand function returns no value.

EXAMPLE The following functions define a portable implementation of rand and srand. static unsigned long int next = 1;

        int rand(void)   // RAND_MAX assumed to be 32767
        {
                next = next * 1103515245 + 12345;
                return (unsigned int)(next/65536) % 32768;
        }

        void srand(unsigned int seed)
        {
                next = seed;
        }

7.20.3 Memory management functions

The order and contiguity of storage allocated by successive calls to the calloc, malloc, and realloc functions is unspecified. The pointer returned if the allocation succeeds is suitably aligned so that it may be assigned to a pointer to any type of object and then used to access such an object or an array of such objects in the space allocated (until the space is explicitly freed or reallocated). Each such allocation shall yield a pointer to an object disjoint from any other object. The pointer returned points to the start (lowest byte address) of the allocated space. If the space cannot be allocated, a null pointer is returned. If the size of the space requested is zero, the behavior is implementation-defined: either a null pointer is returned, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object. The value of a pointer that refers to freed space is indeterminate.

7.20.3.1 The calloc function

Synopsis

        #include <stdlib.h>
        void *calloc(size_t nmemb, size_t size);

Description

The calloc function allocates space for an array of nmemb objects, each of whose size is size. The space is initialized to all bits zero.238)

Returns

The calloc function returns either a null pointer or a pointer to the allocated space.

7.20.3.2 The free function

Synopsis

        #include <stdlib.h>
        void free(void *ptr);

Description

The free function causes the space pointed to by ptr to be deallocated, that is, made available for further allocation. If ptr is a null pointer, no action occurs. Otherwise, if the argument does not match a pointer earlier returned by the calloc, malloc, or realloc function, or if the space has been deallocated by a call to free or realloc, the behavior is undefined.

Returns

The free function returns no value.

7.20.3.3 The malloc function

Synopsis

        #include <stdlib.h>
        void *malloc(size_t size);

Description

The malloc function allocates space for an object whose size is specified by size and whose value is indeterminate.

Returns

The malloc function returns either a null pointer or a pointer to the allocated space.

7.20.3.4 The realloc function

Synopsis

        #include <stdlib.h>
        void *realloc(void *ptr, size_t size);

Description

The realloc function deallocates the old object pointed to by ptr and returns a pointer to a new object that has the size specified by size. The contents of the new object shall be the same as that of the old object prior to deallocation, up to the lesser of the new and old sizes. Any bytes in the new object beyond the size of the old object have indeterminate values.

If ptr is a null pointer, the realloc function behaves like the malloc function for the specified size. Otherwise, if ptr does not match a pointer earlier returned by the calloc, malloc, or realloc function, or if the space has been deallocated by a call to the free or realloc function, the behavior is undefined. If memory for the new object cannot be allocated, the old object is not deallocated and its value is unchanged.

Returns

The realloc function returns a pointer to the new object (which may have the same value as a pointer to the old object), or a null pointer if the new object could not be allocated.

7.20.4 Communication with the environment

7.20.4.1 The abort function

Synopsis

        #include <stdlib.h>
        void abort(void);

Description

The abort function causes abnormal program termination to occur, unless the signal SIGABRT is being caught and the signal handler does not return. Whether open output streams are flushed or open streams closed or temporary files removed is implementation-defined. An implementation- defined form of the status unsuccessful termination is returned to the host environment by means of the function call raise(SIGABRT).

Returns

The abort function does not return to its caller.

7.20.4.2 The atexit function

Synopsis

        #include <stdlib.h>
        int atexit(void (*func)(void));

Description

The atexit function registers the function pointed to by func, to be called without arguments at normal program termination.

Environmental limits

The implementation shall support the registration of at least 32 functions.

Returns

The atexit function returns zero if the registration succeeds, nonzero if it fails.

Forward references: the exit function (7.20.4.3).

7.20.4.3 The exit function

Synopsis

        #include <stdlib.h>
        void exit(int status);

Description

The exit function causes normal program termination to occur. If more than one call to the exit function is executed by a program, the behavior is undefined.

First, all functions registered by the atexit function are called, in the reverse order of their registration.239)

Next, all open streams with unwritten buffered data are flushed, all open streams are closed, and all files created by the tmpfile function are removed.

Finally, control is returned to the host environment. If the value of status is zero or EXIT_SUCCESS, an implementation-defined form of the status successful termination is returned. If the value of status is EXIT_FAILURE, an implementation-defined form of the status unsuccessful termination is returned. Otherwise the status returned is implementation-defined.

Returns

The exit function cannot return to its caller.

7.20.4.4 The getenv function

Synopsis

        #include <stdlib.h>
        char *getenv(const char *name);

Description

The getenv function searches an environment list, provided by the host environment, for a string that matches the string pointed to by name. The set of environment names and the method for altering the environment list are implementation-defined.

The implementation shall behave as if no library function calls the getenv function.

Returns

The getenv function returns a pointer to a string associated with the matched list member. The string pointed to shall not be modified by the program, but may be overwritten by a subsequent call to the getenv function. If the specified name cannot be found, a null pointer is returned.

7.20.4.5 The system function

Synopsis

        #include <stdlib.h>
        int system(const char *string);

Description

If string is a null pointer, the system function determines whether the host environment has a command processor. If string is not a null pointer, the system function passes the string pointed to by string to that command processor to be executed in a manner which the implementation shall document; this might then cause the program calling system to behave in a non-conforming manner or to terminate.

Returns

If the argument is a null pointer, the system function returns nonzero only if a command processor is available. If the argument is not a null pointer, and the system function does return, it returns an implementation-defined value.

7.20.5 Searching and sorting utilities

These utilities make use of a comparison function to search or sort arrays of unspecified type. Where an argument declared as size_t nmemb specifies the length of the array for a function, nmemb can have the value zero on a call to that function; the comparison function is not called, a search finds no matching element, and sorting performs no rearrangement. Pointer arguments on such a call shall still have valid values, as described in 7.1.4.

The implementation shall ensure that the second argument of the comparison function (when called from bsearch), or both arguments (when called from qsort), are pointers to elements of the array.240) The first argument when called from bsearch shall equal key.

The comparison function shall not alter the contents of the array. The implementation may reorder elements of the

array between calls to the comparison function, but shall not alter the contents of any individual element.

When the same objects (consisting of size bytes, irrespective of their current positions in the array) are passed more than once to the comparison function, the results shall be consistent with one another. That is, for qsort they shall define a total ordering on the array, and for bsearch the same object shall always compare the same way with the key.

A sequence point occurs immediately before and immediately after each call to the comparison function, and also between any call to the comparison function and any movement of the objects passed as arguments to that call.

7.20.5.1 The bsearch function

Synopsis

        #include <stdlib.h>
        void *bsearch(const void *key, const void *base,
                size_t nmemb, size_t size,
                int (*compar)(const void *, const void *));

Description

The bsearch function searches an array of nmemb objects, the initial element of which is pointed to by base, for an element that matches the object pointed to by key. The size of each element of the array is specified by size.

The comparison function pointed to by compar is called with two arguments that point to the key object and to an array element, in that order. The function shall return an integer less than, equal to, or greater than zero if the key object is considered, respectively, to be less than, to match, or to be greater than the array element. The array shall consist of: all the elements that compare less than, all the elements that compare equal to, and all the elements that compare greater than the key object, in that order.241)

Returns

The bsearch function returns a pointer to a matching element of the array, or a null pointer if no match is found. If two elements compare as equal, which element is matched is unspecified.

7.20.5.2 The qsort function

Synopsis

        #include <stdlib.h>
        void qsort(void *base, size_t nmemb, size_t size,
                int (*compar)(const void *, const void *));

Description

The qsort function sorts an array of nmemb objects, the initial element of which is pointed to by base. The size of each object is specified by size.

The contents of the array are sorted into ascending order according to a comparison function pointed to by compar, which is called with two arguments that point to the objects being compared. The function shall return an integer less than, equal to, or greater than zero if the first argument is considered to be respectively less than, equal to, or greater than the second.

If two elements compare as equal, their order in the resulting sorted array is unspecified.

Returns

The qsort function returns no value.

7.20.6 Integer arithmetic functions

7.20.6.1 The abs, labs and llabs functions

Synopsis

        #include <stdlib.h>
        int abs(int j);
        long int labs(long int j);
        long long int llabs(long long int j);

Description

The abs, labs, and llabs functions compute the absolute value of an integer j. If the result cannot be represented, the behavior is undefined.242)

Returns

The abs, labs, and llabs, functions return the absolute value.

7.20.6.2 The div, ldiv, and lldiv functions

Synopsis

        #include <stdlib.h>
        div_t div(int numer, int denom);
        ldiv_t div(long int numer, long int denom);
        lldiv_t div(long long int numer, long long int denom);

Description

The div, ldiv, and lldiv, functions compute numer / denom and numer % denom in a single operation.

Returns

The div, ldiv, and lldiv functions return a structure of type div_t, ldiv_t, and lldiv_t, respectively, comprising both the quotient and the remainder. The structures shall contain (in either order) the members quot (the quotient) and rem (the remainder), each of which have the same type as the arguments numer and denom. If either part of the result cannot be represented, the behavior is undefined.

7.20.7 Multibyte character functions

The behavior of the multibyte character functions is affected by the LC_CTYPE category of the current locale. For a state-dependent encoding, each function is placed into its initial state by a call for which its character pointer argument, s, is a null pointer. Subsequent calls with s as other than a null pointer cause the internal state of the function to be altered as necessary. A call with s as a null pointer causes these functions to return a nonzero value if encodings have state dependency, and zero otherwise.243) Changing the LC_CTYPE category causes the shift state of these functions to be indeterminate.

7.20.7.1 The mblen function

        #include <stdlib.h>
        int mblen(const char *s, size_t n);

Description

If s is not a null pointer, the mblen function determines the number of bytes contained in the multibyte character pointed to by s. Except that the shift state of the mbtowc function is not affected, it is equivalent to

        mbtowc((wchar_t *)0, s, n);

The implementation shall behave as if no library function calls the mblen function.

Returns

If s is a null pointer, the mblen function returns a nonzero or zero value, if multibyte character encodings, respectively, do or do not have state-dependent encodings. If s is not a null pointer, the mblen function either returns 0 (if s points to the null character), or returns the number of bytes that are contained in the multibyte character (if the next n or fewer bytes form a valid multibyte character), or returns -1 (if they do not form a valid multibyte character).

Forward references: the mbtowc function (7.20.7.2).

7.20.7.2 The mbtowc function

Synopsis

        #include <stdlib.h>
        int mbtowc(wchar_t * restrict pwc,
                const char * restrict s,
                size_t n);

Description

If s is not a null pointer, the mbtowc function determines the number of bytes that are contained in the multibyte character pointed to by s. It then determines the code for the value of type wchar_t that corresponds to that multibyte character. (The value of the code corresponding to the null character is zero.) If the multibyte character is valid and pwc is not a null pointer, the mbtowc function stores the code in the object pointed to by pwc. At most n bytes of the array pointed to by s will be examined.

The implementation shall behave as if no library function calls the mbtowc function.

Returns

If s is a null pointer, the mbtowc function returns a nonzero or zero value, if multibyte character encodings, respectively, do or do not have state-dependent encodings. If s is not a null pointer, the mbtowc function either returns 0 (if s points to the null character), or returns the number of bytes that are contained in the converted multibyte character (if the next n or fewer bytes form a valid multibyte character), or returns -1 (if they do not form a valid multibyte character).

In no case will the value returned be greater than n or the value of the MB_CUR_MAX macro.

7.20.7.3 The wctomb function

Synopsis

        #include <stdlib.h>
        int wctomb(char *s, wchar_t wchar);

Description

The wctomb function determines the number of bytes needed to represent the multibyte character corresponding to the code whose value is wchar (including any change in shift state). It stores the multibyte character representation in the array object pointed to by s (if s is not a null pointer). At most MB_CUR_MAX characters are stored. If the value of wchar is zero, the wctomb function is left in the initial shift state.

The header <string.h> declares one type and several functions, and defines one macro useful for manipulating arrays of character type and other objects treated as arrays of character type.245) The type is size_t and the macro is NULL (both described in 7.17). Various methods are used for determining the lengths of the arrays, but in all cases a char * or void * argument points to the initial (lowest addressed) character of the array. If an array is accessed beyond the end of an object, the behavior is undefined.

Where an argument declared as size_t n specifies the length of the array for a function, n can have the value zero on a call to that function. Unless explicitly stated otherwise in the description of a particular function in this subclause, pointer arguments on such a call shall still have valid values, as described in 7.1.4. On such a call, a function that locates a character finds no occurrence, a function that compares two character sequences returns zero, and a function that copies characters copies zero characters.

7.21.2 Copying functions

7.21.2.1 The memcpy function

Synopsis

        #include <string.h>
        void *memcpy(void * restrict s1,
                const void * restrict s2,
                size_t n);

Description

The memcpy function copies n characters from the object pointed to by s2 into the object pointed to by s1. If copying takes place between objects that overlap, the behavior is undefined.

Returns

The memcpy function returns the value of s1.

7.21.2.2 The memmove function

Synopsis

        #include <string.h>
        void *memmove(void *s1, const void *s2, size_t n);

Description

The memmove function copies n characters from the object pointed to by s2 into the object pointed to by s1. Copying takes place as if the n characters from the object pointed to by s2 are first copied into a temporary array of n characters that does not overlap the objects pointed to by s1 and s2, and then the n characters from the temporary array are copied into the object pointed to by s1.

Returns

The memmove function returns the value of s1.

7.21.2.3 The strcpy function

Synopsis

        #include <string.h>
        char *strcpy(char * restrict s1,
                const char * restrict s2);

Description

The strcpy function copies the string pointed to by s2 (including the terminating null character) into the array pointed to by s1. If copying takes place between objects that overlap, the behavior is undefined.

Returns

The strcpy function returns the value of s1.

7.21.2.4 The strncpy function

Synopsis

        #include <string.h>
        char *strncpy(char * restrict s1,
                const char * restrict s2,
                size_t n);

Description

The strncpy function copies not more than n characters (characters that follow a null character are not copied) from the array pointed to by s2 to the array pointed to by s1.246) If copying takes place between objects that overlap, the behavior is undefined.

If the array pointed to by s2 is a string that is shorter than n characters, null characters are appended to the copy in the array pointed to by s1, until n characters in all have been written.

Returns

The strncpy function returns the value of s1.

7.21.3 Concatenation functions

7.21.3.1 The strcat function

Synopsis

        #include <string.h>
        char *strcat(char * restrict s1,
                const char * restrict s2);

Description

The strcat function appends a copy of the string pointed to by s2 (including the terminating null character) to the end of the string pointed to by s1. The initial character of s2 overwrites the null character at the end of s1. If copying takes place between objects that overlap, the behavior is undefined.

Returns

The strcat function returns the value of s1.

7.21.3.2 The strncat function

Synopsis

        #include <string.h>
        char *strncat(char * restrict s1,
                const char * restrict s2,
                size_t n);

Description

The strncat function appends not more than n characters (a null character and characters that follow it are not appended) from the array pointed to by s2 to the end of the string pointed to by s1. The initial character of s2 overwrites the null character at the end of s1. A terminating null character is always appended to the result.247) If copying takes place between objects that overlap, the behavior is undefined.

Returns

The strncat function returns the value of s1.

Forward references: the strlen function (7.21.6.3).

7.21.4 Comparison functions

The sign of a nonzero value returned by the comparison functions memcmp, strcmp, and strncmp is determined by the sign of the difference between the values of the first pair of characters (both interpreted as unsigned char) that differ in the objects being compared.

7.21.4.1 The memcmp function

Synopsis

        #include <string.h>
        int memcmp(const void *s1, const void *s2, size_t n);

Description

The memcmp function compares the first n characters of the object pointed to by s1 to the first n characters of the object pointed to by s2.248)

Returns

The memcmp function returns an integer greater than, equal to, or less than zero, accordingly as the object pointed to by s1 is greater than, equal to, or less than the object pointed to by s2.

7.21.4.2 The strcmp function

Synopsis

        #include <string.h>
        int strcmp(const char *s1, const char *s2);

Description

The strcmp function compares the string pointed to by s1 to the string pointed to by s2.

Returns

The strcmp function returns an integer greater than, equal to, or less than zero, accordingly as the string pointed to by s1 is greater than, equal to, or less than the string pointed to by s2.

7.21.4.3 The strcoll function

Synopsis

        #include <string.h>
        int strcoll(const char *s1, const char *s2);

Description

The strcoll function compares the string pointed to by s1 to the string pointed to by s2, both interpreted as appropriate to the LC_COLLATE category of the current locale.

Returns

The strcoll function returns an integer greater than, equal to, or less than zero, accordingly as the string pointed to by s1 is greater than, equal to, or less than the string pointed to by s2 when both are interpreted as appropriate to the current locale.

7.21.4.4 The strncmp function

Synopsis

        #include <string.h>
        int strncmp(const char *s1, const char *s2, size_t n);

Description

The strncmp function compares not more than n characters (characters that follow a null character are not compared) from the array pointed to by s1 to the array pointed to by s2.

Returns

The strncmp function returns an integer greater than, equal to, or less than zero, accordingly as the possibly null-terminated array pointed to by s1 is greater than, equal to, or less than the possibly null-terminated array pointed to by s2.

7.21.4.5 The strxfrm function

Synopsis

        #include <string.h>
        size_t strxfrm(char * restrict s1,
                const char * restrict s2,
                size_t n);

Description

The strxfrm function transforms the string pointed to by s2 and places the resulting string into the array pointed to by s1. The transformation is such that if the strcmp function is applied to two transformed strings, it returns a value greater than, equal to, or less than zero, corresponding to the result of the strcoll function applied to the same two original strings. No more than n characters are placed into the resulting array pointed to by s1, including the terminating null character. If n is zero, s1 is permitted to be a null pointer. If copying takes place between objects that overlap, the behavior is undefined.

Returns

The strxfrm function returns the length of the transformed string (not including the terminating null character). If the value returned is n or more, the contents of the array pointed to by s1 are indeterminate.

EXAMPLE The value of the following expression is the size of the array needed to hold the transformation of the string pointed to by s.

        1 + strxfrm(NULL, s, 0)

7.21.5 Search functions

7.21.5.1 The memchr function

Synopsis

        #include <string.h>
        void *memchr(const void *s, int c, size_t n);

Description

The memchr function locates the first occurrence of c (converted to an unsigned char) in the initial n characters (each interpreted as unsigned char) of the object pointed to by s.

Returns

The memchr function returns a pointer to the located character, or a null pointer if the character does not occur in the object.

7.21.5.2 The strchr function

Synopsis

        #include <string.h>
        char *strchr(const char *s, int c);

Description

The strchr function locates the first occurrence of c (converted to a char) in the string pointed to by s. The terminating null character is considered to be part of the string.

Returns

The strchr function returns a pointer to the located character, or a null pointer if the character does not occur in the string.

7.21.5.3 The strcspn function

Synopsis

        #include <string.h>
        size_t strcspn(const char *s1, const char *s2);

Description

The strcspn function computes the length of the maximum initial segment of the string pointed to by s1 which consists entirely of characters not from the string pointed to by s2.

Returns

The strcspn function returns the length of the segment.

7.21.5.4 The strpbrk function

Synopsis

        #include <string.h>
        char *strpbrk(const char *s1, const char *s2);

Description

The strpbrk function locates the first occurrence in the string pointed to by s1 of any character from the string pointed to by s2.

Returns

The strpbrk function returns a pointer to the character, or a null pointer if no character from s2 occurs in s1.

7.21.5.5 The strrchr function

Synopsis

        #include <string.h>
        char *strrchr(const char *s, int c);

Description

The strrchr function locates the last occurrence of c (converted to a char) in the string pointed to by s. The terminating null character is considered to be part of the string.

Returns

The strrchr function returns a pointer to the character, or a null pointer if c does not occur in the string.

7.21.5.6 The strspn function

Synopsis

        #include <string.h>
        size_t strspn(const char *s1, const char *s2);

Description

The strspn function computes the length of the maximum initial segment of the string pointed to by s1 which consists entirely of characters from the string pointed to by s2.

Returns

The strspn function returns the length of the segment.

7.21.5.7 The strstr function

Synopsis

        #include <string.h>
        char *strstr(const char *s1, const char *s2);

Description

The strstr function locates the first occurrence in the string pointed to by s1 of the sequence of characters (excluding the terminating null character) in the string pointed to by s2.

Returns

The strstr function returns a pointer to the located string, or a null pointer if the string is not found. If s2 points to a string with zero length, the function returns s1.

7.21.5.8 The strtok function

Synopsis

        #include <string.h>
        char *strtok(char * restrict s1,
                const char * restrict s2);

Description

A sequence of calls to the strtok function breaks the string pointed to by s1 into a sequence of tokens, each of which is delimited by a character from the string pointed to by s2. The first call in the sequence has a non-null first argument; subsequent calls in the sequence have a null first argument. The separator string pointed to by s2 may be different from call to call.

The first call in the sequence searches the string pointed to by s1 for the first character that is not contained in the current separator string pointed to by s2. If no such character is found, then there are no tokens in the string pointed to by s1 and the strtok function returns a null pointer. If such a character is found, it is the start of the first token.

The strtok function then searches from there for a character that is contained in the current separator string. If no such character is found, the current token extends to the end of the string pointed to by s1, and subsequent searches for a token will return a null pointer. If such a character is found, it is overwritten by a null character, which terminates the current token. The strtok function saves a pointer to the following character, from which the next search for a token will start.

Each subsequent call, with a null pointer as the value of the first argument, starts searching from the saved pointer and behaves as described above.

The implementation shall behave as if no library function calls the strtok function.

Returns

The strtok function returns a pointer to the first character of a token, or a null pointer if there is no token.

EXAMPLE 1

        #include <string.h>
        static char str[] = "?a???b,,,#c";
        char *t;

        t = strtok(str, "?");   // t points to the token "a"
        t = strtok(NULL, ",");  // t points to the token "??b"
        t = strtok(NULL, "#,"); // t points to the token "c"
        t = strtok(NULL, "?");  // t is a null pointer

7.21.6 Miscellaneous functions

7.21.6.1 The memset function

Synopsis

        #include <string.h>
        void *memset(void *s, int c, size_t n);

Description

The memset function copies the value of c (converted to an unsigned char) into each of the first n characters of the object pointed to by s.

Returns

The memset function returns the value of s.

7.21.6.2 The strerror function

Synopsis

        #include <string.h>
        char *strerror(int errnum);

Description

The strerror function maps the number in errnum to a message string. Typically, the values for errnum come from errno, but strerror shall map any value of type int to a message.

The implementation shall behave as if no library function calls the strerror function.

Returns

The strerror function returns a pointer to the string, the contents of which are locale-specific. The array pointed to shall not be modified by the program, but may be overwritten by a subsequent call to the strerror function.

7.21.6.3 The strlen function

Synopsis

        #include <string.h>
        size_t strlen(const char *s);

Description

The strlen function computes the length of the string pointed to by s.

Returns

The strlen function returns the number of characters that precede the terminating null character.

7.22 Type-generic math <tgmath.h>

The header <tgmath.h> includes the headers <math.h> and <complex.h> and defines several type-generic macros.

7.22.1 Type-generic macros

Of the <math.h> and <complex.h> functions without an f (float) or l (long double) suffix, several have one or more parameters whose corresponding real type is double. For each such function, except modf, there is a corresponding type-generic macro.249) The parameters whose corresponding real type is double in the function synopsis are generic parameters. Use of the macro invokes a function whose corresponding real type and type domain are determined by the arguments for the generic parameters.250)

Use of the macro invokes a function whose generic parameters have the corresponding real type determined as follows:

-- First, if any argument for generic parameters has type long double, the type determined is long double.

-- Otherwise, if any argument for generic parameters has type double or is of integer type, the type determined is double.

-- Otherwise, the type determined is float.

For each unsuffixed function in <math.h> for which there is a function in <complex.h> with the same name except for a c prefix, the corresponding type-generic macro (for both functions) has the same name as the function in <math.h>. The corresponding type-generic macro for fabs and cabs is fabs.

           <math.h>     <complex.h>    type-generic
           function       function        macro
         -------------  -------------  -------------
            acos           cacos          acos
            asin           casin          asin
            atan           catan          atan
            acosh          cacosh         acosh
            asinh          casinh         asinh
            atanh          catanh         atanh
            cos            ccos           cos
            sin            csin           sin
            tan            ctan           tan
            cosh           ccosh          cosh
            sinh           csinh          sinh
            tanh           ctanh          tanh
            exp            cexp           exp
            log            clog           log
            pow            cpow           pow
            sqrt           csqrt          sqrt
            fabs           cabs           fabs

If at least one argument for a generic parameter is complex, then use of the macro invokes a complex function; otherwise, use of the macro invokes a real function.

For each unsuffixed function in <math.h> without a c- prefixed counterpart in <complex.h>, the corresponding type- generic macro has the same name as the function. These type-generic macros are:

atan2 fma llround remainder cbrt fmax log10 remquo ceil fmin log1p rint copysign fmod log2 round erf frexp logb scalbn erfc hypot lrint scalbln exp2 ilogb lround tgamma expm1 ldexp nearbyint trunc fdim lgamma nextafter floor llrint nexttoward

If all arguments for generic parameters are real, then use of the macro invokes a real function; otherwise, use of the macro results in undefined behavior.

For each unsuffixed function in <complex.h> that is not a c-prefixed counterpart to a function in <math.h>, the corresponding type-generic macro has the same name as the function. These type-generic macros are:

carg conj creal cimag cproj

Use of the macro with any real or complex argument invokes a complex function.

EXAMPLE With the declarations

        #include <tgmath.h>
        int n;
        float f;
        double d;
        long double ld;
        float complex fc;
        double complex dc;
        long double complex ldc;

functions invoked by use of type-generic macros are shown in the following table:

                macro use                          invokes
     --------------------------------  --------------------------------
            exp(n)                      exp(n), the function
            acosh(f)                    acoshf(f)
            sin(d)                      sin(d), the function
            atan(ld)                    atanl(ld)
            log(fc)                     clogf(fc)
            sqrt(dc)                    csqrt(dc)
            pow(ldc, f)                 cpowl(ldc, f)
            remainder(n, n)             remainder(n, n), the function
            nextafter(d, f)             nextafter(d, f), the function
            nexttoward(f, ld)           nexttowardf(f, ld)   
            copysign(n, ld)             copysignl(n, ld)
            ceil(fc)                    undefined behavior
            rint(dc)                    undefined behavior
            fmax(ldc, ld)               undefined behavior
            carg(n)                     carg(n), the function
            cproj(f)                    cprojf(f)
            creal(d)                    creal(d), the function
            cimag(ld)                   cimagl(ld)
            cabs(fc)                    cabsf(fc)
            carg(dc)                    carg(dc), the function
            cproj(ldc)                  cprojl(ldc)

7.23 Date and time <time.h>

7.23.1 Components of time

The header <time.h> defines four macros, and declares several types and functions for manipulating time. Many functions deal with a calendar time that represents the current date (according to the Gregorian calendar) and time. Some functions deal with local time, which is the calendar time expressed for some specific time zone, and with Daylight Saving Time, which is a temporary change in the algorithm for determining local time. The local time zone and Daylight Saving Time are implementation-defined.

The macros defined are NULL (described in 7.17); and

CLOCKS_PER_SEC

which expands to a constant expression with the type clock_t described below, and which is the number per second of the value returned by the clock function.

The types declared are size_t (described in 7.17);

clock_t

and

time_t

which are arithmetic types capable of representing times; and

        struct tm

which holds the components of a calendar time, called the broken-down time.

The tm structure shall contain at least the following members, in any order. The semantics of the members and their normal ranges are expressed in the comments.251)

        int tm_sec;   // seconds after the minute  --  [0, 60]
        int tm_min;   // minutes after the hour  --  [0, 59]
        int tm_hour;  // hours since midnight  --  [0, 23]
        int tm_mday;  // day of the month  --  [1, 31]
        int tm_mon;   // months since January  --  [0, 11]
        int tm_year;  // years since 1900
        int tm_wday;  // days since Sunday  --  [0, 6]
        int tm_yday;  // days since January 1  --  [0, 365]
        int tm_isdst; // Daylight Saving Time flag

The value of tm_isdst is positive if Daylight Saving Time is in effect, zero if Daylight Saving Time is not in effect, and negative if the information is not available.

7.23.2 Time manipulation functions

7.23.2.1 The clock function

Synopsis

        #include <time.h>
        clock_t clock(void);

Description

The clock function determines the processor time used.

Returns

The clock function returns the implementation's best approximation to the processor time used by the program since the beginning of an implementation-defined era related only to the program invocation. To determine the time in seconds, the value returned by the clock function should be divided by the value of the macro CLOCKS_PER_SEC. If the processor time used is not available or its value cannot be represented, the function returns the value (clock_t)-1.252)

7.23.2.2 The difftime function

Synopsis

        #include <time.h>
        double difftime(time_t time1, time_t time0);

Description

The difftime function computes the difference between two calendar times: time1 - time0.

Returns

The difftime function returns the difference expressed in seconds as a double.

7.23.2.3 The mktime function

Synopsis

        #include <time.h>
        time_t mktime(struct tm *timeptr);

Description

The mktime function converts the broken-down time, expressed as local time, in the structure pointed to by timeptr into a calendar time value with the same encoding as that of the values returned by the time function. The original values of the tm_wday and tm_yday components of the structure are ignored, and the original values of the other components are not restricted to the ranges indicated above.253) On successful completion, the values of the tm_wday and tm_yday components of the structure are set appropriately, and the other components are set to represent the specified calendar time, but with their values forced to the ranges indicated above; the final value of tm_mday is not set until tm_mon and tm_year are determined.

If the call is successful, a second call to the mktime function with the resulting struct tm value shall always

leave it unchanged and return the same value as the first call. Furthermore, if the normalized time is exactly representable as a time_t value, then the normalized broken- down time and the broken-down time generated by converting the result of the mktime function by a call to localtime shall be identical.

Returns

The mktime function returns the specified calendar time encoded as a value of type time_t. If the calendar time cannot be represented, the function returns the value (time_t)-1.

EXAMPLE What day of the week is July 4, 2001?

        #include <stdio.h>
        #include <time.h>
        static const char *const wday[] = {
                "Sunday", "Monday", "Tuesday", "Wednesday",
                "Thursday", "Friday", "Saturday", "-unknown-"
        };
        struct tm time_str;
        /* ... */

        time_str.tm_year   = 2001 - 1900;
        time_str.tm_mon    = 7 - 1;
        time_str.tm_mday   = 4;
        time_str.tm_hour   = 0;
        time_str.tm_min    = 0;
        time_str.tm_sec    = 1;
        time_str.tm_isdst  = -1;
        if (mktime(&time_str) == (time_t)-1)
                time_str.tm_wday = 7;
        printf("%s\n", wday[time_str.tm_wday]);

7.23.2.4 The time function

Synopsis

        #include <time.h>
        time_t time(time_t *timer);

Description

The time function determines the current calendar time. The encoding of the value is unspecified.

Returns

The time function returns the implementation's best approximation to the current calendar time. The value (time_t)-1 is returned if the calendar time is not available. If timer is not a null pointer, the return value is also assigned to the object it points to.

7.23.3 Time conversion functions

Except for the strftime function, these functions each return a pointer to one of two types of static objects: a broken-down time structure or an array of char. Execution of any of the functions that return a pointer to one of these object types may overwrite the information in any object of the same type pointed to by the value returned from any previous call to any of them. The implementation shall behave as if no other library functions call these functions.

7.23.3.1 The asctime function

Synopsis

        #include <time.h>
        char *asctime(const struct tm *timeptr);

Description

The asctime function converts the broken-down time in the structure pointed to by timeptr into a string in the form

Sun Sep 16 01:03:52 1973\n\0

using the equivalent of the following algorithm.

        char *asctime(const struct tm *timeptr)
        {
                static const char wday_name[7][3] = {
                        "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"
                };
                static const char mon_name[12][3] = {
                        "Jan", "Feb", "Mar", "Apr", "May", "Jun",
                        "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"
                };
                static char result[26];
                sprintf(result, "%.3s %.3s%3d %.2d:%.2d:%.2d %d\n",
                        wday_name[timeptr->tm_wday],
                        mon_name[timeptr->tm_mon],
                        timeptr->tm_mday, timeptr->tm_hour,
                        timeptr->tm_min, timeptr->tm_sec,
                        1900 + timeptr->tm_year);
                return result;
        }

Returns

The asctime function returns a pointer to the string.

7.23.3.2 The ctime function

Synopsis

        #include <time.h>
        char *ctime(const time_t *timer);

Description

The ctime function converts the calendar time pointed to by timer to local time in the form of a string. It is equivalent to

        asctime(localtime(timer))

Returns

The ctime function returns the pointer returned by the asctime function with that broken-down time as argument.

Forward references: the localtime function (7.23.3.4).

7.23.3.3 The gmtime function

Synopsis

        #include <time.h>
        struct tm *gmtime(const time_t *timer);

Description

The gmtime function converts the calendar time pointed to by timer into a broken-down time, expressed as UTC.

Returns

The gmtime function returns a pointer to the broken- down time, or a null pointer if the specified time cannot be converted to UTC.

7.23.3.4 The localtime function

Synopsis

        #include <time.h>
        struct tm *localtime(const time_t *timer);

Description

The localtime function converts the calendar time pointed to by timer into a broken-down time, expressed as local time.

Returns

The localtime function returns a pointer to the broken- down time, or a null pointer if the specified time cannot be converted to local time.

7.23.3.5 The strftime function

Synopsis

        #include <time.h>
        size_t strftime(char * restrict s,
                size_t maxsize,
                const char * restrict format,
                const struct tm * restrict timeptr);

Description

The strftime function places characters into the array pointed to by s as controlled by the string pointed to by format. The format shall be a multibyte character sequence, beginning and ending in its initial shift state. The format string consists of zero or more conversion specifiers and ordinary multibyte characters. A conversion specifier consists of a % character, possibly followed by an E or O modifier character (described below), followed by a character that determines the behavior of the conversion specifier. All ordinary multibyte characters (including the terminating null character) are copied unchanged into the array. If copying takes place between objects that overlap, the behavior is undefined. No more than maxsize characters are placed into the array.

Each conversion specifier is replaced by appropriate characters as described in the following list. The appropriate characters are determined using the LC_TIME category of the current locale and by the values of zero or more members of the broken-down time structure pointed to by timeptr, as specified in brackets in the description. If any of the specified values is outside the normal range, the characters stored are unspecified. %a is replaced by the locale's abbreviated weekday name. [tm_wday] %A is replaced by the locale's full weekday name. [tm_wday] %b is replaced by the locale's abbreviated month name. [tm_mon] %B is replaced by the locale's full month name. [tm_mon] %c is replaced by the locale's appropriate date and time representation. [all specified in 7.23.1] %C is replaced by the year divided by 100 and truncated to an integer, as a decimal number (00-99). [tm_year] %d is replaced by the day of the month as a decimal number (01-31). [tm_mday] %D is equivalent to ``%m/%d/%y''. [tm_mon, tm_mday, tm_year] %e is replaced by the day of the month as a decimal number (1-31); a single digit is preceded by a space. [tm_mday] %F is equivalent to ``%Y-%m-%d'' (the ISO 8601 date format). [tm_year, tm_mon, tm_mday] %g is replaced by the last 2 digits of the week-based year (see below) as a decimal number (00-99). [tm_year, tm_wday, tm_yday] %G is replaced by the week-based year (see below) as a decimal number (e.g., 1997). [tm_year, tm_wday, tm_yday] %h is equivalent to ``%b''. [tm_mon] %H is replaced by the hour (24-hour clock) as a decimal number (00-23). [tm_hour] %I is replaced by the hour (12-hour clock) as a decimal number (01-12). [tm_hour] %j is replaced by the day of the year as a decimal number (001-366). [tm_yday] %m is replaced by the month as a decimal number (01-12). [tm_mon] %M is replaced by the minute as a decimal number (00-59). [tm_min] %n is replaced by a new-line character. %p is replaced by the locale's equivalent of the AM/PM designations associated with a 12-hour clock. [tm_hour] %r is replaced by the locale's 12-hour clock time. [tm_hour, tm_min, tm_sec] %R is equivalent to ``%H:%M''. [tm_hour, tm_min] %S is replaced by the second as a decimal number (00-60). [tm_sec] %t is replaced by a horizontal-tab character. %T is equivalent to ``%H:%M:%S'' (the ISO 8601 time format). [tm_hour, tm_min, tm_sec] %u is replaced by the ISO 8601 weekday as a decimal number (1-7), where Monday is 1. [tm_wday] %U is replaced by the week number of the year (the first Sunday as the first day of week 1) as a decimal number (00-53). [tm_year, tm_wday, tm_yday] %V is replaced by the ISO 8601 week number (see below) as a decimal number (01-53). [tm_year, tm_wday, tm_yday] %w is replaced by the weekday as a decimal number (0-6), where Sunday is 0. [tm_wday] %W is replaced by the week number of the year (the first Monday as the first day of week 1) as a decimal number (00-53). [tm_year, tm_wday, tm_yday] %x is replaced by the locale's appropriate date representation. [all specified in 7.23.1] %X is replaced by the locale's appropriate time representation. [all specified in 7.23.1] %y is replaced by the last 2 digits of the year as a decimal number (00-99). [tm_year] %Y is replaced by the year as a decimal number (e.g., 1997). [tm_year] %z is replaced by the offset from UTC in the ISO 8601 format ``-0430'' (meaning 4 hours 30 minutes behind UTC, west of Greenwich), or by no characters if no time zone is determinable. [tm_isdst] %Z is replaced by the locale's time zone name or abbreviation, or by no characters if no time zone is determinable. [tm_isdst] %% is replaced by %.

Some conversion specifiers can be modified by the inclusion of an E or O modifier character to indicate an alternative format or specification. If the alternative format or specification does not exist for the current locale, the modifier is ignored. %Ec is replaced by the locale's alternative date and time representation. %EC is replaced by the name of the base year (period) in the locale's alternative representation. %Ex is replaced by the locale's alternative date representation. %EX is replaced by the locale's alternative time representation. %Ey is replaced by the offset from %EC (year only) in the locale's alternative representation. %EY is replaced by the locale's full alternative year representation. %Od is replaced by the day of the month, using the locale's alternative numeric symbols (filled as needed with leading zeros, or with leading spaces if there is no alternative symbol for zero). %Oe is replaced by the day of the month, using the locale's alternative numeric symbols (filled as needed with leading spaces). %OH is replaced by the hour (24-hour clock), using the locale's alternative numeric symbols. %OI is replaced by the hour (12-hour clock), using the locale's alternative numeric symbols. %Om is replaced by the month, using the locale's alternative numeric symbols. %OM is replaced by the minutes, using the locale's alternative numeric symbols. %OS is replaced by the seconds, using the locale's alternative numeric symbols. %Ou is replaced by the ISO 8601 weekday as a number in the locale's alternative representation, where Monday is 1. %OU is replaced by the week number, using the locale's alternative numeric symbols. %OV is replaced by the ISO 8601 week number, using the locale's alternative numeric symbols. %Ow is replaced by the weekday as a number, using the locale's alternative numeric symbols. %OW is replaced by the week number of the year, using the locale's alternative numeric symbols. %Oy is replaced by the last 2 digits of the year, using the locale's alternative numeric symbols.

%g, %G, and %V give values according to the ISO 8601 week-based year. In this system, weeks begin on a Monday and week 1 of the year is the week that includes January 4th, which is also the week that includes the first Thursday of the year, and is also the first week that contains at least four days in the year. If the first Monday of January is the 2nd, 3rd, or 4th, the preceding days are part of the last week of the preceding year; thus, for Saturday 2nd January 1999, %G is replaced by 1998 and %V is replaced by 53. If December 29th, 30th, or 31st is a Monday, it and any following days are part of week 1 of the following year. Thus, for Tuesday 30th December 1997, %G is replaced by 1998 and %V is replaced by 1.

If a conversion specifier is not one of the above, the behavior is undefined.

In the "C" locale, the E and O modifiers are ignored and the replacement strings for the following specifiers are: %a the first three characters of %A. %A one of ``Sunday'', ``Monday'', ... , ``Saturday''. %b the first three characters of %B. %B one of ``January'', ``February'', ... , ``December''. %c equivalent to ``%A %B %d %T %Y''. %p one of ``am'' or ``pm''. %r equivalent to ``%I:%M:%S %p''. %x equivalent to ``%A %B %d %Y''. %X equivalent to %T. %Z implementation-defined.

Returns

If the total number of resulting characters including the terminating null character is not more than maxsize, the strftime function returns the number of characters placed into the array pointed to by s not including the terminating null character. Otherwise, zero is returned and the contents of the array are indeterminate.

7.24 Extended multibyte and wide-character utilities

<wchar.h>

7.24.1 Introduction

The header <wchar.h> declares four data types, one tag, four macros, and many functions.254)

The types declared are wchar_t and size_t (both described in 7.17);

mbstate_t

which is an object type other than an array type that can hold the conversion state information necessary to convert between sequences of multibyte characters and wide characters;

wint_t

described in 7.25.1; and

        struct tm

which is declared as an incomplete structure type, the contents of which are described in 7.23.1.

The macros defined are NULL (described in 7.17);

WCHAR_MAX

which is the maximum value representable by an object of type wchar_t;255)

WCHAR_MIN

which is the minimum value representable by an object of type wchar_t; and

WEOF

described in 7.25.1.

The functions declared are grouped as follows:

-- Functions that perform input and output of wide -- Functions that provide wide-string numeric conversion;

-- Functions that perform general wide-string manipulation;

-- Functions for wide-string date and time conversion; and

-- Functions that provide extended capabilities for conversion between multibyte and wide-character sequences.

Unless explicitly stated otherwise, if the execution of a function described in this subclause causes copying to take place between objects that overlap, the behavior is undefined.

7.24.2 Formatted wide-character input/output functions

The formatted wide-character input/output functions256) shall behave as if there is a sequence point after the actions associated with each specifier.

7.24.2.1 The fwprintf function

Synopsis

        #include <stdio.h>
        #include <wchar.h>
        int fwprintf(FILE * restrict stream,
                const wchar_t * restrict format, ...);

Description

The fwprintf function writes output to the stream pointed to by stream, under control of the wide string pointed to by format that specifies how subsequent arguments are converted for output. If there are insufficient arguments for the format, the behavior is undefined. If the format is exhausted while arguments remain, the excess arguments are evaluated (as always) but are otherwise ignored. The fwprintf function returns when the end of the format string is encountered.

The format is composed of zero or more directives: ordinary wide characters (not %), which are copied unchanged to the output stream; and conversion specifications, each of which results in fetching zero or more subsequent arguments, converting them, if applicable, according to the

corresponding conversion specifier, and then writing the result to the output stream.

Each conversion specification is introduced by the wide character %. After the %, the following appear in sequence:

-- Zero or more flags (in any order) that modify the meaning of the conversion specification.

-- An optional minimum field width. If the converted value has fewer wide characters than the field width, it is padded with spaces (by default) on the left (or right, if the left adjustment flag, described later, has been given) to the field width. The field width takes the form of an asterisk * (described later) or a decimal integer.257)

-- An optional precision that gives the minimum number of digits to appear for the d, i, o, u, x, and X conversions, the number of digits to appear after the decimal-point wide character for a, A, e, E, f, and F conversions, the maximum number of significant digits for the g and G conversions, or the maximum number of wide characters to be written from a string in s conversions. The precision takes the form of a period (.) followed either by an asterisk * (described later) or by an optional decimal integer; if only the period is specified, the precision is taken as zero. If a precision appears with any other conversion specifier, the behavior is undefined.

-- An optional length modifier that specifies the size of the argument.

-- A conversion specifier wide character that specifies the type of conversion to be applied.

The flag wide characters and their meanings are:

- The result of the conversion is left-justified within the field. (It is right-justified if this flag is not specified.)

+ The result of a signed conversion always begins with a plus or minus sign. (It begins with a sign only when a negative value is converted if this flag is not specified.)258)

space If the first wide character of a signed conversion is not a sign, or if a signed conversion results in no wide characters, a space is prefixed to the result. If the space and + flags both appear, the space flag is ignored.

# The result is converted to an ``alternative form''. For o conversion, it increases the precision, if and only if necessary, to force the first digit of the result to be a zero (if the value and precision are both 0, a single 0 is printed). For x (or X) conversion, a nonzero result has 0x (or 0X) prefixed to it. For a, A, e, E, f, F, g, and G conversions, the result of converting a floating-point number always contains a decimal-point wide character, even if no digits follow it. (Normally, a decimal-point wide character appears in the result of these conversions only if a digit follows it.) For g and G conversions, trailing zeros are not removed from the result. For other conversions, the behavior is undefined.

The length modifiers and their meanings are:

or that a following n conversion specifier applies to a pointer to a signed char argument.

L Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to a long double argument. If a length modifier appears with any conversion specifier other than as specified above, the behavior is undefined.

The conversion specifiers and their meanings are:

f,F A double argument representing a floating-point number is converted to decimal notation in the style [-]ddd.ddd, where the number of digits after the decimal-point wide character is equal to the precision specification. If the precision is missing, it is taken as 6; if the precision is zero and the # flag is not specified, no decimal-point wide character appears. If a decimal-point wide character appears, at least one digit appears before it. The value is rounded to the appropriate number of digits.

A double argument representing an infinity is converted in one of the styles [-]inf or [-]infinity -- which style is implementation-defined. A double argument representing a NaN is converted in one of the styles [-]nan or [-]nan(n-wchar-sequence) -- which style, and the meaning of any n-wchar- sequence, is implementation-defined. The F conversion specifier produces INF, INFINITY, or NAN instead of inf, infinity, or nan, respectively.259)

e,E A double argument representing a floating-point number is converted in the style [-]d.ddde+-dd, where there is one digit (which is nonzero if the argument is nonzero) before the decimal-point wide character and the number of digits after it is equal to the precision; if the precision is missing, it is taken as 6; if the precision is zero and the # flag is not specified, no decimal-point wide character appears. The value is rounded to the appropriate number of digits. The E conversion specifier produces a number with E instead of e introducing the exponent. The exponent always contains at least two digits, and only as many more digits as necessary to represent the exponent. If the value is zero, the exponent is zero.

A double argument representing an infinity or NaN is converted in the style of an f or F conversion specifier.

g,G A double argument representing a floating-point number is converted in style f or e (or in style F or E in the case of a G conversion specifier), with the precision specifying the number of significant digits. If the precision is zero, it is taken as 1. The style used depends on the value converted; style e (or E) is used only if the exponent resulting from such a conversion is less than -4 or greater than or equal to the precision. Trailing zeros are removed from the fractional portion of the result unless the # flag is specified; a decimal-point wide character appears only if it is followed by a digit.

A double argument representing an infinity or NaN is converted in the style of an f or F conversion specifier.

a,A A double argument representing a floating-point number is converted in the style [-]0xh.hhhhp+-d, where there is one hexadecimal digit (which is nonzero if the argument is a normalized floating- point number and is otherwise unspecified) before the decimal-point wide character260) and the number of hexadecimal digits after it is equal to the precision; if the precision is missing and FLT_RADIX is a power of 2, then the precision is sufficient for an exact representation of the value; if the precision is missing and FLT_RADIX is not a power of 2, then the precision is sufficient to

distinguish261) values of type double, except that trailing zeros may be omitted; if the precision is zero and the # flag is not specified, no decimal- point wide character appears. The letters abcdef are used for a conversion and the letters ABCDEF for A conversion. The A conversion specifier produces a number with X and P instead of x and p. The exponent always contains at least one digit, and only as many more digits as necessary to represent the decimal exponent of 2. If the value is zero, the exponent is zero.

A double argument representing an infinity or NaN is converted in the style of an f or F conversion specifier.

c If no l length modifier is present, the int argument is converted to a wide character as if by calling btowc and the resulting wide character is written.

If an l length modifier is present, the wint_t argument is converted to wchar_t and written.

s If no l length modifier is present, the argument shall be a pointer to the initial element of a character array containing a multibyte character sequence beginning in the initial shift state. Characters from the array are converted as if by repeated calls to the mbrtowc function, with the conversion state described by an mbstate_t object initialized to zero before the first multibyte character is converted, and written up to (but not including) the terminating null wide character. If the precision is specified, no more than that many wide characters are written. If the precision is not specified or is greater than the size of the converted array, the converted array shall contain a null wide character.

If an l length modifier is present, the argument shall be a pointer to the initial element of an array of wchar_t type. Wide characters from the array are written up to (but not including) a terminating null wide character. If the precision is specified, no more than that many wide characters are written. If the precision is not specified or

is greater than the size of the array, the array shall contain a null wide character.

p The argument shall be a pointer to void. The value of the pointer is converted to a sequence of printing wide characters, in an implementation- defined manner.

n The argument shall be a pointer to signed integer into which is written the number of wide characters written to the output stream so far by this call to fwprintf. No argument is converted, but one is consumed. If the conversion specification includes any flags, a field width, or a precision, the behavior is undefined.

% A % wide character is written. No argument is converted. The complete conversion specification shall be %%.

If a conversion specification is invalid, the behavior is undefined.262) If any argument is not the correct type for the corresponding coversion specification, the behavior is undefined.

#10

In no case does a nonexistent or small field width cause truncation of a field; if the result of a conversion is wider than the field width, the field is expanded to contain the conversion result.

#11

For a and A conversions, if FLT_RADIX is a power of 2, the value is correctly rounded to a hexadecimal floating number with the given precision.

Recommended practice

#12

#13

For e, E, f, F, g, and G conversions, if the number of significant decimal digits is at most DECIMAL_DIG, then the result should be correctly rounded.263) If the number of

fixed-point conversion by the source value as well. significant decimal digits is more than DECIMAL_DIG but the source value is exactly representable with DECIMAL_DIG digits, then the result should be an exact representation with trailing zeros. Otherwise, the source value is bounded by two adjacent decimal strings L < U, both having DECIMAL_DIG significant digits; the value of the resultant decimal string D should satisfy L <= D <= U, with the extra stipulation that the error should have a correct sign for the current rounding direction.

Returns

#14

The fwprintf function returns the number of wide characters transmitted, or a negative value if an output or encoding error occurred.

Environmental limits

#15

The number of wide characters that can be produced by any single conversion shall be at least 4095.

#16

EXAMPLE To print a date and time in the form ``Sunday, July 3, 10:02'' followed by pi to five decimal places:

        #include <math.h>
        #include <stdio.h>
        #include <wchar.h>
        /* ... */
        wchar_t *weekday, *month;  // pointers to wide strings
        int day, hour, min;
        fwprintf(stdout, L"%ls, %ls %d, %.2d:%.2d\n",
                weekday, month, day, hour, min);
        fwprintf(stdout, L"pi = %.5f\n", 4 * atan(1.0));

Forward references: the btowc function (7.24.6.1.1), the mbrtowc function (7.24.6.3.2).

7.24.2.2 The fwscanf function

Synopsis

        #include <stdio.h>
        #include <wchar.h>
        int fwscanf(FILE * restrict stream,
                const wchar_t * restrict format, ...);

Description

The fwscanf function reads input from the stream pointed to by stream, under control of the wide string pointed to by format that specifies the admissible input sequences and how they are to be converted for assignment, using subsequent arguments as pointers to the objects to receive the converted input. If there are insufficient arguments for the format, the behavior is undefined. If the format is exhausted while arguments remain, the excess arguments are evaluated (as always) but are otherwise ignored.

The format is composed of zero or more directives: one or more white-space wide characters, an ordinary wide character (neither % nor a white-space wide character), or a conversion specification. Each conversion specification is introduced by the wide character %. After the %, the following appear in sequence:

-- An optional assignment-suppressing wide character *.

-- An optional nonzero decimal integer that specifies the maximum field width (in wide characters).

-- An optional length modifier that specifies the size of the receiving object.

-- A conversion specifier wide character that specifies the type of conversion to be applied.

The fwscanf function executes each directive of the format in turn. If a directive fails, as detailed below, the function returns. Failures are described as input failures (due to the occurrence of an encoding error or the unavailability of input characters), or matching failures (due to inappropriate input).

A directive composed of white-space wide character(s) is executed by reading input up to the first non-white-space wide character (which remains unread), or until no more wide characters can be read.

A directive that is an ordinary wide character is executed by reading the next wide character of the stream. If that wide character differs from the directive, the directive fails and the differing and subsequent wide characters remain unread.

A directive that is a conversion specification defines a set of matching input sequences, as described below for each specifier. A conversion specification is executed in the following steps:

Input white-space wide characters (as specified by the iswspace function) are skipped, unless the specification includes a [, c, or n specifier.264)

An input item is read from the stream, unless the specification includes an n specifier. An input item is defined as the longest sequence of input wide characters which does not exceed any specified field width and which is, or is a prefix of, a matching input sequence. The first wide character, if any, after the input item remains unread. If the length of the input item is zero, the execution of the directive fails; this condition is a matching failure unless end-of-file, an encoding error, or a read error prevented input from the stream, in which case it is an input failure.

#10

Except in the case of a % specifier, the input item (or, in the case of a %n directive, the count of input wide characters) is converted to a type appropriate to the conversion specifier. If the input item is not a matching sequence, the execution of the directive fails: this condition is a matching failure. Unless assignment suppression was indicated by a *, the result of the conversion is placed in the object pointed to by the first argument following the format argument that has not already received a conversion result. If this object does not have an appropriate type, or if the result of the conversion cannot be represented in the object, the behavior is undefined.

#11

The length modifiers and their meanings are:

hh Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to signed char or unsigned char.

h Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to short int or unsigned short int.

ll (ell-ell) Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to long long int or unsigned

long long int.

j Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to intmax_t or uintmax_t.

z Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to size_t or the corresponding signed integer type.

t Specifies that a following d, i, o, u, x, X, or n conversion specifier applies to an argument with type pointer to ptrdiff_t or the corresponding unsigned integer type.

L Specifies that a following a, A, e, E, f, F, g, or G conversion specifier applies to an argument with type pointer to long double.

If a length modifier appears with any conversion specifier other than as specified above, the behavior is undefined.

#12

The conversion specifiers and their meanings are:

d Matches an optionally signed decimal integer, whose format is the same as expected for the subject sequence of the wcstol function with the value 10 for the base argument. The corresponding argument shall be a pointer to signed integer.

i Matches an optionally signed integer, whose format is the same as expected for the subject sequence of the wcstol function with the value 0 for the base argument. The corresponding argument shall be a pointer to signed integer.

o Matches an optionally signed octal integer, whose format is the same as expected for the subject sequence of the wcstoul function with the value 8 for the base argument. The corresponding argument shall be a pointer to unsigned integer.

u Matches an optionally signed decimal integer, whose format is the same as expected for the subject sequence of the wcstoul function with the value 10 for the base argument. The corresponding argument shall be a pointer to unsigned integer.

x Matches an optionally signed hexadecimal integer, whose format is the same as expected for the subject sequence of the wcstoul function with the value 16 for the base argument. The corresponding argument shall be a pointer to unsigned integer. a,e,f,g Matches an optionally signed floating-point number, infinity, or NaN, whose format is the same as expected for the subject sequence of the wcstod function. The corresponding argument shall be a pointer to floating.

c Matches a sequence of wide characters of exactly the number specified by the field width (1 if no field width is present in the directive).

If no l length modifier is present, characters from the input field are converted as if by repeated calls to the wcrtomb function, with the conversion state described by an mbstate_t object initialized to zero before the first wide character is converted. The corresponding argument shall be a pointer to the initial element of a character array large enough to accept the sequence. No null character is added.

If an l length modifier is present, the corresponding argument shall be a pointer to the initial element of an array of wchar_t large enough to accept the sequence. No null wide character is added.

s Matches a sequence of non-white-space wide characters.

If an l length modifier is present, the corresponding argument shall be a pointer to the initial element of an array of wchar_t large enough to accept the sequence and the terminating null wide character, which will be added automatically.

[ Matches a nonempty sequence of wide characters from a set of expected characters (the scanset).

The conversion specifier includes all subsequent wide characters in the format string, up to and including the matching right bracket (]). The wide characters between the brackets (the scanlist) compose the scanset, unless the wide character after the left bracket is a circumflex (^), in which case the scanset contains all wide characters that do not appear in the scanlist between the circumflex and the right bracket. If the conversion specifier begins with [] or [^], the right bracket wide character is in the scanlist and the next following right bracket wide character is the matching right bracket that ends the specification; otherwise the first following right bracket wide character is the one that ends the specification. If a - wide character is in the scanlist and is not the first, nor the second where the first wide character is a ^, nor the last character, the behavior is implementation-defined.

p Matches an implementation-defined set of sequences, which should be the same as the set of sequences that may be produced by the %p conversion of the fwprintf function. The corresponding argument shall be a pointer to a pointer to void. The input item is converted to a pointer value in an implementation-defined manner. If the input item is a value converted earlier during the same program execution, the pointer that results shall compare equal to that value; otherwise the behavior of the %p conversion is undefined.

n No input is consumed. The corresponding argument shall be a pointer to signed integer into which is to be written the number of wide characters read from the input stream so far by this call to the fwscanf function. Execution of a %n directive does not increment the assignment count returned at the completion of execution of the fwscanf function. No argument is converted, but one is consumed. If the conversion specification includes an assignment- suppressing wide character or a field width, the behavior is undefined.

% Matches a single % wide character; no conversion or assignment occurs. The complete conversion specification shall be %%.

#13

If a conversion specification is invalid, the behavior is undefined.265)

#14

The conversion specifiers A, E, F, G, and X are also valid and behave the same as, respectively, a, e, f, g, and x.

#15

If end-of-file is encountered during input, conversion is terminated. If end-of-file occurs before any wide characters matching the current directive have been read (other than leading white space, where permitted), execution of the current directive terminates with an input failure; otherwise, unless execution of the current directive is terminated with a matching failure, execution of the following directive (other than %n, if any) is terminated with an input failure.

#16

Trailing white space (including new-line wide characters) is left unread unless matched by a directive. The success of literal matches and suppressed assignments is not directly determinable other than via the %n directive.

#17

If conversion terminates on a conflicting input wide character, the offending input wide character is left unread in the input stream.266)

Returns

#18

The fwscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

#19

EXAMPLE 1 The call:

        #include <stdio.h>
        #include <wchar.h>
        /* ... */
        int n, i; float x; wchar_t name[50];
        n = fwscanf(stdin, L"%d%f%ls", &i, &x, name);

with the input line:

25 54.32E-1 thompson

will assign to n the value 3, to i the value 25, to x the value 5.432, and to name the sequence thompson\0.

#20

EXAMPLE 2 The call:

        #include <stdio.h>
        #include <wchar.h>
        /* ... */
        int i; float x; double y;
        fwscanf(stdin, L"%2d%f%*d %lf", &i, &x, &y);

with input:

56789 0123 56a72

will assign to i the value 56 and to x the value 789.0, will skip past 0123, and will assign to y the value 56.0. The next wide character read from the input stream will be a.

Forward references: the wcstod, wcstof, and wcstold functions (7.24.4.1.1), the wcstol, wcstoll, wcstoul, and wcstoull functions (7.24.4.1.2), the wcrtomb function (7.24.6.3.3).

7.24.2.3 The swprintf function

Synopsis

        #include <wchar.h>
        int swprintf(wchar_t * restrict s,
                size_t n,
                const wchar_t * restrict format, ...);

Description

The swprintf function is equivalent to fwprintf, except that the argument s specifies an array of wide characters into which the generated output is to be written, rather than written to a stream. No more than n wide characters are written, including a terminating null wide character, which is always added (unless n is zero).

Returns

The swprintf function returns the number of wide characters written in the array, not counting the terminating null wide character, or a negative value if an encoding error occurred or if n or more wide characters were requested to be written.

7.24.2.4 The swscanf function

Synopsis

        #include <wchar.h>
        int swscanf(const wchar_t * restrict s,
                const wchar_t * restrict format, ...);

Description

The swscanf function is equivalent to fwscanf, except that the argument s specifies a wide string from which the input is to be obtained, rather than from a stream. Reaching the end of the wide string is equivalent to encountering end-of-file for the fwscanf function.

Returns

The swscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the swscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

7.24.2.5 The vfwprintf function

Synopsis

        #include <stdarg.h>
        #include <stdio.h>
        #include <wchar.h>
        int vfwprintf(FILE * restrict stream,
                const wchar_t * restrict format,
                va_list arg);

Description

The vfwprintf function is equivalent to fwprintf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vfwprintf function does not invoke the va_end macro.267)

Returns

The vfwprintf function returns the number of wide characters transmitted, or a negative value if an output or encoding error occurred.

EXAMPLE The following shows the use of the vfwprintf function in a general error-reporting routine.

        #include <stdarg.h>
        #include <stdio.h>
        #include <wchar.h>

        void error(char *function_name, wchar_t *format, ...)
        {
                va_list args;

                va_start(args, format);
                // print out name of function causing error
                fwprintf(stderr, L"ERROR in %s: ", function_name);
                // print out remainder of message
                vfwprintf(stderr, format, args);
                va_end(args);
        }

7.24.2.6 The vfwscanf function

Synopsis

        #include <stdarg.h>
        #include <stdio.h>
        #include <wchar.h>
        int vfwscanf(FILE * restrict stream,
                const wchar_t * restrict format,
                va_list arg);

Description

The vfwscanf function is equivalent to fwscanf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vfwscanf function does not invoke the va_end macro.267)

Returns

The vfwscanf function returns the value of the macro

EOF if an input failure occurs before any conversion. Otherwise, the vfwscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

7.24.2.7 The vswprintf function

Synopsis

        #include <stdarg.h>
        #include <wchar.h>
        int vswprintf(wchar_t * restrict s,
                size_t n,
                const wchar_t * restrict format,
                va_list arg);

Description

The vswprintf function is equivalent to swprintf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vswprintf function does not invoke the va_end macro.267)

Returns

The vswprintf function returns the number of wide characters written in the array, not counting the terminating null wide character, or a negative value if an encoding error occurred or if n or more wide characters were requested to be generated.

7.24.2.8 The vswscanf function

Synopsis

        #include <stdarg.h>
        #include <wchar.h>
        int vswscanf(const wchar_t * restrict s,
                const wchar_t * restrict format,
                va_list arg);

Description

The vswscanf function is equivalent to swscanf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vswscanf function does not invoke the va_end macro.267)

Returns

The vswscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the vswscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

7.24.2.9 The vwprintf function

Synopsis

        #include <stdarg.h>
        #include <wchar.h>
        int vwprintf(const wchar_t * restrict format,
                va_list arg);

Description

The vwprintf function is equivalent to wprintf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vwprintf function does not invoke the va_end macro.267)

Returns

The vwprintf function returns the number of wide characters transmitted, or a negative value if an output or encoding error occurred.

7.24.2.10 The vwscanf function

Synopsis

        #include <stdarg.h>
        #include <wchar.h>
        int vwscanf(const wchar_t * restrict format,
                va_list arg);

Description

The vwscanf function is equivalent to wscanf, with the variable argument list replaced by arg, which shall have been initialized by the va_start macro (and possibly subsequent va_arg calls). The vwscanf function does not invoke the va_end macro.267)

Returns

The vwscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the vwscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

7.24.2.11 The wprintf function

Synopsis

        #include <wchar.h>
        int wprintf(const wchar_t * restrict format, ...);

Description

The wprintf function is equivalent to fwprintf with the argument stdout interposed before the arguments to wprintf.

Returns

The wprintf function returns the number of wide characters transmitted, or a negative value if an output or encoding error occurred.

7.24.2.12 The wscanf function

Synopsis

        #include <wchar.h>
        int wscanf(const wchar_t * restrict format, ...);

Description

The wscanf function is equivalent to fwscanf with the argument stdin interposed before the arguments to wscanf.

Returns

The wscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the wscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

7.24.3 Wide-character input/output functions

7.24.3.1 The fgetwc function

Synopsis

        #include <stdio.h>
        #include <wchar.h>
        wint_t fgetwc(FILE *stream);

Description

If a next wide character is present from the input stream pointed to by stream, the fgetwc function obtains that wide character and advances the associated file position indicator for the stream (if defined).

Returns

The fgetwc function returns the next wide character from the input stream pointed to by stream. If the stream is at end-of-file, the end-of-file indicator for the stream is set and fgetwc returns WEOF. If a read error occurs, the error indicator for the stream is set and fgetwc returns WEOF. If an encoding error occurs (including too few bytes), the value of the macro EILSEQ is stored in errno and fgetwc returns WEOF.268)

7.24.3.2 The fgetws function

Synopsis

        #include <stdio.h>
        #include <wchar.h>
        wchar_t *fgetws(wchar_t * restrict s,
                int n, FILE * restrict stream);

Description

The fgetws function reads at most one less than the number of wide characters specified by n from the stream pointed to by stream into the array pointed to by s. No additional wide characters are read after a new-line wide character (which is retained) or after end-of-file. A null

wide character is written immediately after the last wide character read into the array.

Returns

The fgetws function returns s if successful. If end- of-file is encountered and no characters have been read into the array, the contents of the array remain unchanged and a null pointer is returned. If a read or encoding error occurs during the operation, the array contents are indeterminate and a null pointer is returned.

7.24.3.3 The fputwc function

Synopsis

        #include <stdio.h>
        #include <wchar.h>
        wint_t fputwc(wchar_t c, FILE *stream);

Description

The fputwc function writes the wide character specified by c to the output stream pointed to by stream, at the position indicated by the associated file position indicator for the stream (if defined), and advances the indicator appropriately. If the file cannot support positioning requests, or if the stream was opened with append mode, the character is appended to the output stream.

Returns

The fputwc function returns the wide character written. If a write error occurs, the error indicator for the stream is set and fputwc returns WEOF. If an encoding error occurs, the value of the macro EILSEQ is stored in errno and fputwc returns WEOF.

7.24.3.4 The fputws function

Synopsis

        #include <stdio.h>
        #include <wchar.h>
        int fputws(const wchar_t * restrict s,
                FILE * restrict stream);

Description

The fputws function writes the wide string pointed to by s to the stream pointed to by stream. The terminating null wide character is not written.

Returns

The fputws function returns EOF if a write or encoding error occurs; otherwise, it returns a nonnegative value.

7.24.3.5 The fwide function

Synopsis

        #include <stdio.h>
        #include <wchar.h>
        int fwide(FILE *stream, int mode);

Description

The fwide function determines the orientation of the stream pointed to by stream. If mode is greater than zero, the function first attempts to make the stream wide oriented. If mode is less than zero, the function first attempts to make the stream byte oriented.269) Otherwise, mode is zero and the function does not alter the orientation of the stream.

Returns

The fwide function returns a value greater than zero if, after the call, the stream has wide orientation, a value less than zero if the stream has byte orientation, or zero if the stream has no orientation.

7.24.3.6 The getwc function

Synopsis

        #include <stdio.h>
        #include <wchar.h>
        wint_t getwc(FILE *stream);

Description

The getwc function is equivalent to fgetwc, except that if it is implemented as a macro, it may evaluate stream more than once, so the argument should never be an expression

with side effects.

Returns

The getwc function returns the next wide character from the input stream pointed to by stream, or WEOF.

7.24.3.7 The getwchar function

Synopsis

        #include <wchar.h>
        wint_t getwchar(void);

Description

The getwchar function is equivalent to getwc with the argument stdin.

Returns

The getwchar function returns the next wide character from the input stream pointed to by stdin, or WEOF.

7.24.3.8 The putwc function

Synopsis

        #include <stdio.h>
        #include <wchar.h>
        wint_t putwc(wchar_t c, FILE *stream);

Description

The putwc function is equivalent to fputwc, except that if it is implemented as a macro, it may evaluate stream more than once, so that argument should never be an expression with side effects.

Returns

The putwc function returns the wide character written, or WEOF.

7.24.3.9 The putwchar function

Synopsis

        #include <wchar.h>
        wint_t putwchar(wchar_t c);

Description

The putwchar function is equivalent to putwc with the second argument stdout.

Returns

The putwchar function returns the character written, or WEOF.

7.24.3.10 The ungetwc function

Synopsis

        #include <stdio.h>
        #include <wchar.h>
        wint_t ungetwc(wint_t c, FILE *stream);

Description

The ungetwc function pushes the wide character specified by c back onto the input stream pointed to by stream. Pushed-back wide characters will be returned by subsequent reads on that stream in the reverse order of their pushing. A successful intervening call (with the stream pointed to by stream) to a file positioning function (fseek, fsetpos, or rewind) discards any pushed-back wide characters for the stream. The external storage corresponding to the stream is unchanged.

One wide character of pushback is guaranteed, even if the call to the ungetwc function follows just after a call to a formatted wide character input function fwscanf, vfwscanf, vwscanf, or wscanf. If the ungetwc function is called too many times on the same stream without an intervening read or file positioning operation on that stream, the operation may fail.

If the value of c equals that of the macro WEOF, the operation fails and the input stream is unchanged.

A successful call to the ungetwc function clears the end-of-file indicator for the stream. The value of the file position indicator for the stream after reading or discarding all pushed-back wide characters is the same as it was before the wide characters were pushed back. For a text or binary stream, the value of its file position indicator after a