Programming languages -- C 1. Scope [#1] 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. [#2] 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. ____________________ 1) This International Standard is designed to promote the portability of C programs among a variety of data- processing systems. It is intended for use by implementors and programmers. 1 General 1 2 Committee Draft -- August 3, 1998 WG14/N843 2. Normative references [#1] 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. [#2] ISO/IEC 646:1991, Information technology -- ISO 7-bit | coded character set for information interchange. [#3] ISO/IEC 2382-1:1993, Information technology -- Vocabulary -- Part 1: Fundamental terms. [#4] ISO 4217:1995, Codes for the representation of currencies and funds. [#5] ISO 8601:1988, Data elements and interchange formats -- Information interchange -- Representation of dates and times. [#6] ISO/IEC 10646:1993, Information technology -- | Universal Multiple-Octet Coded Character Set (UCS). | [#7] IEC 60559:1989, Binary floating-point arithmetic for | microprocessor systems, second edition (previously | designated IEC 559:1989). 3. Terms and definitions [#1] 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 [#1] alignment requirement that objects of a particular type be located on storage boundaries with addresses that are particular multiples of a byte address 1 General 3.1 WG14/N843 Committee Draft -- August 3, 1998 3 3.2 [#1] 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 [#1] bit unit of data storage in the execution environment large enough to hold an object that may have one of two values [#2] NOTE It need not be possible to express the address of each individual bit of an object. 3.4 [#1] byte addressable unit of data storage large enough to hold any member of the basic character set of the execution environment [#2] NOTE 1 It is possible to express the address of each individual byte of an object uniquely. [#3] 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 [#1] character bit representation that fits in a byte * 3.6 [#1] constraints restrictions, both syntactic and semantic, by which the exposition of language elements is to be interpreted 3.7 [#1] 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 [#1] diagnostic message message belonging to an implementation-defined subset of the implementation's message output 3.9 [#1] forward references references to later subclauses of this International 3.2 General 3.9 4 Committee Draft -- August 3, 1998 WG14/N843 Standard that contain additional information relevant to this subclause 3.10 [#1] 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 [#1] implementation-defined behavior unspecified behavior where each implementation documents how the choice is made [#2] EXAMPLE An example of implementation-defined behavior is the propagation of the high-order bit when a signed integer is shifted right. 3.12 [#1] implementation limits restrictions imposed upon programs by the implementation 3.13 [#1] locale-specific behavior behavior that depends on local conventions of nationality, culture, and language that each implementation documents [#2] 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 [#1] multibyte character sequence of one or more bytes representing a member of the extended character set of either the source or the execution environment [#2] NOTE The extended character set is a superset of the basic character set. 3.15 [#1] object region of data storage in the execution environment, the contents of which can represent values | [#2] NOTE When referenced, an object may be interpreted as having a particular type; see 6.3.2.1. 3.9 General 3.15 WG14/N843 Committee Draft -- August 3, 1998 5 3.16 [#1] 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 [#1] 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 [#1] 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 [#2] 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). [#3] EXAMPLE An example of undefined behavior is the behavior on integer overflow. 3.19 [#1] unspecified behavior behavior where this International Standard provides two or more possibilities and imposes no requirements on which is chosen in any instance [#2] 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). 3.16 General 3.19 6 Committee Draft -- August 3, 1998 WG14/N843 4. Conformance [#1] 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. [#2] 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''. [#3] 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. [#4] 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. [#5] 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. [#6] 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 , , , , , , and . A conforming implementation may have extensions (including additional library functions), provided they do not alter ____________________ 2) A strictly conforming program can use conditional | features (such as those in annex F) provided the use is | guarded by a #ifdef directive with the appropriate macro. | For example: #ifdef __STDC_IEC_559__ /* FE_UPWARD defined */ /* ... */ fesetround(FE_UPWARD); /* ... */ #endif 4 General 4 WG14/N843 Committee Draft -- August 3, 1998 7 the behavior of any strictly conforming program.3) [#7] A conforming program is one that is acceptable to a conforming implementation.4) [#8] 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 (7.7), | alternative spellings (7.9), sizes of integer types (7.10), variable arguments (7.15), boolean type and values (7.16), common | definitions (7.17), integer types | (7.18). ____________________ 3) This implies that a conforming implementation reserves no identifiers other than those explicitly reserved in this International Standard. 4) Strictly conforming programs are intended to be maximally portable among conforming implementations. Conforming programs may depend upon nonportable features of a conforming implementation. 4 General 4 8 Committee Draft -- August 3, 1998 WG14/N843 5. Environment [#1] 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 [#1] 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 [#1] 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. | ____________________ 5) Implementations shall behave as if these separate phases occur, even though many are typically folded together in practice. 5 Environment 5.1.1.2 WG14/N843 Committee Draft -- August 3, 1998 9 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, escape sequence, and universal character name 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 ____________________ 6) As described in 6.4, the process of dividing a source file's characters into preprocessing tokens is context- dependent. For example, see the handling of < within a #include preprocessing directive. 5.1.1.2 Environment 5.1.1.2 10 Committee Draft -- August 3, 1998 WG14/N843 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 [#1] 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) [#2] 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 [#1] 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. ____________________ 7) The intent is that an implementation should identify the nature of, and where possible localize, each violation. Of course, an implementation is free to produce any number of diagnostics as long as a valid program is still correctly translated. It may also successfully translate an invalid program. 5.1.1.2 Environment 5.1.2 WG14/N843 Committee Draft -- August 3, 1998 11 Forward references: initialization (6.7.8). 5.1.2.1 Freestanding environment [#1] 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. [#2] The effect of program termination in a freestanding environment is implementation-defined. 5.1.2.2 Hosted environment [#1] A hosted environment need not be provided, but shall conform to the following specifications if present. 5.1.2.2.1 Program startup [#1] 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. [#2] 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 ____________________ 8) Thus, int can be replaced by a typedef name defined as int, or the type of argv can be written as char ** argv, and so on. 5.1.2 Environment 5.1.2.2.1 12 Committee Draft -- August 3, 1998 WG14/N843 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 [#1] 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 [#1] 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 [#1] The semantic descriptions in this International Standard describe the behavior of an abstract machine in which issues of optimization are irrelevant. [#2] Accessing a volatile object, modifying an object, modifying a file, or calling a function that does any of ____________________ 9) In accordance with 6.2.4, objects with automatic storage duration declared in main will no longer have storage guaranteed to be reserved in the former case even where they would in the latter. 5.1.2.2.1 Environment 5.1.2.3 WG14/N843 Committee Draft -- August 3, 1998 13 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.) [#3] 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). [#4] 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. [#5] 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). [#6] 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. ____________________ 10)The IEC 60559 standard for binary floating-point arithmetic requires certain user-accessible status flags and control modes. Floating-point operations implicitly set the status flags; modes affect result values of floating-point operations. Implementations that support such floating-point state are required to regard changes to it as side effects -- see annex F for details. The floating-point environment library provides a programming facility for indicating when these side effects matter, freeing the implementations in other cases. 5.1.2.3 Environment 5.1.2.3 14 Committee Draft -- August 3, 1998 WG14/N843 -- 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. [#7] What constitutes an interactive device is implementation-defined. [#8] More stringent correspondences between abstract and actual semantics may be defined by each implementation. [#9] 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. 5.1.2.3 Environment 5.1.2.3 WG14/N843 Committee Draft -- August 3, 1998 15 [#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; | 5.1.2.3 Environment 5.1.2.3 16 Committee Draft -- August 3, 1998 WG14/N843 [#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); or 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 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 5.1.2.3 Environment 5.1.2.3 WG14/N843 Committee Draft -- August 3, 1998 17 ;), 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). 5.1.2.3 Environment 5.1.2.3 18 Committee Draft -- August 3, 1998 WG14/N843 5.2 Environmental considerations 5.2.1 Character sets [#1] 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. [#2] 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. [#3] 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 5.2 Environment 5.2.1 WG14/N843 Committee Draft -- August 3, 1998 19 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. [#4] 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 [#1] 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. [#2] EXAMPLE The following source line printf("Eh???/n"); becomes (after replacement of the trigraph sequence ??/) printf("Eh?\n"); 5.2.1.2 Multibyte characters [#1] 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. ____________________ 11)The trigraph sequences enable the input of characters that are not defined in the Invariant Code Set as described in ISO/IEC 646, which is a subset of the seven- | bit US ASCII code set. | 5.2.1 Environment 5.2.1.2 20 Committee Draft -- August 3, 1998 WG14/N843 -- 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. [#2] 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 [#1] 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 printable 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. [#2] 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. 5.2.1.2 Environment 5.2.2 WG14/N843 Committee Draft -- August 3, 1998 21 \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. [#3] 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 [#1] 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 [#1] 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. 5.2.2 Environment 5.2.4 22 Committee Draft -- August 3, 1998 WG14/N843 5.2.4.1 Translation limits [#1] 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 compound statements, iteration statements, and selection statements -- 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 ____________________ 12)Implementations should avoid imposing fixed translation limits whenever possible. 5.2.4.1 Environment 5.2.4.1 WG14/N843 Committee Draft -- August 3, 1998 23 -- 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 [#1] A conforming implementation shall document all the limits specified in this subclause, which are specified in | the headers and . Additional limits are | specified in . 5.2.4.2.1 Sizes of integer types [#1] 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 5.2.4.1 Environment 5.2.4.2.1 24 Committee Draft -- August 3, 1998 WG14/N843 -- 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 [#2] 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 5.2.4.2.1 Environment 5.2.4.2.1 WG14/N843 Committee Draft -- August 3, 1998 25 CHAR_BIT. 5.2.4.2.2 Characteristics of floating types [#1] 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) [#2] 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 [#3] 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) [#4] The accuracy of the floating-point operations (+, -, *, /) and of the library functions in and | that return floating-point results is implementation | defined. The implementation may state that the accuracy is | unknown. [#5] All integer values in the header, except ____________________ 13)See 6.2.5. 14)The floating-point model is intended to clarify the description of each floating-point characteristic and does not require the floating-point arithmetic of the implementation to be identical. 15)Although they are stored in floating types, infinities and NaNs are not floating-point numbers. 16)IEC 60559:1989 specifies quiet and signaling NaNs. For implementations that do not support IEC 60559:1989, the terms quiet NaN and signaling NaN are intended to apply to encodings with similar behavior. 5.2.4.2.1 Environment 5.2.4.2.2 26 Committee Draft -- August 3, 1998 WG14/N843 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. [#6] 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. [#7] 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. ____________________ 17)Evaluation of FLT_ROUNDS correctly reflects any execution-time change of rounding mode through the function fesetround in . 18)The evaluation method determines evaluation formats of expressions involving all floating types, not just real types. For example, if FLT_EVAL_METHOD is 1, then the product of two float _Complex operands is represented in the double _Complex format, and its parts are evaluated to double. 5.2.4.2.2 Environment 5.2.4.2.2 WG14/N843 Committee Draft -- August 3, 1998 27 [#8] 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, | 5.2.4.2.2 Environment 5.2.4.2.2 28 Committee Draft -- August 3, 1998 WG14/N843 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)| 5.2.4.2.2 Environment 5.2.4.2.2 WG14/N843 Committee Draft -- August 3, 1998 29 FLT_MAX_10_EXP +37 DBL_MAX_10_EXP +37 LDBL_MAX_10_EXP +37 [#9] 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 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 5.2.4.2.2 Environment 5.2.4.2.2 30 Committee Draft -- August 3, 1998 WG14/N843 [#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 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 (7.3), mathematics (7.12), | integer types (7.18). ____________________ 19)The floating-point model in that standard sums powers of b from zero, so the values of the exponent limits are one less than shown here. 5.2.4.2.2 Environment 5.2.4.2.2 WG14/N843 Committee Draft -- August 3, 1998 31 6. Language 6.1 Notation [#1] 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. [#2] A summary of the language syntax is given in annex A. 6.2 Concepts 6.2.1 Scopes of identifiers [#1] 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. | [#2] 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.) [#3] 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). Label names shall be unique within a function. [#4] 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 6 Language 6.2.1 32 Committee Draft -- August 3, 1998 WG14/N843 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 } that closes 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. [#5] 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. [#6] Two identifiers have the same scope if and only if their scopes terminate at the same point. [#7] 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 [#1] 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. [#2] In the set of translation units and libraries that constitutes an entire program, each declaration of a | 6.2.1 Language 6.2.2 WG14/N843 Committee Draft -- August 3, 1998 33 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. | [#3] 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) [#4] 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. [#5] 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. [#6] 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. [#7] 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). ____________________ 20)A function declaration can contain the storage-class specifier static only if it is at file scope; see 6.7.1. 21)As specified in 6.2.1, the later declaration might hide the prior declaration. 6.2.2 Language 6.2.2 34 Committee Draft -- August 3, 1998 WG14/N843 6.2.3 Name spaces of identifiers [#1] 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 [#1] 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. [#2] 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) [#3] An object whose identifier is declared with no linkage and without the storage-class specifier static has automatic storage duration. For objects that do 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.) | WG14/N843 Committee Draft -- August 3, 1998 35 [#4] For objects that do 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) | [#5] 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 [#1] 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). [#2] An object declared as type _Bool is large enough to | store the values 0 and 1. | [#3] 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 ____________________ 22)There is only one name space for tags even though three are possible. 23)The term constant address means that two pointers to the object constructed at possibly different times will compare equal. The address may be different during two different executions of the same program. In the case of a volatile object, the last store need not be explicit in the program. 24)Leaving the innermost block containing the declaration, or jumping to a point in that block or an embedded block prior to the declaration, leaves the scope of the declaration. 6.2.4 Language 6.2.5 36 Committee Draft -- August 3, 1998 WG14/N843 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. [#4] 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) | [#5] 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 ). [#6] 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. [#7] 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. [#8] 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. [#9] 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.27) A computation involving unsigned operands ____________________ 25)Implementation-defined keywords shall have the form of an identifier reserved for any use as described in 7.1.3. 26)Therefore, any statement in this Standard about signed integer types also applies to the extended signed integer types. 6.2.5 Language 6.2.5 WG14/N843 Committee Draft -- August 3, 1998 37 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 unsigned integer type. [#10] There are three real floating types, designated as float, double, and long double. 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.28) 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.29) [#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, ____________________ 27)The same representation and alignment requirements are meant to imply interchangeability as arguments to functions, return values from functions, and members of unions. 28)A specification for imaginary types is in informative annex G. 29)An implementation may define new keywords that provide alternative ways to designate a basic (or any other) type; this does not violate the requirement that all basic types be different. Implementation-defined keywords shall have the form of an identifier reserved for any use as described in 7.1.3. 6.2.5 Language 6.2.5 38 Committee Draft -- August 3, 1998 WG14/N843 representation, and behavior as either signed char or unsigned char.30) [#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.31) 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 ____________________ 30)CHAR_MIN, defined in , will have one of the values 0 or SCHAR_MIN, and this can be used to distinguish the two options. Irrespective of the choice made, char is a separate type from the other two and is not compatible with either. 31)Since object types do not include incomplete types, an array of incomplete type cannot be constructed. 6.2.5 Language 6.2.5 WG14/N843 Committee Draft -- August 3, 1998 39 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.32) [#22] Each arithmetic type belongs to one typedomain. 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,33) corresponding to the combinations of one, two, or ____________________ 32)Note that aggregate type does not include union type because an object with union type can only contain one member at a time. 6.2.5 Language 6.2.5 40 Committee Draft -- August 3, 1998 WG14/N843 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.27) 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.27) 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 [#1] The representations of all types are unspecified except as stated in this subclause. 6.2.6.1 General [#1] 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. | [#2] Values stored in objects of type unsigned char shall be ____________________ 33)See 6.7.3 regarding qualified array and function types. 6.2.5 Language 6.2.6.1 WG14/N843 Committee Draft -- August 3, 1998 41 represented using a pure binary notation.34) [#3] 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. [#4] 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.35) Such a representation is called a trap representation. [#5] 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.36) 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. [#6] 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 ____________________ 34)A positional representation for integers that uses the binary digits 0 and 1, in which the values represented by successive bits are additive, begin with 1, and are multiplied by successive integral powers of 2, except perhaps the bit with the highest position. (Adapted from the American National Dictionary for Information Processing Systems.) A byte contains CHAR_BIT bits, and the values of type unsigned char range from 0 to 2CHAR_BIT-1. 35)Thus an automatic variable can be initialized to a trap representation without causing undefined behavior, but the value of the variable cannot be used until a proper value is stored in it. 36)Thus, for example, structure assignment may be implemented element-at-a-time or via memcpy. 6.2.6.1 Language 6.2.6.1 42 Committee Draft -- August 3, 1998 WG14/N843 members take unspecified values, but the value of the union object shall not thereby become a trap representation. [#7] 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 [#1] 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.37) [#2] 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. [#3] The values of any padding bits are unspecified.37) A valid (non-trap) object representation of a signed integer ____________________ 37)Some combinations of padding bits might generate trap representations, for example, if one padding bit is a parity bit. Regardless, no arithmetic operation on valid values can generate a trap representation other than as part of an exception such as an overflow, and this cannot occur with unsigned types. All other combinations of padding bits are alternative object representations of the value specified by the value bits. 6.2.6.1 Language 6.2.6.2 WG14/N843 Committee Draft -- August 3, 1998 43 type where the sign bit is zero is a valid object representation of the corresponding unsigned type, and shall represent the same value. [#4] 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 [#1] 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.38) 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. [#2] All declarations that refer to the same object or function shall have compatible type; otherwise, the behavior is undefined. [#3] 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. ____________________ 38)Two types need not be identical to be compatible. 6.2.6.2 Language 6.2.7 44 Committee Draft -- August 3, 1998 WG14/N843 -- 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. [#4] For an identifier with internal or external linkage declared in a scope in which a prior declaration of that identifier is visible,39) if the prior declaration specifies internal or external linkage, the type of the identifier at the later declaration becomes the composite type. [#5] 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). ____________________ 39)As specified in 6.2.1, the later declaration might hide the prior declaration. 6.2.7 Language 6.2.7 WG14/N843 Committee Draft -- August 3, 1998 45 6.3 Conversions [#1] 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. [#2] 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 | [#1] 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. -- 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 6.3 Language 6.3.1.1 46 Committee Draft -- August 3, 1998 WG14/N843 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. [#2] 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.40) All other types are unchanged by the integer promotions. [#3] 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 | [#1] 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 [#1] 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. [#2] 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. ____________________ 40)The integer promotions are applied only: as part of the | usual arithmetic conversions, to certain argument expressions, to the operands of the unary +, -, and ~ operators, and to both operands of the shift operators, as specified by their respective subclauses. 6.3.1.1 Language 6.3.1.3 WG14/N843 Committee Draft -- August 3, 1998 47 [#3] 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 [#1] When a finite value of real floating type is converted | to 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.41) [#2] When a value of integer type is converted to 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 [#1] When a float is promoted to double or long double, or a double is promoted to long double, its value is unchanged. [#2] When a double is demoted to float or a long double to double or float, 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 value, chosen in an implementation-defined manner. 6.3.1.6 Complex types [#1] 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 [#1] 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. [#2] When a value of complex type is converted to a real ____________________ 41)The remaindering operation performed when a value of integer type is converted to unsigned type need not be performed when a value of real floating type is converted to unsigned type. Thus, the range of portable real floating values is (-1, Utype_MAX+1). 6.3.1.3 Language 6.3.1.7 48 Committee Draft -- August 3, 1998 WG14/N843 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 [#1] 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 determined by the operator. 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.42) 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 ____________________ 42)For example, addition of a double _Complex and a float entails just the conversion of the float operand to double (and yields a double _Complex result). 6.3.1.7 Language 6.3.1.8 WG14/N843 Committee Draft -- August 3, 1998 49 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. [#2] 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.43) 6.3.2 Other operands 6.3.2.1 Lvalues and function designators [#1] An lvalue is an expression with an object type or an | incomplete type other than void;44) 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. [#2] Except when it is the operand of the sizeof operator, the unary & operator, the ++ operator, the -- operator, or ____________________ 43)The cast and assignment operators are still required to perform their specified conversions as described in 6.3.1.4 and 6.3.1.5. 44)The name ``lvalue'' comes originally from the assignment expression E1 = E2, in which the left operand E1 is required to be a (modifiable) lvalue. It is perhaps better considered as representing an object ``locator value''. What is sometimes called ``rvalue'' is in this International Standard described as the ``value of an expression''. An obvious example of an lvalue is an identifier of an object. As a further example, if E is a unary expression that is a pointer to an object, *E is an lvalue that designates the object to which E points. 6.3.1.8 Language 6.3.2.1 50 Committee Draft -- August 3, 1998 WG14/N843 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. [#3] 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. [#4] A function designator is an expression that has function type. Except when it is the operand of the sizeof operator45) 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 (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 [#1] 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 [#1] 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. ____________________ 45)Because this conversion does not occur, the operand of the sizeof operator remains a function designator and violates the constraint in 6.5.3.4. 6.3.2.1 Language 6.3.2.3 WG14/N843 Committee Draft -- August 3, 1998 51 [#2] 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. [#3] An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant.46) If a null pointer constant is assigned to or compared for equality to a pointer, the constant is converted to a pointer of that type. Such a pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function. [#4] Conversion of a null pointer to another pointer type yields a null pointer of that type. Any two null pointers shall compare equal. [#5] An integer may be converted to any pointer type. The result is implementation-defined, might not be properly aligned, and might not point to an entity of the referenced type.47) [#6] Any pointer type may be converted to an integer type; 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. [#7] 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 aligned48) 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. ____________________ 46)The macro NULL is defined in as a null pointer constant; see 7.17. 47)The mapping functions for converting a pointer to an integer or an integer to a pointer are intended to be consistent with the addressing structure of the execution environment. 48)In general, the concept ``correctly aligned'' is | transitive: if a pointer to type A is correctly aligned for a pointer to type B, which in turn is correctly aligned for a pointer to type C, then a pointer to type A is correctly aligned for a pointer to type C. 6.3.2.3 Language 6.3.2.3 52 Committee Draft -- August 3, 1998 WG14/N843 [#8] 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). 6.3.2.3 Language 6.3.2.3 WG14/N843 Committee Draft -- August 3, 1998 53 6.4 Lexical elements Syntax [#1] token: keyword identifier constant string-literal punctuator preprocessing-token: header-name identifier pp-number character-constant string-literal punctuator each universal-character-name that cannot be one of the above| each non-white-space character that cannot be one of the above Constraints [#2] 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 [#3] 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.49) 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 ____________________ 49)An additional category, placemarkers, is used internally in translation phase 4 (see 6.10.3.3); it cannot occur in source files. 6.4 Language 6.4 54 Committee Draft -- August 3, 1998 WG14/N843 token only as part of a header name or between the quotation characters in a character constant or string literal. [#4] 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. [#5] 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. [#6] 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 [#1] 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 6.4 Language 6.4.1 WG14/N843 Committee Draft -- August 3, 1998 55 Semantics [#2] 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 [#1] 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 | [#2] 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. | Lower-case and upper-case letters are distinct. There is no | specific limit on the maximum length of an identifier. | [#3] 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 I.50) The initial ____________________ 50)On systems in which linkers cannot accept extended characters, an encoding of the universal character name may be used in forming valid external identifiers. For example, some otherwise unused character or sequence of characters may be used to encode the \u in a universal character name. Extended characters may produce a long external identifier. 6.4.1 Language 6.4.2.1 56 Committee Draft -- August 3, 1998 WG14/N843 nondigit 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. [#4] 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 [#5] 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. [#6] 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 [#1] 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.51) * [#2] 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. [#3] EXAMPLE Consider the code fragment: ____________________ 51)Note that since the name __func__ is reserved for any use by the implementation (7.1.3), if any other identifier is explicitly declared using the name __func__, the behavior is undefined. 6.4.2.1 Language 6.4.2.2 WG14/N843 Committee Draft -- August 3, 1998 57 #include 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 [#1] universal-character-name: \u hex-quad \U hex-quad hex-quad hex-quad: hexadecimal-digit hexadecimal-digit hexadecimal-digit hexadecimal-digit Constraints [#2] 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 [#3] 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 [#4] 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. 6.4.2.2 Language 6.4.3 58 Committee Draft -- August 3, 1998 WG14/N843 6.4.4 Constants Syntax [#1] constant: integer-constant floating-constant enumeration-constant character-constant Constraints [#2] The value of a constant shall be in the range of representable values for its type. Semantics [#3] Each constant has a type, determined by its form and value, as detailed later. 6.4.4.1 Integer constants Syntax [#1] 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: 0 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 6.4.4 Language 6.4.4.1 WG14/N843 Committee Draft -- August 3, 1998 59 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 [#2] 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. [#3] 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 [#4] 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. [#5] The type of an integer constant is the first of the corresponding list in which its value can be represented. 6.4.4.1 Language 6.4.4.1 60 Committee Draft -- August 3, 1998 WG14/N843 || | || | 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 [#1] 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 6.4.4.1 Language 6.4.4.2 WG14/N843 Committee Draft -- August 3, 1998 61 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 [#2] 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 [#3] The significand part is interpreted as a (decimal or 6.4.4.2 Language 6.4.4.2 62 Committee Draft -- August 3, 1998 WG14/N843 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. [#4] 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 [#5] 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. [#6] 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.52) 6.4.4.3 Enumeration constants Syntax [#1] enumeration-constant: identifier Semantics [#2] An identifier declared as an enumeration constant has type int. Forward references: enumeration specifiers (6.7.2.2). ____________________ 52)The specification for the library functions recommends more accurate conversion than required for floating constants (see 7.20.1.3). 6.4.4.2 Language 6.4.4.3 WG14/N843 Committee Draft -- August 3, 1998 63 6.4.4.4 Character constants Syntax [#1] 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 [#2] 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. [#3] 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: 6.4.4.4 Language 6.4.4.4 64 Committee Draft -- August 3, 1998 WG14/N843 single quote ' \' double quote " \" question mark ? \? backslash \ \\ octal character \octal digits hexadecimal character \xhexadecimal digits [#4] 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 \\. [#5] 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. [#6] 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. [#7] Each octal or hexadecimal escape sequence is the longest sequence of characters that can constitute the escape sequence. [#8] In addition, graphic 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.53) Constraints [#9] 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. ____________________ 53)The semantics of these characters were discussed in 5.2.2. If any other character follows a backslash, the result is not a token and a diagnostic is required. See ``future language directions'' (6.11.1). 6.4.4.4 Language 6.4.4.4 WG14/N843 Committee Draft -- August 3, 1998 65 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 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 6.4.4.4 Language 6.4.4.4 66 Committee Draft -- August 3, 1998 WG14/N843 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 (7.17), * the mbtowc function (7.20.7.2). 6.4.5 String literals Syntax [#1] 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 [#2] 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. [#3] 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 [#4] 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. [#5] In translation phase 7, a byte or code of value zero is appended to each multibyte character sequence that results 6.4.4.4 Language 6.4.5 WG14/N843 Committee Draft -- August 3, 1998 67 from a string literal or literals.54) 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. [#6] 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. [#7] 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 (7.17). 6.4.6 Punctuators Syntax [#1] punctuator: one of [ ] ( ) { } . -> ++ -- & * + - ~ ! / % << >> < > <= >= == != ^ | && || ? : ; ... = *= /= %= += -= <<= >>= &= ^= |= , # ## | <: :> <% %> %: %:%: | ____________________ 54)A character string literal need not be a string (see 7.1.1), because a null character may be embedded in it by a \0 escape sequence. 6.4.5 Language 6.4.6 68 Committee Draft -- August 3, 1998 WG14/N843 Semantics [#2] 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. [#3] In all aspects of the language, these six tokens <: :> <% %> %: %:%: behave, respectively, the same as these six tokens [ ] { } # ## except for their spelling.55) Forward references: expressions (6.5), declarations (6.7), preprocessing directives (6.10), statements (6.8). 6.4.7 Header names Syntax [#1] header-name: "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 " ____________________ 55)Thus [ and <: behave differently when ``stringized'' (see 6.10.3.2), but can otherwise be freely interchanged. 6.4.6 Language 6.4.7 WG14/N843 Committee Draft -- August 3, 1998 69 Semantics [#2] 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. [#3] 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.56) A header name preprocessing token is recognized only within a #include preprocessing directive. [#4] 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 [#1] pp-number: digit . digit pp-number digit pp-number identifier-nondigit | pp-number e sign pp-number E sign pp-number p sign pp-number P sign pp-number . Description ____________________ 56)Thus, sequences of characters that resemble escape sequences cause undefined behavior. 6.4.7 Language 6.4.8 70 Committee Draft -- August 3, 1998 WG14/N843 [#2] 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. [#3] Preprocessing number tokens lexically include all floating and integer constant tokens. Semantics [#4] 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 [#1] 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.57) [#2] 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. [#3] 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; | ____________________ 57)Thus, /* ... */ comments do not nest. 6.4.8 Language 6.4.9 WG14/N843 Committee Draft -- August 3, 1998 71 6.5 Expressions [#1] 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. [#2] 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.58) Informative annex D presents an algorithm for | determining whether an expression or set of expressions | meets these requirements. [#3] The grouping of operators and operands is indicated by | the syntax.59) 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. [#4] 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 ____________________ 58)This paragraph renders undefined statement expressions such as i = ++i + 1; a[i++] = i; | while allowing i = i + 1; a[i] = i; | 59)The syntax specifies the precedence of operators in the evaluation of an expression, which is the same as the order of the major subclauses of this subclause, highest precedence first. Thus, for example, the expressions allowed as the operands of the binary + operator (6.5.6) | are those expressions defined in 6.5.1 through 6.5.6. The exceptions are cast expressions (6.5.4) as operands of unary operators (6.5.3), and an operand contained between any of the following pairs of operators: grouping parentheses () (6.5.1), subscripting brackets [] (6.5.2.1), function-call parentheses () (6.5.2.2), and the conditional operator ?: (6.5.15). Within each major subclause, the operators have the same precedence. Left- or right-associativity is indicated in each subclause by the syntax for the expressions discussed therein. 6.5 Language 6.5 72 Committee Draft -- August 3, 1998 WG14/N843 types. [#5] 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. [#6] The effective type of an object for an access to its stored value is the declared type of the object, if any.60) | 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. [#7] An object shall have its stored value accessed only by an lvalue expression that has one of the following types:61) -- 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. [#8] A floating expression may be contracted, that is, evaluated as though it were an atomic operation, thereby ____________________ 60)Allocated objects have no declared type. 61)The intent of this list is to specify those circumstances in which an object may or may not be aliased. 6.5 Language 6.5 WG14/N843 Committee Draft -- August 3, 1998 73 omitting rounding errors implied by the source code and the expression evaluation method.62) The FP_CONTRACT pragma in provides a way to disallow contracted expressions. Otherwise, whether and how expressions are contracted is implementation-defined.63) 6.5.1 Primary expressions Syntax [#1] primary-expr: identifier constant string-literal ( expression ) Semantics [#2] 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).64) [#3] A constant is a primary expression. Its type depends on its form and value, as detailed in 6.4.4. [#4] A string literal is a primary expression. It is an lvalue with type as detailed in 6.4.5. [#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. ____________________ 62)A contracted expression might also omit the raising of floating-point exception flags. 63)This license is specifically intended to allow implementations to exploit fast machine instructions that combine multiple C operators. As contractions potentially undermine predictability, and can even decrease accuracy for containing expressions, their use needs to be well-defined and clearly documented. 64)Thus, an undeclared identifier is a violation of the syntax. 6.5 Language 6.5.1 74 Committee Draft -- August 3, 1998 WG14/N843 Forward references: declarations (6.7). 6.5.2 Postfix operators Syntax [#1] postfix-expr: primary-expr postfix-expr [ expression ] postfix-expr ( argument-expression-list-opt ) postfix-expr . identifier postfix-expr -> identifier postfix-expr ++ postfix-expr -- ( type-name ) { initializer-list } ( type-name ) { initializer-list , } argument-expression-list: assignment-expr argument-expression-list , assignment-expr 6.5.2.1 Array subscripting Constraints [#1] 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 [#2] 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). [#3] 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). 6.5.1 Language 6.5.2.1 WG14/N843 Committee Draft -- August 3, 1998 75 [#4] 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 [#1] The expression that denotes the called function65) shall have type pointer to function returning void or returning an object type other than an array type. [#2] 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 [#3] 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. [#4] 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.66) | ____________________ 65)Most often, this is the result of converting an identifier that is a function designator. 6.5.2.1 Language 6.5.2.2 76 Committee Draft -- August 3, 1998 WG14/N843 [#5] 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. [#6] 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. [#7] 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. [#8] 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 ____________________ 66)A function may change the values of its parameters, but these changes cannot affect the values of the arguments. On the other hand, it is possible to pass a pointer to an object, and the function may change the value of the object pointed to. A parameter declared to have array or function type is converted to a parameter with a pointer type as described in 6.9.1. 6.5.2.2 Language 6.5.2.2 WG14/N843 Committee Draft -- August 3, 1998 77 declarator. [#9] 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 [#1] 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. [#2] 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 [#3] 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. [#4] 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 6.5.2.2 Language 6.5.2.3 78 Committee Draft -- August 3, 1998 WG14/N843 to which the first expression points, and is an lvalue.67) 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. [#5] 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.68) 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. [#6] 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. [#7] 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 | ____________________ 67)If &E is a valid pointer expression (where & is the ``address-of'' operator, which generates a pointer to its operand), the expression (&E)->MOS is the same as E.MOS. 68)The ``byte orders'' for scalar types are invisible to isolated programs that do not indulge in type punning (for example, by assigning to one member of a union and inspecting the storage by accessing another member that is an appropriately sized array of character type), but have to be accounted for when conforming to externally imposed storage layouts. 6.5.2.3 Language 6.5.2.3 WG14/N843 Committee Draft -- August 3, 1998 79 [#8] 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). 6.5.2.3 Language 6.5.2.3 80 Committee Draft -- August 3, 1998 WG14/N843 6.5.2.4 Postfix increment and decrement operators Constraints [#1] 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 [#2] 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. [#3] 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 [#1] The type name shall specify an object type or an array of unknown size. [#2] No initializer shall attempt to provide a value for an object not contained within the entire unnamed object specified by the compound literal. [#3] If the compound literal occurs outside the body of a function, the initializer list shall consist of constant expressions. Semantics [#4] 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.69) [#5] 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. [#6] The value of the compound literal is that of an unnamed WG14/N843 Committee Draft -- August 3, 1998 81 object initialized by the initializer list. The object has static storage duration if and only if the compound literal occurs outside the body of a function; otherwise, it has automatic storage duration associated with the enclosing block. [#7] All the semantic rules and constraints for initializer lists in 6.7.8 are applicable to compound literals.70) [#8] String literals, and compound literals with const- qualified types, need not designate distinct objects.71) [#9] 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 ____________________ 69)Note that this differs from a cast expression. For example, a cast specifies a conversion to scalar types or void only, and the result of a cast expression is not an lvalue. 70)For example, subobjects without explicit initializers are initialized to zero. 71)This allows implementations to share storage for string literals and constant compound literals with the same or overlapping representations. 6.5.2.5 Language 6.5.2.5 82 Committee Draft -- August 3, 1998 WG14/N843 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); 6.5.2.5 Language 6.5.2.5 WG14/N843 Committee Draft -- August 3, 1998 83 [#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; | while (j < 2) | q = p, p = &((struct s){ j++ }); | return p == q && q->i == 1; | } The function f() always returns the value 1. [#17] Note that if a for loop were used instead of a while | loop, 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 [#1] 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 [#1] 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 [#2] The value of the operand of the prefix ++ operator is 6.5.2.5 Language 6.5.3.1 84 Committee Draft -- August 3, 1998 WG14/N843 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. [#3] 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 [#1] 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. [#2] The operand of the unary * operator shall have pointer type. Semantics [#3] The result of the unary & (address-of) operator is a pointer to the object or function designated by 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. [#4] 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.72) Forward references: storage-class specifiers (6.7.1), structure and union specifiers (6.7.2.1). 6.5.3.1 Language 6.5.3.2 WG14/N843 Committee Draft -- August 3, 1998 85 6.5.3.3 Unary arithmetic operators Constraints [#1] The operand of the unary + or - operator shall have arithmetic type; of the ~ operator, integer type; of the ! operator, scalar type. Semantics [#2] 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. [#3] 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. [#4] 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. [#5] 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 (7.7), sizes of integer types (7.10). ____________________ 72)Thus, &*E is equivalent to E (even if E is a null | pointer), and &(E1[E2]) to ((E1)+(E2)). It is always | true that if E is a function designator or an lvalue that is a valid operand of the unary & operator, *&E is a function designator or an lvalue equal to E. If *P is an lvalue and T is the name of an object pointer type, *(T)P is an lvalue that has a type compatible with that to which T points. Among the invalid values for dereferencing a pointer by the unary * operator are a null pointer, an address inappropriately aligned for the type of object pointed to, and the address of an automatic storage duration object when execution of the block with which the object is associated has terminated. 6.5.3.3 Language 6.5.3.3 86 Committee Draft -- August 3, 1998 WG14/N843 6.5.3.4 The sizeof operator Constraints [#1] 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 lvalue that designates a bit-field object. Semantics [#2] 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. [#3] 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.73) 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. [#4] The value of the result is implementation-defined, and its type (an unsigned integer type) is size_t, defined in | the header. [#5] 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. [#6] EXAMPLE 2 Another use of the sizeof operator is to compute the number of elements in an array: ____________________ 73)When applied to a parameter declared to have array or function type, the sizeof operator yields the size of the pointer obtained by converting as in 6.3.2.1; see 6.9.1. 6.5.3.3 Language 6.5.3.4 WG14/N843 Committee Draft -- August 3, 1998 87 sizeof array / sizeof array[0] [#7] 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 (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 [#1] cast-expr: unary-expr ( type-name ) cast-expr Constraints [#2] 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. [#3] 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 [#4] Preceding an expression by a parenthesized type name converts the value of the expression to the named type. This construction is called a cast.74) A cast that specifies no conversion has no effect on the type or value of an expression.75) 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). 88 Committee Draft -- August 3, 1998 WG14/N843 6.5.5 Multiplicative operators Syntax [#1] multiplicative-expr: cast-expr multiplicative-expr * cast-expr multiplicative-expr / cast-expr multiplicative-expr % cast-expr Constraints [#2] Each of the operands shall have arithmetic type. The operands of the % operator shall have integer type. Semantics [#3] The usual arithmetic conversions are performed on the operands. If either operand has complex type, the result has complex type. [#4] The result of the binary * operator is the product of the operands. [#5] 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. [#6] When integers are divided, the result of the / operator is the algebraic quotient with any fractional part discarded.76) If the quotient a/b is representable, the expression (a/b)*b + a%b shall equal a. ____________________ 75)If the value of the expression is represented with greater precision or range than required by the type named by the cast (6.3.1.8), then the cast specifies a conversion even if the type of the expression is the same as the named type. 76)This is often called ``truncation toward zero''. 6.5.5 Language 6.5.5 WG14/N843 Committee Draft -- August 3, 1998 89 6.5.6 Additive operators Syntax [#1] additive-expr: multiplicative-expr additive-expr + multiplicative-expr additive-expr - multiplicative-expr Constraints [#2] 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.) [#3] 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 [#4] If both operands have arithmetic type, the usual arithmetic conversions are performed on them. If either operand has complex type, the result has complex type. [#5] The result of the binary + operator is the sum of the operands. [#6] The result of the binary - operator is the difference resulting from the subtraction of the second operand from the first. [#7] 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. [#8] 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 6.5.6 Language 6.5.6 90 Committee Draft -- August 3, 1998 WG14/N843 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. [#9] 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 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.77) | ____________________ 77)Another way to approach pointer arithmetic is first to convert the pointer(s) to character pointer(s): In this scheme the integer expression added to or subtracted from the converted pointer is first multiplied by the size of the object originally pointed to, and the resulting pointer is converted back to the original type. For pointer subtraction, the result of the difference between the character pointers is similarly divided by the size of the object originally pointed to. When viewed in this way, an implementation need only provide one extra byte (which may overlap another object in the program) just after the end of the object in order to satisfy the ``one past the last element'' requirements. 6.5.6 Language 6.5.6 WG14/N843 Committee Draft -- August 3, 1998 91 [#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 (7.17). 6.5.7 Bitwise shift operators Syntax [#1] shift-expr: additive-expr shift-expr << additive-expr shift-expr >> additive-expr Constraints [#2] Each of the operands shall have integer type. Semantics [#3] 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. [#4] 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. [#5] The result of E1 >> E2 is E1 right-shifted E2 bit positions. If E1 has an unsigned type or if E1 has a signed 6.5.6 Language 6.5.7 92 Committee Draft -- August 3, 1998 WG14/N843 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 [#1] relational-expr: shift-expr relational-expr < shift-expr relational-expr > shift-expr relational-expr <= shift-expr relational-expr >= shift-expr Constraints [#2] 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 [#3] If both of the operands have arithmetic type, the usual arithmetic conversions are performed. [#4] 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. [#5] 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 6.5.7 Language 6.5.8 WG14/N843 Committee Draft -- August 3, 1998 93 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. [#6] 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.78) The result has type int. 6.5.9 Equality operators Syntax [#1] equality-expr: relational-expr equality-expr == relational-expr equality-expr != relational-expr Constraints [#2] 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 [#3] The == (equal to) and != (not equal to) operators are | analogous to the relational operators except for their lower precedence.79) 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. ____________________ 78)The expression a>= &= ^= |= Constraints [#2] An assignment operator shall have a modifiable lvalue as its left operand. Semantics [#3] 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. [#4] 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 [#1] One of the following shall hold:82) -- 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 ____________________ 82)The asymmetric appearance of these constraints with respect to type qualifiers is due to the conversion (specified in 6.3.2.1) that changes lvalues to ``the value of the expression'' which removes any type qualifiers from the type category of the expression. 6.5.16 Language 6.5.16.1 100 Committee Draft -- August 3, 1998 WG14/N843 by the right; or -- the left operand is a pointer and the right is a null pointer constant. Semantics [#2] 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. [#3] 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. [#4] 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. [#5] 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. | [#6] EXAMPLE 3 Consider the fragment: | 6.5.16.1 Language 6.5.16.1 WG14/N843 Committee Draft -- August 3, 1998 101 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 [#1] 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. [#2] For the other operators, each operand shall have arithmetic type consistent with those allowed by the corresponding binary operator. Semantics [#3] 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 [#1] expression: assignment-expr expression , assignment-expr Semantics [#2] 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.83) 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. ____________________ 83)A comma operator does not yield an lvalue. 6.5.16.1 Language 6.5.17 102 Committee Draft -- August 3, 1998 WG14/N843 [#3] 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). 6.5.17 Language 6.5.17 WG14/N843 Committee Draft -- August 3, 1998 103 6.6 Constant expressions Syntax [#1] constant-expression: conditional-expression Description [#2] 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 [#3] 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.84) [#4] Each constant expression shall evaluate to a constant that is in the range of representable values for its type. Semantics [#5] 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. [#6] An integer constant expression85) 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. ____________________ 84)The operand of a sizeof operator is usually not evaluated | (6.5.3.4). 85)An integer constant expression is used to specify the size of a bit-field member of a structure, the value of an enumeration constant, the size of an array, or the value of a case constant. Further constraints that apply to the integer constant expressions used in conditional- inclusion preprocessing directives are discussed in 6.10.1. 6.6 Language 6.6 104 Committee Draft -- August 3, 1998 WG14/N843 [#7] 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. [#8] 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. [#9] 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.86) Forward references: array declarators (6.7.5.2), initialization (6.7.8). ____________________ 86)Thus, in the following initialization, static int i = 2 || 1 / 0; the expression is a valid integer constant expression with value one. 6.6 Language 6.6 WG14/N843 Committee Draft -- August 3, 1998 105 6.7 Declarations Syntax [#1] 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 [#2] 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. [#3] 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. [#4] All declarations in the same scope that refer to the same object or function shall specify compatible types. Semantics [#5] 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;87) ____________________ 87)Function definitions have a different syntax, described in 6.9.1. 6.7 Language 6.7 106 Committee Draft -- August 3, 1998 WG14/N843 -- for an enumeration constant or typedef name, is the (only) declaration of the identifier. [#6] 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. [#7] 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 [#1] storage-class-specifier: typedef extern static auto register Constraints [#2] At most, one storage-class specifier may be given in the declaration specifiers in a declaration.88) Semantics [#3] 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. [#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.89) [#5] The declaration of an identifier for a function that ____________________ 88)See ``future language directions'' (6.11.2). 6.7 Language 6.7.1 WG14/N843 Committee Draft -- August 3, 1998 107 has block scope shall have no explicit storage-class specifier other than extern. [#6] 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 [#1] type-specifier: void char short int long float double signed unsigned _Bool | _Complex _Imaginary struct-or-union-specifier enum-specifier typedef-name Constraints [#2] 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 ____________________ 89)The implementation may treat any register declaration simply as an auto declaration. However, whether or not addressable storage is actually used, the address of any part of an object declared with storage-class specifier register cannot be computed, either explicitly (by use of the unary & operator as discussed in 6.5.3.2) or implicitly (by converting an array name to a pointer as discussed in 6.3.2.1). Thus the only operator that can be applied to an array declared with storage-class specifier register is sizeof. 6.7.1 Language 6.7.2 108 Committee Draft -- August 3, 1998 WG14/N843 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 | 6.7.2 Language 6.7.2 WG14/N843 Committee Draft -- August 3, 1998 109 -- typedef name | [#3] The type specifiers _Complex and _Imaginary shall not be used if the implementation does not provide those types.90) Semantics [#4] 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. [#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 [#1] 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 ____________________ 90)Implementations are not required to provide imaginary types. Freestanding implementations are not required to provide complex types. 6.7.2 Language 6.7.2.1 110 Committee Draft -- August 3, 1998 WG14/N843 struct-declarator-list: struct-declarator struct-declarator-list , struct-declarator struct-declarator: declarator declarator-opt : constant-expression Constraints [#2] 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. [#3] 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 [#4] 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. [#5] Structure and union specifiers have the same form. [#6] 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. [#7] A member of a structure or union may have any object type other than a variably modified type.91) 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;92) its width is preceded by a colon. ____________________ 91)A structure or union can not contain a member with a variably modified type because member names are not ordinary identifiers as defined in 6.2.3. 6.7.2.1 Language 6.7.2.1 WG14/N843 Committee Draft -- August 3, 1998 111 [#8] 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.93) 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. [#9] 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.94) 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 ____________________ 92)The unary & (address-of) operator cannot be applied to a bit-field object; thus, there are no pointers to or arrays of bit-field objects. 93)As specified in 6.7.2 above, if the actual type specifier used is int or a typedef-name defined as int, then it is implementation-defined whether the bit-field is signed or unsigned. 94)An unnamed bit-field structure member is useful for padding to conform to externally imposed layouts. 6.7.2.1 Language 6.7.2.1 112 Committee Draft -- August 3, 1998 WG14/N843 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.95) 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); ____________________ 95)The length is unspecified to allow for the fact that implementations may give array members different alignments according to their lengths. 6.7.2.1 Language 6.7.2.1 WG14/N843 Committee Draft -- August 3, 1998 113 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 [#1] 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-expression Constraints [#2] The expression that defines the value of an enumeration constant shall be an integer constant expression that has a value representable as an int. 6.7.2.1 Language 6.7.2.2 114 Committee Draft -- August 3, 1998 WG14/N843 Semantics [#3] The identifiers in an enumerator list are declared as constants that have type int and may appear wherever such are permitted.96) 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. [#4] Each enumerated type shall be compatible with an integer type. The choice of type is | implementation-defined,97) 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. [#5] 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 [#1] A specific type shall have its content defined at most once. ____________________ 96)Thus, the identifiers of enumeration constants declared in the same scope shall all be distinct from each other and from other identifiers declared in ordinary declarators. 97)An implementation may delay the choice of which integer type until all enumeration constants have been seen. 6.7.2.2 Language 6.7.2.3 WG14/N843 Committee Draft -- August 3, 1998 115 [#2] A type specifier of the form enum identifier without an enumerator list shall only appear after the type it specifies is completed. Semantics [#3] 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 incomplete98) until the closing brace of the list defining the content, and complete thereafter. [#4] 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. [#5] A type specifier of the form struct-or-union identifier-opt { struct-declaration-list } or enum identifier { enumerator-list } or 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,99) the type specifier also declares the identifier to be the tag of that type. [#6] A declaration of the form ____________________ 98)An incomplete type may only by used when the size of an object of that type is not needed. It is not needed, for example, when a typedef name is declared to be a specifier for a structure or union, or when a pointer to or a function returning a structure or union is being declared. (See incomplete types in 6.2.5.) The | specification has to be complete before such a function is called or defined. 99)If there is no identifier, the type can, within the translation unit, only be referred to by the declaration of which it is a part. Of course, when the declaration is of a typedef name, subsequent declarations can make use of that typedef name to declare objects having the specified structure, union, or enumerated type. 6.7.2.3 Language 6.7.2.3 116 Committee Draft -- August 3, 1998 WG14/N843 struct-or-union identifier ; specifies a structure or union type and declares the identifier as a tag of that type.100) [#7] 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.100) [#8] If a type specifier of the form struct-or-union identifier or 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. [#9] 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: ____________________ 100A similar construction with enum does not exist. 6.7.2.3 Language 6.7.2.3 WG14/N843 Committee Draft -- August 3, 1998 117 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 [#1] type-qualifier: const restrict volatile Constraints [#2] Types other than pointer types derived from object or incomplete types shall not be restrict-qualified. Semantics [#3] The properties associated with qualified types are meaningful only for expressions that are lvalues.101) [#4] If the same qualifier appears more than once in the 6.7.2.3 Language 6.7.3 118 Committee Draft -- August 3, 1998 WG14/N843 same specifier-qualifier-list, either directly or via one or more typedefs, the behavior is the same as if it appeared only once. [#5] 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.102) [#6] 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.103) What constitutes an access to an object that has volatile-qualified type is implementation- defined. [#7] 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.104) 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 ____________________ 101The implementation may place a const object that is not volatile in a read-only region of storage. Moreover, the implementation need not allocate storage for such an object if its address is never used. 102This applies to those objects that behave as if they were defined with qualified types, even if they are never actually defined as objects in the program (such as an object at a memory-mapped input/output address). 103A 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. 104For example, a statement that assigns a value returned by | malloc to a single pointer establishes this association | between the allocated object and the pointer. || 6.7.3 Language 6.7.3 WG14/N843 Committee Draft -- August 3, 1998 119 meaning (i.e., observable behavior). [#8] If the specification of an array type includes any type qualifiers, the element type is so-qualified, not the array type. If the specification of a function type includes any type qualifiers, the behavior is undefined.105) [#9] 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 [#1] Let D be a declaration of an ordinary identifier that provides a means of designating an object P as a restrict- qualified pointer. [#2] 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 ____________________ | 105Both of these can occur through the use of typedefs. || 6.7.3 Language 6.7.3.1 120 Committee Draft -- August 3, 1998 WG14/N843 block of main (or the block of whatever function is called at program startup in a freestanding environment). [#3] 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.106) Note that | ``based'' is defined only for expressions with pointer types. [#4] 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. [#5] 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. [#6] A translator is free to ignore any or all aliasing implications of uses of restrict. [#7] 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. [#8] EXAMPLE 2 The function parameter declarations in the following example ____________________ | 106In other words, E depends on the value of P itself rather || than on the value of an object referenced indirectly || through P. For example, if identifier p has type (int || **restrict), then the pointer expressions p and p+1 are || based on the restricted pointer object designated by p, || but the pointer expressions *p and p[1] are not. || 6.7.3.1 Language 6.7.3.1 WG14/N843 Committee Draft -- August 3, 1998 121 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. [#9] 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. 6.7.3.1 Language 6.7.3.1 122 Committee Draft -- August 3, 1998 WG14/N843 { 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 [#1] function-specifier: inline Constraints [#2] Function specifiers shall be used only in the declaration of an identifier for a function. [#3] 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. [#4] The inline function specifier shall not appear in a declaration of main. Semantics [#5] A function declared with an inline function specifier 6.7.3.1 Language 6.7.4 WG14/N843 Committee Draft -- August 3, 1998 123 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.107) The | extent to which such suggestions are effective is implementation-defined.108) [#6] 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.109) [#7] 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. ____________________ 107By using, for example, an alternative to the usual | function call mechanism, such as ``inline substitution''. | Inline substitution is not textual substitution, nor does it create a new function. Therefore, for example, the expansion of a macro used within the body of the function uses the definition it had at the point the function body appears, and not where the function is called; and identifiers refer to the declarations in scope where the body occurs. Similarly, the address of the function is not affected by the function's being inlined. 108For example, an implementation might never perform inline substitution, or might only perform inline substitutions to calls in the scope of an inline declaration. 109Since an inline definition is distinct from the corresponding external definition, and from any other corresponding inline definition in another translation unit, all corresponding objects with static storage duration are also distinct in each of the definitions. 6.7.4 Language 6.7.4 124 Committee Draft -- August 3, 1998 WG14/N843 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); } [#8] 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 [#1] declarator: pointer-opt direct-declarator direct-declarator: identifier ( declarator ) direct-declarator [ assignment-expression-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 6.7.4 Language 6.7.5 WG14/N843 Committee Draft -- August 3, 1998 125 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 [#2] 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. [#3] 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. [#4] 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. [#5] If, in the declaration ``T D1'', D1 has the form identifier then the type specified for ident is T. [#6] 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 6.7.5 Language 6.7.5 126 Committee Draft -- August 3, 1998 WG14/N843 declarators may be altered by parentheses. Implementation limits [#7] 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 [#1] 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. [#2] For two pointer types to be compatible, both shall be identically qualified and both shall be pointers to compatible types. [#3] 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. [#4] 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 Language 6.7.5.1 WG14/N843 Committee Draft -- August 3, 1998 127 6.7.5.2 Array declarators Constraints [#1] 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. [#2] 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 [#3] If, in the declaration ``T D1'', D1 has the form D[assignment-expr-opt] or 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''.110) 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.111) 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. ____________________ 110When several ``array of'' specifications are adjacent, a multidimensional array is declared. 111Thus, * can be used only in function declarations that are not definitions (see 6.7.5.3). 6.7.5.1 Language 6.7.5.2 128 Committee Draft -- August 3, 1998 WG14/N843 [#4] For two array types to be compatible, both shall have 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. [#5] EXAMPLE 1 float fa[11], *afp[17]; declares an array of float numbers and an array of pointers to float numbers. [#6] 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. [#7] 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. } [#8] 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. 6.7.5.2 Language 6.7.5.2 WG14/N843 Committee Draft -- August 3, 1998 129 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 [#1] A function declarator shall not specify a return type that is a function type or an array type. [#2] The only storage-class specifier that shall occur in a parameter declaration is register. [#3] An identifier list in a function declarator that is not | part of a definition of that function shall be empty. [#4] 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.112) Semantics [#5] If, in the declaration ``T D1'', D1 has the form ____________________ 112Arrays and functions are rewritten as pointers. 6.7.5.2 Language 6.7.5.3 130 Committee Draft -- August 3, 1998 WG14/N843 D(parameter-type-list) or 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''. [#6] 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.113) The special case of an unnamed parameter of type void as the only item in the list specifies that the function has no parameters. [#7] If, in a parameter declaration, an identifier can be treated as a typedef name or as a parameter name, it shall be taken as a typedef name. [#8] 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. [#9] 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.114) [#11] For two function types to be compatible, both shall specify compatible return types.115) 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 WG14/N843 Committee Draft -- August 3, 1998 131 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 ____________________ 113The macros defined in the header (7.15) may be used to access arguments that correspond to the ellipsis. 114See ``future language directions'' (6.11.3). 115If both function types are ``old style'', parameter types are not compared. 6.7.5.3 Language 6.7.5.3 132 Committee Draft -- August 3, 1998 WG14/N843 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). 6.7.5.3 Language 6.7.5.3 WG14/N843 Committee Draft -- August 3, 1998 133 6.7.6 Type names Syntax [#1] 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-expression-opt ] direct-abstract-declarator-opt [ * ] direct-abstract-declarator-opt ( parameter-type-list-opt ) Semantics [#2] 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.116) [#3] 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. ____________________ 116As indicated by the syntax, empty parentheses in a type name are interpreted as ``function with no parameter specification'', rather than redundant parentheses around the omitted identifier. 6.7.6 Language 6.7.6 134 Committee Draft -- August 3, 1998 WG14/N843 6.7.7 Type definitions Syntax [#1] typedef-name: identifier Constraints [#2] If a typedef name specifies a variably modified type then it shall have block scope. Semantics [#3] 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. * [#4] 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. 6.7.7 Language 6.7.7 WG14/N843 Committee Draft -- August 3, 1998 135 The object distance has a type compatible with any other int object. [#5] 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. | [#6] 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. [#7] 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. 6.7.7 Language 6.7.7 136 Committee Draft -- August 3, 1998 WG14/N843 typedef void fv(int), (*pfv)(int); void (*signal(int, void (*)(int)))(int); fv *signal(int, fv *); pfv signal(int, pfv); [#8] 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 [#1] initializer: assignment-expression { initializer-list } { initializer-list , } initializer-list: designation-opt initializer initializer-list , designation-opt initializer designation: designator-list = designator-list: designator designator-list designator designator: [ constant-expression ] . identifier Constraints 6.7.7 Language 6.7.8 WG14/N843 Committee Draft -- August 3, 1998 137 [#2] No initializer shall attempt to provide a value for an object not contained within the entity being initialized. [#3] 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. [#4] All the expressions in an initializer for an object that has static storage duration shall be constant expressions or string literals. [#5] 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. [#6] If a designator has the form [ constant-expression ] 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. [#7] 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 [#8] An initializer specifies the initial value stored in an object. [#9] 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; 6.7.8 Language 6.7.8 138 Committee Draft -- August 3, 1998 WG14/N843 -- 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.117) 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.118) [#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.119) The current object that results at the end of WG14/N843 Committee Draft -- August 3, 1998 139 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 contains elements or members that are | aggregates or unions, or if the first member of a union is an aggregate or union, 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 first member of 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.120) ____________________ 118After a union member is initialized, the next object is not the next member of the union; instead, it is the next subobject of an object containing the union. 119Thus, a designator can only specify a strict subobject of the aggregate or union that is associated with the surrounding brace pair. Note, too, that each separate designator list is independent. 120In particular, the evaluation order need not be the same as the order of subobject initialization. 6.7.8 Language 6.7.8 140 Committee Draft -- August 3, 1998 WG14/N843 [#24] EXAMPLE 1 Provided that 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 6.7.8 Language 6.7.8 WG14/N843 Committee Draft -- August 3, 1998 141 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. 6.7.8 Language 6.7.8 142 Committee Draft -- August 3, 1998 WG14/N843 [#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", }; 6.7.8 Language 6.7.8 WG14/N843 Committee Draft -- August 3, 1998 143 [#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 (7.17). 6.7.8 Language 6.7.8 144 Committee Draft -- August 3, 1998 WG14/N843 6.8 Statements Syntax [#1] statement: labeled-statement compound-statement expression-statement selection-statement iteration-statement jump-statement Semantics [#2] A statement specifies an action to be performed. Except as indicated, statements are executed in sequence. [#3] 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 [#1] labeled-statement: identifier : statement case constant-expr : statement default : statement Constraints [#2] A case or default label shall appear only in a switch statement. Further constraints on such labels are discussed under the switch statement. Semantics [#3] Any statement may be preceded by a prefix that declares an identifier as a label name. Labels in themselves do not 6.8 Language 6.8.1 WG14/N843 Committee Draft -- August 3, 1998 145 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 [#1] compound-statement: { block-item-list-opt } block-item-list: block-item block-item-list block-item block-item: declaration statement Semantics [#2] A compound statement (also called 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 that 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. 6.8.3 Expression and null statements Syntax [#1] expression-statement: expression-opt ; Semantics [#2] The expression in an expression statement is evaluated as a void expression for its side effects.121) ____________________ 121Such as assignments, and function calls which have side effects. 6.8.1 Language 6.8.3 146 Committee Draft -- August 3, 1998 WG14/N843 [#3] A null statement (consisting of just a semicolon) performs no operations. [#4] 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); [#5] 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. [#6] 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 [#1] 6.8.3 Language 6.8.4 WG14/N843 Committee Draft -- August 3, 1998 147 selection-statement: if ( expression ) statement if ( expression ) statement else statement switch ( expression ) statement Semantics [#2] A selection statement selects among a set of statements depending on the value of a controlling expression. 6.8.4.1 The if statement Constraints [#1] The controlling expression of an if statement shall have scalar type. Semantics [#2] 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. [#3] An else is associated with the lexically nearest preceding if that is allowed by the syntax. 6.8.4.2 The switch statement Constraints [#1] The controlling expression of a switch statement shall | have integer type. | [#2] If the switch statement causes a jump to within the | scope of an identifier with a variably modified type, the | entire switch statement shall be within the scope of that | identifier.122) | [#3] 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.) ____________________ 122That is, the declaration either precedes the switch statement, or it follows the last case or default label associated with the switch that is in the block containing the declaration. 6.8.4 Language 6.8.4.2 148 Committee Draft -- August 3, 1998 WG14/N843 Semantics [#4] 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. [#5] 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 [#6] As discussed in 5.2.4.1, the implementation may limit | the number of case values in a switch statement. [#7] 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. 6.8.4.2 Language 6.8.4.2 WG14/N843 Committee Draft -- August 3, 1998 149 6.8.5 Iteration statements Syntax [#1] 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 [#2] The controlling expression of an iteration statement shall have scalar type. [#3] The declaration part of a for statement shall only declare identifiers for objects having storage class auto or register. Semantics [#4] An iteration statement causes a statement called the loop body to be executed repeatedly until the controlling expression compares equal to 0. 6.8.5.1 The while statement [#1] The evaluation of the controlling expression takes place before each execution of the loop body. 6.8.5.2 The do statement [#1] The evaluation of the controlling expression takes place after each execution of the loop body. 6.8.5.3 The for statement [#1] Except for the behavior of a continue statement in the | loop body, the statement for ( clause-1 ; expr-2 ; expr-3 ) statement and the sequence of statements { clause-1 ; while ( expr-2 ) { statement expr-3 ; } } 6.8.5 Language 6.8.5.3 150 Committee Draft -- August 3, 1998 WG14/N843 are equivalent (where clause-1 can be an expression or a declaration).123) Unlike the other iteration statements, | the for statement introduces new blocks that limit the scope | of declarations and compound literals occurring in the loop. [#2] Both clause-1 and expr-3 can be omitted. If clause-1 | is an expression, it is evaluated as a void expression, as | is expr-3. An omitted expr-2 is replaced by a nonzero constant. Forward references: the continue statement (6.8.6.2). 6.8.6 Jump statements Syntax [#1] jump-statement: goto identifier ; continue ; break ; return expression-opt ; Semantics [#2] A jump statement causes an unconditional jump to another place. ____________________ 123Thus, clause-1 specifies initialization for the loop, possibly declaring one or more variables for use in the loop; expr-2, the controlling expression, specifies an evaluation made before each iteration, such that execution of the loop continues until the expression compares equal to 0; expr-3 specifies an operation (such as incrementing) that is performed after each iteration. If clause-1 is a declaration, then the scope of any variable it declares is the remainder of the declaration and the entire loop, including the other two expressions. 6.8.5.3 Language 6.8.6 WG14/N843 Committee Draft -- August 3, 1998 151 6.8.6.1 The goto statement Constraints [#1] 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 [#2] A goto statement causes an unconditional jump to the statement prefixed by the named label in the enclosing function. [#3] 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 /* ... */ } [#4] 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. 6.8.6 Language 6.8.6.1 152 Committee Draft -- August 3, 1998 WG14/N843 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 [#1] A continue statement shall appear only in or as a loop body. Semantics [#2] 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;.124) ____________________ 124Following the contin: label is a null statement. 6.8.6.1 Language 6.8.6.2 WG14/N843 Committee Draft -- August 3, 1998 153 6.8.6.3 The break statement Constraints [#1] A break statement shall appear only in or as a switch body or loop body. Semantics [#2] A break statement terminates execution of the smallest enclosing switch or iteration statement. 6.8.6.4 The return statement Constraints [#1] 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 [#2] A return statement terminates execution of the current function and returns control to its caller. A function may have any number of return statements. [#3] 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.125) * [#4] EXAMPLE In: ____________________ 125The return statement is not an assignment. The overlap restriction of subclause 6.5.16.1 does not apply to the case of function return. 6.8.6.2 Language 6.8.6.4 154 Committee Draft -- August 3, 1998 WG14/N843 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). 6.8.6.4 Language 6.8.6.4 WG14/N843 Committee Draft -- August 3, 1998 155 6.9 External definitions Syntax [#1] translation-unit: external-declaration translation-unit external-declaration external-declaration: function-definition declaration Constraints [#2] The storage-class specifiers auto and register shall not appear in the declaration specifiers in an external declaration. [#3] 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 [#4] 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. [#5] 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.126) ____________________ 126Thus, if an identifier declared with external linkage is not used in an expression, there need be no external definition for it. 6.9 Language 6.9 156 Committee Draft -- August 3, 1998 WG14/N843 6.9.1 Function definitions Syntax [#1] function-definition: declaration-specifiers declarator declaration-list-opt compound-statement declaration-list: declaration declaration-list declaration Constraints [#2] 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.127) [#3] The return type of a function shall be void or an object type other than array type. [#4] The storage-class specifier, if any, in the declaration specifiers shall be either extern or static. [#5] 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. [#6] 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 ____________________ 127The intent is that the type category in a function definition cannot be inherited from a typedef: typedef int F(void); /* type F is ``function of no arguments returning int'' */ F f, g; /* f and g both have type compatible with F */ F f { /* ... */ } /* WRONG: syntax/constraint error */ F g() { /* ... */ } /* WRONG: declares that g returns a function */ int f(void) { /* ... */ } /* RIGHT: f has type compatible with F */ int g() { /* ... */ } /* RIGHT: g has type compatible with F */ F *e(void) { /* ... */ } /* e returns a pointer to a function */ F *((e))(void) { /* ... */ } /* same: parentheses irrelevant */ int (*fp)(void); /* fp points to a function that has type F */ F *Fp; /* Fp points to a function that has type F */ 6.9.1 Language 6.9.1 WG14/N843 Committee Draft -- August 3, 1998 157 storage-class specifier other than register and no initializations. Semantics [#7] 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,128) 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. [#8] 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. [#9] 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, all size expressions of | variably modified parameters 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; } ____________________ 128See ``future language directions'' (6.11.4). 6.9.1 Language 6.9.1 158 Committee Draft -- August 3, 1998 WG14/N843 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 [#1] If the declaration of an identifier for an object has file scope and an initializer, the declaration is an external definition for the identifier. [#2] A declaration of an identifier for an object that has file scope without an initializer, and without a storage- 6.9.1 Language 6.9.2 WG14/N843 Committee Draft -- August 3, 1998 159 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. [#3] 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. [#4] 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 [#5] 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. 6.9.2 Language 6.9.2 160 Committee Draft -- August 3, 1998 WG14/N843 6.10 Preprocessing directives Syntax [#1] preprocessing-file: group-opt group: group-part group group-part group-part: 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 6.10 Language 6.10 WG14/N843 Committee Draft -- August 3, 1998 161 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 [#2] 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.129) 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 [#3] 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 [#4] 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. [#5] The preprocessing tokens within a preprocessing directive are not subject to macro expansion unless ____________________ 129Thus, preprocessing directives are commonly called ``lines''. These ``lines'' have no other syntactic significance, as all white space is equivalent except in certain situations during preprocessing (see the # character string literal creation operator in 6.10.3.2, for example). 6.10 Language 6.10 162 Committee Draft -- August 3, 1998 WG14/N843 otherwise stated. [#6] EXAMPLE In: #define EMPTY EMPTY # include 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 [#1] 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;130) and it may contain unary operator expressions of the form defined identifier or 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 [#2] 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. [#3] 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 ____________________ 130Because the controlling constant expression is evaluated during translation phase 4, all identifiers either are or are not macro names -- there simply are no keywords, enumeration constants, etc. 6.10 Language 6.10.1 WG14/N843 Committee Draft -- August 3, 1998 163 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 . 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.131) Also, whether a single- character character constant may have a negative value is implementation-defined. [#4] 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. [#5] 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.132) ____________________ 131Thus, the constant expression in the following #if directive and if statement is not guaranteed to evaluate to the same value in these two contexts. #if 'z' - 'a' == 25 if ('z' - 'a' == 25) 6.10.1 Language 6.10.1 164 Committee Draft -- August 3, 1998 WG14/N843 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 [#1] A #include directive shall identify a header or source file that can be processed by the implementation. Semantics [#2] A preprocessing directive of the form # include 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. [#3] 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 new-line with the identical contained sequence (including > characters, if any) from the original directive. [#4] 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 ____________________ 132As indicated by the syntax, a preprocessing token shall not follow a #else or #endif directive before the terminating new-line character. However, comments may appear anywhere in a source file, including within a preprocessing directive. 6.10.1 Language 6.10.2 WG14/N843 Committee Draft -- August 3, 1998 165 its replacement list of preprocessing tokens.) The directive resulting after all replacements shall match one of the two previous forms.133) 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. [#5] 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. [#6] 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). [#7] EXAMPLE 1 The most common uses of #include preprocessing directives are as in the following: #include #include "myprog.h" [#8] 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). ____________________ 133Note that adjacent string literals are not concatenated into a single string literal (see the translation phases in 5.1.1.2); thus, an expansion that results in two string literals is an invalid directive. 6.10.2 Language 6.10.2 166 Committee Draft -- August 3, 1998 WG14/N843 6.10.3 Macro replacement Constraints [#1] 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. [#2] 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. [#3] 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. [#4] 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. [#5] The identifier __VA_ARGS__ shall only occur in the replacement-list of a #define preprocessing directive using the ellipsis notation in the arguments. [#6] A parameter identifier in a function-like macro shall be uniquely declared within its scope. Semantics [#7] 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. [#8] 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. [#9] A preprocessing directive of the form 6.10.3 Language 6.10.3 WG14/N843 Committee Draft -- August 3, 1998 167 # define identifier replacement-list new-line defines an object-like macro that causes each subsequent instance of the macro name134) 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 ...). ____________________ 134Since, by macro-replacement time, all character constants and string literals are preprocessing tokens, not sequences possibly containing identifier-like subsequences (see 5.1.1.2, translation phases), they are never scanned for macro names or parameters. 6.10.3 Language 6.10.3 168 Committee Draft -- August 3, 1998 WG14/N843 6.10.3.1 Argument substitution [#1] 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. [#2] 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 [#1] 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 [#2] 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 Language 6.10.3.2 WG14/N843 Committee Draft -- August 3, 1998 169 6.10.3.3 The ## operator Constraints [#1] A ## preprocessing token shall not occur at the beginning or at the end of a replacement list for either form of macro definition. Semantics [#2] 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. [#3] 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. [#4] 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: 6.10.3.2 Language 6.10.3.3 170 Committee Draft -- August 3, 1998 WG14/N843 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 [#1] 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. [#2] 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. [#3] 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 [#1] A macro definition lasts (independent of block structure) until a corresponding #undef directive is encountered or (if none is encountered) until the end of translation phase 4. [#2] 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. 6.10.3.3 Language 6.10.3.5 WG14/N843 Committee Draft -- August 3, 1998 171 [#3] EXAMPLE 1 The simplest use of this facility is to define a ``manifest constant'', as in #define TABSIZE 100 int table[TABSIZE]; [#4] 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. [#5] 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 6.10.3.5 Language 6.10.3.5 172 Committee Draft -- August 3, 1998 WG14/N843 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", "" }; [#6] 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. [#7] EXAMPLE 5 To illustrate the rules for 6.10.3.5 Language 6.10.3.5 WG14/N843 Committee Draft -- August 3, 1998 173 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, }; [#8] 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 */ [#9] 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)); 6.10.3.5 Language 6.10.3.5 174 Committee Draft -- August 3, 1998 WG14/N843 6.10.4 Line control Constraints [#1] The string literal of a #line directive, if present, shall be a character string literal. Semantics [#2] 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. [#3] 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. [#4] 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. [#5] 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. 6.10.3.5 Language 6.10.4 WG14/N843 Committee Draft -- August 3, 1998 175 6.10.5 Error directive Semantics [#1] 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 [#1] 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)135) causes the implementation to behave in a manner which it shall document. 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. [#2] 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 ____________________ 135An implementation is not required to perform macro replacement in pragmas, but it is permitted except for in standard pragmas (where STDC immediately follows pragma). If the result of macro replacement in a non-standard pragma has the same form as a standard pragma, the behavior is still implementation-defined; an implementation is permitted to behave as if it were the standard pragma, but is not required to. 6.10.5 Language 6.10.6 176 Committee Draft -- August 3, 1998 WG14/N843 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 [#1] A preprocessing directive of the form # new-line has no effect. 6.10.8 Predefined macro names [#1] 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).136) __FILE__ The presumed name of the current source file (a | character string literal).136) __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.137) ____________________ 136The presumed line number and source file name can be changed by the #line directive. 137This macro was not specified in ISO/IEC 9899:1990 and was | specified as 199409L in ISO/IEC 9899:AMD1:1995 6.10.6 Language 6.10.8 WG14/N843 Committee Draft -- August 3, 1998 177 [#2] 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). [#3] The values of the predefined macros (except for __LINE__ and __FILE__) remain constant throughout the translation unit. [#4] 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 [#1] 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. [#2] EXAMPLE A directive of the form: 6.10.8 Language 6.10.9 178 Committee Draft -- August 3, 1998 WG14/N843 #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 ) | 6.10.9 Language 6.10.9 WG14/N843 Committee Draft -- August 3, 1998 179 6.11 Future language directions 6.11.1 Character escape sequences [#1] Lowercase letters as escape sequences are reserved for future standardization. Other characters may be used in extensions. 6.11.2 Storage-class specifiers [#1] The placement of a storage-class specifier other than at the beginning of the declaration specifiers in a declaration is an obsolescent feature. 6.11.3 Function declarators [#1] The use of function declarators with empty parentheses (not prototype-format parameter type declarators) is an obsolescent feature. 6.11.4 Function definitions [#1] The use of function definitions with separate parameter identifier and declaration lists (not prototype-format parameter type and identifier declarators) is an obsolescent feature. 6.11.5 Pragma directives [#1] Pragmas whose first pp-token is STDC are reserved for future standardization. 6.11 Language 6.11.5 180 Committee Draft -- August 3, 1998 WG14/N843 7. Library 7.1 Introduction 7.1.1 Definitions of terms [#1] 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. [#2] 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. [#3] 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.138) It is represented in the text and examples by a period, but may be changed by the setlocale function. [#4] 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.139) [#5] A null wide character is a wide character with code value zero. [#6] 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 null wide character and the value of a wide string is the sequence of code values of the contained wide characters, in ____________________ 138The functions that make use of the decimal-point character are the string conversion functions (7.20.1), the wide-string numeric conversion functions (7.24.4.1), the formatted input/output functions (7.19.6), and the formatted wide-character input/output functions (7.24.2). 139An equivalent definition can be found in 6.4.4.4. 7 Library 7.1.1 WG14/N843 Committee Draft -- August 3, 1998 181 order. [#7] 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.140) Forward references: character handling (7.4), the setlocale function (7.11.1.1). 7.1.2 Standard headers [#1] Each library function is declared, with a type that includes a prototype, in a header,141) 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. [#2] The standard headers are [#3] 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. [#4] 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 depends on the definition of | NDEBUG (see 7.2). If used, a header shall be included ____________________ 140For state-dependent encodings, the values for MB_CUR_MAX and MB_LEN_MAX shall thus be large enough to count all the bytes in any complete multibyte character plus at least one adjacent shift sequence of maximum length. Whether these counts provide for more than one shift sequence is the implementation's choice. 141A header is not necessarily a source file, nor are the < and > delimited sequences in header names necessarily valid source file names. 7.1.1 Library 7.1.2 182 Committee Draft -- August 3, 1998 WG14/N843 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. [#5] 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. [#6] Any declaration of a library function shall have external linkage. [#7] A summary of the contents of the standard headers is given in annex B. Forward references: diagnostics (7.2). 7.1.3 Reserved identifiers [#1] 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.142) ____________________ 142The list of reserved identifiers with external linkage includes errno, setjmp, and va_end. 7.1.2 Library 7.1.3 WG14/N843 Committee Draft -- August 3, 1998 183 -- 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. [#2] 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. [#3] 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 [#1] 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.143) 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.144) Likewise, those function-like macros described in the ____________________ 143This means that an implementation shall provide an actual function for each library function, even if it also provides a macro for that function. 7.1.3 Library 7.1.4 184 Committee Draft -- August 3, 1998 WG14/N843 following subclauses may be invoked in an expression anywhere a function with a compatible return type could be called.145) All object-like macros listed as expanding to integer constant expressions shall additionally be suitable for use in #if preprocessing directives. [#2] 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. [#3] There is a sequence point immediately before a library function returns. [#4] The functions in the standard library are not guaranteed to be reentrant and may modify objects with static storage duration.146) [#5] EXAMPLE The function atoi may be used in any of several ways: -- by use of its associated header (possibly generating a macro expansion) #include const char *str; /* ... */ i = atoi(str); ____________________ 144Such macros might not contain the sequence points that the corresponding function calls do. 145Because external identifiers and some macro names beginning with an underscore are reserved, implementations may provide special semantics for such names. For example, the identifier _BUILTIN_abs could be 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. 146Thus, a signal handler cannot, in general, call standard library functions. 7.1.4 Library 7.1.4 WG14/N843 Committee Draft -- August 3, 1998 185 -- by use of its associated header (assuredly generating a true function reference) #include #undef atoi const char *str; /* ... */ i = atoi(str); or #include const char *str; /* ... */ i = (atoi)(str); -- by explicit declaration extern int atoi(const char *); const char *str; /* ... */ i = atoi(str); 7.1.4 Library 7.1.4 186 Committee Draft -- August 3, 1998 WG14/N843 7.2 Diagnostics [#1] The header defines the assert macro and refers to another macro, NDEBUG which is not defined by . If NDEBUG is defined as a macro name at the point in the source file where 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 is included. [#2] 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 [#1] #include void assert(_Bool expression); | Description [#2] The assert macro puts diagnostic tests into programs. When it is executed, if expression 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.147) It then calls the abort function. Returns ____________________ 147The message written might be of the form: | Assertion failed: expression, function abc, file xyz, | line nnn . | 7.2 Library 7.2.1.1 WG14/N843 Committee Draft -- August 3, 1998 187 [#3] The assert macro returns no value. Forward references: the abort function (7.20.4.1). 7.2.1.1 Library 7.2.1.1 188 Committee Draft -- August 3, 1998 WG14/N843 7.3 Complex arithmetic 7.3.1 Introduction [#1] The header defines macros and declares functions that support complex arithmetic.148) 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 same name but with f and l suffixes which are corresponding functions with float and long double parameters and return values. [#2] 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.149) [#3] The macros imaginary and _Imaginary_I are defined if and only if the implementation supports imaginary types;150) if defined, they expand to _Imaginary and a constant expression of type const float _Imaginary with the value of the imaginary unit. [#4] The macro I expands to either _Imaginary_I or _Complex_I. If _Imaginary_I is not defined, I shall expand to _Complex_I. [#5] Notwithstanding the provisions of 7.1.3, a program is permitted to undefine and perhaps then redefine the macros ____________________ 148See ``future library directions'' (7.26.1). 149The imaginary unit is a number i such that i2=-1. | 150A specification for imaginary types is in informative annex G. 7.3 Library 7.3.1 WG14/N843 Committee Draft -- August 3, 1998 189 complex, imaginary, and I. | Forward references: IEC 60559-compatible complex arithmetic | (annex G). 7.3.2 Conventions [#1] Values are interpreted as radians, not degrees. An implementation may set errno but is not required to. 7.3.3 Branch cuts [#1] 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. [#2] 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 [#1] #include #pragma STDC CX_LIMITED_RANGE on-off-switch Description [#2] 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.151) The pragma can occur either outside external declarations or preceding 7.3.1 Library 7.3.4 190 Committee Draft -- August 3, 1998 WG14/N843 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 (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 [#1] #include double complex cacos(double complex z); float complex cacosf(float complex z); long double complex cacosl(long double complex z); Description [#2] The cacos functions compute the complex arc cosine of z, with branch cuts outside the interval [-1, 1] along the real axis. Returns [#3] 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. ____________________ 151The purpose of the pragma is to allow the implementation to use the formulas: | (x+iy)×(u+iv)=(xu-yv)+i(yu+xv) | (x+iy)/(u+iv)=[(xu+yv)+i(yu-xv)]/(u2+v2) | |x+iy|=x2+y2 | where the programmer can determine they are safe. 7.3.4 Library 7.3.5.1 WG14/N843 Committee Draft -- August 3, 1998 191 7.3.5.2 The casin functions Synopsis [#1] #include double complex casin(double complex z); float complex casinf(float complex z); long double complex casinl(long double complex z); Description [#2] The casin functions compute the complex arc sine of z, with branch cuts outside the interval [-1, 1] along the real axis. Returns [#3] 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 [#1] #include double complex catan(double complex z); float complex catanf(float complex z); long double complex catanl(long double complex z); Description [#2] The catan functions compute the complex arc tangent of z, with branch cuts outside the interval [-i, i] along the imaginary axis. Returns [#3] 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. 7.3.5.1 Library 7.3.5.3 192 Committee Draft -- August 3, 1998 WG14/N843 7.3.5.4 The ccos functions Synopsis [#1] #include double complex ccos(double complex z); float complex ccosf(float complex z); long double complex ccosl(long double complex z); Description [#2] The ccos function computes the complex cosine of z. Returns [#3] The ccos functions return the complex cosine value. 7.3.5.5 The csin functions Synopsis [#1] #include double complex csin(double complex z); float complex csinf(float complex z); long double complex csinl(long double complex z); Description [#2] The csin functions compute the complex sine of z. Returns [#3] The csin functions return the complex sine value. 7.3.5.6 The ctan functions Synopsis [#1] #include double complex ctan(double complex z); float complex ctanf(float complex z); long double complex ctanl(long double complex z); Description [#2] The ctan functions compute the complex tangent of z. 7.3.5.3 Library 7.3.5.6 WG14/N843 Committee Draft -- August 3, 1998 193 Returns [#3] The ctan functions return the complex tangent value. 7.3.6 Hyperbolic functions 7.3.6.1 The cacosh functions Synopsis [#1] #include double complex cacosh(double complex z); float complex cacoshf(float complex z); long double complex cacoshl(long double complex z); Description [#2] 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 [#3] 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 [#1] #include double complex casinh(double complex z); float complex casinhf(float complex z); long double complex casinhl(long double complex z); Description [#2] The casinh functions compute the complex arc hyperbolic sine of z, with branch cuts outside the interval [-i, i] along the imaginary axis. Returns [#3] 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. 7.3.5.6 Library 7.3.6.2 194 Committee Draft -- August 3, 1998 WG14/N843 7.3.6.3 The catanh functions Synopsis [#1] #include double complex catanh(double complex z); float complex catanhf(float complex z); long double complex catanhl(long double complex z); Description [#2] The catanh functions compute the complex arc hyperbolic tangent of z, with branch cuts outside the interval [-1, 1] along the real axis. Returns [#3] 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 [#1] #include double complex ccosh(double complex z); float complex ccoshf(float complex z); long double complex ccoshl(long double complex z); Description [#2] The ccosh functions compute the complex hyperbolic cosine of z. Returns [#3] The ccosh functions return the complex hyperbolic cosine value. 7.3.6.2 Library 7.3.6.4 WG14/N843 Committee Draft -- August 3, 1998 195 7.3.6.5 The csinh functions Synopsis [#1] #include double complex csinh(double complex z); float complex csinhf(float complex z); long double complex csinhl(long double complex z); Description [#2] The csinh functions compute the complex hyperbolic sine of z. Returns [#3] The csinh functions return the complex hyperbolic sine value. 7.3.6.6 The ctanh functions Synopsis [#1] #include double complex ctanh(double complex z); float complex ctanhf(float complex z); long double complex ctanhl(long double complex z); Description [#2] The ctanh functions compute the complex hyperbolic tangent of z. Returns [#3] The ctanh functions return the complex hyperbolic tangent value. 7.3.7 Exponential and logarithmic functions 7.3.6.4 Library 7.3.7 196 Committee Draft -- August 3, 1998 WG14/N843 7.3.7.1 The cexp functions Synopsis [#1] #include double complex cexp(double complex z); float complex cexpf(float complex z); long double complex cexpl(long double complex z); Description [#2] The cexp functions compute the complex base-e exponential of z. Returns [#3] The cexp functions return the complex base-e exponential value. 7.3.7.2 The clog functions Synopsis [#1] #include double complex clog(double complex z); float complex clogf(float complex z); long double complex clogl(long double complex z); Description [#2] The clog functions compute the complex natural (base-e) logarithm of z, with a branch cut along the negative real axis. Returns [#3] 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 7.3.7 Library 7.3.8 WG14/N843 Committee Draft -- August 3, 1998 197 7.3.8.1 The cabs functions Synopsis [#1] #include double cabs(double complex z); float cabsf(float complex z); long double cabsl(long double complex z); Description [#2] The cabs functions compute the complex absolute value (also called norm, modulus, or magnitude) of z. Returns [#3] The cabs functions return the complex absolute value. 7.3.8.2 The cpow functions Synopsis [#1] #include 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 [#2] The cpow functions compute the complex power function xy, with a branch cut for the first parameter along the negative real axis. Returns [#3] The cpow functions return the complex power function value. 7.3.8 Library 7.3.8.2 198 Committee Draft -- August 3, 1998 WG14/N843 7.3.8.3 The csqrt functions Synopsis [#1] #include double complex csqrt(double complex z); float complex csqrtf(float complex z); long double complex csqrtl(long double complex z); Description [#2] The csqrt functions compute the complex square root of z, with a branch cut along the negative real axis. Returns [#3] 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 [#1] #include double carg(double complex z); float cargf(float complex z); long double cargl(long double complex z); Description [#2] The carg functions compute the argument (also called phase angle) of z, with a branch cut along the negative real axis. Returns [#3] The carg functions return the value of the argument in the range [-pi, pi]. 7.3.8.2 Library 7.3.9.1 WG14/N843 Committee Draft -- August 3, 1998 199 7.3.9.2 The cimag functions Synopsis [#1] #include double cimag(double complex z); float cimagf(float complex z); long double cimagl(long double complex z); Description [#2] The cimag functions compute the imaginary part of z.152) Returns [#3] The cimag functions return the imaginary part value (as a real). 7.3.9.3 The conj functions Synopsis [#1] #include double complex conj(double complex z); float complex conjf(float complex z); long double complex conjl(long double complex z); Description [#2] The conj functions compute the complex conjugate of z, by reversing the sign of its imaginary part. Returns [#3] The conj functions return the complex conjugate value. ____________________ 152For a variable z of complex type, z == creal(z) + cimag(z)*I. 7.3.9.1 Library 7.3.9.3 200 Committee Draft -- August 3, 1998 WG14/N843 7.3.9.4 The cproj functions Synopsis [#1] #include double complex cproj(double complex z); float complex cprojf(float complex z); long double complex cprojl(long double complex z); Description [#2] 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 [#3] The cproj functions return the value of the projection onto the Riemann sphere. 7.3.9.5 The creal functions Synopsis [#1] #include double creal(double complex z); float crealf(float complex z); long double creall(long double complex z); Description [#2] The creal functions compute the real part of z.153) Returns [#3] The creal functions return the real part value. ____________________ 153For a variable z of complex type, z == creal(z) + cimag(z)*I. 7.3.9.3 Library 7.3.9.5 WG14/N843 Committee Draft -- August 3, 1998 201 7.4 Character handling [#1] The header declares several functions useful for testing and mapping characters.154) 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. [#2] 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. [#3] 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.155) Forward references: EOF (7.19.1), localization (7.11). 7.4.1 Character testing functions [#1] 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 [#1] #include int isalnum(int c); Description [#2] The isalnum function tests for any character for which isalpha or isdigit is true. ____________________ 154See ``future library directions'' (7.26.2). 155In an implementation that uses the seven-bit US ASCII | character set, the printing characters are those whose values lie from 0x20 (space) through 0x7E (tilde); the control characters are those whose values lie from 0 (NUL) through 0x1F (US), and the character 0x7F (DEL). 7.4 Library 7.4.1.1 202 Committee Draft -- August 3, 1998 WG14/N843 7.4.1.2 The isalpha function Synopsis [#1] #include int isalpha(int c); Description [#2] 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.156) 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 [#1] #include int iscntrl(int c); Description [#2] The iscntrl function tests for any control character. 7.4.1.4 The isdigit function Synopsis [#1] #include int isdigit(int c); Description [#2] The isdigit function tests for any decimal-digit character (as defined in 5.2.1). ____________________ 156The functions islower and isupper test true or false separately for each of these additional characters; all four combinations are possible. 7.4.1.1 Library 7.4.1.4 WG14/N843 Committee Draft -- August 3, 1998 203 7.4.1.5 The isgraph function Synopsis [#1] #include int isgraph(int c); Description [#2] The isgraph function tests for any printing character except space (' '). 7.4.1.6 The islower function Synopsis [#1] #include int islower(int c); Description [#2] 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 [#1] #include int isprint(int c); Description [#2] The isprint function tests for any printing character including space (' '). 7.4.1.4 Library 7.4.1.7 204 Committee Draft -- August 3, 1998 WG14/N843 7.4.1.8 The ispunct function Synopsis [#1] #include int ispunct(int c); Description [#2] 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. 7.4.1.9 The isspace function Synopsis [#1] #include int isspace(int c); Description [#2] 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 [#1] #include int isupper(int c); Description [#2] 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). 7.4.1.7 Library 7.4.1.10 WG14/N843 Committee Draft -- August 3, 1998 205 7.4.1.11 The isxdigit function Synopsis [#1] #include int isxdigit(int c); Description [#2] 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 [#1] #include int tolower(int c); Description [#2] The tolower function converts an uppercase letter to a corresponding lowercase letter. Returns [#3] 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 [#1] #include int toupper(int c); Description [#2] The toupper function converts a lowercase letter to a corresponding uppercase letter. 7.4.1.10 Library 7.4.2.2 206 Committee Draft -- August 3, 1998 WG14/N843 Returns [#3] 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. 7.4.2.2 Library 7.4.2.2 WG14/N843 Committee Draft -- August 3, 1998 207 7.5 Errors [#1] The header defines several macros, all relating to the reporting of error conditions. [#2] 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 lvalue157) 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. [#3] The value of errno is zero at program startup, but is never set to zero by any library function.158) 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. [#4] Additional macro definitions, beginning with E and a digit or E and an uppercase letter,159) may also be specified by the implementation. ____________________ 157The macro errno need not be the identifier of an object. It might expand to a modifiable lvalue resulting from a function call (for example, *errno()). 158Thus, a program that uses errno for error checking should set it to zero before a library function call, then inspect it before a subsequent library function call. Of course, a library function can save the value of errno on 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. 159See ``future library directions'' (7.26.3). 7.5 Library 7.5 208 Committee Draft -- August 3, 1998 WG14/N843 7.6 Floating-point environment [#1] The header 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.160) A floating-point status flag is a system variable whose value is set 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. | [#2] Certain programming conventions support the intended model of use for the floating-point environment:161) -- 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. [#3] The type fenv_t represents the entire floating-point environment. [#4] The type fexcept_t represents the floating-point exception flags collectively, ____________________ 160This header is designed to support the exception status flags and directed-rounding control modes required by IEC 60559, and other similar floating-point state information. Also it is designed to facilitate code portability among all systems. 161With these conventions, a programmer can safely assume default modes (or be unaware of them). The responsibilities associated with accessing the floating- point environment fall on the programmer or program that does so explicitly. 7.6 Library 7.6 WG14/N843 Committee Draft -- August 3, 1998 209 including any status the implementation associates with the flags. [#5] 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. [#6] The macro FE_ALL_EXCEPT is simply the bitwise OR of all exception macros defined by the implementation. [#7] 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.162) [#8] 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 ____________________ 162Even though the rounding direction macros may expand to constants corresponding to the values of FLT_ROUNDS, they are not required to do so. 7.6 Library 7.6 210 Committee Draft -- August 3, 1998 WG14/N843 functions that manage the floating-point environment. [#9] 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 [#1] #include #pragma STDC FENV_ACCESS on-off-switch Description [#2] 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.163) 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 (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. [#3] EXAMPLE ____________________ 163The purpose of the FENV_ACCESS pragma is to allow certain optimizations, for example global common subexpression elimination, code motion, and constant folding, that could subvert flag tests and mode changes. In general, if the state of FENV_ACCESS is off then the translator can assume that default modes are in effect and the flags are not tested. 7.6 Library 7.6.1 WG14/N843 Committee Draft -- August 3, 1998 211 #include void f(double x) { #pragma STDC FENV_ACCESS ON void g(double); void h(double); /* ... */ g(x + 1); h(x + 1); /* ... */ } [#4] 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.164) 7.6.2 Exceptions [#1] The following functions provide access to the exception flags.165) 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. ____________________ 164The side effects impose a temporal ordering that requires two evaluations of x + 1. On the other hand, without the #pragma STDC FENV_ACCESS ON pragma, and assuming the default state is off, just one evaluation of x + 1 would suffice. 165The functions fetestexcept, feraiseexcept, and feclearexcept support the basic abstraction of flags that are either set or clear. An implementation may endow exception flags with more information -- for example, the address of the code which first raised the exception; the functions fegetexceptflag and fesetexceptflag deal with the full content of flags. 7.6.1 Library 7.6.2 212 Committee Draft -- August 3, 1998 WG14/N843 7.6.2.1 The feclearexcept function Synopsis [#1] #include void feclearexcept(int excepts); Description [#2] The feclearexcept function clears the supported exceptions represented by its argument. 7.6.2.2 The fegetexceptflag function Synopsis [#1] #include void fegetexceptflag(fexcept_t *flagp, int excepts); Description [#2] 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 [#1] #include void feraiseexcept(int excepts); Description [#2] The feraiseexcept function raises the supported exceptions represented by its argument.166) 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- ____________________ 166The effect is intended to be similar to that of exceptions raised by arithmetic operations. Hence, enabled traps for exceptions raised by this function are taken. The specification in F.7.6 is in the same spirit. 7.6.2 Library 7.6.2.3 WG14/N843 Committee Draft -- August 3, 1998 213 defined. 7.6.2.4 The fesetexceptflag function Synopsis [#1] #include void fesetexceptflag(const fexcept_t *flagp, int excepts); Description [#2] 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 [#1] #include int fetestexcept(int excepts); Description [#2] 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.167) Returns [#3] The fetestexcept function returns the value of the bitwise OR of the exception macros corresponding to the currently set exceptions included in excepts. [#4] EXAMPLE Call f if invalid is set, then g if overflow is set: ____________________ 167This mechanism allows testing several exceptions with just one function call. 7.6.2.3 Library 7.6.2.5 214 Committee Draft -- August 3, 1998 WG14/N843 #include /* ... */ { #pragma STDC FENV_ACCESS ON int set_excepts; // 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 [#1] The fegetround and fesetround functions provide control of rounding direction modes. 7.6.3.1 The fegetround function Synopsis [#1] #include int fegetround(void); Description [#2] The fegetround function gets the current rounding direction. Returns [#3] The fegetround function returns the value of the rounding direction macro representing the current rounding direction. 7.6.3.2 The fesetround function Synopsis [#1] #include int fesetround(int round); Description [#2] 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 7.6.2.5 Library 7.6.3.2 WG14/N843 Committee Draft -- August 3, 1998 215 macro, the rounding direction is not changed. Returns [#3] 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 can be established). [#4] EXAMPLE 1 Save, set, and restore the rounding direction. Report an error and abort if setting the rounding direction fails. #include #include /* ... */ { #pragma STDC FENV_ACCESS ON int save_round; int setround_ok; save_round = fegetround(); setround_ok = fesetround(FE_UPWARD); assert(setround_ok); /* ... */ fesetround(save_round); /* ... */ } 7.6.4 Environment [#1] 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 [#1] #include void fegetenv(fenv_t *envp); Description [#2] The fegetenv function stores the current floating-point environment in the object pointed to by envp. 7.6.3.2 Library 7.6.4.1 216 Committee Draft -- August 3, 1998 WG14/N843 7.6.4.2 The feholdexcept function Synopsis [#1] #include int feholdexcept(fenv_t *envp); Description [#2] 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.168) Returns [#3] The feholdexcept function returns zero if and only if | non-stop exception handling was successfully installed. 7.6.4.3 The fesetenv function Synopsis [#1] #include void fesetenv(const fenv_t *envp); Description [#2] 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. ____________________ 168IEC 60559 systems have a default non-stop mode, and typically at least one other mode for trap handling or aborting; if the system provides only the non-stop mode then installing it is trivial. For such systems, the feholdexcept function can be used in conjunction with the feupdateenv function to write routines that hide spurious exceptions from their callers. 7.6.4.1 Library 7.6.4.3 WG14/N843 Committee Draft -- August 3, 1998 217 7.6.4.4 The feupdateenv function Synopsis [#1] #include void feupdateenv(const fenv_t *envp); Description [#2] 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. [#3] EXAMPLE 1 Hide spurious underflow exceptions: #include 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; } 7.6.4.3 Library 7.6.4.4 218 Committee Draft -- August 3, 1998 WG14/N843 7.7 Characteristics of floating types [#1] The header defines several macros that expand to various limits and parameters of the standard floating- point types. [#2] The macros, their meanings, and the constraints (or restrictions) on their values are listed in 5.2.4.2.2. 7.7 Library 7.7 WG14/N843 Committee Draft -- August 3, 1998 219 7.8 Format conversion of integer types [#1] The header includes the header and extends it with additional facilities provided by hosted implementations. [#2] It declares four functions for converting numeric character strings to greatest-width integers and, for each type declared in , it defines corresponding macros for conversion specifiers for use with the formatted input/output functions.169) Forward references: integer types (7.18). 7.8.1 Macros for format specifiers [#1] Each of the following object-like macros170) 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),171) followed by the conversion specifier, followed by a name corresponding to a similar type name 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. [#2] The fprintf macros for signed integers are: PRId8 PRId16 PRId32 PRId64 PRIdLEAST8 PRIdLEAST16 PRIdLEAST32 PRIdLEAST64 PRIdFAST8 PRIdFAST16 PRIdFAST32 PRIdFAST64 PRIdMAX PRIdPTR PRIi8 PRIi16 PRIi32 PRIi64 PRIiLEAST8 PRIiLEAST16 PRIiLEAST32 PRIiLEAST64 PRIiFAST8 PRIiFAST16 PRIiFAST32 PRIiFAST64 PRIiMAX PRIiPTR [#3] The fprintf macros for unsigned integers are: ____________________ 169See ``future library directions'' (7.26.4). 170C++ implementations should define these macros only when __STDC_FORMAT_MACROS is defined before is included. 171Separate 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. 7.8 Library 7.8.1 220 Committee Draft -- August 3, 1998 WG14/N843 PRIo8 PRIo16 PRIo32 PRIo64 PRIoLEAST8 PRIoLEAST16 PRIoLEAST32 PRIoLEAST64 PRIoFAST8 PRIoFAST16 PRIoFAST32 PRIoFAST64 PRIoMAX PRIoPTR PRIu8 PRIu16 PRIu32 PRIu64 PRIuLEAST8 PRIuLEAST16 PRIuLEAST32 PRIuLEAST64 PRIuFAST8 PRIuFAST16 PRIuFAST32 PRIuFAST64 PRIuMAX PRIuPTR PRIx8 PRIx16 PRIx32 PRIx64 PRIxLEAST8 PRIxLEAST16 PRIxLEAST32 PRIxLEAST64 PRIxFAST8 PRIxFAST16 PRIxFAST32 PRIxFAST64 PRIxMAX PRIxPTR PRIX8 PRIX16 PRIX32 PRIX64 PRIXLEAST8 PRIXLEAST16 PRIXLEAST32 PRIXLEAST64 PRIXFAST8 PRIXFAST16 PRIXFAST32 PRIXFAST64 PRIXMAX PRIXPTR [#4] The fscanf macros for signed integers are: SCNd8 SCNd16 SCNd32 SCNd64 SCNdLEAST8 SCNdLEAST16 SCNdLEAST32 SCNdLEAST64 SCNdFAST8 SCNdFAST16 SCNdFAST32 SCNdFAST64 SCNdMAX SCNdPTR SCNi8 SCNi16 SCNi32 SCNi64 SCNiLEAST8 SCNiLEAST16 SCNiLEAST32 SCNiLEAST64 SCNiFAST8 SCNiFAST16 SCNiFAST32 SCNiFAST64 SCNiMAX SCNiPTR [#5] The fscanf macros for unsigned integers are: SCNo8 SCNo16 SCNo32 SCNo64 SCNoLEAST8 SCNoLEAST16 SCNoLEAST32 SCNoLEAST64 SCNoFAST8 SCNoFAST16 SCNoFAST32 SCNoFAST64 SCNoMAX SCNoPTR SCNu8 SCNu16 SCNu32 SCNu64 SCNuLEAST8 SCNuLEAST16 SCNuLEAST32 SCNuLEAST64 SCNuFAST8 SCNuFAST16 SCNuFAST32 SCNuFAST64 SCNuMAX SCNuPTR SCNx8 SCNx16 SCNx32 SCNx64 SCNxLEAST8 SCNxLEAST16 SCNxLEAST32 SCNxLEAST64 SCNxFAST8 SCNxFAST16 SCNxFAST32 SCNxFAST64 SCNxMAX SCNxPTR [#6] Because the default argument promotions do not affect pointer parameters, there might not exist suitable fscanf format specifiers for some of the types defined in this header. Consequently, as a special exception to the requirement that the implementation define all macros 7.8.1 Library 7.8.1 WG14/N843 Committee Draft -- August 3, 1998 221 associated with each type defined by this header, in such a case the problematic fscanf macros may be left undefined. [#7] EXAMPLE #include #include 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 [#1] #include 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 [#2] 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 [#3] 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 [#1] 7.8.1 Library 7.8.2.2 222 Committee Draft -- August 3, 1998 WG14/N843 #include // for wchar_t #include 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 [#2] 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 [#3] 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. * 7.8.2.2 Library 7.8.2.2 WG14/N843 Committee Draft -- August 3, 1998 223 7.9 Alternative spellings [#1] The header 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 ^= 7.9 Library 7.9 224 Committee Draft -- August 3, 1998 WG14/N843 7.10 Sizes of integer types [#1] The header defines several macros that expand to various limits and parameters of the standard integer types. [#2] The macros, their meanings, and the constraints (or restrictions) on their values are listed in 5.2.4.2.1. 7.10 Library 7.10 WG14/N843 Committee Draft -- August 3, 1998 225 7.11 Localization [#1] The header declares two functions, one type, and defines several macros. [#2] 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 | [#3] 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.172) Additional macro definitions, beginning with the characters LC_ and an uppercase 7.11 Library 7.11 226 Committee Draft -- August 3, 1998 WG14/N843 letter,173) may also be specified by the implementation. 7.11.1 Locale control 7.11.1.1 The setlocale function Synopsis [#1] #include char *setlocale(int category, const char *locale); Description [#2] 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 functions174) 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 and strfxtime functions. [#3] 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. [#4] At program startup, the equivalent of setlocale(LC_ALL, "C"); is executed. ____________________ 172ISO/IEC 9945-2 specifies locale and charmap formats that may be used to specify locales for C. 173See ``future library directions'' (7.26.5). 174The only functions in 7.4 whose behavior is not affected by the current locale are isdigit and isxdigit. 7.11 Library 7.11.1.1 WG14/N843 Committee Draft -- August 3, 1998 227 [#5] The implementation shall behave as if no library function calls the setlocale function. Returns [#6] 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. [#7] 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.175) [#8] 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 strfxtime function (7.23.3.6), the strxfrm function (7.21.4.5). 7.11.2 Numeric formatting convention inquiry 7.11.2.1 The localeconv function Synopsis [#1] #include struct lconv *localeconv(void); Description [#2] 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. ____________________ 175The implementation shall arrange to encode in a string the various categories due to a heterogeneous locale when category has the value LC_ALL. 7.11.1.1 Library 7.11.2.1 228 Committee Draft -- August 3, 1998 WG14/N843 [#3] 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 | 7.11.2.1 Library 7.11.2.1 WG14/N843 Committee Draft -- August 3, 1998 229 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 | 7.11.2.1 Library 7.11.2.1 230 Committee Draft -- August 3, 1998 WG14/N843 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. [#4] 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. [#5] 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 A space separates the currency symbol and value. | 2 A space separates the currency symbol and the sign | string, if adjacent. | [#6] 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. 7.11.2.1 Library 7.11.2.1 WG14/N843 Committee Draft -- August 3, 1998 231 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. | [#7] The implementation shall behave as if no library function calls the localeconv function. Returns [#8] 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. [#9] EXAMPLE 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 1.234,56 | FIM -1.234,56| Italy ||L.1.234 | -L.1.234 | ITL 1.234 | -ITL 1.234| Netherlands ||f 1.234,56 | f -1.234,56 | NLG 1.234,56 | NLG -1.234,56| Switzerland ||SFrs.1,234.56 | SFrs.1,234.56C | CHF 1,234.56 | CHF 1,234.56C| [#10] For these four countries, the respective values for the monetary members of the structure returned by localeconv are: 7.11.2.1 Library 7.11.2.1 232 Committee Draft -- August 3, 1998 WG14/N843 || | | | ||Finland | Italy | Netherlands | Switzerland| -------------------++------------+-------------+-------------+------------| 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 ||1 | 1 | 1 | 1| int_n_sep_by_space ||1 | 1 | 1 | 1| int_p_sign_posn ||1 | 1 | 1 | 1| int_n_sign_posn ||4 | 1 | 4 | 2| 7.11.2.1 Library 7.11.2.1 WG14/N843 Committee Draft -- August 3, 1998 233 7.12 Mathematics [#1] The header 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.176) Integer arithmetic functions and conversion functions are discussed later. [#2] 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.177) [#3] 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.178) ____________________ 176Particularly on systems with wide expression evaluation, a function might pass arguments and return values in wider format than the synopsis prototype indicates. 177The types float_t and double_t are intended to be the implementation's most efficient types at least as wide as float and double, respectively. For FLT_EVAL_METHOD equal 0, 1, or 2, the type float_t is the narrowest type used by the implementation to evaluate floating expressions. 178HUGE_VAL, HUGE_VALF, and HUGE_VALL can be positive infinities in an implementation that supports infinities. 7.12 Library 7.12 234 Committee Draft -- August 3, 1998 WG14/N843 [#4] The macro INFINITY expands to a constant expression of type float representing an implementation-defined positive or unsigned infinity, if | available; else to a positive constant of type float that | overflows at translation time.179) [#5] 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 an implementation- defined quiet NaN. [#6] 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. [#7] 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.180) The macros FP_FAST_FMAF FP_FAST_FMAL are, respectively, float and long double analogs of FP_FAST_FMA. ____________________ 179In this case, using INFINITY will violate the constraint in 6.4.4 and thus require a diagnostic. 180Typically, the FP_FAST_FMA macro is defined if and only if the fma function is implemented directly with a hardware multiply-add instruction. Software implementations are expected to be substantially slower. 7.12 Library 7.12 WG14/N843 Committee Draft -- August 3, 1998 235 [#8] 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 [#9] 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 [#1] The behavior of each of the functions in is specified for all representable values of its input arguments, except where stated otherwise. [#2] 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.181) On a domain error, the function returns an implementation-defined value; whether the integer expression errno acquires the value EDOM is implementation-defined. [#3] 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 ____________________ 181In an implementation that supports infinities, this allows an infinity as an argument to be a domain error if the mathematical domain of the function does not include the infinity. 7.12 Library 7.12.1 236 Committee Draft -- August 3, 1998 WG14/N843 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.182) 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 [#1] #include #pragma STDC FP_CONTRACT on-off-switch Description [#2] 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 (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 [#1] In the synopses in this subclause, real-floating indicates that the argument shall be an expression of real floating type. The result is undefined if an argument is not of real floating type. ____________________ 182The term underflow here is intended to encompass both gradual underflow as in IEC 60559 and also flush-to-zero underflow. 7.12.1 Library 7.12.3 WG14/N843 Committee Draft -- August 3, 1998 237 7.12.3.1 The fpclassify macro Synopsis [#1] #include int fpclassify(real-floating x); Description [#2] 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.183) Returns [#3] The fpclassify macro returns the value of the number classification macro appropriate to the value of its argument. [#4] 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)) ____________________ 183Since an expression can be evaluated with more range and precision than its type has, it is important to know the type that classification is based on. For example, a normal long double value might become subnormal when converted to double, and zero when converted to float. 7.12.3 Library 7.12.3.1 238 Committee Draft -- August 3, 1998 WG14/N843 7.12.3.2 The isfinite macro Synopsis [#1] #include int isfinite(real-floating x); Description [#2] The isfinite macro determines whether its argument has a finite value (zero, subnormal, or normal, and not