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; __________ 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. General 1 Working Draft, 1997-11-21, WG14/N794 J11/97-158 - all minimal requirements of a data-processing system that is capable of supporting a conforming implementation. 2. Normative references [#1] The following standards contain provisions which, through reference in this text, constitute provisions of this International Standard. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this International Standard are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain registers of currently valid International Standards. IEC 559:1993, Binary floating-point arithmetic for microprocessor systems, second edition. ISO 646:1983, Information processing - ISO 7-bit coded character set for information interchange. ISO/IEC 2382-1:1993, Information technology - Vocabulary - Part 1: Fundamental terms. ISO 4217:1987, Codes for the representation of currencies and funds. ISO 8601:1988, Data elements and interchange formats - Information interchange - Representation of dates and times. ISO/IEC TR 10176, Information technology - Guidelines for the preparation of programming language standards. ISO/IEC 10646-1:1993, Information technology - Universal Multiple-Octet Coded Character Set (UCS) - Part 1: Architecture and Basic Multilingual Plane. 2 General Working Draft, 1997-11-21, WG14/N794 J11/97-158 3. Definitions and conventions [#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] For the purposes of this International Standard, the following definitions apply. Other terms are defined where they appear in italic type or being 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 2382-1. 3.1 Alignment [#1] A requirement that objects of a particular type be located on storage boundaries with addresses that are particular multiples of a byte address. 3.2 Argument [#1] An 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. Also known as ``actual argument'' or ``actual parameter.'' 3.3 Bit [#1] The unit of data storage in the execution environment large enough to hold an object that may have one of two values. It need not be possible to express the address of each individual bit of an object. General 3 Working Draft, 1997-11-21, WG14/N794 J11/97-158 3.4 Byte [#1] The unit of data storage large enough to hold any member of the basic character set of the execution environment. It shall be possible to express the address of each individual byte of an object uniquely. A byte is composed of a contiguous sequence of bits, the number of which is implementation-defined. The least significant bit is called the low-order bit; the most significant bit is called the high-order bit. 3.5 Character [#1] A bit representation that fits in a byte. The representation of each member of the basic character set in both the source and execution environments shall fit in a byte. 3.6 Constraints [#1] Restrictions, both syntactic and semantic, by which the exposition of language elements is to be interpreted. 3.7 Correctly rounded result [#1] A representation in the result format that is nearest in value, subject to the effective rounding mode, to what the result would be given unlimited range and precision. 3.8 Diagnostic message [#1] A message belonging to an implementation-defined subset of the implementation's message output. 3.9 Forward references [#1] References to later subclauses of this International Standard that contain additional information relevant to this subclause. 4 General Working Draft, 1997-11-21, WG14/N794 J11/97-158 3.10 Implementation [#1] A particular set of software, running in a particular translation environment under particular control options, that performs translation of programs for, and supports execution of functions in, a particular execution environment. 3.11 Implementation-defined behavior [#1] Unspecified behavior where each implementation shall document how the choice is made. 3.12 Implementation limits [#1] Restrictions imposed upon programs by the implementation. 3.13 Locale-specific behavior [#1] Behavior that depends on local conventions of nationality, culture, and language that each implementation shall document. 3.14 Multibyte character [#1] A sequence of one or more bytes representing a member of the extended character set of either the source or the execution environment. The extended character set is a superset of the basic character set. 3.15 Object [#1] A region of data storage in the execution environment, the contents of which can represent values. 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. When referenced, an object may be interpreted as having a particular type; see 6.2.2.1. General 5 Working Draft, 1997-11-21, WG14/N794 J11/97-158 3.16 Parameter [#1] An 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. Also known as ``formal argument'' or ``formal parameter.'' 3.17 Recommended practice [#1] Sections so entitled contain specification that is strongly recommended as being in keeping with the intent of the standard, but that may be impractical for some implementations. 3.18 Undefined behavior [#1] 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. Permissible 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). [#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] The implementation must successfully translate a given program unless a syntax error is detected, a constraint is violated, or it can determine that every possible execution of that program would result in undefined behavior. 6 General Working Draft, 1997-11-21, WG14/N794 J11/97-158 3.19 Unspecified behavior [#1] Behavior where this International Standard provides two or more possibilities and imposes no requirements on which is chosen in any instance. 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 subclause 5.1.2.3. Examples [#2] 1. An example of unspecified behavior is the order in which the arguments to a function are evaluated. 2. An example of undefined behavior is the behavior on integer overflow. 3. An example of implementation-defined behavior is the propagation of the high-order bit when a signed integer is shifted right. 4. An example of locale-specific behavior is whether the islower function returns true for characters other than the 26 lowercase Latin letters. Forward references: bitwise shift operators (6.3.7), expressions (6.3), function calls (6.3.2.2), the islower function (7.3.1.7), localization (7.5). 4. Compliance [#1] 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 __________ 2. This implies that a strictly conforming program can use features in a conditionally normative annex provided the use is conditioned by a #ifdef directive with the conformance macro for the annex, as in #ifdef __STDC_IEC_559__ /* FE_UPWARD defined */ /* ... */ fesetround(FE_UPWARD); /* ... */ General 7 Working Draft, 1997-11-21, WG14/N794 J11/97-158 dependent on any unspecified, undefined, or implementation- defined behavior, and shall not exceed any minimum implementation limit. [#2] 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 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 the behavior of any strictly conforming program.3 [#3] A conforming program is one that is acceptable to a conforming implementation.4 [#4] An implementation shall be accompanied by a document that defines all implementation-defined characteristics and all extensions. Forward references: limits and (7.1.5), variable arguments (7.12), common definitions (7.1.6), alternate spellings (7.17). ____________________________________________________________ #endif 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. 8 General Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 the environment clause (clause 5), 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, also known as 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 (6.8.1), linkages of identifiers (6.1.2.2), source file inclusion (6.8.2), external definitions (6.7), preprocessing directives (6.8). Environment 9 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. Any multibyte source file character not in the basic source character set is replaced by the universal- character-name that designates that multibyte character.6 Then, trigraph sequences are replaced by corresponding single-character internal representations. 2. Each instance of a backslash character immediately followed by a newline character is deleted, splicing physical source lines to form logical source lines. 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 tokens7 and sequences of white-space characters __________ 5. Implementations must behave as if these separate phases occur, even though many are typically folded together in practice. 6. The process of handling extended characters is specified in terms of mapping to an encoding that uses only the basic source character set, and, in the case of character literals and strings, further mapping to the execution character set. In practical terms, however, any internal encoding may be used, so long as an actual extended character encountered in the input, and the same extended character expressed in the input as a universal-character-name (i.e., using the \U or \u notation), are handled equivalently. 7. As described in 6.1, 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. 10 Environment Working Draft, 1997-11-21, WG14/N794 J11/97-158 (including comments). A source file shall not end in a partial preprocessing token or 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 (6.8.3.3), 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 a member of the execution character set. 6. Adjacent character string literal tokens are concatenated and adjacent wide string literal tokens are concatenated. 7. White-space characters separating tokens are no longer significant. Each preprocessing token is converted into a token. The resulting tokens are syntactically and semantically analyzed and translated as a translation unit. 8. All external object and function references are resolved. Library components are linked to satisfy external references to functions and objects not defined in the current translation. All such translator output is collected into a program image which contains information needed for execution in its execution environment. Constraints [#2] A universal-character-name shall not specify a character short identifier in the range 0000 through 0020 or 007F through 009F inclusive. A universal-character-name shall not designate a character in the basic source character set. Environment 11 Working Draft, 1997-11-21, WG14/N794 J11/97-158 Forward references: lexical elements (6.1), preprocessing directives (6.8), trigraph sequences (5.2.1.1), external definitions (6.7). 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.8 Examples [#2] 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. __________ 8. 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. 12 Environment Working Draft, 1997-11-21, WG14/N794 J11/97-158 Forward references: initialization (6.5.8). 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 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 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,9 or in some other implementation-defined manner. [#2] If they are defined, 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. __________ 9. 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. Environment 13 Working Draft, 1997-11-21, WG14/N794 J11/97-158 - If the value of argc is greater than zero, the array members argv[0] through argv[argc-1] inclusive shall contain pointers to strings, which are given implementation-defined values by the host environment prior to program startup. The intent is to supply to the program information determined prior to program startup from elsewhere in the hosted environment. If the host environment is not capable of supplying strings with letters in both uppercase and lowercase, the implementation shall ensure that the strings are received in lowercase. - If the value of argc is greater than zero, the string pointed to by argv[0] represents the program name; argv[0][0] shall be the null character if the program name is not available from the host environment. If the value of argc is greater than one, the strings pointed to by argv[1] through argv[argc-1] represent the program parameters. - The parameters argc and argv and the strings pointed to by the argv array shall be modifiable by the program, and retain their last-stored values between program startup and program termination. 5.1.2.2.2 Program execution [#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] 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.10 If the } that terminates the main function is reached, the termination status returned to the host environment is unspecified. __________ 10. In accordance with subclause 6.1.2.4, 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. 14 Environment Working Draft, 1997-11-21, WG14/N794 J11/97-158 Forward references: definition of terms (7.1.1), the exit function (7.14.4.3). 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 those operations are all side effects,11 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. [#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 __________ 11. The IEC 559 standard for binary floating-point arithmetic requires certain status flags and control modes, with user access. Floating-point operations implicitly set the status flags; modes affect result values of floating-point operations. Implementations that support such floating-point state will need 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. Environment 15 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 evaluations are complete and subsequent evaluations have not yet occurred. - At program termination, all data written into files shall be identical to the result that execution of the program according to the abstract semantics would have produced. - The input and output dynamics of interactive devices shall take place as specified in 7.13.3. 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. Examples 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. 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 16 Environment Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. 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. 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). 4. Implementations employing wide registers must take care to honor appropriate semantics. Values must be independent of whether they are represented in a register or in memory. For example, an implicit spilling of a register must not alter the value. Also, an explicit store and load must round to the precision of the storage type. In particular, casts and assignments must perform their specified conversion: for the fragment double d1, d2; float f; d1 = f = expression; d2 = (float) expressions; Environment 17 Working Draft, 1997-11-21, WG14/N794 J11/97-158 the values assigned to d1 and d2 must have been converted to float. 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 Annex 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 of y = x * 0.2; 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 18 Environment Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. 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 ;), and the call to getchar can occur at any point prior to the need of its returned value. Forward references: compound statement, or block (6.6.2), expressions (6.3), files (7.13.3), sequence points (6.3, 6.6), the signal function (7.11), type qualifiers (6.5.3). Environment 19 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. 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 20 Environment Working Draft, 1997-11-21, WG14/N794 J11/97-158 control characters representing alert, backspace, carriage return, and new line. If any other characters are encountered in a source file (except in 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. hex-quad: hexadecimal-digit hexadecimal-digit hexadecimal-digit hexadecimal-digit universal-character-name: \u hex-quad \U hex-quad hex-quad [#5] The character designated by the universal-character- name \Unnnnnnnn is that character whose character short identifier is nnnnnnnn specified by ISO/IEC 10646-1; the character designated by the universal-character-name \unnnn is that character whose character short identifier is 0000nnnn specified by ISO/IEC 10646-1. Forward references: identifiers (6.1.2), character constants (6.1.3.4), preprocessing directives (6.8), string literals (6.1.4), comments (6.1.9), 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 sequences12) are replaced with the corresponding single character. __________ 12. The trigraph sequences enable the input of characters that are not defined in the Invariant Code Set as described in ISO/IEC 646:1991, which is a subset of the seven-bit ASCII code set. Environment 21 Working Draft, 1997-11-21, WG14/N794 J11/97-158 ??= # ??( [ ??/ \ ??) ] ??' ^ ??< { ??! | ??> } ??- ~ No other trigraph sequences exist. Each ? that does not begin one of the trigraphs listed above is not changed. Examples [#2] 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. - The presence, meaning, and representation of any additional members is locale-specific. - A multibyte character 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. 22 Environment Working Draft, 1997-11-21, WG14/N794 J11/97-158 - A byte with all bits zero shall not occur in the second or subsequent bytes of a multibyte character. [#2] For the source character set, the following shall hold: - A comment, string literal, character constant, or header name shall begin and end in the initial shift state. - A 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 function would appear. The intent of writing a printable character (as defined by the isprint 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. \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 Environment 23 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 (7.3.1.8), the fputc function (7.13.7.3). 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 comprise 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 environmental limits on a conforming implementation. 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:13 - 127 nesting levels of compound statements, iteration statements, and selection statements __________ 13. Implementations should avoid imposing fixed translation limits whenever possible. 24 Environment Working Draft, 1997-11-21, WG14/N794 J11/97-158 - 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 - 31 significant initial characters in an external identifier - 4095 external identifiers in one translation unit - 511 identifiers with block scope declared in one block - 4095 macro identifiers simultaneously defined in one preprocessing translation unit - 127 parameters in one function definition - 127 arguments in one function call - 127 parameters in one macro definition - 127 arguments in one macro invocation - 4095 characters in a logical source line - 4095 characters in a character string literal or wide string literal (after concatenation) - 65535 bytes in an object (in a hosted environment only) - 15 nesting levels for #included files - 1023 case labels for a switch statement (excluding those for any nested switch statements) - 1023 members in a single structure or union - 1023 enumeration constants in a single enumeration - 63 levels of nested structure or union definitions in a single struct-declaration-list Environment 25 Working Draft, 1997-11-21, WG14/N794 J11/97-158 5.2.4.2 Numerical limits [#1] A conforming implementation shall document all the limits specified in this subclause, which shall be specified in the headers and . 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 - maximum value for an object of type signed char SCHAR_MAX +127 - maximum value for an object of type unsigned char UCHAR_MAX 255 - 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 - maximum value for an object of type short int SHRT_MAX +32767 - maximum value for an object of type unsigned short int USHRT_MAX 65535 26 Environment Working Draft, 1997-11-21, WG14/N794 J11/97-158 - minimum value for an object of type int INT_MIN -32767 - maximum value for an object of type int INT_MAX +32767 - maximum value for an object of type unsigned int UINT_MAX 65535 - minimum value for an object of type long int LONG_MIN -2147483647 - maximum value for an object of type long int LONG_MAX +2147483647 - maximum value for an object of type unsigned long int ULONG_MAX 4294967295 - minimum value for an object of type long long int LLONG_MIN -9223372036854775807 - maximum value for an object of type long long int LLONG_MAX +9223372036854775807 - maximum value for an object of type unsigned long long int ULLONG_MAX 18446744073709551615 [#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.14 The value UCHAR_MAX+1 shall equal 2 raised to the power 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.15 The __________ 14. See 6.1.2.5. 15. The floating-point model is intended to clarify the description of each floating-point characteristic and Environment 27 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 e and a maximum e ) p precision (the number of base-b digits inithe significand) max f nonnegative integers less than b (the significand digits) k [#2] A normalized floating-point number x (f > 0 if x / 0) is defined by the following model: 1 e p -k x = s x b x R fk x b , emin < e < emax k=1 [#3] Floating types might include values that are not normalized floating-point numbers, for example subnormal numbers (x / 0, e = e , f = 0), infinities, and NaNs. A NaN is an encodingmisignifying 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] All integer values in the header, except FLT_ROUNDS, shall be constant expressions suitable for use in #if preprocessing directives; all floating values shall be constant expressions. All except 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. [#5] The rounding mode for floating-point addition is characterized by the value of FLT_ROUNDS:17 ____________________________________________________________ does not require the floating-point arithmetic of the implementation to be identical. 16. IEC 559:1993 specifies quiet and signaling NaNs. For implementations that do not support IEC 559:1993, the terms quiet NaN and signaling NaN are intended to apply to encodings with similar behavior. 17. Evaluation of FLT_ROUNDS correctly reflects any execution-time change of rounding mode through the function fesetround in . 28 Environment Working Draft, 1997-11-21, WG14/N794 J11/97-158 -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. [#6] 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. [#7] The values given in the following list shall be replaced by implementation-defined expressions that shall be equal or greater in magnitude (absolute value) to those shown, with the same sign: - radix of exponent representation, b FLT_RADIX 2 __________ 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. Environment 29 Working Draft, 1997-11-21, WG14/N794 J11/97-158 - 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, 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, (1 if b is a power of 10 | (p - 1) x log b | + | | 10 | (0 otherwise FLT_DIG 6 DBL_DIG 10 LDBL_DIG 10 - minimum negative integer such that FLT_RADIX raised to that power minus 1 is a normalized floating-point number, e min 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 | e -1 | numbers, | log b min | | 10 | FLT_MIN_10_EXP -37 DBL_MIN_10_EXP -37 LDBL_MIN_10_EXP -37 - maximum integer such that FLT_RADIX raised to that power minus 1 is a representable finite floating-point number, e max 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 | e | numbers, | log ((1 - b-p) x b max) | | 10 | 30 Environment Working Draft, 1997-11-21, WG14/N794 J11/97-158 FLT_MAX_10_EXP +37 DBL_MAX_10_EXP +37 LDBL_MAX_10_EXP +37 [#8] The values given in the following list shall be replaced by implementation-defined expressions with values that shall be equal to or greater than those shown: - max-pum reemaxentable finite floating-point number, (1 - b ) x b FLT_MAX 1E+37 DBL_MAX 1E+37 LDBL_MAX 1E+37 [#9] The values given in the following list shall be replaced by implementation-defined expressions with values that shall be equal to or less than those shown: - the difference between 1 and the least value greater than 1 that1-ps representable in the given floating point type, b FLT_EPSILON 1E-5 DBL_EPSILON 1E-9 LDBL_EPSILON 1E-9 - eini-1m normalized positive floating-point number, b min FLT_MIN 1E-37 DBL_MIN 1E-37 LDBL_MIN 1E-37 Examples [#10] 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: e 6 -k x = s x 16 x R fk x 16 , -31 < e < +32 k=1 Environment 31 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 2. The following describes floating-point representations that also meet the requirements for single-precision and double-precision normalized numbers in IEC 559,19 and the appropriate values in a header for types float and double: e 24 -k xf = s x 2 x R fk x 2 , -125 < e < +128 k=1 e 53 -k xd = s x 2 x R fk x 2 , -1021 < e < +1024 k=1 __________ 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. 32 Environment Working Draft, 1997-11-21, WG14/N794 J11/97-158 FLT_RADIX 2 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 Forward references: conditional inclusion (6.8.1). Environment 33 Working Draft, 1997-11-21, WG14/N794 J11/97-158 6. Language [#1] In the syntax notation used in the language clause (clause 6), 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. 6.1 Lexical elements Syntax [#1] token: keyword identifier constant string-literal operator punctuator preprocessing-token: header-name identifier pp-number character-constant string-literal operator punctuator 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, an operator, or a punctuator. 34 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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, operators, 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, operators, punctuators, and single non-white-space characters that do not lexically match the other preprocessing token categories. 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 6.8, in certain circumstances during translation phase 4, white space (or the absence thereof) serves as more than preprocessing token separation. White space may appear within a preprocessing token only as part of a header name or between the quotation characters in a character constant or string literal. [#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. [#5] 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. Examples 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. 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. Language 35 Working Draft, 1997-11-21, WG14/N794 J11/97-158 Forward references: character constants (6.1.3.4), comments (6.1.9), expressions (6.3), floating constants (6.1.3.1), header names (6.1.7), macro replacement (6.8.3), postfix increment and decrement operators (6.3.2.4), prefix increment and decrement operators (6.3.3.1), preprocessing directives (6.8), preprocessing numbers (6.1.8), string literals (6.1.4). 6.1.1 Keywords Syntax [#1] keyword: one of auto break case char complex const continue default do double else enum extern float for goto if imaginary inline int long register restrict return short signed sizeof static struct switch typedef union unsigned void volatile while Semantics [#2] The token complex is reserved in translation units where the header is included; the token imaginary is reserved in translation units where both the header is included and the macro _Imaginary_I is defined; all other keyword tokens are reserved in all translation units. When reserved, the above tokens (entirely in lowercase) are keywords (in translation phases 7 and 8), and shall not be used otherwise. When the token complex or imaginary is reserved, its use prior to the first inclusion of the header results in undefined behavior. 36 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.1.2 Identifiers Syntax [#1] identifier: nondigit identifier nondigit identifier digit nondigit: one of universal-character-name _ 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 Description [#2] An identifier is a sequence of nondigit characters (including the underscore _ and the lowercase and uppercase letters) and digits. Each universal-character-name in an identifier shall designate a character whose encoding in ISO 10646-1 falls into one of the ranges specified in Annex H.20 The first character shall be a nondigit character. Semantics [#3] An identifier can denote an object, a function, or one of the following entities that will be described later: 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 __________ 20. 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. Language 37 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. [#4] There is no specific limit on the maximum length of an identifier. [#5] 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 [#6] The implementation shall treat at least the first 63 characters of an internal name (a macro name or an identifier that does not have external linkage) as significant. The implementation may further restrict the significance of an external name (an identifier that has external linkage) to 31 characters. In both external and internal names, lower-case and upper-case letters are different. The number of significant characters in an identifier is implementation-defined. [#7] Any identifiers that differ in a significant character are different identifiers. If two identifiers differ in a nonsignificant character, the behavior is undefined. Forward references: linkages of identifiers (6.1.2.2), macro replacement (6.8.3). 6.1.2.1 Scopes of identifiers [#1] 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 non-overlapping 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.) [#2] 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. 38 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#3] Every other identifier has scope determined by the placement of its declaration (in a declarator or type specifier). If the declarator or type specifier that declares the identifier appears outside of any block or list of parameters, the identifier has file scope, which terminates at the end of the translation unit. If the declarator or type specifier that declares the identifier appears inside a block or within the list of parameter declarations in a function definition, the identifier has block scope, which terminates at the } 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. [#4] 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. [#5] Two identifiers have the same scope if and only if their scopes terminate at the same point. [#6] 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 (6.6.2), declarations (6.5), enumeration specifiers (6.5.2.2), function calls (6.3.2.2), function declarators (including prototypes) (6.5.5.3), function definitions (6.7.1), the goto statement (6.6.6.1), labeled statements (6.6.1), name spaces of identifiers (6.1.2.3), scope of macro definitions (6.8.3.5), source file inclusion (6.8.2), tags (6.5.2.3), type specifiers (6.5.2). Language 39 Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.1.2.2 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 instance of a particular identifier with external linkage denotes the same object or function. Within one translation unit, each instance of an identifier with internal linkage denotes the same object or function. Identifiers with no linkage denote unique entities. [#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.21 [#4] For an identifier declared with the storage-class specifier extern in a scope in which a prior declaration of that identifier is visible,22 if the prior declaration specifies internal or external linkage, the linkage of the identifier at the later declaration becomes 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. __________ 21. A function declaration can contain the storage-class specifier static only if it is at file scope; see 6.5.1. 22. As specified in 6.1.2.1, the later declaration might hide the prior declaration. 40 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#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 (6.6.2), declarations (6.5), expressions (6.3), external definitions (6.7). 6.1.2.3 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 any23 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 (6.5.2.2), labeled statements (6.6.1), structure and union specifiers (6.5.2.1), structure and union members (6.3.2.3), tags (6.5.2.3). __________ 23. There is only one name space for tags even though three are possible. Language 41 Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.1.2.4 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 7.14.3. [#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.24 [#3] An object whose identifier is declared with no linkage and without the storage-class specifier static has automatic storage duration. Storage is guaranteed to be reserved for a new instance of such an object on each normal entry into the block with which it is associated. If the block with which the object is associated is entered by a jump from outside the block to a labeled statement in the block or in an enclosed block, then storage is guaranteed to be reserved provided the object does not have a variable length array type. If the object is variably modified and the block is entered by a jump to a labeled statement, then the behavior is undefined. If an initialization is specified for the value stored in the object, it is performed on each normal entry, but not if the block is entered by a jump to a labeled statement beyond the declaration. A backwards jump might cause the initializer to be evaluated more than once; if so, a new value will be stored each time. Storage for the object is no longer guaranteed to be reserved when execution of the block ends in any way. (Entering an enclosed block suspends but does not end execution of the enclosing block. Calling a function suspends but does not end execution of the block containing the call.) The value of a pointer that referred to an object with automatic storage duration that is no longer guaranteed to be reserved __________ 24. 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 may not be explicit in the program. 42 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 is indeterminate. During execution of the associated block, the object has a constant address. Forward references: compound statement, or block (6.6.2), function calls (6.3.2.2), declarators (6.5.5), array declarators (6.5.5.2), initialization (6.5.8). 6.1.2.5 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 char is large enough to store any member of the basic execution character set. If a member of the required source character set enumerated in 5.2.1 is stored in a char object, its value is guaranteed to be positive. If any other character is stored in a char object, the resulting value is implementation-defined but shall be within the range of values that can be represented in that type. [#3] 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 6.5.2.) There may also be implementation-defined extended signed integer types.25 The standard and extended signed integer types are collectively called just signed integer types.26 [#4] 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 __________ 25. Implementation-defined keywords must 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. Language 43 Working Draft, 1997-11-21, WG14/N794 J11/97-158 contain any value in the range INT_MIN to INT_MAX as defined in the header ). [#5] 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 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. [#6] The extended signed integer types and extended unsigned integer types are collectively called the extended integer types. [#7] For any two types with the same signedness and different integer conversion rank, the range of values of the type with smaller integer conversion rank is a subrange of the values of the other type. [#8] 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 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. [#9] 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. [#10] There are three complex types, designated as float complex, double complex, and long double complex.28 The __________ 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. 44 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 real floating and complex types are collectively called the floating types. [#11] 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. [#12] 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. [#13] 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 [#14] The three types char, signed char, and unsigned char are collectively called the character types. The implementation shall define char to have the same range, representation, and behavior as one of signed char and unsigned char.30 [#15] An enumeration comprises a set of named integer constant values. Each distinct enumeration constitutes a different enumerated type. [#16] The void type comprises an empty set of values; it is an incomplete type that cannot be completed. __________ 29. An implementation may define new keywords that provide alternative ways to designate a basic (or any other) type. An alternate way to designed a basic type does not violate the requirement that all basic types be different. Implementation-defined keywords must have the form of an identifier reserved for any use as described in 7.1.3. 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 it not compatible with either. Language 45 Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#17] 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, each of which has an optionally specified name and possibly distinct type. - A union type describes an overlapping nonempty set of member objects, each of which has an optionally specified name and possibly distinct type. - A function type describes a function with specified return type. A function type is characterized by its return type and the number and types of its parameters. A function type is said to be derived from its return type, and if its return type is T, the function type is sometimes called ``function returning T.'' The construction of a function type from a return type is called ``function type derivation.'' - A pointer type may be derived from a function type, an object type, or an incomplete type, called the referenced type. A pointer type describes an object whose value provides a reference to an entity of the referenced type. A pointer type derived from the referenced type T is sometimes called ``pointer to T.'' The construction of a pointer type from a referenced type is called ``pointer type derivation.'' [#18] These methods of constructing derived types can be applied recursively. [#19] The type char, the signed and unsigned integer types, and the enumerated types are collectively called integer __________ 31. Since object types do not include incomplete types, an array of incomplete type cannot be constructed. 46 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 types. The integer and real floating types are collectively called real types. [#20] 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 [#21] Each arithmetic type belongs to one type-domain. The real type-domain comprises the real types. The complex type-domain comprises the complex types. [#22] 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 6.5.2.3) 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. [#23] 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. [#24] 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. [#25] 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 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. __________ 32. Note that aggregate type does not include union type because an object with union type can only contain one member at a time. 33. See 6.5.3 regarding qualified array and function types. Language 47 Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#26] 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. Examples [#27] 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. 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 (6.1.3.4), compatible type and composite type (6.1.2.6), integer conversion rank (6.2.1.1), declarations (6.5), tags (6.5.2.3), type qualifiers (6.5.3). 6.1.2.6 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 6.5.2 for type specifiers, in 6.5.3 for type qualifiers, and in 6.5.5 for declarators.34 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 __________ 34. Two types need not be identical to be compatible. 48 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. - 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,35 if the prior declaration specifies internal or external linkage, the type of the identifier at the later declaration becomes the composite type. __________ 35. As specified in 6.1.2.1, the later declaration might hide the prior declaration. Language 49 Working Draft, 1997-11-21, WG14/N794 J11/97-158 Examples [#5] 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 (6.5.5), enumeration specifiers (6.5.2.2), structure and union specifiers (6.5.2.1), type definitions (6.5.7), type qualifiers (6.5.3), type specifiers (6.5.2). 6.1.2.7 Predefined identifiers [#1] The following identifier shall be defined by the implementation: __func__ The name of the lexically-enclosing function. Forward references: the identifier __func__ (6.3.1.1). 6.1.2.8 Representations of types [#1] The representations of all types are unspecified except as stated in this subclause. 6.1.2.8.1 General [#1] Values of type unsigned char shall be represented using a pure binary notation.36 [#2] When stored in objects of any other object type, values of that type consist of n*CHAR_BIT bits, where n is the size __________ 36. 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. 50 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 with the same object representation shall compare equal, but values that compare equal might have different object representations. [#3] Certain object representations might not represent a value of that type. If the stored value of an object has such a representation and is accessed by an lvalue expression that does not have character type, the behavior is undefined. If such a representation is produced by a side effect that modifies all or any part of the object by an lvalue expression that does not have character type, the behavior is undefined.37 Such representations are called trap representations. [#4] When a value is stored in an object of structure or union type, including in a member object, the bytes of the object representation that correspond to any padding bytes take unspecified values.38 The values of padding bytes shall not affect whether the value of such an object is a trap representation. Those bits of a structure or union object that are in the same byte as a bit-field member, but are not part of that member, shall similarly not affect whether the value of such an object is a trap representation. [#5] When a value is stored in a member of an object of union type, the bytes of the object representation that do not correspond to that member but do correspond to other members take unspecified values, but the value of the union object shall not thereby become a trap representation. [#6] 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 __________ 37. 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. 38. Thus, for example, structure assignment may be implemented element-at-a-time or via memcpy. Language 51 Working Draft, 1997-11-21, WG14/N794 J11/97-158 which representation is used, but a trap representation shall not be generated. 6.1.2.8.2 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.39 [#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 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. __________ 41. The semantics of these characters were discussed in 5.2.2. 42. See ``future language directions'' (6.9.1). 60 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 Examples [#12] 1. The construction '\0' is commonly used to represent the null character. 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 . 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.) 4. Even if 12 or more bits are used for objects that have type wchar_t, the construction L'\1234' specifies the implementation-defined value that results from the combination of the values 0123 and '4'. Forward references: characters and integers (6.2.1.1), common definitions (7.1.6), the mbtowc function (7.14.7.2). 6.1.4 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 Language 61 Working Draft, 1997-11-21, WG14/N794 J11/97-158 s-char: any member of the source character set except the double-quote ", backslash \, or new-line character escape-sequence universal-character-name 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 from a string literal or literals.43 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. __________ 43. 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. 62 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#6] These arrays need not be distinct provided their elements have the appropriate values. If the program attempts to modify such an array, the behavior is undefined. Examples [#7] This pair of adjacent character string literals "\x12" "3" produces a single character string literal containing the two characters whose values are 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 (7.1.6). 6.1.5 Operators Syntax [#1] operator: one of [ ] ( ) . -> ++ -- & * + - ~ ! sizeof / % << >> < > <= >= == != ^ | && || ? : = *= /= %= += -= <<= >>= &= ^= |= , # ## <: :> %: %:%: Constraints [#2] The operators [ ], ( ), and ? : (independent of spelling) shall occur in pairs, possibly separated by expressions. The operators # and ## (also spelled %: and %:%:, respectively) shall occur in macro-defining preprocessing directives only. Semantics [#3] An operator specifies an operation to be performed (an evaluation) that yields a value, or yields a designator, or produces a side effect, or a combination thereof. An operand is an entity on which an operator acts. [#4] In all aspects of the language, these six tokens <: :> <% %> %: %:%: Language 63 Working Draft, 1997-11-21, WG14/N794 J11/97-158 behave, respectively, the same as these six tokens [ ] { } # ## except for their spelling.44 Forward references: expressions (6.3), macro replacement (6.8.3). 6.1.6 Punctuators Syntax [#1] punctuator: one of [ ] ( ) { } * , : = ; ... # <: :> <% %> %: Constraints [#2] The punctuators [ ], ( ), and { } (independent of spelling) shall occur (after translation phase 4) in pairs, possibly separated by expressions, declarations, or statements. The punctuator # (also spelled %:) shall occur in preprocessing directives only. Semantics [#3] A punctuator is a symbol that has independent syntactic and semantic significance but does not specify an operation to be performed that yields a value. Depending on context, the same symbol may also represent an operator or part of an operator. Forward references: expressions (6.3), declarations (6.5), preprocessing directives (6.8), statements (6.6). __________ 44. Thus [ and <: behave differently when ``stringized'' (see subclause 6.8.3.2), but can otherwise be freely interchanged. 64 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.1.7 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 " 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 6.8.2. [#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.45 A header name preprocessing token is recognized only within a #include preprocessing directive. __________ 45. Thus, sequences of characters that resemble escape sequences cause undefined behavior. Language 65 Working Draft, 1997-11-21, WG14/N794 J11/97-158 Examples [#4] 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 (6.8.2). 6.1.8 Preprocessing numbers Syntax [#1] pp-number: digit . digit pp-number digit pp-number nondigit pp-number e sign pp-number E sign pp-number p sign pp-number P sign pp-number . Description [#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. 66 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.1.9 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.46 [#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. Examples [#3] "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; __________ 46. Thus, /* ... */ comments do not nest. Language 67 Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.2 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 6.2.1.7 summarizes the conversions performed by most ordinary operators; it is supplemented as required by the discussion of each operator in 6.3. [#2] Conversion of an operand value to a compatible type causes no change to the value or the representation. Forward references: cast operators (6.3.4). 6.2.1 Arithmetic operands 6.2.1.1 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 any standard signed integer type shall be greater than the rank of any extended signed integer type with the same 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. - The rank of char shall equal the rank of signed char and unsigned char. - 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 68 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 int, signed int, or unsigned int. [#3] 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.47 All other types are unchanged by the integer promotions. [#4] 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 (6.5.2.2), structure and union specifiers (6.5.2.1). 6.2.1.2 Signed and unsigned integers [#1] When a value with integer type is converted to another integer type, 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. __________ 47. 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. Language 69 Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#3] Otherwise, the new type is signed and the value cannot be represented in it; the result is implementation-defined. 6.2.1.3 Real floating and integer [#1] When a value of real floating type is converted to integer type, the fractional part is discarded. If the value of the integral part cannot be represented by the integer type, the behavior is undefined.48 [#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. 6.2.1.4 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. 6.2.1.5 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. __________ 48. 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). 70 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.2.1.6 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 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. 6.2.1.7 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.49 Otherwise, the integer promotions are performed on both operands. Then the following rules are applied to the __________ 49. 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). Language 71 Working Draft, 1997-11-21, WG14/N794 J11/97-158 promoted operands: If both operands have the same type, then no further conversion is needed. Otherwise, if both operands have signed integer types or both have unsigned integer types, the operand with the type of lesser integer conversion rank is converted to the type of the operand with greater rank. Otherwise, if the operand that has unsigned integer type has rank greater or equal to the rank of the type of the other operand, then the operand with signed integer type is converted to the type of the operand with unsigned integer type. Otherwise, if the type of the operand with signed integer type can represent all of the values of the type of the operand with unsigned integer type, then the operand with unsigned integer type is converted to the type of the operand with signed integer type. Otherwise, both operands are converted to the unsigned integer type corresponding to the type of the operand with signed integer type. [#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.50 __________ 50. The cast and assignment operators still must perform their specified conversions, as described in 6.2.1.3 and 6.2.1.4. 72 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.2.2 Other operands 6.2.2.1 Lvalues and function designators [#1] An lvalue is an expression (with an object type or an incomplete type other than void) that designates an object.51 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 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 character string literal used to initialize an array of character type, or is a wide string literal used to initialize an array with element type compatible with wchar_t, an lvalue that has type ``array of type'' is converted to an expression that has 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. __________ 51. The name ``lvalue'' comes originally from the assignment expression E1 = E2, in which the left operand E1 must 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. Language 73 Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#4] A function designator is an expression that has function type. Except when it is the operand of the sizeof operator52 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 (6.3.3.2), assignment operators (6.3.16), common definitions (7.1.6), initialization (6.5.8), postfix increment and decrement operators (6.3.2.4), prefix increment and decrement operators (6.3.3.1), the sizeof operator (6.3.3.4), structure and union members (6.3.2.3). 6.2.2.2 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 occurs in a context where a void expression is required, its value or designator is discarded. (A void expression is evaluated for its side effects.) 6.2.2.3 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. [#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.53 If a null pointer constant is assigned __________ 52. Because this conversion does not occur, the operand of the sizeof operator remains a function designator and violates the constraint in 6.3.3.4. 53. The macro NULL is defined in as a null pointer constant; see 7.1.6. 74 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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.54 [#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.55 [#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 aligned56 for the pointed to type, the behavior is undefined. Otherwise, when converted back again, the result shall compare equal to the original pointer. [#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 that has a type that is not compatible with the type of the called function, the behavior is undefined. __________ 54. 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. 55. Thus, if the conversion is to unsigned int but yields a negative value, the behavior is undefined. 56. 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. Language 75 Working Draft, 1997-11-21, WG14/N794 J11/97-158 Forward references: cast operators (6.3.4), equality operators (6.3.9), simple assignment (6.3.16.1). 76 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.3 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.57 [#3] Except as indicated by the syntax58 or otherwise specified later (for the function-call operator (), &&, ||, ?:, and comma operators), the order of evaluation of subexpressions and the order in which side effects take place are both unspecified. __________ 57. This paragraph renders undefined statement expressions such as i = ++i + 1; while allowing i = i + 1; 58. 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 (6.3.6) shall be those expressions defined in 6.3.1 through 6.3.6. The exceptions are cast expressions (6.3.4) as operands of unary operators (6.3.3), and an operand contained between any of the following pairs of operators: grouping parentheses () (6.3.1), subscripting brackets [] (6.3.2.1), function-call parentheses () (6.3.2.2), and the conditional operator ?: (6.3.15). 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. Language 77 Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#4] Some operators (the unary operator ~, and the binary operators <<, >>, &, ^, and |, collectively described as bitwise operators) shall have operands that have integer type. These operators return values that depend on the internal representations of integers, and have implementation-defined and undefined aspects for signed types. [#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 it has one. 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:59 - 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 __________ 59. The intent of this list is to specify those circumstances in which an object may or may not be aliased. 78 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 omitting rounding errors implied by the source code and the expression evaluation method.60 The FP_CONTRACT pragma in provides a way to disallow contracted expressions. Otherwise, whether and how expressions are contracted is implementation-defined.61 6.3.1 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).62 __________ 60. A contracted expression might also omit the raising of floating-point exception flags. 61. 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 must be well-defined and clearly documented. 62. Thus, an undeclared identifier is a violation of the syntax. Language 79 Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#3] A constant is a primary expression. Its type depends on its form and value, as detailed in 6.1.3. [#4] A string literal is a primary expression. It is an lvalue with type as detailed in 6.1.4. [#5] A parenthesized expression is a primary expression. Its type and value are identical to those of the unparenthesized expression. It is an lvalue, a function designator, or a void expression if the unparenthesized expression is, respectively, an lvalue, a function designator, or a void expression. Forward references: declarations (6.5). 6.3.1.1 The identifier __func__ Semantics [#1] The identifier __func__ is 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.63 This name is the unadorned name of the function. [#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. Examples [#3] Consider the code fragment: __________ 63. 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. 80 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 #include void myfunc(void) { printf("%s\n", __func__); /* ... */ } Each time the function is called, it will print to the standard output stream: myfunc 6.3.2 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 6.3.2.1 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 Language 81 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 ixjx ... xk, then E (used as other than an lvalue) is converted to a pointer to an (n- 1)-dimensional array with dimensions jx ... x 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). Examples [#4] Consider the array object defined by the declaration int x[3][5]; Here x is a 3x5 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 in turn is converted to a pointer to the first of the ints, so x[i][j] yields an int. Forward references: additive operators (6.3.6), address and indirection operators (6.3.3.2), array declarators (6.5.5.2). 6.3.2.2 Function calls Constraints [#1] The expression that denotes the called function64 shall have type pointer to function returning void or returning an object type other than an array type. __________ 64. Most often, this is the result of converting an identifier that is a function designator. 82 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#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.65 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 6.6.6.4. Otherwise, the function call has type void. [#5] 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 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 __________ 65. 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 6.7.1. Language 83 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. [#6] If the function is defined with a type that includes a prototype, and the types of the arguments after promotion are not compatible with the types of the parameters, or if the prototype ends with an ellipsis (, ...), the behavior is undefined. [#7] If the expression that denotes the called function has a type that includes 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] 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. [#9] No other conversions are performed implicitly; in particular, the number and types of arguments are not compared with those of the parameters in a function definition that does not include a function prototype declarator. [#10] The order of evaluation of the function designator, the arguments, and subexpressions within the 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. Examples [#12] 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 shall be completed before the function pointed to by pf[f1()] is entered. 84 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 Forward references: function declarators (including prototypes) (6.5.5.3), function definitions (6.7.1), the return statement (6.6.6.4), simple assignment (6.3.16.1). 6.3.2.3 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 to which the first expression points, and is an lvalue.66 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.67 One special guarantee is made in order to __________ 66. 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. 67. 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 Language 85 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. Examples [#6] 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. 2. 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) /* ... */ ____________________________________________________________ is an appropriately sized array of character type), but must be accounted for when conforming to externally imposed storage layouts. 86 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 3. 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 (6.3.3.2), structure and union specifiers (6.5.2.1). 6.3.2.4 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). Language 87 Working Draft, 1997-11-21, WG14/N794 J11/97-158 Forward references: additive operators (6.3.6), compound assignment (6.3.16.2). 6.3.2.5 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.68 [#5] If the type name specifies an array of unknown size, the size is determined by the initializer list as specified in 6.5.7, 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 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 6.5.8 are applicable to compound literals.69 __________ 68. 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. 88 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#8] String literals, and compound literals with const- qualified types, need not designate distinct objects.70 Examples [#9] 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 must be constant. The unnamed object has static storage duration. 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. 3. Initializers with designations can be combined with compound literals. Structure objects created using compound literals can be passed to functions without depending on member order: ____________________________________________________________ 69. For example, subobjects without explicit initializers are initialized to zero. 70. This allows implementations to share storage for string literals and constant compound literals with the same or overlapping representations. Language 89 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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}); 4. A read-only compound literal can be specified through constructions like: (const float []){1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6} 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. 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. 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); 8. Outside the body of a function, a compound literal is an initialization of a static object; however, inside a function body, it is an assignment to an automatic object. Therefore, the following two loops produce the same sequence of values for the objects associated 90 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 with their respective compound literals. for (int i = 0; i < 10; i++) { f((struct s){.a = i, .b = 42}); } for (int i = 0; i < 10; i++) f((struct s){.a = i, .b = 42}); 9. 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; for (j = 0; j < 2; j++) q = p, p = &((struct s){ j }); return p == q && q.i == 1; } The function f() always returns the value 1. Note that if the body of the for loop were enclosed in braces, 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 is undefined behavior. 6.3.3 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 & * + - ~ ! Language 91 Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.3.3.1 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 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 (6.3.6), compound assignment (6.3.16.2). 6.3.3.2 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 are evaluated, and the result shall be as if both were omitted, even if the intermediate object does not exist, 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 [] are evaluated, and the 92 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 result shall be as if the & operator was removed and the [] operator was 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.71 Forward references: storage-class specifiers (6.5.1), structure and union specifiers (6.5.2.1). 6.3.3.3 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 operand. The integer promotion is performed on the operand, and the result has the promoted type. [#3] The result of the unary - operator is the negative of its operand. The integer promotion is performed on the operand, and the result has the promoted type. __________ 71. 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. Language 93 Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#4] The result of the ~ operator is the bitwise complement of its 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 promotion is performed on the operand, and the result has the promoted type. The expression ~E is equivalent to (ULLONG_MAX-E) if E is promoted to type unsigned long long, to (ULONG_MAX-E) if E is promoted to type unsigned long, to (UINT_MAX-E) if E is promoted to type unsigned int. (The constants ULLONG_MAX, ULONG_MAX, and UINT_MAX are defined in the header .) [#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: limits and (7.1.5). 6.3.3.4 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.72 When applied to an operand that has structure or __________ 72. 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 6.2.2.1; see 94 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. Examples [#5] 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. 2. Another use of the sizeof operator is to compute the number of elements in an array: sizeof array / sizeof array[0] 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; } ____________________________________________________________ 6.7.1. Language 95 Working Draft, 1997-11-21, WG14/N794 J11/97-158 Forward references: common definitions (7.1.6), declarations (6.5), structure and union specifiers (6.5.2.1), type names (6.5.6), array declarators (6.5.5.2). 6.3.4 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 6.3.16.1, 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.73 A cast that specifies no conversion has no effect on the type or value of an expression.74 __________ 73. A cast does not yield an lvalue. Thus, a cast to a qualified type has the same effect as a cast to the unqualified version of the type. 74. If the value of the expression is represented with greater precision or range than required by the type named by the cast (6.2.1.7), then the cast specifies a conversion even if the type of the expression is the same as the named type. 96 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 Forward references: equality operators (6.3.9), function declarators (including prototypes) (6.5.5.3), simple assignment (6.3.16.1), type names (6.5.6). 6.3.5 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. [#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.75 If the quotient a/b is representable, the expression (a/b)*b + a%b shall equal a. [#7] If either operand has complex type, the result has complex type. __________ 75. This is often called ``truncation toward zero''. Language 97 Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.3.6 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. [#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 98 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 element such that the difference of the subscripts of the resulting and original array elements equals the integer expression. In other words, if the expression P points to the i-th element of an array object, the expressions (P)+N (equivalently, N+(P)) and (P)-N (where N has the value n) point to, respectively, the i+n-th and i- n-th elements of the array object, provided they exist. Moreover, if the expression P points to the last element of an array object, the expression (P)+1 points one past the last element of the array object, and if the expression Q points one past the last element of an array object, the expression (Q)-1 points to the last element of the array object. If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined. Unless both the pointer operand and the result point to elements of the same array object, or the pointer operand points one past the last element of an array object and the result points to an element of the same array object, the behavior is undefined if the result is used as an operand of a unary * operator that is actually evaluated. [#9] When two pointers to elements of the same array object are subtracted, 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. Unless both pointers point to elements of the same array object, or one past the last element of the array object, the behavior is undefined.76 __________ 76. 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 Language 99 Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#10] If either operand has complex type, the result has complex type. Examples [#11] 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 } [#12] If array a in the above example is declared to be an array of known constant size, and pointer p is declared to be a pointer to an array of the same know constant size that points to a, the results are the same. Forward references: array declarators (6.5.5.2), common definitions (7.1.6). ____________________________________________________________ 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. 100 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.3.7 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 number of value and sign bits in the object representation 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 x 2E2, reduced modulo ULLONG_MAX+1 if E1 has type unsigned long long, ULONG_MAX+1 if E1 has type unsigned long, UINT_MAX+1 otherwise. (The constants ULLONG_MAX, ULONG_MAX, and UINT_MAX are defined in the header .) If E1 has a signed type and nonnegative value, and E1x2E2 is less than or equal to INT_MAX (if E1 has type int), LONG_MAX (if E1 has type long int), or LLONG_MAX (if E1 has type long long int), 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 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. Language 101 Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.3.8 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 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. 102 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#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.77 The result has type int. 6.3.9 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 the != (not equal to) operators are analogous to the relational operators except for their lower precedence.78 Where the operands have types and values suitable for the relational operators, the semantics detailed in 6.3.8 apply. __________ 77. 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. 6.3.16.1 Simple assignment Constraints [#1] One of the following shall hold:81 - the left operand has qualified or unqualified arithmetic type and the right has arithmetic type; __________ 81. The asymmetric appearance of these constraints with respect to type qualifiers is due to the conversion (specified in 6.2.2.1) that changes lvalues to ``the value of the expression'' which removes any type qualifiers from the type category of the expression. Language 109 Working Draft, 1997-11-21, WG14/N794 J11/97-158 - the left operand has a qualified or unqualified version of a structure or union type compatible with the type of the right; - both operands are pointers to qualified or unqualified versions of compatible types, and the type pointed to by the left has all the qualifiers of the type pointed to by the right; - one operand is a pointer to an object or incomplete type and the other is a pointer to a qualified or unqualified version of void, and the type pointed to by the left has all the qualifiers of the type pointed to by the right; or - the left operand is a pointer and the right is a null pointer constant. 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. Examples [#4] 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 110 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 variable c should be declared as int. 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 type. 6.3.16.2 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. 6.3.17 Comma operator Syntax [#1] expression: assignment-expr expression , assignment-expr Language 111 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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.82 Examples [#3] As indicated by the syntax, in contexts where a comma is a punctuator (in lists of arguments to functions and lists of initializers) the comma operator as described in this subclause cannot appear. 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 (6.5.8). __________ 82. A comma operator does not yield an lvalue. 112 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.4 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 the operand of a sizeof operator.83 [#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.84 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 expression shall have integer type and shall only have operands that are integer constants, __________ 83. The operand of a sizeof operator is not evaluated (6.3.3.4), and thus any operator in 6.3 may be used. 84. An integer constant expression must be 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 6.8.1. Language 113 Working Draft, 1997-11-21, WG14/N794 J11/97-158 enumeration constants, character constants, sizeof expressions whose operand does not have variable length array type or a parenthesized name of such a type, 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. [#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.85 __________ 85. Thus, in the following initialization, 114 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 Forward references: array declarators (6.5.5.2), initialization (6.5.8). ____________________________________________________________ static int i = 2 || 1 / 0; the expression is a valid integer constant expression with value one. Language 115 Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.5 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-specifiers 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 (excluding 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 6.5.2.3. [#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;86 116 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 - 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 (6.5.5), enumeration specifiers (6.5.2.2), initialization (6.5.8), tags (6.5.2.3). 6.5.1 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.87 __________ 86. Function definitions have a different syntax, described in 6.7.1. 87. See ``future language directions'' (6.9.2). Language 117 Working Draft, 1997-11-21, WG14/N794 J11/97-158 Semantics [#3] The typedef specifier is called a ``storage-class specifier'' for syntactic convenience only; it is discussed in 6.5.7. The meanings of the various linkages and storage durations were discussed in 6.1.2.2 and 6.1.2.4. [#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.88 [#5] The declaration of an identifier for a function that 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 (6.5.7). __________ 88. 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 may not be computed, either explicitly (by use of the unary & operator as discussed in 6.3.3.2) or implicitly (by converting an array name to a pointer as discussed in 6.2.2.1). Thus the only operator that can be applied to an array declared with storage-class specifier register is sizeof. 118 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.5.2 Type specifiers Syntax [#1] type-specifier: void char short int long float double complex signed unsigned struct-or-union-specifier enum-specifier typedef-name Constraints [#2] At least one type specifier shall be given in the declaration specifiers in a declaration. Each list of type specifiers shall be one of the following sets (delimited by commas, when there is more than one set on a line); the type specifiers may occur in any order, possibly intermixed with the other declaration specifiers. - void - char - signed char - unsigned char - short, signed short, short int, or signed short int - unsigned short, or unsigned short int - int, signed, or signed int - unsigned, or unsigned int - long, signed long, long int, or signed long int - unsigned long, or unsigned long int Language 119 Working Draft, 1997-11-21, WG14/N794 J11/97-158 - 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 - float complex - double complex - long double complex - struct-or-union specifier - enum-specifier - typedef-name Semantics [#3] Specifiers for structures, unions, and enumerations are discussed in 6.5.2.1 through 6.5.2.3. Declarations of typedef names are discussed in 6.5.7. The characteristics of the other types are discussed in 6.1.2.5. [#4] Each of the comma-separated sets designates the same type, except that for bit-fields, it is implementation- defined whether the specified int is the same type as signed int or is the same type as unsigned int. Forward references: enumeration specifiers (6.5.2.2), structure and union specifiers (6.5.2.1), tags (6.5.2.3), type definitions (6.5.7). 6.5.2.1 Structure and union specifiers Syntax [#1] struct-or-union-specifier: struct-or-union identifier-opt { struct-declaration-list } struct-or-union identifier 120 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 struct-or-union: struct union struct-declaration-list: struct-declaration struct-declaration-list struct-declaration struct-declaration: specifier-qualifier-list struct-declarator-list ; specifier-qualifier-list: type-specifier specifier-qualifier-list-opt type-qualifier specifier-qualifier-list-opt struct-declarator-list: struct-declarator struct-declarator-list , struct-declarator struct-declarator: declarator declarator-opt : constant-expression Constraints [#2] A structure or union shall not contain a member with incomplete or function type, except that the last element of a structure may have incomplete array type. Hence it shall not contain an instance of itself (but may contain a pointer to an instance of itself). [#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 6.1.2.5, 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 Language 121 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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.89 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;90 its width is preceded by a colon. [#8] A bit-field shall have a type that is a qualified or unqualified version of signed int or unsigned int. A bit- field is interpreted as a signed or unsigned integer type consisting of the specified number of bits.91 [#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.92 As a special case of this, a bit-field structure member with a __________ 89. A structure or union can not contain a member with a variably modified type because member names are not ordinary identifiers as defined in 6.1.2.3. 90. The unary & (address-of) operator may not be applied to a bit-field object; thus, there are no pointers to or arrays of bit-field objects. 91. As specified in 6.5.2 above, if the actual type specifier used is int or there is no type specifier, or is a typedef-name defined using either of these, then it is implementation-defined whether the bit-field is signed or unsigned. 92. An unnamed bit-field structure member is useful for padding to conform to externally imposed layouts. 122 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 width of 0 indicates that no further bit-field is to be packed into the unit in which the previous bit-field, if any, was placed. [#11] Each non-bit-field member of a structure or union object is aligned in an implementation-defined manner appropriate to its type. [#12] Within a structure object, the non-bit-field members and the units in which bit-fields reside have addresses that increase in the order in which they are declared. A pointer to a structure object, suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in which it resides), and vice versa. There may be unnamed padding within a structure object, but not at its beginning. [#13] The size of a union is sufficient to contain the largest of its members. The value of at most one of the members can be stored in a union object at any time. A pointer to a union object, suitably converted, points to each of its members (or if a member is a bit-field, then to the unit in which it resides), and vice versa. [#14] There may be unnamed padding at the end of a structure or union, were the structure or union to be an element of an array. [#15] As a special case, the last element of a structure may be 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 one element. 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 by the longest array that would not make the structure larger than the object being accessed. If this array would have no elements, then it behaves as if it has one element, but the behavior is undefined if any attempt is made to access that element. Examples [#16] After the declarations: struct s { int n; double d[]; }; struct ss { int n; double d[1]; }; the three expressions: Language 123 Working Draft, 1997-11-21, WG14/N794 J11/97-158 sizeof (struct s) offsetof(struct s, d) offsetof(struct ss, d) have the same value. The structure struct s has a flexible array member d. [#17] If sizeof (double) is 8, then after the following code is executed: struct s *s1; struct s *s2; s1 = malloc(sizeof (struct s) + 64); s2 = malloc(sizeof (struct s) + 46); and assuming that the calls to malloc succeed, s1 and s2 behave as if they 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 they had been declared as: 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 (6.5.2.3). 6.5.2.2 Enumeration specifiers Syntax [#1] enum-specifier: enum identifier-opt { enumerator-list } enum identifier-opt { enumerator-list , } enum identifier 124 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. Semantics [#3] The identifiers in an enumerator list are declared as constants that have type int and may appear wherever such are permitted.93 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, but shall be capable of representing the values of all the members of the enumeration. [#5] The enumerated type is complete at the } that terminates the list of enumerator declarations. __________ 93. 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. Language 125 Working Draft, 1997-11-21, WG14/N794 J11/97-158 Examples [#6] 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 (6.5.2.3). 6.5.2.3 Tags Constraints [#1] A specific type shall have its content defined at most once. [#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 complete94 until the closing brace of the list defining the content, and complete __________ 94. 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 6.1.2.5.) The specification shall be complete before such a function is called or defined. 126 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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,95 the type specifier also declares the identifier to be the tag of that type. [#6] A declaration of the form struct-or-union identifier specifies a structure of union type and declares the identifier as a tag of that type.96 [#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.96 __________ 95. 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. 96. A similar construction with enum does not exist. Language 127 Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#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. Examples [#9] 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. The following alternative formulation uses the typedef mechanism: typedef struct tnode TNODE; struct tnode { int count; TNODE *left, *right; }; TNODE s, *sp; 2. To illustrate the use of prior declaration of a tag to specify a pair of mutually referential structures, the declarations 128 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. 3. An enumeration type is compatible with some integer type. An implementation may delay the choice of which integer type until all enumeration constants have been seen. Thus in: enum f { c = sizeof (enum f) }; the behavior is undefined since the size of the respective enumeration type is not necessarily known when sizeof is encountered. Forward references: declarators (6.5.5), array declarators (6.5.5.2), type definitions (6.5.7). 6.5.3 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. Language 129 Working Draft, 1997-11-21, WG14/N794 J11/97-158 Semantics [#3] The properties associated with qualified types are meaningful only for expressions that are lvalues.97 [#4] If the same qualifier appears more than once in the same specifier-qualifier-list, either directly or via one or more typedefs, the behavior is the same as if it appeared only once. [#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.98 [#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.99 What constitutes an access to an object that has volatile-qualified type is implementation- defined. __________ 97. The 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. 98. This 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). 99. A 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. 130 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#7] An object that is referenced through a restrict- qualified pointer has a special association with that pointer. This association, defined in 6.5.3.1 below, requires that all references to that object shall use, directly or indirectly, the value of that pointer. For example, a statement that assigns a value returned by malloc to a single pointer establishes this association between the allocated object and the pointer. The intended use of the restrict qualifier (like the register storage class) is to promote optimization, and deleting all instances of the qualifier from a conforming program does not change its meaning (i.e., observable behavior). [#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.100 [#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. Examples [#10] 1. An object declared extern const volatile int real_time_clock; may be modifiable by hardware, but cannot be assigned to, incremented, or decremented. 2. The following declarations and expressions illustrate the behavior when type qualifiers modify an aggregate type: __________ 100. Both of these can occur through the use of typedefs. Language 131 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 *'' 6.5.3.1 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 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. (In 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.) 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 references through pointer expressions based on P. Then all references 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. 132 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#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. A reference to a value means either an access to or a modification of the value. During an execution of B, attention is confined to those references that are actually evaluated. (This excludes references that appear in unevaluated expressions, and also excludes references that are ``available'', in the sense of employing visible identifiers, but do not actually appear in the text of B.) [#6] A translator is free to ignore any or all aliasing implications of uses of restrict. Examples [#7] 1. The file scope declarations int * restrict a; int * restrict b; extern int c[]; assert that if an object is referenced using the value of one of a, b, or c, then it is never referenced using the value of either of the other two. 2. The function parameter declarations in the following example void f(int n, int * restrict p, int * restrict q) { while (n-- > 0) *p++ = *q++; } assert that, during each execution of the function, if an object is referenced through one of the pointer parameters, then it is not also referenced through the other. The benefit of the restrict qualifiers is that they enable a translator to make an effective dependence analysis of function f without examining any of the calls of f in the program. The cost is that the programmer must 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 referenced through both Language 133 Working Draft, 1997-11-21, WG14/N794 J11/97-158 p and q. void g(void) { extern float d[100]; f(50, d + 50, d); // ok f(50, d + 1, d); // undefined behavior } 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 referenced through p is never referenced 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. 4. The rule limiting assignments between restricted pointers does not distinguish between a function call and an equivalent nested block. With one exception, only ``outer-to-inner'' assignments between restricted pointers declared in nested blocks have defined behavior. { int * restrict p1; int * restrict q1; p1 = q1; // undefined behavior { int * restrict p2 = p1; // ok int * restrict q2 = q1; // ok p1 = q2; // undefined behavior p2 = q2; // undefined behavior } } The exception allows the value of a restricted pointer to be carried out of the block in which it (or, more 134 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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; } 6.5.4 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 (see below) of a function with external linkage shall not contain a definition of an object with static storage duration that can be modified, 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 is an inline function. Making a function an inline function suggests that calls to the function be as fast as possible by using, for example, an alternative to the usual function call mechanism known as ``inline substitution''.101 The __________ 101. 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 Language 135 Working Draft, 1997-11-21, WG14/N794 J11/97-158 extent to which such suggestions are effective is implementation-defined.102 [#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. Examples [#7] 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. ____________________________________________________________ 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. 102. For example, an implementation might never perform inline substitution, or might only perform inline substitutions to calls in the scope of an inline declaration. 136 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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; } /* Creates an external definition. */ extern double fahr(double); 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 there is a call to cels, an external definition of cels in another translation unit is still required by 6.7. 6.5.5 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 Language 137 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 ) 138 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 then ident has the type specified by the declaration ``T D''. Thus, a declarator in parentheses is identical to the unparenthesized declarator, but the binding of complicated declarators may be altered by parentheses. Implementation limits [#7] The implementation shall allow the specification of types that have at least 12 pointer, array, and function declarators (in any valid combinations) modifying an arithmetic, structure, union, or incomplete type, either directly or via one or more typedefs. Forward references: array declarators (6.5.5.2), type definitions (6.5.7). 6.5.5.1 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. Examples [#3] 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 an 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. Language 139 Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#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''. 6.5.5.2 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 6.1.2.3) with block scope or function prototype scope and without linkage can 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''.103 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 __________ 103. When several ``array of'' specifications are adjacent, a multidimensional array is declared. 140 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 with function prototype scope. 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. [#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. Examples [#5] 1. float fa[11], *afp[17]; declares an array of float numbers and an array of pointers to float numbers. 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. 3. The following declarations demonstrate the compatibility rules for variably modified types. Language 141 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. } 4. All declarations of variably modified (VM) types must be declared 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 must be ordinary identifiers, and can not, therefore, be members of structures or unions. extern int n; int A[n]; // Error - file scope VLA. extern int (*p2)[n]; // Error - file scope VM. int B[100]; // OK - file scope but not VM. void fvla(int n, 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. } 142 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 Forward references: function definitions (6.7.1), initialization (6.5.8). 6.5.5.3 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 function definition shall be empty. [#4] After all rewrites, the parameters in a parameter- type-list that is part of a function definition shall not have incomplete type.104 Semantics [#5] If, in the declaration ``T D1'', D1 has the form 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 declared parameter that is a member of a parameter type list that is not part of a function definition, may use the [*] notation in its sequence of declarator specifiers to specify a variable length array type. If the list terminates with an ellipsis (, ...), no information about the number or types of the parameters after the comma is supplied.105 The __________ 104. Arrays and functions are rewritten as pointers. 105. The macros defined in the header (7.12) may be used to access arguments that correspond to the ellipsis. Language 143 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 function definition, the parameters may have incomplete type. [#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 function definition specifies that the function has no parameters. The empty list in a function declarator that is not part of a function definition specifies that no information about the number or types of the parameters is supplied.106 [#11] For two function types to be compatible, both shall specify compatible return types.107 Moreover, the parameter type lists, if both are present, shall agree in the number of parameters and in use of the ellipsis terminator; corresponding parameters shall have compatible types. If one type has a parameter type list and the other type is specified by a function declarator that is not part of a function definition and that contains an empty identifier list, the parameter list shall not have an ellipsis terminator and the type of each parameter shall be compatible with the type that results from the application of the default argument promotions. If one type has a parameter type list and the other type is specified by a function definition that contains a (possibly empty) identifier list, both shall agree in the number of parameters, and the type of each prototype parameter shall be compatible with the type that results from the application of the default argument promotions to the type __________ 106. See ``future language directions'' (6.9.3). 107. If both function types are ``old style'', parameter types are not compared. 144 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 type that results from conversion to a pointer type, as in 6.7.1, and each parameter declared with qualified type is taken as having the unqualified version of its declared type.) Examples [#12] 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. 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. 2. The declaration int (*apfi[3])(int *x, int *y); declares an array apfi of three pointers to functions returning int. Each of these functions has two parameters that are pointers to int. The identifiers x and y are declared for descriptive purposes only and go out of scope at the end of the declaration of apfi. Language 145 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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), 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. 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; } 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 (6.7.1), type names (6.5.6). 146 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.5.6 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 desired 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.108 Examples [#3] The constructions (a) int (b) int * (c) int *[3] (d) int (*)[3] (e) int *() (f) int (*)(void) (g) 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) function with no parameter specification returning a pointer to int, (f) pointer to function with no __________ 108. As 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. Language 147 Working Draft, 1997-11-21, WG14/N794 J11/97-158 parameters returning an int, and (g) array of an unspecified number of constant pointers to functions, each with one parameter that has type unsigned int and an unspecified number of other parameters, returning an int. 6.5.7 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 specifies the type specified for the identifier in the way described in 6.5.5. Any array size expressions associated with variable length array declarators shall be evaluated with the typedef name at the beginning of its scope upon each normal entry to the block. 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. If the identifier is redeclared in an inner scope or is declared as a member of a structure or union in the same or an inner scope, the type specifiers shall not be omitted in the inner declaration. 148 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 Examples [#4] 1. After typedef int MILES, KLICKSP(); typedef struct { double hi, lo; } range; the constructions MILES distance; extern KLICKSP *metricp; range x; range z, *zp; are all valid declarations. The type of distance is int, that of metricp is ``pointer to function with no parameter specification returning int'', and that of x and z is the specified structure; zp is a pointer to such a structure. The object distance has a type compatible with any other int object. 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, and int. 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 Language 149 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. 4. On the other hand, typedef names can be used to improve code readability. All three of the following declarations of the signal function specify exactly the same type, the first without making use of any typedef names. typedef void fv(int), (*pfv)(int); void (*signal(int, void (*)(int)))(int); fv *signal(int, fv *); pfv signal(int, pfv); 5. The following is a block scope declaration of a typedef name A with a variable length array type. void tdef(int n) { typedef int A[n]; A a; A *p; p = &a; } 6. The size expression that is part of the variable length array type named by typedef name B is evaluated each time function copyt is entered. However, the size of the variable length array type does not change if the value of n is subsequently changed. 150 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 (i = 1; i < n; i++) a[i-1] = b[i]; } } Forward references: the signal function (7.11.1.1). 6.5.8 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 [#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 Language 151 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 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. A union object containing only unnamed members has 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 zero; - if it is an aggregate, every member is initialized (recursively) according to these rules; - if it is a union, the first named member is initialized (recursively) according to these rules. 152 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#11] The initializer for a scalar shall be a single expression, optionally enclosed in braces. The initial value of the object, including unnamed members, is that of the expression; 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] 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.109 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.110 [#13] 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.111 The current object that results at the end of the designator list is the subobject to be initialized by the following initializer. [#14] The initialization shall occur in initializer list order, each initializer provided for a particular subobject __________ 109. If the initializer list for a subaggregate or contained union does not begin with a left brace, its subobjects are initialized as usual, but the subaggregate or contained union does not become the current object: current objects are associated only with brace-enclosed initializer lists. 110. After 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. 111. Thus, 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. Language 153 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. [#15] The initializer for a structure or union object that has automatic storage duration either shall be an initializer list as described below, or shall be a single expression that has compatible structure or union type. In the latter case, the initial value of the object is that of the expression. [#16] The rest of this subclause deals with initializers for objects that have aggregate or union type. [#17] 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. [#18] An array with element type compatible with wchar_t may be initialized by a wide string literal, optionally enclosed in braces. Successive codes of the wide string literal (including the terminating zero-valued code if there is room or if the array is of unknown size) initialize the elements of the array. [#19] Otherwise, the initializer for an object that has aggregate type shall be a brace-enclosed list of initializers for the named members of the aggregate, written in increasing subscript or member order; and the initializer for an object that has union type shall be a brace-enclosed initializer for the first named member of the union. [#20] If the aggregate contains members that are aggregates or unions, or if the first member of a union is an aggregate or union, the 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 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 members of the subaggregate or the first member of the contained union; any remaining initializers are left to initialize the next member of the aggregate of which the current subaggregate or contained union is a part. 154 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#21] If there are fewer initializers in a brace-enclosed list than there are 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.112 Examples [#24] 1. 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. 2. 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 __________ 112. In particular, the evaluation order need not be the same as the order of subobject initialization. Language 155 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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]. 3. The declaration int z[4][3] = { { 1 }, { 2 }, { 3 }, { 4 } }; initializes the first column of z as specified and initializes the rest with zeros. 4. The declaration struct { int a[3], b; } w[] = { { 1 }, 2 }; is a definition with an inconsistently bracketed initialization. It defines an array with two element structures: w[0].a[0] is 1 and w[1].a[0] is 2; all the other elements are zero. 5. 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: 156 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. Note that the fully bracketed and minimally bracketed forms of initialization are, in general, less likely to cause confusion. 6. 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. 7. 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 Language 157 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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'' that is initialized 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. 8. 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", }; 9. Structure members can be initialized to nonzero values without depending on their order: div_t answer = { .quot = 2, .rem = -1 }; 10. 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 }; 11. 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 }; 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. 12. Any member of a union can be initialized: 158 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 union { /* ... */ } u = { .any_member = 42 }; Forward references: common definitions (7.1.6). Language 159 Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.6 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. 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 (6.6.3), selection statements (6.6.4), iteration statements (6.6.5), the return statement (6.6.6.4). 6.6.1 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. 160 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 Semantics [#3] Any statement may be preceded by a prefix that declares an identifier as a label name. Labels in themselves do not alter the flow of control, which continues unimpeded across them. Forward references: the goto statement (6.6.6.1), the switch statement (6.6.4.2). 6.6.2 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 statements to be grouped into one syntactic unit, which may have its own set of declarations and initializations (as discussed in 6.1.2.4). 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. Language 161 Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.6.3 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.113 [#3] A null statement (consisting of just a semicolon) performs no operations. Examples [#4] 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); 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. 3. A null statement may also be used to carry a label just before the closing } of a compound statement. __________ 113. Such as assignments, and function calls which have side effects. 162 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 while (loop1) { /* ... */ while (loop2) { /* ... */ if (want_out) goto end_loop1; /* ... */ } /* ... */ end_loop1: ; } Forward references: iteration statements (6.6.5). 6.6.4 Selection statements Syntax [#1] 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. 6.6.4.1 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 preceeding if that is allowed by the grammar. Language 163 Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.6.4.2 The switch statement Constraints [#1] The controlling expression of a switch statement shall have integer type, and shall not cause a block to be entered by a jump from outside the block to a statement that follows a case or default label in the block (or an enclosed block) if that block contains the declaration of a variably modified object or variably modified typedef name. The expression of each case label shall be an integer constant expression. No two of the case constant expressions in the same switch statement shall have the same value after conversion. There may be at most one default label in a switch statement. (Any enclosed switch statement may have a default label or case constant expressions with values that duplicate case constant expressions in the enclosing switch statement.) Semantics [#2] 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. [#3] 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 [#4] As discussed previously (5.2.4.1), the implementation may limit the number of case values in a switch statement. Examples [#5] In the artificial program fragment 164 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. 6.6.5 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. Language 165 Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.6.5.1 The while statement [#1] The evaluation of the controlling expression takes place before each execution of the loop body. 6.6.5.2 The do statement [#1] The evaluation of the controlling expression takes place after each execution of the loop body. 6.6.5.3 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 ; } } are equivalent (where clause-1 can be an expression or a declaration).114 [#2] Both clause-1 and expr-3 can be omitted. If either or both are an expression, they are evaluated as a void expression. An omitted expr-2 is replaced by a nonzero constant. __________ 114. Thus, 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. 166 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 Forward references: the continue statement (6.6.6.2). 6.6.6 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. 6.6.6.1 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 cause a block to be entered by a jump from outside the block to a labeled statement in the block (or an enclosed block) if that block contains the declaration of a variably modified object or variably modified typedef name. Semantics [#2] A goto statement causes an unconditional jump to the statement prefixed by the named label in the enclosing function. Examples [#3] 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. Language 167 Working Draft, 1997-11-21, WG14/N794 J11/97-158 3. The code to determine the next operation must be 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 /* ... */ } 2. A goto statement is not allowed to jump past any declarations of objects with variably modified types. A jump within the block, however, is permitted. goto lab3; // Error: going INTO scope of VLA. { double a[n]; a[j] = 4.4; lab3: a[j] = 3.3; goto lab 4; // OK, going WITHIN scope of VLA. a[j] = 5.5; lab4: a[j] = 6.6; } goto lab4; // Error: going INTO scope of VLA. 6.6.6.2 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 168 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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;.115 6.6.6.3 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. 6.6.6.4 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.116 __________ 115. Following the contin: label is a null statement. 116. The return statement is not an assignment. The overlap Language 169 Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#4] If a return statement without an expression is executed, and the value of the function call is used by the caller, the behavior is undefined. Examples [#5] In: struct s { double i; } f(void); union { struct { int f1; struct s f2; } u1; struct { struct s f3; int f4; } u2; } g; struct s f(void) { return g.u1.f2; } /* ... */ g.u2.f3 = f(); there is no undefined behavior. ____________________________________________________________ restriction of subclause 6.3.16.1 does not apply to the case of function return. 170 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.7 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 6.5, 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.117 __________ 117. Thus, if an identifier declared with external linkage is not used in an expression, there need be no external Language 171 Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.7.1 Function definitions Syntax [#1] function-definition: declaration-specifiers declarator declaration-list-opt compound-statement 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.118 [#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 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 ____________________________________________________________ definition for it. 118. The 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 */ 172 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 from the identifier list, and every identifier in the identifier list shall be declared. An identifier declared as a typedef name shall not be redeclared as a parameter. The declarations in the declaration list shall contain no storage-class specifier other than register and no initializations. Semantics [#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,119 the types of the parameters shall be declared in a following declaration list. [#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.120 The layout of the storage for parameters is unspecified. [#10] On entry to the function all size expressions of its variably modified parameters are evaluated, and the value of each argument expression shall be converted to the type of its corresponding parameter, as if by assignment to the parameter. Array expressions and function designators as arguments are converted to pointers before the call. 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 6.2.2.1. The resulting parameter type shall be an object type. __________ 119. See ``future language directions'' (6.9.4). 120. A parameter is in effect declared at the head of the compound statement that constitutes the function body, and therefore may not be redeclared in the function body (except in an enclosed block). Language 173 Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#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. Examples [#13] 1. In the following: extern int max(int a, int b) { return a > b ? a : b; } extern is the storage-class specifier and int is the type specifier; max(int a, int b) is the function declarator; and { return a > b ? a : b; } is the function body. The following similar definition uses the identifier-list form for the parameter declarations: extern int max(a, b) int a, b; { return a > b ? a : b; } Here int a, b; is the declaration list for the parameters. The difference between these two definitions is that the first form acts as a prototype declaration that forces conversion of the arguments of subsequent calls to the function, whereas the second form may not. 2. To pass one function to another, one might say int f(void); /* ... */ g(f); Then the definition of g might read 174 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 void g(int (*funcp)(void)) { /* ... */ (*funcp)() /* or funcp() ... */ } or, equivalently, void g(int func(void)) { /* ... */ func() /* or (*func)() ... */ } 6.7.2 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- 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. Examples [#4] 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 Language 175 Working Draft, 1997-11-21, WG14/N794 J11/97-158 int i1; // valid tentative definition, refers to previous int i2; // 6.1.2.2 renders undefined, linkage disagreement int i3; // valid tentative definition, refers to previous int i4; // valid tentative definition, refers to previous int i5; // 6.1.2.2 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 2. If at the end of the translation unit containing int i[]; the array i still has incomplete type, the array is assumed to have one element. This element is initialized to zero on program startup. 176 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.8 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 Language 177 Working Draft, 1997-11-21, WG14/N794 J11/97-158 control-line: # include pp-tokens new-line # define identifier replacement-list new-line # define identifier lparen identifier-list-opt ) replacement-list new-line # define identifier lparen ... ) replacement-list new-line # define identifier lparen identifier-list , ... ) replacement-list new-line # undef identifier new-line # line pp-tokens new-line # error pp-tokens-opt new-line # pragma pp-tokens-opt new-line # new-line lparen: the left-parenthesis character without preceding 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.121 A new-line character ends the preprocessing directive even if it occurs within what would otherwise be an invocation of a function-like macro. __________ 121. Thus, 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 6.8.3.2, for example). 178 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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). [#4] In the definition of an object-like macro, if the first character of a replacement list is not a character required by subclause 5.2.1, then there shall be white-space separation between the identifier and the replacement list.122 Semantics [#5] 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. [#6] The preprocessing tokens within a preprocessing directive are not subject to macro expansion unless otherwise stated. Examples [#7] 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. __________ 122. This allows an implementation to choose to interpret the directive: #define THIS$AND$THAT(a, b) ((a) + (b)) as defining a function-like macro THIS$AND$THAT, rather than an object-like macro THIS. Whichever choice it makes, it must also issue a diagnostic. Language 179 Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.8.1 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;123 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 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 __________ 123. Because 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. 180 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 token is converted into a token. The resulting tokens compose the controlling constant expression which is evaluated according to the rules of 6.4, 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.124 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.125 __________ 124. Thus, 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) 125. As indicated by the syntax, a preprocessing token shall not follow a #else or #endif directive before the Language 181 Working Draft, 1997-11-21, WG14/N794 J11/97-158 Forward references: macro replacement (6.8.3), source file inclusion (6.8.2), largest integer types (7.4.1.5). 6.8.2 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 ____________________________________________________________ terminating new-line character. However, comments may appear anywhere in a source file, including within a preprocessing directive. 182 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 (that does not match one of the two previous forms) is permitted. The preprocessing tokens after include in the directive are processed just as in normal text. (Each identifier currently defined as a macro name is replaced by its replacement list of preprocessing tokens.) The directive resulting after all replacements shall match one of the two previous forms.126 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). Examples [#7] 1. The most common uses of #include preprocessing directives are as in the following: #include #include "myprog.h" 2. This illustrates macro-replaced #include directives: __________ 126. Note 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. Language 183 Working Draft, 1997-11-21, WG14/N794 J11/97-158 #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 (6.8.3). 6.8.3 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. 184 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 # define identifier replacement-list new-line defines an object-like macro that causes each subsequent instance of the macro name127 to be replaced by the replacement list of preprocessing tokens that constitute the remainder of the directive. The replacement list is then rescanned for more macro names as specified below. [#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 __________ 127. Since, 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. Language 185 Working Draft, 1997-11-21, WG14/N794 J11/97-158 matching ) preprocessing token, skipping intervening matched pairs of left and right parenthesis preprocessing tokens. Within the sequence of preprocessing tokens making up an invocation of a function-like macro, new-line is considered a normal white-space character. [#11] The sequence of preprocessing tokens bounded by the outside-most matching parentheses forms the list of arguments for the function-like macro. The individual arguments within the list are separated by comma preprocessing tokens, but comma preprocessing tokens between matching inner parentheses do not separate arguments. If there are sequences of preprocessing tokens within the list of arguments that would otherwise act as preprocessing directives, the behavior is undefined. [#12] If there is a ... in the identifier-list in the macro definition, then the trailing arguments, including any separating comma preprocessing tokens, are merged to form a single item: the variable arguments. The number of arguments so combined is such that, following merger, the number of arguments is one more than the number of parameters in the macro definition (excluding the ...). 6.8.3.1 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. 186 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.8.3.2 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 comprising 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). 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. 6.8.3.3 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, 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 ## Language 187 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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; 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. Examples [#4] #define hash_hash # ## # #define mkstr(a) # a #define in_between(a) mkstr(a) #define join(c, d) in_between(c hash_hash d) char p[] = join(x, y); // equivalent to // char p[] = "x ## y"; The expansion produces, at various stages: join(x, y) in_between(x hash_hash y) in_between(x ## y) mkstr(x ## y) "x ## y" In other words, expanding hash_hash produces a new token, consisting of two adjacent sharp signs, but this new token is not the catenation operator. 6.8.3.4 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 with all subsequent preprocessing tokens of the source file for more macro names to replace. 188 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#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 6.8.9 below. 6.8.3.5 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. Examples [#3] 1. The simplest use of this facility is to define a ``manifest constant,'' as in #define TABSIZE 100 int table[TABSIZE]; 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. Language 189 Working Draft, 1997-11-21, WG14/N794 J11/97-158 #define max(a, b) ((a) > (b) ? (a) : (b)) The parentheses ensure that the arguments and the resulting expression are bound properly. 3. To illustrate the rules for redefinition and reexamination, the sequence #define x 3 #define f(a) f(x * (a)) #undef x #define x 2 #define g f #define z z[0] #define h g(~ #define m(a) a(w) #define w 0,1 #define t(a) a #define p() int #define q(x) x #define r(x,y) x ## y #define str(x) # x f(y+1) + f(f(z)) % t(t(g)(0) + t)(1); g(x+(3,4)-w) | h 5) & m (f)^m(m); p() i[q()] = { q(1), r(2,3), r(4,), r(,5), r(,) }; char c[2][6] = { str(hello), str() }; results in f(2 * (y+1)) + f(2 * (f(2 * (z[0])))) % f(2 * (0)) + t(1); f(2 * (2+(3,4)-0,1)) | f(2 * (~ 5)) & f(2 * (0,1))^m(0,1); int i[] = { 1, 23, 4, 5, }; char c[2][6] = { "hello", "" }; 4. To illustrate the rules for creating character string literals and concatenating tokens, the sequence 190 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 #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. 5. To illustrate the rules for placemarker ## placemarker the sequence #define t(x,y,z) x ## y ## z int j[] = { t(1,2,3), t(,4,5), t(6,,7), t(8,9,), t(10,,), t(,11,), t(,,12), t(,,) }; results in Language 191 Working Draft, 1997-11-21, WG14/N794 J11/97-158 int j[] = { 123, 45, 67, 89, 10, 11, 12, }; 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 */ 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)); 192 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.8.4 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 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. Language 193 Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.8.5 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. 6.8.6 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 the pragma on the directive causes the implementation to behave in a manner which it shall document. The behavior might cause translation to fail 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 the pragma on the directive, then no macro replacements are performed on the directive, and the directive shall have one of the following forms whose meaning is described elsewhere: #pragma STDC FP_CONTRACT on-off-switch #pragma STDC FENV_ACCESS on-off-switch #pragma STDC CX_LIMITED_RANGE on-off-switch on-off-switch: one of ON OFF DEFAULT Forward references: the FP_CONTRACT pragma (7.7.2), the FENV_ACCESS pragma (7.6.1), the CX_LIMITED_RANGE pragma (7.8.1). 194 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.8.7 Null directive Semantics [#1] A preprocessing directive of the form # new-line has no effect. 6.8.8 Predefined macro names [#1] The following macro names shall be defined by the implementation: __LINE__ The line number of the current source line (a decimal constant). __FILE__ The presumed name of the source file (a character string literal). __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.128 [#2] The following macro names are conditionally defined by the implementation: 624.nr:c 0u+000m'unu __________ 128. The value in ISO/IEC 9899:1994 was 199409L. Language 195 Working Draft, 1997-11-21, WG14/N794 J11/97-158 __STDC_IEC_559__ The decimal constant 1, intended to indicate conformance to the specifications in Annex F (IEC 559 floating-point arithmetic). __STDC_IEC_559_COMPLEX__ The decimal constant 1, intended to indicate adherence to the specifications in informative Annex G (IEC 559 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. All predefined macro names shall begin with a leading underscore followed by an uppercase letter or a second underscore. Forward references: the asctime function (7.16.3.1). 6.8.9 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. Examples [#2] A directive of the form: #pragma list 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 196 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 replacement, as in: #define LISTING(x) PRAGMA(listing on #x) #define PRAGMA(x) _Pragma(#x) LISTING ( ..\listing.dir ) Language 197 Working Draft, 1997-11-21, WG14/N794 J11/97-158 6.9 Future language directions 6.9.1 Character escape sequences [#1] Lowercase letters as escape sequences are reserved for future standardization. Other characters may be used in extensions. 6.9.2 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. 6.9.3 Function declarators [#1] The use of function declarators with empty parentheses (not prototype-format parameter type declarators) is an obsolescent feature. 6.9.4 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. 6.9.5 Pragma directives [#1] Pragmas whose first pp-token is STDC are reserved for future standardization. 198 Language Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. 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 its ``value'' 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.129 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.130 [#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 __________ 129. The functions that make use of the decimal-point character are atof, fprintf, fscanf, fwprintf, fwscanf, localeconv, printf, scanf, sprintf, sscanf, strtod, swprintf, swscanf, vfprintf, vfscanf, vfwprintf, vfwscanf, vprintf, vscanf, vsprintf, vsscanf, vswprintf, vswscanf, vwprintf, vwscanf, wprintf, and wscanf. 130. An equivalent definition can be found in subclause 6.1.3.4. Library 199 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 order. [#7] A shift sequence is a contiguous sequence of bytes within a multibyte string that (potentially) causes a change in shift state. (See subclause 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.131 Forward references: character handling (7.3), the setlocale function (7.5.1.1). 7.1.2 Standard headers [#1] Each library function is declared, with a type that includes a prototype, in a header,132 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 explicity stated otherwise. [#2] The standard headers are __________ 131. For state-dependent encodings, the values for MB_CUR_MAX and MB_LEN_MAX must 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. 132. A header is not necessarily a source file, nor are the < and > delimited sequences in header names necessarily valid source file names. 200 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#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 for a source file to be included, 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. If used, a header shall be included outside of any external declaration or definition, and it shall first be included before the first reference to any of the functions or objects it declares, or to any of the types or macros it defines. However, if an identifier is declared or defined in more than one header, the second and subsequent associated headers may be included after the initial reference to the identifier. The program shall not have any macros with names lexically identical to keywords currently defined prior to the inclusion. [#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. Forward references: diagnostics (7.2). Library 201 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 macros and 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 7.1.8). - 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.133 - 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 that is reserved in that context (other than as allowed by 7.1.8), the behavior is undefined.134 __________ 133. The list of reserved identifiers with external linkage includes errno, setjmp, and va_end. 134. Since macro names are replaced whenever found, independent of scope and name space, macro names matching any of the reserved identifier names must not be defined if an associated header, if any, is included. 202 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#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 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 lvalue135 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.136 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. __________ 135. The 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()). 136. Thus, 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. Library 203 Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#4] Additional macro definitions, beginning with E and a digit or E and an uppercase letter,137 may also be specified by the implementation. 7.1.5 Limits and [#1] The headers and define several macros that expand to various limits and parameters. [#2] The macros, their meanings, and the constraints (or restrictions) on their values are listed in 5.2.4.2. 7.1.6 Common definitions [#1] The following types and macros are defined in the standard header . Some are also defined in other headers, as noted in their respective subclauses. [#2] The types are ptrdiff_t which is the signed integer type of the result of subtracting two pointers; size_t which is the unsigned integer type of the result of the sizeof operator; and wchar_t which is an integer type whose range of values can represent distinct codes for all members of the largest extended character set specified among the supported locales; the null character shall have the code value zero and each member of the basic character set defined in 5.2.1 shall have a code value equal to its value when used as the lone character in an integer character constant. [#3] The macros are NULL which expands to an implementation-defined null pointer __________ 137. See ``future library directions'' (7.20.1). 204 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 constant; and offsetof(type, member-designator) which expands to an integer constant expression that has type size_t, the value of which is the offset in bytes, to the structure member (designated by member-designator), from the beginning of its structure (designated by type). The member-designator shall be such that given static type t; then the expression &(t.member-designator) evaluates to an address constant. (If the specified member is a bit-field, the behavior is undefined.) Forward references: localization (7.5). 7.1.7 Boolean type and values [#1] The header defines one type and three macros. [#2] The type is bool which is an integer type that promotes to int or unsigned int, and that is suitable to be used as the type of a bit- field. A bit-field of any width and type bool shall be capable for representing the value 1.138 __________ 138. The traditional choice for type bool has been int, but this is not a requirement of this International Standard. Other available choices include, but are not limited to, char, unsigned int, and an enumeration type. If an enumeration type is chosen, the names of its true and false members are "masked" by the macros true and false, but the member names might be available to the debugger: typedef enum { false=0, true=1 } bool; #define false 0 #define true 1 The type is suitable for bit-fields if it is int, unsigned int, signed int, or some type allowed by an Library 205 Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#3] The macros are true which expands to the decimal constant 1, false which expands to the decimal constant 0, and __bool_true_false_are_defined which expands to the decimal constant 1. The macros are suitable for use in #if preprocessing directives. 7.1.8 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.139 The use of #undef to ____________________________________________________________ implementation extension. It is required that a bool bit-field of width 1 be unsigned. Thus, bool cannot be signed int, nor can it be plain int if width 1 plain int bit-fields are signed. 139. This means that an implementation must provide an 206 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 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.140 Likewise, those function-like macros described in the following subclauses may be invoked in an expression anywhere a function with a compatible return type could be called.141 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. ____________________________________________________________ actual function for each library function, even if it also provides a macro for that function. 140. Such macros might not contain the sequence points that the corresponding function calls do. 141. Because 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. Library 207 Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#3] There is a sequence point immediately before a library function return. [#4] The functions in the standard library are not guaranteed to be reentrant and may modify objects with static storage duration.142 Examples [#5] 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); - 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); __________ 142. Thus, a signal handler cannot, in general, call standard library functions. 208 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 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) [#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(int 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, and the source line number -- the latter are respectively the values of the preprocessing macros __FILE__ and __LINE__ and the identifier __func__) on the standard error file in an implementation-defined format.143 It then calls the abort function. __________ 143. The message written might be of the form Assertion failed: expression, function abc, file xyz, line nnn Library 209 Working Draft, 1997-11-21, WG14/N794 J11/97-158 Returns [#3] The assert macro returns no value. Forward references: the abort function (7.14.4.1). 210 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 7.3 Character handling [#1] The header declares several functions useful for testing and mapping characters.144 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.145 Forward references: EOF (7.13.1), localization (7.5). 7.3.1 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. 7.3.1.1 The isalnum function Synopsis [#1] #include int isalnum(int c); __________ 144. See ``future library directions'' (7.20.2). 145. In an implementation that uses the seven-bit 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). Library 211 Working Draft, 1997-11-21, WG14/N794 J11/97-158 Description [#2] The isalnum function tests for any character for which isalpha or isdigit is true. 7.3.1.2 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 characters for which none of iscntrl, isdigit, ispunct, or isspace is true.146 In the "C" locale, isalpha returns true only for the characters for which isupper or islower is true. 7.3.1.3 The isblank function Synopsis [#1] #include int isblank(int c); Description [#2] The isblank function tests for any character for that is a standard blank character or is one of a locale-specific set of characters, for which isalnum is false. The standard blank characters are the following: space (' '), and horizontal tab ('\t'). In the "C" locale, isblank returns true only for the standard blank characters. __________ 146. The functions islower and isupper test true or false separately for each of these additional characters; all four combinations are possible. 212 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 7.3.1.4 The iscntrl function Synopsis [#1] #include int iscntrl(int c); Description [#2] The iscntrl function tests for any control character. 7.3.1.5 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). 7.3.1.6 The isgraph function Synopsis [#1] #include int isgraph(int c); Description [#2] The isgraph function tests for any printing character except space (' '). 7.3.1.7 The islower function Synopsis [#1] #include int islower(int c); Library 213 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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). 7.3.1.8 The isprint function Synopsis [#1] #include int isprint(int c); Description [#2] The isprint function tests for any printing character including space (' '). 7.3.1.9 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 characters for which neither isspace nor isalnum is true. 7.3.1.10 The isspace function Synopsis [#1] #include int isspace(int c); 214 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. 7.3.1.11 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). 7.3.1.12 The isxdigit function Synopsis [#1] #include int isxdigit(int c); Description [#2] The isxdigit function tests for any hexadecimal-digit character (as defined in 6.1.3.1). Library 215 Working Draft, 1997-11-21, WG14/N794 J11/97-158 7.3.2 Character case mapping functions 7.3.2.1 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. 7.3.2.2 The toupper function Synopsis [#1] #include int toupper(int c); Description [#2] The toupper function converts a lowercase letter to a corresponding uppercase letter. 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. 216 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 7.4 Integer types [#1] The header defines sets of typedef names for integer types having specified widths, and defines corresponding sets of macros. It also defines macros that specify limits of integer types corresponding to typedef names defined in other standard headers, and declares four functions for converting numeric character strings to greatest-width integers. [#2] Typedef names are defined in the following categories: - integer types having certain exact widths; - integer types having at least certain specified widths; - fastest integer types having at least certain specified widths; - integer types wide enough to hold pointers to objects; - integer types having greatest width. (Some of these typedef names may denote the same type.) [#3] Corresponding macros specify limits of the defined types, construct suitable character constants, and provide conversion specifiers for use with the formatted input/output functions. [#4] For each typedef name described herein that can be defined as a type existing in the implementation,147 shall define that typedef name, and it shall define the associated macros. Conversely, for each typedef name described herein that cannot be defined as a type existing in the implementation, shall not define that typedef name, nor shall it define the associated macros. __________ 147. Some of these typedef names may denote implementation- defined extended integer types. Library 217 Working Draft, 1997-11-21, WG14/N794 J11/97-158 7.4.1 Typedef names for integer types [#1] When typedef names differing only in the absence or presence of the initial u are defined, they shall denote corresponding signed and unsigned types as described in subclause 6.1.2.5. 7.4.1.1 Exact-width integer types [#1] Each of the following typedef names designates an integer type that has exactly the specified width. These typedef names have the general form of intn_t or uintn_t where n is the required width. For example, uint8_t denotes an unsigned integer type that has a width of exactly 8 bits. [#2] The following designate exact-width signed integer types: int8_t int16_t int32_t int64_t [#3] The following designate exact-width unsigned integer types: uint8_t uint16_t uint32_t uint64_t (Any of these types might not exist.) 7.4.1.2 Minimum-width integer types [#1] Each of the following typedef names designates an integer type that has at least the specified width, such that no integer type of lesser size has at least the specified width. These typedef names have the general form of int_leastn_t or uint_leastn_t where n is the minimum required width. For example, int_least32_t denotes a signed integer type that has a width of at least 32 bits. [#2] The following designate minimum-width signed integer types: int_least8_t int_least16_t int_least32_t int_least64_t [#3] The following designate minimum-width unsigned integer types: uint_least8_t uint_least16_t uint_least32_t uint_least64_t (These types must exist.) 218 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 7.4.1.3 Fastest minimum-width integer types [#1] Each of the following typedef names designates an integer type that is usually fastest148 to operate with among all integer types that have at least the specified width. These typedef names have the general form of int_fastn_t or uint_fastn_t where n is the minimum required width. For example, int_fast16_t denotes the fastest signed integer type that has a width of at least 16 bits. [#2] The following designate fastest minimum-width signed integer types: int_fast8_t int_fast16_t int_fast32_t int_fast64_t [#3] The following designate fastest minimum-width unsigned integer types: uint_fast8_t uint_fast16_t uint_fast32_t uint_fast64_t (These types must exist.) 7.4.1.4 Integer types capable of holding object pointers [#1] The following typedef name designates a signed integer type with the property that any valid pointer to void can be converted to this type, then converted back to pointer to void, and the result will compare equal to the original pointer: intptr_t [#2] The following typedef name designates an unsigned integer type with the property that any valid pointer to void can be converted to this type, then converted back to pointer to void, and the result will compare equal to the original pointer: __________ 148. The designated type is not guaranteed to be fastest for all purposes; if the implementation has no clear grounds for choosing one type over another, it will simply pick some integer type satisfying the signedness and width requirements. Library 219 Working Draft, 1997-11-21, WG14/N794 J11/97-158 uintptr_t (Either or both of these types might not exist.) 7.4.1.5 Greatest-width integer types [#1] The following typedef name designates a signed integer type capable of representing any value of any signed integer type: intmax_t [#2] The following typedef name designates an unsigned integer type capable of representing any value of any unsigned integer type: uintmax_t (These types must exist.) 7.4.2 Limits of specified-width integer types [#1] The following object-like macros149 specify the minimum and maximum limits of integer types corresponding to the typedef names defined in . Each macro name corresponds to a similar typedef name in subclause 7.4.1. [#2] Each instance of any defined macro shall be replaced by a constant expression suitable for use in #if preprocessing directives, and this expression shall have the same type as would an expression that is an object of the corresponding type converted according to the integer promotions. Its implementation-defined value shall be equal to or greater in magnitude (absolute value) than the corresponding value given below, with the same sign. __________ 149. C++ implementations should define these macros only when __STDC_LIMIT_MACROS is defined before is included. 220 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 7.4.2.1 Limits of exact-width integer types - minimum values of exact-width signed integer types INT8_MIN -127 INT16_MIN -32767 INT32_MIN -2147483647 INT64_MIN -9223372036854775807 (The value must be either that given or exactly 1 less.) - maximum values of exact-width signed integer types INT8_MAX +127 INT16_MAX +32767 INT32_MAX +2147483647 INT64_MAX +9223372036854775807 (The value must be exactly that given.) - maximum values of exact-width unsigned integer types UINT8_MAX 255 UINT16_MAX 65535 UINT32_MAX 4294967295 UINT64_MAX 18446744073709551615 (The value must be exactly that given.) 7.4.2.2 Limits of minimum-width integer types - minimum values of minimum-width signed integer types INT_LEAST8_MIN -127 INT_LEAST16_MIN -32767 INT_LEAST32_MIN -2147483647 INT_LEAST64_MIN -9223372036854775807 - maximum values of minimum-width signed integer types INT_LEAST8_MAX +127 INT_LEAST16_MAX +32767 INT_LEAST32_MAX +2147483647 INT_LEAST64_MAX +9223372036854775807 - maximum values of minimum-width unsigned integer types UINT_LEAST8_MAX 255 UINT_LEAST16_MAX 65535 UINT_LEAST32_MAX 4294967295 UINT_LEAST64_MAX 18446744073709551615 Library 221 Working Draft, 1997-11-21, WG14/N794 J11/97-158 7.4.2.3 Limits of fastest minimum-width integer types - minimum values of fastest minimum-width signed integer types INT_FAST8_MIN -127 INT_FAST16_MIN -32767 INT_FAST32_MIN -2147483647 INT_FAST64_MIN -9223372036854775807 - maximum values of fastest minimum-width signed integer types INT_FAST8_MAX +127 INT_FAST16_MAX +32767 INT_FAST32_MAX +2147483647 INT_FAST64_MAX +9223372036854775807 - maximum values of fastest minimum-width unsigned integer types UINT_FAST8_MAX 255 UINT_FAST16_MAX 65535 UINT_FAST32_MAX 4294967295 UINT_FAST64_MAX 18446744073709551615 7.4.2.4 Limits of integer types capable of holding object pointers - minimum value of pointer-holding signed integer type INTPTR_MIN -32767 - maximum value of pointer-holding signed integer type INTPTR_MAX +32767 - maximum value of pointer-holding unsigned integer type UINTPTR_MAX 65535 7.4.2.5 Limits of greatest-width integer types - minimum value of greatest-width signed integer type INTMAX_MIN -9223372036854775807 - maximum value of greatest-width signed integer type INTMAX_MAX +9223372036854775807 - maximum value of greatest-width unsigned integer type UINTMAX_MAX 18446744073709551615 222 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 7.4.3 Macros for integer constants [#1] The following function-like macros150 expand to integer constants suitable for initializing objects that have integer types corresponding to typedef names defined in . Each macro name corresponds to a similar typedef name in subclause 7.4.1.2 or 7.4.1.5. [#2] The argument in any instance of these macros shall be a decimal, octal, or hexadecimal constant (as defined in subclause 6.1.3.2) with a value that does not exceed the limits for the corresponding type. 7.4.3.1 Macros for minimum-width integer constants [#1] Each of the following macros expands to an integer constant having the value specified by its argument and a type with at least the specified width. These macro names have the general form of INTn_C or UINTn_C where n is the minimum required width. For example, UINT64_C(0x123) might expand to the integer constant 0x123ULL. [#2] The following expand to integer constants that have signed integer types: INT8_C(value) INT16_C(value) INT32_C(value) INT64_C(value) [#3] The following expand to integer constants that have unsigned integer types: UINT8_C(value) UINT16_C(value) UINT32_C(value) UINT64_C(value) __________ 150. C++ implementations should define these macros only when __STDC_CONSTANT_MACROS is defined before is included. Library 223 Working Draft, 1997-11-21, WG14/N794 J11/97-158 7.4.3.2 Macros for greatest-width integer constants [#1] The following macro expands to an integer constant having the value specified by its argument and the type intmax_t: INTMAX_C(value) [#2] The following macro expands to an integer constant having the value specified by its argument and the type uintmax_t: UINTMAX_C(value) 7.4.4 Macros for format specifiers [#1] Each of the following object-like macros151 expands to a string literal containing a conversion specifier, possibly modified by a prefix such as hh, h, l, or ll, 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),152 followed by the conversion specifier, followed by a name corresponding to a similar typedef name in subclause 7.4.1. 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: __________ 151. C++ implementations should define these macros only when __STDC_FORMAT_MACROS is defined before is included. 152. Separate macros are given for use with fprintf and fscanf functions because, typically, different format specifiers are required for fprintf and fscanf even when the type is the same. 224 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 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: 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: Library 225 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 typedef names defined in this header. Consequently, as a special exception to the requirement that the implementation shall define all macros associated with each typedef name defined in this header, in such a case the problematic fscanf macros may be left undefined. Examples #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; } 7.4.5 Limits of other integer types [#1] The following object-like macros151 specify the minimum and maximum limits of integer types corresponding to typedef names defined in other standard headers. [#2] Each instance of these macros shall be replaced by a constant expression suitable for use in #if preprocessing directives, and this expression shall have the same type as would an expression that is an object of the corresponding type converted according to the integer promotions. Its implementation-defined value shall be equal to or greater in magnitude (absolute value) than the corresponding value given below, with the same sign. 226 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 - limits of ptrdiff_t PTRDIFF_MIN -65535 PTRDIFF_MAX +65535 - limits of sig_atomic_t SIG_ATOMIC_MIN see below SIG_ATOMIC_MAX see below - limit of size_t SIZE_MAX 65535 - limits of wchar_t WCHAR_MIN see below WCHAR_MAX see below - limits of wint_t WINT_MIN see below WINT_MAX see below [#3] If sig_atomic_t is defined as a signed integer type, the value of SIG_ATOMIC_MIN shall be no greater than -127 and the value of SIG_ATOMIC_MAX shall be no less than 127; otherwise, sig_atomic_t is defined as an unsigned integer type, and the value of SIG_ATOMIC_MIN shall be 0 and the value of SIG_ATOMIC_MAX shall be no less than 255. [#4] If wchar_t is defined as a signed integer type, the value of WCHAR_MIN shall be no greater than -127 and the value of WCHAR_MAX shall be no less than 127; otherwise, wchar_t is defined as an unsigned integer type, and the value of WCHAR_MIN shall be 0 and the value of WCHAR_MAX shall be no less than 255. [#5] If wint_t is defined as a signed integer type, the value of WINT_MIN shall be no greater than -32767 and the value of WINT_MAX shall be no less than 32767; otherwise, wint_t is defined as an unsigned integer type, and the value of WINT_MIN shall be 0 and the value of WINT_MAX shall be no less than 65535. Library 227 Working Draft, 1997-11-21, WG14/N794 J11/97-158 7.4.6 Conversion functions for greatest-width integer types 7.4.6.1 The strtoimax function Synopsis [#1] #include intmax_t strtoimax(const char * restrict nptr, char ** restrict endptr, int base); Description [#2] The strtoimax function is equivalent to strtol, except that the initial portion of the string is converted to intmax_t representation. Returns [#3] The strtoimax 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 or INTMAX_MIN is returned (according to the sign of the value), and the value of the macro ERANGE is stored in errno. 7.4.6.2 The strtoumax function Synopsis [#1] #include uintmax_t strtoumax(const char * restrict nptr, char ** restrict endptr, int base); Description [#2] The strtoumax function is equivalent to strtoul, except that the initial portion of the string is converted to uintmax_t representation. Returns [#3] The strtoumax 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, UINTMAX_MAX is returned, and the value of the macro ERANGE is stored in errno. 228 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 7.4.6.3 The wcstoimax function Synopsis [#1] #include // for wchar_t #include intmax_t wcstoimax(const wchar_t * restrict nptr, wchar_t ** restrict endptr, int base); Description [#2] The wcstoimax function is equivalent to wcstol, except that the initial portion of the wide string is converted to intmax_t representation. 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 or INTMAX_MIN is returned (according to the sign of the value), and the value of the macro ERANGE is stored in errno. 7.4.6.4 The wcstoumax function Synopsis [#1] #include // for wchar_t #include uintmax_t wcstoumax(const wchar_t * restrict nptr, wchar_t ** restrict endptr, int base); Description [#2] The wcstoumax function is equivalent to wcstoul, except that the initial portion of the wide string is converted to uintmax_t representation. Returns [#3] The wcstoumax 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, UINTMAX_MAX is returned, and the value of the macro ERANGE is stored in errno. Library 229 Working Draft, 1997-11-21, WG14/N794 J11/97-158 7.5 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 is explained in 7.5.2.1. In the "C" locale, the members shall have the values specified in the comments. char *decimal_point; // "." char *thousands_sep; // "" char *grouping; // "" char *int_curr_symbol; // "" char *currency_symbol; // "" char *mon_decimal_point; // "" char *mon_thousands_sep; // "" char *mon_grouping; // "" char *positive_sign; // "" char *negative_sign; // "" char int_frac_digits; // CHAR_MAX char frac_digits; // CHAR_MAX char p_cs_precedes; // CHAR_MAX char p_sep_by_space; // CHAR_MAX char n_cs_precedes; // CHAR_MAX char n_sep_by_space; // CHAR_MAX char p_sign_posn; // CHAR_MAX char n_sign_posn; // CHAR_MAX [#3] The macros defined are NULL (described in 7.1.6); and153 __________ 153. ISO/IEC 9945-2, Information technology - Portable operating system interface (POSIX) - Part 2: shell and utilities specifies locale and charmap formats that may be used to specify locales for C. 230 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 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. Additional macro definitions, beginning with the characters LC_ and an uppercase letter,154 may also be specified by the implementation. 7.5.1 Locale control 7.5.1.1 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 functions155 and the multibyte functions. LC_MONETARY affects the monetary formatting information returned by the localeconv function. LC_NUMERIC affects the decimal-point character for the formatted input/output functions and the string conversion functions, as well as the nonmonetary formatting information returned by the localeconv function. LC_TIME affects the behavior of the strftime function. __________ 154. See ``future library directions'' (7.20.4). 155. The only functions in 7.3 whose behavior is not affected by the current locale are isdigit and isxdigit. Library 231 Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#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. [#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.156 [#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 (7.13.6), the multibyte character functions (7.14.7), the multibyte string functions (7.14.8), string conversion functions (7.14.1), the strcoll function (7.15.4.3), the strftime function (7.16.3.6), the strxfrm function (7.15.4.5). __________ 156. The implementation must arrange to encode in a string the various categories due to a heterogeneous locale when category has the value LC_ALL. 232 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 7.5.2 Numeric formatting convention inquiry 7.5.2.1 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. [#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 *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 4217:1987. The fourth character (immediately preceding the null character) is the character used to separate the international currency symbol from the monetary quantity. Library 233 Working Draft, 1997-11-21, WG14/N794 J11/97-158 char *currency_symbol The local currency symbol applicable to the current locale. 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 int_frac_digits The number of fractional digits (those after the decimal-point) to be displayed in an internationally formatted monetary quantity. char frac_digits The number of fractional digits (those after the decimal-point) to be displayed in a 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 formatted monetary quantity. char p_sep_by_space Set to 1 or 0 if the currency_symbol respectively is or is not separated by a space from the value for a nonnegative 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 formatted monetary quantity. char n_sep_by_space Set to 1 or 0 if the currency_symbol respectively is or is not separated by a space from the value for a 234 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 negative formatted monetary quantity. char p_sign_posn Set to a value indicating the positioning of the positive_sign for a nonnegative formatted monetary quantity. char n_sign_posn Set to a value indicating the positioning of the negative_sign for a negative 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 comprise the current group. The next element is examined to determine the size of the next group of digits before the current group. [#5] The value of p_sign_posn and n_sign_posn is interpreted according to the following: 0 Parentheses surround the quantity and currency_symbol. 1 The sign string precedes the quantity and currency_symbol. 2 The sign string succeeds the quantity and currency_symbol. 3 The sign string immediately precedes the currency_symbol. 4 The sign string immediately succeeds the currency_symbol. [#6] The implementation shall behave as if no library function calls the localeconv function. Returns [#7] 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 Library 235 Working Draft, 1997-11-21, WG14/N794 J11/97-158 contents of the structure. Examples [#8] The following table illustrates the rules which may well be used by four countries to format monetary quantities. Country Positive format Negative formatInternational format Italy L.1.234 -L.1.234 ITL.1.234 Netherlands F 1.234,56 F -1.234,56 NLG 1.234,56 Norway kr1.234,56 kr1.234,56- NOK 1.234,56 Switzerland SFrs.1,234.56 SFrs.1,234.56C CHF 1,234.56 [#9] For these four countries, the respective values for the monetary members of the structure returned by localeconv are: Italy Netherlands NorwaySwitzerland int_curr_symbol "ITL." "NLG " "NOK " "CHF " currency_symbol "L." "F" "kr" "SFrs." mon_decimal_point "" "," ",""." mon_thousands_sep "." "." ".""," mon_grouping "\3" "\3" "\3" "\3" positive_sign "" "" "" "" negative_sign "-" "-" "-" "C" int_frac_digits 0 2 2 2 frac_digits 0 2 2 2 p_cs_precedes 1 1 1 1 p_sep_by_space 0 1 0 0 n_cs_precedes 1 1 1 1 n_sep_by_space 0 1 0 0 p_sign_posn 1 1 1 1 n_sign_posn 1 4 2 2 236 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 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.157 A floating-point status flag is a system variable whose value is set as a side effect of the 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 the arithmetic. [#2] Certain programming conventions support the intended model of use for the floating-point environment:158 - a function call must 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. __________ 157. This header is designed to support the exception status flags and directed-rounding control modes required by IEC 559, and other similar floating-point state information. Also it is designed to facilitate code portability among all systems. 158. With 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. Library 237 Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#4] The type fexcept_t represents the floating-point exception flags collectively, 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. 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. The defined macros expand to integer constant expressions whose values are distinct nonnegative values.159 __________ 159. Even though the rounding direction macros may expand to constants corresponding to the values of FLT_ROUNDS, they are not required to do so. 238 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 [#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 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.160 The pragma can occur either outside external declarations or preceding all explicit declarations and statements inside a compound statement. When outside external declarations, the pragma takes effect from its occurrence until another 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 __________ 160. The 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. Library 239 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 of that program is undefined. The default state (on or off) for the pragma is implementation- defined. Examples [#3] #include void f(double x) { #pragma STDC FENV_ACCESS ON void g(double); void h(double); /* ... */ g(x + 1); h(x + 1); /* ... */ } If the function g might depend on status flags set as a side effect of the first x + 1, or if the second x + 1 might depend on control modes set as a side effect of the call to function g, then the program must contain an appropriately placed invocation of #pragma STDC FENV_ACCESS ON.161 __________ 161. The 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. 240 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 7.6.2 Exceptions [#1] The following functions provide access to the exception flags.162 The int input argument for the functions represents a subset of floating-point exceptions, and can be constructed by bitwise ORs of the exception macros, for example FE_OVERFLOW | FE_INEXACT. For other argument values the behavior of these functions is undefined. 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. __________ 162. The 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. Library 241 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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.163 The order in which these exceptions are raised is unspecified, except as stated in F.7.6. Whether the feraiseexcept function additionally raises the inexact exception whenever it raises the overflow or underflow exception is implementation- defined. 7.6.2.4 The fesetexceptflag function Synopsis [#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 must have been set by a previous call to fegetexceptflag whose second argument represented at least those exceptions represented by the argument excepts; if not, the effect on the indicated exception flags is undefined. This function does not raise exceptions, but only sets the state of the flags. __________ 163. The 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. 242 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 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.164 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. Examples [#4] Call f if invalid is set, then g if overflow is set: #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(); /* ... */ } __________ 164. This mechanism allows testing several exceptions with just one function call. Library 243 Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 does not match a rounding direction macro, the rounding direction is not changed. Returns [#3] The fesetround function returns a nonzero value if and only if the argument matches a rounding direction macro (that is, if and only if the requested rounding direction can be established). 244 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 Examples [#4] 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. 7.6.4.2 The feholdexcept function Synopsis [#1] #include int feholdexcept(fenv_t *envp); Library 245 Working Draft, 1997-11-21, WG14/N794 J11/97-158 Description [#2] The feholdexcept function saves the current environment in the object pointed to by envp, clears the exception flags, and installs a non-stop (continue on exceptions) mode, if available, for all exceptions.165 Returns [#3] The feholdexcept function returns nonzero 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 must 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. __________ 165. IEC 559 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. 246 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 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 environment represented through envp, and then raises the saved exceptions. The argument envp must 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. Examples [#3] 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; } Library 247 Working Draft, 1997-11-21, WG14/N794 J11/97-158 7.7 Mathematics [#1] The header declares two types and several mathematical functions and defines several macros. Most synopses specify a function which takes one or more double arguments and returns a double value; for each such function, there are functions with the same name but with f and l suffixes which are corresponding functions with float and long double arguments and return values.166 Some synopses specify a function which takes a double argument and returns an integer-type value; for each such function, there are functions with the same name but with f and l suffixes which are corresponding functions with float and long double arguments. 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.167 [#3] The macro HUGE_VAL expands to a positive double constant expression, not __________ 166. Particularly on systems with wide expression evaluation, a function might pass arguments and return values in wider format than the synopsis prototype indicates. 167. The 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. 248 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 necessarily representable as a float. The macros HUGE_VALF HUGE_VALL are respectively float and long double analogs of HUGE_VAL.168 [#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. [#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 the fma __________ 168. HUGE_VAL, HUGE_VALF, and HUGE_VALL can be positive infinities in an implementation that supports infinities. Library 249 Working Draft, 1997-11-21, WG14/N794 J11/97-158 function generally executes about as fast as a multiply and an add of double operands.169 The macros FP_FAST_FMAF FP_FAST_FMAL are, respectively, float and long double analogs of FP_FAST_FMA. [#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. [#9] The macro DECIMAL_DIG expands to an integer constant expression whose value is implementation-defined. It represents a number of decimal digits supported by conversion between decimal and all internal floating-point formats.170 Recommended practice [#10] Conversion from (at least) double to decimal with DECIMAL_DIG digits and back is the identity function.171 __________ 169. Typically, 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. 170. DECIMAL_DIG is intended to give an appropriate number of digits to carry in canonical decimal representations. 171. In order that correctly rounded conversion from an internal floating-point format with precision m to decimal with DECIMAL_DIG digits and back be the identity function, DECIMAL_DIG should be a positive integer n satisfying the inequality: n>m if FLT_RADIX is 10 n-1 m otherwise 10 >FLT_RADIX 250 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 7.7.1 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.172 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 appropriate to the specified result type, with the same sign as the correct value of the function; whether errno acquires the value ERANGE when a range error occurs is implementation-defined. The result underflows if the magnitude of the mathematical result is so small that the mathematical result cannot be represented, without extraordinary roundoff error, in an object of the specified type.173 If the result underflows, the function returns a value whose magnitude is no greater than the smallest normalized positive number in the ____________________________________________________________ 172. In 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. 173. The term underflow here is intended to encompass both gradual underflow as in IEC 559 and also flush-to-zero underflow. Library 251 Working Draft, 1997-11-21, WG14/N794 J11/97-158 specified type and is otherwise implementation-defined; whether errno acquires the value ERANGE is implementation- defined. 7.7.2 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 (6.3). 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. 7.7.3 Classification macros [#1] In the synopses in this subclause, real-floating indicates that the argument must be an expression of real floating type. The result is undefined if an argument is not of real floating type. 7.7.3.1 The fpclassify macro Synopsis [#1] #include int fpclassify(real-floating x); 252 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 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.174 Returns [#3] The fpclassify macro returns the value of the number classification macro appropriate to the value of its argument. Examples [#4] The fpclassify macro might be implemented in terms of ordinary functions as #define fpclassify(x) \ ((sizeof (x) == sizeof (float)) ? \ __fpclassifyf(x) \ : (sizeof (x) == sizeof (double)) ? \ __fpclassifyd(x) \ : __fpclassifyl(x)) 7.7.3.2 The signbit macro Synopsis [#1] #include int signbit(real-floating x); Description [#2] The signbit macro determines whether the sign of its argument value is negative.175 __________ 174. Since 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. 175. The signbit macro reports the sign of all values, including infinities, zeros, and NaNs. Library 253 Working Draft, 1997-11-21, WG14/N794 J11/97-158 Returns [#3] The signbit macro returns a nonzero value if and only if the sign of its argument value is negative. 7.7.3.3 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 infinite or NaN). First, an argument represented in a format wider than its semantic type is converted to its semantic type. Then determination is based on the type of the argument. Returns [#3] The isfinite macro returns a nonzero value if and only if its argument has a finite value. 7.7.3.4 The isnormal macro Synopsis [#1] #include int isnormal(real-floating x); Description [#2] The isnormal macro determines whether its argument value is normal (neither zero, subnormal, infinite, nor NaN). First, an argument represented in a format wider than its semantic type is converted to its semantic type. Then determination is based on the type of the argument. Returns [#3] The isnormal macro returns a nonzero value if and only if its argument has a normal value. 254 Library Working Draft, 1997-11-21, WG14/N794 J11/97-158 7.7.3.5 The isnan macro Synopsis [#1] #include int isnan(real-floating x); Description [#2] The isnan macro determines whether its argument value is a NaN. First, an argument represented in a format wider than its semantic type is converted to its semantic type. Then determination is based on the type of the argument.176 Returns [#3] The isnan macro returns a nonzero value if and only if its argument has a NaN value. 7.7.3.6 The isinf macro Synopsis [#1] #include int isinf(real-floating x); Description [#2] The isinf macro determines whether its argument value is an infinity (positive or negative). First, an argument represented in a format wider than its semantic type is converted to its semantic type. Then determination is based on the type of the argument. __________ 176. For the isnan macro, the type for determination doesn't matter unless the implementation supports NaNs in the evaluation type but not in the semantic type. Library 255 Working Draft, 1997-11-21, WG14/N794 J11/97-158 Returns [#3] The isinf macro returns a nonzero value if and only if its argument has an infinite value. 7.7.4 Trigonometric functions 7.7.4.1 The acos function Synopsis [#1] #include double acos(double x); Description [#2] The acos function computes the principal value of the arc cosine of x. A domain error occurs for arguments not in the range [-1, +1]. Returns [#3] The acos function returns the arc cosine in the range [0, pi] radians. 7.7.4.2 The asin function Synopsis [#1] #include double asin(double x); Description [#2] The asin function computes the principal value of the arc sine of x. A domain error occurs for arguments not in the range [-1, +1]. Returns [#3] The asin function returns the arc sine in the range [ - pi/2, +pi/2] radians. 256 Library Working Draft, 1997-11-