(This foreword is not a part of American National Standard for Information Systems --- Programming Language C, X3.???-1988.) American National Standard Programming Language C specifies the syntax and semantics of programs written in the C programming language. It specifies the C program's interactions with the execution environment via input and output data. It also specifies restrictions and limits imposed upon conforming implementations of C language translators. The standard was developed by the X3J11 Technical Committee on the C Programming Language under project 381-D by American National Standards Committee on Computers and Information Processing (X3). SPARC document number 83-079 describes the purpose of this project to ``provide an unambiguous and machine-independent definition of the language C.'' The need for a single clearly defined standard had arisen in the C community due to a rapidly expanding use of the C programming language and the variety of differing translator implementations that had been and were being developed. The existence of similar but incompatible implementations was a serious problem for program developers who wished to develop code that would compile and execute as expected in several different environments. Part of this problem could be traced to the fact that implementors did not have an adequate definition of the C language upon which to base their implementations. The de facto C programming language standard, The C Programming Language by Brian W. Kernighan and Dennis M. Ritchie, is an excellent book; however, it is not precise or complete enough to specify the C language fully. In addition, the language has grown over years of use to incorporate new ideas in programming and to address some of the weaknesses of the original language. American National Standard Programming Language C addresses the problems of both the program developer and the translator implementor by specifying the C language precisely. The work of X3J11 began in the summer of 1983, based on the several documents that were made available to the Committee (see $1.5, Base Documents). The Committee divided the effort into three pieces: the environment, the language, and the library. A complete specification in each of these areas is necessary if truly portable programs are to be developed. Each of these areas is addressed in the Standard. The Committee evaluated many proposals for additions, deletions, and changes to the base documents during its deliberations. A concerted effort was made to codify existing practice wherever unambiguous and consistent practice could be identified. However, where no consistent practice could be identified, the Committee worked to establish clear rules that were consistent with the overall flavor of the language. This document was approved as an American National Standard by the American National Standards Institute (ANSI) on DD MM, 1988. Suggestions for improvement of this Standard are welcome. They should be sent to the American National Standards Institute, 1430 Broadway, New York, NY 10018. The Standard was processed and approved for submittal to ANSI by the American National Standards Committee on Computers and Information Processing, X3. Committee approval of the Standard does not necessarily imply that all members voted for its approval. At the time that it approved this Standard, the X3 Committee had the following members: Organization Name of Representative (To be completed on approval of the Standard.) Technical Committee X3J11 on the C Programming Language had the following members at the time they forwarded this document to X3 for processing as an American National Standard: Chair Jim Brodie Vice-Chair Thomas Plum Plum Hall Secretary P. J. Plauger Whitesmiths, Ltd. International Representative P. J. Plauger Whitesmiths, Ltd. Steve Hersee Lattice, Inc. Vocabulary Representative Andrew Johnson Prime Computer Environment Subcommittee Chairs Ralph Ryan Microsoft Ralph Phraner Phraner Associates Language Subcommittee Chair Lawrence Rosler AT&T Library Subcommittee Chair P. J. Plauger Whitesmiths, Ltd. Draft Redactor David F. Prosser AT&T Lawrence Rosler AT&T Rationale Redactor Randy Hudson Intermetrics, Inc. In the following list, unmarked names denote principal members and * denotes alternate members. David F. Prosser, AT&T Steven J. Adamski, AT&T* (X3H2 SQL liaison) Bob Gottlieb, Alliant Computer Systems Kevin Brosnan, Alliant Computer Systems Neal Weidenhofer, Amdahl Philip C. Steel, American Cimflex Eric McGlohon, American Cimflex* Stephen Kafka, Analog Devices Kevin Leary, Analog Devices* Gordon Sterling, Analog Devices* John Peyton, Apollo Computer Elizabeth Crockett, Apple Computers Ed Wells, Arinc Tom Ketterhagen, Arinc* Vaughn Vernon, Aspen Scientific Craig Bordelon, Bell Communications Research Steve Carter, Bell Communications Research* William Puig, Bell Communications Research* Bob Jervis, Borland International Yom-Tov Meged, Boston Systems Office Rose Thomson, Boston Systems Office* Maurice Fathi, COSMIC John Wu, Charles River Data Systems Daniel Mickey, Chemical Abstracts Service Thomas Mimlitch, Chemical Abstracts Service* Alan Losoff, Chicago Research & Trading Group Edward Briggs, Citibank Firmo Freire, Cobra S/A Jim Patterson, Cognos Bruce Tetelman, Columbia U. Center for Computing Terry Moore, CompuDas Mark Barrenechea, Computer Associates George Eberhardt, Computer Innovations Dave Neathery, Computer Innovations* Joseph Bibbo, Computrition Steve Davies, Concurrent Computer Corporation Don Fosbury, Control Data George VandeBunte, Control Data* Lloyd Irons, Cormorant Communications Tom MacDonald, Cray Research Lynne Johnson, Cray Research* Dave Becker, Cray Research* Jean Risley, Custom Development Environments Rex Jaeschke, DEC Professional Mike Terrazas, DECUS Representative Michael Meissner, Data General Mark Harris, Data General* Leonard Ohmes, Datapoint James Stanley, Data Systems Analysts Samuel J. Kendall, Delft Consulting Randy Meyers, Digital Equipment Corporation Art Bjork, Digital Equipment Corporation* Lu Anne Van de Pas, Digital Equipment Corporation* Ben Patel, EDS Richard Relph, EPI Graham Andrews, Edinburgh Portable Compilers Colin McPhail, Edinburgh Portable Compilers* J. Stephen Adamczyk, Edison Design Group Eric Schwarz, Edison Design Group* Dmitry Lenkov, Everest Solutions Frank Farance, Farance Inc. Peter Hayes, Farance Inc.* Florin Jordan, Floradin Philip Provin, General Electric Information Services Liz Sanville, Gould CSD Tina Aleksa, Gould CSD* Thomas Kelly, HCR Corporation Paul Jackson, HCR Corporation* Gary Jeter, Harris Computer Systems Sue Meloy, Hewlett Packard Larry Rosler, Hewlett Packard* Michelle Ruscetta, Hewlett Packard* Thomas E. Osten, Honeywell Information Systems David Kayden, Honeywell Information Systems* Shawn Elliott, IBM Larry Breed, IBM* Mel Goldberg, IBM* Mike Banahan, Instruction Set Clark Nelson, Intel Dan Lau, Intel* John Wolfe, InterACT Lillian Toll, InterACT* Randy Hudson, Intermetrics Keith Winter, International Computers Ltd. Honey M. Schrecker, International Computers Ltd.* Jim Brodie, J. Brodie & Associates Jacklin Kotikian, Kendall Square Research W. Peter Hesse, LSI Logic Europe Ltd. John Kaminski, Language Processors Inc. David Yost, Laurel Arts Mike Branstetter, Lawrence Livermore National Laboratory Bob Weaver, Los Alamos National Laboratory Lidia Eberhart, Modcomp Robert Sherry, Manx Software Courtney Meissen, Mark Williams Co. Patricia Jenkins, Masscomp Dave Hinman, Masscomp* Michael Kearns, MetaLink Tom Pennello, MetaWare Incorporated David F. Weil, Microsoft Mitch Harder, Microsoft* Kim Kempf, Microware Systems Shane McCarron, Minnesota Educational Computing Bruce Olsen, Mosaic Technologies Michael Paton, Motorola Rick Schubert, NCR Brian Johnson, NCR* Joseph Mueller, National Semiconductor Derek Godfrey, National Semiconductor* Jim Upperman, National Bureau of Standards James W. Williams, Naval Research Laboratory Lisa Simon, OCLC Paul Amaranth, Oakland University August R. Hansen, Omniware Michael Rolle, Oracle Carl Ellis, Oregon Software Barry Hedquist, Perennial Sassan Hazeghi, Peritus International James Holmlund, Peritus International* Thomas Plum, Plum Hall Christopher Skelly, Plum Hall* Andrew Johnson, Prime Computer Fran Litterio, Prime Computer* Daniel J. Conrad, Prismatics David Fritz, Production Languages Kenneth Pugh, Pugh Killeen Ed Ramsey, Purdue University Stephen Roberts, Purdue University* Kevin Nolan, Quantitative Technology Corp. Robert Mueller, Quantitative Technology Corp.* Chris DeVoney, Que Corporation Jon Tulk, Rabbit Software Terry Colligan, Rational Systems Daniel Saks, Saks & Associates Nancy Saks, Saks & Associates* Oliver Bradley, SAS Institute Alan Beale, SAS Institute* Larry Jones, SDRC Donald Kossman, SEI Information Technology Kenneth Harrenstien, SRI International Larry Rosenthal, Sierra Systems Phil Hempfner, Southern Bell Telephone Purshotam Rajani, Spruce Technology Savu Savulescu, Stagg Systems Peter Darnell, Stellar Computer Lee W. Cooprider, Stellar Computer* Paul Gilmartin, Storage Technology Corp. Steve Muchnick, Sun Microsystems Chuck Rasbold, Supercomputer Systems, Inc. Kelly O'Hair, Supercomputer Systems, Inc.* Henry Richardson, Tandem John M. Hausman, Tandem* Samuel Harbison, Tartan Laboratories Michael S. Ball, TauMetric Carl Sutton, Tektronix Jim Besemer, Tektronix* Reid Tatge, Texas Instruments Ed Brower, Tokheim Robert Mansfield, Tokheim* Monika Khushf, Tymlabs Morgan Jones, Tymlabs* Don Bixler, Unisys Steve Bartels, Unisys* Glenda Berkheimer, Unisys* Annice Jackson, Unisys* Fred Blonder, University of Maryland Fred Schwarz, University of Michigan R. Jordan Kreindler, University of Southern California CTC Mike Carmody, University of Waterloo Douglas Gwyn, US Army BRL (IEEE P1003 liaison) C. Dale Pierce, US Army Management Engineering* John C. Black, VideoFinancial Joseph Musacchia, Wang Labs Fred Rozakis, Wang Labs* P. J. Plauger, Whitesmiths, Ltd. Kim Leeper, Wick Hill Mark Wittenberg, Zehntel Jim Balter Robert Bradbury Edward Chin Neil Daniels Stephen Desofi Michael Duffy Phillip Escue Ralph Phraner D. Hugh Redelmeier Arnold Davi Robbins Roger Wilks Michael J. Young purpose: 1.1 scope: 1.2 references: 1.3 organization of the document: 1.4 base documents: 1.5 definitions of terms: 1.6 compliance: 1.7 translation environment: 2. execution environment: 2. separate compilation: 2.1.1.1 separate translation: 2.1.1.1 source file: 2.1.1.1 translation unit: 2.1.1.1 program execution: 2.1.2.3 side effects: 2.1.2.3 sequence point: 2.1.2.3 character set: 2.2.1 signals: 2.2.3 interrupts: 2.2.3 syntax notation: 3. lexical elements: 3.1 comment: 3.1 white space: 3.1 list of keywords: 3.1.1 reserved words: 3.1.1 underscore character: 3.1.2 enumeration constant: 3.1.2 length of names: 3.1.2 internal name, length of: 3.1.2 external name, length of: 3.1.2 function name, length of: 3.1.2 scopes: 3.1.2.1 prototype, function: 3.1.2.1 function scope: 3.1.2.1 file scope: 3.1.2.1 block scope: 3.1.2.1 block structure: 3.1.2.1 function prototype scope: 3.1.2.1 linkage: 3.1.2.2 external linkage: 3.1.2.2 internal linkage: 3.1.2.2 no linkage: 3.1.2.2 name spaces: 3.1.2.3 named label: 3.1.2.3 structure tag: 3.1.2.3 union tag: 3.1.2.3 enumeration tag: 3.1.2.3 structure member name: 3.1.2.3 union member name: 3.1.2.3 storage duration: 3.1.2.4 static storage duration: 3.1.2.4 automatic storage duration: 3.1.2.4 types: 3.1.2.5 object types: 3.1.2.5 function types: 3.1.2.5 incomplete types: 3.1.2.5 char type: 3.1.2.5 signed character: 3.1.2.5 signed char type: 3.1.2.5 short type: 3.1.2.5 long type: 3.1.2.5 unsigned type: 3.1.2.5 float type: 3.1.2.5 double type: 3.1.2.5 long double type: 3.1.2.5 basic types: 3.1.2.5 character types: 3.1.2.5 enumerated type: 3.1.2.5 void type: 3.1.2.5 derived types: 3.1.2.5 integral types: 3.1.2.5 arithmetic types: 3.1.2.5 scalar types: 3.1.2.5 aggregate types: 3.1.2.5 constants: 3.1.3 floating constant: 3.1.3.1 double constant: 3.1.3.1 integer constant: 3.1.3.2 decimal constant: 3.1.3.2 octal constant: 3.1.3.2 hexadecimal constant: 3.1.3.2 unsigned constant: 3.1.3.2 long constant: 3.1.3.2 enumeration constant: 3.1.3.3 character constant: 3.1.3.4 backslash character: 3.1.3.4 escape character: 3.1.3.4 escape sequence: 3.1.3.4 string literal: 3.1.4 character string: 3.1.4 operator: 3.1.5 evaluation: 3.1.5 operand: 3.1.5 punctuator: 3.1.6 character-integer conversion: 3.2.1.1 integer-character conversion: 3.2.1.1 integral promotions: 3.2.1.1 integer-long conversion: 3.2.1.1 signed character: 3.2.1.1 unsigned-integer conversion: 3.2.1.2 integer-unsigned conversion: 3.2.1.2 long-unsigned conversion: 3.2.1.2 long-integer conversion: 3.2.1.2 floating-integer conversion: 3.2.1.3 integer-floating conversion: 3.2.1.3 float-double conversion: 3.2.1.4 double-float conversion: 3.2.1.4 arithmetic conversions: 3.2.1.5 type conversion rules: 3.2.1.5 lvalue: 3.2.2.1 function designator: 3.2.2.1 conversion of array: 3.2.2.1 conversion of function name: 3.2.2.1 void type: 3.2.2.2 pointer-pointer conversion: 3.2.2.3 integer-pointer conversion: 3.2.2.3 null pointer: 3.2.2.3 expression: 3.3 precedence of operators: 3.3 associativity of operators: 3.3 order of evaluation of expressions: 3.3 order of evaluation: 3.3 bitwise operators: 3.3 exceptions: 3.3 primary expression: 3.3.1 type of string: 3.3.1 parenthesized expression: 3.3.1 subscript operator: 3.3.2 function call: 3.3.2 structure member operator: 3.3.2 structure pointer operator: 3.3.2 ++ increment operator: 3.3.2 -- decrement operator: 3.3.2 array, explanation of subscripting: 3.3.2.1 subscripting, explanation of: 3.3.2.1 multi-dimensional array: 3.3.2.1 storage order of array: 3.3.2.1 function call: 3.3.2.2 implicit declaration of function: 3.3.2.2 function argument: 3.3.2.2 call by value: 3.3.2.2 recursion: 3.3.2.2 structure reference: 3.3.2.3 union reference: 3.3.2.3 common initial sequence: 3.3.2.3 postfix ++ and --: 3.3.2.4 -- decrement operator: 3.3.2.4 unary expression: 3.3.3 ++ increment operator: 3.3.3 -- decrement operator: 3.3.3 sizeof operator: 3.3.3 & address operator: 3.3.3 * indirection operator: 3.3.3 + unary plus operator: 3.3.3 - unary minus operator: 3.3.3 ~ bitwise complement operator: 3.3.3 ! logical negation operator: 3.3.3 ++ increment operator: 3.3.3.1 prefix ++ and --: 3.3.3.1 -- decrement operator: 3.3.3.1 + unary plus operator: 3.3.3.3 - unary minus operator: 3.3.3.3 ~ bitwise complement operator: 3.3.3.3 ! logical negation operator: 3.3.3.3 byte: 3.3.3.4 storage allocator: 3.3.3.4 cast expression: 3.3.4 cast operator: 3.3.4 explicit conversion operator: 3.3.4 cast operator: 3.3.4 pointer conversion: 3.3.4 explicit conversion operator: 3.3.4 pointer-integer conversion: 3.3.4 integer-pointer conversion: 3.3.4 alignment restriction: 3.3.4 arithmetic operators: 3.3.5 multiplicative operators: 3.3.5 * multiplication operator: 3.3.5 / division operator: 3.3.5 % modulus operator: 3.3.5 additive operators: 3.3.6 + addition operator: 3.3.6 - subtraction operator: 3.3.6 pointer arithmetic: 3.3.6 pointer arithmetic: 3.3.6 shift operators: 3.3.7 << left shift operator: 3.3.7 >> right shift operator: 3.3.7 relational operators: 3.3.8 < less than operator: 3.3.8 > greater than operator: 3.3.8 <= less than or equal to operator: 3.3.8 >= greater than or equal to operator: 3.3.8 pointer comparison: 3.3.8 equality operators: 3.3.9 == equality operator: 3.3.9 != inequality operator: 3.3.9 & bitwise AND operator: 3.3.10 ^ bitwise exclusive OR operator: 3.3.11 | bitwise inclusive OR operator: 3.3.12 && logical AND operator: 3.3.13 || logical OR operator: 3.3.14 ?: conditional expression: 3.3.15 assignment operators: 3.3.16 assignment expression: 3.3.16 simple assignment: 3.3.16.1 conversion by assignment: 3.3.16.1 compound assignment: 3.3.16.2 comma operator: 3.3.17 constant expression: 3.4 permitted form of initializer: 3.4 declarations: 3.5 storage-class specifier: 3.5.1 storage-class declaration: 3.5.1 typedef declaration: 3.5.1 extern storage class: 3.5.1 static storage class: 3.5.1 auto storage class: 3.5.1 register storage class: 3.5.1 type specifier: 3.5.2 void type: 3.5.2 char type: 3.5.2 short type: 3.5.2 int type: 3.5.2 long type: 3.5.2 float type: 3.5.2 double type: 3.5.2 signed type: 3.5.2 unsigned type: 3.5.2 structure declaration: 3.5.2.1 union declaration: 3.5.2.1 bit-field declaration: 3.5.2.1 bit-field: 3.5.2.1 member alignment: 3.5.2.1 enumeration: 3.5.2.2 enum-specifier: 3.5.2.2 enumerator: 3.5.2.2 structure tag: 3.5.2.3 union tag: 3.5.2.3 structure content: 3.5.2.3 union content: 3.5.2.3 enumeration content: 3.5.2.3 self-referential structure: 3.5.2.3 type qualifier: 3.5.3 const type qualifier: 3.5.3 volatile type qualifier: 3.5.3 declarator: 3.5.4 type declaration: 3.5.4 declaration of pointer: 3.5.4.1 array declaration: 3.5.4.2 declaration of function: 3.5.4.3 type names: 3.5.5 abstract declarator: 3.5.5 typedef declaration: 3.5.6 initialization: 3.5.7 initialization of statics: 3.5.7 implicit initialization: 3.5.7 default initialization: 3.5.7 initialization of automatics: 3.5.7 aggregate initialization: 3.5.7 array initialization: 3.5.7 structure initialization: 3.5.7 character array initialization: 3.5.7 wchar_t array initialization: 3.5.7 statements: 3.6 sequencing of statements: 3.6 full expression: 3.6 labeled statement: 3.6.1 named label: 3.6.1 case label: 3.6.1 default label: 3.6.1 compound statement: 3.6.2 block: 3.6.2 block structure: 3.6.2 initialization in blocks: 3.6.2 expression statement: 3.6.3 null statement: 3.6.3 empty statement: 3.6.3 if-else statement: 3.6.4.1 switch statement: 3.6.4.2 switch body: 3.6.4.2 loop body: 3.6.5 while statement: 3.6.5.1 do statement: 3.6.5.2 for statement: 3.6.5.3 goto statement: 3.6.6.1 continue statement: 3.6.6.2 break statement: 3.6.6.3 return statement: 3.6.6.4 type conversion by return: 3.6.6.4 conversion by return: 3.6.6.4 external definition: 3.7 function definition: 3.7.1 parameter: 3.7.1 array argument: 3.7.1 function name argument: 3.7.1 pointer to function: 3.7.1 object definitions: 3.7.2 scope of externals: 3.7.2 tentative definition: 3.7.2 preprocessing directives: 3.8 macro preprocessor: 3.8 preprocessing directive lines: 3.8 conditional inclusion: 3.8.1 #if: 3.8.1 #elif 3.8.1 #ifdef: 3.8.1 #ifndef: 3.8.1 #else: 3.8.1 #endif: 3.8.1 #include: 3.8.2 source file inclusion: 3.8.2 macro replacement: 3.8.3 object-like macro: 3.8.3 function-like macro: 3.8.3 macro name: 3.8.3 #define: 3.8.3 macro parameters: 3.8.3 macro invocation: 3.8.3 argument substitution: 3.8.3.1 # operator: 3.8.3.2 ## operator: 3.8.3.3 rescanning and replacement: 3.8.3.4 macro definition scope: 3.8.3.5 #undef: 3.8.3.5 #line: 3.8.4 error directive: 3.8.5 pragma directive: 3.8.6 null directive: 3.8.7 introduction: 4.1 string definition: 4.1.1 letter definition: 4.1.1 decimal-point definition: 4.1.1 reserved identifier: 4.1.2 printing character: 4.3 control character: 4.3 variable arguments: 4.8 unbuffered stream: 4.9.3 fully buffered stream: 4.9.3 line buffered stream: 4.9.3 appendices: A. language syntax summary: A.1 sequence points: A.2 library summary: A.3 implementation limits: A.4 warnings: A.5 portability: A.6 order of evaluation: A.6.1 machine dependency: A.6.3 restrictions on registers: A.6.3.7 function pointer casts: A.6.5.7 bit-field types: A.6.5.8 fortran keyword: A.6.5.9 asm keyword: A.6.5.10 multiple external definitions: A.6.5.11 empty macro arguments: A.6.5.12 predefined macro names: A.6.5.13 signal handler arguments: A.6.5.14 stream types: A.6.5.15 file-opening modes: A.6.5.15 file position indicator: A.6.5.16 foreword: A.7 1. INTRODUCTION 1.1 PURPOSE This Standard specifies the form and establishes the interpretation of programs written in the C programming language./1/ 1.2 SCOPE This Standard specifies: * the representation of C programs; * the syntax and constraints of the C language; * the semantic rules for interpreting C programs; * the representation of input data to be processed by C programs; * the representation of output data produced by C programs; * the restrictions and limits imposed by a conforming implementation of C. This Standard does not specify: * the mechanism by which C programs are transformed for use by a data-processing system; * the mechanism by which C programs are invoked for use by a data-processing system; * the mechanism by which input data are transformed for use by a C program; * the mechanism by which output data are transformed after being produced by a C program; * the size or complexity of a program and its data that will exceed the capacity of any specific data-processing system or the capacity of a particular processor; * all minimal requirements of a data-processing system that is capable of supporting a conforming implementation. 1.3 REFERENCES 1. ``The C Reference Manual'' by Dennis M. Ritchie, a version of which was published in The C Programming Language by Brian W. Kernighan and Dennis M. Ritchie, Prentice-Hall, Inc., (1978). Copyright owned by AT&T. 2. 1984 /usr/group Standard by the /usr/group Standards Committee, Santa Clara, California, USA (November, 1984). 3. American National Dictionary for Information Processing Systems, Information Processing Systems Technical Report ANSI X3/TR-1-82 (1982). 4. ISO 646-1983 Invariant Code Set. 5. IEEE Standard for Binary Floating-Point Arithmetic (ANSI/IEEE Std 754-1985). 6. ISO 4217 Codes for the Representation of Currency and Funds. 1.4 ORGANIZATION OF THE DOCUMENT This document is divided into four major sections: 1. this introduction; 2. the characteristics of environments that translate and execute C programs; 3. the language syntax, constraints, and semantics; 4. the library facilities. Examples are provided to illustrate possible forms of the constructions described. Footnotes are provided to emphasize consequences of the rules described in the section or elsewhere in the Standard. References are used to refer to other related sections. A set of appendices summarizes information contained in the Standard. The abstract, the foreword, the examples, the footnotes, the references, and the appendices are not part of the Standard. 1.5 BASE DOCUMENTS The language section ($3) is derived from ``The C Reference Manual'' by Dennis M. Ritchie, a version of which was published as Appendix A of The C Programming Language by Brian W. Kernighan and Dennis M. Ritchie, Prentice-Hall, Inc., 1978; copyright owned by AT&T. The library section ($4) is based on the 1984 /usr/group Standard by the /usr/group Standards Committee, Santa Clara, California, USA (November 14, 1984). 1.6 DEFINITIONS OF TERMS In this 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. The following terms are used in this document: * Implementation --- a particular set of software, running in a particular translation environment under particular control options, that performs translation of programs for, and supports execution of functions in, a particular execution environment. * Bit --- 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. * Byte --- the unit of data storage in the execution environment 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. * Object --- 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. * Character --- a single byte representing a member of the basic character set of either the source or the execution environment. * Multibyte character --- 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. * Alignment --- a requirement that objects of a particular type be located on storage boundaries with addresses that are particular multiples of a byte address. * Argument --- 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.'' * Parameter --- 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.'' * Unspecified behavior --- behavior, for a correct program construct and correct data, for which the Standard imposes no requirements. * Undefined behavior --- behavior, upon use of a nonportable or erroneous program construct, of erroneous data, or of indeterminately-valued objects, for which the 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). 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 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.'' * Implementation-defined behavior --- behavior, for a correct program construct and correct data, that depends on the characteristics of the implementation and that each implementation shall document. * Locale-specific behavior --- behavior that depends on local conventions of nationality, culture, and language that each implementation shall document. * Diagnostic message --- a message belonging to an implementation-defined subset of the implementation's message output. * Constraints --- syntactic and semantic restrictions by which the exposition of language elements is to be interpreted. * Implementation limits --- restrictions imposed upon programs by the implementation. * Forward references --- references to later sections of the Standard that contain additional information relevant to this section. Other terms are defined at their first appearance, indicated by italic type. Terms explicitly defined in this Standard are not to be presumed to refer implicitly to similar terms defined elsewhere. Terms not defined in this Standard are to be interpreted according to the American National Dictionary for Information Processing Systems, Information Processing Systems Technical Report ANSI X3/TR-1-82 (1982). Forward references: localization ($4.4). "Examples" An example of unspecified behavior is the order in which the arguments to a function are evaluated. An example of undefined behavior is the behavior on integer overflow. An example of implementation-defined behavior is the propagation of the high-order bit when a signed integer is shifted right. An example of locale-specific behavior is whether the islower function returns true for characters other than the 26 lower-case English letters. Forward references: bitwise shift operators ($3.3.7), expressions ($3.3), function calls ($3.3.2.2), the islower function ($4.3.1.6). 1.7 COMPLIANCE A strictly conforming program shall use only those features of the language and library specified in this Standard. It shall not produce output dependent on any unspecified, undefined, or implementation-defined behavior, and shall not exceed any minimum implementation limit. The two forms of conforming implementation are hosted and freestanding. A conforming hosted implementation shall accept any strictly conforming program. A conforming freestanding implementation shall accept any strictly conforming program in which the use of the features specified in the library section ($4) 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. A conforming program is one that is acceptable to a conforming implementation./2/ An implementation shall be accompanied by a document that defines all implementation-defined characteristics and all extensions. Forward references: limits and ($4.1.4), variable arguments ($4.8), common definitions ($4.1.5). 1.8 FUTURE DIRECTIONS With the introduction of new devices and extended character sets, new features may be added to the Standard. Subsections in the language and library sections warn implementors and programmers of usages which, though valid in themselves, may conflict with future additions. Certain features are obsolescent , which means that they may be considered for withdrawal in future revisions of the Standard. They are retained in the Standard because of their widespread use, but their use in new implementations (for implementation features) or new programs (for language or library features) is discouraged. Forward references: future language directions ($3.9.9), future library directions ($4.13). 1.9 ABOUT THIS DRAFT Symbols in the right margin mark substantive differences between this draft and its predecessor (ANSI X3J11/88-001, January 11, 1988). A plus sign indicates an addition, a minus sign a deletion, and a vertical bar a replacement. This section and the difference marks themselves will not appear in the published document. 2. ENVIRONMENT An implementation translates C source files and executes C programs in two data-processing-system environments, which will be called the translation environment and the execution environment in this 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 section ($2), only a few of many possible forward references have been noted. 2.1 CONCEPTUAL MODELS 2.1.1 Translation environment 2.1.1.1 Program structure A C program need not all be translated at the same time. The text of the program is kept in units called source files in this Standard. A source file together with all the headers and source files included via the preprocessing directive #include , less any source lines skipped by any of the conditional inclusion preprocessing directives, 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, by manipulation of objects whose identifiers have external linkage, and by manipulation of data files. Translation units may be separately translated and then later linked to produce an executable program. Forward references: conditional inclusion ($3.8.1), linkages of identifiers ($3.1.2.2), source file inclusion ($3.8.2). 2.1.1.2 Translation phases The precedence among the syntax rules of translation is specified by the following phases./3/ 1. Physical source file characters are mapped to the source character set (introducing new-line characters for end-of-line indicators) if necessary. Trigraph sequences are replaced by corresponding single-character internal representations. 2. Each instance of a new-line character and an immediately preceding backslash character is deleted, splicing physical source lines to form logical source lines. A source file that is not empty shall end in a new-line character, which shall not be immediately preceded by a backslash character. 3. The source file is decomposed into preprocessing tokens/4/ and sequences of white-space characters (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 other white-space characters is retained or replaced by one space character is implementation-defined. 4. Preprocessing directives are executed and macro invocations are expanded. A #include preprocessing directive causes the named header or source file to be processed from phase 1 through phase 4, recursively. 5. Each escape sequence 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. Preprocessing tokens are converted into tokens. The resulting tokens are syntactically and semantically analyzed and translated. 8. All external object and function references are resolved. Library components are linked to satisfy external references to functions and objects not defined in the current translation. All such translator output is collected into a program image which contains information needed for execution in its execution environment. Forward references: lexical elements ($3.1), preprocessing directives ($3.8), trigraph sequences ($2.2.1.1). 2.1.1.3 Diagnostics A conforming implementation shall produce at least one diagnostic message (identified in an implementation-defined manner) for every translation unit that contains a violation of any syntax rule or constraint. Diagnostic messages need not be produced in other circumstances. 2.1.2 Execution environments Two execution environments are defined: freestanding and hosted. In both cases, program startup occurs when a designated C function is called by the execution environment. All objects in static storage shall be initialized (set to their initial values) before program startup. The manner and timing of such initialization are otherwise unspecified. Program termination returns control to the execution environment. Forward references: initialization ($3.5.7). 2.1.2.1 Freestanding environment In a freestanding environment (in which C program execution may take place without any benefit of an operating system), the name and type of the function called at program startup are implementation-defined. There are otherwise no reserved external identifiers. Any library facilities available to a freestanding program are implementation-defined. The effect of program termination in a freestanding environment is implementation-defined. 2.1.2.2 Hosted environment A hosted environment need not be provided, but shall conform to the following specifications if present. "Program startup" The function called at program startup is named main . The implementation declares no prototype for this function. It can 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[]) { /*...*/ } 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. * 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 upper-case and lower-case, the implementation shall ensure that the strings are received in lower-case. * 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. "Program execution" In a hosted environment, a program may use all the functions, macros, type definitions, and objects described in the library section ($4). "Program termination" 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. If the main function executes a return that specifies no value, the termination status returned to the host environment is undefined. Forward references: definition of terms ($4.1.1), the exit function ($4.10.4.3). 2.1.2.3 Program execution The semantic descriptions in this Standard describe the behavior of an abstract machine in which issues of optimization are irrelevant. Accessing a volatile object, modifying an object, modifying a file, or calling a function that does any of those operations are all side effects ,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. In the abstract machine, all expressions are evaluated as specified by the semantics. An actual implementation need not evaluate part of an expression if it can deduce that its value is not used and that no needed side effects are produced (including any caused by calling a function or accessing a volatile object). When the processing of the abstract machine is interrupted by receipt of a signal, only the values of objects as of the previous sequence point may be relied on. Objects that may be modified between the previous sequence point and the next sequence point need not have received their correct values yet. An instance of each object with automatic storage duration is associated with each entry into a block. Such an object exists and retains its last-stored value during the execution of the block and while the block is suspended (by a call of a function or receipt of a signal). The least requirements on a conforming implementation are: * At sequence points, volatile objects are stable in the sense that previous 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 $4.9.3. The intent of these requirements is that unbuffered or line-buffered output appear as soon as possible, to ensure that prompting messages actually appear prior to a program waiting for input. What constitutes an interactive device is implementation-defined. More stringent correspondences between abstract and actual semantics may be defined by each implementation. "Examples" 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 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. In executing the fragment char c1, c2; /*...*/ c1 = c1 + c2; the ``integral promotions'' require that the abstract machine promote the value of each variable to int size and then add the two int s and truncate the sum. Provided the addition of two char s can be done without creating an overflow exception, the actual execution need only produce the same result, possibly omitting the promotions. 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 ). Alternatively, an operation involving only int s or float s may be executed using double-precision operations if neither range nor precision is lost thereby. Forward references: compound statement, or block ($3.6.2), files ($4.9.3), sequence points ($3.3, $3.6), the signal function ($4.7), type qualifiers ($3.5.3). 2.2 ENVIRONMENTAL CONSIDERATIONS 2.2.1 Character sets Two sets of characters and their associated collating sequences shall be defined: the set in which source files are written, and the set interpreted in the execution environment. The values of the members of the execution character set are implementation-defined; any additional members beyond those required by this section are locale-specific. In a character constant or string literal, members of the execution character set shall be represented by corresponding members of the source character set or by escape sequences consisting of the backslash \ followed by one or more characters. A byte with all bits set to 0, called the null character, shall exist in the basic execution character set; it is used to terminate a character string literal. Both the basic source and basic execution character sets shall have at least the following members: the 26 upper-case letters of the English 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 lower-case letters of the English 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 Standard treats such an end-of-line indicator as if it were a single new-line character. In the execution character set, there shall be control characters representing alert, backspace, carriage return, and new line. If any other characters are encountered in a source file (except in a preprocessing token that is never converted to a token, a character constant, a string literal, or a comment), the behavior is undefined. Forward references: character constants ($3.1.3.4), preprocessing directives ($3.8), string literals ($3.1.4), comments ($3.1.9). 2.2.1.1 Trigraph sequences All occurrences in a source file of the following sequences of three characters (called trigraph sequences /5/)are replaced with the corresponding single character. ??= # ??( [ ??/ \ ??) ] ??' ^ ??< { ??! | ??> } ??- ~ No other trigraph sequences exist. Each ? that does not begin one of the trigraphs listed above is not changed. Example The following source line printf("Eh???/n"); becomes (after replacement of the trigraph sequence ??/ ) printf("Eh?\n"); 2.2.1.2 Multibyte characters The source character set may contain multibyte characters, used to represent members of the extended character set. The execution character set may also contain multibyte characters, which need not have the same encoding as for the source character set. For both character sets, the following shall hold: * The single-byte characters defined in $2.2.1 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 implementation-defined shift states when specific multibyte characters are encountered in the sequence. While in the initial shift state, all single-byte characters retain their usual interpretation and do not alter the shift state. The interpretation for subsequent bytes in the sequence is a function of the current shift state. * A byte with all bits zero shall be interpreted as a null character independent of shift state. * A byte with all bits zero shall not occur in the second or subsequent bytes of a multibyte character. For 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. 2.2.2 Character display semantics 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 printing is locale-specific. If the active position is at the final position of a line (if there is one), the behavior is unspecified. Alphabetic escape sequences representing nongraphic characters in the execution character set are intended to produce actions on display devices as follows: ( alert ) Produces an audible or visible alert. The active position shall not be changed. ( 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. ( "form feed" ) Moves the active position to the initial position at the start of the next logical page. ( "new line" ) Moves the active position to the initial position of the next line. ( "carriage return" ) Moves the active position to the initial position of the current line. ( "horizontal tab" ) Moves the active position to the next horizontal tabulation position on the current line. If the active position is at or past the last defined horizontal tabulation position, the behavior is unspecified. ( "vertical tab" ) Moves the active position to the initial position of the next vertical tabulation position. If the active position is at or past the last defined vertical tabulation position, the behavior is unspecified. Each of these escape sequences shall produce a unique implementation-defined value which can be stored in a single char object. The external representations in a text file need not be identical to the internal representations, and are outside the scope of this Standard. Forward references: the fputc function ($4.9.7.3), the isprint function ($4.3.1.7). 2.2.3 Signals and interrupts Functions shall be implemented such that they may be interrupted at any time by a signal, or may be called by a signal handler, or both, with no alteration to earlier, but still active, invocations' control flow (after the interruption), function return values, or objects with automatic storage duration. All such objects shall be maintained outside the function image (the instructions that comprise the executable representation of a function) on a per-invocation basis. The functions in the standard library are not guaranteed to be reentrant and may modify objects with static storage duration. 2.2.4 Environmental limits Both the translation and execution environments constrain the implementation of language translators and libraries. The following summarizes the environmental limits on a conforming implementation. 2.2.4.1 Translation limits The implementation shall be able to translate and execute at least one program that contains at least one instance of every one of the following limits:/6/ * 15 nesting levels of compound statements, iteration control structures, and selection control structures * 8 nesting levels of conditional inclusion * 12 pointer, array, and function declarators (in any combinations) modifying an arithmetic, a structure, a union, or an incomplete type in a declaration * 31 declarators nested by parentheses within a full declarator * 32 expressions nested by parentheses within a full expression * 31 significant initial characters in an internal identifier or a macro name * 6 significant initial characters in an external identifier * 511 external identifiers in one translation unit * 127 identifiers with block scope declared in one block * 1024 macro identifiers simultaneously defined in one translation unit * 31 parameters in one function definition * 31 arguments in one function call * 31 parameters in one macro definition * 31 arguments in one macro invocation * 509 characters in a logical source line * 509 characters in a character string literal or wide string literal (after concatenation) * 32767 bytes in an object (in a hosted environment only) * 8 nesting levels for #include'd files * 257 case labels for a switch statement (excluding those for any nested switch statements) * 127 members in a single structure or union * 127 enumeration constants in a single enumeration * 15 levels of nested structure or union definitions in a single struct-declaration-list 2.2.4.2 Numerical limits A conforming implementation shall document all the limits specified in this section, which shall be specified in the headers and . "Sizes of integral types " The values given below shall be replaced by constant expressions suitable for use in #if preprocessing directives. Their implementation-defined values shall be equal or greater in magnitude (absolute value) to those shown, with the same sign. * maximum 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 * 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 If the value of an object of type char sign-extends 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 . If the value of an object of type char does not sign-extend when used in an expression, the value of CHAR_MIN shall be 0 and the value of CHAR_MAX shall be the same as that of UCHAR_MAX ./7/ "Characteristics of floating types " delim $$ 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. The following parameters are used to define the model for each floating-point type: A normalized floating-point number x ($f sub 1$ > 0 if x is defined by the following model:/8/ $x~=~s~times~b sup e~times~sum from k=1 to p~f sub k~times~b sup -k~,~~~e sub min~<=~e~<=~e sub max$ Of the values in the header, FLT_RADIX shall be a constant expression suitable for use in #if preprocessing directives; all other values need not be constant expressions. All except 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_ROUNDS . The rounding mode for floating-point addition is characterized by the value of FLT_ROUNDS : -1 indeterminable, 0 toward zero, 1 to nearest, 2 toward positive infinity, 3 toward negative infinity. All other values for FLT_ROUNDS characterize implementation-defined rounding behavior. The values 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 * number of base- FLT_RADIX digits in the floating-point mantissa, p FLT_MANT_DIG DBL_MANT_DIG LDBL_MANT_DIG * number of decimal digits of precision, $left floor~(p~-~1)~times~{ log sub 10 } b~right floor ~+~ left { lpile { 1 above 0 } ~~ lpile { roman "if " b roman " is a power of 10" above roman otherwise }$ 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, $e sub min$ 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 numbers, 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, $e sub max$ 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 numbers, FLT_MAX_10_EXP +37 DBL_MAX_10_EXP +37 LDBL_MAX_10_EXP +37 The values given in the following list shall be replaced by implementation-defined expressions with values that shall be equal to or greater than those shown. * maximum representable finite floating-point number, FLT_MAX 1E+37 DBL_MAX 1E+37 LDBL_MAX 1E+37 The values given in the following list shall be replaced by implementation-defined expressions with values that shall be equal to or smaller than those shown. * minimum positive floating-point number x such that 1.0 + x FLT_EPSILON 1E-5 DBL_EPSILON 1E-9 LDBL_EPSILON 1E-9 * minimum normalized positive floating-point number, $b sup { e sub min - 1 }$ FLT_MIN 1E-37 DBL_MIN 1E-37 LDBL_MIN 1E-37 Examples The following describes an artificial floating-point representation that meets the minimum requirements of the Standard, and the appropriate values in a header for type float : $x~=~s~times~16 sup e~times~sum from k=1 to 6~f sub k~times~16 sup -k~,~~~-31~<=~e~<=~+32$ FLT_RADIX 16 FLT_MANT_DIG 6 FLT_EPSILON 9.53674316E-07F FLT_DIG 6 FLT_MIN_EXP -31 FLT_MIN 2.93873588E-39F FLT_MIN_10_EXP -38 FLT_MAX_EXP +32 FLT_MAX 3.40282347E+38F FLT_MAX_10_EXP +38 The following describes floating-point representations that also meet the requirements for single-precision and double-precision normalized numbers in the IEEE Standard for Binary Floating-Point Arithmetic (ANSI/IEEE Std 754-1985),/9/ b and the appropriate values in a header for types float and double : $x sub f~=~s~times~2 sup e~times~{ sum from k=1 to 24~f sub k~times~2 sup -k },~~~-125~<=~e~<=~+128$ $x sub d~=~s~times~2 sup e~times~{ sum from k=1 to 53~f sub k~times~2 sup -k },~~~-1021~<=~e~<=~+1024$ FLT_RADIX 2 FLT_MANT_DIG 24 FLT_EPSILON 1.19209290E-07F FLT_DIG 6 FLT_MIN_EXP -125 FLT_MIN 1.17549435E-38F FLT_MIN_10_EXP -37 FLT_MAX_EXP +128 FLT_MAX 3.40282347E+38F FLT_MAX_10_EXP +38 DBL_MANT_DIG 53 DBL_EPSILON 2.2204460492503131E-16 DBL_DIG 15 DBL_MIN_EXP -1021 DBL_MIN 2.2250738585072016E-308 DBL_MIN_10_EXP -307 DBL_MAX_EXP +1024 DBL_MAX 1.7976931348623157E+308 DBL_MAX_10_EXP +308 The values shown above for FLT_EPSILON and DBL_EPSILON are appropriate for the ANSI/IEEE Std 754-1985 default rounding mode (to nearest). Their values may differ for other rounding modes. Forward references: conditional inclusion ($3.8.1). conditional inclusion ($3.8.1). 3. LANGUAGE In the syntax notation used in the language section ($3), 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 so that { expression } indicates an optional expression enclosed in braces. 3.1 LEXICAL ELEMENTS Syntax 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 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. Semantics A token is the minimal lexical element of the language in translation phases 7 and 8. The categories of tokens are: keywords, identifiers, constants, string literals, 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. Comments (described later) and the characters space, horizontal tab, new-line, vertical tab, and form-feed---collectively called white space ---canseparate preprocessing tokens. As described in $3.8, 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. 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. Examples 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. The program fragment x+++++y is parsed as x ++ ++ + y, which violates a constraint on increment operators, even though the parse x ++ + ++ y might yield a correct expression. Forward references: character constants ($3.1.3.4), comments ($3.1.9), expressions ($3.3), floating constants ($3.1.3.1), header names ($3.1.7), macro replacement ($3.8.3), postfix increment and decrement operators ($3.3.2.4), prefix increment and decrement operators ($3.3.3.1), preprocessing directives ($3.8), preprocessing numbers ($3.1.8), string literals ($3.1.4). 3.1.1 Keywords Syntax keyword: one of auto double int struct break else long switch case enum register typedef char extern return union const float short unsigned continue for signed void default goto sizeof volatile do if static while Semantics The above tokens (entirely in lower-case) are reserved (in translation phases 7 and 8) for use as keywords, and shall not be used otherwise. 3.1.2 Identifiers Syntax identifier: nondigit identifier nondigit identifier digit nondigit: one of _ a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z digit: one of 0 1 2 3 4 5 6 7 8 9 Description An identifier is a sequence of nondigit characters (including the underscore _ and the lower-case and upper-case letters) and digits. The first character shall be a nondigit character. Constraints In translation phases 7 and 8, an identifier shall not consist of the same sequence of characters as a keyword. Semantics An identifier denotes 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. A member of an enumeration is called an enumeration constant. Macro names and macro parameters are not considered further here, because prior to the semantic phase of program translation any occurrences of macro names in the source file are replaced by the preprocessing token sequences that constitute their macro definitions. There is no specific limit on the maximum length of an identifier. "Implementation limits" The implementation shall treat at least the first 31 characters of an internal name (a macro name or an identifier that does not have external linkage) as significant. Corresponding lower-case and upper-case letters are different. The implementation may further restrict the significance of an external name (an identifier that has external linkage) to six characters and may ignore distinctions of alphabetical case for such names./10/ These limitations on identifiers are all implementation-defined. Any identifiers that differ in a significant character are different identifiers. If two identifiers differ in a non-significant character, the behavior is undefined. Forward references: linkages of identifiers ($3.1.2.2), macro replacement ($3.8.3). 3.1.2.1 Scopes of identifiers An identifier is visible (i.e., can be used) only within a region of program text called its scope . There are four kinds of scopes: function, file, block, and function prototype. (A function prototype is a declaration of a function that declares the types of its parameters.) A label name is the only kind of identifier that has function scope. It can be used (in a goto statement) anywhere in the function in which it appears, and is declared implicitly by its syntactic appearance (followed by a : and a statement). Label names shall be unique within a function. 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 outer declaration of a lexically identical identifier exists in the same name space, it is hidden until the current scope terminates, after which it again becomes visible. 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 ($3.6.2), declarations ($3.5), enumeration specifiers ($3.5.2.2), function calls ($3.3.2.2), function declarators (including prototypes) ($3.5.4.3), function definitions ($3.7.1), the goto statement ($3.6.6.1), labeled statements ($3.6.1), name spaces of identifiers ($3.1.2.3), scope of macro definitions ($3.8.3.5), source file inclusion ($3.8.2), tags ($3.5.2.3), type specifiers ($3.5.2). 3.1.2.2 Linkages of identifiers An identifier declared in different scopes or in the same scope more than once can be made to refer to the same object or function by a process called linkage . There are three kinds of linkage: external, internal, and none. In the set of translation units and libraries that constitutes an entire program, each 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. If the declaration of an identifier for an object or a function has file scope and contains the storage-class specifier static, the identifier has internal linkage. If the declaration of an identifier for an object or a function contains the storage-class specifier extern , the identifier has the same linkage as any visible declaration of the identifier with file scope. If there is no visible declaration with file scope, the identifier has external linkage. If the declaration of an identifier for a function has no storage-class specifier, its linkage is determined exactly as if it were declared with the storage-class specifier extern . If the declaration of an identifier for an object has file scope and no storage-class specifier, its linkage is external. The following identifiers have no linkage: an identifier declared to be anything other than an object or a function; an identifier declared to be a function parameter; an identifier declared to be an object inside a block without the storage-class specifier extern. If, within a translation unit, the same identifier appears with both internal and external linkage, the behavior is undefined. Forward references: compound statement, or block ($3.6.2), declarations ($3.5), expressions ($3.3), external definitions ($3.7). 3.1.2.3 Name spaces of identifiers If more than one declaration of a particular identifier is visible at any point in a translation unit, the syntactic context disambiguates uses that refer to different entities. Thus, there are separate name spaces for various categories of identifiers, as follows: * label names (disambiguated by the syntax of the label declaration and use); * the tags of structures, unions, and enumerations (disambiguated by following any/11/ 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: declarators ($3.5.4), enumeration specifiers ($3.5.2.2), labeled statements ($3.6.1), structure and union specifiers ($3.5.2.1), structure and union members ($3.3.2.3), tags ($3.5.2.3). 3.1.2.4 Storage durations of objects An object has a storage duration that determines its lifetime. There are two storage durations: static and automatic. An object 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 and retains its last-stored value throughout the execution of the entire program./12/ An object 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 in which it is declared, or on a jump from outside the block to a label in the block or in an enclosed block. 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 label. 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 that returns 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 is indeterminate. Forward references: compound statement, or block ($3.6.2), function calls ($3.3.2.2), initialization ($3.5.7). 3.1.2.5 Types The meaning of a value stored in an object or returned by a function is determined by the type of the expression used to access it. (An identifier declared to be an object is the simplest such expression; the type is specified in the declaration of the identifier.) Types are partitioned into object types (types that describe objects), function types (types that describe functions), and incomplete types (types that describe objects but lack information needed to determine their sizes). An object declared as type 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 $2.2.1 is stored in a char object, its value is guaranteed to be positive. If other quantities are stored in a char object, the behavior is implementation-defined: the values are treated as either signed or nonnegative integers. There are four signed integer types, designated as signed char, short int, int, and long int. (The signed integer and other types may be designated in several additional ways, as described in $3.5.2.) An object declared as type signed char occupies the same amount of storage as a ``plain'' char object. A ``plain'' int object has the natural size suggested by the architecture of the execution environment (large enough to contain any value in the range INT_MIN to INT_MAX as defined in the header ). In the list of signed integer types above, the range of values of each type is a subrange of the values of the next type in the list. 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 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. 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. There are three 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. 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. There are three character types, designated as char , signed char , and unsigned char. An enumeration comprises a set of named integer constant values. Each distinct enumeration constitutes a different enumerated type. The void type comprises an empty set of values; it is an incomplete type that cannot be completed. Any number of derived types can be constructed from the basic, enumerated, and incomplete types, as follows: * An array type describes a contiguously allocated set of objects with a particular member object type, called the element type .Array types are characterized by their element type and by the number of members of 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 set of member objects, each of which has an optionally specified name and possibly distinct type. * A union type describes an overlapping 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.'' These methods of constructing derived types can be applied recursively. The type char, the signed and unsigned integer types, and the enumerated types are collectively called integral types. The representations of integral types shall define values by use of a pure binary numeration system./13/ American National Dictionary for Information Processing Systems.) The representations of floating types are unspecified. Integral 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. /14/ A pointer to void shall have the same representation and alignment requirements as a pointer to a character type. Other pointer types need not have the same representation or alignment requirements. 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 $3.5.2.3) 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. 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, a function, or a pointer type derivation to T. A type is characterized by its top type, which is either the first type named in describing a derived type (as noted above in the construction of derived types), or the type itself if the type consists of no derived types. A type has qualified type if its top type is specified with a type qualifier; otherwise it has unqualified type. The type qualifiers const and volatile respectively designate const-qualified type and volatile-qualified type. /15/ For each qualified type there is an unqualified type that is specified the same way as the qualified type, but without any type qualifiers in its top type. This type is known as the unqualified version of the qualified type. Similarly, there are appropriately qualified versions of types (such as a const-qualified version of a type), just as there are appropriately non-qualified versions of types (such as a non-const-qualified version of a type). Examples The type designated as ``float *'' is called ``pointer to float'' and its top type is a pointer type, 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 --- it is called ``pointer to const float '' and is a pointer to a qualified type. Finally, the type designated as `` struct tag (*[5])(float) '' is called ``array of pointer to function returning struct tag.'' Its top type is array type. The array has length five and the function has a single parameter of type float. Forward references: character constants ($3.1.3.4), declarations ($3.5), tags ($3.5.2.3), type qualifiers ($3.5.3). 3.1.2.6 Compatible type and composite type Two types have compatible type if their types are the same. Additional rules for determining whether two types are compatible are described in $3.5.2 for type specifiers, in $3.5.3 for type qualifiers, and in $3.5.4 for declarators. /16/ Moreover, two structure, union, or enumeration types declared in separate translation units are compatible if they have the same number of members, the same member names, and compatible member types; for two structures, the members shall be in the same order; for two enumerations, the members shall have the same values. All declarations that refer to the same object or function shall have compatible type; otherwise the behavior is undefined. A composite type can be constructed from two types that are compatible; it is a type that is compatible with both of the two types and has the following additions: * If one type is an array of known size, the composite type is an array of that size. * 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 have parameter type lists, the type of each parameter in the composite parameter type list is the composite type of the corresponding parameters. These rules apply recursively to the types from which the two types are derived. For an identifier with external or internal linkage declared in the same scope as another declaration for that identifier, the type of the identifier becomes the composite type. Example Given the following two file scope declarations: int f(int (*)(), double (*)[3]); int f(int (*)(char *), double (*)[]); The resulting composite type for the function is: int f(int (*)(char *), double (*)[3]); Forward references: declarators ($3.5.4), enumeration specifiers ($3.5.2.2), structure and union specifiers ($3.5.2.1), type definitions ($3.5.6), type qualifiers ($3.5.3), type specifiers ($3.5.2). 3.1.3 Constants Syntax constant: floating-constant integer-constant enumeration-constant character-constant Constraints The value of a constant shall be in the range of representable values for its type. Semantics Each constant has a type, determined by its form and value, as detailed later. 3.1.3.1 Floating constants Syntax floating-constant: fractional-constant exponent-part floating-suffix digit-sequence exponent-part floating-suffix fractional-constant: digit-sequence.digit-sequence digit-sequence. exponent-part: e sign digit-sequence E sign digit-sequence sign: one of + - digit-sequence: digit digit-sequence digit floating-suffix: one of f l F L Description A floating constant has a value part that may be followed by an exponent part and a suffix that specifies its type. The components of the value part may include a digit sequence representing the whole-number part, followed by a period (.), followed by a digit sequence representing the fraction part. The components of the exponent part are an e or E followed by an exponent consisting of an optionally signed digit sequence. Either the whole-number part or the fraction part shall be present; either the period or the exponent part shall be present. Semantics The value part is interpreted as a decimal rational number; the digit sequence in the exponent part is interpreted as a decimal integer. The exponent indicates the power of 10 by which the value part is to be scaled. If the scaled value is in the range of representable values (for its type) but cannot be represented exactly, the result is either the nearest higher or nearest lower value, chosen in an implementation-defined manner. An unsuffixed floating constant has type double. If suffixed by the letter f or F, it has type float. If suffixed by the letter l or L, it has type long double. 3.1.3.2 Integer constants Syntax integer-constant: decimal-constant integer-suffix octal-constant integer-suffix hexadecimal-constant integer-suffix decimal-constant: nonzero-digit decimal-constant digit octal-constant: 0 octal-constant octal-digit hexadecimal-constant: 0x hexadecimal-digit 0X hexadecimal-digit hexadecimal-constant hexadecimal-digit nonzero-digit: one of 1 2 3 4 5 6 7 8 9 octal-digit: one of 0 1 2 3 4 5 6 7 hexadecimal-digit: one of 0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F integer-suffix: unsigned-suffix long-suffix long-suffix unsigned-suffix unsigned-suffix: one of u U long-suffix: one of l L Description An integer constant begins with a digit, but has no period or exponent part. It may have a prefix that specifies its base and a suffix that specifies its type. A decimal constant begins with a nonzero digit and consists of a sequence of decimal digits. An octal constant consists of the prefix 0 optionally followed by a sequence of the digits 0 through 7 only. A hexadecimal constant consists of the prefix 0x or 0X followed by a sequence of the decimal digits and the letters a (or A ) through f (or F) with values 10 through 15 respectively. Semantics The value of a decimal constant is computed base 10; that of an octal constant, base 8; that of a hexadecimal constant, base 16. The lexically first digit is the most significant. The type of an integer constant is the first of the corresponding list in which its value can be represented. Unsuffixed decimal: int, long int, unsigned long int; unsuffixed octal or hexadecimal: int, unsigned int, long int, unsigned long int; suffixed by the letter u or U: unsigned int, unsigned long int; suffixed by the letter l or L: long int, unsigned long int; suffixed by both the letters u or U and l or L: unsigned long int . 3.1.3.3 Enumeration constants Syntax enumeration-constant: identifier Semantics An identifier declared as an enumeration constant has type int. Forward references: enumeration specifiers ($3.5.2.2). 3.1.3.4 Character constants Syntax character-constant: ' c-char-sequence' L' c-char-sequence' c-char-sequence: c-char c-char-sequence c-char c-char: any member of the source character set except the single-quote ', backslash \, or new-line character escape-sequence escape-sequence: simple-escape-sequence octal-escape-sequence hexadecimal-escape-sequence simple-escape-sequence: one of \' \" \? \\ \a \b \f \n \r \t \v octal-escape-sequence: \ octal-digit \ octal-digit octal-digit \ octal-digit octal-digit octal-digit hexadecimal-escape-sequence: \x hexadecimal-digit hexadecimal-escape-sequence hexadecimal-digit Description An integer character constant is a sequence of one or more multibyte characters enclosed in single-quotes, as in 'x' or 'ab'. A wide character constant is the same, except prefixed by the letter L . With a few exceptions detailed later, the elements of the sequence are any members of the source character set; they are mapped in an implementation-defined manner to members of the execution character set. The single-quote ', the double-quote , the question-mark ?, the backslash \ , and arbitrary integral values, are representable according to the following table of escape sequences: single-quote ' \' double-quote " \" question-mark ? \? backslash \ \\ octal integer \ octal digits hexadecimal integer \x hexadecimal digits The double-quote and question-mark ? are representable either by themselves or by the escape sequences \" and \? respectively, but the single-quote ' and the backslash \ shall be represented, respectively, by the escape sequences \' and \\ . The octal digits that follow the backslash in an octal escape sequence are taken to be part of the construction of a single character for an integer character constant or of a single wide character for a wide character constant. The numerical value of the octal integer so formed specifies the value of the desired character. The hexadecimal digits that follow the backslash and the letter x in a hexadecimal escape sequence are taken to be part of the construction of a single character for an integer character constant or of a single wide character for a wide character constant. The numerical value of the hexadecimal integer so formed specifies the value of the desired character. Each octal or hexadecimal escape sequence is the longest sequence of characters that can constitute the escape sequence. In addition, certain nongraphic characters are representable by escape sequences consisting of the backslash \ followed by a lower-case letter: \a , \b , \f , \n , \r , \t , and \v ./17/ If any other escape sequence is encountered, the behavior is undefined./18/ Constraints The value of an octal or hexadecimal escape sequence shall be in the range of representable values for the unsigned type corresponding to its type. Semantics An integer character constant has type int. The value of an integer character constant containing a single character that maps into a member of the basic execution character set is the numerical value of the representation of the mapped character interpreted as an integer. The value of an integer character constant containing more than one character, or containing a character or escape sequence not represented in the basic execution character set, is implementation-defined. In particular, in an implementation in which type char has the same range of values as signed char, the high-order bit position of a single-character integer character constant is treated as a sign bit. A wide character constant has type wchar_t , an integral type defined in the header. The value of a wide character constant containing a single multibyte character that maps into 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. Examples The construction '\0' is commonly used to represent the null character. 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 if type char has the same range of values as unsigned char, the character constant '\xFF' has the value Even if eight bits are used for objects that have type char , the construction '\x123' specifies an integer character constant containing only one character. (The value of this single-character integer character constant is implementation-defined and violates the above constraint.) To specify an integer character constant containing the two characters whose values are 0x12 and '3', the construction '\0223' may be used, since a hexadecimal escape sequence is terminated only by a non-hexadecimal character. (The value of this two-character integer character constant is implementation-defined also.) 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 ($3.2.1.1) common definitions ($4.1.5), the mbtowc function ($4.10.7.2). 3.1.4 String literals Syntax string-literal: " s-char-sequence" L" s-char-sequence" s-char-sequence: s-char s-char-sequence s-char s-char: any member of the source character set except the double-quote ", backslash \, or new-line character escape-sequence Description A character string literal is a sequence of zero or more multibyte characters enclosed in double-quotes, as in xyz. A wide string literal is the same, except prefixed by the letter L. The same considerations apply to each element of the sequence in a character string literal or a wide string literal as if it were in an integer character constant or a wide character constant, except that the single-quote ' is representable either by itself or by the escape sequence \', but the double-quote shall be represented by the escape sequence \. Semantics A character string literal has static storage duration and type ``array of char ,'' and is initialized with the given characters. A wide string literal has static storage duration and type ``array of wchar_t,'' and is initialized with the wide characters corresponding to the given multibyte characters. Character string literals that are adjacent tokens are concatenated into a single character string literal. A null character is then appended. /19/ Likewise, adjacent wide string literal tokens are concatenated into a single wide string literal to which a code with value zero is then appended. If a character string literal token is adjacent to a wide string literal token, the behavior is undefined. Identical string literals of either form need not be distinct. If the program attempts to modify a string literal of either form, the behavior is undefined. Example This pair of adjacent character string literals "\x12" "3" produces a single character string literal containing the two characters whose values are \x12 and '3', because escape sequences are converted into single members of the execution character set just prior to adjacent string literal concatenation. Forward references: common definitions ($4.1.5). 3.1.5 Operators Syntax operator: one of [ ] ( ) . -> ++ -- & * + - ~ ! sizeof / % << >> < > <= >= == != ^ | && || ? : = *= /= %= += -= <<= >>= &= ^= |= , # ## Constraints The operators [ ] , ( ) , and ? : shall occur in pairs, possibly separated by expressions. The operators # and ## shall occur in macro-defining preprocessing directives only. Semantics 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. Forward references: expressions ($3.3), macro replacement ($3.8.3). 3.1.6 Punctuators Syntax punctuator: one of [ ] ( ) { } * , : = ; ... # Constraints The punctuators [ ] , ( ) , and { } shall occur in pairs, possibly separated by expressions, declarations, or statements. The punctuator # shall occur in preprocessing directives only. Semantics 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 ($3.3), declarations ($3.5), preprocessing directives ($3.8), statements ($3.6). 3.1.7 Header names Syntax header-name: < h-char-sequence> " q-char-sequence" h-char-sequence: h-char h-char-sequence h-char h-char: any member of the source character set except the new-line character and > q-char-sequence: q-char q-char-sequence q-char q-char: any member of the source character set except the new-line character and " Constraints Header name preprocessing tokens shall only appear within a #include preprocessing directive. Semantics The sequences in both forms of header names are mapped in an implementation-defined manner to headers or external source file names as specified in $3.8.2. If the characters ', \ , , or /* occur in the sequence between the < and > delimiters, the behavior is undefined. Similarly, if the characters ', \ , or /* occur in the sequence between the " delimiters, the behavior is undefined. /20/ Example The following sequence of characters: 0x3<1/a.h>1e2 #include <1/a.h> #define const.member@$ forms the following sequence of preprocessing tokens (with each individual preprocessing token delimited by a { on the left and a } on the right). {0x3}{<}{1}{/}{a}{.}{h}{>}{1e2} {#}{include} {<1/a.h>} {#}{define} {const}{.}{member}{@}{$} Forward references: source file inclusion ($3.8.2). 3.1.8 Preprocessing numbers Syntax pp-number: digit . digit pp-number digit pp-number nondigit pp-number e sign pp-number E sign pp-number . Description A preprocessing number begins with a digit optionally preceded by a period (.) and may be followed by letters, underscores, digits, periods, and e+, e-, E+, or E- character sequences. Preprocessing number tokens lexically include all floating and integer constant tokens. Semantics A preprocessing number does not have type or a value; it acquires both after a successful conversion (as part of translation phase 7) to a floating constant token or an integer constant token. 3.1.9 Comments Except within a character constant, a string literal, or a comment, the characters /* introduce a comment. The contents of a comment are examined only to identify multibyte characters and to find the characters */ that terminate it. /21/ 3.2 CONVERSIONS Several operators convert operand values from one type to another automatically. This section 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 $3.2.1.5 summarizes the conversions performed by most ordinary operators; it is supplemented as required by the discussion of each operator in $3.3. Conversion of an operand value to a compatible type causes no change. Forward references: cast operators ($3.3.4). 3.2.1 Arithmetic operands 3.2.1.1 Characters and integers A char, a short int, or an int bit-field, or their signed or unsigned varieties, or an object that has enumeration type, may be used in an expression wherever an int or unsigned int may be used. 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 integral promotions. The integral promotions preserve value including sign. As discussed earlier, whether a ``plain'' char is treated as signed is implementation-defined. Forward references: enumeration specifiers ($3.5.2.2), structure and union specifiers ($3.5.2.1). 3.2.1.2 Signed and unsigned integers When an unsigned integer is converted to another integral type, if the value can be represented by the new type, its value is unchanged. When a signed integer is converted to an unsigned integer with equal or greater size, if the value of the signed integer is nonnegative, its value is unchanged. Otherwise: if the unsigned integer has greater size, the signed integer is first promoted to the signed integer corresponding to the unsigned integer; the value is converted to unsigned by adding to it one greater than the largest number that can be represented in the unsigned integer type. /22/ When an integer is demoted to an unsigned integer with smaller size, the result is the nonnegative remainder on division by the number one greater than the largest unsigned number that can be represented in the type with smaller size. When an integer is demoted to a signed integer with smaller size, or an unsigned integer is converted to its corresponding signed integer, if the value cannot be represented the result is implementation-defined. 3.2.1.3 Floating and integral When a value of floating type is converted to integral type, the fractional part is discarded. If the value of the integral part cannot be represented by the integral type, the behavior is undefined. /23/ When a value of integral type is converted to 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. 3.2.1.4 Floating types When a float is promoted to double or long double , or a double is promoted to long double , its value is unchanged. When a double is demoted to float 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. 3.2.1.5 Usual arithmetic conversions Many binary operators that expect operands of arithmetic type cause conversions and yield result types in a similar way. The purpose is to yield a common type, which is also the type of the result. This pattern is called the usual arithmetic conversions: First, if either operand has type long double, the other operand is converted to long double . Otherwise, if either operand has type double, the other operand is converted to double. Otherwise, if either operand has type float, the other operand is converted to float. Otherwise, the integral promotions are performed on both operands. Then the following rules are applied: If either operand has type unsigned long int, the other operand is converted to unsigned long int. Otherwise, if one operand has type long int and the other has type unsigned int, if a long int can represent all values of an unsigned int, the operand of type unsigned int is converted to long int ; if a long int cannot represent all the values of an unsigned int, both operands are converted to unsigned long int. Otherwise, if either operand has type long int, the other operand is converted to long int. Otherwise, if either operand has type unsigned int, the other operand is converted to unsigned int. Otherwise, both operands have type int. The values of operands and of the results of expressions may be represented in greater precision and range than that required by the type; the types are not changed thereby. 3.2.2 Other operands 3.2.2.1 Lvalues and function designators An lvalue is an expression (with an object type or an incomplete type other than void) that designates an object. /24/ 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 of all contained structures or unions) with a const-qualified type. Except when it is the operand of the sizeof operator, the unary & operator, the ++ operator, the -- operator, or the left operand of the . operator or an assignment operator, an lvalue that does not have array type is converted to the value stored in the designated object (and is no longer an lvalue). If the lvalue has qualified type, the value has the unqualified version of the type of the lvalue; otherwise the value has the type of the lvalue. If the lvalue has an incomplete type and does not have array type, the behavior is undefined. Except when it is the operand of the sizeof operator or the unary & operator, or is a 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 member of the array object and is not an lvalue. A function designator is an expression that has function type. Except when it is the operand of the sizeof operator /25/ 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 ($3.3.3.2), assignment operators ($3.3.16), common definitions ($4.1.5), initialization ($3.5.7), postfix increment and decrement operators ($3.3.2.4), prefix increment and decrement operators ($3.3.3.1), the sizeof operator ($3.3.3.4), structure and union members ($3.3.2.3). 3.2.2.2 void The (nonexistent) value of a void expression (an expression that has type void) shall not be used in any way, and implicit or explicit conversions (except to void ) shall not be applied to such an expression. If an expression of any other type 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.) 3.2.2.3 Pointers A pointer to void may be converted to or from a pointer to any incomplete or object type. A pointer to any incomplete or object type may be converted to a pointer to void and back again; the result shall compare equal to the original pointer. A pointer to a non-q-qualified type may be converted to a pointer to the q-qualified version of the type; the values stored in the original and converted pointers shall compare equal. An integral constant expression with the value 0, or such an expression cast to type void * , is called a null pointer constant. If a null pointer constant is assigned to or compared for equality to a pointer, the constant is converted to a pointer of that type. Such a pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function. Two null pointers, converted through possibly different sequences of casts to pointer types, shall compare equal. Forward references: cast operators ($3.3.4), equality operators ($3.3.9), simple assignment ($3.3.16.1). 3.3 EXPRESSIONS An expression is a sequence of operators and operands that specifies computation of a value, or that designates an object or a function, or that generates side effects, or that performs a combination thereof. Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be accessed only to determine the value to be stored. /26/ Except as indicated by the syntax /27/ 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. Some operators (the unary operator ~ , and the binary operators << , >> , & , ^ , and | , collectively described as bitwise operators )shall have operands that have integral type. These operators return values that depend on the internal representations of integers, and thus have implementation-defined aspects for signed types. If an exception occurs during the evaluation of an expression (that is, if the result is not mathematically defined or not representable), the behavior is undefined. An object shall have its stored value accessed only by an lvalue that has one of the following types: /28/ * the declared type of the object, * a qualified version of the declared type of the object, * a type that is the signed or unsigned type corresponding to the declared type of the object, * a type that is the signed or unsigned type corresponding to a qualified version of the declared 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. 3.3.1 Primary expressions Syntax primary-expression: identifier constant string-literal ( expression ) Semantics An identifier is a primary expression, provided it has been declared as designating an object (in which case it is an lvalue) or a function (in which case it is a function designator). A constant is a primary expression. Its type depends on its form, as detailed in $3.1.3. A string literal is a primary expression. It is an lvalue with type as detailed in $3.1.4. 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 ($3.5). 3.3.2 Postfix operators Syntax postfix-expression: primary-expression postfix-expression [ expression ] postfix-expression ( argument-expression-list ) postfix-expression . identifier postfix-expression -> identifier postfix-expression ++ postfix-expression -- argument-expression-list: assignment-expression argument-expression-list , assignment-expression 3.3.2.1 Array subscripting Constraints One of the expressions shall have type ``pointer to object type ,'' the other expression shall have integral type, and the result has type `` type .'' Semantics A postfix expression followed by an expression in square brackets [] is a subscripted designation of a member 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 member of an array object) and E2 is an integer, E1[E2] designates the E2 -th member of E1 (counting from zero). Successive subscript operators designate a member of a multi-dimensional array object. If E is an n -dimensional array ( n >=2) with dimensions i x j "x ... x" k , then E (used as other than an lvalue) is converted to a pointer to an ( n -1)-dimensional array with dimensions j "x ... 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). Example Consider the array object defined by the declaration int x[3][5]; Here x is a 3x5 array of int s; more precisely, x is an array of three member objects, each of which is an array of five int s. In the expression x[i] , which is equivalent to (*(x+(i))) , x is first converted to a pointer to the initial array of five int s. 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 int s. When used in the expression x[i][j] , that in turn is converted to a pointer to the first of the int s, so x[i][j] yields an int. Forward references: additive operators ($3.3.6), address and indirection operators ($3.3.3.2), array declarators ($3.5.4.2). 3.3.2.2 Function calls Constraints The expression that denotes the called function/29/ shall have type pointer to function returning void or returning an object type other than array. If the expression that denotes the called function has a type that includes a prototype, the number of arguments shall agree with the number of parameters. Each argument shall have a type such that its value may be assigned to an object with the unqualified version of the type of its corresponding parameter. Semantics A postfix expression followed by parentheses () containing a possibly empty, comma-separated list of expressions is a function call. The postfix expression denotes the called function. The list of expressions specifies the arguments to the function. If the expression that precedes the parenthesized argument list in a function call consists solely of an identifier, and if no declaration is visible for this identifier, the identifier is implicitly declared exactly as if, in the innermost block containing the function call, the declaration extern int identifier(); appeared. /30/ 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./31/ The value of the function call expression is specified in $3.6.6.4. If the expression that denotes the called function has a type that does not include a prototype, the integral 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. 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. 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. 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. 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. 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. 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. Recursive function calls shall be permitted, both directly and indirectly through any chain of other functions. Example In the function call (*pf[f1()]) (f2(), f3() + f4()) the functions f1 , f2 , f3 , and f4 may be called in any order. All side effects shall be completed before the function pointed to by pf[f1()] is entered. Forward references: function declarators (including prototypes) ($3.5.4.3), function definitions ($3.7.1), the return statement ($3.6.6.4), simple assignment ($3.3.16.1). 3.3.2.3 Structure and union members Constraints The first operand of the . operator shall have a qualified or unqualified structure or union type, and the second operand shall name a member of that type. The first operand of the -> operator shall have type ``pointer to qualified or unqualified structure'' or ``pointer to qualified or unqualified union,'' and the second operand shall name a member of the type pointed to. Semantics A postfix expression followed by a dot . and an identifier designates a member of a structure or union object. The value is that of the named member, and is an lvalue if the first expression is an lvalue. If the first expression has qualified type, the result has the so-qualified version of the type of the designated member. A postfix expression followed by an arrow -> 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./32/ If the first expression is a pointer to a qualified type, the result has the so-qualified version of the type of the designated member. With one exception, if a member of a union object is accessed after a value has been stored in a different member of the object, the behavior is implementation-defined./33/ One special guarantee is made in order to simplify the use of unions: If a union contains several structures that share a common initial sequence, and if the union object currently contains one of these structures, it is permitted to inspect the common initial part of any of them. Two structures share a common initial sequence if corresponding members have compatible types for a sequence of one or more initial members. Example 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. 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) /*...*/ sin(u.nf.doublenode) /*...*/ Forward references: address and indirection operators ($3.3.3.2), structure and union specifiers ($3.5.2.1). 3.3.2.4 Postfix increment and decrement operators Constraints The operand of the postfix increment or decrement operator shall have qualified or unqualified scalar type and shall be a modifiable lvalue. Semantics The result of the postfix ++ operator is the value of the operand. After the result is obtained, the value of the operand is incremented. (That is, the value 1 of the appropriate type is added to it.) See the discussions of additive operators and compound assignment for information on constraints, types and conversions and the effects of operations on pointers. The side effect of updating the stored value of the operand shall occur between the previous and the next sequence point. The postfix -- operator is analogous to the postfix ++ operator, except that the value of the operand is decremented (that is, the value 1 of the appropriate type is subtracted from it). Forward references: additive operators ($3.3.6), compound assignment ($3.3.16.2). 3.3.3 Unary operators Syntax unary-expression: postfix-expression ++ unary-expression -- unary-expression unary-operator cast-expression sizeof unary-expression sizeof ( type-name ) unary-operator: one of & * + - ~ ! 3.3.3.1 Prefix increment and decrement operators Constraints The operand of the prefix increment or decrement operator shall have qualified or unqualified scalar type and shall be a modifiable lvalue. Semantics The value of the operand of the prefix ++ operator is incremented. The result is the new value of the operand after incrementation. The expression ++E is equivalent to (E+=1) . See the discussions of additive operators and compound assignment for information on constraints, types, side effects, and conversions and the effects of operations on pointers. The prefix -- operator is analogous to the prefix ++ operator, except that the value of the operand is decremented. Forward references: additive operators ($3.3.6), compound assignment ($3.3.16.2). 3.3.3.2 Address and indirection operators Constraints The operand of the unary & operator shall be either a function designator or an lvalue that designates an object that is not a bit-field and is not declared with the register storage-class specifier. The operand of the unary * operator shall have pointer type. Semantics The 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 .'' 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./34/ Forward references: storage-class specifiers ($3.5.1), structure and union specifiers ($3.5.2.1). 3.3.3.3 Unary arithmetic operators Constraints The operand of the unary + or - operator shall have arithmetic type; of the ~ operator, integral type; of the ! operator, scalar type. Semantics The result of the unary + operator is the value of its operand. The integral promotion is performed on the operand, and the result has the promoted type. The result of the unary - operator is the negative of its operand. The integral promotion is performed on the operand, and the result has the promoted type. 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 integral promotion is performed on the operand, and the result has the promoted type. The expression ~E is equivalent 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 ULONG_MAX and UINT_MAX are defined in the header .) 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 ($4.1.4). 3.3.3.4 The sizeof operator Constraints The sizeof operator shall not be applied to an expression that has function type or an incomplete type, to the parenthesized name of such a type, or to an lvalue that designates a bit-field object. Semantics The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand, which is not itself evaluated. The result is an integer constant. When applied to an operand that has type char , unsigned char , or signed char , (or a qualified version thereof) the result is 1. When applied to an operand that has array type, the result is the total number of bytes in the array./35/ When applied to an operand that has structure or union type, the result is the total number of bytes in such an object, including internal and trailing padding. The value of the result is implementation-defined, and its type (an unsigned integral type) is size_t defined in the header. Examples 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(); 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. Another use of the sizeof operator is to compute the number of members in an array: sizeof array / sizeof array[0] Forward references: common definitions ($4.1.5), declarations ($3.5), structure and union specifiers ($3.5.2.1), type names ($3.5.5). 3.3.4 Cast operators Syntax cast-expression: unary-expression ( type-name ) cast-expression Constraints Unless the type name specifies void type, the type name shall specify qualified or unqualified scalar type and the operand shall have scalar type. Semantics Preceding an expression by a parenthesized type name converts the value of the expression to the named type. This construction is called a cast. /36/ A cast that specifies an implicit conversion or no conversion has no effect on the type or value of an expression. Conversions that involve pointers (other than as permitted by the constraints of $3.3.16.1) shall be specified by means of an explicit cast; they have implementation-defined aspects: A pointer may be converted to an integral type. The size of integer required and the result are implementation-defined. If the space provided is not long enough, the behavior is undefined. An arbitrary integer may be converted to a pointer. The result is implementation-defined./37/ A pointer to an object or incomplete type may be converted to a pointer to a different object type or a different incomplete type. The resulting pointer might not be valid if it is improperly aligned for the type pointed to. It is guaranteed, however, that a pointer to an object of a given alignment may be converted to a pointer to an object of the same alignment or a less strict alignment and back again; the result shall compare equal to the original pointer. (An object that has character type has the least strict alignment.) 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. Forward references: equality operators ($3.3.9), function declarators (including prototypes) ($3.5.4.3), simple assignment ($3.3.16.1), type names ($3.5.5). 3.3.5 Multiplicative operators Syntax multiplicative-expression: cast-expression multiplicative-expression * cast-expression multiplicative-expression / cast-expression multiplicative-expression % cast-expression Constraints Each of the operands shall have arithmetic type. The operands of the % operator shall have integral type. Semantics The usual arithmetic conversions are performed on the operands. The result of the binary * operator is the product of the operands. The result of the / operator is the quotient from the division of the first operand by the second; the result of the % operator is the remainder. In both operations, if the value of the second operand is zero, the behavior is undefined. When integers are divided and the division is inexact, if both operands are positive the result of the / operator is the largest integer less than the algebraic quotient and the result of the % operator is positive. If either operand is negative, whether the result of the / operator is the largest integer less than the algebraic quotient or the smallest integer greater than the algebraic quotient is implementation-defined, as is the sign of the result of the % operator. If the quotient a/b is representable, the expression (a/b)*b + a%b shall equal a . 3.3.6 Additive operators Syntax additive-expression: multiplicative-expression additive-expression + multiplicative-expression additive-expression - multiplicative-expression Constraints For addition, either both operands shall have arithmetic type, or one operand shall be a pointer to an object type and the other shall have integral type. (Incrementing is equivalent to adding 1.) For subtraction, one of the following shall hold: * both operands have arithmetic type; * both operands are pointers to qualified or unqualified versions of compatible object types; or * the left operand is a pointer to an object type and the right operand has integral type. (Decrementing is equivalent to subtracting 1.) Semantics If both operands have arithmetic type, the usual arithmetic conversions are performed on them. The result of the binary + operator is the sum of the operands. The result of the binary - operator is the difference resulting from the subtraction of the second operand from the first. When an expression that has integral type is added to or subtracted from a pointer, the integral value is first multiplied by the size of the object pointed to. The result has the type of the pointer operand. If the pointer operand points to a member of an array object, and the array object is large enough, the result points to a member of the same array object, appropriately offset from the original member. Thus if P points to a member of an array object, the expression P+1 points to the next member of the array object. Unless both the pointer operand and the result point to a member of the same array object, or one past the last member of the array object, the behavior is undefined. Unless both the pointer operand and the result point to a member of the same array object, or the pointer operand points one past the last member of an array object and the result points to a member of the same array object, the behavior is undefined if the result is used as the operand of a unary * operator. When two pointers to members of the same array object are subtracted, the difference is divided by the size of a member. The result represents the difference of the subscripts of the two array members. The size of the result is implementation-defined, and its type (a signed integral type) is ptrdiff_t defined in the header. As with any other arithmetic overflow, if the result does not fit in the space provided, the behavior is undefined. If two pointers that do not point to members of the same array object are subtracted, the behavior is undefined. However, if P points either to a member of an array object or one past the last member of an array object, and Q points to the last member of the same array object, the expression (Q+1) - P has the same value as (Q-P) + 1 , even though Q+1 does not point to a member of the array object. Forward references: common definitions ($4.1.5). 3.3.7 Bitwise shift operators Syntax shift-expression: additive-expression shift-expression << additive-expression shift-expression >> additive-expression Constraints Each of the operands shall have integral type. Semantics The integral promotions are performed on each of the operands. The type of the result is that of the promoted left operand. If the value of the right operand is negative or is greater than or equal to the width in bits of the promoted left operand, the behavior is undefined. The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with zeros. If E1 has an unsigned type, the value of the result is E1 multiplied by the quantity, 2 raised to the power E2, reduced modulo ULONG_MAX+1 if E1 has type unsigned long, UINT_MAX+1 otherwise. (The constants ULONG_MAX and UINT_MAX are defined in the header .) 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. 3.3.8 Relational operators Syntax relational-expression: shift-expression relational-expression < shift-expression relational-expression > shift-expression relational-expression <= shift-expression relational-expression >= shift-expression Constraints 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 * both operands are pointers to qualified or unqualified versions of compatible incomplete types. Semantics If both of the operands have arithmetic type, the usual arithmetic conversions are performed. When two pointers are compared, the result depends on the relative locations in the address space of the objects pointed to. If the objects pointed to are members of the same aggregate object, pointers to structure members declared later compare higher than pointers to members declared earlier in the structure, and pointers to array elements with larger subscript values compare higher 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 objects pointed to are not members of the same aggregate or union object, the result is undefined, with the following exception. If P points to the last member of an array object and Q points to a member of the same array object, the pointer expression P+1 compares higher than Q , even though P+1 does not point to a member of the array object. 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./38/ The result has type int. 3.3.9 Equality operators Syntax equality-expression: relational-expression equality-expression == relational-expression equality-expression != relational-expression Constraints One of the following shall hold: * both operands have arithmetic type; * both operands are pointers to qualified or unqualified versions of compatible types; * one operand is a pointer to an object or incomplete type and the other is a qualified or unqualified version of void ; or * one operand is a pointer and the other is a null pointer constant. Semantics The == (equal to) and the != (not equal to) operators are analogous to the relational operators except for their lower precedence./39/ If two pointers to object or incomplete types compare equal, they point to the same object. If two pointers to functions compare equal, they point to the same function. If two pointers point to the same object or function, they compare equal./40/ If one of the operands is a pointer to an object or incomplete type and the other has type pointer to a qualified or unqualified version of void , the pointer to an object or incomplete type is converted to the type of the other operand. 3.3.10 Bitwise AND operator Syntax AND-expression: equality-expression AND-expression & equality-expression Constraints Each of the operands shall have integral type. Semantics The usual arithmetic conversions are performed on the operands. The result of the binary & operator is the bitwise AND of the operands (that is, each bit in the result is set if and only if each of the corresponding bits in the converted operands is set). 3.3.11 Bitwise exclusive OR operator Syntax exclusive-OR-expression: AND-expression exclusive-OR-expression ^ AND-expression Constraints Each of the operands shall have integral type. Semantics The usual arithmetic conversions are performed on the operands. The result of the ^ operator is the bitwise exclusive OR of the operands (that is, each bit in the result is set if and only if exactly one of the corresponding bits in the converted operands is set). 3.3.12 Bitwise inclusive OR operator Syntax inclusive-OR-expression: exclusive-OR-expression inclusive-OR-expression | exclusive-OR-expression Constraints Each of the operands shall have integral type. Semantics The usual arithmetic conversions are performed on the operands. The result of the | operator is the bitwise inclusive OR of the operands (that is, each bit in the result is set if and only if at least one of the corresponding bits in the converted operands is set). 3.3.13 Logical AND operator Syntax logical-AND-expression: inclusive-OR-expression logical-AND-expression && inclusive-OR-expression Constraints Each of the operands shall have scalar type. Semantics The && operator shall yield 1 if both of its operands compare unequal to 0, otherwise it yields 0. The result has type int. Unlike the bitwise binary & operator, the && operator guarantees left-to-right evaluation; there is a sequence point after the evaluation of the first operand. If the first operand compares equal to 0, the second operand is not evaluated. 3.3.14 Logical OR operator Syntax logical-OR-expression: logical-AND-expression logical-OR-expression || logical-AND-expression Constraints Each of the operands shall have scalar type. Semantics The || operator shall yield 1 if either of its operands compare unequal to 0, otherwise it yields 0. The result has type int. Unlike the bitwise | operator, the || operator guarantees left-to-right evaluation; there is a sequence point after the evaluation of the first operand. If the first operand compares unequal to 0, the second operand is not evaluated. 3.3.15 Conditional operator Syntax conditional-expression: logical-OR-expression logical-OR-expression ? expression : conditional-expression Constraints The first operand shall have scalar type. One of the following shall hold for the second and third operands: * both operands have arithmetic type; * both operands have compatible structure or union types; * both operands have void type; * both operands are pointers to qualified or unqualified versions of compatible types; * one operand is a pointer and the other is a null pointer constant; or * one operand is a pointer to an object or incomplete type and the other is a pointer to a qualified or unqualified version of void . Semantics The first operand is evaluated; there is a sequence point after its evaluation. The second operand is evaluated only if the first compares unequal to 0; the third operand is evaluated only if the first compares equal to 0; the value of the second or third operand (whichever is evaluated) is the result./41/ If both the second and third operands have arithmetic type, the usual arithmetic conversions are performed to bring them to a common type and the result has that type. If both the operands have structure or union type, the result has that type. If both operands have void type, the result has void type. If both the second and third operands are pointers or one is a null pointer constant and the other is a pointer, the result type is a pointer to a type qualified with all the type qualifiers of the types pointed-to by both operands. Furthermore, if both operands are pointers to compatible types or differently qualified versions of a compatible type, the result has the composite type; if one operand is a null pointer constant, the result has the type of the other operand; otherwise, one operand is a pointer to void or a qualified version of void, in which case the other operand is converted to type pointer to void, and the result has that type. 3.3.16 Assignment operators Syntax assignment-expression: conditional-expression unary-expression assignment-operator assignment-expression assignment-operator: one of = *= /= %= += -= <<= >>= &= ^= |= Constraints An assignment operator shall have a modifiable lvalue as its left operand. Semantics An assignment operator stores a value in the object designated by the left operand. An assignment expression has the value of the left operand after the assignment, but is not an lvalue. The type of an assignment expression is the type of the left operand unless the left operand has qualified type, in which case it is the unqualified version of the type of the left operand. The side effect of updating the stored value of the left operand shall occur between the previous and the next sequence point. The order of evaluation of the operands is unspecified. 3.3.16.1 Simple assignment Constraints One of the following shall hold:/42/ * the left operand has qualified or unqualified arithmetic type and the right has arithmetic type; * the left operand has a qualified or unqualified version of a structure or union type compatible with the type of the right; * both operands are pointers to qualified or unqualified versions of compatible types, and the type pointed to by the left has all the qualifiers of the type pointed to by the right; * one operand is a pointer to an object or incomplete type and the other is a pointer to a qualified or unqualified version of void, and the type pointed to by the left has all the qualifiers of the type pointed to by the right; or * the left operand is a pointer and the right is a null pointer constant. Semantics In simple assignment ( = ), the value of the right operand is converted to the type of the assignment expression and replaces the value stored in the object designated by the left operand. If the value being stored in an object is accessed from another object that overlaps in any way the storage of the first object, then the overlap shall be exact and the two objects shall have qualified or unqualified versions of a compatible type; otherwise the behavior is undefined. Example In the program fragment int f(void); char c; /*...*/ /*...*/ ((c = f()) == -1) /*...*/ the int value returned by the function may be truncated when stored in the char, and then converted back to int width prior to the comparison. In an implementation in which ``plain'' char has the same range of values as unsigned char (and char is narrower than int ), the result of the conversion cannot be negative, so the operands of the comparison can never compare equal. Therefore, for full portability the variable c should be declared as int. 3.3.16.2 Compound assignment Constraints For the operators += and -= only, either the left operand shall be a pointer to an object type and the right shall have integral type, or the left operand shall have qualified or unqualified arithmetic type and the right shall have arithmetic type. For the other operators, each operand shall have arithmetic type consistent with those allowed by the corresponding binary operator. Semantics A compound assignment of the form E1 op = E2 differs from the simple assignment expression E1 = E1 op (E2) only in that the lvalue E1 is evaluated only once. 3.3.17 Comma operator Syntax expression: assignment-expression expression , assignment-expression Semantics The left operand of a comma operator is evaluated as a void expression; there is a sequence point after its evaluation. Then the right operand is evaluated; the result has its type and value./43/ Example 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 section 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 ($3.5.7). 3.4 CONSTANT EXPRESSIONS Syntax constant-expression: conditional-expression Description A constant expression can be evaluated during translation rather than runtime, and accordingly may be used in any place that a constant may be. Constraints Constant expressions shall not contain assignment, increment, decrement, function-call, or comma operators, except when they are contained within the operand of a sizeof operator./44/ Each constant expression shall evaluate to a constant that is in the range of representable values for its type. Semantics An expression that evaluates to a constant is required in several contexts./45/ If the expression is evaluated in the translation environment, the arithmetic precision and range shall be at least as great as if the expression were being evaluated in the execution environment. An integral constant expression shall have integral type and shall only have operands that are integer constants, enumeration constants, character constants, sizeof expressions, and floating constants that are the immediate operands of casts. Cast operators in an integral constant expression shall only convert arithmetic types to integral types, except as part of an operand to the sizeof operator. More latitude is permitted for constant expressions in initializers. Such a constant expression shall evaluate to one of the following: * an arithmetic constant expression, * an address constant, or * an address constant for an object type plus or minus an integral constant expression. An arithmetic constant expression shall have arithmetic type and shall only have operands that are integer constants, floating constants, enumeration constants, character constants, and sizeof expressions. Cast operators in an arithmetic constant expression shall only convert arithmetic types to arithmetic types, except as part of an operand to the sizeof operator. An address constant is a 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 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 an address constant, but the value of an object shall not be accessed by use of these operators. The semantic rules for the evaluation of a constant expression are the same as for non-constant expressions./46/ Forward references: initialization ($3.5.7). 3.5 DECLARATIONS Syntax declaration: declaration-specifiers init-declarator-list ; declaration-specifiers: storage-class-specifier declaration-specifiers type-specifier declaration-specifiers type-qualifier declaration-specifiers init-declarator-list: init-declarator init-declarator-list , init-declarator init-declarator: declarator declarator = initializer Constraints A declaration shall declare at least a declarator, a tag, or the members of an enumeration. If an identifier has no linkage, there shall be no more than one declaration of the identifier (in a declarator or type specifier) with the same scope and in the same name space, except for tags as specified in $3.5.2.3. All declarations in the same scope that refer to the same object or function shall specify compatible types. Semantics A declaration specifies the interpretation and attributes of a set of identifiers. A declaration that also causes storage to be reserved for an object or function named by an identifier is a definition ./47/ The declaration specifiers consist of a sequence of specifiers that indicate the linkage, storage duration, and part of the type of the entities that the declarators denote. The init-declarator-list is a comma-separated sequence of declarators, each of which may have additional type information, or an initializer, or both. The declarators contain the identifiers (if any) being declared. If an identifier for an object is declared with no linkage, the type for the object shall be complete by the end of its declarator, or by the end of its init-declarator if it has an initializer. Forward references: declarators ($3.5.4), enumeration specifiers ($3.5.2.2), initialization ($3.5.7), tags ($3.5.2.3). 3.5.1 Storage-class specifiers Syntax storage-class-specifier: typedef extern static auto register Constraints At most one storage-class specifier may be given in the declaration specifiers in a declaration./48/ Semantics The typedef specifier is called a ``storage-class specifier'' for syntactic convenience only; it is discussed in $3.5.6. The meanings of the various linkages and storage durations were discussed in $3.1.2.2 and $3.1.2.4. A declaration of an identifier for an object with storage-class specifier register suggests that access to the object be as fast as possible. The extent to which such suggestions are effective is implementation-defined./49/ The declaration of an identifier for a function that has block scope shall have no explicit storage-class specifier other than extern. Forward references: type definitions ($3.5.6). 3.5.2 Type specifiers Syntax type-specifier: void char short int long float double signed unsigned struct-or-union-specifier enum-specifier typedef-name Constraints Each list of type specifiers shall be one of the following sets; 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 , signed int , or no type specifiers * unsigned , or unsigned int * long , signed long , long int , or signed long int * unsigned long , or unsigned long int * float * double * long double * struct-or-union specifier * enum-specifier * typedef-name Semantics Specifiers for structures, unions, and enumerations are discussed in $3.5.2.1 through $3.5.2.3. Declarations of typedef names are discussed in $3.5.6. The characteristics of the other types are discussed in $3.1.2.5. Each of the above comma-separated lists designates the same type, except that for bit-field declarations, signed int (or signed ) may differ from int (or no type specifiers). Forward references: enumeration specifiers ($3.5.2.2), structure and union specifiers ($3.5.2.1), tags ($3.5.2.3), type definitions ($3.5.6). 3.5.2.1 Structure and union specifiers Syntax struct-or-union-specifier: struct-or-union identifier { struct-declaration-list } struct-or-union identifier struct-or-union: struct union struct-declaration-list: struct-declaration struct-declaration-list struct-declaration struct-declaration: specifier-qualifier-list struct-declarator-list ; specifier-qualifier-list: type-specifier specifier-qualifier-list type-qualifier specifier-qualifier-list struct-declarator-list: struct-declarator struct-declarator-list , struct-declarator struct-declarator: declarator declarator : constant-expression Constraints A structure or union shall not contain a member with incomplete or function type. Hence it shall not contain an instance of itself (but may contain a pointer to an instance of itself). The expression that specifies the width of a bit-field shall be an integral constant expression that has nonnegative value that shall not exceed the number of bits in an ordinary object of compatible type. If the value is zero, the declaration shall have no declarator. Semantics As discussed in $3.1.2.5, a structure is a type consisting of a sequence of named members, whose storage is allocated in an ordered sequence, and a union is a type consisting of a sequence of named members, whose storage overlap. Structure and union specifiers have the same form. The presence of a struct-declaration-list in a struct-or-union-specifier declares a new type, within a translation unit. The struct-declaration-list is a sequence of declarations for the members of the structure or union. The type is incomplete until after the } that terminates the list. A member of a structure or union may have any object type. 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 ;/50/ its width is preceded by a colon. A bit-field may have type int , unsigned int , or signed int . Whether the high-order bit position of a ``plain'' int bit-field is treated as a sign bit is implementation-defined. A bit-field is interpreted as an integral type consisting of the specified number of bits. 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. A bit-field declaration with no declarator, but only a colon and a width, indicates an unnamed bit-field./51/ As a special case of this, a bit-field with a width of 0 indicates that no further bit-field is to be packed into the unit in which the previous bit-field, if any, was placed. Each non-bit-field member of a structure or union object is aligned in an implementation-defined manner appropriate to its type. 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 cast, 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 therefore be unnamed holes within a structure object, but not at its beginning, as necessary to achieve the appropriate alignment. 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 cast, 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. There may also be unnamed padding at the end of a structure or union, as necessary to achieve the appropriate alignment were the structure or union to be a member of an array. 3.5.2.2 Enumeration specifiers Syntax enum-specifier: enum identifier { enumerator-list } enum identifier enumerator-list: enumerator enumerator-list , enumerator enumerator: enumeration-constant enumeration-constant = constant-expression Constraints The expression that defines the value of an enumeration constant shall be an integral constant expression that has a value representable as an int. Semantics The identifiers in an enumerator list are declared as constants that have type int and may appear wherever such are permitted./52/ 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. (A combination of both forms of enumerators may produce enumeration constants with values that duplicate other values in the same enumeration.) The enumerators of an enumeration are also known as its members. Each enumerated type shall be compatible with an integer type; the choice of type is implementation-defined. Example enum hue { chartreuse, burgundy, claret=20, winedark }; /*...*/ enum hue col, *cp; /*...*/ col = claret; cp = &col; /*...*/ /*...*/ (*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}. 3.5.2.3 Tags A type specifier of the form struct-or-union identifier { struct-declaration-list } enum identifier { enumerator-list } declares the identifier to be the tag of the structure, union, or enumeration specified by the list. The list defines the structure content ,union content ,or enumeration content .If this declaration of the tag is visible, a subsequent declaration that uses the tag and that omits the bracketed list specifies the declared structure, union, or enumerated type. Subsequent declarations in the same scope shall omit the bracketed list. If a type specifier of the form struct-or-union identifier occurs prior to the declaration that defines the content, the structure or union is an incomplete type./53/ It declares a tag that specifies a type that may be used only when the size of an object of the specified type is not needed./54/ If the type is to be completed, another declaration of the tag in the same scope (but not in an enclosed block, which declares a new type known only within that block) shall define the content. A declaration of the form struct-or-union identifier ; specifies a structure or union type and declares a tag, both visible only within the scope in which the declaration occurs. It specifies a new type distinct from any type with the same tag in an enclosing scope (if any). A type specifier of the form struct-or-union { struct-declaration-list } enum { enumerator-list } specifies a new structure, union, or enumerated type, within the translation unit, that can only be referred to by the declaration of which it is a part./55/ Examples 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; To illustrate the use of prior declaration of a tag to specify a pair of mutually-referential structures, the declarations struct s1 { struct s2 *s2p; /*...*/ }; /* D1 */ struct s2 { struct s1 *s1p; /*...*/ }; /* D2 */ specify a pair of structures that contain pointers to each other. Note, however, that if s2 were already declared as a tag in an enclosing scope, the declaration D1 would refer to it, not to the tag s2 declared in D2 . To eliminate this context sensitivity, the otherwise vacuous declaration struct s2; may be inserted ahead of D1. This declares a new tag s2 in the inner scope; the declaration D2 then completes the specification of the new type. Forward references: type definitions ($3.5.6). 3.5.3 Type qualifiers Syntax type-qualifier: const volatile Constraints The same type qualifier shall not appear more than once in the same specifier list or qualifier list, either directly or via one or more typedef s. Semantics The properties associated with qualified types are meaningful only for expressions that are lvalues./56/ 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./57/ 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 $2.1.2.3. 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./58/ What constitutes an access to an object that has volatile-qualified type is implementation-defined. 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./59/ 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 An object declared extern const volatile int real_time_clock; may be modifiable by hardware, but cannot be assigned to, incremented, or decremented. The following declarations and expressions illustrate the behavior when type qualifiers modify an aggregate type: const struct s { int mem; } cs = { 1 }; struct s ncs; /* the object ncs is modifiable */ typedef int A[2][3]; const A a = {{4, 5, 6}, {7, 8, 9}}; /* array of array of const int */ int *pi; const int *pci; ncs = cs; /* valid */ cs = ncs; /* violates modifiable lvalue constraint for = */ pi = &ncs.mem; /* valid */ pi = &cs.mem; /* violates type constraints for = */ pci = &cs.mem; /* valid */ pi = a[0]; /* invalid: a[0] has type ``const int * '' */ 3.5.4 Declarators Syntax declarator: pointer direct-declarator direct-declarator: identifier ( declarator ) direct-declarator [ constant-expression ] direct-declarator ( parameter-type-list ) direct-declarator ( identifier-list ) pointer: * type-qualifier-list * type-qualifier-list pointer type-qualifier-list: type-qualifier type-qualifier-list type-qualifier parameter-type-list: parameter-list parameter-list , ... parameter-list: parameter-declaration parameter-list , parameter-declaration parameter-declaration: declaration-specifiers declarator declaration-specifiers abstract-declarator identifier-list: identifier identifier-list , identifier Semantics Each declarator declares one identifier, and asserts that when an operand of the same form as the declarator appears in an expression, it designates a function or object with the scope, storage duration, and type indicated by the declaration specifiers. In the following subsections, consider a declaration T D1 where T contains the declaration specifiers that specify a type T (such as int) and D1 is a declarator that contains an identifier ident . The type specified for the identifier ident in the various forms of declarator is described inductively using this notation. If, in the declaration `` T D1 ,'' D1 has the form identifier then the type specified for ident is T . If, in the declaration `` T D1 ,'' D1 has the form ( D ) then ident has the type specified by the declaration `` T D .'' Thus, a declarator in parentheses is identical to the unparenthesized declarator, but the binding of complex declarators may be altered by parentheses. "Implementation limits" 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, a structure, a union, or an incomplete type, either directly or via one or more typedef s. Forward references: type definitions ($3.5.6). 3.5.4.1 Pointer declarators Semantics If, in the declaration `` T D1 ,'' D1 has the form * type-qualifier-list D and the type specified for ident in the declaration `` T D '' is `` "derived-declarator-type-list T" ,'' then the type specified for ident is `` "derived-declarator-type-list type-qualifier-list" pointer to T.'' For each type qualifier in the list, ident is a so-qualified pointer. For two pointer types to be compatible, both shall be identically qualified and both shall be pointers to compatible types. Examples 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 the const int pointed to by ptr_to_constant shall not be modified, but ptr_to_constant itself may be changed to point to another const int . Similarly, the contents of the int pointed to by constant_ptr may be modified, but constant_ptr itself shall always point to the same location. The declaration of the constant pointer constant_ptr may be clarified by including a definition for the type ``pointer to int .'' typedef int *int_ptr; const int_ptr constant_ptr; declares constant_ptr as an object that has type ``const-qualified pointer to int .'' 3.5.4.2 Array declarators Constraints The expression that specifies the size of an array shall be an integral constant expression that has a value greater than zero. Semantics If, in the declaration `` T D1 ,'' D1 has the form D[ constant-expression] 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 .''/60/ If the size is not present, the array type is an incomplete type. For two array types to be compatible, both shall have compatible element types, and if both size specifiers are present, they shall have the same value. Examples float fa[11], *afp[17]; declares an array of float numbers and an array of pointers to float numbers. 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. Forward references: function definitions ($3.7.1), initialization ($3.5.7). 3.5.4.3 Function declarators (including prototypes) Constraints A function declarator shall not specify a return type that is a function type or an array type. The only storage-class specifier that shall occur in a parameter declaration is register. An identifier list in a function declarator that is not part of a function definition shall be empty. Semantics If, in the declaration `` T D1 ,'' D1 has the form D( parameter-type-list) D( identifier-list) and the type specified for ident in the declaration `` T D '' is `` "derived-declarator-type-list T" ,'' then the type specified for ident is `` derived-declarator-type-list function returning T .'' A parameter type list specifies the types of, and may declare identifiers for, the parameters of the function. If the list terminates with an ellipsis ( , ... ), no information about the number or types of the parameters after the comma is supplied./61/ The special case of void as the only item in the list specifies that the function has no parameters. In a parameter declaration, a single typedef name in parentheses is taken to be an abstract declarator that specifies a function with a single parameter, not as redundant parentheses around the identifier for a declarator. 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. 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./62/ For two function types to be compatible, both shall specify compatible return types./63/ Moreover, the parameter type lists, if both are present, shall agree in the number of parameters and in use of the ellipsis terminator; corresponding parameters shall have compatible types. If one type has a parameter type list and the other type is specified by a function declarator that is not part of a function definition and that contains an empty identifier list, the parameter list shall not have an ellipsis terminator and the type of each parameter shall be compatible with the type that results from the application of the default argument promotions. If one type has a parameter type list and the other type is specified by a function definition that contains a (possibly empty) identifier list, both shall agree in the number of parameters, and the type of each prototype parameter shall be compatible with the type that results from the application of the default argument promotions to the type of the corresponding identifier. (For each parameter declared with function or array type, its type for these comparisons is the one that results from conversion to a pointer type, as in $3.7.1. For each parameter declared with qualified type, its type for these comparisons is the unqualified version of its declared type.) Examples 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 external linkage, and the identifier of the pointer pfi has block scope and no linkage. Here are two more intricate examples. 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 . 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 at least one parameter, which has type int . Forward references: function definitions ($3.7.1), type names ($3.5.5). 3.5.5 Type names Syntax type-name: specifier-qualifier-list abstract-declarator abstract-declarator: pointer pointer direct-abstract-declarator direct-abstract-declarator: ( abstract-declarator ) direct-abstract-declarator [ constant-expression ] direct-abstract-declarator ( parameter-type-list ) Semantics 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./64/ Examples 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 int's, (e) function with no parameter specification returning a pointer to int , (f) pointer to function with no 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 . 3.5.6 Type definitions Syntax typedef-name: identifier Semantics 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 $3.5.4. 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. Examples After typedef int MILES, KLICKSP(); typedef struct { double re, im; } complex; the constructions MILES distance; extern KLICKSP *metricp; complex x; complex 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. 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 . The following constructions typedef signed int t; typedef int plain; struct tag { unsigned t:4; const t:5; plain r:5; }; declare a typedef name t with type signed int , a typedef name plain with type int , and a structure with three bit-field members, one named t that contains values in the range [0,15], an unnamed const-qualified bit-field which (if it could be accessed) would contain values in at least the range [-15,+15], and one named r that contains values in the range [0,31] or values in at least the range [-15,+15]. (The choice of range is implementation-defined.) 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 . 3.5.7 Initialization Syntax initializer: assignment-expression { initializer-list } { initializer-list , } initializer-list: initializer initializer-list , initializer Constraints There shall be no more initializers in an initializer list than there are objects to be initialized. The type of the entity to be initialized shall be an object type or an array of unknown size. All the expressions in an initializer for an object that has static storage duration or in an initializer list for an object that has aggregate or union type shall be constant expressions. If the declaration of an identifier has block scope, and the identifier has external or internal linkage, there shall be no initializer for the identifier. Semantics An initializer specifies the initial value stored in an object. All unnamed structure or union members are ignored during initialization. If an object that has static storage duration is not initialized explicitly, it is initialized implicitly as if every member that has arithmetic type were assigned 0 and every member that has pointer type were assigned a null pointer constant. If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate./65/ The initializer for a scalar shall be a single expression, optionally enclosed in braces. The initial value of the object is that of the expression; the same type constraints and conversions as for simple assignment apply. A brace-enclosed initializer for a union object initializes the member that appears first in the declaration list of the union type. 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. The rest of this section deals with initializers for objects that have aggregate or union type. 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 members of the array. 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 members of the array. Otherwise, the initializer for an object that has aggregate type shall be a brace-enclosed list of initializers for the 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 member of the union. 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 first 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. If there are fewer initializers in a list than there are members of an aggregate, the remainder of the aggregate shall be initialized implicitly the same as objects that have static storage duration. If an array of unknown size is initialized, its size is determined by the number of initializers provided for its members. At the end of its initializer list, the array no longer has incomplete type. Examples The declaration int x[] = { 1, 3, 5 }; defines and initializes x as a one-dimensional array object that has three members, as no size was specified and there are three initializers. float 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 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 float 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] . Also, float z[4][3] = { { 1 }, { 2 }, { 3 }, { 4 } }; initializes the first column of z as specified and initializes the rest with zeros. struct { int a[3], b; } w[] = { { 1 }, 2 }; is a definition with an inconsistently bracketed initialization. It defines an array with two member structures: w[0].a[0] is 1 and w[1].a[0] is 2; all the other elements are zero. 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][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 members are initialized with zero. Likewise, the initializers for q[1][0][0] and q[2][0][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 occur. The same initialization result could have been achieved by: short q[4][3][2] = { 1, 0, 0, 0, 0, 0, 2, 3, 0, 0, 0, 0, 4, 5, 6 }; or by: short q[4][3][2] = { { { 1 }, }, { { 2, 3 }, }, { { 4, 5 }, { 6 }, } }; in a fully-bracketed form. Note that the fully-bracketed and minimally-bracketed forms of initialization are, in general, less likely to cause confusion. Finally, the declaration char s[] = "abc", t[3] = "abc"; defines ``plain'' char array objects s and t whose members are initialized with character string literals. This declaration is identical to char s[] = { 'a', 'b', 'c', '\0' }, t[] = { 'a', 'b', 'c' }; The contents of the arrays are modifiable. On the other hand, the declaration char *p = "abc"; defines p with type ``pointer to char '' that is initialized to point to an object with type ``array of char '' whose members 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. Forward references: common definitions ($4.1.5). 3.6 STATEMENTS Syntax statement: labeled-statement compound-statement expression-statement selection-statement iteration-statement jump-statement Semantics A statement specifies an action to be performed. Except as indicated, statements are executed in sequence. 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 three expressions of a for statement; the expression in a return statement. The end of a full expression is a sequence point. Forward references: expression and null statements ($3.6.3), selection statements ($3.6.4), iteration statements ($3.6.5), the return statement ($3.6.6.4). 3.6.1 Labeled statements Syntax labeled-statement: identifier : statement case constant-expression : statement default : statement Constraints A case or default label shall appear only in a switch statement. Further constraints on such labels are discussed under the switch statement. Semantics Any statement may be preceded by a prefix that declares an identifier as a label name. Labels in themselves do not alter the flow of control, which continues unimpeded across them. Forward references: the goto statement ($3.6.6.1), the switch statement ($3.6.4.2). 3.6.2 Compound statement, or block Syntax compound-statement: { declaration-list statement-list } declaration-list: declaration declaration-list declaration statement-list: statement statement-list statement Semantics 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 $3.1.2.4). The initializers of objects that have automatic storage duration are evaluated and the values are stored in the objects in the order their declarators appear in the translation unit. 3.6.3 Expression and null statements Syntax expression-statement: expression ; Semantics The expression in an expression statement is evaluated as a void expression for its side effects./66/ A null statement (consisting of just a semicolon) performs no operations. Examples 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); In the program fragment char *s; /*...*/ while (*s++ != '\0') ; a null statement is used to supply an empty loop body to the iteration statement. A null statement may also be used to carry a label just before the closing } of a compound statement. while (loop1) { /*...*/ while (loop2) { /*...*/ if (want_out) goto end_loop1; /*...*/ } /*...*/ end_loop1: ; } Forward references: iteration statements ($3.6.5). 3.6.4 Selection statements Syntax selection-statement: if ( expression ) statement if ( expression ) statement else statement switch ( expression ) statement Semantics A selection statement selects among a set of statements depending on the value of a controlling expression. 3.6.4.1 The if statement Constraints The controlling expression of an if statement shall have scalar type. Semantics In both forms, the first substatement is executed if the expression compares unequal to 0. In the else form, the second substatement is executed if the expression compares equal to 0. If the first substatement is reached via a label, the second substatement is not executed. An else is associated with the lexically immediately preceding else -less if that is in the same block (but not in an enclosed block). 3.6.4.2 The switch statement Constraints The controlling expression of a switch statement shall have integral type. The expression of each case label shall be an integral 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 A switch statement causes control to jump to, into, or past the statement that is the switch body, depending on the value of a controlling expression, and on the presence of a default label and the values of any case labels on or in the switch body. A case or default label is accessible only within the closest enclosing switch statement. The integral promotions are performed on the controlling expression. The constant expression in each case label is converted to the promoted type of the controlling expression. If a converted value matches that of the promoted controlling expression, control jumps to the statement following the matched case label. Otherwise, if there is a default label, control jumps to the labeled statement. If no converted case constant expression matches and there is no default label, no part of the switch body is executed. "Implementation limits" As discussed previously ($2.2.4.1), the implementation may limit the number of case values in a switch statement. 3.6.5 Iteration statements Syntax iteration-statement: while ( expression ) statement do statement while ( expression ) ; for ( expression ; expression ; expression ) statement Constraints The controlling expression of an iteration statement shall have scalar type. Semantics An iteration statement causes a statement called the loop body to be executed repeatedly until the controlling expression compares equal to 0. 3.6.5.1 The while statement The evaluation of the controlling expression takes place before each execution of the loop body. 3.6.5.2 The do statement The evaluation of the controlling expression takes place after each execution of the loop body. 3.6.5.3 The for statement Except for the behavior of a continue statement in the loop body, the statement for ( expression-1 ; expression-2 ; expression-3 ) statement and the sequence of statements expression-1 ; while ( expression-2) { statement expression-3 ; } are equivalent./67/ expression-1 expression-2 , expression-3 Both expression-1 and expression-3 may be omitted. Each is evaluated as a void expression. An omitted expression-2 is replaced by a nonzero constant. Forward references: the continue statement ($3.6.6.2). 3.6.6 Jump statements Syntax jump-statement: goto identifier ; continue ; break ; return expression ; Semantics A jump statement causes an unconditional jump to another place. 3.6.6.1 The goto statement Constraints The identifier in a goto statement shall name a label located somewhere in the current function. Semantics A goto statement causes an unconditional jump to the statement prefixed by the named label in the current function. 3.6.6.2 The continue statement Constraints A continue statement shall appear only in or as a loop body. Semantics A continue statement causes a jump to the loop-continuation portion of the smallest enclosing iteration statement; that is, to the end of the loop body. More precisely, in each of the statements while (/*...*/) { do { for (/*...*/) { /*...*/ /*...*/ /*...*/ continue; continue; continue; /*...*/ /*...*/ /*...*/ contin: ; contin: ; contin: ; } } while (/*...*/); } unless the continue statement shown is in an enclosed iteration statement (in which case it is interpreted within that statement), it is equivalent to goto contin; ./68/ 3.6.6.3 The break statement Constraints A break statement shall appear only in or as a switch body or loop body. Semantics A break statement terminates execution of the smallest enclosing switch or iteration statement. 3.6.6.4 The return statement Constraints A return statement with an expression shall not appear in a function whose return type is void . Semantics A return statement terminates execution of the current function and returns control to its caller. A function may have any number of return statements, with and without expressions. 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 that of the function in which it appears, it is converted as if it were assigned to an object of that type. 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. Reaching the } that terminates a function is equivalent to executing a return statement without an expression. 3.7 EXTERNAL DEFINITIONS Syntax translation-unit: external-declaration translation-unit external-declaration external-declaration: function-definition declaration Constraints The storage-class specifiers auto and register shall not appear in the declaration specifiers in an external declaration. There shall be no more than one external definition for each identifier declared with internal linkage in a translation unit. Moreover, if an identifier declared with internal linkage is used in an expression (other than as a part of the operand of a sizeof operator), there shall be exactly one external definition for the identifier in the translation unit. Semantics As discussed in $2.1.1.1, 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 $3.5, a declaration that also causes storage to be reserved for an object or a function named by the identifier is a definition. An external definition is an external declaration that is also a definition of a function or an object. If an identifier declared with external linkage is used in an expression (other than as part of the operand of a sizeof operator), somewhere in the entire program there shall be exactly one external definition for the identifier./69/ 3.7.1 Function definitions Syntax function-definition: declaration-specifiers declarator declaration-list compound-statement Constraints The identifier declared in a function definition (which is the name of the function) shall have a function type, as specified by the declarator portion of the function definition./70/ The return type of a function shall be void or an object type other than array. The storage-class specifier, if any, in the declaration specifiers shall be either extern or static . If the declarator includes a parameter type list, the declaration of each parameter shall include an identifier (except for the special case of a parameter list consisting of a single parameter of type void, in which there shall not be an identifier). No declaration list shall follow. If the declarator includes an identifier list, only the identifiers it names shall be declared in the declaration list. An identifier declared as a typedef name shall not be redeclared as a parameter. The declarations in the declaration list shall contain no storage-class specifier other than register and no initializations. Semantics The declarator in a function definition specifies the name of the function being defined and the identifiers of its parameters. If the declarator includes a parameter type list, the list also specifies the types of all the parameters; such a declarator also serves as a function prototype for later calls to the same function in the same translation unit. If the declarator includes an identifier list,/71/ the types of the parameters may be declared in a following declaration list. Any parameter that is not declared has type int . 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. On entry to the function 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 $3.2.2.1. The resulting parameter type shall be an object type. Each parameter has automatic storage duration. Its identifier is an lvalue./72/ The layout of the storage for parameters is unspecified. Examples extern int max(int a, int b) { return a > b ? a : b; } Here extern is the storage-class specifier and int is the type specifier (each of which may be omitted as those are the defaults); 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, which may be omitted because those are the defaults. 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. To pass one function to another, one might say int f(void); /*...*/ g(f); Note that f must be declared explicitly in the calling function, as its appearance in the expression g(f) was not followed by ( . Then the definition of g might read g(int (*funcp)(void)) { /*...*/ (*funcp)() /* or funcp() ... */ } or, equivalently, g(int func(void)) { /*...*/ func() /* or (*func)() ... */ } 3.7.2 External object definitions Semantics If the declaration of an identifier for an object has file scope and an initializer, the declaration is an external definition for the identifier. A declaration of an identifier for an object that has file scope without an initializer, and without a storage-class specifier or with the storage-class specifier static , constitutes a tentative definition. If a translation unit contains one or more tentative definitions for an identifier, and the translation unit contains no external definition for that identifier, then the behavior is exactly as if the translation unit contains a file scope declaration of that identifier, with the composite type as of the end of the translation unit, with an initializer equal to 0. If the declaration of an identifier for an object is a tentative definition and has internal linkage, the declared type shall not be an incomplete type. Examples int i1 = 1; /* definition, external linkage */ static int i2 = 2; /* definition, internal linkage */ extern int i3 = 3; /* definition, external linkage */ int i4; /* tentative definition, external linkage */ static int i5; /* tentative definition, internal linkage */ int i1; /* valid tentative definition, refers to previous */ int i2; /* $3.1.2.2 renders undefined, linkage disagreement */ int i3; /* valid tentative definition, refers to previous */ int i4; /* valid tentative definition, refers to previous */ int i5; /* $3.1.2.2 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 */ 3.8 PREPROCESSING DIRECTIVES Syntax preprocessing-file: group group: group-part group group-part group-part: pp-tokens new-line if-section control-line if-section: if-group elif-groups else-group endif-line if-group: # if constant-expression new-line group # ifdef identifier new-line group # ifndef identifier new-line group elif-groups: elif-group elif-groups elif-group elif-group: # elif constant-expression new-line group else-group: # else new-line group endif-line: # endif new-line control-line: # include pp-tokens new-line # define identifier 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 new-line # pragma pp-tokens new-line # new-line lparen: the left-parenthesis character without preceding white-space replacement-list: pp-tokens pp-tokens: preprocessing-token pp-tokens preprocessing-token new-line: the new-line character Description A preprocessing directive consists of a sequence of preprocessing tokens that begins with a # preprocessing token that 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./73/ Constraints The only white-space characters that shall appear between preprocessing tokens within a preprocessing directive (from just after the introducing # preprocessing token through just before the terminating new-line character) are space and horizontal-tab (including spaces that have replaced comments in translation phase 3). Semantics The implementation can process and skip sections of source files conditionally, include other source files, and replace macros. These capabilities are called preprocessing , because conceptually they occur before translation of the resulting translation unit. The preprocessing tokens within a preprocessing directive are not subject to macro expansion unless otherwise stated. 3.8.1 Conditional inclusion Constraints The expression that controls conditional inclusion shall be an integral constant expression except that: it shall not contain a cast; identifiers (including those lexically identical to keywords) are interpreted as described below;/74/ and it may contain unary operator expressions of the form defined identifier defined ( identifier ) which evaluate to 1 if the identifier is currently defined as a macro name (that is, if it is predefined or if it has been the subject of a #define preprocessing directive without an intervening #undef directive with the same subject identifier), 0 if it is not. Each preprocessing token that remains after all macro replacements have occurred shall be in the lexical form of a token. Semantics Preprocessing directives of the forms # if constant-expression new-line group # elif constant-expression new-line group check whether the controlling constant expression evaluates to nonzero. Prior to evaluation, macro invocations in the list of preprocessing tokens that will become the controlling constant expression are replaced (except for those macro names modified by the defined unary operator), just as in normal text. If the token defined is generated as a result of this replacement process, the behavior is undefined. After all replacements are finished, the resulting preprocessing tokens are converted into tokens, and then all remaining identifiers are replaced with 0 . The resulting tokens comprise the controlling constant expression which is evaluated according to the rules of $3.4 using arithmetic that has at least the ranges specified in $2.2.4.2, except that int and unsigned int act as if they have the same representation as, respectively, long and unsigned long . 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./75/ Also, whether a single-character character constant may have a negative value is implementation-defined. Preprocessing directives of the forms # ifdef identifier new-line group # ifndef identifier new-line group check whether the identifier is or is not currently defined as a macro name. Their conditions are equivalent to #if defined identifier and #if !defined identifier respectively. Each directive's condition is checked in order. If it evaluates to false (zero), the group that it controls is skipped: directives are processed only through the name that determines the directive in order to keep track of the level of nested conditionals; the rest of the directives' preprocessing tokens are ignored, as are the other preprocessing tokens in the group. Only the first group whose control condition evaluates to true (nonzero) is processed. If none of the conditions evaluates to true, and there is a #else directive, the group controlled by the #else is processed; lacking a #else directive, all the groups until the #endif are skipped./76/ Forward references: macro replacement ($3.8.3), source file inclusion ($3.8.2). 3.8.2 Source file inclusion Constraints A #include directive shall identify a header or source file that can be processed by the implementation. Semantics A preprocessing directive of the form # include new-line searches a sequence of implementation-defined places for a header identified uniquely by the specified sequence between the < and > delimiters, and causes the replacement of that directive by the entire contents of the header. How the places are specified or the header identified is implementation-defined. A preprocessing directive of the form # include "q-char-sequence" new-line causes the replacement of that directive by the entire contents of the source file identified by the specified sequence between the delimiters. The named source file is searched for in an implementation-defined manner. If this search is not supported, or if the search fails, the directive is reprocessed as if it read # include new-line with the identical contained sequence (including > characters, if any) from the original directive. A preprocessing directive of the form # include pp-tokens new-line (that does not match one of the two previous forms) is permitted. The preprocessing tokens after include in the directive are processed just as in normal text. (Each identifier currently defined as a macro name is replaced by its replacement list of preprocessing tokens.) The directive resulting after all replacements shall match one of the two previous forms./77/ 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. There shall be an implementation-defined mapping between the delimited sequence and the external source file name. The implementation shall provide unique mappings for sequences consisting of one or more letters (as defined in $2.2.1) followed by a period (.) and a single letter. The implementation may ignore the distinctions of alphabetical case and restrict the mapping to six significant characters before the period. A #include preprocessing directive may appear in a source file that has been read because of a #include directive in another file, up to an implementation-defined nesting limit (see $2.2.4.1). Examples The most common uses of #include preprocessing directives are as in the following: #include #include "myprog.h" This example illustrates a macro-replaced #include directive: #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 ($3.8.3). 3.8.3 Macro replacement Constraints Two replacement lists are identical if and only if the preprocessing tokens in both have the same number, ordering, spelling, and white-space separation, where all white-space separations are considered identical. An identifier currently defined as a macro without use of lparen (an object-like macro) may be redefined by another #define preprocessing directive provided that the second definition is an object-like macro definition and the two replacement lists are identical. An identifier currently defined as a macro using lparen (a function-like macro) may be redefined by another #define preprocessing directive provided that 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. The number of arguments in an invocation of a function-like macro shall agree with the number of parameters in the macro definition, and there shall exist a ) preprocessing token that terminates the invocation. A parameter identifier in a function-like macro shall be uniquely declared within its scope. Semantics The identifier immediately following the define is called the macro name. Any white-space characters preceding or following the replacement list of preprocessing tokens are not considered part of the replacement list for either form of macro. If a # preprocessing token, followed by an identifier, occurs lexically at the point at which a preprocessing directive could begin, the identifier is not subject to macro replacement. A preprocessing directive of the form # define identifier replacement-list new-line defines an object-like macro that causes each subsequent instance of the macro name/78/ 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. A preprocessing directive of the form # define identifier lparen identifier-list ) replacement-list new-line defines a function-like macro with arguments, similar syntactically to a function call. The parameters are specified by the optional list of identifiers, whose scope extends from their declaration in the identifier list until the new-line character that terminates the #define preprocessing directive. Each subsequent instance of the function-like macro name followed by a ( as the next preprocessing token introduces the sequence of preprocessing tokens that is replaced by the replacement list in the definition (an invocation of the macro). The replaced sequence of preprocessing tokens is terminated by the matching ) preprocessing token, skipping intervening matched pairs of left and right parenthesis preprocessing tokens. Within the sequence of preprocessing tokens making up an invocation of a function-like macro, new-line is considered a normal white-space character. 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 bounded by nested parentheses do not separate arguments. If (before argument substitution) any argument consists of no preprocessing tokens, the behavior is undefined. If there are sequences of preprocessing tokens within the list of arguments that would otherwise act as preprocessing directives, the behavior is undefined. 3.8.3.1 Argument substitution After the arguments for the invocation of a function-like macro have been identified, argument substitution takes place. A parameter in the replacement list, unless preceded by a # or ## preprocessing token or followed by a ## preprocessing token (see below), is replaced by the corresponding argument after all macros contained therein have been expanded. Before being substituted, each argument's preprocessing tokens are completely macro replaced as if they formed the rest of the source file; no other preprocessing tokens are available. 3.8.3.2 The # operator Constraints Each # preprocessing token in the replacement list for a function-like macro shall be followed by a parameter as the next preprocessing token in the replacement list. Semantics If, in the replacement list, a parameter is immediately preceded by a # preprocessing token, both are replaced by a single character string literal preprocessing token that contains the spelling of the preprocessing token sequence for the corresponding argument. Each occurrence of white space between the argument's preprocessing tokens becomes a single space character in the character string literal. White space before the first preprocessing token and after the last preprocessing token 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 order of evaluation of # and ## operators is unspecified. 3.8.3.3 The ## operator Constraints A ## preprocessing token shall not occur at the beginning or at the end of a replacement list for either form of macro definition. Semantics If, in the replacement list, a parameter is immediately preceded or followed by a ## preprocessing token, the parameter is replaced by the corresponding argument's preprocessing token sequence. For both object-like and function-like macro invocations, before the replacement list is reexamined for more macro names to replace, each instance of a ## preprocessing token in the replacement list (not from an argument) is deleted and the preceding preprocessing token is concatenated with the following preprocessing token. 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. 3.8.3.4 Rescanning and further replacement After all parameters in the replacement list have been substituted, the resulting preprocessing token sequence is rescanned with the rest of the source file's preprocessing tokens for more macro names to replace. If the name of the macro being replaced is found during this scan of the replacement list (not including the rest of the source file's preprocessing tokens), it is not replaced. Further, if any nested replacements encounter the name of the macro being replaced, it is not replaced. These nonreplaced macro name preprocessing tokens are no longer available for further replacement even if they are later (re)examined in contexts in which that macro name preprocessing token would otherwise have been replaced. The resulting completely macro-replaced preprocessing token sequence is not processed as a preprocessing directive even if it resembles one. 3.8.3.5 Scope of macro definitions A macro definition lasts (independent of block structure) until a corresponding #undef directive is encountered or (if none is encountered) until the end of the translation unit. 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 The simplest use of this facility is to define a ``manifest constant,'' as in #define TABSIZE 100 int table[TABSIZE]; 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 of generating more code than a function if invoked several times. #define max(a, b) ((a) > (b) ? (a) : (b)) The parentheses ensure that the arguments and the resulting expression are bound properly. 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 f(y+1) + f(f(z)) % t(t(g)(0) + t)(1); g(x+(3,4)-w) | h 5) & m (f)^m(m); 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); To illustrate the rules for creating character string literals and concatenating tokens, the sequence #define str(s) # s #define xstr(s) str(s) #define debug(s, t) printf("x" # s "= %d, x" # t "= %s", \ x ## s, x ## t) #define INCFILE(n) vers ## n /* from previous #include example */ #define glue(a, b) a ## b #define xglue(a, b) glue(a, b) #define HIGHLOW "hello" #define LOW LOW ", world" debug(1, 2); fputs(str(strncmp("abc\0d", "abc", '\4') /* this goes away */ == 0) str(: @\n), s); #include xstr(INCFILE(2).h) glue(HIGH, LOW); xglue(HIGH, LOW) results in printf("x" "1" "= %d, x" "2" "= %s", x1, x2); fputs("strncmp(\"abc\\0d\", \"abc\", '\\4') == 0" ": @\n", s); #include "vers2.h" (after macro replacement, before file access) "hello"; "hello" ", world" or, after concatenation of the character string literals, printf("x1= %d, x2= %s", x1, x2); fputs("strncmp(\"abc\\0d\", \"abc\", '\\4') == 0: @\n", s); #include "vers2.h" (after macro replacement, before file access) "hello"; "hello, world" Space around the # and ## tokens in the macro definition is optional. And finally, to demonstrate the redefinition rules, the following sequence is valid. #define OBJ_LIKE (1-1) #define OBJ_LIKE /* white space */ (1-1) /* other */ #define FTN_LIKE(a) ( a ) #define FTN_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 FTN_LIKE(b) ( a ) /* different parameter usage */ #define FTN_LIKE(b) ( b ) /* different parameter spelling */ 3.8.4 Line control Constraints The string literal of a #line directive, if present, shall be a character string literal. Semantics The line number of the current source line is one greater than the number of new-line characters read or introduced in translation phase 1 ($2.1.1.2) while processing the source file to the current token. A preprocessing directive of the form # line digit-sequence new-line causes the implementation to behave as if the following sequence of source lines begins with a source line that has a line number as specified by the digit sequence (interpreted as a decimal integer). A preprocessing directive of the form # line digit-sequence " s-char-sequence" 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. 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. 3.8.5 Error directive Semantics A preprocessing directive of the form # error pp-tokens new-line causes the implementation to produce a diagnostic message that includes the specified sequence of preprocessing tokens. 3.8.6 Pragma directive Semantics A preprocessing directive of the form # pragma pp-tokens new-line causes the implementation to behave in an implementation-defined manner. Any pragma that is not recognized by the implementation is ignored. 3.8.7 Null directive Semantics A preprocessing directive of the form # new-line has no effect. 3.8.8 Predefined macro names The following macro names shall be defined by the implementation: The line number of the current source line (a decimal constant). The presumed name of the source file (a character string literal). 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. 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. the decimal constant 1./79/ The values of the predefined macros (except for __LINE__ and __FILE__ ) remain constant throughout the translation unit. None of these macro names, nor the identifier defined , shall be the subject of a #define or a #undef preprocessing directive. All predefined macro names shall begin with a leading underscore followed by an upper-case letter or a second underscore. Forward references: the asctime function ($4.12.3.1). 3.9 FUTURE LANGUAGE DIRECTIONS 3.9.1 External names Restriction of the significance of an external name to fewer than 31 characters or to only one case is an obsolescent feature that is a concession to existing implementations. 3.9.2 Character escape sequences Lower-case letters as escape sequences are reserved for future standardization. Other characters may be used in extensions. 3.9.3 Storage-class specifiers The placement of a storage-class specifier other than at the beginning of the declaration specifiers in a declaration is an obsolescent feature. 3.9.4 Function declarators The use of function declarators with empty parentheses (not prototype-format parameter type declarators) is an obsolescent feature. 3.9.5 Function definitions The use of function definitions with separate parameter identifier and declaration lists (not prototype-format parameter type and identifier declarators) is an obsolescent feature. 4. LIBRARY 4.1 INTRODUCTION 4.1.1 Definitions of terms A string is a contiguous sequence of characters terminated by and including the first null character. It is represented by a pointer to its initial (lowest addressed) character and its length is the number of characters preceding the null character. A letter is a printing character in the execution character set corresponding to any of the 52 required lower-case and upper-case letters in the source character set, listed in $2.2.1. The decimal-point character is the character used by functions that convert floating-point numbers to or from character sequences to denote the beginning of the fractional part of such character sequences./80/ It is represented in the text and examples by a period, but may be changed by the setlocale function. Forward references: character handling ($4.3), the setlocale function ($4.4.1.1). 4.1.2 Standard headers Each library function is declared in a header, /81/ 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. Each header declares and defines only those identifiers listed in its associated section. All external identifiers declared in any of the headers are reserved, whether or not the associated header is included. All external identifiers that begin with an underscore are reserved. All other identifiers that begin with an underscore and either an upper-case letter or another underscore are reserved. If the program defines an external identifier with the same name as a reserved external identifier, even in a semantically equivalent form, the behavior is undefined./82/ The standard headers are 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. 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. Furthermore, the program shall not have any macros with names lexically identical to keywords currently defined prior to the inclusion. Forward references: diagnostics ($4.2). 4.1.3 Errors The header defines several macros, all relating to the reporting of error conditions. The macros are EDOM ERANGE which expand to distinct nonzero integral constant expressions; and errno which expands to a modifiable lvalue/83/ 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 external identifier with the name errno , the behavior is undefined. The value of errno is zero at program startup, but is never set to zero by any library function./84/ 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 the Standard. Additional macro definitions, beginning with E and a digit or E and an upper-case letter,/85/ may also be specified by the implementation. 4.1.4 Limits and The headers and define several macros that expand to various limits and parameters. The macros, their meanings, and their minimum magnitudes are listed in $2.2.4.2. 4.1.5 Common definitions The following types and macros are defined in the standard header . Some are also defined in other headers, as noted in their respective sections. The types are ptrdiff_t which is the signed integral type of the result of subtracting two pointers; size_t which is the unsigned integral type of the result of the sizeof operator; and wchar_t which is an integral 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 $2.2.1 shall have a code value equal to its value when used as the lone character in an integer character constant. The macros are NULL which expands to an implementation-defined null pointer constant; and offsetof( type, member-designator) which expands to an integral 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 ($4.4). 4.1.6 Use of library functions Each of the following statements applies unless explicitly stated otherwise in the detailed descriptions that follow. If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer outside the address space of the program, or a null pointer), the behavior is undefined. Any function declared in a header may be implemented as a macro defined in the header, so a library function should not be declared explicitly if its header is included. 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./86/ The use of #undef to remove any macro definition will also ensure that an actual function is referred to. Any invocation of a library function that is implemented as a macro will 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. Likewise, those function-like macros described in the following sections may be invoked in an expression anywhere a function with a compatible return type could be called./87/ 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, either explicitly or implicitly, and use it without including its associated header. If a function that accepts a variable number of arguments is not declared (explicitly or by including its associated header), the behavior is undefined. Examples 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); * by implicit declaration const char *str; /*...*/ i = atoi(str); 4.2 DIAGNOSTICS The header defines the assert macro and refers to another macro, NDEBUG which is not defined by . If NDEBUG is defined as a macro name at the point in the source file where is included, the assert macro is defined simply as #define assert(ignore) ((void)0) The assert macro 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. 4.2.1 Program diagnostics 4.2.1.1 The assert macro Synopsis #include void assert(int expression); Description The assert macro puts diagnostics 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 EM the latter are respectively the values of the preprocessing macros __FILE__ and __LINE__ ) on the standard error file in an implementation-defined format./88/ expression , xyz , nnn It then calls the abort function. Returns The assert macro returns no value. Forward references: the abort function ($4.10.4.1). 4.3 CHARACTER HANDLING The header declares several functions useful for testing and mapping characters./89/ In all cases the argument is an int , the value of which shall be representable as an unsigned char or shall equal the value of the macro EOF . If the argument has any other value, the behavior is undefined. The behavior of these functions is affected by the current locale. Those functions that have no implementation-defined aspects in the C locale are noted below. The term printing character refers to a member of an implementation-defined set of characters, each of which occupies one printing position on a display device; the term control character refers to a member of an implementation-defined set of characters that are not printing characters./90/ Forward references: EOF ($4.9.1), localization ($4.4). 4.3.1 Character testing functions The functions in this section return nonzero (true) if and only if the value of the argument c conforms to that in the description of the function. 4.3.1.1 The isalnum function Synopsis #include int isalnum(int c); Description The isalnum function tests for any character for which isalpha or isdigit is true. 4.3.1.2 The isalpha function Synopsis #include int isalpha(int c); Description The isalpha function tests for any character for which isupper or islower is true, or any of an implementation-defined set of characters for which none of iscntrl , isdigit , ispunct , or isspace is true. In the C locale, isalpha returns true only for the characters for which isupper or islower is true. 4.3.1.3 The iscntrl function Synopsis #include int iscntrl(int c); Description The iscntrl function tests for any control character. 4.3.1.4 The isdigit function Synopsis #include int isdigit(int c); Description The isdigit function tests for any decimal-digit character (as defined in $2.2.1). 4.3.1.5 The isgraph function Synopsis #include int isgraph(int c); Description The isgraph function tests for any printing character except space (' '). 4.3.1.6 The islower function Synopsis #include int islower(int c); Description The islower function tests for any lower-case letter or any of an implementation-defined 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 lower-case letters (as defined in $2.2.1). 4.3.1.7 The isprint function Synopsis #include int isprint(int c); Description The isprint function tests for any printing character including space (' '). 4.3.1.8 The ispunct function Synopsis #include int ispunct(int c); Description The ispunct function tests for any printing character except space (' ') or a character for which isalnum is true. 4.3.1.9 The isspace function Synopsis #include int isspace(int c); Description The isspace function tests for the standard white-space characters or for any of an implementation-defined 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. 4.3.1.10 The isupper function Synopsis #include int isupper(int c); Description The isupper function tests for any upper-case letter or any of an implementation-defined 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 upper-case letters (as defined in $2.2.1). 4.3.1.11 The isxdigit function Synopsis #include int isxdigit(int c); Description The isxdigit function tests for any hexadecimal-digit character (as defined in $3.1.3.2). 4.3.2 Character case mapping functions 4.3.2.1 The tolower function Synopsis #include int tolower(int c); Description The tolower function converts an upper-case letter to the corresponding lower-case letter. Returns If the argument is an upper-case letter, the tolower function returns the corresponding lower-case letter if there is one; otherwise the argument is returned unchanged. In the C locale, tolower maps only the characters for which isupper is true to the corresponding characters for which islower is true. 4.3.2.2 The toupper function Synopsis #include int toupper(int c); Description The toupper function converts a lower-case letter to the corresponding upper-case letter. Returns If the argument is a lower-case letter, the toupper function returns the corresponding upper-case letter if there is one; otherwise the argument is returned unchanged. In the C locale, toupper maps only the characters for which islower is true to the corresponding characters for which isupper is true. 4.4 LOCALIZATION The header declares two functions, one type, and defines several macros. The type is struct lconv which contains members related to the formatting of numeric values. The structure shall contain at least the following members, in any order. The semantics of the members and their normal ranges is explained in $4.4.2.1. 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 */ The macros defined are NULL (described in $4.1.5); and LC_ALL LC_COLLATE LC_CTYPE LC_MONETARY LC_NUMERIC LC_TIME which expand to distinct integral constant expressions, suitable for use as the first argument to the setlocale function. Additional macro definitions, beginning with the characters LC_ and an upper-case letter,/91/ may also be specified by the implementation. 4.4.1 Locale control 4.4.1.1 The setlocale function Synopsis #include char *setlocale(int category, const char *locale); Description The setlocale function selects the appropriate portion of the program's locale as specified by the category and locale arguments. The setlocale function may be used to change or query the program's entire current locale or portions thereof. The value LC_ALL for category names the program's entire locale; the other values for category name only a portion of the program's locale. LC_COLLATE affects the behavior of the strcoll and strxfrm functions. LC_CTYPE affects the behavior of the character handling functions/92/ 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 non-monetary formatting information returned by the localeconv function. LC_TIME affects the behavior of the strftime function. A value of "C" for locale specifies the minimal environment for C translation; a value of "" for locale specifies the implementation-defined native environment. Other implementation-defined strings may be passed as the second argument to setlocale . At program startup, the equivalent of setlocale(LC_ALL, "C"); is executed. The implementation shall behave as if no library function calls the setlocale function. Returns If a pointer to a string is given for locale and the selection can be honored, the setlocale function returns the string associated with the specified category for the new locale. If the selection cannot be honored, the setlocale function returns a null pointer and the program's locale is not changed. A null pointer for locale causes the setlocale function to return the string associated with the category for the program's current locale; the program's locale is not changed. The string returned by the setlocale function is such that a subsequent call with that string and its associated category will restore that part of the program's locale. The string returned 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 ($4.9.6), the multibyte character functions ($4.10.7), the multibyte string functions ($4.10.8), string conversion functions ($4.10.1), the strcoll function ($4.11.4.3), the strftime function ($4.12.3.5), the strxfrm function ($4.11.4.5). 4.4.2 Numeric formatting convention inquiry 4.4.2.1 The localeconv function Synopsis #include struct lconv *localeconv(void); Description The localeconv function sets the components of an object with type struct lconv with values appropriate for the formatting of numeric quantities (monetary and otherwise) according to the rules of the current locale. The members of the structure with type char * are 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. 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: The decimal-point character used to format non-monetary quantities. The character used to separate groups of digits to the left of the decimal-point character in formatted non-monetary quantities. A string whose elements indicate the size of each group of digits in formatted non-monetary quantities. 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 Codes for the Representation of Currency and Funds .The fourth character (immediately preceding the null character) is the character used to separate the international currency symbol from the monetary quantity. The local currency symbol applicable to the current locale. The decimal-point used to format monetary quantities. The separator for groups of digits to the left of the decimal-point in formatted monetary quantities. A string whose elements indicate the size of each group of digits in formatted monetary quantities. The string used to indicate a nonnegative-valued formatted monetary quantity. The string used to indicate a negative-valued formatted monetary quantity. The number of fractional digits (those to the right of the decimal-point) to be displayed in a internationally formatted monetary quantity. The number of fractional digits (those to the right of the decimal-point) to be displayed in a formatted monetary quantity. Set to 1 or 0 if the currency_symbol respectively precedes or succeeds the value for a nonnegative formatted monetary quantity. 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. Set to 1 or 0 if the currency_symbol respectively precedes or succeeds the value for a negative formatted monetary quantity. Set to 1 or 0 if the currency_symbol respectively is or is not separated by a space from the value for a negative formatted monetary quantity. Set to a value indicating the positioning of the positive_sign for a nonnegative formatted monetary quantity. Set to a value indicating the positioning of the negative_sign for a negative formatted monetary quantity. The elements of grouping and mon_grouping are interpreted according to the following: No further grouping is to be performed. The previous element is to be repeatedly used for the remainder of the digits. The 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 to the left of the current group. The value of p_sign_posn and n_sign_posn is interpreted according to the following: Parentheses surround the quantity and currency_symbol. The sign string precedes the quantity and currency_symbol. The sign string succeeds the quantity and currency_symbol. The sign string immediately precedes the currency_symbol. The sign string immediately succeeds the currency_symbol. The implementation shall behave as if no library function calls the localeconv function. Returns The localeconv function returns a pointer to the filled-in object. The structure pointed to by the return value shall not be modified by the program, but may be overwritten by a subsequent call to the localeconv function. In addition, calls to the setlocale function with categories LC_ALL , LC_MONETARY , or LC_NUMERIC may overwrite the contents of the structure. Examples The following table illustrates the rules used by four countries to format monetary quantities. Country Positive format Negative format International 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 For these four countries, the respective values for the monetary members of the structure returned by localeconv are: Italy Netherlands Norway Switzerland 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 4.5 MATHEMATICS The header declares several mathematical functions and defines one macro. The functions take double-precision arguments and return double-precision values./93/ Integer arithmetic functions and conversion functions are discussed later. The macro defined is HUGE_VAL which expands to a positive double expression, not necessarily representable as a float . Forward references: integer arithmetic functions ($4.10.6), the atof function ($4.10.1.1), the strtod function ($4.10.1.4). 4.5.1 Treatment of error conditions The behavior of each of these functions is defined for all representable values of its input arguments. Each function shall execute as if it were a single operation, without generating any externally visible exceptions. 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./94/ On a domain error, the function returns an implementation-defined value; the value of the macro EDOM is stored in errno . Similarly, a range error occurs if the result of the function cannot be represented as a double value. If the result overflows (the magnitude of the result is so large that it cannot be represented in an object of the specified type), the function returns the value of the macro HUGE_VAL , with the same sign as the correct value of the function; the value of the macro ERANGE is stored in errno . If the result underflows (the magnitude of the result is so small that it cannot be represented in an object of the specified type), the function returns zero; whether the integer expression errno acquires the value of the macro ERANGE is implementation-defined. 4.5.2 Trigonometric functions 4.5.2.1 The acos function Synopsis #include double acos(double x); Description 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 The acos function returns the arc cosine in the range [0, PI] radians. 4.5.2.2 The asin function Synopsis #include double asin(double x); Description 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 The asin function returns the arc sine in the range [-PI/2, +PI/2] radians. 4.5.2.3 The atan function Synopsis #include double atan(double x); Description The atan function computes the principal value of the arc tangent of x. Returns The atan function returns the arc tangent in the range [-PI/2, +PI/2] radians. 4.5.2.4 The atan2 function Synopsis #include double atan2(double y, double x); Description The atan2 function computes the principal value of the arc tangent of y/x , using the signs of both arguments to determine the quadrant of the return value. A domain error may occur if both arguments are zero. Returns The atan2 function returns the arc tangent of y/x , in the range [-PI, +PI] radians. 4.5.2.5 The cos function Synopsis #include double cos(double x); Description The cos function computes the cosine of x (measured in radians). A large magnitude argument may yield a result with little or no significance. Returns The cos function returns the cosine value. 4.5.2.6 The sin function Synopsis #include double sin(double x); Description The sin function computes the sine of x (measured in radians). A large magnitude argument may yield a result with little or no significance. Returns The sin function returns the sine value. 4.5.2.7 The tan function Synopsis #include double tan(double x); Description The tan function returns the tangent of x (measured in radians). A large magnitude argument may yield a result with little or no significance. Returns The tan function returns the tangent value. 4.5.3 Hyperbolic functions 4.5.3.1 The cosh function Synopsis #include double cosh(double x); Description The cosh function computes the hyperbolic cosine of x. A range error occurs if the magnitude of x is too large. Returns The cosh function returns the hyperbolic cosine value. 4.5.3.2 The sinh function Synopsis #include double sinh(double x); Description The sinh function computes the hyperbolic sine of x . A range error occurs if the magnitude of x is too large. Returns The sinh function returns the hyperbolic sine value. 4.5.3.3 The tanh function Synopsis #include double tanh(double x); Description The tanh function computes the hyperbolic tangent of x . Returns The tanh function returns the hyperbolic tangent value. 4.5.4 Exponential and logarithmic functions 4.5.4.1 The exp function Synopsis #include double exp(double x); Description The exp function computes the exponential function of x . A range error occurs if the magnitude of x is too large. Returns The exp function returns the exponential value. 4.5.4.2 The frexp function Synopsis #include double frexp(double value, int *exp); Description The frexp function breaks a floating-point number into a normalized fraction and an integral power of 2. It stores the integer in the int object pointed to by exp . Returns The frexp function returns the value x , such that x is a double with magnitude in the interval [1/2, 1) or zero, and value equals x times 2 raised to the power *exp . If value is zero, both parts of the result are zero. 4.5.4.3 The ldexp function Synopsis #include double ldexp(double x, int exp); Description The ldexp function multiplies a floating-point number by an integral power of 2. A range error may occur. Returns The ldexp function returns the value of x times 2 raised to the power exp . 4.5.4.4 The log function Synopsis #include double log(double x); Description The log function computes the natural logarithm of x. A domain error occurs if the argument is negative. A range error occurs if the argument is zero and the logarithm of zero cannot be represented. Returns The log function returns the natural logarithm. 4.5.4.5 The log10 function Synopsis #include double log10(double x); Description The log10 function computes the base-ten logarithm of x . A domain error occurs if the argument is negative. A range error occurs if the argument is zero and the logarithm of zero cannot be represented. Returns The log10 function returns the base-ten logarithm. 4.5.4.6 The modf function Synopsis #include double modf(double value, double *iptr); Description The modf function breaks the argument value into integral and fractional parts, each of which has the same sign as the argument. It stores the integral part as a double in the object pointed to by iptr. Returns The modf function returns the signed fractional part of value . 4.5.5 Power functions 4.5.5.1 The pow function Synopsis #include double pow(double x, double y); Description The pow function computes x raised to the power y . A domain error occurs if x is negative and y is not an integer. A domain error occurs if the result cannot be represented when x is zero and y is less than or equal to zero. A range error may occur. Returns The pow function returns the value of x raised to the power y . 4.5.5.2 The sqrt function Synopsis #include double sqrt(double x); Description The sqrt function computes the nonnegative square root of x . A domain error occurs if the argument is negative. Returns The sqrt function returns the value of the square root. 4.5.6 Nearest integer, absolute value, and remainder functions 4.5.6.1 The ceil function Synopsis #include double ceil(double x); Description The ceil function computes the smallest integral value not less than x . Returns The ceil function returns the smallest integral value not less than x , expressed as a double. 4.5.6.2 The fabs function Synopsis #include double fabs(double x); Description The fabs function computes the absolute value of a floating-point number x . Returns The fabs function returns the absolute value of x. 4.5.6.3 The floor function Synopsis #include double floor(double x); Description The floor function computes the largest integral value not greater than x . Returns The floor function returns the largest integral value not greater than x , expressed as a double. 4.5.6.4 The fmod function Synopsis #include double fmod(double x, double y); Description The fmod function computes the floating-point remainder of x/y . Returns The fmod function returns the value x i y , for some integer i such that, if y is nonzero, the result has the same sign as x and magnitude less than the magnitude of y . If y is zero, whether a domain error occurs or the fmod function returns zero is implementation-defined. 4.6 NON-LOCAL JUMPS The header defines the macro setjmp , and declares one function and one type, for bypassing the normal function call and return discipline./95/ The type declared is jmp_buf which is an array type suitable for holding the information needed to restore a calling environment. It is unspecified whether setjmp is a macro or an identifier declared with external linkage. If a macro definition is suppressed in order to access an actual function, or a program defines an external identifier with the name setjmp , the behavior is undefined. 4.6.1 Save calling environment 4.6.1.1 The setjmp macro Synopsis #include int setjmp(jmp_buf env); Description The setjmp macro saves its calling environment in its jmp_buf argument for later use by the longjmp function. Returns If the return is from a direct invocation, the setjmp macro returns the value zero. If the return is from a call to the longjmp function, the setjmp macro returns a nonzero value. "Environmental constraint" An invocation of the setjmp macro shall appear only in one of the following contexts: * the entire controlling expression of a selection or iteration statement; * one operand of a relational or equality operator with the other operand an integral constant expression, with the resulting expression being the entire controlling expression of a selection or iteration statement; * the operand of a unary ! operator with the resulting expression being the entire controlling expression of a selection or iteration statement; or * the entire expression of an expression statement (possibly cast to void). 4.6.2 Restore calling environment 4.6.2.1 The longjmp function Synopsis #include void longjmp(jmp_buf env, int val); Description The longjmp function restores the environment saved by the most recent invocation of the setjmp macro in the same invocation of the program, with the corresponding jmp_buf argument. If there has been no such invocation, or if the function containing the invocation of the setjmp macro has terminated execution/96/ in the interim, the behavior is undefined. All accessible objects have values as of the time longjmp was called, except that the values of objects of automatic storage duration that do not have volatile type and have been changed between the setjmp invocation and longjmp call are indeterminate. As it bypasses the usual function call and return mechanisms, the longjmp function shall execute correctly in contexts of interrupts, signals and any of their associated functions. However, if the longjmp function is invoked from a nested signal handler (that is, from a function invoked as a result of a signal raised during the handling of another signal), the behavior is undefined. Returns After longjmp is completed, program execution continues as if the corresponding invocation of the setjmp macro had just returned the value specified by val . The longjmp function cannot cause the setjmp macro to return the value 0; if val is 0, the setjmp macro returns the value 1. 4.7 SIGNAL HANDLING The header declares a type and two functions and defines several macros, for handling various signals (conditions that may be reported during program execution). The type defined is sig_atomic_t which is the integral type of an object that can be accessed as an atomic entity, even in the presence of asynchronous interrupts. The macros defined are SIG_DFL SIG_ERR SIG_IGN which expand to distinct constant expressions that have type compatible with the second argument to and the return value of the signal function, and whose value compares unequal to the address of any declarable function; and the following, each of which expands to a positive integral constant expression that is the signal number corresponding to the specified condition: SIGABRT abnormal termination, such as is initiated by the abort function SIGFPE an erroneous arithmetic operation, such as zero divide or an operation resulting in overflow SIGILL detection of an invalid function image, such as an illegal instruction SIGINT receipt of an interactive attention signal SIGSEGV an invalid access to storage SIGTERM a termination request sent to the program An implementation need not generate any of these signals, except as a result of explicit calls to the raise function. Additional signals and pointers to undeclarable functions, with macro definitions beginning, respectively, with the letters SIG and an upper-case letter or with SIG_ and an upper-case letter,/97/ may also be specified by the implementation. The complete set of signals, their semantics, and their default handling is implementation-defined; all signal values shall be positive. 4.7.1 Specify signal handling 4.7.1.1 The signal function Synopsis #include void (*signal(int sig, void (*func)(int)))(int); Description The signal function chooses one of three ways in which receipt of the signal number sig is to be subsequently handled. If the value of func is SIG_DFL , default handling for that signal will occur. If the value of func is SIG_IGN , the signal will be ignored. Otherwise, func shall point to a function to be called when that signal occurs. Such a function is called a signal handler . When a signal occurs, if func points to a function, first the equivalent of signal(sig, SIG_DFL); is executed or an implementation-defined blocking of the signal is performed. (If the value of sig is SIGILL, whether the reset to SIG_DFL occurs is implementation-defined.) Next the equivalent of (*func)(sig); is executed. The function func may terminate by executing a return statement or by calling the abort , exit , or longjmp function. If func executes a return statement and the value of sig was SIGFPE or any other implementation-defined value corresponding to a computational exception, the behavior is undefined. Otherwise, the program will resume execution at the point it was interrupted. If the signal occurs other than as the result of calling the abort or raise function, the behavior is undefined if the signal handler calls any function in the standard library other than the signal function itself or refers to any object with static storage duration other than by assigning a value to a static storage duration variable of type volatile sig_atomic_t . Furthermore, if such a call to the signal function results in a SIG_ERR return, the value of errno is indeterminate. At program startup, the equivalent of signal(sig, SIG_IGN); may be executed for some signals selected in an implementation-defined manner; the equivalent of signal(sig, SIG_DFL); is executed for all other signals defined by the implementation. The implementation shall behave as if no library function calls the signal function. Returns If the request can be honored, the signal function returns the value of func for the most recent call to signal for the specified signal sig . Otherwise, a value of SIG_ERR is returned and a positive value is stored in errno . Forward references: the abort function ($4.10.4.1). 4.7.2 Send signal 4.7.2.1 The raise function Synopsis #include int raise(int sig); Description The raise function sends the signal sig to the executing program. Returns The raise function returns zero if successful, nonzero if unsuccessful. 4.8 VARIABLE ARGUMENTS The header declares a type and defines three macros, for advancing through a list of arguments whose number and types are not known to the called function when it is translated. A function may be called with a variable number of arguments of varying types. As described in $3.7.1, its parameter list contains one or more parameters. The rightmost parameter plays a special role in the access mechanism, and will be designated parmN in this description. The type declared is va_list which is a type suitable for holding information needed by the macros va_start , va_arg , and va_end . If access to the varying arguments is desired, the called function shall declare an object (referred to as ap in this section) having type va_list . The object ap may be passed as an argument to another function; if that function invokes the va_arg macro with parameter ap , the value of ap in the calling function is indeterminate and shall be passed to the va_end macro prior to any further reference to ap . 4.8.1 Variable argument list access macros The va_start and va_arg macros described in this section shall be implemented as macros, not as actual functions. It is unspecified whether va_end is a macro or an identifier declared with external linkage. If a macro definition is suppressed in order to access an actual function, or a program defines an external identifier with the name va_end , the behavior is undefined. The va_start and va_end macros shall be invoked in the function accepting a varying number of arguments, if access to the varying arguments is desired. 4.8.1.1 The va_start macro Synopsis #include void va_start(va_list ap, parmN); Description The va_start macro shall be invoked before any access to the unnamed arguments. The va_start macro initializes ap for subsequent use by va_arg and va_end . The parameter parmN is the identifier of the rightmost parameter in the variable parameter list in the function definition (the one just before the , ... ). If the parameter parmN is declared with the register storage class, with a function or array type, or with a type that is not compatible with the type that results after application of the default argument promotions, the behavior is undefined. Returns The va_start macro returns no value. 4.8.1.2 The va_arg macro Synopsis #include type va_arg(va_list ap, type); Description The va_arg macro expands to an expression that has the type and value of the next argument in the call. The parameter ap shall be the same as the va_list ap initialized by va_start . Each invocation of va_arg modifies ap so that the values of successive arguments are returned in turn. The parameter type is a type name specified such that the type of a pointer to an object that has the specified type can be obtained simply by postfixing a * to type . If there is no actual next argument, or if type is not compatible with the type of the actual next argument (as promoted according to the default argument promotions), the behavior is undefined. Returns The first invocation of the va_arg macro after that of the va_start macro returns the value of the argument after that specified by parmN. Successive invocations return the values of the remaining arguments in succession. 4.8.1.3 The va_end macro Synopsis #include void va_end(va_list ap); Description The va_end macro facilitates a normal return from the function whose variable argument list was referred to by the expansion of va_start that initialized the va_list ap . The va_end macro may modify ap so that it is no longer usable (without an intervening invocation of va_start ). If there is no corresponding invocation of the va_start macro, or if the va_end macro is not invoked before the return, the behavior is undefined. Returns The va_end macro returns no value. Example The function f1 gathers into an array a list of arguments that are pointers to strings (but not more than MAXARGS arguments), then passes the array as a single argument to function f2 . The number of pointers is specified by the first argument to f1 . #include #define MAXARGS 31 void f1(int n_ptrs, ...) { va_list ap; char *array[MAXARGS]; int ptr_no = 0; if (n_ptrs > MAXARGS) n_ptrs = MAXARGS; va_start(ap, n_ptrs); while (ptr_no < n_ptrs) array[ptr_no++] = va_arg(ap, char *); va_end(ap); f2(n_ptrs, array); } Each call to f1 shall have visible the definition of the function or a declaration such as void f1(int, ...); 4.9 INPUT/OUTPUT 4.9.1 Introduction The header declares three types, several macros, and many functions for performing input and output. The types declared are size_t (described in $4.1.5); FILE which is an object type capable of recording all the information needed to control a stream, including its file position indicator, a pointer to its associated buffer, an error indicator that records whether a read/write error has occurred, and an end-of-file indicator that records whether the end of the file has been reached; and fpos_t which is an object type capable of recording all the information needed to specify uniquely every position within a file. The macros are NULL (described in $4.1.5); _IOFBF _IOLBF _IONBF which expand to distinct integral constant expressions, suitable for use as the third argument to the setvbuf function; BUFSIZ which expands to an integral constant expression, which is the size of the buffer used by the setbuf function; EOF which expands to a negative integral constant expression that is returned by several functions to indicate end-of-file ,that is, no more input from a stream; FOPEN_MAX which expands to an integral constant expression that is the minimum number of files that the implementation guarantees can be open simultaneously; FILENAME_MAX which expands to an integral constant expression that is the maximum length for a file name string that the implementation guarantees can be opened;/98/ L_tmpnam which expands to an integral constant expression that is the size of an array of char large enough to hold a temporary file name string generated by the tmpnam function; SEEK_CUR SEEK_END SEEK_SET which expand to distinct integral constant expressions, suitable for use as the third argument to the fseek function; TMP_MAX which expands to an integral constant expression that is the minimum number of unique file names that shall be generated by the tmpnam function; stderr stdin stdout which are expressions of type ``pointer to FILE '' that point to the FILE objects associated, respectively, with the standard error, input, and output streams. Forward references: files ($4.9.3), the fseek function ($4.9.9.2), streams ($4.9.2), the tmpnam function ($4.9.4.4). 4.9.2 Streams Input and output, whether to or from physical devices such as terminals and tape drives, or whether to or from files supported on structured storage devices, are mapped into logical data streams ,whose properties are more uniform than their various inputs and outputs. Two forms of mapping are supported, for text streams and for binary streams ./99/ A text stream is an ordered sequence of characters composed into lines , each line consisting of zero or more characters plus a terminating new-line character. Whether the last line requires a terminating new-line character is implementation-defined. Characters may have to be added, altered, or deleted on input and output to conform to differing conventions for representing text in the host environment. Thus, there need not be a one-to-one correspondence between the characters in a stream and those in the external representation. Data read in from a text stream will necessarily compare equal to the data that were earlier written out to that stream only if: the data consist only of printable characters and the control characters horizontal tab and new-line; no new-line character is immediately preceded by space characters; and the last character is a new-line character. Whether space characters that are written out immediately before a new-line character appear when read in is implementation-defined. A binary stream is an ordered sequence of characters that can transparently record internal data. Data read in from a binary stream shall compare equal to the data that were earlier written out to that stream, under the same implementation. Such a stream may, however, have an implementation-defined number of null characters appended. "Environmental limits" An implementation shall support text files with lines containing at least 254 characters, including the terminating new-line character. The value of the macro BUFSIZ shall be at least 256. 4.9.3 Files A stream is associated with an external file (which may be a physical device) by opening a file, which may involve creating a new file. Creating an existing file causes its former contents to be discarded, if necessary, so that it appears as if newly created. If a file can support positioning requests (such as a disk file, as opposed to a terminal), then a file position indicator /100/ associated with the stream is positioned at the start (character number zero) of the file, unless the file is opened with append mode in which case it is implementation-defined whether the file position indicator is positioned at the beginning or the end of the file. The file position indicator is maintained by subsequent reads, writes, and positioning requests, to facilitate an orderly progression through the file. All input takes place as if characters were read by successive calls to the fgetc function; all output takes place as if characters were written by successive calls to the fputc function. Binary files are not truncated, except as defined in $4.9.5.3. Whether a write on a text stream causes the associated file to be truncated beyond that point is implementation-defined. When a stream is unbuffered, characters are intended to appear from the source or at the destination as soon as possible. Otherwise characters may be accumulated and transmitted to or from the host environment as a block. When a stream is fully buffered, characters are intended to be transmitted to or from the host environment as a block when a buffer is filled. When a stream is line buffered, characters are intended to be transmitted to or from the host environment as a block when a new-line character is encountered. Furthermore, characters are intended to be transmitted as a block to the host environment when a buffer is filled, when input is requested on an unbuffered stream, or when input is requested on a line buffered stream that requires the transmission of characters from the host environment. Support for these characteristics is implementation-defined, and may be affected via the setbuf and setvbuf functions. A file may be disassociated from its controlling stream by closing the file. Output streams are flushed (any unwritten buffer contents are transmitted to the host environment) before the stream is disassociated from the file. The value of a pointer to a FILE object is indeterminate after the associated file is closed (including the standard text streams). Whether a file of zero length (on which no characters have been written by an output stream) actually exists is implementation-defined. The file may be subsequently reopened, by the same or another program execution, and its contents reclaimed or modified (if it can be repositioned at its start). If the main function returns to its original caller, or if the exit function is called, all open files are closed (hence all output streams are flushed) before program termination. Other paths to program termination, such as calling the abort function, need not close all files properly. The address of the FILE object used to control a stream may be significant; a copy of a FILE object may not necessarily serve in place of the original. At program startup, three text streams are predefined and need not be opened explicitly --- standard input (for reading conventional input), standard output (for writing conventional output), and standard error (for writing diagnostic output). When opened, the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device. Functions that open additional (nontemporary) files require a file name, which is a string. The rules for composing valid file names are implementation-defined. Whether the same file can be simultaneously open multiple times is also implementation-defined. "Environmental limits" The value of the macro FOPEN_MAX shall be at least eight, including the three standard text streams. Forward references: the exit function ($4.10.4.3), the fgetc function ($4.9.7.1), the fopen function ($4.9.5.3), the fputc function ($4.9.7.3), the setbuf function ($4.9.5.5), the setvbuf function ($4.9.5.6). 4.9.4 Operations on files 4.9.4.1 The remove function Synopsis #include int remove(const char *filename); Description The remove function causes the file whose name is the string pointed to by filename to be no longer accessible by that name. A subsequent attempt to open that file using that name will fail, unless it is created anew. If the file is open, the behavior of the remove function is implementation-defined. Returns The remove function returns zero if the operation succeeds, nonzero if it fails. 4.9.4.2 The rename function Synopsis #include int rename(const char *old, const char *new); Description The rename function causes the file whose name is the string pointed to by old to be henceforth known by the name given by the string pointed to by new . The file named old is effectively removed. If a file named by the string pointed to by new exists prior to the call to the rename function, the behavior is implementation-defined. Returns The rename function returns zero if the operation succeeds, nonzero if it fails,/101/ in which case if the file existed previously it is still known by its original name. 4.9.4.3 The tmpfile function Synopsis #include FILE *tmpfile(void); Description The tmpfile function creates a temporary binary file that will automatically be removed when it is closed or at program termination. If the program terminates abnormally, whether an open temporary file is removed is implementation-defined. The file is opened for update with wb+ mode. Returns The tmpfile function returns a pointer to the stream of the file that it created. If the file cannot be created, the tmpfile function returns a null pointer. Forward references: the fopen function ($4.9.5.3). 4.9.4.4 The tmpnam function Synopsis #include char *tmpnam(char *s); Description The tmpnam function generates a string that is a valid file name and that is not the same as the name of an existing file./102/ The tmpnam function generates a different string each time it is called, up to TMP_MAX times. If it is called more than TMP_MAX times, the behavior is implementation-defined. The implementation shall behave as if no library function calls the tmpnam function. Returns If the argument is a null pointer, the tmpnam function leaves its result in an internal static object and returns a pointer to that object. Subsequent calls to the tmpnam function may modify the same object. If the argument is not a null pointer, it is assumed to point to an array of at least L_tmpnam char s; the tmpnam function writes its result in that array and returns the argument as its value. "Environmental limits" The value of the macro TMP_MAX shall be at least 25. 4.9.5 File access functions 4.9.5.1 The fclose function Synopsis #include int fclose(FILE *stream); Description The fclose function causes the stream pointed to by stream to be flushed and the associated file to be closed. Any unwritten buffered data for the stream are delivered to the host environment to be written to the file; any unread buffered data are discarded. The stream is disassociated from the file. If the associated buffer was automatically allocated, it is deallocated. Returns The fclose function returns zero if the stream was successfully closed, or EOF if any errors were detected. 4.9.5.2 The fflush function Synopsis #include int fflush(FILE *stream); Description If stream points to an output stream or an update stream in which the most recent operation was output, the fflush function causes any unwritten data for that stream to be delivered to the host environment to be written to the file; otherwise, the behavior is undefined. If stream is a null pointer, the fflush function performs this flushing action on all streams for which the behavior is defined above. Returns The fflush function returns EOF if a write error occurs, otherwise zero. Forward references: the ungetc function ($4.9.7.11). 4.9.5.3 The fopen function Synopsis #include FILE *fopen(const char *filename, const char *mode); Description The fopen function opens the file whose name is the string pointed to by filename , and associates a stream with it. The argument mode points to a string beginning with one of the following sequences:/103/ "r" open text file for reading "w" truncate to zero length or create text file for writing "a" append; open or create text file for writing at end-of-file "rb" open binary file for reading "wb" truncate to zero length or create binary file for writing "ab" append; open or create binary file for writing at end-of-file "r+" open text file for update (reading and writing) "w+" truncate to zero length or create text file for update "a+" append; open or create text file for update, writing at end-of-file "r+b" or "rb+" open binary file for update (reading and writing) "w+b" or "wb+" truncate to zero length or create binary file for update "a+b" or "ab+" append; open or create binary file for update, writing at end-of-file Opening a file with read mode ('r' as the first character in the mode argument) fails if the file does not exist or cannot be read. Opening a file with append mode ('a' as the first character in the mode argument) causes all subsequent writes to the file to be forced to the then current end-of-file, regardless of intervening calls to the fseek function. In some implementations, opening a binary file with append mode ('b' as the second or third character in the mode argument) may initially position the file position indicator for the stream beyond the last data written, because of null character padding. When a file is opened with update mode ('+' as the second or third character in the mode argument), both input and output may be performed on the associated stream. However, output may not be directly followed by input without an intervening call to the fflush function or to a file positioning function ( fseek , fsetpos , or rewind ), and input may not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file. Opening a file with update mode may open or create a binary stream in some implementations. When opened, a stream is fully buffered if and only if it can be determined not to refer to an interactive device. The error and end-of-file indicators for the stream are cleared. Returns The fopen function returns a pointer to the object controlling the stream. If the open operation fails, fopen returns a null pointer. Forward references: file positioning functions ($4.9.9). 4.9.5.4 The freopen function Synopsis #include FILE *freopen(const char *filename, const char *mode, FILE *stream); Description The freopen function opens the file whose name is the string pointed to by filename and associates the stream pointed to by stream with it. The mode argument is used just as in the fopen function./104/ The freopen function first attempts to close any file that is associated with the specified stream. Failure to close the file successfully is ignored. The error and end-of-file indicators for the stream are cleared. Returns The freopen function returns a null pointer if the open operation fails. Otherwise, freopen returns the value of stream . 4.9.5.5 The setbuf function Synopsis #include void setbuf(FILE *stream, char *buf); Description Except that it returns no value, the setbuf function is equivalent to the setvbuf function invoked with the values _IOFBF for mode and BUFSIZ for size , or (if buf is a null pointer), with the value _IONBF for mode . Returns The setbuf function returns no value. Forward references: the setvbuf function ($4.9.5.6). 4.9.5.6 The setvbuf function Synopsis #include int setvbuf(FILE *stream, char *buf, int mode, size_t size); Description The setvbuf function may be used after the stream pointed to by stream has been associated with an open file but before any other operation is performed on the stream. The argument mode determines how stream will be buffered, as follows: _IOFBF causes input/output to be fully buffered; _IOLBF causes output to be line buffered; _IONBF causes input/output to be unbuffered. If buf is not a null pointer, the array it points to may be used instead of a buffer allocated by the setvbuf function./105/ The argument size specifies the size of the array. The contents of the array at any time are indeterminate. Returns The setvbuf function returns zero on success, or nonzero if an invalid value is given for mode or if the request cannot be honored. 4.9.6 Formatted input/output functions 4.9.6.1 The fprintf function Synopsis #include int fprintf(FILE *stream, const char *format, ...); Description The fprintf function writes output to the stream pointed to by stream , under control of the string pointed to by format that specifies how subsequent arguments are converted for output. If there are insufficient arguments for the format, the behavior is undefined. If the format is exhausted while arguments remain, the excess arguments are evaluated (as always) but are otherwise ignored. The fprintf function returns when the end of the format string is encountered. The format shall be a multibyte character sequence, beginning and ending in its initial shift state. The format is composed of zero or more directives: ordinary multibyte characters (not % ), which are copied unchanged to the output stream; and conversion specifications, each of which results in fetching zero or more subsequent arguments. Each conversion specification is introduced by the character % . After the % , the following appear in sequence: * Zero or more flags that modify the meaning of the conversion specification. * An optional decimal integer specifying a minimum field width ./106/ If the converted value has fewer characters than the field width, it will be padded with spaces on the left (or right, if the left adjustment flag, described later, has been given) to the field width. * An optional precision that gives the minimum number of digits to appear for the d , i , o , u , x , and X conversions, the number of digits to appear after the decimal-point character for e , E , and f conversions, the maximum number of significant digits for the g and G conversions, or the maximum number of characters to be written from a string in s conversion. The precision takes the form of a period (.) followed by an optional decimal integer; if the integer is omitted, it is treated as zero. * An optional h specifying that a following d , i , o , u , x , or X conversion specifier applies to a short int or unsigned short int argument (the argument will have been promoted according to the integral promotions, and its value shall be converted to short int or unsigned short int before printing); an optional h specifying that a following n conversion specifier applies to a pointer to a short int argument; an optional l (ell) specifying that a following d , i , o , u , x , or X conversion specifier applies to a long int or unsigned long int argument; an optional l specifying that a following n conversion specifier applies to a pointer to a long int argument; or an optional L specifying that a following e , E , f , g , or G conversion specifier applies to a long double argument. If an h , l , or L appears with any other conversion specifier, the behavior is undefined. * A character that specifies the type of conversion to be applied. A field width or precision, or both, may be indicated by an asterisk * instead of a digit string. In this case, an int argument supplies the field width or precision. The arguments specifying field width or precision, or both, shall appear (in that order) before the argument (if any) to be converted. A negative field width argument is taken as a - flag followed by a positive field width. A negative precision argument is taken as if it were missing. The flag characters and their meanings are - The result of the conversion will be left-justified within the field. + The result of a signed conversion will always begin with a plus or minus sign. space If the first character of a signed conversion is not a sign, or if a signed conversion results in no characters, a space will be prepended to the result. If the space and + flags both appear, the space flag will be ignored. # The result is to be converted to an ``alternate form.'' For o conversion, it increases the precision to force the first digit of the result to be a zero. For x (or X ) conversion, a nonzero result will have 0x (or 0X ) prepended to it. For e , E , f , g , and G conversions, the result will always contain a decimal-point character, even if no digits follow it (normally, a decimal-point character appears in the result of these conversions only if a digit follows it). For g and G conversions, trailing zeros will not be removed from the result. For other conversions, the behavior is undefined. 0 For d, i, o, u, x, X, e, E, f, g and G conversions, leading zeros (following any indication of sign or base) are used to pad to the field width; no space padding is performed. If the 0 and - flags both appear, the 0 flag will be ignored. For d, i, o, u, x and X conversions, if a precision is specified, the 0 flag will be ignored. For other conversions, the behavior is undefined. The conversion specifiers and their meanings are d, i, o, u, x, X The int argument is converted to signed decimal ( d or i ), unsigned octal ( o ), unsigned decimal ( u ), or unsigned hexadecimal notation ( x or X ); the letters abcdef are used for x conversion and the letters ABCDEF for X conversion. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it will be expanded with leading zeros. The default precision is 1. The result of converting a zero value with an explicit precision of zero is no characters. f The double argument is converted to decimal notation in the style [-]ddd.ddd , where the number of digits after the decimal-point character is equal to the precision specification. If the precision is missing, it is taken as 6; if the precision is explicitly zero, no decimal-point character appears. If a decimal-point character appears, at least one digit appears before it. The value is rounded to the appropriate number of digits. e, E The double argument is converted in the style [-]d.ddde+- dd , where there is one digit before the decimal-point character (which is nonzero if the argument is nonzero) and the number of digits after it is equal to the precision; if the precision is missing, it is taken as 6; if the precision is zero, no decimal-point character appears. The value is rounded to the appropriate number of digits. The E conversion specifier will produce a number with E instead of e introducing the exponent. The exponent always contains at least two digits. If the value is zero, the exponent is zero. g, G The double argument is converted in style f or e (or in style E in the case of a G conversion specifier), with the precision specifying the number of significant digits. If an explicit precision is zero, it is taken as 1. The style used depends on the value converted; style e will be used only if the exponent resulting from such a conversion is less than -4 or greater than or equal to the precision. Trailing zeros are removed from the fractional portion of the result; a decimal-point character appears only if it is followed by a digit. c The int argument is converted to an unsigned char , and the resulting character is written. s The argument shall be a pointer to an array of character type./107/ Characters from the array are written up to (but not including) a terminating null character; if the precision is specified, no more than that many characters are written. If the precision is not specified or is greater than the size of the array, the array shall contain a null character. p The argument shall be a pointer to void . The value of the pointer is converted to a sequence of printable characters, in an implementation-defined manner. n The argument shall be a pointer to an integer into which is written the number of characters written to the output stream so far by this call to fprintf . No argument is converted. % A % is written. No argument is converted. The complete conversion specification shall be %% . If a conversion specification is invalid, the behavior is undefined./108/ If any argument is, or points to, a union or an aggregate (except for an array of character type using %s conversion, or a pointer cast to be a pointer to void using %p conversion), the behavior is undefined. In no case does a nonexistent or small field width cause truncation of a field; if the result of a conversion is wider than the field width, the field is expanded to contain the conversion result. Returns The fprintf function returns the number of characters transmitted, or a negative value if an output error occurred. "Environmental limit" The minimum value for the maximum number of characters produced by any single conversion shall be 509. Examples To print a date and time in the form ``Sunday, July 3, 10:02,'' where weekday and month are pointers to strings: #include fprintf(stdout, "%s, %s %d, %.2d:%.2d\n", weekday, month, day, hour, min); To print PI to five decimal places: #include #include fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0)); 4.9.6.2 The fscanf function Synopsis #include int fscanf(FILE *stream, const char *format, ...); Description The fscanf function reads input from the stream pointed to by stream , under control of the string pointed to by format that specifies the admissible input sequences and how they are to be converted for assignment, using subsequent arguments as pointers to the objects to receive the converted input. If there are insufficient arguments for the format, the behavior is undefined. If the format is exhausted while arguments remain, the excess arguments are evaluated (as always) but are otherwise ignored. The format shall be a multibyte character sequence, beginning and ending in its initial shift state. The format is composed of zero or more directives: one or more white-space characters; an ordinary multibyte character (not % ); or a conversion specification. Each conversion specification is introduced by the character % . After the %, the following appear in sequence: * An optional assignment-suppressing character * . * An optional decimal integer that specifies the maximum field width. * An optional h , l (ell) or L indicating the size of the receiving object. The conversion specifiers d , i , and n shall be preceded by h if the corresponding argument is a pointer to short int rather than a pointer to int , or by l if it is a pointer to long int . Similarly, the conversion specifiers o , u , and x shall be preceded by h if the corresponding argument is a pointer to unsigned short int rather than a pointer to unsigned int , or by l if it is a pointer to unsigned long int . Finally, the conversion specifiers e , f , and g shall be preceded by l if the corresponding argument is a pointer to double rather than a pointer to float , or by L if it is a pointer to long double . If an h , l , or L appears with any other conversion specifier, the behavior is undefined. * A character that specifies the type of conversion to be applied. The valid conversion specifiers are described below. The fscanf function executes each directive of the format in turn. If a directive fails, as detailed below, the fscanf function returns. Failures are described as input failures (due to the unavailability of input characters), or matching failures (due to inappropriate input). A directive composed of white space is executed by reading input up to the first non-white-space character (which remains unread), or until no more characters can be read. A directive that is an ordinary multibyte character is executed by reading the next characters of the stream. If one of the characters differs from one comprising the directive, the directive fails, and the differing and subsequent characters remain unread. A directive that is a conversion specification defines a set of matching input sequences, as described below for each specifier. A conversion specification is executed in the following steps: Input white-space characters (as specified by the isspace function) are skipped, unless the specification includes a [ , c , or n specifier. An input item is read from the stream, unless the specification includes an n specifier. An input item is defined as the longest sequence of input characters (up to any specified maximum field width) which is an initial subsequence of a matching sequence. The first character, if any, after the input item remains unread. If the length of the input item is zero, the execution of the directive fails: this condition is a matching failure, unless an error prevented input from the stream, in which case it is an input failure. Except in the case of a % specifier, the input item (or, in the case of a %n directive, the count of input characters) is converted to a type appropriate to the conversion specifier. If the input item is not a matching sequence, the execution of the directive fails: this condition is a matching failure. Unless assignment suppression was indicated by a * , the result of the conversion is placed in the object pointed to by the first argument following the format argument that has not already received a conversion result. If this object does not have an appropriate type, or if the result of the conversion cannot be represented in the space provided, the behavior is undefined. The following conversion specifiers are valid: d Matches an optionally signed decimal integer, whose format is the same as expected for the subject sequence of the strtol function with the value 10 for the base argument. The corresponding argument shall be a pointer to integer. i Matches an optionally signed integer, whose format is the same as expected for the subject sequence of the strtol function with the value 0 for the base argument. The corresponding argument shall be a pointer to integer. o Matches an optionally signed octal integer, whose format is the same as expected for the subject sequence of the strtoul function with the value 8 for the base argument. The corresponding argument shall be a pointer to unsigned integer. u Matches an optionally signed decimal integer, whose format is the same as expected for the subject sequence of the strtoul function with the value 10 for the base argument. The corresponding argument shall be a pointer to unsigned integer. x Matches an optionally signed hexadecimal integer, whose format is the same as expected for the subject sequence of the strtoul function with the value 16 for the base argument. The corresponding argument shall be a pointer to unsigned integer. e,f,g Matches an optionally signed floating-point number, whose format is the same as expected for the subject string of the strtod function. The corresponding argument shall be a pointer to floating. s Matches a sequence of non-white-space characters. The corresponding argument shall be a pointer to the initial character of an array large enough to accept the sequence and a terminating null character, which will be added automatically. [ Matches a nonempty sequence of characters from a set of expected characters (the scanset ). The corresponding argument shall be a pointer to the initial character of an array large enough to accept the sequence and a terminating null character, which will be added automatically. The conversion specifier includes all subsequent characters in the format string, up to and including the matching right bracket ( ] ). The characters between the brackets (the scanlist ) comprise the scanset, unless the character after the left bracket is a circumflex ( ^ ), in which case the scanset contains all characters that do not appear in the scanlist between the circumflex and the right bracket. As a special case, if the conversion specifier begins with [] or [^] , the right bracket character is in the scanlist and the next right bracket character is the matching right bracket that ends the specification. If a - character is in the scanlist and is not the first, nor the second where the first character is a ^ , nor the last character, the behavior is implementation-defined. c Matches a sequence of characters of the number specified by the field width (1 if no field width is present in the directive). The corresponding argument shall be a pointer to the initial character of an array large enough to accept the sequence. No null character is added. p Matches an implementation-defined set of sequences, which should be the same as the set of sequences that may be produced by the %p conversion of the fprintf function. The corresponding argument shall be a pointer to a pointer to void . The interpretation of the input item is implementation-defined; however, for any input item other than a value converted earlier during the same program execution, the behavior of the %p conversion is undefined. n No input is consumed. The corresponding argument shall be a pointer to integer into which is to be written the number of characters read from the input stream so far by this call to the fscanf function. Execution of a %n directive does not increment the assignment count returned at the completion of execution of the fscanf function. % Matches a single % ; no conversion or assignment occurs. The complete conversion specification shall be %% . If a conversion specification is invalid, the behavior is undefined./110/ The conversion specifiers E , G , and X are also valid and behave the same as, respectively, e , g , and x . If end-of-file is encountered during input, conversion is terminated. If end-of-file occurs before any characters matching the current directive have been read (other than leading white space, where permitted), execution of the current directive terminates with an input failure; otherwise, unless execution of the current directive is terminated with a matching failure, execution of the following directive (if any) is terminated with an input failure. If conversion terminates on a conflicting input character, the offending input character is left unread in the input stream. Trailing white space (including new-line characters) is left unread unless matched by a directive. The success of literal matches and suppressed assignments is not directly determinable other than via the %n directive. Returns The fscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the fscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure. Examples The call: #include int n, i; float x; char name[50]; n = fscanf(stdin, "%d%f%s", &i, &x, name); with the input line: 25 54.32E-1 thompson will assign to n the value 3, to i the value 25, to x the value 5.432, and name will contain thompson\0 . Or: #include int i; float x; char name[50]; fscanf(stdin, "%2d%f%*d %[0123456789]", &i, &x, name); with input: 56789 0123 56a72 will assign to i the value 56 and to x the value 789.0, will skip 0123, and name will contain 56\0 . The next character read from the input stream will be a . To accept repeatedly from stdin a quantity, a unit of measure and an item name: #include int count; float quant; char units[21], item[21]; while (!feof(stdin) && !ferror(stdin)) { count = fscanf(stdin, "%f%20s of %20s", &quant, units, item); fscanf(stdin,"%*[^\n]"); } If the stdin stream contains the following lines: 2 quarts of oil -12.8degrees Celsius lots of luck 10.0LBS of fertilizer 100ergs of energy the execution of the above example will be equivalent to the following assignments: quant = 2; strcpy(units, "quarts"); strcpy(item, "oil"); count = 3; quant = -12.8; strcpy(units, "degrees"); count = 2; /* "C" fails to match "o" */ count = 0; /* "l" fails to match "%f" */ quant = 10.0; strcpy(units, "LBS"); strcpy(item, "fertilizer"); count = 3; count = 0; /* "100e" fails to match "%f" */ count = EOF; Forward references: the strtod function ($4.10.1.4), the strtol function ($4.10.1.5), the strtoul function ($4.10.1.6). 4.9.6.3 The printf function Synopsis #include int printf(const char *format, ...); Description The printf function is equivalent to fprintf with the argument stdout interposed before the arguments to printf . Returns The printf function returns the number of characters transmitted, or a negative value if an output error occurred. 4.9.6.4 The scanf function Synopsis #include int scanf(const char *format, ...); Description The scanf function is equivalent to fscanf with the argument stdin interposed before the arguments to scanf . Returns The scanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the scanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure. 4.9.6.5 The sprintf function Synopsis #include int sprintf(char *s, const char *format, ...); Description The sprintf function is equivalent to fprintf , except that the argument s specifies an array into which the generated output is to be written, rather than to a stream. A null character is written at the end of the characters written; it is not counted as part of the returned sum. If copying takes place between objects that overlap, the behavior is undefined. Returns The sprintf function returns the number of characters written in the array, not counting the terminating null character. 4.9.6.6 The sscanf function Synopsis #include int sscanf(const char *s, const char *format, ...); Description The sscanf function is equivalent to fscanf , except that the argument s specifies a string from which the input is to be obtained, rather than from a stream. Reaching the end of the string is equivalent to encountering end-of-file for the fscanf function. If copying takes place between objects that overlap, the behavior is undefined. Returns The sscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the sscanf function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure. 4.9.6.7 The vfprintf function Synopsis #include #include int vfprintf(FILE *stream, const char *format, va_list arg); Description The vfprintf function is equivalent to fprintf , with the variable argument list replaced by arg , which has been initialized by the va_start macro (and possibly subsequent va_arg calls). The vfprintf function does not invoke the va_end macro. Returns The vfprintf function returns the number of characters transmitted, or a negative value if an output error occurred. Example The following shows the use of the vfprintf function in a general error-reporting routine. #include #include void error(char *function_name, char *format, ...) { va_list args; va_start(args, format); /* print out name of function causing error */ fprintf(stderr, "ERROR in %s: ", function_name); /* print out remainder of message */ vfprintf(stderr, format, args); va_end(args); } 4.9.6.8 The vprintf function Synopsis #include #include int vprintf(const char *format, va_list arg); Description The vprintf function is equivalent to printf , with the variable argument list replaced by arg , which has been initialized by the va_start macro (and possibly subsequent va_arg calls). The vprintf function does not invoke the va_end macro.rN Returns The vprintf function returns the number of characters transmitted, or a negative value if an output error occurred. 4.9.6.9 The vsprintf function Synopsis #include #include int vsprintf(char *s, const char *format, va_list arg); Description The vsprintf function is equivalent to sprintf , with the variable argument list replaced by arg , which has been initialized by the va_start macro (and possibly subsequent va_arg calls). The vsprintf function does not invoke the va_end macro.rN If copying takes place between objects that overlap, the behavior is undefined. Returns The vsprintf function returns the number of characters written in the array, not counting the terminating null character. 4.9.7 Character input/output functions 4.9.7.1 The fgetc function Synopsis #include int fgetc(FILE *stream); Description The fgetc function obtains the next character (if present) as an unsigned char converted to an int , from the input stream pointed to by stream , and advances the associated file position indicator for the stream (if defined). Returns The fgetc function returns the next character from the input stream pointed to by stream . If the stream is at end-of-file, the end-of-file indicator for the stream is set and fgetc returns EOF . If a read error occurs, the error indicator for the stream is set and fgetc returns EOF ./112/ 4.9.7.2 The fgets function Synopsis #include char *fgets(char *s, int n, FILE *stream); Description The fgets function reads at most one less than the number of characters specified by n from the stream pointed to by stream into the array pointed to by s . No additional characters are read after a new-line character (which is retained) or after end-of-file. A null character is written immediately after the last character read into the array. Returns The fgets function returns s if successful. If end-of-file is encountered and no characters have been read into the array, the contents of the array remain unchanged and a null pointer is returned. If a read error occurs during the operation, the array contents are indeterminate and a null pointer is returned. 4.9.7.3 The fputc function Synopsis #include int fputc(int c, FILE *stream); Description The fputc function writes the character specified by c (converted to an unsigned char ) to the output stream pointed to by stream , at the position indicated by the associated file position indicator for the stream (if defined), and advances the indicator appropriately. If the file cannot support positioning requests, or if the stream was opened with append mode, the character is appended to the output stream. Returns The fputc function returns the character written. If a write error occurs, the error indicator for the stream is set and fputc returns EOF. 4.9.7.4 The fputs function Synopsis #include int fputs(const char *s, FILE *stream); Description The fputs function writes the string pointed to by s to the stream pointed to by stream . The terminating null character is not written. Returns The fputs function returns EOF if a write error occurs; otherwise it returns a nonnegative value. 4.9.7.5 The getc function Synopsis #include int getc(FILE *stream); Description The getc function is equivalent to fgetc , except that if it is implemented as a macro, it may evaluate stream more than once, so the argument should never be an expression with side effects. Returns The getc function returns the next character from the input stream pointed to by stream . If the stream is at end-of-file, the end-of-file indicator for the stream is set and getc returns EOF . If a read error occurs, the error indicator for the stream is set and getc returns EOF . 4.9.7.6 The getchar function Synopsis #include int getchar(void); Description The getchar function is equivalent to getc with the argument stdin . Returns The getchar function returns the next character from the input stream pointed to by stdin . If the stream is at end-of-file, the end-of-file indicator for the stream is set and getchar returns EOF . If a read error occurs, the error indicator for the stream is set and getchar returns EOF . 4.9.7.7 The gets function Synopsis #include char *gets(char *s); Description The gets function reads characters from the input stream pointed to by stdin , into the array pointed to by s , until end-of-file is encountered or a new-line character is read. Any new-line character is discarded, and a null character is written immediately after the last character read into the array. Returns The gets function returns s if successful. If end-of-file is encountered and no characters have been read into the array, the contents of the array remain unchanged and a null pointer is returned. If a read error occurs during the operation, the array contents are indeterminate and a null pointer is returned. 4.9.7.8 The putc function Synopsis #include int putc(int c, FILE *stream); Description The putc function is equivalent to fputc , except that if it is implemented as a macro, it may evaluate stream more than once, so the argument should never be an expression with side effects. Returns The putc function returns the character written. If a write error occurs, the error indicator for the stream is set and putc returns EOF. 4.9.7.9 The putchar function Synopsis #include int putchar(int c); Description The putchar function is equivalent to putc with the second argument stdout. Returns The putchar function returns the character written. If a write error occurs, the error indicator for the stream is set and putchar returns EOF. 4.9.7.10 The puts function Synopsis #include int puts(const char *s); Description The puts function writes the string pointed to by s to the stream pointed to by stdout , and appends a new-line character to the output. The terminating null character is not written. Returns The puts function returns EOF if a write error occurs; otherwise it returns a nonnegative value. 4.9.7.11 The ungetc function Synopsis #include int ungetc(int c, FILE *stream); Description The ungetc function pushes the character specified by c (converted to an unsigned char ) back onto the input stream pointed to by stream. The pushed-back characters will be returned by subsequent reads on that stream in the reverse order of their pushing. A successful intervening call (with the stream pointed to by stream ) to a file positioning function ( fseek , fsetpos , or rewind ) discards any pushed-back characters for the stream. The external storage corresponding to the stream is unchanged. One character of pushback is guaranteed. If the ungetc function is called too many times on the same stream without an intervening read or file positioning operation on that stream, the operation may fail. If the value of c equals that of the macro EOF , the operation fails and the input stream is unchanged. A successful call to the ungetc function clears the end-of-file indicator for the stream. The value of the file position indicator for the stream after reading or discarding all pushed-back characters shall be the same as it was before the characters were pushed back. For a text stream, the value of its file position indicator after a successful call to the ungetc function is unspecified until all pushed-back characters are read or discarded. For a binary stream, its file position indicator is decremented by each successful call to the ungetc function; if its value was zero before a call, it is indeterminate after the call. Returns The ungetc function returns the character pushed back after conversion, or EOF if the operation fails. Forward references: file positioning functions ($4.9.9). 4.9.8 Direct input/output functions 4.9.8.1 The fread function Synopsis #include size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream); Description The fread function reads, into the array pointed to by ptr , up to nmemb members whose size is specified by size , from the stream pointed to by stream . The file position indicator for the stream (if defined) is advanced by the number of characters successfully read. If an error occurs, the resulting value of the file position indicator for the stream is indeterminate. If a partial member is read, its value is indeterminate. Returns The fread function returns the number of members successfully read, which may be less than nmemb if a read error or end-of-file is encountered. If size or nmemb is zero, fread returns zero and the contents of the array and the state of the stream remain unchanged. 4.9.8.2 The fwrite function Synopsis #include size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream); Description The fwrite function writes, from the array pointed to by ptr , up to nmemb members whose size is specified by size , to the stream pointed to by stream . The file position indicator for the stream (if defined) is advanced by the number of characters successfully written. If an error occurs, the resulting value of the file position indicator for the stream is indeterminate. Returns The fwrite function returns the number of members successfully written, which will be less than nmemb only if a write error is encountered. 4.9.9 File positioning functions 4.9.9.1 The fgetpos function Synopsis #include int fgetpos(FILE *stream, fpos_t *pos); Description The fgetpos function stores the current value of the file position indicator for the stream pointed to by stream in the object pointed to by pos . The value stored contains unspecified information usable by the fsetpos function for repositioning the stream to its position at the time of the call to the fgetpos function. Returns If successful, the fgetpos function returns zero; on failure, the fgetpos function returns nonzero and stores an implementation-defined positive value in errno . Forward references: the fsetpos function ($4.9.9.3). 4.9.9.2 The fseek function Synopsis #include int fseek(FILE *stream, long int offset, int whence); Description The fseek function sets the file position indicator for the stream pointed to by stream . For a binary stream, the new position, measured in characters from the beginning of the file, is obtained by adding offset to the position specified by whence. The specified point is the beginning of the file for SEEK_SET, the current value of the file position indicator for SEEK_CUR, or end-of-file for SEEK_END. A binary stream need not meaningfully support fseek calls with a whence value of SEEK_END. For a text stream, either offset shall be zero, or offset shall be a value returned by an earlier call to the ftell function on the same stream and whence shall be SEEK_SET . A successful call to the fseek function clears the end-of-file indicator for the stream and undoes any effects of the ungetc function on the same stream. After an fseek call, the next operation on an update stream may be either input or output. Returns The fseek function returns nonzero only for a request that cannot be satisfied. Forward references: the ftell function ($4.9.9.4). 4.9.9.3 The fsetpos function Synopsis #include int fsetpos(FILE *stream, const fpos_t *pos); Description The fsetpos function sets the file position indicator for the stream pointed to by stream according to the value of the object pointed to by pos , which shall be a value returned by an earlier call to the fgetpos function on the same stream. A successful call to the fsetpos function clears the end-of-file indicator for the stream and undoes any effects of the ungetc function on the same stream. After an fsetpos call, the next operation on an update stream may be either input or output. Returns If successful, the fsetpos function returns zero; on failure, the fsetpos function returns nonzero and stores an implementation-defined positive value in errno . 4.9.9.4 The ftell function Synopsis #include long int ftell(FILE *stream); Description The ftell function obtains the current value of the file position indicator for the stream pointed to by stream . For a binary stream, the value is the number of characters from the beginning of the file. For a text stream, its file position indicator contains unspecified information, usable by the fseek function for returning the file position indicator for the stream to its position at the time of the ftell call; the difference between two such return values is not necessarily a meaningful measure of the number of characters written or read. Returns If successful, the ftell function returns the current value of the file position indicator for the stream. On failure, the ftell function returns -1L and stores an implementation-defined positive value in errno . 4.9.9.5 The rewind function Synopsis #include void rewind(FILE *stream); Description The rewind function sets the file position indicator for the stream pointed to by stream to the beginning of the file. It is equivalent to (void)fseek(stream, 0L, SEEK_SET) except that the error indicator for the stream is also cleared. Returns The rewind function returns no value. 4.9.10 Error-handling functions 4.9.10.1 The clearerr function Synopsis #include void clearerr(FILE *stream); Description The clearerr function clears the end-of-file and error indicators for the stream pointed to by stream . Returns The clearerr function returns no value. 4.9.10.2 The feof function Synopsis #include int feof(FILE *stream); Description The feof function tests the end-of-file indicator for the stream pointed to by stream . Returns The feof function returns nonzero if and only if the end-of-file indicator is set for stream . 4.9.10.3 The ferror function Synopsis #include
