1\input texinfo 2@setfilename cpp.info 3@settitle The C Preprocessor 4@setchapternewpage off 5@c @smallbook 6@c @cropmarks 7@c @finalout 8 9@include gcc-common.texi 10 11@copying 12@c man begin COPYRIGHT 13Copyright @copyright{} 1987-2017 Free Software Foundation, Inc. 14 15Permission is granted to copy, distribute and/or modify this document 16under the terms of the GNU Free Documentation License, Version 1.3 or 17any later version published by the Free Software Foundation. A copy of 18the license is included in the 19@c man end 20section entitled ``GNU Free Documentation License''. 21@ignore 22@c man begin COPYRIGHT 23man page gfdl(7). 24@c man end 25@end ignore 26 27@c man begin COPYRIGHT 28This manual contains no Invariant Sections. The Front-Cover Texts are 29(a) (see below), and the Back-Cover Texts are (b) (see below). 30 31(a) The FSF's Front-Cover Text is: 32 33 A GNU Manual 34 35(b) The FSF's Back-Cover Text is: 36 37 You have freedom to copy and modify this GNU Manual, like GNU 38 software. Copies published by the Free Software Foundation raise 39 funds for GNU development. 40@c man end 41@end copying 42 43@c Create a separate index for command line options. 44@defcodeindex op 45@syncodeindex vr op 46 47@c Used in cppopts.texi and cppenv.texi. 48@set cppmanual 49 50@ifinfo 51@dircategory Software development 52@direntry 53* Cpp: (cpp). The GNU C preprocessor. 54@end direntry 55@end ifinfo 56 57@titlepage 58@title The C Preprocessor 59@versionsubtitle 60@author Richard M. Stallman, Zachary Weinberg 61@page 62@c There is a fill at the bottom of the page, so we need a filll to 63@c override it. 64@vskip 0pt plus 1filll 65@insertcopying 66@end titlepage 67@contents 68@page 69 70@ifnottex 71@node Top 72@top 73The C preprocessor implements the macro language used to transform C, 74C++, and Objective-C programs before they are compiled. It can also be 75useful on its own. 76 77@menu 78* Overview:: 79* Header Files:: 80* Macros:: 81* Conditionals:: 82* Diagnostics:: 83* Line Control:: 84* Pragmas:: 85* Other Directives:: 86* Preprocessor Output:: 87* Traditional Mode:: 88* Implementation Details:: 89* Invocation:: 90* Environment Variables:: 91* GNU Free Documentation License:: 92* Index of Directives:: 93* Option Index:: 94* Concept Index:: 95 96@detailmenu 97 --- The Detailed Node Listing --- 98 99Overview 100 101* Character sets:: 102* Initial processing:: 103* Tokenization:: 104* The preprocessing language:: 105 106Header Files 107 108* Include Syntax:: 109* Include Operation:: 110* Search Path:: 111* Once-Only Headers:: 112* Alternatives to Wrapper #ifndef:: 113* Computed Includes:: 114* Wrapper Headers:: 115* System Headers:: 116 117Macros 118 119* Object-like Macros:: 120* Function-like Macros:: 121* Macro Arguments:: 122* Stringizing:: 123* Concatenation:: 124* Variadic Macros:: 125* Predefined Macros:: 126* Undefining and Redefining Macros:: 127* Directives Within Macro Arguments:: 128* Macro Pitfalls:: 129 130Predefined Macros 131 132* Standard Predefined Macros:: 133* Common Predefined Macros:: 134* System-specific Predefined Macros:: 135* C++ Named Operators:: 136 137Macro Pitfalls 138 139* Misnesting:: 140* Operator Precedence Problems:: 141* Swallowing the Semicolon:: 142* Duplication of Side Effects:: 143* Self-Referential Macros:: 144* Argument Prescan:: 145* Newlines in Arguments:: 146 147Conditionals 148 149* Conditional Uses:: 150* Conditional Syntax:: 151* Deleted Code:: 152 153Conditional Syntax 154 155* Ifdef:: 156* If:: 157* Defined:: 158* Else:: 159* Elif:: 160 161Implementation Details 162 163* Implementation-defined behavior:: 164* Implementation limits:: 165* Obsolete Features:: 166 167Obsolete Features 168 169* Obsolete Features:: 170 171@end detailmenu 172@end menu 173 174@insertcopying 175@end ifnottex 176 177@node Overview 178@chapter Overview 179@c man begin DESCRIPTION 180The C preprocessor, often known as @dfn{cpp}, is a @dfn{macro processor} 181that is used automatically by the C compiler to transform your program 182before compilation. It is called a macro processor because it allows 183you to define @dfn{macros}, which are brief abbreviations for longer 184constructs. 185 186The C preprocessor is intended to be used only with C, C++, and 187Objective-C source code. In the past, it has been abused as a general 188text processor. It will choke on input which does not obey C's lexical 189rules. For example, apostrophes will be interpreted as the beginning of 190character constants, and cause errors. Also, you cannot rely on it 191preserving characteristics of the input which are not significant to 192C-family languages. If a Makefile is preprocessed, all the hard tabs 193will be removed, and the Makefile will not work. 194 195Having said that, you can often get away with using cpp on things which 196are not C@. Other Algol-ish programming languages are often safe 197(Pascal, Ada, etc.) So is assembly, with caution. @option{-traditional-cpp} 198mode preserves more white space, and is otherwise more permissive. Many 199of the problems can be avoided by writing C or C++ style comments 200instead of native language comments, and keeping macros simple. 201 202Wherever possible, you should use a preprocessor geared to the language 203you are writing in. Modern versions of the GNU assembler have macro 204facilities. Most high level programming languages have their own 205conditional compilation and inclusion mechanism. If all else fails, 206try a true general text processor, such as GNU M4. 207 208C preprocessors vary in some details. This manual discusses the GNU C 209preprocessor, which provides a small superset of the features of ISO 210Standard C@. In its default mode, the GNU C preprocessor does not do a 211few things required by the standard. These are features which are 212rarely, if ever, used, and may cause surprising changes to the meaning 213of a program which does not expect them. To get strict ISO Standard C, 214you should use the @option{-std=c90}, @option{-std=c99} or 215@option{-std=c11} options, depending 216on which version of the standard you want. To get all the mandatory 217diagnostics, you must also use @option{-pedantic}. @xref{Invocation}. 218 219This manual describes the behavior of the ISO preprocessor. To 220minimize gratuitous differences, where the ISO preprocessor's 221behavior does not conflict with traditional semantics, the 222traditional preprocessor should behave the same way. The various 223differences that do exist are detailed in the section @ref{Traditional 224Mode}. 225 226For clarity, unless noted otherwise, references to @samp{CPP} in this 227manual refer to GNU CPP@. 228@c man end 229 230@menu 231* Character sets:: 232* Initial processing:: 233* Tokenization:: 234* The preprocessing language:: 235@end menu 236 237@node Character sets 238@section Character sets 239 240Source code character set processing in C and related languages is 241rather complicated. The C standard discusses two character sets, but 242there are really at least four. 243 244The files input to CPP might be in any character set at all. CPP's 245very first action, before it even looks for line boundaries, is to 246convert the file into the character set it uses for internal 247processing. That set is what the C standard calls the @dfn{source} 248character set. It must be isomorphic with ISO 10646, also known as 249Unicode. CPP uses the UTF-8 encoding of Unicode. 250 251The character sets of the input files are specified using the 252@option{-finput-charset=} option. 253 254All preprocessing work (the subject of the rest of this manual) is 255carried out in the source character set. If you request textual 256output from the preprocessor with the @option{-E} option, it will be 257in UTF-8. 258 259After preprocessing is complete, string and character constants are 260converted again, into the @dfn{execution} character set. This 261character set is under control of the user; the default is UTF-8, 262matching the source character set. Wide string and character 263constants have their own character set, which is not called out 264specifically in the standard. Again, it is under control of the user. 265The default is UTF-16 or UTF-32, whichever fits in the target's 266@code{wchar_t} type, in the target machine's byte 267order.@footnote{UTF-16 does not meet the requirements of the C 268standard for a wide character set, but the choice of 16-bit 269@code{wchar_t} is enshrined in some system ABIs so we cannot fix 270this.} Octal and hexadecimal escape sequences do not undergo 271conversion; @t{'\x12'} has the value 0x12 regardless of the currently 272selected execution character set. All other escapes are replaced by 273the character in the source character set that they represent, then 274converted to the execution character set, just like unescaped 275characters. 276 277In identifiers, characters outside the ASCII range can only be 278specified with the @samp{\u} and @samp{\U} escapes, not used 279directly. If strict ISO C90 conformance is specified with an option 280such as @option{-std=c90}, or @option{-fno-extended-identifiers} is 281used, then those escapes are not permitted in identifiers. 282 283@node Initial processing 284@section Initial processing 285 286The preprocessor performs a series of textual transformations on its 287input. These happen before all other processing. Conceptually, they 288happen in a rigid order, and the entire file is run through each 289transformation before the next one begins. CPP actually does them 290all at once, for performance reasons. These transformations correspond 291roughly to the first three ``phases of translation'' described in the C 292standard. 293 294@enumerate 295@item 296@cindex line endings 297The input file is read into memory and broken into lines. 298 299Different systems use different conventions to indicate the end of a 300line. GCC accepts the ASCII control sequences @kbd{LF}, @kbd{@w{CR 301LF}} and @kbd{CR} as end-of-line markers. These are the canonical 302sequences used by Unix, DOS and VMS, and the classic Mac OS (before 303OSX) respectively. You may therefore safely copy source code written 304on any of those systems to a different one and use it without 305conversion. (GCC may lose track of the current line number if a file 306doesn't consistently use one convention, as sometimes happens when it 307is edited on computers with different conventions that share a network 308file system.) 309 310If the last line of any input file lacks an end-of-line marker, the end 311of the file is considered to implicitly supply one. The C standard says 312that this condition provokes undefined behavior, so GCC will emit a 313warning message. 314 315@item 316@cindex trigraphs 317@anchor{trigraphs}If trigraphs are enabled, they are replaced by their 318corresponding single characters. By default GCC ignores trigraphs, 319but if you request a strictly conforming mode with the @option{-std} 320option, or you specify the @option{-trigraphs} option, then it 321converts them. 322 323These are nine three-character sequences, all starting with @samp{??}, 324that are defined by ISO C to stand for single characters. They permit 325obsolete systems that lack some of C's punctuation to use C@. For 326example, @samp{??/} stands for @samp{\}, so @t{'??/n'} is a character 327constant for a newline. 328 329Trigraphs are not popular and many compilers implement them 330incorrectly. Portable code should not rely on trigraphs being either 331converted or ignored. With @option{-Wtrigraphs} GCC will warn you 332when a trigraph may change the meaning of your program if it were 333converted. @xref{Wtrigraphs}. 334 335In a string constant, you can prevent a sequence of question marks 336from being confused with a trigraph by inserting a backslash between 337the question marks, or by separating the string literal at the 338trigraph and making use of string literal concatenation. @t{"(??\?)"} 339is the string @samp{(???)}, not @samp{(?]}. Traditional C compilers 340do not recognize these idioms. 341 342The nine trigraphs and their replacements are 343 344@smallexample 345Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??- 346Replacement: [ ] @{ @} # \ ^ | ~ 347@end smallexample 348 349@item 350@cindex continued lines 351@cindex backslash-newline 352Continued lines are merged into one long line. 353 354A continued line is a line which ends with a backslash, @samp{\}. The 355backslash is removed and the following line is joined with the current 356one. No space is inserted, so you may split a line anywhere, even in 357the middle of a word. (It is generally more readable to split lines 358only at white space.) 359 360The trailing backslash on a continued line is commonly referred to as a 361@dfn{backslash-newline}. 362 363If there is white space between a backslash and the end of a line, that 364is still a continued line. However, as this is usually the result of an 365editing mistake, and many compilers will not accept it as a continued 366line, GCC will warn you about it. 367 368@item 369@cindex comments 370@cindex line comments 371@cindex block comments 372All comments are replaced with single spaces. 373 374There are two kinds of comments. @dfn{Block comments} begin with 375@samp{/*} and continue until the next @samp{*/}. Block comments do not 376nest: 377 378@smallexample 379/* @r{this is} /* @r{one comment} */ @r{text outside comment} 380@end smallexample 381 382@dfn{Line comments} begin with @samp{//} and continue to the end of the 383current line. Line comments do not nest either, but it does not matter, 384because they would end in the same place anyway. 385 386@smallexample 387// @r{this is} // @r{one comment} 388@r{text outside comment} 389@end smallexample 390@end enumerate 391 392It is safe to put line comments inside block comments, or vice versa. 393 394@smallexample 395@group 396/* @r{block comment} 397 // @r{contains line comment} 398 @r{yet more comment} 399 */ @r{outside comment} 400 401// @r{line comment} /* @r{contains block comment} */ 402@end group 403@end smallexample 404 405But beware of commenting out one end of a block comment with a line 406comment. 407 408@smallexample 409@group 410 // @r{l.c.} /* @r{block comment begins} 411 @r{oops! this isn't a comment anymore} */ 412@end group 413@end smallexample 414 415Comments are not recognized within string literals. 416@t{@w{"/* blah */"}} is the string constant @samp{@w{/* blah */}}, not 417an empty string. 418 419Line comments are not in the 1989 edition of the C standard, but they 420are recognized by GCC as an extension. In C++ and in the 1999 edition 421of the C standard, they are an official part of the language. 422 423Since these transformations happen before all other processing, you can 424split a line mechanically with backslash-newline anywhere. You can 425comment out the end of a line. You can continue a line comment onto the 426next line with backslash-newline. You can even split @samp{/*}, 427@samp{*/}, and @samp{//} onto multiple lines with backslash-newline. 428For example: 429 430@smallexample 431@group 432/\ 433* 434*/ # /* 435*/ defi\ 436ne FO\ 437O 10\ 43820 439@end group 440@end smallexample 441 442@noindent 443is equivalent to @code{@w{#define FOO 1020}}. All these tricks are 444extremely confusing and should not be used in code intended to be 445readable. 446 447There is no way to prevent a backslash at the end of a line from being 448interpreted as a backslash-newline. This cannot affect any correct 449program, however. 450 451@node Tokenization 452@section Tokenization 453 454@cindex tokens 455@cindex preprocessing tokens 456After the textual transformations are finished, the input file is 457converted into a sequence of @dfn{preprocessing tokens}. These mostly 458correspond to the syntactic tokens used by the C compiler, but there are 459a few differences. White space separates tokens; it is not itself a 460token of any kind. Tokens do not have to be separated by white space, 461but it is often necessary to avoid ambiguities. 462 463When faced with a sequence of characters that has more than one possible 464tokenization, the preprocessor is greedy. It always makes each token, 465starting from the left, as big as possible before moving on to the next 466token. For instance, @code{a+++++b} is interpreted as 467@code{@w{a ++ ++ + b}}, not as @code{@w{a ++ + ++ b}}, even though the 468latter tokenization could be part of a valid C program and the former 469could not. 470 471Once the input file is broken into tokens, the token boundaries never 472change, except when the @samp{##} preprocessing operator is used to paste 473tokens together. @xref{Concatenation}. For example, 474 475@smallexample 476@group 477#define foo() bar 478foo()baz 479 @expansion{} bar baz 480@emph{not} 481 @expansion{} barbaz 482@end group 483@end smallexample 484 485The compiler does not re-tokenize the preprocessor's output. Each 486preprocessing token becomes one compiler token. 487 488@cindex identifiers 489Preprocessing tokens fall into five broad classes: identifiers, 490preprocessing numbers, string literals, punctuators, and other. An 491@dfn{identifier} is the same as an identifier in C: any sequence of 492letters, digits, or underscores, which begins with a letter or 493underscore. Keywords of C have no significance to the preprocessor; 494they are ordinary identifiers. You can define a macro whose name is a 495keyword, for instance. The only identifier which can be considered a 496preprocessing keyword is @code{defined}. @xref{Defined}. 497 498This is mostly true of other languages which use the C preprocessor. 499However, a few of the keywords of C++ are significant even in the 500preprocessor. @xref{C++ Named Operators}. 501 502In the 1999 C standard, identifiers may contain letters which are not 503part of the ``basic source character set'', at the implementation's 504discretion (such as accented Latin letters, Greek letters, or Chinese 505ideograms). This may be done with an extended character set, or the 506@samp{\u} and @samp{\U} escape sequences. GCC only accepts such 507characters in the @samp{\u} and @samp{\U} forms. 508 509As an extension, GCC treats @samp{$} as a letter. This is for 510compatibility with some systems, such as VMS, where @samp{$} is commonly 511used in system-defined function and object names. @samp{$} is not a 512letter in strictly conforming mode, or if you specify the @option{-$} 513option. @xref{Invocation}. 514 515@cindex numbers 516@cindex preprocessing numbers 517A @dfn{preprocessing number} has a rather bizarre definition. The 518category includes all the normal integer and floating point constants 519one expects of C, but also a number of other things one might not 520initially recognize as a number. Formally, preprocessing numbers begin 521with an optional period, a required decimal digit, and then continue 522with any sequence of letters, digits, underscores, periods, and 523exponents. Exponents are the two-character sequences @samp{e+}, 524@samp{e-}, @samp{E+}, @samp{E-}, @samp{p+}, @samp{p-}, @samp{P+}, and 525@samp{P-}. (The exponents that begin with @samp{p} or @samp{P} are 526used for hexadecimal floating-point constants.) 527 528The purpose of this unusual definition is to isolate the preprocessor 529from the full complexity of numeric constants. It does not have to 530distinguish between lexically valid and invalid floating-point numbers, 531which is complicated. The definition also permits you to split an 532identifier at any position and get exactly two tokens, which can then be 533pasted back together with the @samp{##} operator. 534 535It's possible for preprocessing numbers to cause programs to be 536misinterpreted. For example, @code{0xE+12} is a preprocessing number 537which does not translate to any valid numeric constant, therefore a 538syntax error. It does not mean @code{@w{0xE + 12}}, which is what you 539might have intended. 540 541@cindex string literals 542@cindex string constants 543@cindex character constants 544@cindex header file names 545@c the @: prevents makeinfo from turning '' into ". 546@dfn{String literals} are string constants, character constants, and 547header file names (the argument of @samp{#include}).@footnote{The C 548standard uses the term @dfn{string literal} to refer only to what we are 549calling @dfn{string constants}.} String constants and character 550constants are straightforward: @t{"@dots{}"} or @t{'@dots{}'}. In 551either case embedded quotes should be escaped with a backslash: 552@t{'\'@:'} is the character constant for @samp{'}. There is no limit on 553the length of a character constant, but the value of a character 554constant that contains more than one character is 555implementation-defined. @xref{Implementation Details}. 556 557Header file names either look like string constants, @t{"@dots{}"}, or are 558written with angle brackets instead, @t{<@dots{}>}. In either case, 559backslash is an ordinary character. There is no way to escape the 560closing quote or angle bracket. The preprocessor looks for the header 561file in different places depending on which form you use. @xref{Include 562Operation}. 563 564No string literal may extend past the end of a line. You may use continued 565lines instead, or string constant concatenation. 566 567@cindex punctuators 568@cindex digraphs 569@cindex alternative tokens 570@dfn{Punctuators} are all the usual bits of punctuation which are 571meaningful to C and C++. All but three of the punctuation characters in 572ASCII are C punctuators. The exceptions are @samp{@@}, @samp{$}, and 573@samp{`}. In addition, all the two- and three-character operators are 574punctuators. There are also six @dfn{digraphs}, which the C++ standard 575calls @dfn{alternative tokens}, which are merely alternate ways to spell 576other punctuators. This is a second attempt to work around missing 577punctuation in obsolete systems. It has no negative side effects, 578unlike trigraphs, but does not cover as much ground. The digraphs and 579their corresponding normal punctuators are: 580 581@smallexample 582Digraph: <% %> <: :> %: %:%: 583Punctuator: @{ @} [ ] # ## 584@end smallexample 585 586@cindex other tokens 587Any other single character is considered ``other''. It is passed on to 588the preprocessor's output unmolested. The C compiler will almost 589certainly reject source code containing ``other'' tokens. In ASCII, the 590only other characters are @samp{@@}, @samp{$}, @samp{`}, and control 591characters other than NUL (all bits zero). (Note that @samp{$} is 592normally considered a letter.) All characters with the high bit set 593(numeric range 0x7F--0xFF) are also ``other'' in the present 594implementation. This will change when proper support for international 595character sets is added to GCC@. 596 597NUL is a special case because of the high probability that its 598appearance is accidental, and because it may be invisible to the user 599(many terminals do not display NUL at all). Within comments, NULs are 600silently ignored, just as any other character would be. In running 601text, NUL is considered white space. For example, these two directives 602have the same meaning. 603 604@smallexample 605#define X^@@1 606#define X 1 607@end smallexample 608 609@noindent 610(where @samp{^@@} is ASCII NUL)@. Within string or character constants, 611NULs are preserved. In the latter two cases the preprocessor emits a 612warning message. 613 614@node The preprocessing language 615@section The preprocessing language 616@cindex directives 617@cindex preprocessing directives 618@cindex directive line 619@cindex directive name 620 621After tokenization, the stream of tokens may simply be passed straight 622to the compiler's parser. However, if it contains any operations in the 623@dfn{preprocessing language}, it will be transformed first. This stage 624corresponds roughly to the standard's ``translation phase 4'' and is 625what most people think of as the preprocessor's job. 626 627The preprocessing language consists of @dfn{directives} to be executed 628and @dfn{macros} to be expanded. Its primary capabilities are: 629 630@itemize @bullet 631@item 632Inclusion of header files. These are files of declarations that can be 633substituted into your program. 634 635@item 636Macro expansion. You can define @dfn{macros}, which are abbreviations 637for arbitrary fragments of C code. The preprocessor will replace the 638macros with their definitions throughout the program. Some macros are 639automatically defined for you. 640 641@item 642Conditional compilation. You can include or exclude parts of the 643program according to various conditions. 644 645@item 646Line control. If you use a program to combine or rearrange source files 647into an intermediate file which is then compiled, you can use line 648control to inform the compiler where each source line originally came 649from. 650 651@item 652Diagnostics. You can detect problems at compile time and issue errors 653or warnings. 654@end itemize 655 656There are a few more, less useful, features. 657 658Except for expansion of predefined macros, all these operations are 659triggered with @dfn{preprocessing directives}. Preprocessing directives 660are lines in your program that start with @samp{#}. Whitespace is 661allowed before and after the @samp{#}. The @samp{#} is followed by an 662identifier, the @dfn{directive name}. It specifies the operation to 663perform. Directives are commonly referred to as @samp{#@var{name}} 664where @var{name} is the directive name. For example, @samp{#define} is 665the directive that defines a macro. 666 667The @samp{#} which begins a directive cannot come from a macro 668expansion. Also, the directive name is not macro expanded. Thus, if 669@code{foo} is defined as a macro expanding to @code{define}, that does 670not make @samp{#foo} a valid preprocessing directive. 671 672The set of valid directive names is fixed. Programs cannot define new 673preprocessing directives. 674 675Some directives require arguments; these make up the rest of the 676directive line and must be separated from the directive name by 677whitespace. For example, @samp{#define} must be followed by a macro 678name and the intended expansion of the macro. 679 680A preprocessing directive cannot cover more than one line. The line 681may, however, be continued with backslash-newline, or by a block comment 682which extends past the end of the line. In either case, when the 683directive is processed, the continuations have already been merged with 684the first line to make one long line. 685 686@node Header Files 687@chapter Header Files 688 689@cindex header file 690A header file is a file containing C declarations and macro definitions 691(@pxref{Macros}) to be shared between several source files. You request 692the use of a header file in your program by @dfn{including} it, with the 693C preprocessing directive @samp{#include}. 694 695Header files serve two purposes. 696 697@itemize @bullet 698@item 699@cindex system header files 700System header files declare the interfaces to parts of the operating 701system. You include them in your program to supply the definitions and 702declarations you need to invoke system calls and libraries. 703 704@item 705Your own header files contain declarations for interfaces between the 706source files of your program. Each time you have a group of related 707declarations and macro definitions all or most of which are needed in 708several different source files, it is a good idea to create a header 709file for them. 710@end itemize 711 712Including a header file produces the same results as copying the header 713file into each source file that needs it. Such copying would be 714time-consuming and error-prone. With a header file, the related 715declarations appear in only one place. If they need to be changed, they 716can be changed in one place, and programs that include the header file 717will automatically use the new version when next recompiled. The header 718file eliminates the labor of finding and changing all the copies as well 719as the risk that a failure to find one copy will result in 720inconsistencies within a program. 721 722In C, the usual convention is to give header files names that end with 723@file{.h}. It is most portable to use only letters, digits, dashes, and 724underscores in header file names, and at most one dot. 725 726@menu 727* Include Syntax:: 728* Include Operation:: 729* Search Path:: 730* Once-Only Headers:: 731* Alternatives to Wrapper #ifndef:: 732* Computed Includes:: 733* Wrapper Headers:: 734* System Headers:: 735@end menu 736 737@node Include Syntax 738@section Include Syntax 739 740@findex #include 741Both user and system header files are included using the preprocessing 742directive @samp{#include}. It has two variants: 743 744@table @code 745@item #include <@var{file}> 746This variant is used for system header files. It searches for a file 747named @var{file} in a standard list of system directories. You can prepend 748directories to this list with the @option{-I} option (@pxref{Invocation}). 749 750@item #include "@var{file}" 751This variant is used for header files of your own program. It 752searches for a file named @var{file} first in the directory containing 753the current file, then in the quote directories and then the same 754directories used for @code{<@var{file}>}. You can prepend directories 755to the list of quote directories with the @option{-iquote} option. 756@end table 757 758The argument of @samp{#include}, whether delimited with quote marks or 759angle brackets, behaves like a string constant in that comments are not 760recognized, and macro names are not expanded. Thus, @code{@w{#include 761<x/*y>}} specifies inclusion of a system header file named @file{x/*y}. 762 763However, if backslashes occur within @var{file}, they are considered 764ordinary text characters, not escape characters. None of the character 765escape sequences appropriate to string constants in C are processed. 766Thus, @code{@w{#include "x\n\\y"}} specifies a filename containing three 767backslashes. (Some systems interpret @samp{\} as a pathname separator. 768All of these also interpret @samp{/} the same way. It is most portable 769to use only @samp{/}.) 770 771It is an error if there is anything (other than comments) on the line 772after the file name. 773 774@node Include Operation 775@section Include Operation 776 777The @samp{#include} directive works by directing the C preprocessor to 778scan the specified file as input before continuing with the rest of the 779current file. The output from the preprocessor contains the output 780already generated, followed by the output resulting from the included 781file, followed by the output that comes from the text after the 782@samp{#include} directive. For example, if you have a header file 783@file{header.h} as follows, 784 785@smallexample 786char *test (void); 787@end smallexample 788 789@noindent 790and a main program called @file{program.c} that uses the header file, 791like this, 792 793@smallexample 794int x; 795#include "header.h" 796 797int 798main (void) 799@{ 800 puts (test ()); 801@} 802@end smallexample 803 804@noindent 805the compiler will see the same token stream as it would if 806@file{program.c} read 807 808@smallexample 809int x; 810char *test (void); 811 812int 813main (void) 814@{ 815 puts (test ()); 816@} 817@end smallexample 818 819Included files are not limited to declarations and macro definitions; 820those are merely the typical uses. Any fragment of a C program can be 821included from another file. The include file could even contain the 822beginning of a statement that is concluded in the containing file, or 823the end of a statement that was started in the including file. However, 824an included file must consist of complete tokens. Comments and string 825literals which have not been closed by the end of an included file are 826invalid. For error recovery, they are considered to end at the end of 827the file. 828 829To avoid confusion, it is best if header files contain only complete 830syntactic units---function declarations or definitions, type 831declarations, etc. 832 833The line following the @samp{#include} directive is always treated as a 834separate line by the C preprocessor, even if the included file lacks a 835final newline. 836 837@node Search Path 838@section Search Path 839 840By default, the preprocessor looks for header files included by the quote 841form of the directive @code{@w{#include "@var{file}"}} first relative to 842the directory of the current file, and then in a preconfigured list 843of standard system directories. 844For example, if @file{/usr/include/sys/stat.h} contains 845@code{@w{#include "types.h"}}, GCC looks for @file{types.h} first in 846@file{/usr/include/sys}, then in its usual search path. 847 848For the angle-bracket form @code{@w{#include <@var{file}>}}, the 849preprocessor's default behavior is to look only in the standard system 850directories. The exact search directory list depends on the target 851system, how GCC is configured, and where it is installed. You can 852find the default search directory list for your version of CPP by 853invoking it with the @option{-v} option. For example, 854 855@smallexample 856cpp -v /dev/null -o /dev/null 857@end smallexample 858 859There are a number of command-line options you can use to add 860additional directories to the search path. 861The most commonly-used option is @option{-I@var{dir}}, which causes 862@var{dir} to be searched after the current directory (for the quote 863form of the directive) and ahead of the standard system directories. 864You can specify multiple @option{-I} options on the command line, 865in which case the directories are searched in left-to-right order. 866 867If you need separate control over the search paths for the quote and 868angle-bracket forms of the @samp{#include} directive, you can use the 869@option{-iquote} and/or @option{-isystem} options instead of @option{-I}. 870@xref{Invocation}, for a detailed description of these options, as 871well as others that are less generally useful. 872 873If you specify other options on the command line, such as @option{-I}, 874that affect where the preprocessor searches for header files, the 875directory list printed by the @option{-v} option reflects the actual 876search path used by the preprocessor. 877 878Note that you can also prevent the preprocessor from searching any of 879the default system header directories with the @option{-nostdinc} 880option. This is useful when you are compiling an operating system 881kernel or some other program that does not use the standard C library 882facilities, or the standard C library itself. 883 884@node Once-Only Headers 885@section Once-Only Headers 886@cindex repeated inclusion 887@cindex including just once 888@cindex wrapper @code{#ifndef} 889 890If a header file happens to be included twice, the compiler will process 891its contents twice. This is very likely to cause an error, e.g.@: when the 892compiler sees the same structure definition twice. Even if it does not, 893it will certainly waste time. 894 895The standard way to prevent this is to enclose the entire real contents 896of the file in a conditional, like this: 897 898@smallexample 899@group 900/* File foo. */ 901#ifndef FILE_FOO_SEEN 902#define FILE_FOO_SEEN 903 904@var{the entire file} 905 906#endif /* !FILE_FOO_SEEN */ 907@end group 908@end smallexample 909 910This construct is commonly known as a @dfn{wrapper #ifndef}. 911When the header is included again, the conditional will be false, 912because @code{FILE_FOO_SEEN} is defined. The preprocessor will skip 913over the entire contents of the file, and the compiler will not see it 914twice. 915 916CPP optimizes even further. It remembers when a header file has a 917wrapper @samp{#ifndef}. If a subsequent @samp{#include} specifies that 918header, and the macro in the @samp{#ifndef} is still defined, it does 919not bother to rescan the file at all. 920 921You can put comments outside the wrapper. They will not interfere with 922this optimization. 923 924@cindex controlling macro 925@cindex guard macro 926The macro @code{FILE_FOO_SEEN} is called the @dfn{controlling macro} or 927@dfn{guard macro}. In a user header file, the macro name should not 928begin with @samp{_}. In a system header file, it should begin with 929@samp{__} to avoid conflicts with user programs. In any kind of header 930file, the macro name should contain the name of the file and some 931additional text, to avoid conflicts with other header files. 932 933@node Alternatives to Wrapper #ifndef 934@section Alternatives to Wrapper #ifndef 935 936CPP supports two more ways of indicating that a header file should be 937read only once. Neither one is as portable as a wrapper @samp{#ifndef} 938and we recommend you do not use them in new programs, with the caveat 939that @samp{#import} is standard practice in Objective-C. 940 941@findex #import 942CPP supports a variant of @samp{#include} called @samp{#import} which 943includes a file, but does so at most once. If you use @samp{#import} 944instead of @samp{#include}, then you don't need the conditionals 945inside the header file to prevent multiple inclusion of the contents. 946@samp{#import} is standard in Objective-C, but is considered a 947deprecated extension in C and C++. 948 949@samp{#import} is not a well designed feature. It requires the users of 950a header file to know that it should only be included once. It is much 951better for the header file's implementor to write the file so that users 952don't need to know this. Using a wrapper @samp{#ifndef} accomplishes 953this goal. 954 955In the present implementation, a single use of @samp{#import} will 956prevent the file from ever being read again, by either @samp{#import} or 957@samp{#include}. You should not rely on this; do not use both 958@samp{#import} and @samp{#include} to refer to the same header file. 959 960Another way to prevent a header file from being included more than once 961is with the @samp{#pragma once} directive. If @samp{#pragma once} is 962seen when scanning a header file, that file will never be read again, no 963matter what. 964 965@samp{#pragma once} does not have the problems that @samp{#import} does, 966but it is not recognized by all preprocessors, so you cannot rely on it 967in a portable program. 968 969@node Computed Includes 970@section Computed Includes 971@cindex computed includes 972@cindex macros in include 973 974Sometimes it is necessary to select one of several different header 975files to be included into your program. They might specify 976configuration parameters to be used on different sorts of operating 977systems, for instance. You could do this with a series of conditionals, 978 979@smallexample 980#if SYSTEM_1 981# include "system_1.h" 982#elif SYSTEM_2 983# include "system_2.h" 984#elif SYSTEM_3 985@dots{} 986#endif 987@end smallexample 988 989That rapidly becomes tedious. Instead, the preprocessor offers the 990ability to use a macro for the header name. This is called a 991@dfn{computed include}. Instead of writing a header name as the direct 992argument of @samp{#include}, you simply put a macro name there instead: 993 994@smallexample 995#define SYSTEM_H "system_1.h" 996@dots{} 997#include SYSTEM_H 998@end smallexample 999 1000@noindent 1001@code{SYSTEM_H} will be expanded, and the preprocessor will look for 1002@file{system_1.h} as if the @samp{#include} had been written that way 1003originally. @code{SYSTEM_H} could be defined by your Makefile with a 1004@option{-D} option. 1005 1006You must be careful when you define the macro. @samp{#define} saves 1007tokens, not text. The preprocessor has no way of knowing that the macro 1008will be used as the argument of @samp{#include}, so it generates 1009ordinary tokens, not a header name. This is unlikely to cause problems 1010if you use double-quote includes, which are close enough to string 1011constants. If you use angle brackets, however, you may have trouble. 1012 1013The syntax of a computed include is actually a bit more general than the 1014above. If the first non-whitespace character after @samp{#include} is 1015not @samp{"} or @samp{<}, then the entire line is macro-expanded 1016like running text would be. 1017 1018If the line expands to a single string constant, the contents of that 1019string constant are the file to be included. CPP does not re-examine the 1020string for embedded quotes, but neither does it process backslash 1021escapes in the string. Therefore 1022 1023@smallexample 1024#define HEADER "a\"b" 1025#include HEADER 1026@end smallexample 1027 1028@noindent 1029looks for a file named @file{a\"b}. CPP searches for the file according 1030to the rules for double-quoted includes. 1031 1032If the line expands to a token stream beginning with a @samp{<} token 1033and including a @samp{>} token, then the tokens between the @samp{<} and 1034the first @samp{>} are combined to form the filename to be included. 1035Any whitespace between tokens is reduced to a single space; then any 1036space after the initial @samp{<} is retained, but a trailing space 1037before the closing @samp{>} is ignored. CPP searches for the file 1038according to the rules for angle-bracket includes. 1039 1040In either case, if there are any tokens on the line after the file name, 1041an error occurs and the directive is not processed. It is also an error 1042if the result of expansion does not match either of the two expected 1043forms. 1044 1045These rules are implementation-defined behavior according to the C 1046standard. To minimize the risk of different compilers interpreting your 1047computed includes differently, we recommend you use only a single 1048object-like macro which expands to a string constant. This will also 1049minimize confusion for people reading your program. 1050 1051@node Wrapper Headers 1052@section Wrapper Headers 1053@cindex wrapper headers 1054@cindex overriding a header file 1055@findex #include_next 1056 1057Sometimes it is necessary to adjust the contents of a system-provided 1058header file without editing it directly. GCC's @command{fixincludes} 1059operation does this, for example. One way to do that would be to create 1060a new header file with the same name and insert it in the search path 1061before the original header. That works fine as long as you're willing 1062to replace the old header entirely. But what if you want to refer to 1063the old header from the new one? 1064 1065You cannot simply include the old header with @samp{#include}. That 1066will start from the beginning, and find your new header again. If your 1067header is not protected from multiple inclusion (@pxref{Once-Only 1068Headers}), it will recurse infinitely and cause a fatal error. 1069 1070You could include the old header with an absolute pathname: 1071@smallexample 1072#include "/usr/include/old-header.h" 1073@end smallexample 1074@noindent 1075This works, but is not clean; should the system headers ever move, you 1076would have to edit the new headers to match. 1077 1078There is no way to solve this problem within the C standard, but you can 1079use the GNU extension @samp{#include_next}. It means, ``Include the 1080@emph{next} file with this name''. This directive works like 1081@samp{#include} except in searching for the specified file: it starts 1082searching the list of header file directories @emph{after} the directory 1083in which the current file was found. 1084 1085Suppose you specify @option{-I /usr/local/include}, and the list of 1086directories to search also includes @file{/usr/include}; and suppose 1087both directories contain @file{signal.h}. Ordinary @code{@w{#include 1088<signal.h>}} finds the file under @file{/usr/local/include}. If that 1089file contains @code{@w{#include_next <signal.h>}}, it starts searching 1090after that directory, and finds the file in @file{/usr/include}. 1091 1092@samp{#include_next} does not distinguish between @code{<@var{file}>} 1093and @code{"@var{file}"} inclusion, nor does it check that the file you 1094specify has the same name as the current file. It simply looks for the 1095file named, starting with the directory in the search path after the one 1096where the current file was found. 1097 1098The use of @samp{#include_next} can lead to great confusion. We 1099recommend it be used only when there is no other alternative. In 1100particular, it should not be used in the headers belonging to a specific 1101program; it should be used only to make global corrections along the 1102lines of @command{fixincludes}. 1103 1104@node System Headers 1105@section System Headers 1106@cindex system header files 1107 1108The header files declaring interfaces to the operating system and 1109runtime libraries often cannot be written in strictly conforming C@. 1110Therefore, GCC gives code found in @dfn{system headers} special 1111treatment. All warnings, other than those generated by @samp{#warning} 1112(@pxref{Diagnostics}), are suppressed while GCC is processing a system 1113header. Macros defined in a system header are immune to a few warnings 1114wherever they are expanded. This immunity is granted on an ad-hoc 1115basis, when we find that a warning generates lots of false positives 1116because of code in macros defined in system headers. 1117 1118Normally, only the headers found in specific directories are considered 1119system headers. These directories are determined when GCC is compiled. 1120There are, however, two ways to make normal headers into system headers: 1121 1122@itemize @bullet 1123@item 1124Header files found in directories added to the search path with the 1125@option{-isystem} and @option{-idirafter} command-line options are 1126treated as system headers for the purposes of diagnostics. 1127 1128The @option{-cxx-isystem} command line option adds its argument to the 1129list of C++ system headers, similar to @option{-isystem} for C headers. 1130 1131@item 1132@findex #pragma GCC system_header 1133There is also a directive, @code{@w{#pragma GCC system_header}}, which 1134tells GCC to consider the rest of the current include file a system 1135header, no matter where it was found. Code that comes before the 1136@samp{#pragma} in the file is not affected. @code{@w{#pragma GCC 1137system_header}} has no effect in the primary source file. 1138@end itemize 1139 1140@node Macros 1141@chapter Macros 1142 1143A @dfn{macro} is a fragment of code which has been given a name. 1144Whenever the name is used, it is replaced by the contents of the macro. 1145There are two kinds of macros. They differ mostly in what they look 1146like when they are used. @dfn{Object-like} macros resemble data objects 1147when used, @dfn{function-like} macros resemble function calls. 1148 1149You may define any valid identifier as a macro, even if it is a C 1150keyword. The preprocessor does not know anything about keywords. This 1151can be useful if you wish to hide a keyword such as @code{const} from an 1152older compiler that does not understand it. However, the preprocessor 1153operator @code{defined} (@pxref{Defined}) can never be defined as a 1154macro, and C++'s named operators (@pxref{C++ Named Operators}) cannot be 1155macros when you are compiling C++. 1156 1157@menu 1158* Object-like Macros:: 1159* Function-like Macros:: 1160* Macro Arguments:: 1161* Stringizing:: 1162* Concatenation:: 1163* Variadic Macros:: 1164* Predefined Macros:: 1165* Undefining and Redefining Macros:: 1166* Directives Within Macro Arguments:: 1167* Macro Pitfalls:: 1168@end menu 1169 1170@node Object-like Macros 1171@section Object-like Macros 1172@cindex object-like macro 1173@cindex symbolic constants 1174@cindex manifest constants 1175 1176An @dfn{object-like macro} is a simple identifier which will be replaced 1177by a code fragment. It is called object-like because it looks like a 1178data object in code that uses it. They are most commonly used to give 1179symbolic names to numeric constants. 1180 1181@findex #define 1182You create macros with the @samp{#define} directive. @samp{#define} is 1183followed by the name of the macro and then the token sequence it should 1184be an abbreviation for, which is variously referred to as the macro's 1185@dfn{body}, @dfn{expansion} or @dfn{replacement list}. For example, 1186 1187@smallexample 1188#define BUFFER_SIZE 1024 1189@end smallexample 1190 1191@noindent 1192defines a macro named @code{BUFFER_SIZE} as an abbreviation for the 1193token @code{1024}. If somewhere after this @samp{#define} directive 1194there comes a C statement of the form 1195 1196@smallexample 1197foo = (char *) malloc (BUFFER_SIZE); 1198@end smallexample 1199 1200@noindent 1201then the C preprocessor will recognize and @dfn{expand} the macro 1202@code{BUFFER_SIZE}. The C compiler will see the same tokens as it would 1203if you had written 1204 1205@smallexample 1206foo = (char *) malloc (1024); 1207@end smallexample 1208 1209By convention, macro names are written in uppercase. Programs are 1210easier to read when it is possible to tell at a glance which names are 1211macros. 1212 1213The macro's body ends at the end of the @samp{#define} line. You may 1214continue the definition onto multiple lines, if necessary, using 1215backslash-newline. When the macro is expanded, however, it will all 1216come out on one line. For example, 1217 1218@smallexample 1219#define NUMBERS 1, \ 1220 2, \ 1221 3 1222int x[] = @{ NUMBERS @}; 1223 @expansion{} int x[] = @{ 1, 2, 3 @}; 1224@end smallexample 1225 1226@noindent 1227The most common visible consequence of this is surprising line numbers 1228in error messages. 1229 1230There is no restriction on what can go in a macro body provided it 1231decomposes into valid preprocessing tokens. Parentheses need not 1232balance, and the body need not resemble valid C code. (If it does not, 1233you may get error messages from the C compiler when you use the macro.) 1234 1235The C preprocessor scans your program sequentially. Macro definitions 1236take effect at the place you write them. Therefore, the following input 1237to the C preprocessor 1238 1239@smallexample 1240foo = X; 1241#define X 4 1242bar = X; 1243@end smallexample 1244 1245@noindent 1246produces 1247 1248@smallexample 1249foo = X; 1250bar = 4; 1251@end smallexample 1252 1253When the preprocessor expands a macro name, the macro's expansion 1254replaces the macro invocation, then the expansion is examined for more 1255macros to expand. For example, 1256 1257@smallexample 1258@group 1259#define TABLESIZE BUFSIZE 1260#define BUFSIZE 1024 1261TABLESIZE 1262 @expansion{} BUFSIZE 1263 @expansion{} 1024 1264@end group 1265@end smallexample 1266 1267@noindent 1268@code{TABLESIZE} is expanded first to produce @code{BUFSIZE}, then that 1269macro is expanded to produce the final result, @code{1024}. 1270 1271Notice that @code{BUFSIZE} was not defined when @code{TABLESIZE} was 1272defined. The @samp{#define} for @code{TABLESIZE} uses exactly the 1273expansion you specify---in this case, @code{BUFSIZE}---and does not 1274check to see whether it too contains macro names. Only when you 1275@emph{use} @code{TABLESIZE} is the result of its expansion scanned for 1276more macro names. 1277 1278This makes a difference if you change the definition of @code{BUFSIZE} 1279at some point in the source file. @code{TABLESIZE}, defined as shown, 1280will always expand using the definition of @code{BUFSIZE} that is 1281currently in effect: 1282 1283@smallexample 1284#define BUFSIZE 1020 1285#define TABLESIZE BUFSIZE 1286#undef BUFSIZE 1287#define BUFSIZE 37 1288@end smallexample 1289 1290@noindent 1291Now @code{TABLESIZE} expands (in two stages) to @code{37}. 1292 1293If the expansion of a macro contains its own name, either directly or 1294via intermediate macros, it is not expanded again when the expansion is 1295examined for more macros. This prevents infinite recursion. 1296@xref{Self-Referential Macros}, for the precise details. 1297 1298@node Function-like Macros 1299@section Function-like Macros 1300@cindex function-like macros 1301 1302You can also define macros whose use looks like a function call. These 1303are called @dfn{function-like macros}. To define a function-like macro, 1304you use the same @samp{#define} directive, but you put a pair of 1305parentheses immediately after the macro name. For example, 1306 1307@smallexample 1308#define lang_init() c_init() 1309lang_init() 1310 @expansion{} c_init() 1311@end smallexample 1312 1313A function-like macro is only expanded if its name appears with a pair 1314of parentheses after it. If you write just the name, it is left alone. 1315This can be useful when you have a function and a macro of the same 1316name, and you wish to use the function sometimes. 1317 1318@smallexample 1319extern void foo(void); 1320#define foo() /* @r{optimized inline version} */ 1321@dots{} 1322 foo(); 1323 funcptr = foo; 1324@end smallexample 1325 1326Here the call to @code{foo()} will use the macro, but the function 1327pointer will get the address of the real function. If the macro were to 1328be expanded, it would cause a syntax error. 1329 1330If you put spaces between the macro name and the parentheses in the 1331macro definition, that does not define a function-like macro, it defines 1332an object-like macro whose expansion happens to begin with a pair of 1333parentheses. 1334 1335@smallexample 1336#define lang_init () c_init() 1337lang_init() 1338 @expansion{} () c_init()() 1339@end smallexample 1340 1341The first two pairs of parentheses in this expansion come from the 1342macro. The third is the pair that was originally after the macro 1343invocation. Since @code{lang_init} is an object-like macro, it does not 1344consume those parentheses. 1345 1346@node Macro Arguments 1347@section Macro Arguments 1348@cindex arguments 1349@cindex macros with arguments 1350@cindex arguments in macro definitions 1351 1352Function-like macros can take @dfn{arguments}, just like true functions. 1353To define a macro that uses arguments, you insert @dfn{parameters} 1354between the pair of parentheses in the macro definition that make the 1355macro function-like. The parameters must be valid C identifiers, 1356separated by commas and optionally whitespace. 1357 1358To invoke a macro that takes arguments, you write the name of the macro 1359followed by a list of @dfn{actual arguments} in parentheses, separated 1360by commas. The invocation of the macro need not be restricted to a 1361single logical line---it can cross as many lines in the source file as 1362you wish. The number of arguments you give must match the number of 1363parameters in the macro definition. When the macro is expanded, each 1364use of a parameter in its body is replaced by the tokens of the 1365corresponding argument. (You need not use all of the parameters in the 1366macro body.) 1367 1368As an example, here is a macro that computes the minimum of two numeric 1369values, as it is defined in many C programs, and some uses. 1370 1371@smallexample 1372#define min(X, Y) ((X) < (Y) ? (X) : (Y)) 1373 x = min(a, b); @expansion{} x = ((a) < (b) ? (a) : (b)); 1374 y = min(1, 2); @expansion{} y = ((1) < (2) ? (1) : (2)); 1375 z = min(a + 28, *p); @expansion{} z = ((a + 28) < (*p) ? (a + 28) : (*p)); 1376@end smallexample 1377 1378@noindent 1379(In this small example you can already see several of the dangers of 1380macro arguments. @xref{Macro Pitfalls}, for detailed explanations.) 1381 1382Leading and trailing whitespace in each argument is dropped, and all 1383whitespace between the tokens of an argument is reduced to a single 1384space. Parentheses within each argument must balance; a comma within 1385such parentheses does not end the argument. However, there is no 1386requirement for square brackets or braces to balance, and they do not 1387prevent a comma from separating arguments. Thus, 1388 1389@smallexample 1390macro (array[x = y, x + 1]) 1391@end smallexample 1392 1393@noindent 1394passes two arguments to @code{macro}: @code{array[x = y} and @code{x + 13951]}. If you want to supply @code{array[x = y, x + 1]} as an argument, 1396you can write it as @code{array[(x = y, x + 1)]}, which is equivalent C 1397code. 1398 1399All arguments to a macro are completely macro-expanded before they are 1400substituted into the macro body. After substitution, the complete text 1401is scanned again for macros to expand, including the arguments. This rule 1402may seem strange, but it is carefully designed so you need not worry 1403about whether any function call is actually a macro invocation. You can 1404run into trouble if you try to be too clever, though. @xref{Argument 1405Prescan}, for detailed discussion. 1406 1407For example, @code{min (min (a, b), c)} is first expanded to 1408 1409@smallexample 1410 min (((a) < (b) ? (a) : (b)), (c)) 1411@end smallexample 1412 1413@noindent 1414and then to 1415 1416@smallexample 1417@group 1418((((a) < (b) ? (a) : (b))) < (c) 1419 ? (((a) < (b) ? (a) : (b))) 1420 : (c)) 1421@end group 1422@end smallexample 1423 1424@noindent 1425(Line breaks shown here for clarity would not actually be generated.) 1426 1427@cindex empty macro arguments 1428You can leave macro arguments empty; this is not an error to the 1429preprocessor (but many macros will then expand to invalid code). 1430You cannot leave out arguments entirely; if a macro takes two arguments, 1431there must be exactly one comma at the top level of its argument list. 1432Here are some silly examples using @code{min}: 1433 1434@smallexample 1435min(, b) @expansion{} (( ) < (b) ? ( ) : (b)) 1436min(a, ) @expansion{} ((a ) < ( ) ? (a ) : ( )) 1437min(,) @expansion{} (( ) < ( ) ? ( ) : ( )) 1438min((,),) @expansion{} (((,)) < ( ) ? ((,)) : ( )) 1439 1440min() @error{} macro "min" requires 2 arguments, but only 1 given 1441min(,,) @error{} macro "min" passed 3 arguments, but takes just 2 1442@end smallexample 1443 1444Whitespace is not a preprocessing token, so if a macro @code{foo} takes 1445one argument, @code{@w{foo ()}} and @code{@w{foo ( )}} both supply it an 1446empty argument. Previous GNU preprocessor implementations and 1447documentation were incorrect on this point, insisting that a 1448function-like macro that takes a single argument be passed a space if an 1449empty argument was required. 1450 1451Macro parameters appearing inside string literals are not replaced by 1452their corresponding actual arguments. 1453 1454@smallexample 1455#define foo(x) x, "x" 1456foo(bar) @expansion{} bar, "x" 1457@end smallexample 1458 1459@node Stringizing 1460@section Stringizing 1461@cindex stringizing 1462@cindex @samp{#} operator 1463 1464Sometimes you may want to convert a macro argument into a string 1465constant. Parameters are not replaced inside string constants, but you 1466can use the @samp{#} preprocessing operator instead. When a macro 1467parameter is used with a leading @samp{#}, the preprocessor replaces it 1468with the literal text of the actual argument, converted to a string 1469constant. Unlike normal parameter replacement, the argument is not 1470macro-expanded first. This is called @dfn{stringizing}. 1471 1472There is no way to combine an argument with surrounding text and 1473stringize it all together. Instead, you can write a series of adjacent 1474string constants and stringized arguments. The preprocessor 1475replaces the stringized arguments with string constants. The C 1476compiler then combines all the adjacent string constants into one 1477long string. 1478 1479Here is an example of a macro definition that uses stringizing: 1480 1481@smallexample 1482@group 1483#define WARN_IF(EXP) \ 1484do @{ if (EXP) \ 1485 fprintf (stderr, "Warning: " #EXP "\n"); @} \ 1486while (0) 1487WARN_IF (x == 0); 1488 @expansion{} do @{ if (x == 0) 1489 fprintf (stderr, "Warning: " "x == 0" "\n"); @} while (0); 1490@end group 1491@end smallexample 1492 1493@noindent 1494The argument for @code{EXP} is substituted once, as-is, into the 1495@code{if} statement, and once, stringized, into the argument to 1496@code{fprintf}. If @code{x} were a macro, it would be expanded in the 1497@code{if} statement, but not in the string. 1498 1499The @code{do} and @code{while (0)} are a kludge to make it possible to 1500write @code{WARN_IF (@var{arg});}, which the resemblance of 1501@code{WARN_IF} to a function would make C programmers want to do; see 1502@ref{Swallowing the Semicolon}. 1503 1504Stringizing in C involves more than putting double-quote characters 1505around the fragment. The preprocessor backslash-escapes the quotes 1506surrounding embedded string constants, and all backslashes within string and 1507character constants, in order to get a valid C string constant with the 1508proper contents. Thus, stringizing @code{@w{p = "foo\n";}} results in 1509@t{@w{"p = \"foo\\n\";"}}. However, backslashes that are not inside string 1510or character constants are not duplicated: @samp{\n} by itself 1511stringizes to @t{"\n"}. 1512 1513All leading and trailing whitespace in text being stringized is 1514ignored. Any sequence of whitespace in the middle of the text is 1515converted to a single space in the stringized result. Comments are 1516replaced by whitespace long before stringizing happens, so they 1517never appear in stringized text. 1518 1519There is no way to convert a macro argument into a character constant. 1520 1521If you want to stringize the result of expansion of a macro argument, 1522you have to use two levels of macros. 1523 1524@smallexample 1525#define xstr(s) str(s) 1526#define str(s) #s 1527#define foo 4 1528str (foo) 1529 @expansion{} "foo" 1530xstr (foo) 1531 @expansion{} xstr (4) 1532 @expansion{} str (4) 1533 @expansion{} "4" 1534@end smallexample 1535 1536@code{s} is stringized when it is used in @code{str}, so it is not 1537macro-expanded first. But @code{s} is an ordinary argument to 1538@code{xstr}, so it is completely macro-expanded before @code{xstr} 1539itself is expanded (@pxref{Argument Prescan}). Therefore, by the time 1540@code{str} gets to its argument, it has already been macro-expanded. 1541 1542@node Concatenation 1543@section Concatenation 1544@cindex concatenation 1545@cindex token pasting 1546@cindex token concatenation 1547@cindex @samp{##} operator 1548 1549It is often useful to merge two tokens into one while expanding macros. 1550This is called @dfn{token pasting} or @dfn{token concatenation}. The 1551@samp{##} preprocessing operator performs token pasting. When a macro 1552is expanded, the two tokens on either side of each @samp{##} operator 1553are combined into a single token, which then replaces the @samp{##} and 1554the two original tokens in the macro expansion. Usually both will be 1555identifiers, or one will be an identifier and the other a preprocessing 1556number. When pasted, they make a longer identifier. This isn't the 1557only valid case. It is also possible to concatenate two numbers (or a 1558number and a name, such as @code{1.5} and @code{e3}) into a number. 1559Also, multi-character operators such as @code{+=} can be formed by 1560token pasting. 1561 1562However, two tokens that don't together form a valid token cannot be 1563pasted together. For example, you cannot concatenate @code{x} with 1564@code{+} in either order. If you try, the preprocessor issues a warning 1565and emits the two tokens. Whether it puts white space between the 1566tokens is undefined. It is common to find unnecessary uses of @samp{##} 1567in complex macros. If you get this warning, it is likely that you can 1568simply remove the @samp{##}. 1569 1570Both the tokens combined by @samp{##} could come from the macro body, 1571but you could just as well write them as one token in the first place. 1572Token pasting is most useful when one or both of the tokens comes from a 1573macro argument. If either of the tokens next to an @samp{##} is a 1574parameter name, it is replaced by its actual argument before @samp{##} 1575executes. As with stringizing, the actual argument is not 1576macro-expanded first. If the argument is empty, that @samp{##} has no 1577effect. 1578 1579Keep in mind that the C preprocessor converts comments to whitespace 1580before macros are even considered. Therefore, you cannot create a 1581comment by concatenating @samp{/} and @samp{*}. You can put as much 1582whitespace between @samp{##} and its operands as you like, including 1583comments, and you can put comments in arguments that will be 1584concatenated. However, it is an error if @samp{##} appears at either 1585end of a macro body. 1586 1587Consider a C program that interprets named commands. There probably 1588needs to be a table of commands, perhaps an array of structures declared 1589as follows: 1590 1591@smallexample 1592@group 1593struct command 1594@{ 1595 char *name; 1596 void (*function) (void); 1597@}; 1598@end group 1599 1600@group 1601struct command commands[] = 1602@{ 1603 @{ "quit", quit_command @}, 1604 @{ "help", help_command @}, 1605 @dots{} 1606@}; 1607@end group 1608@end smallexample 1609 1610It would be cleaner not to have to give each command name twice, once in 1611the string constant and once in the function name. A macro which takes the 1612name of a command as an argument can make this unnecessary. The string 1613constant can be created with stringizing, and the function name by 1614concatenating the argument with @samp{_command}. Here is how it is done: 1615 1616@smallexample 1617#define COMMAND(NAME) @{ #NAME, NAME ## _command @} 1618 1619struct command commands[] = 1620@{ 1621 COMMAND (quit), 1622 COMMAND (help), 1623 @dots{} 1624@}; 1625@end smallexample 1626 1627@node Variadic Macros 1628@section Variadic Macros 1629@cindex variable number of arguments 1630@cindex macros with variable arguments 1631@cindex variadic macros 1632 1633A macro can be declared to accept a variable number of arguments much as 1634a function can. The syntax for defining the macro is similar to that of 1635a function. Here is an example: 1636 1637@smallexample 1638#define eprintf(@dots{}) fprintf (stderr, __VA_ARGS__) 1639@end smallexample 1640 1641This kind of macro is called @dfn{variadic}. When the macro is invoked, 1642all the tokens in its argument list after the last named argument (this 1643macro has none), including any commas, become the @dfn{variable 1644argument}. This sequence of tokens replaces the identifier 1645@code{@w{__VA_ARGS__}} in the macro body wherever it appears. Thus, we 1646have this expansion: 1647 1648@smallexample 1649eprintf ("%s:%d: ", input_file, lineno) 1650 @expansion{} fprintf (stderr, "%s:%d: ", input_file, lineno) 1651@end smallexample 1652 1653The variable argument is completely macro-expanded before it is inserted 1654into the macro expansion, just like an ordinary argument. You may use 1655the @samp{#} and @samp{##} operators to stringize the variable argument 1656or to paste its leading or trailing token with another token. (But see 1657below for an important special case for @samp{##}.) 1658 1659If your macro is complicated, you may want a more descriptive name for 1660the variable argument than @code{@w{__VA_ARGS__}}. CPP permits 1661this, as an extension. You may write an argument name immediately 1662before the @samp{@dots{}}; that name is used for the variable argument. 1663The @code{eprintf} macro above could be written 1664 1665@smallexample 1666#define eprintf(args@dots{}) fprintf (stderr, args) 1667@end smallexample 1668 1669@noindent 1670using this extension. You cannot use @code{@w{__VA_ARGS__}} and this 1671extension in the same macro. 1672 1673You can have named arguments as well as variable arguments in a variadic 1674macro. We could define @code{eprintf} like this, instead: 1675 1676@smallexample 1677#define eprintf(format, @dots{}) fprintf (stderr, format, __VA_ARGS__) 1678@end smallexample 1679 1680@noindent 1681This formulation looks more descriptive, but unfortunately it is less 1682flexible: you must now supply at least one argument after the format 1683string. In standard C, you cannot omit the comma separating the named 1684argument from the variable arguments. Furthermore, if you leave the 1685variable argument empty, you will get a syntax error, because 1686there will be an extra comma after the format string. 1687 1688@smallexample 1689eprintf("success!\n", ); 1690 @expansion{} fprintf(stderr, "success!\n", ); 1691@end smallexample 1692 1693GNU CPP has a pair of extensions which deal with this problem. First, 1694you are allowed to leave the variable argument out entirely: 1695 1696@smallexample 1697eprintf ("success!\n") 1698 @expansion{} fprintf(stderr, "success!\n", ); 1699@end smallexample 1700 1701@noindent 1702Second, the @samp{##} token paste operator has a special meaning when 1703placed between a comma and a variable argument. If you write 1704 1705@smallexample 1706#define eprintf(format, @dots{}) fprintf (stderr, format, ##__VA_ARGS__) 1707@end smallexample 1708 1709@noindent 1710and the variable argument is left out when the @code{eprintf} macro is 1711used, then the comma before the @samp{##} will be deleted. This does 1712@emph{not} happen if you pass an empty argument, nor does it happen if 1713the token preceding @samp{##} is anything other than a comma. 1714 1715@smallexample 1716eprintf ("success!\n") 1717 @expansion{} fprintf(stderr, "success!\n"); 1718@end smallexample 1719 1720@noindent 1721The above explanation is ambiguous about the case where the only macro 1722parameter is a variable arguments parameter, as it is meaningless to 1723try to distinguish whether no argument at all is an empty argument or 1724a missing argument. 1725CPP retains the comma when conforming to a specific C 1726standard. Otherwise the comma is dropped as an extension to the standard. 1727 1728The C standard 1729mandates that the only place the identifier @code{@w{__VA_ARGS__}} 1730can appear is in the replacement list of a variadic macro. It may not 1731be used as a macro name, macro argument name, or within a different type 1732of macro. It may also be forbidden in open text; the standard is 1733ambiguous. We recommend you avoid using it except for its defined 1734purpose. 1735 1736Variadic macros became a standard part of the C language with C99. 1737GNU CPP previously supported them 1738with a named variable argument 1739(@samp{args@dots{}}, not @samp{@dots{}} and @code{@w{__VA_ARGS__}}), which 1740is still supported for backward compatibility. 1741 1742@node Predefined Macros 1743@section Predefined Macros 1744 1745@cindex predefined macros 1746Several object-like macros are predefined; you use them without 1747supplying their definitions. They fall into three classes: standard, 1748common, and system-specific. 1749 1750In C++, there is a fourth category, the named operators. They act like 1751predefined macros, but you cannot undefine them. 1752 1753@menu 1754* Standard Predefined Macros:: 1755* Common Predefined Macros:: 1756* System-specific Predefined Macros:: 1757* C++ Named Operators:: 1758@end menu 1759 1760@node Standard Predefined Macros 1761@subsection Standard Predefined Macros 1762@cindex standard predefined macros. 1763 1764The standard predefined macros are specified by the relevant 1765language standards, so they are available with all compilers that 1766implement those standards. Older compilers may not provide all of 1767them. Their names all start with double underscores. 1768 1769@table @code 1770@item __FILE__ 1771This macro expands to the name of the current input file, in the form of 1772a C string constant. This is the path by which the preprocessor opened 1773the file, not the short name specified in @samp{#include} or as the 1774input file name argument. For example, 1775@code{"/usr/local/include/myheader.h"} is a possible expansion of this 1776macro. 1777 1778@item __LINE__ 1779This macro expands to the current input line number, in the form of a 1780decimal integer constant. While we call it a predefined macro, it's 1781a pretty strange macro, since its ``definition'' changes with each 1782new line of source code. 1783@end table 1784 1785@code{__FILE__} and @code{__LINE__} are useful in generating an error 1786message to report an inconsistency detected by the program; the message 1787can state the source line at which the inconsistency was detected. For 1788example, 1789 1790@smallexample 1791fprintf (stderr, "Internal error: " 1792 "negative string length " 1793 "%d at %s, line %d.", 1794 length, __FILE__, __LINE__); 1795@end smallexample 1796 1797An @samp{#include} directive changes the expansions of @code{__FILE__} 1798and @code{__LINE__} to correspond to the included file. At the end of 1799that file, when processing resumes on the input file that contained 1800the @samp{#include} directive, the expansions of @code{__FILE__} and 1801@code{__LINE__} revert to the values they had before the 1802@samp{#include} (but @code{__LINE__} is then incremented by one as 1803processing moves to the line after the @samp{#include}). 1804 1805A @samp{#line} directive changes @code{__LINE__}, and may change 1806@code{__FILE__} as well. @xref{Line Control}. 1807 1808C99 introduced @code{__func__}, and GCC has provided @code{__FUNCTION__} 1809for a long time. Both of these are strings containing the name of the 1810current function (there are slight semantic differences; see the GCC 1811manual). Neither of them is a macro; the preprocessor does not know the 1812name of the current function. They tend to be useful in conjunction 1813with @code{__FILE__} and @code{__LINE__}, though. 1814 1815@table @code 1816 1817@item __DATE__ 1818This macro expands to a string constant that describes the date on which 1819the preprocessor is being run. The string constant contains eleven 1820characters and looks like @code{@w{"Feb 12 1996"}}. If the day of the 1821month is less than 10, it is padded with a space on the left. 1822 1823If GCC cannot determine the current date, it will emit a warning message 1824(once per compilation) and @code{__DATE__} will expand to 1825@code{@w{"??? ?? ????"}}. 1826 1827@item __TIME__ 1828This macro expands to a string constant that describes the time at 1829which the preprocessor is being run. The string constant contains 1830eight characters and looks like @code{"23:59:01"}. 1831 1832If GCC cannot determine the current time, it will emit a warning message 1833(once per compilation) and @code{__TIME__} will expand to 1834@code{"??:??:??"}. 1835 1836@item __STDC__ 1837In normal operation, this macro expands to the constant 1, to signify 1838that this compiler conforms to ISO Standard C@. If GNU CPP is used with 1839a compiler other than GCC, this is not necessarily true; however, the 1840preprocessor always conforms to the standard unless the 1841@option{-traditional-cpp} option is used. 1842 1843This macro is not defined if the @option{-traditional-cpp} option is used. 1844 1845On some hosts, the system compiler uses a different convention, where 1846@code{__STDC__} is normally 0, but is 1 if the user specifies strict 1847conformance to the C Standard. CPP follows the host convention when 1848processing system header files, but when processing user files 1849@code{__STDC__} is always 1. This has been reported to cause problems; 1850for instance, some versions of Solaris provide X Windows headers that 1851expect @code{__STDC__} to be either undefined or 1. @xref{Invocation}. 1852 1853@item __STDC_VERSION__ 1854This macro expands to the C Standard's version number, a long integer 1855constant of the form @code{@var{yyyy}@var{mm}L} where @var{yyyy} and 1856@var{mm} are the year and month of the Standard version. This signifies 1857which version of the C Standard the compiler conforms to. Like 1858@code{__STDC__}, this is not necessarily accurate for the entire 1859implementation, unless GNU CPP is being used with GCC@. 1860 1861The value @code{199409L} signifies the 1989 C standard as amended in 18621994, which is the current default; the value @code{199901L} signifies 1863the 1999 revision of the C standard. Support for the 1999 revision is 1864not yet complete. 1865 1866This macro is not defined if the @option{-traditional-cpp} option is 1867used, nor when compiling C++ or Objective-C@. 1868 1869@item __STDC_HOSTED__ 1870This macro is defined, with value 1, if the compiler's target is a 1871@dfn{hosted environment}. A hosted environment has the complete 1872facilities of the standard C library available. 1873 1874@item __cplusplus 1875This macro is defined when the C++ compiler is in use. You can use 1876@code{__cplusplus} to test whether a header is compiled by a C compiler 1877or a C++ compiler. This macro is similar to @code{__STDC_VERSION__}, in 1878that it expands to a version number. Depending on the language standard 1879selected, the value of the macro is 1880@code{199711L} for the 1998 C++ standard, 1881@code{201103L} for the 2011 C++ standard, 1882@code{201402L} for the 2014 C++ standard, 1883or an unspecified value strictly larger than @code{201402L} for the 1884experimental languages enabled by @option{-std=c++1z} and 1885@option{-std=gnu++1z}. 1886 1887@item __OBJC__ 1888This macro is defined, with value 1, when the Objective-C compiler is in 1889use. You can use @code{__OBJC__} to test whether a header is compiled 1890by a C compiler or an Objective-C compiler. 1891 1892@item __ASSEMBLER__ 1893This macro is defined with value 1 when preprocessing assembly 1894language. 1895 1896@end table 1897 1898@node Common Predefined Macros 1899@subsection Common Predefined Macros 1900@cindex common predefined macros 1901 1902The common predefined macros are GNU C extensions. They are available 1903with the same meanings regardless of the machine or operating system on 1904which you are using GNU C or GNU Fortran. Their names all start with 1905double underscores. 1906 1907@table @code 1908 1909@item __COUNTER__ 1910This macro expands to sequential integral values starting from 0. In 1911conjunction with the @code{##} operator, this provides a convenient means to 1912generate unique identifiers. Care must be taken to ensure that 1913@code{__COUNTER__} is not expanded prior to inclusion of precompiled headers 1914which use it. Otherwise, the precompiled headers will not be used. 1915 1916@item __GFORTRAN__ 1917The GNU Fortran compiler defines this. 1918 1919@item __GNUC__ 1920@itemx __GNUC_MINOR__ 1921@itemx __GNUC_PATCHLEVEL__ 1922These macros are defined by all GNU compilers that use the C 1923preprocessor: C, C++, Objective-C and Fortran. Their values are the major 1924version, minor version, and patch level of the compiler, as integer 1925constants. For example, GCC version @var{x}.@var{y}.@var{z} 1926defines @code{__GNUC__} to @var{x}, @code{__GNUC_MINOR__} to @var{y}, 1927and @code{__GNUC_PATCHLEVEL__} to @var{z}. These 1928macros are also defined if you invoke the preprocessor directly. 1929 1930If all you need to know is whether or not your program is being compiled 1931by GCC, or a non-GCC compiler that claims to accept the GNU C dialects, 1932you can simply test @code{__GNUC__}. If you need to write code 1933which depends on a specific version, you must be more careful. Each 1934time the minor version is increased, the patch level is reset to zero; 1935each time the major version is increased, the 1936minor version and patch level are reset. If you wish to use the 1937predefined macros directly in the conditional, you will need to write it 1938like this: 1939 1940@smallexample 1941/* @r{Test for GCC > 3.2.0} */ 1942#if __GNUC__ > 3 || \ 1943 (__GNUC__ == 3 && (__GNUC_MINOR__ > 2 || \ 1944 (__GNUC_MINOR__ == 2 && \ 1945 __GNUC_PATCHLEVEL__ > 0)) 1946@end smallexample 1947 1948@noindent 1949Another approach is to use the predefined macros to 1950calculate a single number, then compare that against a threshold: 1951 1952@smallexample 1953#define GCC_VERSION (__GNUC__ * 10000 \ 1954 + __GNUC_MINOR__ * 100 \ 1955 + __GNUC_PATCHLEVEL__) 1956@dots{} 1957/* @r{Test for GCC > 3.2.0} */ 1958#if GCC_VERSION > 30200 1959@end smallexample 1960 1961@noindent 1962Many people find this form easier to understand. 1963 1964@item __GNUG__ 1965The GNU C++ compiler defines this. Testing it is equivalent to 1966testing @code{@w{(__GNUC__ && __cplusplus)}}. 1967 1968@item __STRICT_ANSI__ 1969GCC defines this macro if and only if the @option{-ansi} switch, or a 1970@option{-std} switch specifying strict conformance to some version of ISO C 1971or ISO C++, was specified when GCC was invoked. It is defined to @samp{1}. 1972This macro exists primarily to direct GNU libc's header files to use only 1973definitions found in standard C. 1974 1975@item __BASE_FILE__ 1976This macro expands to the name of the main input file, in the form 1977of a C string constant. This is the source file that was specified 1978on the command line of the preprocessor or C compiler. 1979 1980@item __INCLUDE_LEVEL__ 1981This macro expands to a decimal integer constant that represents the 1982depth of nesting in include files. The value of this macro is 1983incremented on every @samp{#include} directive and decremented at the 1984end of every included file. It starts out at 0, its value within the 1985base file specified on the command line. 1986 1987@item __ELF__ 1988This macro is defined if the target uses the ELF object format. 1989 1990@item __VERSION__ 1991This macro expands to a string constant which describes the version of 1992the compiler in use. You should not rely on its contents having any 1993particular form, but it can be counted on to contain at least the 1994release number. 1995 1996@item __OPTIMIZE__ 1997@itemx __OPTIMIZE_SIZE__ 1998@itemx __NO_INLINE__ 1999These macros describe the compilation mode. @code{__OPTIMIZE__} is 2000defined in all optimizing compilations. @code{__OPTIMIZE_SIZE__} is 2001defined if the compiler is optimizing for size, not speed. 2002@code{__NO_INLINE__} is defined if no functions will be inlined into 2003their callers (when not optimizing, or when inlining has been 2004specifically disabled by @option{-fno-inline}). 2005 2006These macros cause certain GNU header files to provide optimized 2007definitions, using macros or inline functions, of system library 2008functions. You should not use these macros in any way unless you make 2009sure that programs will execute with the same effect whether or not they 2010are defined. If they are defined, their value is 1. 2011 2012@item __GNUC_GNU_INLINE__ 2013GCC defines this macro if functions declared @code{inline} will be 2014handled in GCC's traditional gnu90 mode. Object files will contain 2015externally visible definitions of all functions declared @code{inline} 2016without @code{extern} or @code{static}. They will not contain any 2017definitions of any functions declared @code{extern inline}. 2018 2019@item __GNUC_STDC_INLINE__ 2020GCC defines this macro if functions declared @code{inline} will be 2021handled according to the ISO C99 or later standards. Object files will contain 2022externally visible definitions of all functions declared @code{extern 2023inline}. They will not contain definitions of any functions declared 2024@code{inline} without @code{extern}. 2025 2026If this macro is defined, GCC supports the @code{gnu_inline} function 2027attribute as a way to always get the gnu90 behavior. 2028 2029@item __CHAR_UNSIGNED__ 2030GCC defines this macro if and only if the data type @code{char} is 2031unsigned on the target machine. It exists to cause the standard header 2032file @file{limits.h} to work correctly. You should not use this macro 2033yourself; instead, refer to the standard macros defined in @file{limits.h}. 2034 2035@item __WCHAR_UNSIGNED__ 2036Like @code{__CHAR_UNSIGNED__}, this macro is defined if and only if the 2037data type @code{wchar_t} is unsigned and the front-end is in C++ mode. 2038 2039@item __REGISTER_PREFIX__ 2040This macro expands to a single token (not a string constant) which is 2041the prefix applied to CPU register names in assembly language for this 2042target. You can use it to write assembly that is usable in multiple 2043environments. For example, in the @code{m68k-aout} environment it 2044expands to nothing, but in the @code{m68k-coff} environment it expands 2045to a single @samp{%}. 2046 2047@item __USER_LABEL_PREFIX__ 2048This macro expands to a single token which is the prefix applied to 2049user labels (symbols visible to C code) in assembly. For example, in 2050the @code{m68k-aout} environment it expands to an @samp{_}, but in the 2051@code{m68k-coff} environment it expands to nothing. 2052 2053This macro will have the correct definition even if 2054@option{-f(no-)underscores} is in use, but it will not be correct if 2055target-specific options that adjust this prefix are used (e.g.@: the 2056OSF/rose @option{-mno-underscores} option). 2057 2058@item __SIZE_TYPE__ 2059@itemx __PTRDIFF_TYPE__ 2060@itemx __WCHAR_TYPE__ 2061@itemx __WINT_TYPE__ 2062@itemx __INTMAX_TYPE__ 2063@itemx __UINTMAX_TYPE__ 2064@itemx __SIG_ATOMIC_TYPE__ 2065@itemx __INT8_TYPE__ 2066@itemx __INT16_TYPE__ 2067@itemx __INT32_TYPE__ 2068@itemx __INT64_TYPE__ 2069@itemx __UINT8_TYPE__ 2070@itemx __UINT16_TYPE__ 2071@itemx __UINT32_TYPE__ 2072@itemx __UINT64_TYPE__ 2073@itemx __INT_LEAST8_TYPE__ 2074@itemx __INT_LEAST16_TYPE__ 2075@itemx __INT_LEAST32_TYPE__ 2076@itemx __INT_LEAST64_TYPE__ 2077@itemx __UINT_LEAST8_TYPE__ 2078@itemx __UINT_LEAST16_TYPE__ 2079@itemx __UINT_LEAST32_TYPE__ 2080@itemx __UINT_LEAST64_TYPE__ 2081@itemx __INT_FAST8_TYPE__ 2082@itemx __INT_FAST16_TYPE__ 2083@itemx __INT_FAST32_TYPE__ 2084@itemx __INT_FAST64_TYPE__ 2085@itemx __UINT_FAST8_TYPE__ 2086@itemx __UINT_FAST16_TYPE__ 2087@itemx __UINT_FAST32_TYPE__ 2088@itemx __UINT_FAST64_TYPE__ 2089@itemx __INTPTR_TYPE__ 2090@itemx __UINTPTR_TYPE__ 2091These macros are defined to the correct underlying types for the 2092@code{size_t}, @code{ptrdiff_t}, @code{wchar_t}, @code{wint_t}, 2093@code{intmax_t}, @code{uintmax_t}, @code{sig_atomic_t}, @code{int8_t}, 2094@code{int16_t}, @code{int32_t}, @code{int64_t}, @code{uint8_t}, 2095@code{uint16_t}, @code{uint32_t}, @code{uint64_t}, 2096@code{int_least8_t}, @code{int_least16_t}, @code{int_least32_t}, 2097@code{int_least64_t}, @code{uint_least8_t}, @code{uint_least16_t}, 2098@code{uint_least32_t}, @code{uint_least64_t}, @code{int_fast8_t}, 2099@code{int_fast16_t}, @code{int_fast32_t}, @code{int_fast64_t}, 2100@code{uint_fast8_t}, @code{uint_fast16_t}, @code{uint_fast32_t}, 2101@code{uint_fast64_t}, @code{intptr_t}, and @code{uintptr_t} typedefs, 2102respectively. They exist to make the standard header files 2103@file{stddef.h}, @file{stdint.h}, and @file{wchar.h} work correctly. 2104You should not use these macros directly; instead, include the 2105appropriate headers and use the typedefs. Some of these macros may 2106not be defined on particular systems if GCC does not provide a 2107@file{stdint.h} header on those systems. 2108 2109@item __CHAR_BIT__ 2110Defined to the number of bits used in the representation of the 2111@code{char} data type. It exists to make the standard header given 2112numerical limits work correctly. You should not use 2113this macro directly; instead, include the appropriate headers. 2114 2115@item __SCHAR_MAX__ 2116@itemx __WCHAR_MAX__ 2117@itemx __SHRT_MAX__ 2118@itemx __INT_MAX__ 2119@itemx __LONG_MAX__ 2120@itemx __LONG_LONG_MAX__ 2121@itemx __WINT_MAX__ 2122@itemx __SIZE_MAX__ 2123@itemx __PTRDIFF_MAX__ 2124@itemx __INTMAX_MAX__ 2125@itemx __UINTMAX_MAX__ 2126@itemx __SIG_ATOMIC_MAX__ 2127@itemx __INT8_MAX__ 2128@itemx __INT16_MAX__ 2129@itemx __INT32_MAX__ 2130@itemx __INT64_MAX__ 2131@itemx __UINT8_MAX__ 2132@itemx __UINT16_MAX__ 2133@itemx __UINT32_MAX__ 2134@itemx __UINT64_MAX__ 2135@itemx __INT_LEAST8_MAX__ 2136@itemx __INT_LEAST16_MAX__ 2137@itemx __INT_LEAST32_MAX__ 2138@itemx __INT_LEAST64_MAX__ 2139@itemx __UINT_LEAST8_MAX__ 2140@itemx __UINT_LEAST16_MAX__ 2141@itemx __UINT_LEAST32_MAX__ 2142@itemx __UINT_LEAST64_MAX__ 2143@itemx __INT_FAST8_MAX__ 2144@itemx __INT_FAST16_MAX__ 2145@itemx __INT_FAST32_MAX__ 2146@itemx __INT_FAST64_MAX__ 2147@itemx __UINT_FAST8_MAX__ 2148@itemx __UINT_FAST16_MAX__ 2149@itemx __UINT_FAST32_MAX__ 2150@itemx __UINT_FAST64_MAX__ 2151@itemx __INTPTR_MAX__ 2152@itemx __UINTPTR_MAX__ 2153@itemx __WCHAR_MIN__ 2154@itemx __WINT_MIN__ 2155@itemx __SIG_ATOMIC_MIN__ 2156Defined to the maximum value of the @code{signed char}, @code{wchar_t}, 2157@code{signed short}, 2158@code{signed int}, @code{signed long}, @code{signed long long}, 2159@code{wint_t}, @code{size_t}, @code{ptrdiff_t}, 2160@code{intmax_t}, @code{uintmax_t}, @code{sig_atomic_t}, @code{int8_t}, 2161@code{int16_t}, @code{int32_t}, @code{int64_t}, @code{uint8_t}, 2162@code{uint16_t}, @code{uint32_t}, @code{uint64_t}, 2163@code{int_least8_t}, @code{int_least16_t}, @code{int_least32_t}, 2164@code{int_least64_t}, @code{uint_least8_t}, @code{uint_least16_t}, 2165@code{uint_least32_t}, @code{uint_least64_t}, @code{int_fast8_t}, 2166@code{int_fast16_t}, @code{int_fast32_t}, @code{int_fast64_t}, 2167@code{uint_fast8_t}, @code{uint_fast16_t}, @code{uint_fast32_t}, 2168@code{uint_fast64_t}, @code{intptr_t}, and @code{uintptr_t} types and 2169to the minimum value of the @code{wchar_t}, @code{wint_t}, and 2170@code{sig_atomic_t} types respectively. They exist to make the 2171standard header given numerical limits work correctly. You should not 2172use these macros directly; instead, include the appropriate headers. 2173Some of these macros may not be defined on particular systems if GCC 2174does not provide a @file{stdint.h} header on those systems. 2175 2176@item __INT8_C 2177@itemx __INT16_C 2178@itemx __INT32_C 2179@itemx __INT64_C 2180@itemx __UINT8_C 2181@itemx __UINT16_C 2182@itemx __UINT32_C 2183@itemx __UINT64_C 2184@itemx __INTMAX_C 2185@itemx __UINTMAX_C 2186Defined to implementations of the standard @file{stdint.h} macros with 2187the same names without the leading @code{__}. They exist the make the 2188implementation of that header work correctly. You should not use 2189these macros directly; instead, include the appropriate headers. Some 2190of these macros may not be defined on particular systems if GCC does 2191not provide a @file{stdint.h} header on those systems. 2192 2193@item __SCHAR_WIDTH__ 2194@itemx __SHRT_WIDTH__ 2195@itemx __INT_WIDTH__ 2196@itemx __LONG_WIDTH__ 2197@itemx __LONG_LONG_WIDTH__ 2198@itemx __PTRDIFF_WIDTH__ 2199@itemx __SIG_ATOMIC_WIDTH__ 2200@itemx __SIZE_WIDTH__ 2201@itemx __WCHAR_WIDTH__ 2202@itemx __WINT_WIDTH__ 2203@itemx __INT_LEAST8_WIDTH__ 2204@itemx __INT_LEAST16_WIDTH__ 2205@itemx __INT_LEAST32_WIDTH__ 2206@itemx __INT_LEAST64_WIDTH__ 2207@itemx __INT_FAST8_WIDTH__ 2208@itemx __INT_FAST16_WIDTH__ 2209@itemx __INT_FAST32_WIDTH__ 2210@itemx __INT_FAST64_WIDTH__ 2211@itemx __INTPTR_WIDTH__ 2212@itemx __INTMAX_WIDTH__ 2213Defined to the bit widths of the corresponding types. They exist to 2214make the implementations of @file{limits.h} and @file{stdint.h} behave 2215correctly. You should not use these macros directly; instead, include 2216the appropriate headers. Some of these macros may not be defined on 2217particular systems if GCC does not provide a @file{stdint.h} header on 2218those systems. 2219 2220@item __SIZEOF_INT__ 2221@itemx __SIZEOF_LONG__ 2222@itemx __SIZEOF_LONG_LONG__ 2223@itemx __SIZEOF_SHORT__ 2224@itemx __SIZEOF_POINTER__ 2225@itemx __SIZEOF_FLOAT__ 2226@itemx __SIZEOF_DOUBLE__ 2227@itemx __SIZEOF_LONG_DOUBLE__ 2228@itemx __SIZEOF_SIZE_T__ 2229@itemx __SIZEOF_WCHAR_T__ 2230@itemx __SIZEOF_WINT_T__ 2231@itemx __SIZEOF_PTRDIFF_T__ 2232Defined to the number of bytes of the C standard data types: @code{int}, 2233@code{long}, @code{long long}, @code{short}, @code{void *}, @code{float}, 2234@code{double}, @code{long double}, @code{size_t}, @code{wchar_t}, @code{wint_t} 2235and @code{ptrdiff_t}. 2236 2237@item __BYTE_ORDER__ 2238@itemx __ORDER_LITTLE_ENDIAN__ 2239@itemx __ORDER_BIG_ENDIAN__ 2240@itemx __ORDER_PDP_ENDIAN__ 2241@code{__BYTE_ORDER__} is defined to one of the values 2242@code{__ORDER_LITTLE_ENDIAN__}, @code{__ORDER_BIG_ENDIAN__}, or 2243@code{__ORDER_PDP_ENDIAN__} to reflect the layout of multi-byte and 2244multi-word quantities in memory. If @code{__BYTE_ORDER__} is equal to 2245@code{__ORDER_LITTLE_ENDIAN__} or @code{__ORDER_BIG_ENDIAN__}, then 2246multi-byte and multi-word quantities are laid out identically: the 2247byte (word) at the lowest address is the least significant or most 2248significant byte (word) of the quantity, respectively. If 2249@code{__BYTE_ORDER__} is equal to @code{__ORDER_PDP_ENDIAN__}, then 2250bytes in 16-bit words are laid out in a little-endian fashion, whereas 2251the 16-bit subwords of a 32-bit quantity are laid out in big-endian 2252fashion. 2253 2254You should use these macros for testing like this: 2255 2256@smallexample 2257/* @r{Test for a little-endian machine} */ 2258#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ 2259@end smallexample 2260 2261@item __FLOAT_WORD_ORDER__ 2262@code{__FLOAT_WORD_ORDER__} is defined to one of the values 2263@code{__ORDER_LITTLE_ENDIAN__} or @code{__ORDER_BIG_ENDIAN__} to reflect 2264the layout of the words of multi-word floating-point quantities. 2265 2266@item __DEPRECATED 2267This macro is defined, with value 1, when compiling a C++ source file 2268with warnings about deprecated constructs enabled. These warnings are 2269enabled by default, but can be disabled with @option{-Wno-deprecated}. 2270 2271@item __EXCEPTIONS 2272This macro is defined, with value 1, when compiling a C++ source file 2273with exceptions enabled. If @option{-fno-exceptions} is used when 2274compiling the file, then this macro is not defined. 2275 2276@item __GXX_RTTI 2277This macro is defined, with value 1, when compiling a C++ source file 2278with runtime type identification enabled. If @option{-fno-rtti} is 2279used when compiling the file, then this macro is not defined. 2280 2281@item __USING_SJLJ_EXCEPTIONS__ 2282This macro is defined, with value 1, if the compiler uses the old 2283mechanism based on @code{setjmp} and @code{longjmp} for exception 2284handling. 2285 2286@item __GXX_EXPERIMENTAL_CXX0X__ 2287This macro is defined when compiling a C++ source file with the option 2288@option{-std=c++0x} or @option{-std=gnu++0x}. It indicates that some 2289features likely to be included in C++0x are available. Note that these 2290features are experimental, and may change or be removed in future 2291versions of GCC. 2292 2293@item __GXX_WEAK__ 2294This macro is defined when compiling a C++ source file. It has the 2295value 1 if the compiler will use weak symbols, COMDAT sections, or 2296other similar techniques to collapse symbols with ``vague linkage'' 2297that are defined in multiple translation units. If the compiler will 2298not collapse such symbols, this macro is defined with value 0. In 2299general, user code should not need to make use of this macro; the 2300purpose of this macro is to ease implementation of the C++ runtime 2301library provided with G++. 2302 2303@item __NEXT_RUNTIME__ 2304This macro is defined, with value 1, if (and only if) the NeXT runtime 2305(as in @option{-fnext-runtime}) is in use for Objective-C@. If the GNU 2306runtime is used, this macro is not defined, so that you can use this 2307macro to determine which runtime (NeXT or GNU) is being used. 2308 2309@item __LP64__ 2310@itemx _LP64 2311These macros are defined, with value 1, if (and only if) the compilation 2312is for a target where @code{long int} and pointer both use 64-bits and 2313@code{int} uses 32-bit. 2314 2315@item __SSP__ 2316This macro is defined, with value 1, when @option{-fstack-protector} is in 2317use. 2318 2319@item __SSP_ALL__ 2320This macro is defined, with value 2, when @option{-fstack-protector-all} is 2321in use. 2322 2323@item __SSP_STRONG__ 2324This macro is defined, with value 3, when @option{-fstack-protector-strong} is 2325in use. 2326 2327@item __SSP_EXPLICIT__ 2328This macro is defined, with value 4, when @option{-fstack-protector-explicit} is 2329in use. 2330 2331@item __SANITIZE_ADDRESS__ 2332This macro is defined, with value 1, when @option{-fsanitize=address} 2333or @option{-fsanitize=kernel-address} are in use. 2334 2335@item __SANITIZE_THREAD__ 2336This macro is defined, with value 1, when @option{-fsanitize=thread} is in use. 2337 2338@item __TIMESTAMP__ 2339This macro expands to a string constant that describes the date and time 2340of the last modification of the current source file. The string constant 2341contains abbreviated day of the week, month, day of the month, time in 2342hh:mm:ss form, year and looks like @code{@w{"Sun Sep 16 01:03:52 1973"}}. 2343If the day of the month is less than 10, it is padded with a space on the left. 2344 2345If GCC cannot determine the current date, it will emit a warning message 2346(once per compilation) and @code{__TIMESTAMP__} will expand to 2347@code{@w{"??? ??? ?? ??:??:?? ????"}}. 2348 2349@item __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1 2350@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2 2351@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 2352@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8 2353@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 2354These macros are defined when the target processor supports atomic compare 2355and swap operations on operands 1, 2, 4, 8 or 16 bytes in length, respectively. 2356 2357@item __GCC_HAVE_DWARF2_CFI_ASM 2358This macro is defined when the compiler is emitting DWARF CFI directives 2359to the assembler. When this is defined, it is possible to emit those same 2360directives in inline assembly. 2361 2362@item __FP_FAST_FMA 2363@itemx __FP_FAST_FMAF 2364@itemx __FP_FAST_FMAL 2365These macros are defined with value 1 if the backend supports the 2366@code{fma}, @code{fmaf}, and @code{fmal} builtin functions, so that 2367the include file @file{math.h} can define the macros 2368@code{FP_FAST_FMA}, @code{FP_FAST_FMAF}, and @code{FP_FAST_FMAL} 2369for compatibility with the 1999 C standard. 2370 2371@item __GCC_IEC_559 2372This macro is defined to indicate the intended level of support for 2373IEEE 754 (IEC 60559) floating-point arithmetic. It expands to a 2374nonnegative integer value. If 0, it indicates that the combination of 2375the compiler configuration and the command-line options is not 2376intended to support IEEE 754 arithmetic for @code{float} and 2377@code{double} as defined in C99 and C11 Annex F (for example, that the 2378standard rounding modes and exceptions are not supported, or that 2379optimizations are enabled that conflict with IEEE 754 semantics). If 23801, it indicates that IEEE 754 arithmetic is intended to be supported; 2381this does not mean that all relevant language features are supported 2382by GCC. If 2 or more, it additionally indicates support for IEEE 2383754-2008 (in particular, that the binary encodings for quiet and 2384signaling NaNs are as specified in IEEE 754-2008). 2385 2386This macro does not indicate the default state of command-line options 2387that control optimizations that C99 and C11 permit to be controlled by 2388standard pragmas, where those standards do not require a particular 2389default state. It does not indicate whether optimizations respect 2390signaling NaN semantics (the macro for that is 2391@code{__SUPPORT_SNAN__}). It does not indicate support for decimal 2392floating point or the IEEE 754 binary16 and binary128 types. 2393 2394@item __GCC_IEC_559_COMPLEX 2395This macro is defined to indicate the intended level of support for 2396IEEE 754 (IEC 60559) floating-point arithmetic for complex numbers, as 2397defined in C99 and C11 Annex G. It expands to a nonnegative integer 2398value. If 0, it indicates that the combination of the compiler 2399configuration and the command-line options is not intended to support 2400Annex G requirements (for example, because @option{-fcx-limited-range} 2401was used). If 1 or more, it indicates that it is intended to support 2402those requirements; this does not mean that all relevant language 2403features are supported by GCC. 2404 2405@item __NO_MATH_ERRNO__ 2406This macro is defined if @option{-fno-math-errno} is used, or enabled 2407by another option such as @option{-ffast-math} or by default. 2408@end table 2409 2410@node System-specific Predefined Macros 2411@subsection System-specific Predefined Macros 2412 2413@cindex system-specific predefined macros 2414@cindex predefined macros, system-specific 2415@cindex reserved namespace 2416 2417The C preprocessor normally predefines several macros that indicate what 2418type of system and machine is in use. They are obviously different on 2419each target supported by GCC@. This manual, being for all systems and 2420machines, cannot tell you what their names are, but you can use 2421@command{cpp -dM} to see them all. @xref{Invocation}. All system-specific 2422predefined macros expand to a constant value, so you can test them with 2423either @samp{#ifdef} or @samp{#if}. 2424 2425The C standard requires that all system-specific macros be part of the 2426@dfn{reserved namespace}. All names which begin with two underscores, 2427or an underscore and a capital letter, are reserved for the compiler and 2428library to use as they wish. However, historically system-specific 2429macros have had names with no special prefix; for instance, it is common 2430to find @code{unix} defined on Unix systems. For all such macros, GCC 2431provides a parallel macro with two underscores added at the beginning 2432and the end. If @code{unix} is defined, @code{__unix__} will be defined 2433too. There will never be more than two underscores; the parallel of 2434@code{_mips} is @code{__mips__}. 2435 2436When the @option{-ansi} option, or any @option{-std} option that 2437requests strict conformance, is given to the compiler, all the 2438system-specific predefined macros outside the reserved namespace are 2439suppressed. The parallel macros, inside the reserved namespace, remain 2440defined. 2441 2442We are slowly phasing out all predefined macros which are outside the 2443reserved namespace. You should never use them in new programs, and we 2444encourage you to correct older code to use the parallel macros whenever 2445you find it. We don't recommend you use the system-specific macros that 2446are in the reserved namespace, either. It is better in the long run to 2447check specifically for features you need, using a tool such as 2448@command{autoconf}. 2449 2450@node C++ Named Operators 2451@subsection C++ Named Operators 2452@cindex named operators 2453@cindex C++ named operators 2454@cindex @file{iso646.h} 2455 2456In C++, there are eleven keywords which are simply alternate spellings 2457of operators normally written with punctuation. These keywords are 2458treated as such even in the preprocessor. They function as operators in 2459@samp{#if}, and they cannot be defined as macros or poisoned. In C, you 2460can request that those keywords take their C++ meaning by including 2461@file{iso646.h}. That header defines each one as a normal object-like 2462macro expanding to the appropriate punctuator. 2463 2464These are the named operators and their corresponding punctuators: 2465 2466@multitable {Named Operator} {Punctuator} 2467@item Named Operator @tab Punctuator 2468@item @code{and} @tab @code{&&} 2469@item @code{and_eq} @tab @code{&=} 2470@item @code{bitand} @tab @code{&} 2471@item @code{bitor} @tab @code{|} 2472@item @code{compl} @tab @code{~} 2473@item @code{not} @tab @code{!} 2474@item @code{not_eq} @tab @code{!=} 2475@item @code{or} @tab @code{||} 2476@item @code{or_eq} @tab @code{|=} 2477@item @code{xor} @tab @code{^} 2478@item @code{xor_eq} @tab @code{^=} 2479@end multitable 2480 2481@node Undefining and Redefining Macros 2482@section Undefining and Redefining Macros 2483@cindex undefining macros 2484@cindex redefining macros 2485@findex #undef 2486 2487If a macro ceases to be useful, it may be @dfn{undefined} with the 2488@samp{#undef} directive. @samp{#undef} takes a single argument, the 2489name of the macro to undefine. You use the bare macro name, even if the 2490macro is function-like. It is an error if anything appears on the line 2491after the macro name. @samp{#undef} has no effect if the name is not a 2492macro. 2493 2494@smallexample 2495#define FOO 4 2496x = FOO; @expansion{} x = 4; 2497#undef FOO 2498x = FOO; @expansion{} x = FOO; 2499@end smallexample 2500 2501Once a macro has been undefined, that identifier may be @dfn{redefined} 2502as a macro by a subsequent @samp{#define} directive. The new definition 2503need not have any resemblance to the old definition. 2504 2505However, if an identifier which is currently a macro is redefined, then 2506the new definition must be @dfn{effectively the same} as the old one. 2507Two macro definitions are effectively the same if: 2508@itemize @bullet 2509@item Both are the same type of macro (object- or function-like). 2510@item All the tokens of the replacement list are the same. 2511@item If there are any parameters, they are the same. 2512@item Whitespace appears in the same places in both. It need not be 2513exactly the same amount of whitespace, though. Remember that comments 2514count as whitespace. 2515@end itemize 2516 2517@noindent 2518These definitions are effectively the same: 2519@smallexample 2520#define FOUR (2 + 2) 2521#define FOUR (2 + 2) 2522#define FOUR (2 /* @r{two} */ + 2) 2523@end smallexample 2524@noindent 2525but these are not: 2526@smallexample 2527#define FOUR (2 + 2) 2528#define FOUR ( 2+2 ) 2529#define FOUR (2 * 2) 2530#define FOUR(score,and,seven,years,ago) (2 + 2) 2531@end smallexample 2532 2533If a macro is redefined with a definition that is not effectively the 2534same as the old one, the preprocessor issues a warning and changes the 2535macro to use the new definition. If the new definition is effectively 2536the same, the redefinition is silently ignored. This allows, for 2537instance, two different headers to define a common macro. The 2538preprocessor will only complain if the definitions do not match. 2539 2540@node Directives Within Macro Arguments 2541@section Directives Within Macro Arguments 2542@cindex macro arguments and directives 2543 2544Occasionally it is convenient to use preprocessor directives within 2545the arguments of a macro. The C and C++ standards declare that 2546behavior in these cases is undefined. GNU CPP 2547processes arbitrary directives within macro arguments in 2548exactly the same way as it would have processed the directive were the 2549function-like macro invocation not present. 2550 2551If, within a macro invocation, that macro is redefined, then the new 2552definition takes effect in time for argument pre-expansion, but the 2553original definition is still used for argument replacement. Here is a 2554pathological example: 2555 2556@smallexample 2557#define f(x) x x 2558f (1 2559#undef f 2560#define f 2 2561f) 2562@end smallexample 2563 2564@noindent 2565which expands to 2566 2567@smallexample 25681 2 1 2 2569@end smallexample 2570 2571@noindent 2572with the semantics described above. 2573 2574@node Macro Pitfalls 2575@section Macro Pitfalls 2576@cindex problems with macros 2577@cindex pitfalls of macros 2578 2579In this section we describe some special rules that apply to macros and 2580macro expansion, and point out certain cases in which the rules have 2581counter-intuitive consequences that you must watch out for. 2582 2583@menu 2584* Misnesting:: 2585* Operator Precedence Problems:: 2586* Swallowing the Semicolon:: 2587* Duplication of Side Effects:: 2588* Self-Referential Macros:: 2589* Argument Prescan:: 2590* Newlines in Arguments:: 2591@end menu 2592 2593@node Misnesting 2594@subsection Misnesting 2595 2596When a macro is called with arguments, the arguments are substituted 2597into the macro body and the result is checked, together with the rest of 2598the input file, for more macro calls. It is possible to piece together 2599a macro call coming partially from the macro body and partially from the 2600arguments. For example, 2601 2602@smallexample 2603#define twice(x) (2*(x)) 2604#define call_with_1(x) x(1) 2605call_with_1 (twice) 2606 @expansion{} twice(1) 2607 @expansion{} (2*(1)) 2608@end smallexample 2609 2610Macro definitions do not have to have balanced parentheses. By writing 2611an unbalanced open parenthesis in a macro body, it is possible to create 2612a macro call that begins inside the macro body but ends outside of it. 2613For example, 2614 2615@smallexample 2616#define strange(file) fprintf (file, "%s %d", 2617@dots{} 2618strange(stderr) p, 35) 2619 @expansion{} fprintf (stderr, "%s %d", p, 35) 2620@end smallexample 2621 2622The ability to piece together a macro call can be useful, but the use of 2623unbalanced open parentheses in a macro body is just confusing, and 2624should be avoided. 2625 2626@node Operator Precedence Problems 2627@subsection Operator Precedence Problems 2628@cindex parentheses in macro bodies 2629 2630You may have noticed that in most of the macro definition examples shown 2631above, each occurrence of a macro argument name had parentheses around 2632it. In addition, another pair of parentheses usually surround the 2633entire macro definition. Here is why it is best to write macros that 2634way. 2635 2636Suppose you define a macro as follows, 2637 2638@smallexample 2639#define ceil_div(x, y) (x + y - 1) / y 2640@end smallexample 2641 2642@noindent 2643whose purpose is to divide, rounding up. (One use for this operation is 2644to compute how many @code{int} objects are needed to hold a certain 2645number of @code{char} objects.) Then suppose it is used as follows: 2646 2647@smallexample 2648a = ceil_div (b & c, sizeof (int)); 2649 @expansion{} a = (b & c + sizeof (int) - 1) / sizeof (int); 2650@end smallexample 2651 2652@noindent 2653This does not do what is intended. The operator-precedence rules of 2654C make it equivalent to this: 2655 2656@smallexample 2657a = (b & (c + sizeof (int) - 1)) / sizeof (int); 2658@end smallexample 2659 2660@noindent 2661What we want is this: 2662 2663@smallexample 2664a = ((b & c) + sizeof (int) - 1)) / sizeof (int); 2665@end smallexample 2666 2667@noindent 2668Defining the macro as 2669 2670@smallexample 2671#define ceil_div(x, y) ((x) + (y) - 1) / (y) 2672@end smallexample 2673 2674@noindent 2675provides the desired result. 2676 2677Unintended grouping can result in another way. Consider @code{sizeof 2678ceil_div(1, 2)}. That has the appearance of a C expression that would 2679compute the size of the type of @code{ceil_div (1, 2)}, but in fact it 2680means something very different. Here is what it expands to: 2681 2682@smallexample 2683sizeof ((1) + (2) - 1) / (2) 2684@end smallexample 2685 2686@noindent 2687This would take the size of an integer and divide it by two. The 2688precedence rules have put the division outside the @code{sizeof} when it 2689was intended to be inside. 2690 2691Parentheses around the entire macro definition prevent such problems. 2692Here, then, is the recommended way to define @code{ceil_div}: 2693 2694@smallexample 2695#define ceil_div(x, y) (((x) + (y) - 1) / (y)) 2696@end smallexample 2697 2698@node Swallowing the Semicolon 2699@subsection Swallowing the Semicolon 2700@cindex semicolons (after macro calls) 2701 2702Often it is desirable to define a macro that expands into a compound 2703statement. Consider, for example, the following macro, that advances a 2704pointer (the argument @code{p} says where to find it) across whitespace 2705characters: 2706 2707@smallexample 2708#define SKIP_SPACES(p, limit) \ 2709@{ char *lim = (limit); \ 2710 while (p < lim) @{ \ 2711 if (*p++ != ' ') @{ \ 2712 p--; break; @}@}@} 2713@end smallexample 2714 2715@noindent 2716Here backslash-newline is used to split the macro definition, which must 2717be a single logical line, so that it resembles the way such code would 2718be laid out if not part of a macro definition. 2719 2720A call to this macro might be @code{SKIP_SPACES (p, lim)}. Strictly 2721speaking, the call expands to a compound statement, which is a complete 2722statement with no need for a semicolon to end it. However, since it 2723looks like a function call, it minimizes confusion if you can use it 2724like a function call, writing a semicolon afterward, as in 2725@code{SKIP_SPACES (p, lim);} 2726 2727This can cause trouble before @code{else} statements, because the 2728semicolon is actually a null statement. Suppose you write 2729 2730@smallexample 2731if (*p != 0) 2732 SKIP_SPACES (p, lim); 2733else @dots{} 2734@end smallexample 2735 2736@noindent 2737The presence of two statements---the compound statement and a null 2738statement---in between the @code{if} condition and the @code{else} 2739makes invalid C code. 2740 2741The definition of the macro @code{SKIP_SPACES} can be altered to solve 2742this problem, using a @code{do @dots{} while} statement. Here is how: 2743 2744@smallexample 2745#define SKIP_SPACES(p, limit) \ 2746do @{ char *lim = (limit); \ 2747 while (p < lim) @{ \ 2748 if (*p++ != ' ') @{ \ 2749 p--; break; @}@}@} \ 2750while (0) 2751@end smallexample 2752 2753Now @code{SKIP_SPACES (p, lim);} expands into 2754 2755@smallexample 2756do @{@dots{}@} while (0); 2757@end smallexample 2758 2759@noindent 2760which is one statement. The loop executes exactly once; most compilers 2761generate no extra code for it. 2762 2763@node Duplication of Side Effects 2764@subsection Duplication of Side Effects 2765 2766@cindex side effects (in macro arguments) 2767@cindex unsafe macros 2768Many C programs define a macro @code{min}, for ``minimum'', like this: 2769 2770@smallexample 2771#define min(X, Y) ((X) < (Y) ? (X) : (Y)) 2772@end smallexample 2773 2774When you use this macro with an argument containing a side effect, 2775as shown here, 2776 2777@smallexample 2778next = min (x + y, foo (z)); 2779@end smallexample 2780 2781@noindent 2782it expands as follows: 2783 2784@smallexample 2785next = ((x + y) < (foo (z)) ? (x + y) : (foo (z))); 2786@end smallexample 2787 2788@noindent 2789where @code{x + y} has been substituted for @code{X} and @code{foo (z)} 2790for @code{Y}. 2791 2792The function @code{foo} is used only once in the statement as it appears 2793in the program, but the expression @code{foo (z)} has been substituted 2794twice into the macro expansion. As a result, @code{foo} might be called 2795two times when the statement is executed. If it has side effects or if 2796it takes a long time to compute, the results might not be what you 2797intended. We say that @code{min} is an @dfn{unsafe} macro. 2798 2799The best solution to this problem is to define @code{min} in a way that 2800computes the value of @code{foo (z)} only once. The C language offers 2801no standard way to do this, but it can be done with GNU extensions as 2802follows: 2803 2804@smallexample 2805#define min(X, Y) \ 2806(@{ typeof (X) x_ = (X); \ 2807 typeof (Y) y_ = (Y); \ 2808 (x_ < y_) ? x_ : y_; @}) 2809@end smallexample 2810 2811The @samp{(@{ @dots{} @})} notation produces a compound statement that 2812acts as an expression. Its value is the value of its last statement. 2813This permits us to define local variables and assign each argument to 2814one. The local variables have underscores after their names to reduce 2815the risk of conflict with an identifier of wider scope (it is impossible 2816to avoid this entirely). Now each argument is evaluated exactly once. 2817 2818If you do not wish to use GNU C extensions, the only solution is to be 2819careful when @emph{using} the macro @code{min}. For example, you can 2820calculate the value of @code{foo (z)}, save it in a variable, and use 2821that variable in @code{min}: 2822 2823@smallexample 2824@group 2825#define min(X, Y) ((X) < (Y) ? (X) : (Y)) 2826@dots{} 2827@{ 2828 int tem = foo (z); 2829 next = min (x + y, tem); 2830@} 2831@end group 2832@end smallexample 2833 2834@noindent 2835(where we assume that @code{foo} returns type @code{int}). 2836 2837@node Self-Referential Macros 2838@subsection Self-Referential Macros 2839@cindex self-reference 2840 2841A @dfn{self-referential} macro is one whose name appears in its 2842definition. Recall that all macro definitions are rescanned for more 2843macros to replace. If the self-reference were considered a use of the 2844macro, it would produce an infinitely large expansion. To prevent this, 2845the self-reference is not considered a macro call. It is passed into 2846the preprocessor output unchanged. Consider an example: 2847 2848@smallexample 2849#define foo (4 + foo) 2850@end smallexample 2851 2852@noindent 2853where @code{foo} is also a variable in your program. 2854 2855Following the ordinary rules, each reference to @code{foo} will expand 2856into @code{(4 + foo)}; then this will be rescanned and will expand into 2857@code{(4 + (4 + foo))}; and so on until the computer runs out of memory. 2858 2859The self-reference rule cuts this process short after one step, at 2860@code{(4 + foo)}. Therefore, this macro definition has the possibly 2861useful effect of causing the program to add 4 to the value of @code{foo} 2862wherever @code{foo} is referred to. 2863 2864In most cases, it is a bad idea to take advantage of this feature. A 2865person reading the program who sees that @code{foo} is a variable will 2866not expect that it is a macro as well. The reader will come across the 2867identifier @code{foo} in the program and think its value should be that 2868of the variable @code{foo}, whereas in fact the value is four greater. 2869 2870One common, useful use of self-reference is to create a macro which 2871expands to itself. If you write 2872 2873@smallexample 2874#define EPERM EPERM 2875@end smallexample 2876 2877@noindent 2878then the macro @code{EPERM} expands to @code{EPERM}. Effectively, it is 2879left alone by the preprocessor whenever it's used in running text. You 2880can tell that it's a macro with @samp{#ifdef}. You might do this if you 2881want to define numeric constants with an @code{enum}, but have 2882@samp{#ifdef} be true for each constant. 2883 2884If a macro @code{x} expands to use a macro @code{y}, and the expansion of 2885@code{y} refers to the macro @code{x}, that is an @dfn{indirect 2886self-reference} of @code{x}. @code{x} is not expanded in this case 2887either. Thus, if we have 2888 2889@smallexample 2890#define x (4 + y) 2891#define y (2 * x) 2892@end smallexample 2893 2894@noindent 2895then @code{x} and @code{y} expand as follows: 2896 2897@smallexample 2898@group 2899x @expansion{} (4 + y) 2900 @expansion{} (4 + (2 * x)) 2901 2902y @expansion{} (2 * x) 2903 @expansion{} (2 * (4 + y)) 2904@end group 2905@end smallexample 2906 2907@noindent 2908Each macro is expanded when it appears in the definition of the other 2909macro, but not when it indirectly appears in its own definition. 2910 2911@node Argument Prescan 2912@subsection Argument Prescan 2913@cindex expansion of arguments 2914@cindex macro argument expansion 2915@cindex prescan of macro arguments 2916 2917Macro arguments are completely macro-expanded before they are 2918substituted into a macro body, unless they are stringized or pasted 2919with other tokens. After substitution, the entire macro body, including 2920the substituted arguments, is scanned again for macros to be expanded. 2921The result is that the arguments are scanned @emph{twice} to expand 2922macro calls in them. 2923 2924Most of the time, this has no effect. If the argument contained any 2925macro calls, they are expanded during the first scan. The result 2926therefore contains no macro calls, so the second scan does not change 2927it. If the argument were substituted as given, with no prescan, the 2928single remaining scan would find the same macro calls and produce the 2929same results. 2930 2931You might expect the double scan to change the results when a 2932self-referential macro is used in an argument of another macro 2933(@pxref{Self-Referential Macros}): the self-referential macro would be 2934expanded once in the first scan, and a second time in the second scan. 2935However, this is not what happens. The self-references that do not 2936expand in the first scan are marked so that they will not expand in the 2937second scan either. 2938 2939You might wonder, ``Why mention the prescan, if it makes no difference? 2940And why not skip it and make the preprocessor faster?'' The answer is 2941that the prescan does make a difference in three special cases: 2942 2943@itemize @bullet 2944@item 2945Nested calls to a macro. 2946 2947We say that @dfn{nested} calls to a macro occur when a macro's argument 2948contains a call to that very macro. For example, if @code{f} is a macro 2949that expects one argument, @code{f (f (1))} is a nested pair of calls to 2950@code{f}. The desired expansion is made by expanding @code{f (1)} and 2951substituting that into the definition of @code{f}. The prescan causes 2952the expected result to happen. Without the prescan, @code{f (1)} itself 2953would be substituted as an argument, and the inner use of @code{f} would 2954appear during the main scan as an indirect self-reference and would not 2955be expanded. 2956 2957@item 2958Macros that call other macros that stringize or concatenate. 2959 2960If an argument is stringized or concatenated, the prescan does not 2961occur. If you @emph{want} to expand a macro, then stringize or 2962concatenate its expansion, you can do that by causing one macro to call 2963another macro that does the stringizing or concatenation. For 2964instance, if you have 2965 2966@smallexample 2967#define AFTERX(x) X_ ## x 2968#define XAFTERX(x) AFTERX(x) 2969#define TABLESIZE 1024 2970#define BUFSIZE TABLESIZE 2971@end smallexample 2972 2973then @code{AFTERX(BUFSIZE)} expands to @code{X_BUFSIZE}, and 2974@code{XAFTERX(BUFSIZE)} expands to @code{X_1024}. (Not to 2975@code{X_TABLESIZE}. Prescan always does a complete expansion.) 2976 2977@item 2978Macros used in arguments, whose expansions contain unshielded commas. 2979 2980This can cause a macro expanded on the second scan to be called with the 2981wrong number of arguments. Here is an example: 2982 2983@smallexample 2984#define foo a,b 2985#define bar(x) lose(x) 2986#define lose(x) (1 + (x)) 2987@end smallexample 2988 2989We would like @code{bar(foo)} to turn into @code{(1 + (foo))}, which 2990would then turn into @code{(1 + (a,b))}. Instead, @code{bar(foo)} 2991expands into @code{lose(a,b)}, and you get an error because @code{lose} 2992requires a single argument. In this case, the problem is easily solved 2993by the same parentheses that ought to be used to prevent misnesting of 2994arithmetic operations: 2995 2996@smallexample 2997#define foo (a,b) 2998@exdent or 2999#define bar(x) lose((x)) 3000@end smallexample 3001 3002The extra pair of parentheses prevents the comma in @code{foo}'s 3003definition from being interpreted as an argument separator. 3004 3005@end itemize 3006 3007@node Newlines in Arguments 3008@subsection Newlines in Arguments 3009@cindex newlines in macro arguments 3010 3011The invocation of a function-like macro can extend over many logical 3012lines. However, in the present implementation, the entire expansion 3013comes out on one line. Thus line numbers emitted by the compiler or 3014debugger refer to the line the invocation started on, which might be 3015different to the line containing the argument causing the problem. 3016 3017Here is an example illustrating this: 3018 3019@smallexample 3020#define ignore_second_arg(a,b,c) a; c 3021 3022ignore_second_arg (foo (), 3023 ignored (), 3024 syntax error); 3025@end smallexample 3026 3027@noindent 3028The syntax error triggered by the tokens @code{syntax error} results in 3029an error message citing line three---the line of ignore_second_arg--- 3030even though the problematic code comes from line five. 3031 3032We consider this a bug, and intend to fix it in the near future. 3033 3034@node Conditionals 3035@chapter Conditionals 3036@cindex conditionals 3037 3038A @dfn{conditional} is a directive that instructs the preprocessor to 3039select whether or not to include a chunk of code in the final token 3040stream passed to the compiler. Preprocessor conditionals can test 3041arithmetic expressions, or whether a name is defined as a macro, or both 3042simultaneously using the special @code{defined} operator. 3043 3044A conditional in the C preprocessor resembles in some ways an @code{if} 3045statement in C, but it is important to understand the difference between 3046them. The condition in an @code{if} statement is tested during the 3047execution of your program. Its purpose is to allow your program to 3048behave differently from run to run, depending on the data it is 3049operating on. The condition in a preprocessing conditional directive is 3050tested when your program is compiled. Its purpose is to allow different 3051code to be included in the program depending on the situation at the 3052time of compilation. 3053 3054However, the distinction is becoming less clear. Modern compilers often 3055do test @code{if} statements when a program is compiled, if their 3056conditions are known not to vary at run time, and eliminate code which 3057can never be executed. If you can count on your compiler to do this, 3058you may find that your program is more readable if you use @code{if} 3059statements with constant conditions (perhaps determined by macros). Of 3060course, you can only use this to exclude code, not type definitions or 3061other preprocessing directives, and you can only do it if the code 3062remains syntactically valid when it is not to be used. 3063 3064@menu 3065* Conditional Uses:: 3066* Conditional Syntax:: 3067* Deleted Code:: 3068@end menu 3069 3070@node Conditional Uses 3071@section Conditional Uses 3072 3073There are three general reasons to use a conditional. 3074 3075@itemize @bullet 3076@item 3077A program may need to use different code depending on the machine or 3078operating system it is to run on. In some cases the code for one 3079operating system may be erroneous on another operating system; for 3080example, it might refer to data types or constants that do not exist on 3081the other system. When this happens, it is not enough to avoid 3082executing the invalid code. Its mere presence will cause the compiler 3083to reject the program. With a preprocessing conditional, the offending 3084code can be effectively excised from the program when it is not valid. 3085 3086@item 3087You may want to be able to compile the same source file into two 3088different programs. One version might make frequent time-consuming 3089consistency checks on its intermediate data, or print the values of 3090those data for debugging, and the other not. 3091 3092@item 3093A conditional whose condition is always false is one way to exclude code 3094from the program but keep it as a sort of comment for future reference. 3095@end itemize 3096 3097Simple programs that do not need system-specific logic or complex 3098debugging hooks generally will not need to use preprocessing 3099conditionals. 3100 3101@node Conditional Syntax 3102@section Conditional Syntax 3103 3104@findex #if 3105A conditional in the C preprocessor begins with a @dfn{conditional 3106directive}: @samp{#if}, @samp{#ifdef} or @samp{#ifndef}. 3107 3108@menu 3109* Ifdef:: 3110* If:: 3111* Defined:: 3112* Else:: 3113* Elif:: 3114@end menu 3115 3116@node Ifdef 3117@subsection Ifdef 3118@findex #ifdef 3119@findex #endif 3120 3121The simplest sort of conditional is 3122 3123@smallexample 3124@group 3125#ifdef @var{MACRO} 3126 3127@var{controlled text} 3128 3129#endif /* @var{MACRO} */ 3130@end group 3131@end smallexample 3132 3133@cindex conditional group 3134This block is called a @dfn{conditional group}. @var{controlled text} 3135will be included in the output of the preprocessor if and only if 3136@var{MACRO} is defined. We say that the conditional @dfn{succeeds} if 3137@var{MACRO} is defined, @dfn{fails} if it is not. 3138 3139The @var{controlled text} inside of a conditional can include 3140preprocessing directives. They are executed only if the conditional 3141succeeds. You can nest conditional groups inside other conditional 3142groups, but they must be completely nested. In other words, 3143@samp{#endif} always matches the nearest @samp{#ifdef} (or 3144@samp{#ifndef}, or @samp{#if}). Also, you cannot start a conditional 3145group in one file and end it in another. 3146 3147Even if a conditional fails, the @var{controlled text} inside it is 3148still run through initial transformations and tokenization. Therefore, 3149it must all be lexically valid C@. Normally the only way this matters is 3150that all comments and string literals inside a failing conditional group 3151must still be properly ended. 3152 3153The comment following the @samp{#endif} is not required, but it is a 3154good practice if there is a lot of @var{controlled text}, because it 3155helps people match the @samp{#endif} to the corresponding @samp{#ifdef}. 3156Older programs sometimes put @var{MACRO} directly after the 3157@samp{#endif} without enclosing it in a comment. This is invalid code 3158according to the C standard. CPP accepts it with a warning. It 3159never affects which @samp{#ifndef} the @samp{#endif} matches. 3160 3161@findex #ifndef 3162Sometimes you wish to use some code if a macro is @emph{not} defined. 3163You can do this by writing @samp{#ifndef} instead of @samp{#ifdef}. 3164One common use of @samp{#ifndef} is to include code only the first 3165time a header file is included. @xref{Once-Only Headers}. 3166 3167Macro definitions can vary between compilations for several reasons. 3168Here are some samples. 3169 3170@itemize @bullet 3171@item 3172Some macros are predefined on each kind of machine 3173(@pxref{System-specific Predefined Macros}). This allows you to provide 3174code specially tuned for a particular machine. 3175 3176@item 3177System header files define more macros, associated with the features 3178they implement. You can test these macros with conditionals to avoid 3179using a system feature on a machine where it is not implemented. 3180 3181@item 3182Macros can be defined or undefined with the @option{-D} and @option{-U} 3183command-line options when you compile the program. You can arrange to 3184compile the same source file into two different programs by choosing a 3185macro name to specify which program you want, writing conditionals to 3186test whether or how this macro is defined, and then controlling the 3187state of the macro with command-line options, perhaps set in the 3188Makefile. @xref{Invocation}. 3189 3190@item 3191Your program might have a special header file (often called 3192@file{config.h}) that is adjusted when the program is compiled. It can 3193define or not define macros depending on the features of the system and 3194the desired capabilities of the program. The adjustment can be 3195automated by a tool such as @command{autoconf}, or done by hand. 3196@end itemize 3197 3198@node If 3199@subsection If 3200 3201The @samp{#if} directive allows you to test the value of an arithmetic 3202expression, rather than the mere existence of one macro. Its syntax is 3203 3204@smallexample 3205@group 3206#if @var{expression} 3207 3208@var{controlled text} 3209 3210#endif /* @var{expression} */ 3211@end group 3212@end smallexample 3213 3214@var{expression} is a C expression of integer type, subject to stringent 3215restrictions. It may contain 3216 3217@itemize @bullet 3218@item 3219Integer constants. 3220 3221@item 3222Character constants, which are interpreted as they would be in normal 3223code. 3224 3225@item 3226Arithmetic operators for addition, subtraction, multiplication, 3227division, bitwise operations, shifts, comparisons, and logical 3228operations (@code{&&} and @code{||}). The latter two obey the usual 3229short-circuiting rules of standard C@. 3230 3231@item 3232Macros. All macros in the expression are expanded before actual 3233computation of the expression's value begins. 3234 3235@item 3236Uses of the @code{defined} operator, which lets you check whether macros 3237are defined in the middle of an @samp{#if}. 3238 3239@item 3240Identifiers that are not macros, which are all considered to be the 3241number zero. This allows you to write @code{@w{#if MACRO}} instead of 3242@code{@w{#ifdef MACRO}}, if you know that MACRO, when defined, will 3243always have a nonzero value. Function-like macros used without their 3244function call parentheses are also treated as zero. 3245 3246In some contexts this shortcut is undesirable. The @option{-Wundef} 3247option causes GCC to warn whenever it encounters an identifier which is 3248not a macro in an @samp{#if}. 3249@end itemize 3250 3251The preprocessor does not know anything about types in the language. 3252Therefore, @code{sizeof} operators are not recognized in @samp{#if}, and 3253neither are @code{enum} constants. They will be taken as identifiers 3254which are not macros, and replaced by zero. In the case of 3255@code{sizeof}, this is likely to cause the expression to be invalid. 3256 3257The preprocessor calculates the value of @var{expression}. It carries 3258out all calculations in the widest integer type known to the compiler; 3259on most machines supported by GCC this is 64 bits. This is not the same 3260rule as the compiler uses to calculate the value of a constant 3261expression, and may give different results in some cases. If the value 3262comes out to be nonzero, the @samp{#if} succeeds and the @var{controlled 3263text} is included; otherwise it is skipped. 3264 3265@node Defined 3266@subsection Defined 3267 3268@cindex @code{defined} 3269The special operator @code{defined} is used in @samp{#if} and 3270@samp{#elif} expressions to test whether a certain name is defined as a 3271macro. @code{defined @var{name}} and @code{defined (@var{name})} are 3272both expressions whose value is 1 if @var{name} is defined as a macro at 3273the current point in the program, and 0 otherwise. Thus, @code{@w{#if 3274defined MACRO}} is precisely equivalent to @code{@w{#ifdef MACRO}}. 3275 3276@code{defined} is useful when you wish to test more than one macro for 3277existence at once. For example, 3278 3279@smallexample 3280#if defined (__vax__) || defined (__ns16000__) 3281@end smallexample 3282 3283@noindent 3284would succeed if either of the names @code{__vax__} or 3285@code{__ns16000__} is defined as a macro. 3286 3287Conditionals written like this: 3288 3289@smallexample 3290#if defined BUFSIZE && BUFSIZE >= 1024 3291@end smallexample 3292 3293@noindent 3294can generally be simplified to just @code{@w{#if BUFSIZE >= 1024}}, 3295since if @code{BUFSIZE} is not defined, it will be interpreted as having 3296the value zero. 3297 3298If the @code{defined} operator appears as a result of a macro expansion, 3299the C standard says the behavior is undefined. GNU cpp treats it as a 3300genuine @code{defined} operator and evaluates it normally. It will warn 3301wherever your code uses this feature if you use the command-line option 3302@option{-Wpedantic}, since other compilers may handle it differently. The 3303warning is also enabled by @option{-Wextra}, and can also be enabled 3304individually with @option{-Wexpansion-to-defined}. 3305 3306@node Else 3307@subsection Else 3308 3309@findex #else 3310The @samp{#else} directive can be added to a conditional to provide 3311alternative text to be used if the condition fails. This is what it 3312looks like: 3313 3314@smallexample 3315@group 3316#if @var{expression} 3317@var{text-if-true} 3318#else /* Not @var{expression} */ 3319@var{text-if-false} 3320#endif /* Not @var{expression} */ 3321@end group 3322@end smallexample 3323 3324@noindent 3325If @var{expression} is nonzero, the @var{text-if-true} is included and 3326the @var{text-if-false} is skipped. If @var{expression} is zero, the 3327opposite happens. 3328 3329You can use @samp{#else} with @samp{#ifdef} and @samp{#ifndef}, too. 3330 3331@node Elif 3332@subsection Elif 3333 3334@findex #elif 3335One common case of nested conditionals is used to check for more than two 3336possible alternatives. For example, you might have 3337 3338@smallexample 3339#if X == 1 3340@dots{} 3341#else /* X != 1 */ 3342#if X == 2 3343@dots{} 3344#else /* X != 2 */ 3345@dots{} 3346#endif /* X != 2 */ 3347#endif /* X != 1 */ 3348@end smallexample 3349 3350Another conditional directive, @samp{#elif}, allows this to be 3351abbreviated as follows: 3352 3353@smallexample 3354#if X == 1 3355@dots{} 3356#elif X == 2 3357@dots{} 3358#else /* X != 2 and X != 1*/ 3359@dots{} 3360#endif /* X != 2 and X != 1*/ 3361@end smallexample 3362 3363@samp{#elif} stands for ``else if''. Like @samp{#else}, it goes in the 3364middle of a conditional group and subdivides it; it does not require a 3365matching @samp{#endif} of its own. Like @samp{#if}, the @samp{#elif} 3366directive includes an expression to be tested. The text following the 3367@samp{#elif} is processed only if the original @samp{#if}-condition 3368failed and the @samp{#elif} condition succeeds. 3369 3370More than one @samp{#elif} can go in the same conditional group. Then 3371the text after each @samp{#elif} is processed only if the @samp{#elif} 3372condition succeeds after the original @samp{#if} and all previous 3373@samp{#elif} directives within it have failed. 3374 3375@samp{#else} is allowed after any number of @samp{#elif} directives, but 3376@samp{#elif} may not follow @samp{#else}. 3377 3378@node Deleted Code 3379@section Deleted Code 3380@cindex commenting out code 3381 3382If you replace or delete a part of the program but want to keep the old 3383code around for future reference, you often cannot simply comment it 3384out. Block comments do not nest, so the first comment inside the old 3385code will end the commenting-out. The probable result is a flood of 3386syntax errors. 3387 3388One way to avoid this problem is to use an always-false conditional 3389instead. For instance, put @code{#if 0} before the deleted code and 3390@code{#endif} after it. This works even if the code being turned 3391off contains conditionals, but they must be entire conditionals 3392(balanced @samp{#if} and @samp{#endif}). 3393 3394Some people use @code{#ifdef notdef} instead. This is risky, because 3395@code{notdef} might be accidentally defined as a macro, and then the 3396conditional would succeed. @code{#if 0} can be counted on to fail. 3397 3398Do not use @code{#if 0} for comments which are not C code. Use a real 3399comment, instead. The interior of @code{#if 0} must consist of complete 3400tokens; in particular, single-quote characters must balance. Comments 3401often contain unbalanced single-quote characters (known in English as 3402apostrophes). These confuse @code{#if 0}. They don't confuse 3403@samp{/*}. 3404 3405@node Diagnostics 3406@chapter Diagnostics 3407@cindex diagnostic 3408@cindex reporting errors 3409@cindex reporting warnings 3410 3411@findex #error 3412The directive @samp{#error} causes the preprocessor to report a fatal 3413error. The tokens forming the rest of the line following @samp{#error} 3414are used as the error message. 3415 3416You would use @samp{#error} inside of a conditional that detects a 3417combination of parameters which you know the program does not properly 3418support. For example, if you know that the program will not run 3419properly on a VAX, you might write 3420 3421@smallexample 3422@group 3423#ifdef __vax__ 3424#error "Won't work on VAXen. See comments at get_last_object." 3425#endif 3426@end group 3427@end smallexample 3428 3429If you have several configuration parameters that must be set up by 3430the installation in a consistent way, you can use conditionals to detect 3431an inconsistency and report it with @samp{#error}. For example, 3432 3433@smallexample 3434#if !defined(FOO) && defined(BAR) 3435#error "BAR requires FOO." 3436#endif 3437@end smallexample 3438 3439@findex #warning 3440The directive @samp{#warning} is like @samp{#error}, but causes the 3441preprocessor to issue a warning and continue preprocessing. The tokens 3442following @samp{#warning} are used as the warning message. 3443 3444You might use @samp{#warning} in obsolete header files, with a message 3445directing the user to the header file which should be used instead. 3446 3447Neither @samp{#error} nor @samp{#warning} macro-expands its argument. 3448Internal whitespace sequences are each replaced with a single space. 3449The line must consist of complete tokens. It is wisest to make the 3450argument of these directives be a single string constant; this avoids 3451problems with apostrophes and the like. 3452 3453@node Line Control 3454@chapter Line Control 3455@cindex line control 3456 3457The C preprocessor informs the C compiler of the location in your source 3458code where each token came from. Presently, this is just the file name 3459and line number. All the tokens resulting from macro expansion are 3460reported as having appeared on the line of the source file where the 3461outermost macro was used. We intend to be more accurate in the future. 3462 3463If you write a program which generates source code, such as the 3464@command{bison} parser generator, you may want to adjust the preprocessor's 3465notion of the current file name and line number by hand. Parts of the 3466output from @command{bison} are generated from scratch, other parts come 3467from a standard parser file. The rest are copied verbatim from 3468@command{bison}'s input. You would like compiler error messages and 3469symbolic debuggers to be able to refer to @code{bison}'s input file. 3470 3471@findex #line 3472@command{bison} or any such program can arrange this by writing 3473@samp{#line} directives into the output file. @samp{#line} is a 3474directive that specifies the original line number and source file name 3475for subsequent input in the current preprocessor input file. 3476@samp{#line} has three variants: 3477 3478@table @code 3479@item #line @var{linenum} 3480@var{linenum} is a non-negative decimal integer constant. It specifies 3481the line number which should be reported for the following line of 3482input. Subsequent lines are counted from @var{linenum}. 3483 3484@item #line @var{linenum} @var{filename} 3485@var{linenum} is the same as for the first form, and has the same 3486effect. In addition, @var{filename} is a string constant. The 3487following line and all subsequent lines are reported to come from the 3488file it specifies, until something else happens to change that. 3489@var{filename} is interpreted according to the normal rules for a string 3490constant: backslash escapes are interpreted. This is different from 3491@samp{#include}. 3492 3493@item #line @var{anything else} 3494@var{anything else} is checked for macro calls, which are expanded. 3495The result should match one of the above two forms. 3496@end table 3497 3498@samp{#line} directives alter the results of the @code{__FILE__} and 3499@code{__LINE__} predefined macros from that point on. @xref{Standard 3500Predefined Macros}. They do not have any effect on @samp{#include}'s 3501idea of the directory containing the current file. 3502 3503@node Pragmas 3504@chapter Pragmas 3505 3506The @samp{#pragma} directive is the method specified by the C standard 3507for providing additional information to the compiler, beyond what is 3508conveyed in the language itself. The forms of this directive 3509(commonly known as @dfn{pragmas}) specified by C standard are prefixed with 3510@code{STDC}. A C compiler is free to attach any meaning it likes to other 3511pragmas. All GNU-defined, supported pragmas have been given a 3512@code{GCC} prefix. 3513 3514@cindex @code{_Pragma} 3515C99 introduced the @code{@w{_Pragma}} operator. This feature addresses a 3516major problem with @samp{#pragma}: being a directive, it cannot be 3517produced as the result of macro expansion. @code{@w{_Pragma}} is an 3518operator, much like @code{sizeof} or @code{defined}, and can be embedded 3519in a macro. 3520 3521Its syntax is @code{@w{_Pragma (@var{string-literal})}}, where 3522@var{string-literal} can be either a normal or wide-character string 3523literal. It is destringized, by replacing all @samp{\\} with a single 3524@samp{\} and all @samp{\"} with a @samp{"}. The result is then 3525processed as if it had appeared as the right hand side of a 3526@samp{#pragma} directive. For example, 3527 3528@smallexample 3529_Pragma ("GCC dependency \"parse.y\"") 3530@end smallexample 3531 3532@noindent 3533has the same effect as @code{#pragma GCC dependency "parse.y"}. The 3534same effect could be achieved using macros, for example 3535 3536@smallexample 3537#define DO_PRAGMA(x) _Pragma (#x) 3538DO_PRAGMA (GCC dependency "parse.y") 3539@end smallexample 3540 3541The standard is unclear on where a @code{_Pragma} operator can appear. 3542The preprocessor does not accept it within a preprocessing conditional 3543directive like @samp{#if}. To be safe, you are probably best keeping it 3544out of directives other than @samp{#define}, and putting it on a line of 3545its own. 3546 3547This manual documents the pragmas which are meaningful to the 3548preprocessor itself. Other pragmas are meaningful to the C or C++ 3549compilers. They are documented in the GCC manual. 3550 3551GCC plugins may provide their own pragmas. 3552 3553@ftable @code 3554@item #pragma GCC dependency 3555@code{#pragma GCC dependency} allows you to check the relative dates of 3556the current file and another file. If the other file is more recent than 3557the current file, a warning is issued. This is useful if the current 3558file is derived from the other file, and should be regenerated. The 3559other file is searched for using the normal include search path. 3560Optional trailing text can be used to give more information in the 3561warning message. 3562 3563@smallexample 3564#pragma GCC dependency "parse.y" 3565#pragma GCC dependency "/usr/include/time.h" rerun fixincludes 3566@end smallexample 3567 3568@item #pragma GCC poison 3569Sometimes, there is an identifier that you want to remove completely 3570from your program, and make sure that it never creeps back in. To 3571enforce this, you can @dfn{poison} the identifier with this pragma. 3572@code{#pragma GCC poison} is followed by a list of identifiers to 3573poison. If any of those identifiers appears anywhere in the source 3574after the directive, it is a hard error. For example, 3575 3576@smallexample 3577#pragma GCC poison printf sprintf fprintf 3578sprintf(some_string, "hello"); 3579@end smallexample 3580 3581@noindent 3582will produce an error. 3583 3584If a poisoned identifier appears as part of the expansion of a macro 3585which was defined before the identifier was poisoned, it will @emph{not} 3586cause an error. This lets you poison an identifier without worrying 3587about system headers defining macros that use it. 3588 3589For example, 3590 3591@smallexample 3592#define strrchr rindex 3593#pragma GCC poison rindex 3594strrchr(some_string, 'h'); 3595@end smallexample 3596 3597@noindent 3598will not produce an error. 3599 3600@item #pragma GCC system_header 3601This pragma takes no arguments. It causes the rest of the code in the 3602current file to be treated as if it came from a system header. 3603@xref{System Headers}. 3604 3605@item #pragma GCC warning 3606@itemx #pragma GCC error 3607@code{#pragma GCC warning "message"} causes the preprocessor to issue 3608a warning diagnostic with the text @samp{message}. The message 3609contained in the pragma must be a single string literal. Similarly, 3610@code{#pragma GCC error "message"} issues an error message. Unlike 3611the @samp{#warning} and @samp{#error} directives, these pragmas can be 3612embedded in preprocessor macros using @samp{_Pragma}. 3613 3614@end ftable 3615 3616@node Other Directives 3617@chapter Other Directives 3618 3619@findex #ident 3620@findex #sccs 3621The @samp{#ident} directive takes one argument, a string constant. On 3622some systems, that string constant is copied into a special segment of 3623the object file. On other systems, the directive is ignored. The 3624@samp{#sccs} directive is a synonym for @samp{#ident}. 3625 3626These directives are not part of the C standard, but they are not 3627official GNU extensions either. What historical information we have 3628been able to find, suggests they originated with System V@. 3629 3630@cindex null directive 3631The @dfn{null directive} consists of a @samp{#} followed by a newline, 3632with only whitespace (including comments) in between. A null directive 3633is understood as a preprocessing directive but has no effect on the 3634preprocessor output. The primary significance of the existence of the 3635null directive is that an input line consisting of just a @samp{#} will 3636produce no output, rather than a line of output containing just a 3637@samp{#}. Supposedly some old C programs contain such lines. 3638 3639@node Preprocessor Output 3640@chapter Preprocessor Output 3641 3642When the C preprocessor is used with the C, C++, or Objective-C 3643compilers, it is integrated into the compiler and communicates a stream 3644of binary tokens directly to the compiler's parser. However, it can 3645also be used in the more conventional standalone mode, where it produces 3646textual output. 3647@c FIXME: Document the library interface. 3648 3649@cindex output format 3650The output from the C preprocessor looks much like the input, except 3651that all preprocessing directive lines have been replaced with blank 3652lines and all comments with spaces. Long runs of blank lines are 3653discarded. 3654 3655The ISO standard specifies that it is implementation defined whether a 3656preprocessor preserves whitespace between tokens, or replaces it with 3657e.g.@: a single space. In GNU CPP, whitespace between tokens is collapsed 3658to become a single space, with the exception that the first token on a 3659non-directive line is preceded with sufficient spaces that it appears in 3660the same column in the preprocessed output that it appeared in the 3661original source file. This is so the output is easy to read. 3662CPP does not insert any 3663whitespace where there was none in the original source, except where 3664necessary to prevent an accidental token paste. 3665 3666@cindex linemarkers 3667Source file name and line number information is conveyed by lines 3668of the form 3669 3670@smallexample 3671# @var{linenum} @var{filename} @var{flags} 3672@end smallexample 3673 3674@noindent 3675These are called @dfn{linemarkers}. They are inserted as needed into 3676the output (but never within a string or character constant). They mean 3677that the following line originated in file @var{filename} at line 3678@var{linenum}. @var{filename} will never contain any non-printing 3679characters; they are replaced with octal escape sequences. 3680 3681After the file name comes zero or more flags, which are @samp{1}, 3682@samp{2}, @samp{3}, or @samp{4}. If there are multiple flags, spaces 3683separate them. Here is what the flags mean: 3684 3685@table @samp 3686@item 1 3687This indicates the start of a new file. 3688@item 2 3689This indicates returning to a file (after having included another file). 3690@item 3 3691This indicates that the following text comes from a system header file, 3692so certain warnings should be suppressed. 3693@item 4 3694This indicates that the following text should be treated as being 3695wrapped in an implicit @code{extern "C"} block. 3696@c maybe cross reference NO_IMPLICIT_EXTERN_C 3697@end table 3698 3699As an extension, the preprocessor accepts linemarkers in non-assembler 3700input files. They are treated like the corresponding @samp{#line} 3701directive, (@pxref{Line Control}), except that trailing flags are 3702permitted, and are interpreted with the meanings described above. If 3703multiple flags are given, they must be in ascending order. 3704 3705Some directives may be duplicated in the output of the preprocessor. 3706These are @samp{#ident} (always), @samp{#pragma} (only if the 3707preprocessor does not handle the pragma itself), and @samp{#define} and 3708@samp{#undef} (with certain debugging options). If this happens, the 3709@samp{#} of the directive will always be in the first column, and there 3710will be no space between the @samp{#} and the directive name. If macro 3711expansion happens to generate tokens which might be mistaken for a 3712duplicated directive, a space will be inserted between the @samp{#} and 3713the directive name. 3714 3715@node Traditional Mode 3716@chapter Traditional Mode 3717 3718Traditional (pre-standard) C preprocessing is rather different from 3719the preprocessing specified by the standard. When the preprocessor 3720is invoked with the 3721@option{-traditional-cpp} option, it attempts to emulate a traditional 3722preprocessor. 3723 3724This mode is not useful for compiling C code with GCC, 3725but is intended for use with non-C preprocessing applications. Thus 3726traditional mode semantics are supported only when invoking 3727the preprocessor explicitly, and not in the compiler front ends. 3728 3729The implementation does not correspond precisely to the behavior of 3730early pre-standard versions of GCC, nor to any true traditional preprocessor. 3731After all, inconsistencies among traditional implementations were a 3732major motivation for C standardization. However, we intend that it 3733should be compatible with true traditional preprocessors in all ways 3734that actually matter. 3735 3736@menu 3737* Traditional lexical analysis:: 3738* Traditional macros:: 3739* Traditional miscellany:: 3740* Traditional warnings:: 3741@end menu 3742 3743@node Traditional lexical analysis 3744@section Traditional lexical analysis 3745 3746The traditional preprocessor does not decompose its input into tokens 3747the same way a standards-conforming preprocessor does. The input is 3748simply treated as a stream of text with minimal internal form. 3749 3750This implementation does not treat trigraphs (@pxref{trigraphs}) 3751specially since they were an invention of the standards committee. It 3752handles arbitrarily-positioned escaped newlines properly and splices 3753the lines as you would expect; many traditional preprocessors did not 3754do this. 3755 3756The form of horizontal whitespace in the input file is preserved in 3757the output. In particular, hard tabs remain hard tabs. This can be 3758useful if, for example, you are preprocessing a Makefile. 3759 3760Traditional CPP only recognizes C-style block comments, and treats the 3761@samp{/*} sequence as introducing a comment only if it lies outside 3762quoted text. Quoted text is introduced by the usual single and double 3763quotes, and also by an initial @samp{<} in a @code{#include} 3764directive. 3765 3766Traditionally, comments are completely removed and are not replaced 3767with a space. Since a traditional compiler does its own tokenization 3768of the output of the preprocessor, this means that comments can 3769effectively be used as token paste operators. However, comments 3770behave like separators for text handled by the preprocessor itself, 3771since it doesn't re-lex its input. For example, in 3772 3773@smallexample 3774#if foo/**/bar 3775@end smallexample 3776 3777@noindent 3778@samp{foo} and @samp{bar} are distinct identifiers and expanded 3779separately if they happen to be macros. In other words, this 3780directive is equivalent to 3781 3782@smallexample 3783#if foo bar 3784@end smallexample 3785 3786@noindent 3787rather than 3788 3789@smallexample 3790#if foobar 3791@end smallexample 3792 3793Generally speaking, in traditional mode an opening quote need not have 3794a matching closing quote. In particular, a macro may be defined with 3795replacement text that contains an unmatched quote. Of course, if you 3796attempt to compile preprocessed output containing an unmatched quote 3797you will get a syntax error. 3798 3799However, all preprocessing directives other than @code{#define} 3800require matching quotes. For example: 3801 3802@smallexample 3803#define m This macro's fine and has an unmatched quote 3804"/* This is not a comment. */ 3805/* @r{This is a comment. The following #include directive 3806 is ill-formed.} */ 3807#include <stdio.h 3808@end smallexample 3809 3810Just as for the ISO preprocessor, what would be a closing quote can be 3811escaped with a backslash to prevent the quoted text from closing. 3812 3813@node Traditional macros 3814@section Traditional macros 3815 3816The major difference between traditional and ISO macros is that the 3817former expand to text rather than to a token sequence. CPP removes 3818all leading and trailing horizontal whitespace from a macro's 3819replacement text before storing it, but preserves the form of internal 3820whitespace. 3821 3822One consequence is that it is legitimate for the replacement text to 3823contain an unmatched quote (@pxref{Traditional lexical analysis}). An 3824unclosed string or character constant continues into the text 3825following the macro call. Similarly, the text at the end of a macro's 3826expansion can run together with the text after the macro invocation to 3827produce a single token. 3828 3829Normally comments are removed from the replacement text after the 3830macro is expanded, but if the @option{-CC} option is passed on the 3831command-line comments are preserved. (In fact, the current 3832implementation removes comments even before saving the macro 3833replacement text, but it careful to do it in such a way that the 3834observed effect is identical even in the function-like macro case.) 3835 3836The ISO stringizing operator @samp{#} and token paste operator 3837@samp{##} have no special meaning. As explained later, an effect 3838similar to these operators can be obtained in a different way. Macro 3839names that are embedded in quotes, either from the main file or after 3840macro replacement, do not expand. 3841 3842CPP replaces an unquoted object-like macro name with its replacement 3843text, and then rescans it for further macros to replace. Unlike 3844standard macro expansion, traditional macro expansion has no provision 3845to prevent recursion. If an object-like macro appears unquoted in its 3846replacement text, it will be replaced again during the rescan pass, 3847and so on @emph{ad infinitum}. GCC detects when it is expanding 3848recursive macros, emits an error message, and continues after the 3849offending macro invocation. 3850 3851@smallexample 3852#define PLUS + 3853#define INC(x) PLUS+x 3854INC(foo); 3855 @expansion{} ++foo; 3856@end smallexample 3857 3858Function-like macros are similar in form but quite different in 3859behavior to their ISO counterparts. Their arguments are contained 3860within parentheses, are comma-separated, and can cross physical lines. 3861Commas within nested parentheses are not treated as argument 3862separators. Similarly, a quote in an argument cannot be left 3863unclosed; a following comma or parenthesis that comes before the 3864closing quote is treated like any other character. There is no 3865facility for handling variadic macros. 3866 3867This implementation removes all comments from macro arguments, unless 3868the @option{-C} option is given. The form of all other horizontal 3869whitespace in arguments is preserved, including leading and trailing 3870whitespace. In particular 3871 3872@smallexample 3873f( ) 3874@end smallexample 3875 3876@noindent 3877is treated as an invocation of the macro @samp{f} with a single 3878argument consisting of a single space. If you want to invoke a 3879function-like macro that takes no arguments, you must not leave any 3880whitespace between the parentheses. 3881 3882If a macro argument crosses a new line, the new line is replaced with 3883a space when forming the argument. If the previous line contained an 3884unterminated quote, the following line inherits the quoted state. 3885 3886Traditional preprocessors replace parameters in the replacement text 3887with their arguments regardless of whether the parameters are within 3888quotes or not. This provides a way to stringize arguments. For 3889example 3890 3891@smallexample 3892#define str(x) "x" 3893str(/* @r{A comment} */some text ) 3894 @expansion{} "some text " 3895@end smallexample 3896 3897@noindent 3898Note that the comment is removed, but that the trailing space is 3899preserved. Here is an example of using a comment to effect token 3900pasting. 3901 3902@smallexample 3903#define suffix(x) foo_/**/x 3904suffix(bar) 3905 @expansion{} foo_bar 3906@end smallexample 3907 3908@node Traditional miscellany 3909@section Traditional miscellany 3910 3911Here are some things to be aware of when using the traditional 3912preprocessor. 3913 3914@itemize @bullet 3915@item 3916Preprocessing directives are recognized only when their leading 3917@samp{#} appears in the first column. There can be no whitespace 3918between the beginning of the line and the @samp{#}, but whitespace can 3919follow the @samp{#}. 3920 3921@item 3922A true traditional C preprocessor does not recognize @samp{#error} or 3923@samp{#pragma}, and may not recognize @samp{#elif}. CPP supports all 3924the directives in traditional mode that it supports in ISO mode, 3925including extensions, with the exception that the effects of 3926@samp{#pragma GCC poison} are undefined. 3927 3928@item 3929__STDC__ is not defined. 3930 3931@item 3932If you use digraphs the behavior is undefined. 3933 3934@item 3935If a line that looks like a directive appears within macro arguments, 3936the behavior is undefined. 3937 3938@end itemize 3939 3940@node Traditional warnings 3941@section Traditional warnings 3942You can request warnings about features that did not exist, or worked 3943differently, in traditional C with the @option{-Wtraditional} option. 3944GCC does not warn about features of ISO C which you must use when you 3945are using a conforming compiler, such as the @samp{#} and @samp{##} 3946operators. 3947 3948Presently @option{-Wtraditional} warns about: 3949 3950@itemize @bullet 3951@item 3952Macro parameters that appear within string literals in the macro body. 3953In traditional C macro replacement takes place within string literals, 3954but does not in ISO C@. 3955 3956@item 3957In traditional C, some preprocessor directives did not exist. 3958Traditional preprocessors would only consider a line to be a directive 3959if the @samp{#} appeared in column 1 on the line. Therefore 3960@option{-Wtraditional} warns about directives that traditional C 3961understands but would ignore because the @samp{#} does not appear as the 3962first character on the line. It also suggests you hide directives like 3963@samp{#pragma} not understood by traditional C by indenting them. Some 3964traditional implementations would not recognize @samp{#elif}, so it 3965suggests avoiding it altogether. 3966 3967@item 3968A function-like macro that appears without an argument list. In some 3969traditional preprocessors this was an error. In ISO C it merely means 3970that the macro is not expanded. 3971 3972@item 3973The unary plus operator. This did not exist in traditional C@. 3974 3975@item 3976The @samp{U} and @samp{LL} integer constant suffixes, which were not 3977available in traditional C@. (Traditional C does support the @samp{L} 3978suffix for simple long integer constants.) You are not warned about 3979uses of these suffixes in macros defined in system headers. For 3980instance, @code{UINT_MAX} may well be defined as @code{4294967295U}, but 3981you will not be warned if you use @code{UINT_MAX}. 3982 3983You can usually avoid the warning, and the related warning about 3984constants which are so large that they are unsigned, by writing the 3985integer constant in question in hexadecimal, with no U suffix. Take 3986care, though, because this gives the wrong result in exotic cases. 3987@end itemize 3988 3989@node Implementation Details 3990@chapter Implementation Details 3991 3992Here we document details of how the preprocessor's implementation 3993affects its user-visible behavior. You should try to avoid undue 3994reliance on behavior described here, as it is possible that it will 3995change subtly in future implementations. 3996 3997Also documented here are obsolete features still supported by CPP@. 3998 3999@menu 4000* Implementation-defined behavior:: 4001* Implementation limits:: 4002* Obsolete Features:: 4003@end menu 4004 4005@node Implementation-defined behavior 4006@section Implementation-defined behavior 4007@cindex implementation-defined behavior 4008 4009This is how CPP behaves in all the cases which the C standard 4010describes as @dfn{implementation-defined}. This term means that the 4011implementation is free to do what it likes, but must document its choice 4012and stick to it. 4013@c FIXME: Check the C++ standard for more implementation-defined stuff. 4014 4015@itemize @bullet 4016@need 1000 4017@item The mapping of physical source file multi-byte characters to the 4018execution character set. 4019 4020The input character set can be specified using the 4021@option{-finput-charset} option, while the execution character set may 4022be controlled using the @option{-fexec-charset} and 4023@option{-fwide-exec-charset} options. 4024 4025@item Identifier characters. 4026@anchor{Identifier characters} 4027 4028The C and C++ standards allow identifiers to be composed of @samp{_} 4029and the alphanumeric characters. C++ also allows universal character 4030names. C99 and later C standards permit both universal character 4031names and implementation-defined characters. 4032 4033GCC allows the @samp{$} character in identifiers as an extension for 4034most targets. This is true regardless of the @option{std=} switch, 4035since this extension cannot conflict with standards-conforming 4036programs. When preprocessing assembler, however, dollars are not 4037identifier characters by default. 4038 4039Currently the targets that by default do not permit @samp{$} are AVR, 4040IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC targets for the AIX 4041operating system. 4042 4043You can override the default with @option{-fdollars-in-identifiers} or 4044@option{fno-dollars-in-identifiers}. @xref{fdollars-in-identifiers}. 4045 4046@item Non-empty sequences of whitespace characters. 4047 4048In textual output, each whitespace sequence is collapsed to a single 4049space. For aesthetic reasons, the first token on each non-directive 4050line of output is preceded with sufficient spaces that it appears in the 4051same column as it did in the original source file. 4052 4053@item The numeric value of character constants in preprocessor expressions. 4054 4055The preprocessor and compiler interpret character constants in the 4056same way; i.e.@: escape sequences such as @samp{\a} are given the 4057values they would have on the target machine. 4058 4059The compiler evaluates a multi-character character constant a character 4060at a time, shifting the previous value left by the number of bits per 4061target character, and then or-ing in the bit-pattern of the new 4062character truncated to the width of a target character. The final 4063bit-pattern is given type @code{int}, and is therefore signed, 4064regardless of whether single characters are signed or not. 4065If there are more 4066characters in the constant than would fit in the target @code{int} the 4067compiler issues a warning, and the excess leading characters are 4068ignored. 4069 4070For example, @code{'ab'} for a target with an 8-bit @code{char} would be 4071interpreted as @w{@samp{(int) ((unsigned char) 'a' * 256 + (unsigned char) 4072'b')}}, and @code{'\234a'} as @w{@samp{(int) ((unsigned char) '\234' * 4073256 + (unsigned char) 'a')}}. 4074 4075@item Source file inclusion. 4076 4077For a discussion on how the preprocessor locates header files, 4078@ref{Include Operation}. 4079 4080@item Interpretation of the filename resulting from a macro-expanded 4081@samp{#include} directive. 4082 4083@xref{Computed Includes}. 4084 4085@item Treatment of a @samp{#pragma} directive that after macro-expansion 4086results in a standard pragma. 4087 4088No macro expansion occurs on any @samp{#pragma} directive line, so the 4089question does not arise. 4090 4091Note that GCC does not yet implement any of the standard 4092pragmas. 4093 4094@end itemize 4095 4096@node Implementation limits 4097@section Implementation limits 4098@cindex implementation limits 4099 4100CPP has a small number of internal limits. This section lists the 4101limits which the C standard requires to be no lower than some minimum, 4102and all the others known. It is intended that there should be as few limits 4103as possible. If you encounter an undocumented or inconvenient limit, 4104please report that as a bug. @xref{Bugs, , Reporting Bugs, gcc, Using 4105the GNU Compiler Collection (GCC)}. 4106 4107Where we say something is limited @dfn{only by available memory}, that 4108means that internal data structures impose no intrinsic limit, and space 4109is allocated with @code{malloc} or equivalent. The actual limit will 4110therefore depend on many things, such as the size of other things 4111allocated by the compiler at the same time, the amount of memory 4112consumed by other processes on the same computer, etc. 4113 4114@itemize @bullet 4115 4116@item Nesting levels of @samp{#include} files. 4117 4118We impose an arbitrary limit of 200 levels, to avoid runaway recursion. 4119The standard requires at least 15 levels. 4120 4121@item Nesting levels of conditional inclusion. 4122 4123The C standard mandates this be at least 63. CPP is limited only by 4124available memory. 4125 4126@item Levels of parenthesized expressions within a full expression. 4127 4128The C standard requires this to be at least 63. In preprocessor 4129conditional expressions, it is limited only by available memory. 4130 4131@item Significant initial characters in an identifier or macro name. 4132 4133The preprocessor treats all characters as significant. The C standard 4134requires only that the first 63 be significant. 4135 4136@item Number of macros simultaneously defined in a single translation unit. 4137 4138The standard requires at least 4095 be possible. CPP is limited only 4139by available memory. 4140 4141@item Number of parameters in a macro definition and arguments in a macro call. 4142 4143We allow @code{USHRT_MAX}, which is no smaller than 65,535. The minimum 4144required by the standard is 127. 4145 4146@item Number of characters on a logical source line. 4147 4148The C standard requires a minimum of 4096 be permitted. CPP places 4149no limits on this, but you may get incorrect column numbers reported in 4150diagnostics for lines longer than 65,535 characters. 4151 4152@item Maximum size of a source file. 4153 4154The standard does not specify any lower limit on the maximum size of a 4155source file. GNU cpp maps files into memory, so it is limited by the 4156available address space. This is generally at least two gigabytes. 4157Depending on the operating system, the size of physical memory may or 4158may not be a limitation. 4159 4160@end itemize 4161 4162@node Obsolete Features 4163@section Obsolete Features 4164 4165CPP has some features which are present mainly for compatibility with 4166older programs. We discourage their use in new code. In some cases, 4167we plan to remove the feature in a future version of GCC@. 4168 4169@subsection Assertions 4170@cindex assertions 4171 4172@dfn{Assertions} are a deprecated alternative to macros in writing 4173conditionals to test what sort of computer or system the compiled 4174program will run on. Assertions are usually predefined, but you can 4175define them with preprocessing directives or command-line options. 4176 4177Assertions were intended to provide a more systematic way to describe 4178the compiler's target system and we added them for compatibility with 4179existing compilers. In practice they are just as unpredictable as the 4180system-specific predefined macros. In addition, they are not part of 4181any standard, and only a few compilers support them. 4182Therefore, the use of assertions is @strong{less} portable than the use 4183of system-specific predefined macros. We recommend you do not use them at 4184all. 4185 4186@cindex predicates 4187An assertion looks like this: 4188 4189@smallexample 4190#@var{predicate} (@var{answer}) 4191@end smallexample 4192 4193@noindent 4194@var{predicate} must be a single identifier. @var{answer} can be any 4195sequence of tokens; all characters are significant except for leading 4196and trailing whitespace, and differences in internal whitespace 4197sequences are ignored. (This is similar to the rules governing macro 4198redefinition.) Thus, @code{(x + y)} is different from @code{(x+y)} but 4199equivalent to @code{@w{( x + y )}}. Parentheses do not nest inside an 4200answer. 4201 4202@cindex testing predicates 4203To test an assertion, you write it in an @samp{#if}. For example, this 4204conditional succeeds if either @code{vax} or @code{ns16000} has been 4205asserted as an answer for @code{machine}. 4206 4207@smallexample 4208#if #machine (vax) || #machine (ns16000) 4209@end smallexample 4210 4211@noindent 4212You can test whether @emph{any} answer is asserted for a predicate by 4213omitting the answer in the conditional: 4214 4215@smallexample 4216#if #machine 4217@end smallexample 4218 4219@findex #assert 4220Assertions are made with the @samp{#assert} directive. Its sole 4221argument is the assertion to make, without the leading @samp{#} that 4222identifies assertions in conditionals. 4223 4224@smallexample 4225#assert @var{predicate} (@var{answer}) 4226@end smallexample 4227 4228@noindent 4229You may make several assertions with the same predicate and different 4230answers. Subsequent assertions do not override previous ones for the 4231same predicate. All the answers for any given predicate are 4232simultaneously true. 4233 4234@cindex assertions, canceling 4235@findex #unassert 4236Assertions can be canceled with the @samp{#unassert} directive. It 4237has the same syntax as @samp{#assert}. In that form it cancels only the 4238answer which was specified on the @samp{#unassert} line; other answers 4239for that predicate remain true. You can cancel an entire predicate by 4240leaving out the answer: 4241 4242@smallexample 4243#unassert @var{predicate} 4244@end smallexample 4245 4246@noindent 4247In either form, if no such assertion has been made, @samp{#unassert} has 4248no effect. 4249 4250You can also make or cancel assertions using command-line options. 4251@xref{Invocation}. 4252 4253@node Invocation 4254@chapter Invocation 4255@cindex invocation 4256@cindex command line 4257 4258Most often when you use the C preprocessor you do not have to invoke it 4259explicitly: the C compiler does so automatically. However, the 4260preprocessor is sometimes useful on its own. You can invoke the 4261preprocessor either with the @command{cpp} command, or via @command{gcc -E}. 4262In GCC, the preprocessor is actually integrated with the compiler 4263rather than a separate program, and both of these commands invoke 4264GCC and tell it to stop after the preprocessing phase. 4265 4266The @command{cpp} options listed here are also accepted by 4267@command{gcc} and have the same meaning. Likewise the @command{cpp} 4268command accepts all the usual @command{gcc} driver options, although those 4269pertaining to compilation phases after preprocessing are ignored. 4270 4271Only options specific to preprocessing behavior are documented here. 4272Refer to the GCC manual for full documentation of other driver options. 4273 4274@ignore 4275@c man begin SYNOPSIS 4276cpp [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}] 4277 [@option{-I}@var{dir}@dots{}] [@option{-iquote}@var{dir}@dots{}] 4278 [@option{-iremap}@var{src}:@var{dst}] 4279 [@option{-W}@var{warn}@dots{}] 4280 [@option{-M}|@option{-MM}] [@option{-MG}] [@option{-MF} @var{filename}] 4281 [@option{-MP}] [@option{-MQ} @var{target}@dots{}] 4282 [@option{-MT} @var{target}@dots{}] 4283 @var{infile} [[@option{-o}] @var{outfile}] 4284 4285Only the most useful options are given above; see below for a more 4286complete list of preprocessor-specific options. 4287In addition, @command{cpp} accepts most @command{gcc} driver options, which 4288are not listed here. Refer to the GCC documentation for details. 4289@c man end 4290@c man begin SEEALSO 4291gpl(7), gfdl(7), fsf-funding(7), 4292gcc(1), and the Info entries for @file{cpp} and @file{gcc}. 4293@c man end 4294@end ignore 4295 4296@c man begin OPTIONS 4297The @command{cpp} command expects two file names as arguments, @var{infile} and 4298@var{outfile}. The preprocessor reads @var{infile} together with any 4299other files it specifies with @samp{#include}. All the output generated 4300by the combined input files is written in @var{outfile}. 4301 4302Either @var{infile} or @var{outfile} may be @option{-}, which as 4303@var{infile} means to read from standard input and as @var{outfile} 4304means to write to standard output. If either file is omitted, it 4305means the same as if @option{-} had been specified for that file. 4306You can also use the @option{-o @var{outfile}} option to specify the 4307output file. 4308 4309Unless otherwise noted, or the option ends in @samp{=}, all options 4310which take an argument may have that argument appear either immediately 4311after the option, or with a space between option and argument: 4312@option{-Ifoo} and @option{-I foo} have the same effect. 4313 4314@cindex grouping options 4315@cindex options, grouping 4316Many options have multi-letter names; therefore multiple single-letter 4317options may @emph{not} be grouped: @option{-dM} is very different from 4318@w{@samp{-d -M}}. 4319 4320@cindex options 4321 4322@table @gcctabopt 4323@include cppopts.texi 4324@include cppdiropts.texi 4325@include cppwarnopts.texi 4326@end table 4327@c man end 4328 4329@node Environment Variables 4330@chapter Environment Variables 4331@cindex environment variables 4332@c man begin ENVIRONMENT 4333 4334This section describes the environment variables that affect how CPP 4335operates. You can use them to specify directories or prefixes to use 4336when searching for include files, or to control dependency output. 4337 4338Note that you can also specify places to search using options such as 4339@option{-I}, and control dependency output with options like 4340@option{-M} (@pxref{Invocation}). These take precedence over 4341environment variables, which in turn take precedence over the 4342configuration of GCC@. 4343 4344@include cppenv.texi 4345@c man end 4346 4347@page 4348@include fdl.texi 4349 4350@page 4351@node Index of Directives 4352@unnumbered Index of Directives 4353@printindex fn 4354 4355@node Option Index 4356@unnumbered Option Index 4357@noindent 4358CPP's command-line options and environment variables are indexed here 4359without any initial @samp{-} or @samp{--}. 4360@printindex op 4361 4362@page 4363@node Concept Index 4364@unnumbered Concept Index 4365@printindex cp 4366 4367@bye 4368