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{} 1987-2015 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* Stringification:: 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* Differences from previous versions:: 167 168Obsolete Features 169 170* Obsolete Features:: 171 172@end detailmenu 173@end menu 174 175@insertcopying 176@end ifnottex 177 178@node Overview 179@chapter Overview 180@c man begin DESCRIPTION 181The C preprocessor, often known as @dfn{cpp}, is a @dfn{macro processor} 182that is used automatically by the C compiler to transform your program 183before compilation. It is called a macro processor because it allows 184you to define @dfn{macros}, which are brief abbreviations for longer 185constructs. 186 187The C preprocessor is intended to be used only with C, C++, and 188Objective-C source code. In the past, it has been abused as a general 189text processor. It will choke on input which does not obey C's lexical 190rules. For example, apostrophes will be interpreted as the beginning of 191character constants, and cause errors. Also, you cannot rely on it 192preserving characteristics of the input which are not significant to 193C-family languages. If a Makefile is preprocessed, all the hard tabs 194will be removed, and the Makefile will not work. 195 196Having said that, you can often get away with using cpp on things which 197are not C@. Other Algol-ish programming languages are often safe 198(Pascal, Ada, etc.) So is assembly, with caution. @option{-traditional-cpp} 199mode preserves more white space, and is otherwise more permissive. Many 200of the problems can be avoided by writing C or C++ style comments 201instead of native language comments, and keeping macros simple. 202 203Wherever possible, you should use a preprocessor geared to the language 204you are writing in. Modern versions of the GNU assembler have macro 205facilities. Most high level programming languages have their own 206conditional compilation and inclusion mechanism. If all else fails, 207try a true general text processor, such as GNU M4. 208 209C preprocessors vary in some details. This manual discusses the GNU C 210preprocessor, which provides a small superset of the features of ISO 211Standard C@. In its default mode, the GNU C preprocessor does not do a 212few things required by the standard. These are features which are 213rarely, if ever, used, and may cause surprising changes to the meaning 214of a program which does not expect them. To get strict ISO Standard C, 215you should use the @option{-std=c90}, @option{-std=c99} or 216@option{-std=c11} options, depending 217on which version of the standard you want. To get all the mandatory 218diagnostics, you must also use @option{-pedantic}. @xref{Invocation}. 219 220This manual describes the behavior of the ISO preprocessor. To 221minimize gratuitous differences, where the ISO preprocessor's 222behavior does not conflict with traditional semantics, the 223traditional preprocessor should behave the same way. The various 224differences that do exist are detailed in the section @ref{Traditional 225Mode}. 226 227For clarity, unless noted otherwise, references to @samp{CPP} in this 228manual refer to GNU CPP@. 229@c man end 230 231@menu 232* Character sets:: 233* Initial processing:: 234* Tokenization:: 235* The preprocessing language:: 236@end menu 237 238@node Character sets 239@section Character sets 240 241Source code character set processing in C and related languages is 242rather complicated. The C standard discusses two character sets, but 243there are really at least four. 244 245The files input to CPP might be in any character set at all. CPP's 246very first action, before it even looks for line boundaries, is to 247convert the file into the character set it uses for internal 248processing. That set is what the C standard calls the @dfn{source} 249character set. It must be isomorphic with ISO 10646, also known as 250Unicode. CPP uses the UTF-8 encoding of Unicode. 251 252The character sets of the input files are specified using the 253@option{-finput-charset=} option. 254 255All preprocessing work (the subject of the rest of this manual) is 256carried out in the source character set. If you request textual 257output from the preprocessor with the @option{-E} option, it will be 258in UTF-8. 259 260After preprocessing is complete, string and character constants are 261converted again, into the @dfn{execution} character set. This 262character set is under control of the user; the default is UTF-8, 263matching the source character set. Wide string and character 264constants have their own character set, which is not called out 265specifically in the standard. Again, it is under control of the user. 266The default is UTF-16 or UTF-32, whichever fits in the target's 267@code{wchar_t} type, in the target machine's byte 268order.@footnote{UTF-16 does not meet the requirements of the C 269standard for a wide character set, but the choice of 16-bit 270@code{wchar_t} is enshrined in some system ABIs so we cannot fix 271this.} Octal and hexadecimal escape sequences do not undergo 272conversion; @t{'\x12'} has the value 0x12 regardless of the currently 273selected execution character set. All other escapes are replaced by 274the character in the source character set that they represent, then 275converted to the execution character set, just like unescaped 276characters. 277 278In identifiers, characters outside the ASCII range can only be 279specified with the @samp{\u} and @samp{\U} escapes, not used 280directly. If strict ISO C90 conformance is specified with an option 281such as @option{-std=c90}, or @option{-fno-extended-identifiers} is 282used, then those escapes are not permitted in identifiers. 283 284@node Initial processing 285@section Initial processing 286 287The preprocessor performs a series of textual transformations on its 288input. These happen before all other processing. Conceptually, they 289happen in a rigid order, and the entire file is run through each 290transformation before the next one begins. CPP actually does them 291all at once, for performance reasons. These transformations correspond 292roughly to the first three ``phases of translation'' described in the C 293standard. 294 295@enumerate 296@item 297@cindex line endings 298The input file is read into memory and broken into lines. 299 300Different systems use different conventions to indicate the end of a 301line. GCC accepts the ASCII control sequences @kbd{LF}, @kbd{@w{CR 302LF}} and @kbd{CR} as end-of-line markers. These are the canonical 303sequences used by Unix, DOS and VMS, and the classic Mac OS (before 304OSX) respectively. You may therefore safely copy source code written 305on any of those systems to a different one and use it without 306conversion. (GCC may lose track of the current line number if a file 307doesn't consistently use one convention, as sometimes happens when it 308is edited on computers with different conventions that share a network 309file system.) 310 311If the last line of any input file lacks an end-of-line marker, the end 312of the file is considered to implicitly supply one. The C standard says 313that this condition provokes undefined behavior, so GCC will emit a 314warning message. 315 316@item 317@cindex trigraphs 318@anchor{trigraphs}If trigraphs are enabled, they are replaced by their 319corresponding single characters. By default GCC ignores trigraphs, 320but if you request a strictly conforming mode with the @option{-std} 321option, or you specify the @option{-trigraphs} option, then it 322converts them. 323 324These are nine three-character sequences, all starting with @samp{??}, 325that are defined by ISO C to stand for single characters. They permit 326obsolete systems that lack some of C's punctuation to use C@. For 327example, @samp{??/} stands for @samp{\}, so @t{'??/n'} is a character 328constant for a newline. 329 330Trigraphs are not popular and many compilers implement them 331incorrectly. Portable code should not rely on trigraphs being either 332converted or ignored. With @option{-Wtrigraphs} GCC will warn you 333when a trigraph may change the meaning of your program if it were 334converted. @xref{Wtrigraphs}. 335 336In a string constant, you can prevent a sequence of question marks 337from being confused with a trigraph by inserting a backslash between 338the question marks, or by separating the string literal at the 339trigraph and making use of string literal concatenation. @t{"(??\?)"} 340is the string @samp{(???)}, not @samp{(?]}. Traditional C compilers 341do not recognize these idioms. 342 343The nine trigraphs and their replacements are 344 345@smallexample 346Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??- 347Replacement: [ ] @{ @} # \ ^ | ~ 348@end smallexample 349 350@item 351@cindex continued lines 352@cindex backslash-newline 353Continued lines are merged into one long line. 354 355A continued line is a line which ends with a backslash, @samp{\}. The 356backslash is removed and the following line is joined with the current 357one. No space is inserted, so you may split a line anywhere, even in 358the middle of a word. (It is generally more readable to split lines 359only at white space.) 360 361The trailing backslash on a continued line is commonly referred to as a 362@dfn{backslash-newline}. 363 364If there is white space between a backslash and the end of a line, that 365is still a continued line. However, as this is usually the result of an 366editing mistake, and many compilers will not accept it as a continued 367line, GCC will warn you about it. 368 369@item 370@cindex comments 371@cindex line comments 372@cindex block comments 373All comments are replaced with single spaces. 374 375There are two kinds of comments. @dfn{Block comments} begin with 376@samp{/*} and continue until the next @samp{*/}. Block comments do not 377nest: 378 379@smallexample 380/* @r{this is} /* @r{one comment} */ @r{text outside comment} 381@end smallexample 382 383@dfn{Line comments} begin with @samp{//} and continue to the end of the 384current line. Line comments do not nest either, but it does not matter, 385because they would end in the same place anyway. 386 387@smallexample 388// @r{this is} // @r{one comment} 389@r{text outside comment} 390@end smallexample 391@end enumerate 392 393It is safe to put line comments inside block comments, or vice versa. 394 395@smallexample 396@group 397/* @r{block comment} 398 // @r{contains line comment} 399 @r{yet more comment} 400 */ @r{outside comment} 401 402// @r{line comment} /* @r{contains block comment} */ 403@end group 404@end smallexample 405 406But beware of commenting out one end of a block comment with a line 407comment. 408 409@smallexample 410@group 411 // @r{l.c.} /* @r{block comment begins} 412 @r{oops! this isn't a comment anymore} */ 413@end group 414@end smallexample 415 416Comments are not recognized within string literals. 417@t{@w{"/* blah */"}} is the string constant @samp{@w{/* blah */}}, not 418an empty string. 419 420Line comments are not in the 1989 edition of the C standard, but they 421are recognized by GCC as an extension. In C++ and in the 1999 edition 422of the C standard, they are an official part of the language. 423 424Since these transformations happen before all other processing, you can 425split a line mechanically with backslash-newline anywhere. You can 426comment out the end of a line. You can continue a line comment onto the 427next line with backslash-newline. You can even split @samp{/*}, 428@samp{*/}, and @samp{//} onto multiple lines with backslash-newline. 429For example: 430 431@smallexample 432@group 433/\ 434* 435*/ # /* 436*/ defi\ 437ne FO\ 438O 10\ 43920 440@end group 441@end smallexample 442 443@noindent 444is equivalent to @code{@w{#define FOO 1020}}. All these tricks are 445extremely confusing and should not be used in code intended to be 446readable. 447 448There is no way to prevent a backslash at the end of a line from being 449interpreted as a backslash-newline. This cannot affect any correct 450program, however. 451 452@node Tokenization 453@section Tokenization 454 455@cindex tokens 456@cindex preprocessing tokens 457After the textual transformations are finished, the input file is 458converted into a sequence of @dfn{preprocessing tokens}. These mostly 459correspond to the syntactic tokens used by the C compiler, but there are 460a few differences. White space separates tokens; it is not itself a 461token of any kind. Tokens do not have to be separated by white space, 462but it is often necessary to avoid ambiguities. 463 464When faced with a sequence of characters that has more than one possible 465tokenization, the preprocessor is greedy. It always makes each token, 466starting from the left, as big as possible before moving on to the next 467token. For instance, @code{a+++++b} is interpreted as 468@code{@w{a ++ ++ + b}}, not as @code{@w{a ++ + ++ b}}, even though the 469latter tokenization could be part of a valid C program and the former 470could not. 471 472Once the input file is broken into tokens, the token boundaries never 473change, except when the @samp{##} preprocessing operator is used to paste 474tokens together. @xref{Concatenation}. For example, 475 476@smallexample 477@group 478#define foo() bar 479foo()baz 480 @expansion{} bar baz 481@emph{not} 482 @expansion{} barbaz 483@end group 484@end smallexample 485 486The compiler does not re-tokenize the preprocessor's output. Each 487preprocessing token becomes one compiler token. 488 489@cindex identifiers 490Preprocessing tokens fall into five broad classes: identifiers, 491preprocessing numbers, string literals, punctuators, and other. An 492@dfn{identifier} is the same as an identifier in C: any sequence of 493letters, digits, or underscores, which begins with a letter or 494underscore. Keywords of C have no significance to the preprocessor; 495they are ordinary identifiers. You can define a macro whose name is a 496keyword, for instance. The only identifier which can be considered a 497preprocessing keyword is @code{defined}. @xref{Defined}. 498 499This is mostly true of other languages which use the C preprocessor. 500However, a few of the keywords of C++ are significant even in the 501preprocessor. @xref{C++ Named Operators}. 502 503In the 1999 C standard, identifiers may contain letters which are not 504part of the ``basic source character set'', at the implementation's 505discretion (such as accented Latin letters, Greek letters, or Chinese 506ideograms). This may be done with an extended character set, or the 507@samp{\u} and @samp{\U} escape sequences. GCC only accepts such 508characters in the @samp{\u} and @samp{\U} forms. 509 510As an extension, GCC treats @samp{$} as a letter. This is for 511compatibility with some systems, such as VMS, where @samp{$} is commonly 512used in system-defined function and object names. @samp{$} is not a 513letter in strictly conforming mode, or if you specify the @option{-$} 514option. @xref{Invocation}. 515 516@cindex numbers 517@cindex preprocessing numbers 518A @dfn{preprocessing number} has a rather bizarre definition. The 519category includes all the normal integer and floating point constants 520one expects of C, but also a number of other things one might not 521initially recognize as a number. Formally, preprocessing numbers begin 522with an optional period, a required decimal digit, and then continue 523with any sequence of letters, digits, underscores, periods, and 524exponents. Exponents are the two-character sequences @samp{e+}, 525@samp{e-}, @samp{E+}, @samp{E-}, @samp{p+}, @samp{p-}, @samp{P+}, and 526@samp{P-}. (The exponents that begin with @samp{p} or @samp{P} are new 527to C99. They are used for hexadecimal floating-point constants.) 528 529The purpose of this unusual definition is to isolate the preprocessor 530from the full complexity of numeric constants. It does not have to 531distinguish between lexically valid and invalid floating-point numbers, 532which is complicated. The definition also permits you to split an 533identifier at any position and get exactly two tokens, which can then be 534pasted back together with the @samp{##} operator. 535 536It's possible for preprocessing numbers to cause programs to be 537misinterpreted. For example, @code{0xE+12} is a preprocessing number 538which does not translate to any valid numeric constant, therefore a 539syntax error. It does not mean @code{@w{0xE + 12}}, which is what you 540might have intended. 541 542@cindex string literals 543@cindex string constants 544@cindex character constants 545@cindex header file names 546@c the @: prevents makeinfo from turning '' into ". 547@dfn{String literals} are string constants, character constants, and 548header file names (the argument of @samp{#include}).@footnote{The C 549standard uses the term @dfn{string literal} to refer only to what we are 550calling @dfn{string constants}.} String constants and character 551constants are straightforward: @t{"@dots{}"} or @t{'@dots{}'}. In 552either case embedded quotes should be escaped with a backslash: 553@t{'\'@:'} is the character constant for @samp{'}. There is no limit on 554the length of a character constant, but the value of a character 555constant that contains more than one character is 556implementation-defined. @xref{Implementation Details}. 557 558Header file names either look like string constants, @t{"@dots{}"}, or are 559written with angle brackets instead, @t{<@dots{}>}. In either case, 560backslash is an ordinary character. There is no way to escape the 561closing quote or angle bracket. The preprocessor looks for the header 562file in different places depending on which form you use. @xref{Include 563Operation}. 564 565No string literal may extend past the end of a line. Older versions 566of GCC accepted multi-line string constants. You may use continued 567lines instead, or string constant concatenation. @xref{Differences 568from previous versions}. 569 570@cindex punctuators 571@cindex digraphs 572@cindex alternative tokens 573@dfn{Punctuators} are all the usual bits of punctuation which are 574meaningful to C and C++. All but three of the punctuation characters in 575ASCII are C punctuators. The exceptions are @samp{@@}, @samp{$}, and 576@samp{`}. In addition, all the two- and three-character operators are 577punctuators. There are also six @dfn{digraphs}, which the C++ standard 578calls @dfn{alternative tokens}, which are merely alternate ways to spell 579other punctuators. This is a second attempt to work around missing 580punctuation in obsolete systems. It has no negative side effects, 581unlike trigraphs, but does not cover as much ground. The digraphs and 582their corresponding normal punctuators are: 583 584@smallexample 585Digraph: <% %> <: :> %: %:%: 586Punctuator: @{ @} [ ] # ## 587@end smallexample 588 589@cindex other tokens 590Any other single character is considered ``other''. It is passed on to 591the preprocessor's output unmolested. The C compiler will almost 592certainly reject source code containing ``other'' tokens. In ASCII, the 593only other characters are @samp{@@}, @samp{$}, @samp{`}, and control 594characters other than NUL (all bits zero). (Note that @samp{$} is 595normally considered a letter.) All characters with the high bit set 596(numeric range 0x7F--0xFF) are also ``other'' in the present 597implementation. This will change when proper support for international 598character sets is added to GCC@. 599 600NUL is a special case because of the high probability that its 601appearance is accidental, and because it may be invisible to the user 602(many terminals do not display NUL at all). Within comments, NULs are 603silently ignored, just as any other character would be. In running 604text, NUL is considered white space. For example, these two directives 605have the same meaning. 606 607@smallexample 608#define X^@@1 609#define X 1 610@end smallexample 611 612@noindent 613(where @samp{^@@} is ASCII NUL)@. Within string or character constants, 614NULs are preserved. In the latter two cases the preprocessor emits a 615warning message. 616 617@node The preprocessing language 618@section The preprocessing language 619@cindex directives 620@cindex preprocessing directives 621@cindex directive line 622@cindex directive name 623 624After tokenization, the stream of tokens may simply be passed straight 625to the compiler's parser. However, if it contains any operations in the 626@dfn{preprocessing language}, it will be transformed first. This stage 627corresponds roughly to the standard's ``translation phase 4'' and is 628what most people think of as the preprocessor's job. 629 630The preprocessing language consists of @dfn{directives} to be executed 631and @dfn{macros} to be expanded. Its primary capabilities are: 632 633@itemize @bullet 634@item 635Inclusion of header files. These are files of declarations that can be 636substituted into your program. 637 638@item 639Macro expansion. You can define @dfn{macros}, which are abbreviations 640for arbitrary fragments of C code. The preprocessor will replace the 641macros with their definitions throughout the program. Some macros are 642automatically defined for you. 643 644@item 645Conditional compilation. You can include or exclude parts of the 646program according to various conditions. 647 648@item 649Line control. If you use a program to combine or rearrange source files 650into an intermediate file which is then compiled, you can use line 651control to inform the compiler where each source line originally came 652from. 653 654@item 655Diagnostics. You can detect problems at compile time and issue errors 656or warnings. 657@end itemize 658 659There are a few more, less useful, features. 660 661Except for expansion of predefined macros, all these operations are 662triggered with @dfn{preprocessing directives}. Preprocessing directives 663are lines in your program that start with @samp{#}. Whitespace is 664allowed before and after the @samp{#}. The @samp{#} is followed by an 665identifier, the @dfn{directive name}. It specifies the operation to 666perform. Directives are commonly referred to as @samp{#@var{name}} 667where @var{name} is the directive name. For example, @samp{#define} is 668the directive that defines a macro. 669 670The @samp{#} which begins a directive cannot come from a macro 671expansion. Also, the directive name is not macro expanded. Thus, if 672@code{foo} is defined as a macro expanding to @code{define}, that does 673not make @samp{#foo} a valid preprocessing directive. 674 675The set of valid directive names is fixed. Programs cannot define new 676preprocessing directives. 677 678Some directives require arguments; these make up the rest of the 679directive line and must be separated from the directive name by 680whitespace. For example, @samp{#define} must be followed by a macro 681name and the intended expansion of the macro. 682 683A preprocessing directive cannot cover more than one line. The line 684may, however, be continued with backslash-newline, or by a block comment 685which extends past the end of the line. In either case, when the 686directive is processed, the continuations have already been merged with 687the first line to make one long line. 688 689@node Header Files 690@chapter Header Files 691 692@cindex header file 693A header file is a file containing C declarations and macro definitions 694(@pxref{Macros}) to be shared between several source files. You request 695the use of a header file in your program by @dfn{including} it, with the 696C preprocessing directive @samp{#include}. 697 698Header files serve two purposes. 699 700@itemize @bullet 701@item 702@cindex system header files 703System header files declare the interfaces to parts of the operating 704system. You include them in your program to supply the definitions and 705declarations you need to invoke system calls and libraries. 706 707@item 708Your own header files contain declarations for interfaces between the 709source files of your program. Each time you have a group of related 710declarations and macro definitions all or most of which are needed in 711several different source files, it is a good idea to create a header 712file for them. 713@end itemize 714 715Including a header file produces the same results as copying the header 716file into each source file that needs it. Such copying would be 717time-consuming and error-prone. With a header file, the related 718declarations appear in only one place. If they need to be changed, they 719can be changed in one place, and programs that include the header file 720will automatically use the new version when next recompiled. The header 721file eliminates the labor of finding and changing all the copies as well 722as the risk that a failure to find one copy will result in 723inconsistencies within a program. 724 725In C, the usual convention is to give header files names that end with 726@file{.h}. It is most portable to use only letters, digits, dashes, and 727underscores in header file names, and at most one dot. 728 729@menu 730* Include Syntax:: 731* Include Operation:: 732* Search Path:: 733* Once-Only Headers:: 734* Alternatives to Wrapper #ifndef:: 735* Computed Includes:: 736* Wrapper Headers:: 737* System Headers:: 738@end menu 739 740@node Include Syntax 741@section Include Syntax 742 743@findex #include 744Both user and system header files are included using the preprocessing 745directive @samp{#include}. It has two variants: 746 747@table @code 748@item #include <@var{file}> 749This variant is used for system header files. It searches for a file 750named @var{file} in a standard list of system directories. You can prepend 751directories to this list with the @option{-I} option (@pxref{Invocation}). 752 753@item #include "@var{file}" 754This variant is used for header files of your own program. It 755searches for a file named @var{file} first in the directory containing 756the current file, then in the quote directories and then the same 757directories used for @code{<@var{file}>}. You can prepend directories 758to the list of quote directories with the @option{-iquote} option. 759@end table 760 761The argument of @samp{#include}, whether delimited with quote marks or 762angle brackets, behaves like a string constant in that comments are not 763recognized, and macro names are not expanded. Thus, @code{@w{#include 764<x/*y>}} specifies inclusion of a system header file named @file{x/*y}. 765 766However, if backslashes occur within @var{file}, they are considered 767ordinary text characters, not escape characters. None of the character 768escape sequences appropriate to string constants in C are processed. 769Thus, @code{@w{#include "x\n\\y"}} specifies a filename containing three 770backslashes. (Some systems interpret @samp{\} as a pathname separator. 771All of these also interpret @samp{/} the same way. It is most portable 772to use only @samp{/}.) 773 774It is an error if there is anything (other than comments) on the line 775after the file name. 776 777@node Include Operation 778@section Include Operation 779 780The @samp{#include} directive works by directing the C preprocessor to 781scan the specified file as input before continuing with the rest of the 782current file. The output from the preprocessor contains the output 783already generated, followed by the output resulting from the included 784file, followed by the output that comes from the text after the 785@samp{#include} directive. For example, if you have a header file 786@file{header.h} as follows, 787 788@smallexample 789char *test (void); 790@end smallexample 791 792@noindent 793and a main program called @file{program.c} that uses the header file, 794like this, 795 796@smallexample 797int x; 798#include "header.h" 799 800int 801main (void) 802@{ 803 puts (test ()); 804@} 805@end smallexample 806 807@noindent 808the compiler will see the same token stream as it would if 809@file{program.c} read 810 811@smallexample 812int x; 813char *test (void); 814 815int 816main (void) 817@{ 818 puts (test ()); 819@} 820@end smallexample 821 822Included files are not limited to declarations and macro definitions; 823those are merely the typical uses. Any fragment of a C program can be 824included from another file. The include file could even contain the 825beginning of a statement that is concluded in the containing file, or 826the end of a statement that was started in the including file. However, 827an included file must consist of complete tokens. Comments and string 828literals which have not been closed by the end of an included file are 829invalid. For error recovery, they are considered to end at the end of 830the file. 831 832To avoid confusion, it is best if header files contain only complete 833syntactic units---function declarations or definitions, type 834declarations, etc. 835 836The line following the @samp{#include} directive is always treated as a 837separate line by the C preprocessor, even if the included file lacks a 838final newline. 839 840@node Search Path 841@section Search Path 842 843GCC looks in several different places for headers. On a normal Unix 844system, if you do not instruct it otherwise, it will look for headers 845requested with @code{@w{#include <@var{file}>}} in: 846 847@smallexample 848/usr/local/include 849@var{libdir}/gcc/@var{target}/@var{version}/include 850/usr/@var{target}/include 851/usr/include 852@end smallexample 853 854For C++ programs, it will also look in 855@file{@var{libdir}/../include/c++/@var{version}}, 856first. In the above, @var{target} is the canonical name of the system 857GCC was configured to compile code for; often but not always the same as 858the canonical name of the system it runs on. @var{version} is the 859version of GCC in use. 860 861You can add to this list with the @option{-I@var{dir}} command-line 862option. All the directories named by @option{-I} are searched, in 863left-to-right order, @emph{before} the default directories. The only 864exception is when @file{dir} is already searched by default. In 865this case, the option is ignored and the search order for system 866directories remains unchanged. 867 868Duplicate directories are removed from the quote and bracket search 869chains before the two chains are merged to make the final search chain. 870Thus, it is possible for a directory to occur twice in the final search 871chain if it was specified in both the quote and bracket chains. 872 873You can prevent GCC from searching any of the default directories with 874the @option{-nostdinc} option. This is useful when you are compiling an 875operating system kernel or some other program that does not use the 876standard C library facilities, or the standard C library itself. 877@option{-I} options are not ignored as described above when 878@option{-nostdinc} is in effect. 879 880GCC looks for headers requested with @code{@w{#include "@var{file}"}} 881first in the directory containing the current file, then in the 882directories as specified by @option{-iquote} options, then in the same 883places it would have looked for a header requested with angle 884brackets. For example, if @file{/usr/include/sys/stat.h} contains 885@code{@w{#include "types.h"}}, GCC looks for @file{types.h} first in 886@file{/usr/include/sys}, then in its usual search path. 887 888@samp{#line} (@pxref{Line Control}) does not change GCC's idea of the 889directory containing the current file. 890 891You may put @option{-I-} at any point in your list of @option{-I} options. 892This has two effects. First, directories appearing before the 893@option{-I-} in the list are searched only for headers requested with 894quote marks. Directories after @option{-I-} are searched for all 895headers. Second, the directory containing the current file is not 896searched for anything, unless it happens to be one of the directories 897named by an @option{-I} switch. @option{-I-} is deprecated, @option{-iquote} 898should be used instead. 899 900@option{-I. -I-} is not the same as no @option{-I} options at all, and does 901not cause the same behavior for @samp{<>} includes that @samp{""} 902includes get with no special options. @option{-I.} searches the 903compiler's current working directory for header files. That may or may 904not be the same as the directory containing the current file. 905 906If you need to look for headers in a directory named @file{-}, write 907@option{-I./-}. 908 909There are several more ways to adjust the header search path. They are 910generally less useful. @xref{Invocation}. 911 912@node Once-Only Headers 913@section Once-Only Headers 914@cindex repeated inclusion 915@cindex including just once 916@cindex wrapper @code{#ifndef} 917 918If a header file happens to be included twice, the compiler will process 919its contents twice. This is very likely to cause an error, e.g.@: when the 920compiler sees the same structure definition twice. Even if it does not, 921it will certainly waste time. 922 923The standard way to prevent this is to enclose the entire real contents 924of the file in a conditional, like this: 925 926@smallexample 927@group 928/* File foo. */ 929#ifndef FILE_FOO_SEEN 930#define FILE_FOO_SEEN 931 932@var{the entire file} 933 934#endif /* !FILE_FOO_SEEN */ 935@end group 936@end smallexample 937 938This construct is commonly known as a @dfn{wrapper #ifndef}. 939When the header is included again, the conditional will be false, 940because @code{FILE_FOO_SEEN} is defined. The preprocessor will skip 941over the entire contents of the file, and the compiler will not see it 942twice. 943 944CPP optimizes even further. It remembers when a header file has a 945wrapper @samp{#ifndef}. If a subsequent @samp{#include} specifies that 946header, and the macro in the @samp{#ifndef} is still defined, it does 947not bother to rescan the file at all. 948 949You can put comments outside the wrapper. They will not interfere with 950this optimization. 951 952@cindex controlling macro 953@cindex guard macro 954The macro @code{FILE_FOO_SEEN} is called the @dfn{controlling macro} or 955@dfn{guard macro}. In a user header file, the macro name should not 956begin with @samp{_}. In a system header file, it should begin with 957@samp{__} to avoid conflicts with user programs. In any kind of header 958file, the macro name should contain the name of the file and some 959additional text, to avoid conflicts with other header files. 960 961@node Alternatives to Wrapper #ifndef 962@section Alternatives to Wrapper #ifndef 963 964CPP supports two more ways of indicating that a header file should be 965read only once. Neither one is as portable as a wrapper @samp{#ifndef} 966and we recommend you do not use them in new programs, with the caveat 967that @samp{#import} is standard practice in Objective-C. 968 969@findex #import 970CPP supports a variant of @samp{#include} called @samp{#import} which 971includes a file, but does so at most once. If you use @samp{#import} 972instead of @samp{#include}, then you don't need the conditionals 973inside the header file to prevent multiple inclusion of the contents. 974@samp{#import} is standard in Objective-C, but is considered a 975deprecated extension in C and C++. 976 977@samp{#import} is not a well designed feature. It requires the users of 978a header file to know that it should only be included once. It is much 979better for the header file's implementor to write the file so that users 980don't need to know this. Using a wrapper @samp{#ifndef} accomplishes 981this goal. 982 983In the present implementation, a single use of @samp{#import} will 984prevent the file from ever being read again, by either @samp{#import} or 985@samp{#include}. You should not rely on this; do not use both 986@samp{#import} and @samp{#include} to refer to the same header file. 987 988Another way to prevent a header file from being included more than once 989is with the @samp{#pragma once} directive. If @samp{#pragma once} is 990seen when scanning a header file, that file will never be read again, no 991matter what. 992 993@samp{#pragma once} does not have the problems that @samp{#import} does, 994but it is not recognized by all preprocessors, so you cannot rely on it 995in a portable program. 996 997@node Computed Includes 998@section Computed Includes 999@cindex computed includes 1000@cindex macros in include 1001 1002Sometimes it is necessary to select one of several different header 1003files to be included into your program. They might specify 1004configuration parameters to be used on different sorts of operating 1005systems, for instance. You could do this with a series of conditionals, 1006 1007@smallexample 1008#if SYSTEM_1 1009# include "system_1.h" 1010#elif SYSTEM_2 1011# include "system_2.h" 1012#elif SYSTEM_3 1013@dots{} 1014#endif 1015@end smallexample 1016 1017That rapidly becomes tedious. Instead, the preprocessor offers the 1018ability to use a macro for the header name. This is called a 1019@dfn{computed include}. Instead of writing a header name as the direct 1020argument of @samp{#include}, you simply put a macro name there instead: 1021 1022@smallexample 1023#define SYSTEM_H "system_1.h" 1024@dots{} 1025#include SYSTEM_H 1026@end smallexample 1027 1028@noindent 1029@code{SYSTEM_H} will be expanded, and the preprocessor will look for 1030@file{system_1.h} as if the @samp{#include} had been written that way 1031originally. @code{SYSTEM_H} could be defined by your Makefile with a 1032@option{-D} option. 1033 1034You must be careful when you define the macro. @samp{#define} saves 1035tokens, not text. The preprocessor has no way of knowing that the macro 1036will be used as the argument of @samp{#include}, so it generates 1037ordinary tokens, not a header name. This is unlikely to cause problems 1038if you use double-quote includes, which are close enough to string 1039constants. If you use angle brackets, however, you may have trouble. 1040 1041The syntax of a computed include is actually a bit more general than the 1042above. If the first non-whitespace character after @samp{#include} is 1043not @samp{"} or @samp{<}, then the entire line is macro-expanded 1044like running text would be. 1045 1046If the line expands to a single string constant, the contents of that 1047string constant are the file to be included. CPP does not re-examine the 1048string for embedded quotes, but neither does it process backslash 1049escapes in the string. Therefore 1050 1051@smallexample 1052#define HEADER "a\"b" 1053#include HEADER 1054@end smallexample 1055 1056@noindent 1057looks for a file named @file{a\"b}. CPP searches for the file according 1058to the rules for double-quoted includes. 1059 1060If the line expands to a token stream beginning with a @samp{<} token 1061and including a @samp{>} token, then the tokens between the @samp{<} and 1062the first @samp{>} are combined to form the filename to be included. 1063Any whitespace between tokens is reduced to a single space; then any 1064space after the initial @samp{<} is retained, but a trailing space 1065before the closing @samp{>} is ignored. CPP searches for the file 1066according to the rules for angle-bracket includes. 1067 1068In either case, if there are any tokens on the line after the file name, 1069an error occurs and the directive is not processed. It is also an error 1070if the result of expansion does not match either of the two expected 1071forms. 1072 1073These rules are implementation-defined behavior according to the C 1074standard. To minimize the risk of different compilers interpreting your 1075computed includes differently, we recommend you use only a single 1076object-like macro which expands to a string constant. This will also 1077minimize confusion for people reading your program. 1078 1079@node Wrapper Headers 1080@section Wrapper Headers 1081@cindex wrapper headers 1082@cindex overriding a header file 1083@findex #include_next 1084 1085Sometimes it is necessary to adjust the contents of a system-provided 1086header file without editing it directly. GCC's @command{fixincludes} 1087operation does this, for example. One way to do that would be to create 1088a new header file with the same name and insert it in the search path 1089before the original header. That works fine as long as you're willing 1090to replace the old header entirely. But what if you want to refer to 1091the old header from the new one? 1092 1093You cannot simply include the old header with @samp{#include}. That 1094will start from the beginning, and find your new header again. If your 1095header is not protected from multiple inclusion (@pxref{Once-Only 1096Headers}), it will recurse infinitely and cause a fatal error. 1097 1098You could include the old header with an absolute pathname: 1099@smallexample 1100#include "/usr/include/old-header.h" 1101@end smallexample 1102@noindent 1103This works, but is not clean; should the system headers ever move, you 1104would have to edit the new headers to match. 1105 1106There is no way to solve this problem within the C standard, but you can 1107use the GNU extension @samp{#include_next}. It means, ``Include the 1108@emph{next} file with this name''. This directive works like 1109@samp{#include} except in searching for the specified file: it starts 1110searching the list of header file directories @emph{after} the directory 1111in which the current file was found. 1112 1113Suppose you specify @option{-I /usr/local/include}, and the list of 1114directories to search also includes @file{/usr/include}; and suppose 1115both directories contain @file{signal.h}. Ordinary @code{@w{#include 1116<signal.h>}} finds the file under @file{/usr/local/include}. If that 1117file contains @code{@w{#include_next <signal.h>}}, it starts searching 1118after that directory, and finds the file in @file{/usr/include}. 1119 1120@samp{#include_next} does not distinguish between @code{<@var{file}>} 1121and @code{"@var{file}"} inclusion, nor does it check that the file you 1122specify has the same name as the current file. It simply looks for the 1123file named, starting with the directory in the search path after the one 1124where the current file was found. 1125 1126The use of @samp{#include_next} can lead to great confusion. We 1127recommend it be used only when there is no other alternative. In 1128particular, it should not be used in the headers belonging to a specific 1129program; it should be used only to make global corrections along the 1130lines of @command{fixincludes}. 1131 1132@node System Headers 1133@section System Headers 1134@cindex system header files 1135 1136The header files declaring interfaces to the operating system and 1137runtime libraries often cannot be written in strictly conforming C@. 1138Therefore, GCC gives code found in @dfn{system headers} special 1139treatment. All warnings, other than those generated by @samp{#warning} 1140(@pxref{Diagnostics}), are suppressed while GCC is processing a system 1141header. Macros defined in a system header are immune to a few warnings 1142wherever they are expanded. This immunity is granted on an ad-hoc 1143basis, when we find that a warning generates lots of false positives 1144because of code in macros defined in system headers. 1145 1146Normally, only the headers found in specific directories are considered 1147system headers. These directories are determined when GCC is compiled. 1148There are, however, two ways to make normal headers into system headers. 1149 1150The @option{-isystem} command-line option adds its argument to the list of 1151directories to search for headers, just like @option{-I}. Any headers 1152found in that directory will be considered system headers. 1153 1154All directories named by @option{-isystem} are searched @emph{after} all 1155directories named by @option{-I}, no matter what their order was on the 1156command line. If the same directory is named by both @option{-I} and 1157@option{-isystem}, the @option{-I} option is ignored. GCC provides an 1158informative message when this occurs if @option{-v} is used. 1159 1160@findex #pragma GCC system_header 1161There is also a directive, @code{@w{#pragma GCC system_header}}, which 1162tells GCC to consider the rest of the current include file a system 1163header, no matter where it was found. Code that comes before the 1164@samp{#pragma} in the file will not be affected. @code{@w{#pragma GCC 1165system_header}} has no effect in the primary source file. 1166 1167On very old systems, some of the pre-defined system header directories 1168get even more special treatment. GNU C++ considers code in headers 1169found in those directories to be surrounded by an @code{@w{extern "C"}} 1170block. There is no way to request this behavior with a @samp{#pragma}, 1171or from the command line. 1172 1173@node Macros 1174@chapter Macros 1175 1176A @dfn{macro} is a fragment of code which has been given a name. 1177Whenever the name is used, it is replaced by the contents of the macro. 1178There are two kinds of macros. They differ mostly in what they look 1179like when they are used. @dfn{Object-like} macros resemble data objects 1180when used, @dfn{function-like} macros resemble function calls. 1181 1182You may define any valid identifier as a macro, even if it is a C 1183keyword. The preprocessor does not know anything about keywords. This 1184can be useful if you wish to hide a keyword such as @code{const} from an 1185older compiler that does not understand it. However, the preprocessor 1186operator @code{defined} (@pxref{Defined}) can never be defined as a 1187macro, and C++'s named operators (@pxref{C++ Named Operators}) cannot be 1188macros when you are compiling C++. 1189 1190@menu 1191* Object-like Macros:: 1192* Function-like Macros:: 1193* Macro Arguments:: 1194* Stringification:: 1195* Concatenation:: 1196* Variadic Macros:: 1197* Predefined Macros:: 1198* Undefining and Redefining Macros:: 1199* Directives Within Macro Arguments:: 1200* Macro Pitfalls:: 1201@end menu 1202 1203@node Object-like Macros 1204@section Object-like Macros 1205@cindex object-like macro 1206@cindex symbolic constants 1207@cindex manifest constants 1208 1209An @dfn{object-like macro} is a simple identifier which will be replaced 1210by a code fragment. It is called object-like because it looks like a 1211data object in code that uses it. They are most commonly used to give 1212symbolic names to numeric constants. 1213 1214@findex #define 1215You create macros with the @samp{#define} directive. @samp{#define} is 1216followed by the name of the macro and then the token sequence it should 1217be an abbreviation for, which is variously referred to as the macro's 1218@dfn{body}, @dfn{expansion} or @dfn{replacement list}. For example, 1219 1220@smallexample 1221#define BUFFER_SIZE 1024 1222@end smallexample 1223 1224@noindent 1225defines a macro named @code{BUFFER_SIZE} as an abbreviation for the 1226token @code{1024}. If somewhere after this @samp{#define} directive 1227there comes a C statement of the form 1228 1229@smallexample 1230foo = (char *) malloc (BUFFER_SIZE); 1231@end smallexample 1232 1233@noindent 1234then the C preprocessor will recognize and @dfn{expand} the macro 1235@code{BUFFER_SIZE}. The C compiler will see the same tokens as it would 1236if you had written 1237 1238@smallexample 1239foo = (char *) malloc (1024); 1240@end smallexample 1241 1242By convention, macro names are written in uppercase. Programs are 1243easier to read when it is possible to tell at a glance which names are 1244macros. 1245 1246The macro's body ends at the end of the @samp{#define} line. You may 1247continue the definition onto multiple lines, if necessary, using 1248backslash-newline. When the macro is expanded, however, it will all 1249come out on one line. For example, 1250 1251@smallexample 1252#define NUMBERS 1, \ 1253 2, \ 1254 3 1255int x[] = @{ NUMBERS @}; 1256 @expansion{} int x[] = @{ 1, 2, 3 @}; 1257@end smallexample 1258 1259@noindent 1260The most common visible consequence of this is surprising line numbers 1261in error messages. 1262 1263There is no restriction on what can go in a macro body provided it 1264decomposes into valid preprocessing tokens. Parentheses need not 1265balance, and the body need not resemble valid C code. (If it does not, 1266you may get error messages from the C compiler when you use the macro.) 1267 1268The C preprocessor scans your program sequentially. Macro definitions 1269take effect at the place you write them. Therefore, the following input 1270to the C preprocessor 1271 1272@smallexample 1273foo = X; 1274#define X 4 1275bar = X; 1276@end smallexample 1277 1278@noindent 1279produces 1280 1281@smallexample 1282foo = X; 1283bar = 4; 1284@end smallexample 1285 1286When the preprocessor expands a macro name, the macro's expansion 1287replaces the macro invocation, then the expansion is examined for more 1288macros to expand. For example, 1289 1290@smallexample 1291@group 1292#define TABLESIZE BUFSIZE 1293#define BUFSIZE 1024 1294TABLESIZE 1295 @expansion{} BUFSIZE 1296 @expansion{} 1024 1297@end group 1298@end smallexample 1299 1300@noindent 1301@code{TABLESIZE} is expanded first to produce @code{BUFSIZE}, then that 1302macro is expanded to produce the final result, @code{1024}. 1303 1304Notice that @code{BUFSIZE} was not defined when @code{TABLESIZE} was 1305defined. The @samp{#define} for @code{TABLESIZE} uses exactly the 1306expansion you specify---in this case, @code{BUFSIZE}---and does not 1307check to see whether it too contains macro names. Only when you 1308@emph{use} @code{TABLESIZE} is the result of its expansion scanned for 1309more macro names. 1310 1311This makes a difference if you change the definition of @code{BUFSIZE} 1312at some point in the source file. @code{TABLESIZE}, defined as shown, 1313will always expand using the definition of @code{BUFSIZE} that is 1314currently in effect: 1315 1316@smallexample 1317#define BUFSIZE 1020 1318#define TABLESIZE BUFSIZE 1319#undef BUFSIZE 1320#define BUFSIZE 37 1321@end smallexample 1322 1323@noindent 1324Now @code{TABLESIZE} expands (in two stages) to @code{37}. 1325 1326If the expansion of a macro contains its own name, either directly or 1327via intermediate macros, it is not expanded again when the expansion is 1328examined for more macros. This prevents infinite recursion. 1329@xref{Self-Referential Macros}, for the precise details. 1330 1331@node Function-like Macros 1332@section Function-like Macros 1333@cindex function-like macros 1334 1335You can also define macros whose use looks like a function call. These 1336are called @dfn{function-like macros}. To define a function-like macro, 1337you use the same @samp{#define} directive, but you put a pair of 1338parentheses immediately after the macro name. For example, 1339 1340@smallexample 1341#define lang_init() c_init() 1342lang_init() 1343 @expansion{} c_init() 1344@end smallexample 1345 1346A function-like macro is only expanded if its name appears with a pair 1347of parentheses after it. If you write just the name, it is left alone. 1348This can be useful when you have a function and a macro of the same 1349name, and you wish to use the function sometimes. 1350 1351@smallexample 1352extern void foo(void); 1353#define foo() /* @r{optimized inline version} */ 1354@dots{} 1355 foo(); 1356 funcptr = foo; 1357@end smallexample 1358 1359Here the call to @code{foo()} will use the macro, but the function 1360pointer will get the address of the real function. If the macro were to 1361be expanded, it would cause a syntax error. 1362 1363If you put spaces between the macro name and the parentheses in the 1364macro definition, that does not define a function-like macro, it defines 1365an object-like macro whose expansion happens to begin with a pair of 1366parentheses. 1367 1368@smallexample 1369#define lang_init () c_init() 1370lang_init() 1371 @expansion{} () c_init()() 1372@end smallexample 1373 1374The first two pairs of parentheses in this expansion come from the 1375macro. The third is the pair that was originally after the macro 1376invocation. Since @code{lang_init} is an object-like macro, it does not 1377consume those parentheses. 1378 1379@node Macro Arguments 1380@section Macro Arguments 1381@cindex arguments 1382@cindex macros with arguments 1383@cindex arguments in macro definitions 1384 1385Function-like macros can take @dfn{arguments}, just like true functions. 1386To define a macro that uses arguments, you insert @dfn{parameters} 1387between the pair of parentheses in the macro definition that make the 1388macro function-like. The parameters must be valid C identifiers, 1389separated by commas and optionally whitespace. 1390 1391To invoke a macro that takes arguments, you write the name of the macro 1392followed by a list of @dfn{actual arguments} in parentheses, separated 1393by commas. The invocation of the macro need not be restricted to a 1394single logical line---it can cross as many lines in the source file as 1395you wish. The number of arguments you give must match the number of 1396parameters in the macro definition. When the macro is expanded, each 1397use of a parameter in its body is replaced by the tokens of the 1398corresponding argument. (You need not use all of the parameters in the 1399macro body.) 1400 1401As an example, here is a macro that computes the minimum of two numeric 1402values, as it is defined in many C programs, and some uses. 1403 1404@smallexample 1405#define min(X, Y) ((X) < (Y) ? (X) : (Y)) 1406 x = min(a, b); @expansion{} x = ((a) < (b) ? (a) : (b)); 1407 y = min(1, 2); @expansion{} y = ((1) < (2) ? (1) : (2)); 1408 z = min(a + 28, *p); @expansion{} z = ((a + 28) < (*p) ? (a + 28) : (*p)); 1409@end smallexample 1410 1411@noindent 1412(In this small example you can already see several of the dangers of 1413macro arguments. @xref{Macro Pitfalls}, for detailed explanations.) 1414 1415Leading and trailing whitespace in each argument is dropped, and all 1416whitespace between the tokens of an argument is reduced to a single 1417space. Parentheses within each argument must balance; a comma within 1418such parentheses does not end the argument. However, there is no 1419requirement for square brackets or braces to balance, and they do not 1420prevent a comma from separating arguments. Thus, 1421 1422@smallexample 1423macro (array[x = y, x + 1]) 1424@end smallexample 1425 1426@noindent 1427passes two arguments to @code{macro}: @code{array[x = y} and @code{x + 14281]}. If you want to supply @code{array[x = y, x + 1]} as an argument, 1429you can write it as @code{array[(x = y, x + 1)]}, which is equivalent C 1430code. 1431 1432All arguments to a macro are completely macro-expanded before they are 1433substituted into the macro body. After substitution, the complete text 1434is scanned again for macros to expand, including the arguments. This rule 1435may seem strange, but it is carefully designed so you need not worry 1436about whether any function call is actually a macro invocation. You can 1437run into trouble if you try to be too clever, though. @xref{Argument 1438Prescan}, for detailed discussion. 1439 1440For example, @code{min (min (a, b), c)} is first expanded to 1441 1442@smallexample 1443 min (((a) < (b) ? (a) : (b)), (c)) 1444@end smallexample 1445 1446@noindent 1447and then to 1448 1449@smallexample 1450@group 1451((((a) < (b) ? (a) : (b))) < (c) 1452 ? (((a) < (b) ? (a) : (b))) 1453 : (c)) 1454@end group 1455@end smallexample 1456 1457@noindent 1458(Line breaks shown here for clarity would not actually be generated.) 1459 1460@cindex empty macro arguments 1461You can leave macro arguments empty; this is not an error to the 1462preprocessor (but many macros will then expand to invalid code). 1463You cannot leave out arguments entirely; if a macro takes two arguments, 1464there must be exactly one comma at the top level of its argument list. 1465Here are some silly examples using @code{min}: 1466 1467@smallexample 1468min(, b) @expansion{} (( ) < (b) ? ( ) : (b)) 1469min(a, ) @expansion{} ((a ) < ( ) ? (a ) : ( )) 1470min(,) @expansion{} (( ) < ( ) ? ( ) : ( )) 1471min((,),) @expansion{} (((,)) < ( ) ? ((,)) : ( )) 1472 1473min() @error{} macro "min" requires 2 arguments, but only 1 given 1474min(,,) @error{} macro "min" passed 3 arguments, but takes just 2 1475@end smallexample 1476 1477Whitespace is not a preprocessing token, so if a macro @code{foo} takes 1478one argument, @code{@w{foo ()}} and @code{@w{foo ( )}} both supply it an 1479empty argument. Previous GNU preprocessor implementations and 1480documentation were incorrect on this point, insisting that a 1481function-like macro that takes a single argument be passed a space if an 1482empty argument was required. 1483 1484Macro parameters appearing inside string literals are not replaced by 1485their corresponding actual arguments. 1486 1487@smallexample 1488#define foo(x) x, "x" 1489foo(bar) @expansion{} bar, "x" 1490@end smallexample 1491 1492@node Stringification 1493@section Stringification 1494@cindex stringification 1495@cindex @samp{#} operator 1496 1497Sometimes you may want to convert a macro argument into a string 1498constant. Parameters are not replaced inside string constants, but you 1499can use the @samp{#} preprocessing operator instead. When a macro 1500parameter is used with a leading @samp{#}, the preprocessor replaces it 1501with the literal text of the actual argument, converted to a string 1502constant. Unlike normal parameter replacement, the argument is not 1503macro-expanded first. This is called @dfn{stringification}. 1504 1505There is no way to combine an argument with surrounding text and 1506stringify it all together. Instead, you can write a series of adjacent 1507string constants and stringified arguments. The preprocessor will 1508replace the stringified arguments with string constants. The C 1509compiler will then combine all the adjacent string constants into one 1510long string. 1511 1512Here is an example of a macro definition that uses stringification: 1513 1514@smallexample 1515@group 1516#define WARN_IF(EXP) \ 1517do @{ if (EXP) \ 1518 fprintf (stderr, "Warning: " #EXP "\n"); @} \ 1519while (0) 1520WARN_IF (x == 0); 1521 @expansion{} do @{ if (x == 0) 1522 fprintf (stderr, "Warning: " "x == 0" "\n"); @} while (0); 1523@end group 1524@end smallexample 1525 1526@noindent 1527The argument for @code{EXP} is substituted once, as-is, into the 1528@code{if} statement, and once, stringified, into the argument to 1529@code{fprintf}. If @code{x} were a macro, it would be expanded in the 1530@code{if} statement, but not in the string. 1531 1532The @code{do} and @code{while (0)} are a kludge to make it possible to 1533write @code{WARN_IF (@var{arg});}, which the resemblance of 1534@code{WARN_IF} to a function would make C programmers want to do; see 1535@ref{Swallowing the Semicolon}. 1536 1537Stringification in C involves more than putting double-quote characters 1538around the fragment. The preprocessor backslash-escapes the quotes 1539surrounding embedded string constants, and all backslashes within string and 1540character constants, in order to get a valid C string constant with the 1541proper contents. Thus, stringifying @code{@w{p = "foo\n";}} results in 1542@t{@w{"p = \"foo\\n\";"}}. However, backslashes that are not inside string 1543or character constants are not duplicated: @samp{\n} by itself 1544stringifies to @t{"\n"}. 1545 1546All leading and trailing whitespace in text being stringified is 1547ignored. Any sequence of whitespace in the middle of the text is 1548converted to a single space in the stringified result. Comments are 1549replaced by whitespace long before stringification happens, so they 1550never appear in stringified text. 1551 1552There is no way to convert a macro argument into a character constant. 1553 1554If you want to stringify the result of expansion of a macro argument, 1555you have to use two levels of macros. 1556 1557@smallexample 1558#define xstr(s) str(s) 1559#define str(s) #s 1560#define foo 4 1561str (foo) 1562 @expansion{} "foo" 1563xstr (foo) 1564 @expansion{} xstr (4) 1565 @expansion{} str (4) 1566 @expansion{} "4" 1567@end smallexample 1568 1569@code{s} is stringified when it is used in @code{str}, so it is not 1570macro-expanded first. But @code{s} is an ordinary argument to 1571@code{xstr}, so it is completely macro-expanded before @code{xstr} 1572itself is expanded (@pxref{Argument Prescan}). Therefore, by the time 1573@code{str} gets to its argument, it has already been macro-expanded. 1574 1575@node Concatenation 1576@section Concatenation 1577@cindex concatenation 1578@cindex token pasting 1579@cindex token concatenation 1580@cindex @samp{##} operator 1581 1582It is often useful to merge two tokens into one while expanding macros. 1583This is called @dfn{token pasting} or @dfn{token concatenation}. The 1584@samp{##} preprocessing operator performs token pasting. When a macro 1585is expanded, the two tokens on either side of each @samp{##} operator 1586are combined into a single token, which then replaces the @samp{##} and 1587the two original tokens in the macro expansion. Usually both will be 1588identifiers, or one will be an identifier and the other a preprocessing 1589number. When pasted, they make a longer identifier. This isn't the 1590only valid case. It is also possible to concatenate two numbers (or a 1591number and a name, such as @code{1.5} and @code{e3}) into a number. 1592Also, multi-character operators such as @code{+=} can be formed by 1593token pasting. 1594 1595However, two tokens that don't together form a valid token cannot be 1596pasted together. For example, you cannot concatenate @code{x} with 1597@code{+} in either order. If you try, the preprocessor issues a warning 1598and emits the two tokens. Whether it puts white space between the 1599tokens is undefined. It is common to find unnecessary uses of @samp{##} 1600in complex macros. If you get this warning, it is likely that you can 1601simply remove the @samp{##}. 1602 1603Both the tokens combined by @samp{##} could come from the macro body, 1604but you could just as well write them as one token in the first place. 1605Token pasting is most useful when one or both of the tokens comes from a 1606macro argument. If either of the tokens next to an @samp{##} is a 1607parameter name, it is replaced by its actual argument before @samp{##} 1608executes. As with stringification, the actual argument is not 1609macro-expanded first. If the argument is empty, that @samp{##} has no 1610effect. 1611 1612Keep in mind that the C preprocessor converts comments to whitespace 1613before macros are even considered. Therefore, you cannot create a 1614comment by concatenating @samp{/} and @samp{*}. You can put as much 1615whitespace between @samp{##} and its operands as you like, including 1616comments, and you can put comments in arguments that will be 1617concatenated. However, it is an error if @samp{##} appears at either 1618end of a macro body. 1619 1620Consider a C program that interprets named commands. There probably 1621needs to be a table of commands, perhaps an array of structures declared 1622as follows: 1623 1624@smallexample 1625@group 1626struct command 1627@{ 1628 char *name; 1629 void (*function) (void); 1630@}; 1631@end group 1632 1633@group 1634struct command commands[] = 1635@{ 1636 @{ "quit", quit_command @}, 1637 @{ "help", help_command @}, 1638 @dots{} 1639@}; 1640@end group 1641@end smallexample 1642 1643It would be cleaner not to have to give each command name twice, once in 1644the string constant and once in the function name. A macro which takes the 1645name of a command as an argument can make this unnecessary. The string 1646constant can be created with stringification, and the function name by 1647concatenating the argument with @samp{_command}. Here is how it is done: 1648 1649@smallexample 1650#define COMMAND(NAME) @{ #NAME, NAME ## _command @} 1651 1652struct command commands[] = 1653@{ 1654 COMMAND (quit), 1655 COMMAND (help), 1656 @dots{} 1657@}; 1658@end smallexample 1659 1660@node Variadic Macros 1661@section Variadic Macros 1662@cindex variable number of arguments 1663@cindex macros with variable arguments 1664@cindex variadic macros 1665 1666A macro can be declared to accept a variable number of arguments much as 1667a function can. The syntax for defining the macro is similar to that of 1668a function. Here is an example: 1669 1670@smallexample 1671#define eprintf(@dots{}) fprintf (stderr, __VA_ARGS__) 1672@end smallexample 1673 1674This kind of macro is called @dfn{variadic}. When the macro is invoked, 1675all the tokens in its argument list after the last named argument (this 1676macro has none), including any commas, become the @dfn{variable 1677argument}. This sequence of tokens replaces the identifier 1678@code{@w{__VA_ARGS__}} in the macro body wherever it appears. Thus, we 1679have this expansion: 1680 1681@smallexample 1682eprintf ("%s:%d: ", input_file, lineno) 1683 @expansion{} fprintf (stderr, "%s:%d: ", input_file, lineno) 1684@end smallexample 1685 1686The variable argument is completely macro-expanded before it is inserted 1687into the macro expansion, just like an ordinary argument. You may use 1688the @samp{#} and @samp{##} operators to stringify the variable argument 1689or to paste its leading or trailing token with another token. (But see 1690below for an important special case for @samp{##}.) 1691 1692If your macro is complicated, you may want a more descriptive name for 1693the variable argument than @code{@w{__VA_ARGS__}}. CPP permits 1694this, as an extension. You may write an argument name immediately 1695before the @samp{@dots{}}; that name is used for the variable argument. 1696The @code{eprintf} macro above could be written 1697 1698@smallexample 1699#define eprintf(args@dots{}) fprintf (stderr, args) 1700@end smallexample 1701 1702@noindent 1703using this extension. You cannot use @code{@w{__VA_ARGS__}} and this 1704extension in the same macro. 1705 1706You can have named arguments as well as variable arguments in a variadic 1707macro. We could define @code{eprintf} like this, instead: 1708 1709@smallexample 1710#define eprintf(format, @dots{}) fprintf (stderr, format, __VA_ARGS__) 1711@end smallexample 1712 1713@noindent 1714This formulation looks more descriptive, but unfortunately it is less 1715flexible: you must now supply at least one argument after the format 1716string. In standard C, you cannot omit the comma separating the named 1717argument from the variable arguments. Furthermore, if you leave the 1718variable argument empty, you will get a syntax error, because 1719there will be an extra comma after the format string. 1720 1721@smallexample 1722eprintf("success!\n", ); 1723 @expansion{} fprintf(stderr, "success!\n", ); 1724@end smallexample 1725 1726GNU CPP has a pair of extensions which deal with this problem. First, 1727you are allowed to leave the variable argument out entirely: 1728 1729@smallexample 1730eprintf ("success!\n") 1731 @expansion{} fprintf(stderr, "success!\n", ); 1732@end smallexample 1733 1734@noindent 1735Second, the @samp{##} token paste operator has a special meaning when 1736placed between a comma and a variable argument. If you write 1737 1738@smallexample 1739#define eprintf(format, @dots{}) fprintf (stderr, format, ##__VA_ARGS__) 1740@end smallexample 1741 1742@noindent 1743and the variable argument is left out when the @code{eprintf} macro is 1744used, then the comma before the @samp{##} will be deleted. This does 1745@emph{not} happen if you pass an empty argument, nor does it happen if 1746the token preceding @samp{##} is anything other than a comma. 1747 1748@smallexample 1749eprintf ("success!\n") 1750 @expansion{} fprintf(stderr, "success!\n"); 1751@end smallexample 1752 1753@noindent 1754The above explanation is ambiguous about the case where the only macro 1755parameter is a variable arguments parameter, as it is meaningless to 1756try to distinguish whether no argument at all is an empty argument or 1757a missing argument. In this case the C99 standard is clear that the 1758comma must remain, however the existing GCC extension used to swallow 1759the comma. So CPP retains the comma when conforming to a specific C 1760standard, and drops it otherwise. 1761 1762C99 mandates that the only place the identifier @code{@w{__VA_ARGS__}} 1763can appear is in the replacement list of a variadic macro. It may not 1764be used as a macro name, macro argument name, or within a different type 1765of macro. It may also be forbidden in open text; the standard is 1766ambiguous. We recommend you avoid using it except for its defined 1767purpose. 1768 1769Variadic macros are a new feature in C99. GNU CPP has supported them 1770for a long time, but only with a named variable argument 1771(@samp{args@dots{}}, not @samp{@dots{}} and @code{@w{__VA_ARGS__}}). If you are 1772concerned with portability to previous versions of GCC, you should use 1773only named variable arguments. On the other hand, if you are concerned 1774with portability to other conforming implementations of C99, you should 1775use only @code{@w{__VA_ARGS__}}. 1776 1777Previous versions of CPP implemented the comma-deletion extension 1778much more generally. We have restricted it in this release to minimize 1779the differences from C99. To get the same effect with both this and 1780previous versions of GCC, the token preceding the special @samp{##} must 1781be a comma, and there must be white space between that comma and 1782whatever comes immediately before it: 1783 1784@smallexample 1785#define eprintf(format, args@dots{}) fprintf (stderr, format , ##args) 1786@end smallexample 1787 1788@noindent 1789@xref{Differences from previous versions}, for the gory details. 1790 1791@node Predefined Macros 1792@section Predefined Macros 1793 1794@cindex predefined macros 1795Several object-like macros are predefined; you use them without 1796supplying their definitions. They fall into three classes: standard, 1797common, and system-specific. 1798 1799In C++, there is a fourth category, the named operators. They act like 1800predefined macros, but you cannot undefine them. 1801 1802@menu 1803* Standard Predefined Macros:: 1804* Common Predefined Macros:: 1805* System-specific Predefined Macros:: 1806* C++ Named Operators:: 1807@end menu 1808 1809@node Standard Predefined Macros 1810@subsection Standard Predefined Macros 1811@cindex standard predefined macros. 1812 1813The standard predefined macros are specified by the relevant 1814language standards, so they are available with all compilers that 1815implement those standards. Older compilers may not provide all of 1816them. Their names all start with double underscores. 1817 1818@table @code 1819@item __FILE__ 1820This macro expands to the name of the current input file, in the form of 1821a C string constant. This is the path by which the preprocessor opened 1822the file, not the short name specified in @samp{#include} or as the 1823input file name argument. For example, 1824@code{"/usr/local/include/myheader.h"} is a possible expansion of this 1825macro. 1826 1827@item __LINE__ 1828This macro expands to the current input line number, in the form of a 1829decimal integer constant. While we call it a predefined macro, it's 1830a pretty strange macro, since its ``definition'' changes with each 1831new line of source code. 1832@end table 1833 1834@code{__FILE__} and @code{__LINE__} are useful in generating an error 1835message to report an inconsistency detected by the program; the message 1836can state the source line at which the inconsistency was detected. For 1837example, 1838 1839@smallexample 1840fprintf (stderr, "Internal error: " 1841 "negative string length " 1842 "%d at %s, line %d.", 1843 length, __FILE__, __LINE__); 1844@end smallexample 1845 1846An @samp{#include} directive changes the expansions of @code{__FILE__} 1847and @code{__LINE__} to correspond to the included file. At the end of 1848that file, when processing resumes on the input file that contained 1849the @samp{#include} directive, the expansions of @code{__FILE__} and 1850@code{__LINE__} revert to the values they had before the 1851@samp{#include} (but @code{__LINE__} is then incremented by one as 1852processing moves to the line after the @samp{#include}). 1853 1854A @samp{#line} directive changes @code{__LINE__}, and may change 1855@code{__FILE__} as well. @xref{Line Control}. 1856 1857C99 introduces @code{__func__}, and GCC has provided @code{__FUNCTION__} 1858for a long time. Both of these are strings containing the name of the 1859current function (there are slight semantic differences; see the GCC 1860manual). Neither of them is a macro; the preprocessor does not know the 1861name of the current function. They tend to be useful in conjunction 1862with @code{__FILE__} and @code{__LINE__}, though. 1863 1864@table @code 1865 1866@item __DATE__ 1867This macro expands to a string constant that describes the date on which 1868the preprocessor is being run. The string constant contains eleven 1869characters and looks like @code{@w{"Feb 12 1996"}}. If the day of the 1870month is less than 10, it is padded with a space on the left. 1871 1872If GCC cannot determine the current date, it will emit a warning message 1873(once per compilation) and @code{__DATE__} will expand to 1874@code{@w{"??? ?? ????"}}. 1875 1876@item __TIME__ 1877This macro expands to a string constant that describes the time at 1878which the preprocessor is being run. The string constant contains 1879eight characters and looks like @code{"23:59:01"}. 1880 1881If GCC cannot determine the current time, it will emit a warning message 1882(once per compilation) and @code{__TIME__} will expand to 1883@code{"??:??:??"}. 1884 1885@item __STDC__ 1886In normal operation, this macro expands to the constant 1, to signify 1887that this compiler conforms to ISO Standard C@. If GNU CPP is used with 1888a compiler other than GCC, this is not necessarily true; however, the 1889preprocessor always conforms to the standard unless the 1890@option{-traditional-cpp} option is used. 1891 1892This macro is not defined if the @option{-traditional-cpp} option is used. 1893 1894On some hosts, the system compiler uses a different convention, where 1895@code{__STDC__} is normally 0, but is 1 if the user specifies strict 1896conformance to the C Standard. CPP follows the host convention when 1897processing system header files, but when processing user files 1898@code{__STDC__} is always 1. This has been reported to cause problems; 1899for instance, some versions of Solaris provide X Windows headers that 1900expect @code{__STDC__} to be either undefined or 1. @xref{Invocation}. 1901 1902@item __STDC_VERSION__ 1903This macro expands to the C Standard's version number, a long integer 1904constant of the form @code{@var{yyyy}@var{mm}L} where @var{yyyy} and 1905@var{mm} are the year and month of the Standard version. This signifies 1906which version of the C Standard the compiler conforms to. Like 1907@code{__STDC__}, this is not necessarily accurate for the entire 1908implementation, unless GNU CPP is being used with GCC@. 1909 1910The value @code{199409L} signifies the 1989 C standard as amended in 19111994, which is the current default; the value @code{199901L} signifies 1912the 1999 revision of the C standard. Support for the 1999 revision is 1913not yet complete. 1914 1915This macro is not defined if the @option{-traditional-cpp} option is 1916used, nor when compiling C++ or Objective-C@. 1917 1918@item __STDC_HOSTED__ 1919This macro is defined, with value 1, if the compiler's target is a 1920@dfn{hosted environment}. A hosted environment has the complete 1921facilities of the standard C library available. 1922 1923@item __cplusplus 1924This macro is defined when the C++ compiler is in use. You can use 1925@code{__cplusplus} to test whether a header is compiled by a C compiler 1926or a C++ compiler. This macro is similar to @code{__STDC_VERSION__}, in 1927that it expands to a version number. Depending on the language standard 1928selected, the value of the macro is @code{199711L}, as mandated by the 19291998 C++ standard; @code{201103L}, per the 2011 C++ standard; an 1930unspecified value strictly larger than @code{201103L} for the experimental 1931languages enabled by @option{-std=c++1y} and @option{-std=gnu++1y}. 1932 1933@item __OBJC__ 1934This macro is defined, with value 1, when the Objective-C compiler is in 1935use. You can use @code{__OBJC__} to test whether a header is compiled 1936by a C compiler or an Objective-C compiler. 1937 1938@item __ASSEMBLER__ 1939This macro is defined with value 1 when preprocessing assembly 1940language. 1941 1942@end table 1943 1944@node Common Predefined Macros 1945@subsection Common Predefined Macros 1946@cindex common predefined macros 1947 1948The common predefined macros are GNU C extensions. They are available 1949with the same meanings regardless of the machine or operating system on 1950which you are using GNU C or GNU Fortran. Their names all start with 1951double underscores. 1952 1953@table @code 1954 1955@item __COUNTER__ 1956This macro expands to sequential integral values starting from 0. In 1957conjunction with the @code{##} operator, this provides a convenient means to 1958generate unique identifiers. Care must be taken to ensure that 1959@code{__COUNTER__} is not expanded prior to inclusion of precompiled headers 1960which use it. Otherwise, the precompiled headers will not be used. 1961 1962@item __GFORTRAN__ 1963The GNU Fortran compiler defines this. 1964 1965@item __GNUC__ 1966@itemx __GNUC_MINOR__ 1967@itemx __GNUC_PATCHLEVEL__ 1968These macros are defined by all GNU compilers that use the C 1969preprocessor: C, C++, Objective-C and Fortran. Their values are the major 1970version, minor version, and patch level of the compiler, as integer 1971constants. For example, GCC 3.2.1 will define @code{__GNUC__} to 3, 1972@code{__GNUC_MINOR__} to 2, and @code{__GNUC_PATCHLEVEL__} to 1. These 1973macros are also defined if you invoke the preprocessor directly. 1974 1975@code{__GNUC_PATCHLEVEL__} is new to GCC 3.0; it is also present in the 1976widely-used development snapshots leading up to 3.0 (which identify 1977themselves as GCC 2.96 or 2.97, depending on which snapshot you have). 1978 1979If all you need to know is whether or not your program is being compiled 1980by GCC, or a non-GCC compiler that claims to accept the GNU C dialects, 1981you can simply test @code{__GNUC__}. If you need to write code 1982which depends on a specific version, you must be more careful. Each 1983time the minor version is increased, the patch level is reset to zero; 1984each time the major version is increased (which happens rarely), the 1985minor version and patch level are reset. If you wish to use the 1986predefined macros directly in the conditional, you will need to write it 1987like this: 1988 1989@smallexample 1990/* @r{Test for GCC > 3.2.0} */ 1991#if __GNUC__ > 3 || \ 1992 (__GNUC__ == 3 && (__GNUC_MINOR__ > 2 || \ 1993 (__GNUC_MINOR__ == 2 && \ 1994 __GNUC_PATCHLEVEL__ > 0)) 1995@end smallexample 1996 1997@noindent 1998Another approach is to use the predefined macros to 1999calculate a single number, then compare that against a threshold: 2000 2001@smallexample 2002#define GCC_VERSION (__GNUC__ * 10000 \ 2003 + __GNUC_MINOR__ * 100 \ 2004 + __GNUC_PATCHLEVEL__) 2005@dots{} 2006/* @r{Test for GCC > 3.2.0} */ 2007#if GCC_VERSION > 30200 2008@end smallexample 2009 2010@noindent 2011Many people find this form easier to understand. 2012 2013@item __GNUG__ 2014The GNU C++ compiler defines this. Testing it is equivalent to 2015testing @code{@w{(__GNUC__ && __cplusplus)}}. 2016 2017@item __STRICT_ANSI__ 2018GCC defines this macro if and only if the @option{-ansi} switch, or a 2019@option{-std} switch specifying strict conformance to some version of ISO C 2020or ISO C++, was specified when GCC was invoked. It is defined to @samp{1}. 2021This macro exists primarily to direct GNU libc's header files to 2022restrict their definitions to the minimal set found in the 1989 C 2023standard. 2024 2025@item __BASE_FILE__ 2026This macro expands to the name of the main input file, in the form 2027of a C string constant. This is the source file that was specified 2028on the command line of the preprocessor or C compiler. 2029 2030@item __INCLUDE_LEVEL__ 2031This macro expands to a decimal integer constant that represents the 2032depth of nesting in include files. The value of this macro is 2033incremented on every @samp{#include} directive and decremented at the 2034end of every included file. It starts out at 0, its value within the 2035base file specified on the command line. 2036 2037@item __ELF__ 2038This macro is defined if the target uses the ELF object format. 2039 2040@item __VERSION__ 2041This macro expands to a string constant which describes the version of 2042the compiler in use. You should not rely on its contents having any 2043particular form, but it can be counted on to contain at least the 2044release number. 2045 2046@item __OPTIMIZE__ 2047@itemx __OPTIMIZE_SIZE__ 2048@itemx __NO_INLINE__ 2049These macros describe the compilation mode. @code{__OPTIMIZE__} is 2050defined in all optimizing compilations. @code{__OPTIMIZE_SIZE__} is 2051defined if the compiler is optimizing for size, not speed. 2052@code{__NO_INLINE__} is defined if no functions will be inlined into 2053their callers (when not optimizing, or when inlining has been 2054specifically disabled by @option{-fno-inline}). 2055 2056These macros cause certain GNU header files to provide optimized 2057definitions, using macros or inline functions, of system library 2058functions. You should not use these macros in any way unless you make 2059sure that programs will execute with the same effect whether or not they 2060are defined. If they are defined, their value is 1. 2061 2062@item __GNUC_GNU_INLINE__ 2063GCC defines this macro if functions declared @code{inline} will be 2064handled in GCC's traditional gnu90 mode. Object files will contain 2065externally visible definitions of all functions declared @code{inline} 2066without @code{extern} or @code{static}. They will not contain any 2067definitions of any functions declared @code{extern inline}. 2068 2069@item __GNUC_STDC_INLINE__ 2070GCC defines this macro if functions declared @code{inline} will be 2071handled according to the ISO C99 standard. Object files will contain 2072externally visible definitions of all functions declared @code{extern 2073inline}. They will not contain definitions of any functions declared 2074@code{inline} without @code{extern}. 2075 2076If this macro is defined, GCC supports the @code{gnu_inline} function 2077attribute as a way to always get the gnu90 behavior. Support for 2078this and @code{__GNUC_GNU_INLINE__} was added in GCC 4.1.3. If 2079neither macro is defined, an older version of GCC is being used: 2080@code{inline} functions will be compiled in gnu90 mode, and the 2081@code{gnu_inline} function attribute will not be recognized. 2082 2083@item __CHAR_UNSIGNED__ 2084GCC defines this macro if and only if the data type @code{char} is 2085unsigned on the target machine. It exists to cause the standard header 2086file @file{limits.h} to work correctly. You should not use this macro 2087yourself; instead, refer to the standard macros defined in @file{limits.h}. 2088 2089@item __WCHAR_UNSIGNED__ 2090Like @code{__CHAR_UNSIGNED__}, this macro is defined if and only if the 2091data type @code{wchar_t} is unsigned and the front-end is in C++ mode. 2092 2093@item __REGISTER_PREFIX__ 2094This macro expands to a single token (not a string constant) which is 2095the prefix applied to CPU register names in assembly language for this 2096target. You can use it to write assembly that is usable in multiple 2097environments. For example, in the @code{m68k-aout} environment it 2098expands to nothing, but in the @code{m68k-coff} environment it expands 2099to a single @samp{%}. 2100 2101@item __USER_LABEL_PREFIX__ 2102This macro expands to a single token which is the prefix applied to 2103user labels (symbols visible to C code) in assembly. For example, in 2104the @code{m68k-aout} environment it expands to an @samp{_}, but in the 2105@code{m68k-coff} environment it expands to nothing. 2106 2107This macro will have the correct definition even if 2108@option{-f(no-)underscores} is in use, but it will not be correct if 2109target-specific options that adjust this prefix are used (e.g.@: the 2110OSF/rose @option{-mno-underscores} option). 2111 2112@item __SIZE_TYPE__ 2113@itemx __PTRDIFF_TYPE__ 2114@itemx __WCHAR_TYPE__ 2115@itemx __WINT_TYPE__ 2116@itemx __INTMAX_TYPE__ 2117@itemx __UINTMAX_TYPE__ 2118@itemx __SIG_ATOMIC_TYPE__ 2119@itemx __INT8_TYPE__ 2120@itemx __INT16_TYPE__ 2121@itemx __INT32_TYPE__ 2122@itemx __INT64_TYPE__ 2123@itemx __UINT8_TYPE__ 2124@itemx __UINT16_TYPE__ 2125@itemx __UINT32_TYPE__ 2126@itemx __UINT64_TYPE__ 2127@itemx __INT_LEAST8_TYPE__ 2128@itemx __INT_LEAST16_TYPE__ 2129@itemx __INT_LEAST32_TYPE__ 2130@itemx __INT_LEAST64_TYPE__ 2131@itemx __UINT_LEAST8_TYPE__ 2132@itemx __UINT_LEAST16_TYPE__ 2133@itemx __UINT_LEAST32_TYPE__ 2134@itemx __UINT_LEAST64_TYPE__ 2135@itemx __INT_FAST8_TYPE__ 2136@itemx __INT_FAST16_TYPE__ 2137@itemx __INT_FAST32_TYPE__ 2138@itemx __INT_FAST64_TYPE__ 2139@itemx __UINT_FAST8_TYPE__ 2140@itemx __UINT_FAST16_TYPE__ 2141@itemx __UINT_FAST32_TYPE__ 2142@itemx __UINT_FAST64_TYPE__ 2143@itemx __INTPTR_TYPE__ 2144@itemx __UINTPTR_TYPE__ 2145These macros are defined to the correct underlying types for the 2146@code{size_t}, @code{ptrdiff_t}, @code{wchar_t}, @code{wint_t}, 2147@code{intmax_t}, @code{uintmax_t}, @code{sig_atomic_t}, @code{int8_t}, 2148@code{int16_t}, @code{int32_t}, @code{int64_t}, @code{uint8_t}, 2149@code{uint16_t}, @code{uint32_t}, @code{uint64_t}, 2150@code{int_least8_t}, @code{int_least16_t}, @code{int_least32_t}, 2151@code{int_least64_t}, @code{uint_least8_t}, @code{uint_least16_t}, 2152@code{uint_least32_t}, @code{uint_least64_t}, @code{int_fast8_t}, 2153@code{int_fast16_t}, @code{int_fast32_t}, @code{int_fast64_t}, 2154@code{uint_fast8_t}, @code{uint_fast16_t}, @code{uint_fast32_t}, 2155@code{uint_fast64_t}, @code{intptr_t}, and @code{uintptr_t} typedefs, 2156respectively. They exist to make the standard header files 2157@file{stddef.h}, @file{stdint.h}, and @file{wchar.h} work correctly. 2158You should not use these macros directly; instead, include the 2159appropriate headers and use the typedefs. Some of these macros may 2160not be defined on particular systems if GCC does not provide a 2161@file{stdint.h} header on those systems. 2162 2163@item __CHAR_BIT__ 2164Defined to the number of bits used in the representation of the 2165@code{char} data type. It exists to make the standard header given 2166numerical limits work correctly. You should not use 2167this macro directly; instead, include the appropriate headers. 2168 2169@item __SCHAR_MAX__ 2170@itemx __WCHAR_MAX__ 2171@itemx __SHRT_MAX__ 2172@itemx __INT_MAX__ 2173@itemx __LONG_MAX__ 2174@itemx __LONG_LONG_MAX__ 2175@itemx __WINT_MAX__ 2176@itemx __SIZE_MAX__ 2177@itemx __PTRDIFF_MAX__ 2178@itemx __INTMAX_MAX__ 2179@itemx __UINTMAX_MAX__ 2180@itemx __SIG_ATOMIC_MAX__ 2181@itemx __INT8_MAX__ 2182@itemx __INT16_MAX__ 2183@itemx __INT32_MAX__ 2184@itemx __INT64_MAX__ 2185@itemx __UINT8_MAX__ 2186@itemx __UINT16_MAX__ 2187@itemx __UINT32_MAX__ 2188@itemx __UINT64_MAX__ 2189@itemx __INT_LEAST8_MAX__ 2190@itemx __INT_LEAST16_MAX__ 2191@itemx __INT_LEAST32_MAX__ 2192@itemx __INT_LEAST64_MAX__ 2193@itemx __UINT_LEAST8_MAX__ 2194@itemx __UINT_LEAST16_MAX__ 2195@itemx __UINT_LEAST32_MAX__ 2196@itemx __UINT_LEAST64_MAX__ 2197@itemx __INT_FAST8_MAX__ 2198@itemx __INT_FAST16_MAX__ 2199@itemx __INT_FAST32_MAX__ 2200@itemx __INT_FAST64_MAX__ 2201@itemx __UINT_FAST8_MAX__ 2202@itemx __UINT_FAST16_MAX__ 2203@itemx __UINT_FAST32_MAX__ 2204@itemx __UINT_FAST64_MAX__ 2205@itemx __INTPTR_MAX__ 2206@itemx __UINTPTR_MAX__ 2207@itemx __WCHAR_MIN__ 2208@itemx __WINT_MIN__ 2209@itemx __SIG_ATOMIC_MIN__ 2210Defined to the maximum value of the @code{signed char}, @code{wchar_t}, 2211@code{signed short}, 2212@code{signed int}, @code{signed long}, @code{signed long long}, 2213@code{wint_t}, @code{size_t}, @code{ptrdiff_t}, 2214@code{intmax_t}, @code{uintmax_t}, @code{sig_atomic_t}, @code{int8_t}, 2215@code{int16_t}, @code{int32_t}, @code{int64_t}, @code{uint8_t}, 2216@code{uint16_t}, @code{uint32_t}, @code{uint64_t}, 2217@code{int_least8_t}, @code{int_least16_t}, @code{int_least32_t}, 2218@code{int_least64_t}, @code{uint_least8_t}, @code{uint_least16_t}, 2219@code{uint_least32_t}, @code{uint_least64_t}, @code{int_fast8_t}, 2220@code{int_fast16_t}, @code{int_fast32_t}, @code{int_fast64_t}, 2221@code{uint_fast8_t}, @code{uint_fast16_t}, @code{uint_fast32_t}, 2222@code{uint_fast64_t}, @code{intptr_t}, and @code{uintptr_t} types and 2223to the minimum value of the @code{wchar_t}, @code{wint_t}, and 2224@code{sig_atomic_t} types respectively. They exist to make the 2225standard header given numerical limits work correctly. You should not 2226use these macros directly; instead, include the appropriate headers. 2227Some of these macros may not be defined on particular systems if GCC 2228does not provide a @file{stdint.h} header on those systems. 2229 2230@item __INT8_C 2231@itemx __INT16_C 2232@itemx __INT32_C 2233@itemx __INT64_C 2234@itemx __UINT8_C 2235@itemx __UINT16_C 2236@itemx __UINT32_C 2237@itemx __UINT64_C 2238@itemx __INTMAX_C 2239@itemx __UINTMAX_C 2240Defined to implementations of the standard @file{stdint.h} macros with 2241the same names without the leading @code{__}. They exist the make the 2242implementation of that header work correctly. You should not use 2243these macros directly; instead, include the appropriate headers. Some 2244of these macros may not be defined on particular systems if GCC does 2245not provide a @file{stdint.h} header on those systems. 2246 2247@item __SIZEOF_INT__ 2248@itemx __SIZEOF_LONG__ 2249@itemx __SIZEOF_LONG_LONG__ 2250@itemx __SIZEOF_SHORT__ 2251@itemx __SIZEOF_POINTER__ 2252@itemx __SIZEOF_FLOAT__ 2253@itemx __SIZEOF_DOUBLE__ 2254@itemx __SIZEOF_LONG_DOUBLE__ 2255@itemx __SIZEOF_SIZE_T__ 2256@itemx __SIZEOF_WCHAR_T__ 2257@itemx __SIZEOF_WINT_T__ 2258@itemx __SIZEOF_PTRDIFF_T__ 2259Defined to the number of bytes of the C standard data types: @code{int}, 2260@code{long}, @code{long long}, @code{short}, @code{void *}, @code{float}, 2261@code{double}, @code{long double}, @code{size_t}, @code{wchar_t}, @code{wint_t} 2262and @code{ptrdiff_t}. 2263 2264@item __BYTE_ORDER__ 2265@itemx __ORDER_LITTLE_ENDIAN__ 2266@itemx __ORDER_BIG_ENDIAN__ 2267@itemx __ORDER_PDP_ENDIAN__ 2268@code{__BYTE_ORDER__} is defined to one of the values 2269@code{__ORDER_LITTLE_ENDIAN__}, @code{__ORDER_BIG_ENDIAN__}, or 2270@code{__ORDER_PDP_ENDIAN__} to reflect the layout of multi-byte and 2271multi-word quantities in memory. If @code{__BYTE_ORDER__} is equal to 2272@code{__ORDER_LITTLE_ENDIAN__} or @code{__ORDER_BIG_ENDIAN__}, then 2273multi-byte and multi-word quantities are laid out identically: the 2274byte (word) at the lowest address is the least significant or most 2275significant byte (word) of the quantity, respectively. If 2276@code{__BYTE_ORDER__} is equal to @code{__ORDER_PDP_ENDIAN__}, then 2277bytes in 16-bit words are laid out in a little-endian fashion, whereas 2278the 16-bit subwords of a 32-bit quantity are laid out in big-endian 2279fashion. 2280 2281You should use these macros for testing like this: 2282 2283@smallexample 2284/* @r{Test for a little-endian machine} */ 2285#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ 2286@end smallexample 2287 2288@item __FLOAT_WORD_ORDER__ 2289@code{__FLOAT_WORD_ORDER__} is defined to one of the values 2290@code{__ORDER_LITTLE_ENDIAN__} or @code{__ORDER_BIG_ENDIAN__} to reflect 2291the layout of the words of multi-word floating-point quantities. 2292 2293@item __DEPRECATED 2294This macro is defined, with value 1, when compiling a C++ source file 2295with warnings about deprecated constructs enabled. These warnings are 2296enabled by default, but can be disabled with @option{-Wno-deprecated}. 2297 2298@item __EXCEPTIONS 2299This macro is defined, with value 1, when compiling a C++ source file 2300with exceptions enabled. If @option{-fno-exceptions} is used when 2301compiling the file, then this macro is not defined. 2302 2303@item __GXX_RTTI 2304This macro is defined, with value 1, when compiling a C++ source file 2305with runtime type identification enabled. If @option{-fno-rtti} is 2306used when compiling the file, then this macro is not defined. 2307 2308@item __USING_SJLJ_EXCEPTIONS__ 2309This macro is defined, with value 1, if the compiler uses the old 2310mechanism based on @code{setjmp} and @code{longjmp} for exception 2311handling. 2312 2313@item __GXX_EXPERIMENTAL_CXX0X__ 2314This macro is defined when compiling a C++ source file with the option 2315@option{-std=c++0x} or @option{-std=gnu++0x}. It indicates that some 2316features likely to be included in C++0x are available. Note that these 2317features are experimental, and may change or be removed in future 2318versions of GCC. 2319 2320@item __GXX_WEAK__ 2321This macro is defined when compiling a C++ source file. It has the 2322value 1 if the compiler will use weak symbols, COMDAT sections, or 2323other similar techniques to collapse symbols with ``vague linkage'' 2324that are defined in multiple translation units. If the compiler will 2325not collapse such symbols, this macro is defined with value 0. In 2326general, user code should not need to make use of this macro; the 2327purpose of this macro is to ease implementation of the C++ runtime 2328library provided with G++. 2329 2330@item __NEXT_RUNTIME__ 2331This macro is defined, with value 1, if (and only if) the NeXT runtime 2332(as in @option{-fnext-runtime}) is in use for Objective-C@. If the GNU 2333runtime is used, this macro is not defined, so that you can use this 2334macro to determine which runtime (NeXT or GNU) is being used. 2335 2336@item __LP64__ 2337@itemx _LP64 2338These macros are defined, with value 1, if (and only if) the compilation 2339is for a target where @code{long int} and pointer both use 64-bits and 2340@code{int} uses 32-bit. 2341 2342@item __SSP__ 2343This macro is defined, with value 1, when @option{-fstack-protector} is in 2344use. 2345 2346@item __SSP_ALL__ 2347This macro is defined, with value 2, when @option{-fstack-protector-all} is 2348in use. 2349 2350@item __SSP_STRONG__ 2351This macro is defined, with value 3, when @option{-fstack-protector-strong} is 2352in use. 2353 2354@item __SSP_EXPLICIT__ 2355This macro is defined, with value 4, when @option{-fstack-protector-explicit} is 2356in use. 2357 2358@item __SANITIZE_ADDRESS__ 2359This macro is defined, with value 1, when @option{-fsanitize=address} 2360or @option{-fsanitize=kernel-address} are in use. 2361 2362@item __TIMESTAMP__ 2363This macro expands to a string constant that describes the date and time 2364of the last modification of the current source file. The string constant 2365contains abbreviated day of the week, month, day of the month, time in 2366hh:mm:ss form, year and looks like @code{@w{"Sun Sep 16 01:03:52 1973"}}. 2367If the day of the month is less than 10, it is padded with a space on the left. 2368 2369If GCC cannot determine the current date, it will emit a warning message 2370(once per compilation) and @code{__TIMESTAMP__} will expand to 2371@code{@w{"??? ??? ?? ??:??:?? ????"}}. 2372 2373@item __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1 2374@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2 2375@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 2376@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8 2377@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 2378These macros are defined when the target processor supports atomic compare 2379and swap operations on operands 1, 2, 4, 8 or 16 bytes in length, respectively. 2380 2381@item __GCC_HAVE_DWARF2_CFI_ASM 2382This macro is defined when the compiler is emitting Dwarf2 CFI directives 2383to the assembler. When this is defined, it is possible to emit those same 2384directives in inline assembly. 2385 2386@item __FP_FAST_FMA 2387@itemx __FP_FAST_FMAF 2388@itemx __FP_FAST_FMAL 2389These macros are defined with value 1 if the backend supports the 2390@code{fma}, @code{fmaf}, and @code{fmal} builtin functions, so that 2391the include file @file{math.h} can define the macros 2392@code{FP_FAST_FMA}, @code{FP_FAST_FMAF}, and @code{FP_FAST_FMAL} 2393for compatibility with the 1999 C standard. 2394 2395@item __GCC_IEC_559 2396This macro is defined to indicate the intended level of support for 2397IEEE 754 (IEC 60559) floating-point arithmetic. It expands to a 2398nonnegative integer value. If 0, it indicates that the combination of 2399the compiler configuration and the command-line options is not 2400intended to support IEEE 754 arithmetic for @code{float} and 2401@code{double} as defined in C99 and C11 Annex F (for example, that the 2402standard rounding modes and exceptions are not supported, or that 2403optimizations are enabled that conflict with IEEE 754 semantics). If 24041, it indicates that IEEE 754 arithmetic is intended to be supported; 2405this does not mean that all relevant language features are supported 2406by GCC. If 2 or more, it additionally indicates support for IEEE 2407754-2008 (in particular, that the binary encodings for quiet and 2408signaling NaNs are as specified in IEEE 754-2008). 2409 2410This macro does not indicate the default state of command-line options 2411that control optimizations that C99 and C11 permit to be controlled by 2412standard pragmas, where those standards do not require a particular 2413default state. It does not indicate whether optimizations respect 2414signaling NaN semantics (the macro for that is 2415@code{__SUPPORT_SNAN__}). It does not indicate support for decimal 2416floating point or the IEEE 754 binary16 and binary128 types. 2417 2418@item __GCC_IEC_559_COMPLEX 2419This macro is defined to indicate the intended level of support for 2420IEEE 754 (IEC 60559) floating-point arithmetic for complex numbers, as 2421defined in C99 and C11 Annex G. It expands to a nonnegative integer 2422value. If 0, it indicates that the combination of the compiler 2423configuration and the command-line options is not intended to support 2424Annex G requirements (for example, because @option{-fcx-limited-range} 2425was used). If 1 or more, it indicates that it is intended to support 2426those requirements; this does not mean that all relevant language 2427features are supported by GCC. 2428 2429@item __NO_MATH_ERRNO__ 2430This macro is defined if @option{-fno-math-errno} is used, or enabled 2431by another option such as @option{-ffast-math} or by default. 2432@end table 2433 2434@node System-specific Predefined Macros 2435@subsection System-specific Predefined Macros 2436 2437@cindex system-specific predefined macros 2438@cindex predefined macros, system-specific 2439@cindex reserved namespace 2440 2441The C preprocessor normally predefines several macros that indicate what 2442type of system and machine is in use. They are obviously different on 2443each target supported by GCC@. This manual, being for all systems and 2444machines, cannot tell you what their names are, but you can use 2445@command{cpp -dM} to see them all. @xref{Invocation}. All system-specific 2446predefined macros expand to a constant value, so you can test them with 2447either @samp{#ifdef} or @samp{#if}. 2448 2449The C standard requires that all system-specific macros be part of the 2450@dfn{reserved namespace}. All names which begin with two underscores, 2451or an underscore and a capital letter, are reserved for the compiler and 2452library to use as they wish. However, historically system-specific 2453macros have had names with no special prefix; for instance, it is common 2454to find @code{unix} defined on Unix systems. For all such macros, GCC 2455provides a parallel macro with two underscores added at the beginning 2456and the end. If @code{unix} is defined, @code{__unix__} will be defined 2457too. There will never be more than two underscores; the parallel of 2458@code{_mips} is @code{__mips__}. 2459 2460When the @option{-ansi} option, or any @option{-std} option that 2461requests strict conformance, is given to the compiler, all the 2462system-specific predefined macros outside the reserved namespace are 2463suppressed. The parallel macros, inside the reserved namespace, remain 2464defined. 2465 2466We are slowly phasing out all predefined macros which are outside the 2467reserved namespace. You should never use them in new programs, and we 2468encourage you to correct older code to use the parallel macros whenever 2469you find it. We don't recommend you use the system-specific macros that 2470are in the reserved namespace, either. It is better in the long run to 2471check specifically for features you need, using a tool such as 2472@command{autoconf}. 2473 2474@node C++ Named Operators 2475@subsection C++ Named Operators 2476@cindex named operators 2477@cindex C++ named operators 2478@cindex @file{iso646.h} 2479 2480In C++, there are eleven keywords which are simply alternate spellings 2481of operators normally written with punctuation. These keywords are 2482treated as such even in the preprocessor. They function as operators in 2483@samp{#if}, and they cannot be defined as macros or poisoned. In C, you 2484can request that those keywords take their C++ meaning by including 2485@file{iso646.h}. That header defines each one as a normal object-like 2486macro expanding to the appropriate punctuator. 2487 2488These are the named operators and their corresponding punctuators: 2489 2490@multitable {Named Operator} {Punctuator} 2491@item Named Operator @tab Punctuator 2492@item @code{and} @tab @code{&&} 2493@item @code{and_eq} @tab @code{&=} 2494@item @code{bitand} @tab @code{&} 2495@item @code{bitor} @tab @code{|} 2496@item @code{compl} @tab @code{~} 2497@item @code{not} @tab @code{!} 2498@item @code{not_eq} @tab @code{!=} 2499@item @code{or} @tab @code{||} 2500@item @code{or_eq} @tab @code{|=} 2501@item @code{xor} @tab @code{^} 2502@item @code{xor_eq} @tab @code{^=} 2503@end multitable 2504 2505@node Undefining and Redefining Macros 2506@section Undefining and Redefining Macros 2507@cindex undefining macros 2508@cindex redefining macros 2509@findex #undef 2510 2511If a macro ceases to be useful, it may be @dfn{undefined} with the 2512@samp{#undef} directive. @samp{#undef} takes a single argument, the 2513name of the macro to undefine. You use the bare macro name, even if the 2514macro is function-like. It is an error if anything appears on the line 2515after the macro name. @samp{#undef} has no effect if the name is not a 2516macro. 2517 2518@smallexample 2519#define FOO 4 2520x = FOO; @expansion{} x = 4; 2521#undef FOO 2522x = FOO; @expansion{} x = FOO; 2523@end smallexample 2524 2525Once a macro has been undefined, that identifier may be @dfn{redefined} 2526as a macro by a subsequent @samp{#define} directive. The new definition 2527need not have any resemblance to the old definition. 2528 2529However, if an identifier which is currently a macro is redefined, then 2530the new definition must be @dfn{effectively the same} as the old one. 2531Two macro definitions are effectively the same if: 2532@itemize @bullet 2533@item Both are the same type of macro (object- or function-like). 2534@item All the tokens of the replacement list are the same. 2535@item If there are any parameters, they are the same. 2536@item Whitespace appears in the same places in both. It need not be 2537exactly the same amount of whitespace, though. Remember that comments 2538count as whitespace. 2539@end itemize 2540 2541@noindent 2542These definitions are effectively the same: 2543@smallexample 2544#define FOUR (2 + 2) 2545#define FOUR (2 + 2) 2546#define FOUR (2 /* @r{two} */ + 2) 2547@end smallexample 2548@noindent 2549but these are not: 2550@smallexample 2551#define FOUR (2 + 2) 2552#define FOUR ( 2+2 ) 2553#define FOUR (2 * 2) 2554#define FOUR(score,and,seven,years,ago) (2 + 2) 2555@end smallexample 2556 2557If a macro is redefined with a definition that is not effectively the 2558same as the old one, the preprocessor issues a warning and changes the 2559macro to use the new definition. If the new definition is effectively 2560the same, the redefinition is silently ignored. This allows, for 2561instance, two different headers to define a common macro. The 2562preprocessor will only complain if the definitions do not match. 2563 2564@node Directives Within Macro Arguments 2565@section Directives Within Macro Arguments 2566@cindex macro arguments and directives 2567 2568Occasionally it is convenient to use preprocessor directives within 2569the arguments of a macro. The C and C++ standards declare that 2570behavior in these cases is undefined. 2571 2572Versions of CPP prior to 3.2 would reject such constructs with an 2573error message. This was the only syntactic difference between normal 2574functions and function-like macros, so it seemed attractive to remove 2575this limitation, and people would often be surprised that they could 2576not use macros in this way. Moreover, sometimes people would use 2577conditional compilation in the argument list to a normal library 2578function like @samp{printf}, only to find that after a library upgrade 2579@samp{printf} had changed to be a function-like macro, and their code 2580would no longer compile. So from version 3.2 we changed CPP to 2581successfully process arbitrary directives within macro arguments in 2582exactly the same way as it would have processed the directive were the 2583function-like macro invocation not present. 2584 2585If, within a macro invocation, that macro is redefined, then the new 2586definition takes effect in time for argument pre-expansion, but the 2587original definition is still used for argument replacement. Here is a 2588pathological example: 2589 2590@smallexample 2591#define f(x) x x 2592f (1 2593#undef f 2594#define f 2 2595f) 2596@end smallexample 2597 2598@noindent 2599which expands to 2600 2601@smallexample 26021 2 1 2 2603@end smallexample 2604 2605@noindent 2606with the semantics described above. 2607 2608@node Macro Pitfalls 2609@section Macro Pitfalls 2610@cindex problems with macros 2611@cindex pitfalls of macros 2612 2613In this section we describe some special rules that apply to macros and 2614macro expansion, and point out certain cases in which the rules have 2615counter-intuitive consequences that you must watch out for. 2616 2617@menu 2618* Misnesting:: 2619* Operator Precedence Problems:: 2620* Swallowing the Semicolon:: 2621* Duplication of Side Effects:: 2622* Self-Referential Macros:: 2623* Argument Prescan:: 2624* Newlines in Arguments:: 2625@end menu 2626 2627@node Misnesting 2628@subsection Misnesting 2629 2630When a macro is called with arguments, the arguments are substituted 2631into the macro body and the result is checked, together with the rest of 2632the input file, for more macro calls. It is possible to piece together 2633a macro call coming partially from the macro body and partially from the 2634arguments. For example, 2635 2636@smallexample 2637#define twice(x) (2*(x)) 2638#define call_with_1(x) x(1) 2639call_with_1 (twice) 2640 @expansion{} twice(1) 2641 @expansion{} (2*(1)) 2642@end smallexample 2643 2644Macro definitions do not have to have balanced parentheses. By writing 2645an unbalanced open parenthesis in a macro body, it is possible to create 2646a macro call that begins inside the macro body but ends outside of it. 2647For example, 2648 2649@smallexample 2650#define strange(file) fprintf (file, "%s %d", 2651@dots{} 2652strange(stderr) p, 35) 2653 @expansion{} fprintf (stderr, "%s %d", p, 35) 2654@end smallexample 2655 2656The ability to piece together a macro call can be useful, but the use of 2657unbalanced open parentheses in a macro body is just confusing, and 2658should be avoided. 2659 2660@node Operator Precedence Problems 2661@subsection Operator Precedence Problems 2662@cindex parentheses in macro bodies 2663 2664You may have noticed that in most of the macro definition examples shown 2665above, each occurrence of a macro argument name had parentheses around 2666it. In addition, another pair of parentheses usually surround the 2667entire macro definition. Here is why it is best to write macros that 2668way. 2669 2670Suppose you define a macro as follows, 2671 2672@smallexample 2673#define ceil_div(x, y) (x + y - 1) / y 2674@end smallexample 2675 2676@noindent 2677whose purpose is to divide, rounding up. (One use for this operation is 2678to compute how many @code{int} objects are needed to hold a certain 2679number of @code{char} objects.) Then suppose it is used as follows: 2680 2681@smallexample 2682a = ceil_div (b & c, sizeof (int)); 2683 @expansion{} a = (b & c + sizeof (int) - 1) / sizeof (int); 2684@end smallexample 2685 2686@noindent 2687This does not do what is intended. The operator-precedence rules of 2688C make it equivalent to this: 2689 2690@smallexample 2691a = (b & (c + sizeof (int) - 1)) / sizeof (int); 2692@end smallexample 2693 2694@noindent 2695What we want is this: 2696 2697@smallexample 2698a = ((b & c) + sizeof (int) - 1)) / sizeof (int); 2699@end smallexample 2700 2701@noindent 2702Defining the macro as 2703 2704@smallexample 2705#define ceil_div(x, y) ((x) + (y) - 1) / (y) 2706@end smallexample 2707 2708@noindent 2709provides the desired result. 2710 2711Unintended grouping can result in another way. Consider @code{sizeof 2712ceil_div(1, 2)}. That has the appearance of a C expression that would 2713compute the size of the type of @code{ceil_div (1, 2)}, but in fact it 2714means something very different. Here is what it expands to: 2715 2716@smallexample 2717sizeof ((1) + (2) - 1) / (2) 2718@end smallexample 2719 2720@noindent 2721This would take the size of an integer and divide it by two. The 2722precedence rules have put the division outside the @code{sizeof} when it 2723was intended to be inside. 2724 2725Parentheses around the entire macro definition prevent such problems. 2726Here, then, is the recommended way to define @code{ceil_div}: 2727 2728@smallexample 2729#define ceil_div(x, y) (((x) + (y) - 1) / (y)) 2730@end smallexample 2731 2732@node Swallowing the Semicolon 2733@subsection Swallowing the Semicolon 2734@cindex semicolons (after macro calls) 2735 2736Often it is desirable to define a macro that expands into a compound 2737statement. Consider, for example, the following macro, that advances a 2738pointer (the argument @code{p} says where to find it) across whitespace 2739characters: 2740 2741@smallexample 2742#define SKIP_SPACES(p, limit) \ 2743@{ char *lim = (limit); \ 2744 while (p < lim) @{ \ 2745 if (*p++ != ' ') @{ \ 2746 p--; break; @}@}@} 2747@end smallexample 2748 2749@noindent 2750Here backslash-newline is used to split the macro definition, which must 2751be a single logical line, so that it resembles the way such code would 2752be laid out if not part of a macro definition. 2753 2754A call to this macro might be @code{SKIP_SPACES (p, lim)}. Strictly 2755speaking, the call expands to a compound statement, which is a complete 2756statement with no need for a semicolon to end it. However, since it 2757looks like a function call, it minimizes confusion if you can use it 2758like a function call, writing a semicolon afterward, as in 2759@code{SKIP_SPACES (p, lim);} 2760 2761This can cause trouble before @code{else} statements, because the 2762semicolon is actually a null statement. Suppose you write 2763 2764@smallexample 2765if (*p != 0) 2766 SKIP_SPACES (p, lim); 2767else @dots{} 2768@end smallexample 2769 2770@noindent 2771The presence of two statements---the compound statement and a null 2772statement---in between the @code{if} condition and the @code{else} 2773makes invalid C code. 2774 2775The definition of the macro @code{SKIP_SPACES} can be altered to solve 2776this problem, using a @code{do @dots{} while} statement. Here is how: 2777 2778@smallexample 2779#define SKIP_SPACES(p, limit) \ 2780do @{ char *lim = (limit); \ 2781 while (p < lim) @{ \ 2782 if (*p++ != ' ') @{ \ 2783 p--; break; @}@}@} \ 2784while (0) 2785@end smallexample 2786 2787Now @code{SKIP_SPACES (p, lim);} expands into 2788 2789@smallexample 2790do @{@dots{}@} while (0); 2791@end smallexample 2792 2793@noindent 2794which is one statement. The loop executes exactly once; most compilers 2795generate no extra code for it. 2796 2797@node Duplication of Side Effects 2798@subsection Duplication of Side Effects 2799 2800@cindex side effects (in macro arguments) 2801@cindex unsafe macros 2802Many C programs define a macro @code{min}, for ``minimum'', like this: 2803 2804@smallexample 2805#define min(X, Y) ((X) < (Y) ? (X) : (Y)) 2806@end smallexample 2807 2808When you use this macro with an argument containing a side effect, 2809as shown here, 2810 2811@smallexample 2812next = min (x + y, foo (z)); 2813@end smallexample 2814 2815@noindent 2816it expands as follows: 2817 2818@smallexample 2819next = ((x + y) < (foo (z)) ? (x + y) : (foo (z))); 2820@end smallexample 2821 2822@noindent 2823where @code{x + y} has been substituted for @code{X} and @code{foo (z)} 2824for @code{Y}. 2825 2826The function @code{foo} is used only once in the statement as it appears 2827in the program, but the expression @code{foo (z)} has been substituted 2828twice into the macro expansion. As a result, @code{foo} might be called 2829two times when the statement is executed. If it has side effects or if 2830it takes a long time to compute, the results might not be what you 2831intended. We say that @code{min} is an @dfn{unsafe} macro. 2832 2833The best solution to this problem is to define @code{min} in a way that 2834computes the value of @code{foo (z)} only once. The C language offers 2835no standard way to do this, but it can be done with GNU extensions as 2836follows: 2837 2838@smallexample 2839#define min(X, Y) \ 2840(@{ typeof (X) x_ = (X); \ 2841 typeof (Y) y_ = (Y); \ 2842 (x_ < y_) ? x_ : y_; @}) 2843@end smallexample 2844 2845The @samp{(@{ @dots{} @})} notation produces a compound statement that 2846acts as an expression. Its value is the value of its last statement. 2847This permits us to define local variables and assign each argument to 2848one. The local variables have underscores after their names to reduce 2849the risk of conflict with an identifier of wider scope (it is impossible 2850to avoid this entirely). Now each argument is evaluated exactly once. 2851 2852If you do not wish to use GNU C extensions, the only solution is to be 2853careful when @emph{using} the macro @code{min}. For example, you can 2854calculate the value of @code{foo (z)}, save it in a variable, and use 2855that variable in @code{min}: 2856 2857@smallexample 2858@group 2859#define min(X, Y) ((X) < (Y) ? (X) : (Y)) 2860@dots{} 2861@{ 2862 int tem = foo (z); 2863 next = min (x + y, tem); 2864@} 2865@end group 2866@end smallexample 2867 2868@noindent 2869(where we assume that @code{foo} returns type @code{int}). 2870 2871@node Self-Referential Macros 2872@subsection Self-Referential Macros 2873@cindex self-reference 2874 2875A @dfn{self-referential} macro is one whose name appears in its 2876definition. Recall that all macro definitions are rescanned for more 2877macros to replace. If the self-reference were considered a use of the 2878macro, it would produce an infinitely large expansion. To prevent this, 2879the self-reference is not considered a macro call. It is passed into 2880the preprocessor output unchanged. Consider an example: 2881 2882@smallexample 2883#define foo (4 + foo) 2884@end smallexample 2885 2886@noindent 2887where @code{foo} is also a variable in your program. 2888 2889Following the ordinary rules, each reference to @code{foo} will expand 2890into @code{(4 + foo)}; then this will be rescanned and will expand into 2891@code{(4 + (4 + foo))}; and so on until the computer runs out of memory. 2892 2893The self-reference rule cuts this process short after one step, at 2894@code{(4 + foo)}. Therefore, this macro definition has the possibly 2895useful effect of causing the program to add 4 to the value of @code{foo} 2896wherever @code{foo} is referred to. 2897 2898In most cases, it is a bad idea to take advantage of this feature. A 2899person reading the program who sees that @code{foo} is a variable will 2900not expect that it is a macro as well. The reader will come across the 2901identifier @code{foo} in the program and think its value should be that 2902of the variable @code{foo}, whereas in fact the value is four greater. 2903 2904One common, useful use of self-reference is to create a macro which 2905expands to itself. If you write 2906 2907@smallexample 2908#define EPERM EPERM 2909@end smallexample 2910 2911@noindent 2912then the macro @code{EPERM} expands to @code{EPERM}. Effectively, it is 2913left alone by the preprocessor whenever it's used in running text. You 2914can tell that it's a macro with @samp{#ifdef}. You might do this if you 2915want to define numeric constants with an @code{enum}, but have 2916@samp{#ifdef} be true for each constant. 2917 2918If a macro @code{x} expands to use a macro @code{y}, and the expansion of 2919@code{y} refers to the macro @code{x}, that is an @dfn{indirect 2920self-reference} of @code{x}. @code{x} is not expanded in this case 2921either. Thus, if we have 2922 2923@smallexample 2924#define x (4 + y) 2925#define y (2 * x) 2926@end smallexample 2927 2928@noindent 2929then @code{x} and @code{y} expand as follows: 2930 2931@smallexample 2932@group 2933x @expansion{} (4 + y) 2934 @expansion{} (4 + (2 * x)) 2935 2936y @expansion{} (2 * x) 2937 @expansion{} (2 * (4 + y)) 2938@end group 2939@end smallexample 2940 2941@noindent 2942Each macro is expanded when it appears in the definition of the other 2943macro, but not when it indirectly appears in its own definition. 2944 2945@node Argument Prescan 2946@subsection Argument Prescan 2947@cindex expansion of arguments 2948@cindex macro argument expansion 2949@cindex prescan of macro arguments 2950 2951Macro arguments are completely macro-expanded before they are 2952substituted into a macro body, unless they are stringified or pasted 2953with other tokens. After substitution, the entire macro body, including 2954the substituted arguments, is scanned again for macros to be expanded. 2955The result is that the arguments are scanned @emph{twice} to expand 2956macro calls in them. 2957 2958Most of the time, this has no effect. If the argument contained any 2959macro calls, they are expanded during the first scan. The result 2960therefore contains no macro calls, so the second scan does not change 2961it. If the argument were substituted as given, with no prescan, the 2962single remaining scan would find the same macro calls and produce the 2963same results. 2964 2965You might expect the double scan to change the results when a 2966self-referential macro is used in an argument of another macro 2967(@pxref{Self-Referential Macros}): the self-referential macro would be 2968expanded once in the first scan, and a second time in the second scan. 2969However, this is not what happens. The self-references that do not 2970expand in the first scan are marked so that they will not expand in the 2971second scan either. 2972 2973You might wonder, ``Why mention the prescan, if it makes no difference? 2974And why not skip it and make the preprocessor faster?'' The answer is 2975that the prescan does make a difference in three special cases: 2976 2977@itemize @bullet 2978@item 2979Nested calls to a macro. 2980 2981We say that @dfn{nested} calls to a macro occur when a macro's argument 2982contains a call to that very macro. For example, if @code{f} is a macro 2983that expects one argument, @code{f (f (1))} is a nested pair of calls to 2984@code{f}. The desired expansion is made by expanding @code{f (1)} and 2985substituting that into the definition of @code{f}. The prescan causes 2986the expected result to happen. Without the prescan, @code{f (1)} itself 2987would be substituted as an argument, and the inner use of @code{f} would 2988appear during the main scan as an indirect self-reference and would not 2989be expanded. 2990 2991@item 2992Macros that call other macros that stringify or concatenate. 2993 2994If an argument is stringified or concatenated, the prescan does not 2995occur. If you @emph{want} to expand a macro, then stringify or 2996concatenate its expansion, you can do that by causing one macro to call 2997another macro that does the stringification or concatenation. For 2998instance, if you have 2999 3000@smallexample 3001#define AFTERX(x) X_ ## x 3002#define XAFTERX(x) AFTERX(x) 3003#define TABLESIZE 1024 3004#define BUFSIZE TABLESIZE 3005@end smallexample 3006 3007then @code{AFTERX(BUFSIZE)} expands to @code{X_BUFSIZE}, and 3008@code{XAFTERX(BUFSIZE)} expands to @code{X_1024}. (Not to 3009@code{X_TABLESIZE}. Prescan always does a complete expansion.) 3010 3011@item 3012Macros used in arguments, whose expansions contain unshielded commas. 3013 3014This can cause a macro expanded on the second scan to be called with the 3015wrong number of arguments. Here is an example: 3016 3017@smallexample 3018#define foo a,b 3019#define bar(x) lose(x) 3020#define lose(x) (1 + (x)) 3021@end smallexample 3022 3023We would like @code{bar(foo)} to turn into @code{(1 + (foo))}, which 3024would then turn into @code{(1 + (a,b))}. Instead, @code{bar(foo)} 3025expands into @code{lose(a,b)}, and you get an error because @code{lose} 3026requires a single argument. In this case, the problem is easily solved 3027by the same parentheses that ought to be used to prevent misnesting of 3028arithmetic operations: 3029 3030@smallexample 3031#define foo (a,b) 3032@exdent or 3033#define bar(x) lose((x)) 3034@end smallexample 3035 3036The extra pair of parentheses prevents the comma in @code{foo}'s 3037definition from being interpreted as an argument separator. 3038 3039@end itemize 3040 3041@node Newlines in Arguments 3042@subsection Newlines in Arguments 3043@cindex newlines in macro arguments 3044 3045The invocation of a function-like macro can extend over many logical 3046lines. However, in the present implementation, the entire expansion 3047comes out on one line. Thus line numbers emitted by the compiler or 3048debugger refer to the line the invocation started on, which might be 3049different to the line containing the argument causing the problem. 3050 3051Here is an example illustrating this: 3052 3053@smallexample 3054#define ignore_second_arg(a,b,c) a; c 3055 3056ignore_second_arg (foo (), 3057 ignored (), 3058 syntax error); 3059@end smallexample 3060 3061@noindent 3062The syntax error triggered by the tokens @code{syntax error} results in 3063an error message citing line three---the line of ignore_second_arg--- 3064even though the problematic code comes from line five. 3065 3066We consider this a bug, and intend to fix it in the near future. 3067 3068@node Conditionals 3069@chapter Conditionals 3070@cindex conditionals 3071 3072A @dfn{conditional} is a directive that instructs the preprocessor to 3073select whether or not to include a chunk of code in the final token 3074stream passed to the compiler. Preprocessor conditionals can test 3075arithmetic expressions, or whether a name is defined as a macro, or both 3076simultaneously using the special @code{defined} operator. 3077 3078A conditional in the C preprocessor resembles in some ways an @code{if} 3079statement in C, but it is important to understand the difference between 3080them. The condition in an @code{if} statement is tested during the 3081execution of your program. Its purpose is to allow your program to 3082behave differently from run to run, depending on the data it is 3083operating on. The condition in a preprocessing conditional directive is 3084tested when your program is compiled. Its purpose is to allow different 3085code to be included in the program depending on the situation at the 3086time of compilation. 3087 3088However, the distinction is becoming less clear. Modern compilers often 3089do test @code{if} statements when a program is compiled, if their 3090conditions are known not to vary at run time, and eliminate code which 3091can never be executed. If you can count on your compiler to do this, 3092you may find that your program is more readable if you use @code{if} 3093statements with constant conditions (perhaps determined by macros). Of 3094course, you can only use this to exclude code, not type definitions or 3095other preprocessing directives, and you can only do it if the code 3096remains syntactically valid when it is not to be used. 3097 3098GCC version 3 eliminates this kind of never-executed code even when 3099not optimizing. Older versions did it only when optimizing. 3100 3101@menu 3102* Conditional Uses:: 3103* Conditional Syntax:: 3104* Deleted Code:: 3105@end menu 3106 3107@node Conditional Uses 3108@section Conditional Uses 3109 3110There are three general reasons to use a conditional. 3111 3112@itemize @bullet 3113@item 3114A program may need to use different code depending on the machine or 3115operating system it is to run on. In some cases the code for one 3116operating system may be erroneous on another operating system; for 3117example, it might refer to data types or constants that do not exist on 3118the other system. When this happens, it is not enough to avoid 3119executing the invalid code. Its mere presence will cause the compiler 3120to reject the program. With a preprocessing conditional, the offending 3121code can be effectively excised from the program when it is not valid. 3122 3123@item 3124You may want to be able to compile the same source file into two 3125different programs. One version might make frequent time-consuming 3126consistency checks on its intermediate data, or print the values of 3127those data for debugging, and the other not. 3128 3129@item 3130A conditional whose condition is always false is one way to exclude code 3131from the program but keep it as a sort of comment for future reference. 3132@end itemize 3133 3134Simple programs that do not need system-specific logic or complex 3135debugging hooks generally will not need to use preprocessing 3136conditionals. 3137 3138@node Conditional Syntax 3139@section Conditional Syntax 3140 3141@findex #if 3142A conditional in the C preprocessor begins with a @dfn{conditional 3143directive}: @samp{#if}, @samp{#ifdef} or @samp{#ifndef}. 3144 3145@menu 3146* Ifdef:: 3147* If:: 3148* Defined:: 3149* Else:: 3150* Elif:: 3151@end menu 3152 3153@node Ifdef 3154@subsection Ifdef 3155@findex #ifdef 3156@findex #endif 3157 3158The simplest sort of conditional is 3159 3160@smallexample 3161@group 3162#ifdef @var{MACRO} 3163 3164@var{controlled text} 3165 3166#endif /* @var{MACRO} */ 3167@end group 3168@end smallexample 3169 3170@cindex conditional group 3171This block is called a @dfn{conditional group}. @var{controlled text} 3172will be included in the output of the preprocessor if and only if 3173@var{MACRO} is defined. We say that the conditional @dfn{succeeds} if 3174@var{MACRO} is defined, @dfn{fails} if it is not. 3175 3176The @var{controlled text} inside of a conditional can include 3177preprocessing directives. They are executed only if the conditional 3178succeeds. You can nest conditional groups inside other conditional 3179groups, but they must be completely nested. In other words, 3180@samp{#endif} always matches the nearest @samp{#ifdef} (or 3181@samp{#ifndef}, or @samp{#if}). Also, you cannot start a conditional 3182group in one file and end it in another. 3183 3184Even if a conditional fails, the @var{controlled text} inside it is 3185still run through initial transformations and tokenization. Therefore, 3186it must all be lexically valid C@. Normally the only way this matters is 3187that all comments and string literals inside a failing conditional group 3188must still be properly ended. 3189 3190The comment following the @samp{#endif} is not required, but it is a 3191good practice if there is a lot of @var{controlled text}, because it 3192helps people match the @samp{#endif} to the corresponding @samp{#ifdef}. 3193Older programs sometimes put @var{MACRO} directly after the 3194@samp{#endif} without enclosing it in a comment. This is invalid code 3195according to the C standard. CPP accepts it with a warning. It 3196never affects which @samp{#ifndef} the @samp{#endif} matches. 3197 3198@findex #ifndef 3199Sometimes you wish to use some code if a macro is @emph{not} defined. 3200You can do this by writing @samp{#ifndef} instead of @samp{#ifdef}. 3201One common use of @samp{#ifndef} is to include code only the first 3202time a header file is included. @xref{Once-Only Headers}. 3203 3204Macro definitions can vary between compilations for several reasons. 3205Here are some samples. 3206 3207@itemize @bullet 3208@item 3209Some macros are predefined on each kind of machine 3210(@pxref{System-specific Predefined Macros}). This allows you to provide 3211code specially tuned for a particular machine. 3212 3213@item 3214System header files define more macros, associated with the features 3215they implement. You can test these macros with conditionals to avoid 3216using a system feature on a machine where it is not implemented. 3217 3218@item 3219Macros can be defined or undefined with the @option{-D} and @option{-U} 3220command-line options when you compile the program. You can arrange to 3221compile the same source file into two different programs by choosing a 3222macro name to specify which program you want, writing conditionals to 3223test whether or how this macro is defined, and then controlling the 3224state of the macro with command-line options, perhaps set in the 3225Makefile. @xref{Invocation}. 3226 3227@item 3228Your program might have a special header file (often called 3229@file{config.h}) that is adjusted when the program is compiled. It can 3230define or not define macros depending on the features of the system and 3231the desired capabilities of the program. The adjustment can be 3232automated by a tool such as @command{autoconf}, or done by hand. 3233@end itemize 3234 3235@node If 3236@subsection If 3237 3238The @samp{#if} directive allows you to test the value of an arithmetic 3239expression, rather than the mere existence of one macro. Its syntax is 3240 3241@smallexample 3242@group 3243#if @var{expression} 3244 3245@var{controlled text} 3246 3247#endif /* @var{expression} */ 3248@end group 3249@end smallexample 3250 3251@var{expression} is a C expression of integer type, subject to stringent 3252restrictions. It may contain 3253 3254@itemize @bullet 3255@item 3256Integer constants. 3257 3258@item 3259Character constants, which are interpreted as they would be in normal 3260code. 3261 3262@item 3263Arithmetic operators for addition, subtraction, multiplication, 3264division, bitwise operations, shifts, comparisons, and logical 3265operations (@code{&&} and @code{||}). The latter two obey the usual 3266short-circuiting rules of standard C@. 3267 3268@item 3269Macros. All macros in the expression are expanded before actual 3270computation of the expression's value begins. 3271 3272@item 3273Uses of the @code{defined} operator, which lets you check whether macros 3274are defined in the middle of an @samp{#if}. 3275 3276@item 3277Identifiers that are not macros, which are all considered to be the 3278number zero. This allows you to write @code{@w{#if MACRO}} instead of 3279@code{@w{#ifdef MACRO}}, if you know that MACRO, when defined, will 3280always have a nonzero value. Function-like macros used without their 3281function call parentheses are also treated as zero. 3282 3283In some contexts this shortcut is undesirable. The @option{-Wundef} 3284option causes GCC to warn whenever it encounters an identifier which is 3285not a macro in an @samp{#if}. 3286@end itemize 3287 3288The preprocessor does not know anything about types in the language. 3289Therefore, @code{sizeof} operators are not recognized in @samp{#if}, and 3290neither are @code{enum} constants. They will be taken as identifiers 3291which are not macros, and replaced by zero. In the case of 3292@code{sizeof}, this is likely to cause the expression to be invalid. 3293 3294The preprocessor calculates the value of @var{expression}. It carries 3295out all calculations in the widest integer type known to the compiler; 3296on most machines supported by GCC this is 64 bits. This is not the same 3297rule as the compiler uses to calculate the value of a constant 3298expression, and may give different results in some cases. If the value 3299comes out to be nonzero, the @samp{#if} succeeds and the @var{controlled 3300text} is included; otherwise it is skipped. 3301 3302@node Defined 3303@subsection Defined 3304 3305@cindex @code{defined} 3306The special operator @code{defined} is used in @samp{#if} and 3307@samp{#elif} expressions to test whether a certain name is defined as a 3308macro. @code{defined @var{name}} and @code{defined (@var{name})} are 3309both expressions whose value is 1 if @var{name} is defined as a macro at 3310the current point in the program, and 0 otherwise. Thus, @code{@w{#if 3311defined MACRO}} is precisely equivalent to @code{@w{#ifdef MACRO}}. 3312 3313@code{defined} is useful when you wish to test more than one macro for 3314existence at once. For example, 3315 3316@smallexample 3317#if defined (__vax__) || defined (__ns16000__) 3318@end smallexample 3319 3320@noindent 3321would succeed if either of the names @code{__vax__} or 3322@code{__ns16000__} is defined as a macro. 3323 3324Conditionals written like this: 3325 3326@smallexample 3327#if defined BUFSIZE && BUFSIZE >= 1024 3328@end smallexample 3329 3330@noindent 3331can generally be simplified to just @code{@w{#if BUFSIZE >= 1024}}, 3332since if @code{BUFSIZE} is not defined, it will be interpreted as having 3333the value zero. 3334 3335If the @code{defined} operator appears as a result of a macro expansion, 3336the C standard says the behavior is undefined. GNU cpp treats it as a 3337genuine @code{defined} operator and evaluates it normally. It will warn 3338wherever your code uses this feature if you use the command-line option 3339@option{-pedantic}, since other compilers may handle it differently. 3340 3341@node Else 3342@subsection Else 3343 3344@findex #else 3345The @samp{#else} directive can be added to a conditional to provide 3346alternative text to be used if the condition fails. This is what it 3347looks like: 3348 3349@smallexample 3350@group 3351#if @var{expression} 3352@var{text-if-true} 3353#else /* Not @var{expression} */ 3354@var{text-if-false} 3355#endif /* Not @var{expression} */ 3356@end group 3357@end smallexample 3358 3359@noindent 3360If @var{expression} is nonzero, the @var{text-if-true} is included and 3361the @var{text-if-false} is skipped. If @var{expression} is zero, the 3362opposite happens. 3363 3364You can use @samp{#else} with @samp{#ifdef} and @samp{#ifndef}, too. 3365 3366@node Elif 3367@subsection Elif 3368 3369@findex #elif 3370One common case of nested conditionals is used to check for more than two 3371possible alternatives. For example, you might have 3372 3373@smallexample 3374#if X == 1 3375@dots{} 3376#else /* X != 1 */ 3377#if X == 2 3378@dots{} 3379#else /* X != 2 */ 3380@dots{} 3381#endif /* X != 2 */ 3382#endif /* X != 1 */ 3383@end smallexample 3384 3385Another conditional directive, @samp{#elif}, allows this to be 3386abbreviated as follows: 3387 3388@smallexample 3389#if X == 1 3390@dots{} 3391#elif X == 2 3392@dots{} 3393#else /* X != 2 and X != 1*/ 3394@dots{} 3395#endif /* X != 2 and X != 1*/ 3396@end smallexample 3397 3398@samp{#elif} stands for ``else if''. Like @samp{#else}, it goes in the 3399middle of a conditional group and subdivides it; it does not require a 3400matching @samp{#endif} of its own. Like @samp{#if}, the @samp{#elif} 3401directive includes an expression to be tested. The text following the 3402@samp{#elif} is processed only if the original @samp{#if}-condition 3403failed and the @samp{#elif} condition succeeds. 3404 3405More than one @samp{#elif} can go in the same conditional group. Then 3406the text after each @samp{#elif} is processed only if the @samp{#elif} 3407condition succeeds after the original @samp{#if} and all previous 3408@samp{#elif} directives within it have failed. 3409 3410@samp{#else} is allowed after any number of @samp{#elif} directives, but 3411@samp{#elif} may not follow @samp{#else}. 3412 3413@node Deleted Code 3414@section Deleted Code 3415@cindex commenting out code 3416 3417If you replace or delete a part of the program but want to keep the old 3418code around for future reference, you often cannot simply comment it 3419out. Block comments do not nest, so the first comment inside the old 3420code will end the commenting-out. The probable result is a flood of 3421syntax errors. 3422 3423One way to avoid this problem is to use an always-false conditional 3424instead. For instance, put @code{#if 0} before the deleted code and 3425@code{#endif} after it. This works even if the code being turned 3426off contains conditionals, but they must be entire conditionals 3427(balanced @samp{#if} and @samp{#endif}). 3428 3429Some people use @code{#ifdef notdef} instead. This is risky, because 3430@code{notdef} might be accidentally defined as a macro, and then the 3431conditional would succeed. @code{#if 0} can be counted on to fail. 3432 3433Do not use @code{#if 0} for comments which are not C code. Use a real 3434comment, instead. The interior of @code{#if 0} must consist of complete 3435tokens; in particular, single-quote characters must balance. Comments 3436often contain unbalanced single-quote characters (known in English as 3437apostrophes). These confuse @code{#if 0}. They don't confuse 3438@samp{/*}. 3439 3440@node Diagnostics 3441@chapter Diagnostics 3442@cindex diagnostic 3443@cindex reporting errors 3444@cindex reporting warnings 3445 3446@findex #error 3447The directive @samp{#error} causes the preprocessor to report a fatal 3448error. The tokens forming the rest of the line following @samp{#error} 3449are used as the error message. 3450 3451You would use @samp{#error} inside of a conditional that detects a 3452combination of parameters which you know the program does not properly 3453support. For example, if you know that the program will not run 3454properly on a VAX, you might write 3455 3456@smallexample 3457@group 3458#ifdef __vax__ 3459#error "Won't work on VAXen. See comments at get_last_object." 3460#endif 3461@end group 3462@end smallexample 3463 3464If you have several configuration parameters that must be set up by 3465the installation in a consistent way, you can use conditionals to detect 3466an inconsistency and report it with @samp{#error}. For example, 3467 3468@smallexample 3469#if !defined(FOO) && defined(BAR) 3470#error "BAR requires FOO." 3471#endif 3472@end smallexample 3473 3474@findex #warning 3475The directive @samp{#warning} is like @samp{#error}, but causes the 3476preprocessor to issue a warning and continue preprocessing. The tokens 3477following @samp{#warning} are used as the warning message. 3478 3479You might use @samp{#warning} in obsolete header files, with a message 3480directing the user to the header file which should be used instead. 3481 3482Neither @samp{#error} nor @samp{#warning} macro-expands its argument. 3483Internal whitespace sequences are each replaced with a single space. 3484The line must consist of complete tokens. It is wisest to make the 3485argument of these directives be a single string constant; this avoids 3486problems with apostrophes and the like. 3487 3488@node Line Control 3489@chapter Line Control 3490@cindex line control 3491 3492The C preprocessor informs the C compiler of the location in your source 3493code where each token came from. Presently, this is just the file name 3494and line number. All the tokens resulting from macro expansion are 3495reported as having appeared on the line of the source file where the 3496outermost macro was used. We intend to be more accurate in the future. 3497 3498If you write a program which generates source code, such as the 3499@command{bison} parser generator, you may want to adjust the preprocessor's 3500notion of the current file name and line number by hand. Parts of the 3501output from @command{bison} are generated from scratch, other parts come 3502from a standard parser file. The rest are copied verbatim from 3503@command{bison}'s input. You would like compiler error messages and 3504symbolic debuggers to be able to refer to @code{bison}'s input file. 3505 3506@findex #line 3507@command{bison} or any such program can arrange this by writing 3508@samp{#line} directives into the output file. @samp{#line} is a 3509directive that specifies the original line number and source file name 3510for subsequent input in the current preprocessor input file. 3511@samp{#line} has three variants: 3512 3513@table @code 3514@item #line @var{linenum} 3515@var{linenum} is a non-negative decimal integer constant. It specifies 3516the line number which should be reported for the following line of 3517input. Subsequent lines are counted from @var{linenum}. 3518 3519@item #line @var{linenum} @var{filename} 3520@var{linenum} is the same as for the first form, and has the same 3521effect. In addition, @var{filename} is a string constant. The 3522following line and all subsequent lines are reported to come from the 3523file it specifies, until something else happens to change that. 3524@var{filename} is interpreted according to the normal rules for a string 3525constant: backslash escapes are interpreted. This is different from 3526@samp{#include}. 3527 3528Previous versions of CPP did not interpret escapes in @samp{#line}; 3529we have changed it because the standard requires they be interpreted, 3530and most other compilers do. 3531 3532@item #line @var{anything else} 3533@var{anything else} is checked for macro calls, which are expanded. 3534The result should match one of the above two forms. 3535@end table 3536 3537@samp{#line} directives alter the results of the @code{__FILE__} and 3538@code{__LINE__} predefined macros from that point on. @xref{Standard 3539Predefined Macros}. They do not have any effect on @samp{#include}'s 3540idea of the directory containing the current file. This is a change 3541from GCC 2.95. Previously, a file reading 3542 3543@smallexample 3544#line 1 "../src/gram.y" 3545#include "gram.h" 3546@end smallexample 3547 3548would search for @file{gram.h} in @file{../src}, then the @option{-I} 3549chain; the directory containing the physical source file would not be 3550searched. In GCC 3.0 and later, the @samp{#include} is not affected by 3551the presence of a @samp{#line} referring to a different directory. 3552 3553We made this change because the old behavior caused problems when 3554generated source files were transported between machines. For instance, 3555it is common practice to ship generated parsers with a source release, 3556so that people building the distribution do not need to have yacc or 3557Bison installed. These files frequently have @samp{#line} directives 3558referring to the directory tree of the system where the distribution was 3559created. If GCC tries to search for headers in those directories, the 3560build is likely to fail. 3561 3562The new behavior can cause failures too, if the generated file is not 3563in the same directory as its source and it attempts to include a header 3564which would be visible searching from the directory containing the 3565source file. However, this problem is easily solved with an additional 3566@option{-I} switch on the command line. The failures caused by the old 3567semantics could sometimes be corrected only by editing the generated 3568files, which is difficult and error-prone. 3569 3570@node Pragmas 3571@chapter Pragmas 3572 3573The @samp{#pragma} directive is the method specified by the C standard 3574for providing additional information to the compiler, beyond what is 3575conveyed in the language itself. Three forms of this directive 3576(commonly known as @dfn{pragmas}) are specified by the 1999 C standard. 3577A C compiler is free to attach any meaning it likes to other pragmas. 3578 3579GCC has historically preferred to use extensions to the syntax of the 3580language, such as @code{__attribute__}, for this purpose. However, GCC 3581does define a few pragmas of its own. These mostly have effects on the 3582entire translation unit or source file. 3583 3584In GCC version 3, all GNU-defined, supported pragmas have been given a 3585@code{GCC} prefix. This is in line with the @code{STDC} prefix on all 3586pragmas defined by C99. For backward compatibility, pragmas which were 3587recognized by previous versions are still recognized without the 3588@code{GCC} prefix, but that usage is deprecated. Some older pragmas are 3589deprecated in their entirety. They are not recognized with the 3590@code{GCC} prefix. @xref{Obsolete Features}. 3591 3592@cindex @code{_Pragma} 3593C99 introduces the @code{@w{_Pragma}} operator. This feature addresses a 3594major problem with @samp{#pragma}: being a directive, it cannot be 3595produced as the result of macro expansion. @code{@w{_Pragma}} is an 3596operator, much like @code{sizeof} or @code{defined}, and can be embedded 3597in a macro. 3598 3599Its syntax is @code{@w{_Pragma (@var{string-literal})}}, where 3600@var{string-literal} can be either a normal or wide-character string 3601literal. It is destringized, by replacing all @samp{\\} with a single 3602@samp{\} and all @samp{\"} with a @samp{"}. The result is then 3603processed as if it had appeared as the right hand side of a 3604@samp{#pragma} directive. For example, 3605 3606@smallexample 3607_Pragma ("GCC dependency \"parse.y\"") 3608@end smallexample 3609 3610@noindent 3611has the same effect as @code{#pragma GCC dependency "parse.y"}. The 3612same effect could be achieved using macros, for example 3613 3614@smallexample 3615#define DO_PRAGMA(x) _Pragma (#x) 3616DO_PRAGMA (GCC dependency "parse.y") 3617@end smallexample 3618 3619The standard is unclear on where a @code{_Pragma} operator can appear. 3620The preprocessor does not accept it within a preprocessing conditional 3621directive like @samp{#if}. To be safe, you are probably best keeping it 3622out of directives other than @samp{#define}, and putting it on a line of 3623its own. 3624 3625This manual documents the pragmas which are meaningful to the 3626preprocessor itself. Other pragmas are meaningful to the C or C++ 3627compilers. They are documented in the GCC manual. 3628 3629GCC plugins may provide their own pragmas. 3630 3631@ftable @code 3632@item #pragma GCC dependency 3633@code{#pragma GCC dependency} allows you to check the relative dates of 3634the current file and another file. If the other file is more recent than 3635the current file, a warning is issued. This is useful if the current 3636file is derived from the other file, and should be regenerated. The 3637other file is searched for using the normal include search path. 3638Optional trailing text can be used to give more information in the 3639warning message. 3640 3641@smallexample 3642#pragma GCC dependency "parse.y" 3643#pragma GCC dependency "/usr/include/time.h" rerun fixincludes 3644@end smallexample 3645 3646@item #pragma GCC poison 3647Sometimes, there is an identifier that you want to remove completely 3648from your program, and make sure that it never creeps back in. To 3649enforce this, you can @dfn{poison} the identifier with this pragma. 3650@code{#pragma GCC poison} is followed by a list of identifiers to 3651poison. If any of those identifiers appears anywhere in the source 3652after the directive, it is a hard error. For example, 3653 3654@smallexample 3655#pragma GCC poison printf sprintf fprintf 3656sprintf(some_string, "hello"); 3657@end smallexample 3658 3659@noindent 3660will produce an error. 3661 3662If a poisoned identifier appears as part of the expansion of a macro 3663which was defined before the identifier was poisoned, it will @emph{not} 3664cause an error. This lets you poison an identifier without worrying 3665about system headers defining macros that use it. 3666 3667For example, 3668 3669@smallexample 3670#define strrchr rindex 3671#pragma GCC poison rindex 3672strrchr(some_string, 'h'); 3673@end smallexample 3674 3675@noindent 3676will not produce an error. 3677 3678@item #pragma GCC system_header 3679This pragma takes no arguments. It causes the rest of the code in the 3680current file to be treated as if it came from a system header. 3681@xref{System Headers}. 3682 3683@item #pragma GCC warning 3684@itemx #pragma GCC error 3685@code{#pragma GCC warning "message"} causes the preprocessor to issue 3686a warning diagnostic with the text @samp{message}. The message 3687contained in the pragma must be a single string literal. Similarly, 3688@code{#pragma GCC error "message"} issues an error message. Unlike 3689the @samp{#warning} and @samp{#error} directives, these pragmas can be 3690embedded in preprocessor macros using @samp{_Pragma}. 3691 3692@end ftable 3693 3694@node Other Directives 3695@chapter Other Directives 3696 3697@findex #ident 3698@findex #sccs 3699The @samp{#ident} directive takes one argument, a string constant. On 3700some systems, that string constant is copied into a special segment of 3701the object file. On other systems, the directive is ignored. The 3702@samp{#sccs} directive is a synonym for @samp{#ident}. 3703 3704These directives are not part of the C standard, but they are not 3705official GNU extensions either. What historical information we have 3706been able to find, suggests they originated with System V@. 3707 3708@cindex null directive 3709The @dfn{null directive} consists of a @samp{#} followed by a newline, 3710with only whitespace (including comments) in between. A null directive 3711is understood as a preprocessing directive but has no effect on the 3712preprocessor output. The primary significance of the existence of the 3713null directive is that an input line consisting of just a @samp{#} will 3714produce no output, rather than a line of output containing just a 3715@samp{#}. Supposedly some old C programs contain such lines. 3716 3717@node Preprocessor Output 3718@chapter Preprocessor Output 3719 3720When the C preprocessor is used with the C, C++, or Objective-C 3721compilers, it is integrated into the compiler and communicates a stream 3722of binary tokens directly to the compiler's parser. However, it can 3723also be used in the more conventional standalone mode, where it produces 3724textual output. 3725@c FIXME: Document the library interface. 3726 3727@cindex output format 3728The output from the C preprocessor looks much like the input, except 3729that all preprocessing directive lines have been replaced with blank 3730lines and all comments with spaces. Long runs of blank lines are 3731discarded. 3732 3733The ISO standard specifies that it is implementation defined whether a 3734preprocessor preserves whitespace between tokens, or replaces it with 3735e.g.@: a single space. In GNU CPP, whitespace between tokens is collapsed 3736to become a single space, with the exception that the first token on a 3737non-directive line is preceded with sufficient spaces that it appears in 3738the same column in the preprocessed output that it appeared in the 3739original source file. This is so the output is easy to read. 3740@xref{Differences from previous versions}. CPP does not insert any 3741whitespace where there was none in the original source, except where 3742necessary to prevent an accidental token paste. 3743 3744@cindex linemarkers 3745Source file name and line number information is conveyed by lines 3746of the form 3747 3748@smallexample 3749# @var{linenum} @var{filename} @var{flags} 3750@end smallexample 3751 3752@noindent 3753These are called @dfn{linemarkers}. They are inserted as needed into 3754the output (but never within a string or character constant). They mean 3755that the following line originated in file @var{filename} at line 3756@var{linenum}. @var{filename} will never contain any non-printing 3757characters; they are replaced with octal escape sequences. 3758 3759After the file name comes zero or more flags, which are @samp{1}, 3760@samp{2}, @samp{3}, or @samp{4}. If there are multiple flags, spaces 3761separate them. Here is what the flags mean: 3762 3763@table @samp 3764@item 1 3765This indicates the start of a new file. 3766@item 2 3767This indicates returning to a file (after having included another file). 3768@item 3 3769This indicates that the following text comes from a system header file, 3770so certain warnings should be suppressed. 3771@item 4 3772This indicates that the following text should be treated as being 3773wrapped in an implicit @code{extern "C"} block. 3774@c maybe cross reference NO_IMPLICIT_EXTERN_C 3775@end table 3776 3777As an extension, the preprocessor accepts linemarkers in non-assembler 3778input files. They are treated like the corresponding @samp{#line} 3779directive, (@pxref{Line Control}), except that trailing flags are 3780permitted, and are interpreted with the meanings described above. If 3781multiple flags are given, they must be in ascending order. 3782 3783Some directives may be duplicated in the output of the preprocessor. 3784These are @samp{#ident} (always), @samp{#pragma} (only if the 3785preprocessor does not handle the pragma itself), and @samp{#define} and 3786@samp{#undef} (with certain debugging options). If this happens, the 3787@samp{#} of the directive will always be in the first column, and there 3788will be no space between the @samp{#} and the directive name. If macro 3789expansion happens to generate tokens which might be mistaken for a 3790duplicated directive, a space will be inserted between the @samp{#} and 3791the directive name. 3792 3793@node Traditional Mode 3794@chapter Traditional Mode 3795 3796Traditional (pre-standard) C preprocessing is rather different from 3797the preprocessing specified by the standard. When GCC is given the 3798@option{-traditional-cpp} option, it attempts to emulate a traditional 3799preprocessor. 3800 3801GCC versions 3.2 and later only support traditional mode semantics in 3802the preprocessor, and not in the compiler front ends. This chapter 3803outlines the traditional preprocessor semantics we implemented. 3804 3805The implementation does not correspond precisely to the behavior of 3806earlier versions of GCC, nor to any true traditional preprocessor. 3807After all, inconsistencies among traditional implementations were a 3808major motivation for C standardization. However, we intend that it 3809should be compatible with true traditional preprocessors in all ways 3810that actually matter. 3811 3812@menu 3813* Traditional lexical analysis:: 3814* Traditional macros:: 3815* Traditional miscellany:: 3816* Traditional warnings:: 3817@end menu 3818 3819@node Traditional lexical analysis 3820@section Traditional lexical analysis 3821 3822The traditional preprocessor does not decompose its input into tokens 3823the same way a standards-conforming preprocessor does. The input is 3824simply treated as a stream of text with minimal internal form. 3825 3826This implementation does not treat trigraphs (@pxref{trigraphs}) 3827specially since they were an invention of the standards committee. It 3828handles arbitrarily-positioned escaped newlines properly and splices 3829the lines as you would expect; many traditional preprocessors did not 3830do this. 3831 3832The form of horizontal whitespace in the input file is preserved in 3833the output. In particular, hard tabs remain hard tabs. This can be 3834useful if, for example, you are preprocessing a Makefile. 3835 3836Traditional CPP only recognizes C-style block comments, and treats the 3837@samp{/*} sequence as introducing a comment only if it lies outside 3838quoted text. Quoted text is introduced by the usual single and double 3839quotes, and also by an initial @samp{<} in a @code{#include} 3840directive. 3841 3842Traditionally, comments are completely removed and are not replaced 3843with a space. Since a traditional compiler does its own tokenization 3844of the output of the preprocessor, this means that comments can 3845effectively be used as token paste operators. However, comments 3846behave like separators for text handled by the preprocessor itself, 3847since it doesn't re-lex its input. For example, in 3848 3849@smallexample 3850#if foo/**/bar 3851@end smallexample 3852 3853@noindent 3854@samp{foo} and @samp{bar} are distinct identifiers and expanded 3855separately if they happen to be macros. In other words, this 3856directive is equivalent to 3857 3858@smallexample 3859#if foo bar 3860@end smallexample 3861 3862@noindent 3863rather than 3864 3865@smallexample 3866#if foobar 3867@end smallexample 3868 3869Generally speaking, in traditional mode an opening quote need not have 3870a matching closing quote. In particular, a macro may be defined with 3871replacement text that contains an unmatched quote. Of course, if you 3872attempt to compile preprocessed output containing an unmatched quote 3873you will get a syntax error. 3874 3875However, all preprocessing directives other than @code{#define} 3876require matching quotes. For example: 3877 3878@smallexample 3879#define m This macro's fine and has an unmatched quote 3880"/* This is not a comment. */ 3881/* @r{This is a comment. The following #include directive 3882 is ill-formed.} */ 3883#include <stdio.h 3884@end smallexample 3885 3886Just as for the ISO preprocessor, what would be a closing quote can be 3887escaped with a backslash to prevent the quoted text from closing. 3888 3889@node Traditional macros 3890@section Traditional macros 3891 3892The major difference between traditional and ISO macros is that the 3893former expand to text rather than to a token sequence. CPP removes 3894all leading and trailing horizontal whitespace from a macro's 3895replacement text before storing it, but preserves the form of internal 3896whitespace. 3897 3898One consequence is that it is legitimate for the replacement text to 3899contain an unmatched quote (@pxref{Traditional lexical analysis}). An 3900unclosed string or character constant continues into the text 3901following the macro call. Similarly, the text at the end of a macro's 3902expansion can run together with the text after the macro invocation to 3903produce a single token. 3904 3905Normally comments are removed from the replacement text after the 3906macro is expanded, but if the @option{-CC} option is passed on the 3907command-line comments are preserved. (In fact, the current 3908implementation removes comments even before saving the macro 3909replacement text, but it careful to do it in such a way that the 3910observed effect is identical even in the function-like macro case.) 3911 3912The ISO stringification operator @samp{#} and token paste operator 3913@samp{##} have no special meaning. As explained later, an effect 3914similar to these operators can be obtained in a different way. Macro 3915names that are embedded in quotes, either from the main file or after 3916macro replacement, do not expand. 3917 3918CPP replaces an unquoted object-like macro name with its replacement 3919text, and then rescans it for further macros to replace. Unlike 3920standard macro expansion, traditional macro expansion has no provision 3921to prevent recursion. If an object-like macro appears unquoted in its 3922replacement text, it will be replaced again during the rescan pass, 3923and so on @emph{ad infinitum}. GCC detects when it is expanding 3924recursive macros, emits an error message, and continues after the 3925offending macro invocation. 3926 3927@smallexample 3928#define PLUS + 3929#define INC(x) PLUS+x 3930INC(foo); 3931 @expansion{} ++foo; 3932@end smallexample 3933 3934Function-like macros are similar in form but quite different in 3935behavior to their ISO counterparts. Their arguments are contained 3936within parentheses, are comma-separated, and can cross physical lines. 3937Commas within nested parentheses are not treated as argument 3938separators. Similarly, a quote in an argument cannot be left 3939unclosed; a following comma or parenthesis that comes before the 3940closing quote is treated like any other character. There is no 3941facility for handling variadic macros. 3942 3943This implementation removes all comments from macro arguments, unless 3944the @option{-C} option is given. The form of all other horizontal 3945whitespace in arguments is preserved, including leading and trailing 3946whitespace. In particular 3947 3948@smallexample 3949f( ) 3950@end smallexample 3951 3952@noindent 3953is treated as an invocation of the macro @samp{f} with a single 3954argument consisting of a single space. If you want to invoke a 3955function-like macro that takes no arguments, you must not leave any 3956whitespace between the parentheses. 3957 3958If a macro argument crosses a new line, the new line is replaced with 3959a space when forming the argument. If the previous line contained an 3960unterminated quote, the following line inherits the quoted state. 3961 3962Traditional preprocessors replace parameters in the replacement text 3963with their arguments regardless of whether the parameters are within 3964quotes or not. This provides a way to stringize arguments. For 3965example 3966 3967@smallexample 3968#define str(x) "x" 3969str(/* @r{A comment} */some text ) 3970 @expansion{} "some text " 3971@end smallexample 3972 3973@noindent 3974Note that the comment is removed, but that the trailing space is 3975preserved. Here is an example of using a comment to effect token 3976pasting. 3977 3978@smallexample 3979#define suffix(x) foo_/**/x 3980suffix(bar) 3981 @expansion{} foo_bar 3982@end smallexample 3983 3984@node Traditional miscellany 3985@section Traditional miscellany 3986 3987Here are some things to be aware of when using the traditional 3988preprocessor. 3989 3990@itemize @bullet 3991@item 3992Preprocessing directives are recognized only when their leading 3993@samp{#} appears in the first column. There can be no whitespace 3994between the beginning of the line and the @samp{#}, but whitespace can 3995follow the @samp{#}. 3996 3997@item 3998A true traditional C preprocessor does not recognize @samp{#error} or 3999@samp{#pragma}, and may not recognize @samp{#elif}. CPP supports all 4000the directives in traditional mode that it supports in ISO mode, 4001including extensions, with the exception that the effects of 4002@samp{#pragma GCC poison} are undefined. 4003 4004@item 4005__STDC__ is not defined. 4006 4007@item 4008If you use digraphs the behavior is undefined. 4009 4010@item 4011If a line that looks like a directive appears within macro arguments, 4012the behavior is undefined. 4013 4014@end itemize 4015 4016@node Traditional warnings 4017@section Traditional warnings 4018You can request warnings about features that did not exist, or worked 4019differently, in traditional C with the @option{-Wtraditional} option. 4020GCC does not warn about features of ISO C which you must use when you 4021are using a conforming compiler, such as the @samp{#} and @samp{##} 4022operators. 4023 4024Presently @option{-Wtraditional} warns about: 4025 4026@itemize @bullet 4027@item 4028Macro parameters that appear within string literals in the macro body. 4029In traditional C macro replacement takes place within string literals, 4030but does not in ISO C@. 4031 4032@item 4033In traditional C, some preprocessor directives did not exist. 4034Traditional preprocessors would only consider a line to be a directive 4035if the @samp{#} appeared in column 1 on the line. Therefore 4036@option{-Wtraditional} warns about directives that traditional C 4037understands but would ignore because the @samp{#} does not appear as the 4038first character on the line. It also suggests you hide directives like 4039@samp{#pragma} not understood by traditional C by indenting them. Some 4040traditional implementations would not recognize @samp{#elif}, so it 4041suggests avoiding it altogether. 4042 4043@item 4044A function-like macro that appears without an argument list. In some 4045traditional preprocessors this was an error. In ISO C it merely means 4046that the macro is not expanded. 4047 4048@item 4049The unary plus operator. This did not exist in traditional C@. 4050 4051@item 4052The @samp{U} and @samp{LL} integer constant suffixes, which were not 4053available in traditional C@. (Traditional C does support the @samp{L} 4054suffix for simple long integer constants.) You are not warned about 4055uses of these suffixes in macros defined in system headers. For 4056instance, @code{UINT_MAX} may well be defined as @code{4294967295U}, but 4057you will not be warned if you use @code{UINT_MAX}. 4058 4059You can usually avoid the warning, and the related warning about 4060constants which are so large that they are unsigned, by writing the 4061integer constant in question in hexadecimal, with no U suffix. Take 4062care, though, because this gives the wrong result in exotic cases. 4063@end itemize 4064 4065@node Implementation Details 4066@chapter Implementation Details 4067 4068Here we document details of how the preprocessor's implementation 4069affects its user-visible behavior. You should try to avoid undue 4070reliance on behavior described here, as it is possible that it will 4071change subtly in future implementations. 4072 4073Also documented here are obsolete features and changes from previous 4074versions of CPP@. 4075 4076@menu 4077* Implementation-defined behavior:: 4078* Implementation limits:: 4079* Obsolete Features:: 4080* Differences from previous versions:: 4081@end menu 4082 4083@node Implementation-defined behavior 4084@section Implementation-defined behavior 4085@cindex implementation-defined behavior 4086 4087This is how CPP behaves in all the cases which the C standard 4088describes as @dfn{implementation-defined}. This term means that the 4089implementation is free to do what it likes, but must document its choice 4090and stick to it. 4091@c FIXME: Check the C++ standard for more implementation-defined stuff. 4092 4093@itemize @bullet 4094@need 1000 4095@item The mapping of physical source file multi-byte characters to the 4096execution character set. 4097 4098The input character set can be specified using the 4099@option{-finput-charset} option, while the execution character set may 4100be controlled using the @option{-fexec-charset} and 4101@option{-fwide-exec-charset} options. 4102 4103@item Identifier characters. 4104@anchor{Identifier characters} 4105 4106The C and C++ standards allow identifiers to be composed of @samp{_} 4107and the alphanumeric characters. C++ and C99 also allow universal 4108character names, and C99 further permits implementation-defined 4109characters. 4110 4111GCC allows the @samp{$} character in identifiers as an extension for 4112most targets. This is true regardless of the @option{std=} switch, 4113since this extension cannot conflict with standards-conforming 4114programs. When preprocessing assembler, however, dollars are not 4115identifier characters by default. 4116 4117Currently the targets that by default do not permit @samp{$} are AVR, 4118IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC targets for the AIX 4119operating system. 4120 4121You can override the default with @option{-fdollars-in-identifiers} or 4122@option{fno-dollars-in-identifiers}. @xref{fdollars-in-identifiers}. 4123 4124@item Non-empty sequences of whitespace characters. 4125 4126In textual output, each whitespace sequence is collapsed to a single 4127space. For aesthetic reasons, the first token on each non-directive 4128line of output is preceded with sufficient spaces that it appears in the 4129same column as it did in the original source file. 4130 4131@item The numeric value of character constants in preprocessor expressions. 4132 4133The preprocessor and compiler interpret character constants in the 4134same way; i.e.@: escape sequences such as @samp{\a} are given the 4135values they would have on the target machine. 4136 4137The compiler evaluates a multi-character character constant a character 4138at a time, shifting the previous value left by the number of bits per 4139target character, and then or-ing in the bit-pattern of the new 4140character truncated to the width of a target character. The final 4141bit-pattern is given type @code{int}, and is therefore signed, 4142regardless of whether single characters are signed or not (a slight 4143change from versions 3.1 and earlier of GCC)@. If there are more 4144characters in the constant than would fit in the target @code{int} the 4145compiler issues a warning, and the excess leading characters are 4146ignored. 4147 4148For example, @code{'ab'} for a target with an 8-bit @code{char} would be 4149interpreted as @w{@samp{(int) ((unsigned char) 'a' * 256 + (unsigned char) 4150'b')}}, and @code{'\234a'} as @w{@samp{(int) ((unsigned char) '\234' * 4151256 + (unsigned char) 'a')}}. 4152 4153@item Source file inclusion. 4154 4155For a discussion on how the preprocessor locates header files, 4156@ref{Include Operation}. 4157 4158@item Interpretation of the filename resulting from a macro-expanded 4159@samp{#include} directive. 4160 4161@xref{Computed Includes}. 4162 4163@item Treatment of a @samp{#pragma} directive that after macro-expansion 4164results in a standard pragma. 4165 4166No macro expansion occurs on any @samp{#pragma} directive line, so the 4167question does not arise. 4168 4169Note that GCC does not yet implement any of the standard 4170pragmas. 4171 4172@end itemize 4173 4174@node Implementation limits 4175@section Implementation limits 4176@cindex implementation limits 4177 4178CPP has a small number of internal limits. This section lists the 4179limits which the C standard requires to be no lower than some minimum, 4180and all the others known. It is intended that there should be as few limits 4181as possible. If you encounter an undocumented or inconvenient limit, 4182please report that as a bug. @xref{Bugs, , Reporting Bugs, gcc, Using 4183the GNU Compiler Collection (GCC)}. 4184 4185Where we say something is limited @dfn{only by available memory}, that 4186means that internal data structures impose no intrinsic limit, and space 4187is allocated with @code{malloc} or equivalent. The actual limit will 4188therefore depend on many things, such as the size of other things 4189allocated by the compiler at the same time, the amount of memory 4190consumed by other processes on the same computer, etc. 4191 4192@itemize @bullet 4193 4194@item Nesting levels of @samp{#include} files. 4195 4196We impose an arbitrary limit of 200 levels, to avoid runaway recursion. 4197The standard requires at least 15 levels. 4198 4199@item Nesting levels of conditional inclusion. 4200 4201The C standard mandates this be at least 63. CPP is limited only by 4202available memory. 4203 4204@item Levels of parenthesized expressions within a full expression. 4205 4206The C standard requires this to be at least 63. In preprocessor 4207conditional expressions, it is limited only by available memory. 4208 4209@item Significant initial characters in an identifier or macro name. 4210 4211The preprocessor treats all characters as significant. The C standard 4212requires only that the first 63 be significant. 4213 4214@item Number of macros simultaneously defined in a single translation unit. 4215 4216The standard requires at least 4095 be possible. CPP is limited only 4217by available memory. 4218 4219@item Number of parameters in a macro definition and arguments in a macro call. 4220 4221We allow @code{USHRT_MAX}, which is no smaller than 65,535. The minimum 4222required by the standard is 127. 4223 4224@item Number of characters on a logical source line. 4225 4226The C standard requires a minimum of 4096 be permitted. CPP places 4227no limits on this, but you may get incorrect column numbers reported in 4228diagnostics for lines longer than 65,535 characters. 4229 4230@item Maximum size of a source file. 4231 4232The standard does not specify any lower limit on the maximum size of a 4233source file. GNU cpp maps files into memory, so it is limited by the 4234available address space. This is generally at least two gigabytes. 4235Depending on the operating system, the size of physical memory may or 4236may not be a limitation. 4237 4238@end itemize 4239 4240@node Obsolete Features 4241@section Obsolete Features 4242 4243CPP has some features which are present mainly for compatibility with 4244older programs. We discourage their use in new code. In some cases, 4245we plan to remove the feature in a future version of GCC@. 4246 4247@subsection Assertions 4248@cindex assertions 4249 4250@dfn{Assertions} are a deprecated alternative to macros in writing 4251conditionals to test what sort of computer or system the compiled 4252program will run on. Assertions are usually predefined, but you can 4253define them with preprocessing directives or command-line options. 4254 4255Assertions were intended to provide a more systematic way to describe 4256the compiler's target system and we added them for compatibility with 4257existing compilers. In practice they are just as unpredictable as the 4258system-specific predefined macros. In addition, they are not part of 4259any standard, and only a few compilers support them. 4260Therefore, the use of assertions is @strong{less} portable than the use 4261of system-specific predefined macros. We recommend you do not use them at 4262all. 4263 4264@cindex predicates 4265An assertion looks like this: 4266 4267@smallexample 4268#@var{predicate} (@var{answer}) 4269@end smallexample 4270 4271@noindent 4272@var{predicate} must be a single identifier. @var{answer} can be any 4273sequence of tokens; all characters are significant except for leading 4274and trailing whitespace, and differences in internal whitespace 4275sequences are ignored. (This is similar to the rules governing macro 4276redefinition.) Thus, @code{(x + y)} is different from @code{(x+y)} but 4277equivalent to @code{@w{( x + y )}}. Parentheses do not nest inside an 4278answer. 4279 4280@cindex testing predicates 4281To test an assertion, you write it in an @samp{#if}. For example, this 4282conditional succeeds if either @code{vax} or @code{ns16000} has been 4283asserted as an answer for @code{machine}. 4284 4285@smallexample 4286#if #machine (vax) || #machine (ns16000) 4287@end smallexample 4288 4289@noindent 4290You can test whether @emph{any} answer is asserted for a predicate by 4291omitting the answer in the conditional: 4292 4293@smallexample 4294#if #machine 4295@end smallexample 4296 4297@findex #assert 4298Assertions are made with the @samp{#assert} directive. Its sole 4299argument is the assertion to make, without the leading @samp{#} that 4300identifies assertions in conditionals. 4301 4302@smallexample 4303#assert @var{predicate} (@var{answer}) 4304@end smallexample 4305 4306@noindent 4307You may make several assertions with the same predicate and different 4308answers. Subsequent assertions do not override previous ones for the 4309same predicate. All the answers for any given predicate are 4310simultaneously true. 4311 4312@cindex assertions, canceling 4313@findex #unassert 4314Assertions can be canceled with the @samp{#unassert} directive. It 4315has the same syntax as @samp{#assert}. In that form it cancels only the 4316answer which was specified on the @samp{#unassert} line; other answers 4317for that predicate remain true. You can cancel an entire predicate by 4318leaving out the answer: 4319 4320@smallexample 4321#unassert @var{predicate} 4322@end smallexample 4323 4324@noindent 4325In either form, if no such assertion has been made, @samp{#unassert} has 4326no effect. 4327 4328You can also make or cancel assertions using command-line options. 4329@xref{Invocation}. 4330 4331@node Differences from previous versions 4332@section Differences from previous versions 4333@cindex differences from previous versions 4334 4335This section details behavior which has changed from previous versions 4336of CPP@. We do not plan to change it again in the near future, but 4337we do not promise not to, either. 4338 4339The ``previous versions'' discussed here are 2.95 and before. The 4340behavior of GCC 3.0 is mostly the same as the behavior of the widely 4341used 2.96 and 2.97 development snapshots. Where there are differences, 4342they generally represent bugs in the snapshots. 4343 4344@itemize @bullet 4345 4346@item -I- deprecated 4347 4348This option has been deprecated in 4.0. @option{-iquote} is meant to 4349replace the need for this option. 4350 4351@item Order of evaluation of @samp{#} and @samp{##} operators 4352 4353The standard does not specify the order of evaluation of a chain of 4354@samp{##} operators, nor whether @samp{#} is evaluated before, after, or 4355at the same time as @samp{##}. You should therefore not write any code 4356which depends on any specific ordering. It is possible to guarantee an 4357ordering, if you need one, by suitable use of nested macros. 4358 4359An example of where this might matter is pasting the arguments @samp{1}, 4360@samp{e} and @samp{-2}. This would be fine for left-to-right pasting, 4361but right-to-left pasting would produce an invalid token @samp{e-2}. 4362 4363GCC 3.0 evaluates @samp{#} and @samp{##} at the same time and strictly 4364left to right. Older versions evaluated all @samp{#} operators first, 4365then all @samp{##} operators, in an unreliable order. 4366 4367@item The form of whitespace between tokens in preprocessor output 4368 4369@xref{Preprocessor Output}, for the current textual format. This is 4370also the format used by stringification. Normally, the preprocessor 4371communicates tokens directly to the compiler's parser, and whitespace 4372does not come up at all. 4373 4374Older versions of GCC preserved all whitespace provided by the user and 4375inserted lots more whitespace of their own, because they could not 4376accurately predict when extra spaces were needed to prevent accidental 4377token pasting. 4378 4379@item Optional argument when invoking rest argument macros 4380 4381As an extension, GCC permits you to omit the variable arguments entirely 4382when you use a variable argument macro. This is forbidden by the 1999 C 4383standard, and will provoke a pedantic warning with GCC 3.0. Previous 4384versions accepted it silently. 4385 4386@item @samp{##} swallowing preceding text in rest argument macros 4387 4388Formerly, in a macro expansion, if @samp{##} appeared before a variable 4389arguments parameter, and the set of tokens specified for that argument 4390in the macro invocation was empty, previous versions of CPP would 4391back up and remove the preceding sequence of non-whitespace characters 4392(@strong{not} the preceding token). This extension is in direct 4393conflict with the 1999 C standard and has been drastically pared back. 4394 4395In the current version of the preprocessor, if @samp{##} appears between 4396a comma and a variable arguments parameter, and the variable argument is 4397omitted entirely, the comma will be removed from the expansion. If the 4398variable argument is empty, or the token before @samp{##} is not a 4399comma, then @samp{##} behaves as a normal token paste. 4400 4401@item @samp{#line} and @samp{#include} 4402 4403The @samp{#line} directive used to change GCC's notion of the 4404``directory containing the current file'', used by @samp{#include} with 4405a double-quoted header file name. In 3.0 and later, it does not. 4406@xref{Line Control}, for further explanation. 4407 4408@item Syntax of @samp{#line} 4409 4410In GCC 2.95 and previous, the string constant argument to @samp{#line} 4411was treated the same way as the argument to @samp{#include}: backslash 4412escapes were not honored, and the string ended at the second @samp{"}. 4413This is not compliant with the C standard. In GCC 3.0, an attempt was 4414made to correct the behavior, so that the string was treated as a real 4415string constant, but it turned out to be buggy. In 3.1, the bugs have 4416been fixed. (We are not fixing the bugs in 3.0 because they affect 4417relatively few people and the fix is quite invasive.) 4418 4419@end itemize 4420 4421@node Invocation 4422@chapter Invocation 4423@cindex invocation 4424@cindex command line 4425 4426Most often when you use the C preprocessor you will not have to invoke it 4427explicitly: the C compiler will do so automatically. However, the 4428preprocessor is sometimes useful on its own. All the options listed 4429here are also acceptable to the C compiler and have the same meaning, 4430except that the C compiler has different rules for specifying the output 4431file. 4432 4433@emph{Note:} Whether you use the preprocessor by way of @command{gcc} 4434or @command{cpp}, the @dfn{compiler driver} is run first. This 4435program's purpose is to translate your command into invocations of the 4436programs that do the actual work. Their command-line interfaces are 4437similar but not identical to the documented interface, and may change 4438without notice. 4439 4440@ignore 4441@c man begin SYNOPSIS 4442cpp [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}] 4443 [@option{-I}@var{dir}@dots{}] [@option{-iquote}@var{dir}@dots{}] 4444 [@option{-W}@var{warn}@dots{}] 4445 [@option{-M}|@option{-MM}] [@option{-MG}] [@option{-MF} @var{filename}] 4446 [@option{-MP}] [@option{-MQ} @var{target}@dots{}] 4447 [@option{-MT} @var{target}@dots{}] 4448 [@option{-P}] [@option{-fno-working-directory}] 4449 [@option{-x} @var{language}] [@option{-std=}@var{standard}] 4450 @var{infile} @var{outfile} 4451 4452Only the most useful options are listed here; see below for the remainder. 4453@c man end 4454@c man begin SEEALSO 4455gpl(7), gfdl(7), fsf-funding(7), 4456gcc(1), as(1), ld(1), and the Info entries for @file{cpp}, @file{gcc}, and 4457@file{binutils}. 4458@c man end 4459@end ignore 4460 4461@c man begin OPTIONS 4462The C preprocessor expects two file names as arguments, @var{infile} and 4463@var{outfile}. The preprocessor reads @var{infile} together with any 4464other files it specifies with @samp{#include}. All the output generated 4465by the combined input files is written in @var{outfile}. 4466 4467Either @var{infile} or @var{outfile} may be @option{-}, which as 4468@var{infile} means to read from standard input and as @var{outfile} 4469means to write to standard output. Also, if either file is omitted, it 4470means the same as if @option{-} had been specified for that file. 4471 4472Unless otherwise noted, or the option ends in @samp{=}, all options 4473which take an argument may have that argument appear either immediately 4474after the option, or with a space between option and argument: 4475@option{-Ifoo} and @option{-I foo} have the same effect. 4476 4477@cindex grouping options 4478@cindex options, grouping 4479Many options have multi-letter names; therefore multiple single-letter 4480options may @emph{not} be grouped: @option{-dM} is very different from 4481@w{@samp{-d -M}}. 4482 4483@cindex options 4484@include cppopts.texi 4485@c man end 4486 4487@node Environment Variables 4488@chapter Environment Variables 4489@cindex environment variables 4490@c man begin ENVIRONMENT 4491 4492This section describes the environment variables that affect how CPP 4493operates. You can use them to specify directories or prefixes to use 4494when searching for include files, or to control dependency output. 4495 4496Note that you can also specify places to search using options such as 4497@option{-I}, and control dependency output with options like 4498@option{-M} (@pxref{Invocation}). These take precedence over 4499environment variables, which in turn take precedence over the 4500configuration of GCC@. 4501 4502@include cppenv.texi 4503@c man end 4504 4505@page 4506@include fdl.texi 4507 4508@page 4509@node Index of Directives 4510@unnumbered Index of Directives 4511@printindex fn 4512 4513@node Option Index 4514@unnumbered Option Index 4515@noindent 4516CPP's command-line options and environment variables are indexed here 4517without any initial @samp{-} or @samp{--}. 4518@printindex op 4519 4520@page 4521@node Concept Index 4522@unnumbered Concept Index 4523@printindex cp 4524 4525@bye 4526