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 1160The @option{-cxx-isystem} command line option adds its argument to the 1161list of C++ system headers, similar to @option{-isystem} for C headers. 1162 1163@findex #pragma GCC system_header 1164There is also a directive, @code{@w{#pragma GCC system_header}}, which 1165tells GCC to consider the rest of the current include file a system 1166header, no matter where it was found. Code that comes before the 1167@samp{#pragma} in the file will not be affected. @code{@w{#pragma GCC 1168system_header}} has no effect in the primary source file. 1169 1170On very old systems, some of the pre-defined system header directories 1171get even more special treatment. GNU C++ considers code in headers 1172found in those directories to be surrounded by an @code{@w{extern "C"}} 1173block. There is no way to request this behavior with a @samp{#pragma}, 1174or from the command line. 1175 1176@node Macros 1177@chapter Macros 1178 1179A @dfn{macro} is a fragment of code which has been given a name. 1180Whenever the name is used, it is replaced by the contents of the macro. 1181There are two kinds of macros. They differ mostly in what they look 1182like when they are used. @dfn{Object-like} macros resemble data objects 1183when used, @dfn{function-like} macros resemble function calls. 1184 1185You may define any valid identifier as a macro, even if it is a C 1186keyword. The preprocessor does not know anything about keywords. This 1187can be useful if you wish to hide a keyword such as @code{const} from an 1188older compiler that does not understand it. However, the preprocessor 1189operator @code{defined} (@pxref{Defined}) can never be defined as a 1190macro, and C++'s named operators (@pxref{C++ Named Operators}) cannot be 1191macros when you are compiling C++. 1192 1193@menu 1194* Object-like Macros:: 1195* Function-like Macros:: 1196* Macro Arguments:: 1197* Stringification:: 1198* Concatenation:: 1199* Variadic Macros:: 1200* Predefined Macros:: 1201* Undefining and Redefining Macros:: 1202* Directives Within Macro Arguments:: 1203* Macro Pitfalls:: 1204@end menu 1205 1206@node Object-like Macros 1207@section Object-like Macros 1208@cindex object-like macro 1209@cindex symbolic constants 1210@cindex manifest constants 1211 1212An @dfn{object-like macro} is a simple identifier which will be replaced 1213by a code fragment. It is called object-like because it looks like a 1214data object in code that uses it. They are most commonly used to give 1215symbolic names to numeric constants. 1216 1217@findex #define 1218You create macros with the @samp{#define} directive. @samp{#define} is 1219followed by the name of the macro and then the token sequence it should 1220be an abbreviation for, which is variously referred to as the macro's 1221@dfn{body}, @dfn{expansion} or @dfn{replacement list}. For example, 1222 1223@smallexample 1224#define BUFFER_SIZE 1024 1225@end smallexample 1226 1227@noindent 1228defines a macro named @code{BUFFER_SIZE} as an abbreviation for the 1229token @code{1024}. If somewhere after this @samp{#define} directive 1230there comes a C statement of the form 1231 1232@smallexample 1233foo = (char *) malloc (BUFFER_SIZE); 1234@end smallexample 1235 1236@noindent 1237then the C preprocessor will recognize and @dfn{expand} the macro 1238@code{BUFFER_SIZE}. The C compiler will see the same tokens as it would 1239if you had written 1240 1241@smallexample 1242foo = (char *) malloc (1024); 1243@end smallexample 1244 1245By convention, macro names are written in uppercase. Programs are 1246easier to read when it is possible to tell at a glance which names are 1247macros. 1248 1249The macro's body ends at the end of the @samp{#define} line. You may 1250continue the definition onto multiple lines, if necessary, using 1251backslash-newline. When the macro is expanded, however, it will all 1252come out on one line. For example, 1253 1254@smallexample 1255#define NUMBERS 1, \ 1256 2, \ 1257 3 1258int x[] = @{ NUMBERS @}; 1259 @expansion{} int x[] = @{ 1, 2, 3 @}; 1260@end smallexample 1261 1262@noindent 1263The most common visible consequence of this is surprising line numbers 1264in error messages. 1265 1266There is no restriction on what can go in a macro body provided it 1267decomposes into valid preprocessing tokens. Parentheses need not 1268balance, and the body need not resemble valid C code. (If it does not, 1269you may get error messages from the C compiler when you use the macro.) 1270 1271The C preprocessor scans your program sequentially. Macro definitions 1272take effect at the place you write them. Therefore, the following input 1273to the C preprocessor 1274 1275@smallexample 1276foo = X; 1277#define X 4 1278bar = X; 1279@end smallexample 1280 1281@noindent 1282produces 1283 1284@smallexample 1285foo = X; 1286bar = 4; 1287@end smallexample 1288 1289When the preprocessor expands a macro name, the macro's expansion 1290replaces the macro invocation, then the expansion is examined for more 1291macros to expand. For example, 1292 1293@smallexample 1294@group 1295#define TABLESIZE BUFSIZE 1296#define BUFSIZE 1024 1297TABLESIZE 1298 @expansion{} BUFSIZE 1299 @expansion{} 1024 1300@end group 1301@end smallexample 1302 1303@noindent 1304@code{TABLESIZE} is expanded first to produce @code{BUFSIZE}, then that 1305macro is expanded to produce the final result, @code{1024}. 1306 1307Notice that @code{BUFSIZE} was not defined when @code{TABLESIZE} was 1308defined. The @samp{#define} for @code{TABLESIZE} uses exactly the 1309expansion you specify---in this case, @code{BUFSIZE}---and does not 1310check to see whether it too contains macro names. Only when you 1311@emph{use} @code{TABLESIZE} is the result of its expansion scanned for 1312more macro names. 1313 1314This makes a difference if you change the definition of @code{BUFSIZE} 1315at some point in the source file. @code{TABLESIZE}, defined as shown, 1316will always expand using the definition of @code{BUFSIZE} that is 1317currently in effect: 1318 1319@smallexample 1320#define BUFSIZE 1020 1321#define TABLESIZE BUFSIZE 1322#undef BUFSIZE 1323#define BUFSIZE 37 1324@end smallexample 1325 1326@noindent 1327Now @code{TABLESIZE} expands (in two stages) to @code{37}. 1328 1329If the expansion of a macro contains its own name, either directly or 1330via intermediate macros, it is not expanded again when the expansion is 1331examined for more macros. This prevents infinite recursion. 1332@xref{Self-Referential Macros}, for the precise details. 1333 1334@node Function-like Macros 1335@section Function-like Macros 1336@cindex function-like macros 1337 1338You can also define macros whose use looks like a function call. These 1339are called @dfn{function-like macros}. To define a function-like macro, 1340you use the same @samp{#define} directive, but you put a pair of 1341parentheses immediately after the macro name. For example, 1342 1343@smallexample 1344#define lang_init() c_init() 1345lang_init() 1346 @expansion{} c_init() 1347@end smallexample 1348 1349A function-like macro is only expanded if its name appears with a pair 1350of parentheses after it. If you write just the name, it is left alone. 1351This can be useful when you have a function and a macro of the same 1352name, and you wish to use the function sometimes. 1353 1354@smallexample 1355extern void foo(void); 1356#define foo() /* @r{optimized inline version} */ 1357@dots{} 1358 foo(); 1359 funcptr = foo; 1360@end smallexample 1361 1362Here the call to @code{foo()} will use the macro, but the function 1363pointer will get the address of the real function. If the macro were to 1364be expanded, it would cause a syntax error. 1365 1366If you put spaces between the macro name and the parentheses in the 1367macro definition, that does not define a function-like macro, it defines 1368an object-like macro whose expansion happens to begin with a pair of 1369parentheses. 1370 1371@smallexample 1372#define lang_init () c_init() 1373lang_init() 1374 @expansion{} () c_init()() 1375@end smallexample 1376 1377The first two pairs of parentheses in this expansion come from the 1378macro. The third is the pair that was originally after the macro 1379invocation. Since @code{lang_init} is an object-like macro, it does not 1380consume those parentheses. 1381 1382@node Macro Arguments 1383@section Macro Arguments 1384@cindex arguments 1385@cindex macros with arguments 1386@cindex arguments in macro definitions 1387 1388Function-like macros can take @dfn{arguments}, just like true functions. 1389To define a macro that uses arguments, you insert @dfn{parameters} 1390between the pair of parentheses in the macro definition that make the 1391macro function-like. The parameters must be valid C identifiers, 1392separated by commas and optionally whitespace. 1393 1394To invoke a macro that takes arguments, you write the name of the macro 1395followed by a list of @dfn{actual arguments} in parentheses, separated 1396by commas. The invocation of the macro need not be restricted to a 1397single logical line---it can cross as many lines in the source file as 1398you wish. The number of arguments you give must match the number of 1399parameters in the macro definition. When the macro is expanded, each 1400use of a parameter in its body is replaced by the tokens of the 1401corresponding argument. (You need not use all of the parameters in the 1402macro body.) 1403 1404As an example, here is a macro that computes the minimum of two numeric 1405values, as it is defined in many C programs, and some uses. 1406 1407@smallexample 1408#define min(X, Y) ((X) < (Y) ? (X) : (Y)) 1409 x = min(a, b); @expansion{} x = ((a) < (b) ? (a) : (b)); 1410 y = min(1, 2); @expansion{} y = ((1) < (2) ? (1) : (2)); 1411 z = min(a + 28, *p); @expansion{} z = ((a + 28) < (*p) ? (a + 28) : (*p)); 1412@end smallexample 1413 1414@noindent 1415(In this small example you can already see several of the dangers of 1416macro arguments. @xref{Macro Pitfalls}, for detailed explanations.) 1417 1418Leading and trailing whitespace in each argument is dropped, and all 1419whitespace between the tokens of an argument is reduced to a single 1420space. Parentheses within each argument must balance; a comma within 1421such parentheses does not end the argument. However, there is no 1422requirement for square brackets or braces to balance, and they do not 1423prevent a comma from separating arguments. Thus, 1424 1425@smallexample 1426macro (array[x = y, x + 1]) 1427@end smallexample 1428 1429@noindent 1430passes two arguments to @code{macro}: @code{array[x = y} and @code{x + 14311]}. If you want to supply @code{array[x = y, x + 1]} as an argument, 1432you can write it as @code{array[(x = y, x + 1)]}, which is equivalent C 1433code. 1434 1435All arguments to a macro are completely macro-expanded before they are 1436substituted into the macro body. After substitution, the complete text 1437is scanned again for macros to expand, including the arguments. This rule 1438may seem strange, but it is carefully designed so you need not worry 1439about whether any function call is actually a macro invocation. You can 1440run into trouble if you try to be too clever, though. @xref{Argument 1441Prescan}, for detailed discussion. 1442 1443For example, @code{min (min (a, b), c)} is first expanded to 1444 1445@smallexample 1446 min (((a) < (b) ? (a) : (b)), (c)) 1447@end smallexample 1448 1449@noindent 1450and then to 1451 1452@smallexample 1453@group 1454((((a) < (b) ? (a) : (b))) < (c) 1455 ? (((a) < (b) ? (a) : (b))) 1456 : (c)) 1457@end group 1458@end smallexample 1459 1460@noindent 1461(Line breaks shown here for clarity would not actually be generated.) 1462 1463@cindex empty macro arguments 1464You can leave macro arguments empty; this is not an error to the 1465preprocessor (but many macros will then expand to invalid code). 1466You cannot leave out arguments entirely; if a macro takes two arguments, 1467there must be exactly one comma at the top level of its argument list. 1468Here are some silly examples using @code{min}: 1469 1470@smallexample 1471min(, b) @expansion{} (( ) < (b) ? ( ) : (b)) 1472min(a, ) @expansion{} ((a ) < ( ) ? (a ) : ( )) 1473min(,) @expansion{} (( ) < ( ) ? ( ) : ( )) 1474min((,),) @expansion{} (((,)) < ( ) ? ((,)) : ( )) 1475 1476min() @error{} macro "min" requires 2 arguments, but only 1 given 1477min(,,) @error{} macro "min" passed 3 arguments, but takes just 2 1478@end smallexample 1479 1480Whitespace is not a preprocessing token, so if a macro @code{foo} takes 1481one argument, @code{@w{foo ()}} and @code{@w{foo ( )}} both supply it an 1482empty argument. Previous GNU preprocessor implementations and 1483documentation were incorrect on this point, insisting that a 1484function-like macro that takes a single argument be passed a space if an 1485empty argument was required. 1486 1487Macro parameters appearing inside string literals are not replaced by 1488their corresponding actual arguments. 1489 1490@smallexample 1491#define foo(x) x, "x" 1492foo(bar) @expansion{} bar, "x" 1493@end smallexample 1494 1495@node Stringification 1496@section Stringification 1497@cindex stringification 1498@cindex @samp{#} operator 1499 1500Sometimes you may want to convert a macro argument into a string 1501constant. Parameters are not replaced inside string constants, but you 1502can use the @samp{#} preprocessing operator instead. When a macro 1503parameter is used with a leading @samp{#}, the preprocessor replaces it 1504with the literal text of the actual argument, converted to a string 1505constant. Unlike normal parameter replacement, the argument is not 1506macro-expanded first. This is called @dfn{stringification}. 1507 1508There is no way to combine an argument with surrounding text and 1509stringify it all together. Instead, you can write a series of adjacent 1510string constants and stringified arguments. The preprocessor will 1511replace the stringified arguments with string constants. The C 1512compiler will then combine all the adjacent string constants into one 1513long string. 1514 1515Here is an example of a macro definition that uses stringification: 1516 1517@smallexample 1518@group 1519#define WARN_IF(EXP) \ 1520do @{ if (EXP) \ 1521 fprintf (stderr, "Warning: " #EXP "\n"); @} \ 1522while (0) 1523WARN_IF (x == 0); 1524 @expansion{} do @{ if (x == 0) 1525 fprintf (stderr, "Warning: " "x == 0" "\n"); @} while (0); 1526@end group 1527@end smallexample 1528 1529@noindent 1530The argument for @code{EXP} is substituted once, as-is, into the 1531@code{if} statement, and once, stringified, into the argument to 1532@code{fprintf}. If @code{x} were a macro, it would be expanded in the 1533@code{if} statement, but not in the string. 1534 1535The @code{do} and @code{while (0)} are a kludge to make it possible to 1536write @code{WARN_IF (@var{arg});}, which the resemblance of 1537@code{WARN_IF} to a function would make C programmers want to do; see 1538@ref{Swallowing the Semicolon}. 1539 1540Stringification in C involves more than putting double-quote characters 1541around the fragment. The preprocessor backslash-escapes the quotes 1542surrounding embedded string constants, and all backslashes within string and 1543character constants, in order to get a valid C string constant with the 1544proper contents. Thus, stringifying @code{@w{p = "foo\n";}} results in 1545@t{@w{"p = \"foo\\n\";"}}. However, backslashes that are not inside string 1546or character constants are not duplicated: @samp{\n} by itself 1547stringifies to @t{"\n"}. 1548 1549All leading and trailing whitespace in text being stringified is 1550ignored. Any sequence of whitespace in the middle of the text is 1551converted to a single space in the stringified result. Comments are 1552replaced by whitespace long before stringification happens, so they 1553never appear in stringified text. 1554 1555There is no way to convert a macro argument into a character constant. 1556 1557If you want to stringify the result of expansion of a macro argument, 1558you have to use two levels of macros. 1559 1560@smallexample 1561#define xstr(s) str(s) 1562#define str(s) #s 1563#define foo 4 1564str (foo) 1565 @expansion{} "foo" 1566xstr (foo) 1567 @expansion{} xstr (4) 1568 @expansion{} str (4) 1569 @expansion{} "4" 1570@end smallexample 1571 1572@code{s} is stringified when it is used in @code{str}, so it is not 1573macro-expanded first. But @code{s} is an ordinary argument to 1574@code{xstr}, so it is completely macro-expanded before @code{xstr} 1575itself is expanded (@pxref{Argument Prescan}). Therefore, by the time 1576@code{str} gets to its argument, it has already been macro-expanded. 1577 1578@node Concatenation 1579@section Concatenation 1580@cindex concatenation 1581@cindex token pasting 1582@cindex token concatenation 1583@cindex @samp{##} operator 1584 1585It is often useful to merge two tokens into one while expanding macros. 1586This is called @dfn{token pasting} or @dfn{token concatenation}. The 1587@samp{##} preprocessing operator performs token pasting. When a macro 1588is expanded, the two tokens on either side of each @samp{##} operator 1589are combined into a single token, which then replaces the @samp{##} and 1590the two original tokens in the macro expansion. Usually both will be 1591identifiers, or one will be an identifier and the other a preprocessing 1592number. When pasted, they make a longer identifier. This isn't the 1593only valid case. It is also possible to concatenate two numbers (or a 1594number and a name, such as @code{1.5} and @code{e3}) into a number. 1595Also, multi-character operators such as @code{+=} can be formed by 1596token pasting. 1597 1598However, two tokens that don't together form a valid token cannot be 1599pasted together. For example, you cannot concatenate @code{x} with 1600@code{+} in either order. If you try, the preprocessor issues a warning 1601and emits the two tokens. Whether it puts white space between the 1602tokens is undefined. It is common to find unnecessary uses of @samp{##} 1603in complex macros. If you get this warning, it is likely that you can 1604simply remove the @samp{##}. 1605 1606Both the tokens combined by @samp{##} could come from the macro body, 1607but you could just as well write them as one token in the first place. 1608Token pasting is most useful when one or both of the tokens comes from a 1609macro argument. If either of the tokens next to an @samp{##} is a 1610parameter name, it is replaced by its actual argument before @samp{##} 1611executes. As with stringification, the actual argument is not 1612macro-expanded first. If the argument is empty, that @samp{##} has no 1613effect. 1614 1615Keep in mind that the C preprocessor converts comments to whitespace 1616before macros are even considered. Therefore, you cannot create a 1617comment by concatenating @samp{/} and @samp{*}. You can put as much 1618whitespace between @samp{##} and its operands as you like, including 1619comments, and you can put comments in arguments that will be 1620concatenated. However, it is an error if @samp{##} appears at either 1621end of a macro body. 1622 1623Consider a C program that interprets named commands. There probably 1624needs to be a table of commands, perhaps an array of structures declared 1625as follows: 1626 1627@smallexample 1628@group 1629struct command 1630@{ 1631 char *name; 1632 void (*function) (void); 1633@}; 1634@end group 1635 1636@group 1637struct command commands[] = 1638@{ 1639 @{ "quit", quit_command @}, 1640 @{ "help", help_command @}, 1641 @dots{} 1642@}; 1643@end group 1644@end smallexample 1645 1646It would be cleaner not to have to give each command name twice, once in 1647the string constant and once in the function name. A macro which takes the 1648name of a command as an argument can make this unnecessary. The string 1649constant can be created with stringification, and the function name by 1650concatenating the argument with @samp{_command}. Here is how it is done: 1651 1652@smallexample 1653#define COMMAND(NAME) @{ #NAME, NAME ## _command @} 1654 1655struct command commands[] = 1656@{ 1657 COMMAND (quit), 1658 COMMAND (help), 1659 @dots{} 1660@}; 1661@end smallexample 1662 1663@node Variadic Macros 1664@section Variadic Macros 1665@cindex variable number of arguments 1666@cindex macros with variable arguments 1667@cindex variadic macros 1668 1669A macro can be declared to accept a variable number of arguments much as 1670a function can. The syntax for defining the macro is similar to that of 1671a function. Here is an example: 1672 1673@smallexample 1674#define eprintf(@dots{}) fprintf (stderr, __VA_ARGS__) 1675@end smallexample 1676 1677This kind of macro is called @dfn{variadic}. When the macro is invoked, 1678all the tokens in its argument list after the last named argument (this 1679macro has none), including any commas, become the @dfn{variable 1680argument}. This sequence of tokens replaces the identifier 1681@code{@w{__VA_ARGS__}} in the macro body wherever it appears. Thus, we 1682have this expansion: 1683 1684@smallexample 1685eprintf ("%s:%d: ", input_file, lineno) 1686 @expansion{} fprintf (stderr, "%s:%d: ", input_file, lineno) 1687@end smallexample 1688 1689The variable argument is completely macro-expanded before it is inserted 1690into the macro expansion, just like an ordinary argument. You may use 1691the @samp{#} and @samp{##} operators to stringify the variable argument 1692or to paste its leading or trailing token with another token. (But see 1693below for an important special case for @samp{##}.) 1694 1695If your macro is complicated, you may want a more descriptive name for 1696the variable argument than @code{@w{__VA_ARGS__}}. CPP permits 1697this, as an extension. You may write an argument name immediately 1698before the @samp{@dots{}}; that name is used for the variable argument. 1699The @code{eprintf} macro above could be written 1700 1701@smallexample 1702#define eprintf(args@dots{}) fprintf (stderr, args) 1703@end smallexample 1704 1705@noindent 1706using this extension. You cannot use @code{@w{__VA_ARGS__}} and this 1707extension in the same macro. 1708 1709You can have named arguments as well as variable arguments in a variadic 1710macro. We could define @code{eprintf} like this, instead: 1711 1712@smallexample 1713#define eprintf(format, @dots{}) fprintf (stderr, format, __VA_ARGS__) 1714@end smallexample 1715 1716@noindent 1717This formulation looks more descriptive, but unfortunately it is less 1718flexible: you must now supply at least one argument after the format 1719string. In standard C, you cannot omit the comma separating the named 1720argument from the variable arguments. Furthermore, if you leave the 1721variable argument empty, you will get a syntax error, because 1722there will be an extra comma after the format string. 1723 1724@smallexample 1725eprintf("success!\n", ); 1726 @expansion{} fprintf(stderr, "success!\n", ); 1727@end smallexample 1728 1729GNU CPP has a pair of extensions which deal with this problem. First, 1730you are allowed to leave the variable argument out entirely: 1731 1732@smallexample 1733eprintf ("success!\n") 1734 @expansion{} fprintf(stderr, "success!\n", ); 1735@end smallexample 1736 1737@noindent 1738Second, the @samp{##} token paste operator has a special meaning when 1739placed between a comma and a variable argument. If you write 1740 1741@smallexample 1742#define eprintf(format, @dots{}) fprintf (stderr, format, ##__VA_ARGS__) 1743@end smallexample 1744 1745@noindent 1746and the variable argument is left out when the @code{eprintf} macro is 1747used, then the comma before the @samp{##} will be deleted. This does 1748@emph{not} happen if you pass an empty argument, nor does it happen if 1749the token preceding @samp{##} is anything other than a comma. 1750 1751@smallexample 1752eprintf ("success!\n") 1753 @expansion{} fprintf(stderr, "success!\n"); 1754@end smallexample 1755 1756@noindent 1757The above explanation is ambiguous about the case where the only macro 1758parameter is a variable arguments parameter, as it is meaningless to 1759try to distinguish whether no argument at all is an empty argument or 1760a missing argument. In this case the C99 standard is clear that the 1761comma must remain, however the existing GCC extension used to swallow 1762the comma. So CPP retains the comma when conforming to a specific C 1763standard, and drops it otherwise. 1764 1765C99 mandates that the only place the identifier @code{@w{__VA_ARGS__}} 1766can appear is in the replacement list of a variadic macro. It may not 1767be used as a macro name, macro argument name, or within a different type 1768of macro. It may also be forbidden in open text; the standard is 1769ambiguous. We recommend you avoid using it except for its defined 1770purpose. 1771 1772Variadic macros are a new feature in C99. GNU CPP has supported them 1773for a long time, but only with a named variable argument 1774(@samp{args@dots{}}, not @samp{@dots{}} and @code{@w{__VA_ARGS__}}). If you are 1775concerned with portability to previous versions of GCC, you should use 1776only named variable arguments. On the other hand, if you are concerned 1777with portability to other conforming implementations of C99, you should 1778use only @code{@w{__VA_ARGS__}}. 1779 1780Previous versions of CPP implemented the comma-deletion extension 1781much more generally. We have restricted it in this release to minimize 1782the differences from C99. To get the same effect with both this and 1783previous versions of GCC, the token preceding the special @samp{##} must 1784be a comma, and there must be white space between that comma and 1785whatever comes immediately before it: 1786 1787@smallexample 1788#define eprintf(format, args@dots{}) fprintf (stderr, format , ##args) 1789@end smallexample 1790 1791@noindent 1792@xref{Differences from previous versions}, for the gory details. 1793 1794@node Predefined Macros 1795@section Predefined Macros 1796 1797@cindex predefined macros 1798Several object-like macros are predefined; you use them without 1799supplying their definitions. They fall into three classes: standard, 1800common, and system-specific. 1801 1802In C++, there is a fourth category, the named operators. They act like 1803predefined macros, but you cannot undefine them. 1804 1805@menu 1806* Standard Predefined Macros:: 1807* Common Predefined Macros:: 1808* System-specific Predefined Macros:: 1809* C++ Named Operators:: 1810@end menu 1811 1812@node Standard Predefined Macros 1813@subsection Standard Predefined Macros 1814@cindex standard predefined macros. 1815 1816The standard predefined macros are specified by the relevant 1817language standards, so they are available with all compilers that 1818implement those standards. Older compilers may not provide all of 1819them. Their names all start with double underscores. 1820 1821@table @code 1822@item __FILE__ 1823This macro expands to the name of the current input file, in the form of 1824a C string constant. This is the path by which the preprocessor opened 1825the file, not the short name specified in @samp{#include} or as the 1826input file name argument. For example, 1827@code{"/usr/local/include/myheader.h"} is a possible expansion of this 1828macro. 1829 1830@item __LINE__ 1831This macro expands to the current input line number, in the form of a 1832decimal integer constant. While we call it a predefined macro, it's 1833a pretty strange macro, since its ``definition'' changes with each 1834new line of source code. 1835@end table 1836 1837@code{__FILE__} and @code{__LINE__} are useful in generating an error 1838message to report an inconsistency detected by the program; the message 1839can state the source line at which the inconsistency was detected. For 1840example, 1841 1842@smallexample 1843fprintf (stderr, "Internal error: " 1844 "negative string length " 1845 "%d at %s, line %d.", 1846 length, __FILE__, __LINE__); 1847@end smallexample 1848 1849An @samp{#include} directive changes the expansions of @code{__FILE__} 1850and @code{__LINE__} to correspond to the included file. At the end of 1851that file, when processing resumes on the input file that contained 1852the @samp{#include} directive, the expansions of @code{__FILE__} and 1853@code{__LINE__} revert to the values they had before the 1854@samp{#include} (but @code{__LINE__} is then incremented by one as 1855processing moves to the line after the @samp{#include}). 1856 1857A @samp{#line} directive changes @code{__LINE__}, and may change 1858@code{__FILE__} as well. @xref{Line Control}. 1859 1860C99 introduces @code{__func__}, and GCC has provided @code{__FUNCTION__} 1861for a long time. Both of these are strings containing the name of the 1862current function (there are slight semantic differences; see the GCC 1863manual). Neither of them is a macro; the preprocessor does not know the 1864name of the current function. They tend to be useful in conjunction 1865with @code{__FILE__} and @code{__LINE__}, though. 1866 1867@table @code 1868 1869@item __DATE__ 1870This macro expands to a string constant that describes the date on which 1871the preprocessor is being run. The string constant contains eleven 1872characters and looks like @code{@w{"Feb 12 1996"}}. If the day of the 1873month is less than 10, it is padded with a space on the left. 1874 1875If GCC cannot determine the current date, it will emit a warning message 1876(once per compilation) and @code{__DATE__} will expand to 1877@code{@w{"??? ?? ????"}}. 1878 1879@item __TIME__ 1880This macro expands to a string constant that describes the time at 1881which the preprocessor is being run. The string constant contains 1882eight characters and looks like @code{"23:59:01"}. 1883 1884If GCC cannot determine the current time, it will emit a warning message 1885(once per compilation) and @code{__TIME__} will expand to 1886@code{"??:??:??"}. 1887 1888@item __STDC__ 1889In normal operation, this macro expands to the constant 1, to signify 1890that this compiler conforms to ISO Standard C@. If GNU CPP is used with 1891a compiler other than GCC, this is not necessarily true; however, the 1892preprocessor always conforms to the standard unless the 1893@option{-traditional-cpp} option is used. 1894 1895This macro is not defined if the @option{-traditional-cpp} option is used. 1896 1897On some hosts, the system compiler uses a different convention, where 1898@code{__STDC__} is normally 0, but is 1 if the user specifies strict 1899conformance to the C Standard. CPP follows the host convention when 1900processing system header files, but when processing user files 1901@code{__STDC__} is always 1. This has been reported to cause problems; 1902for instance, some versions of Solaris provide X Windows headers that 1903expect @code{__STDC__} to be either undefined or 1. @xref{Invocation}. 1904 1905@item __STDC_VERSION__ 1906This macro expands to the C Standard's version number, a long integer 1907constant of the form @code{@var{yyyy}@var{mm}L} where @var{yyyy} and 1908@var{mm} are the year and month of the Standard version. This signifies 1909which version of the C Standard the compiler conforms to. Like 1910@code{__STDC__}, this is not necessarily accurate for the entire 1911implementation, unless GNU CPP is being used with GCC@. 1912 1913The value @code{199409L} signifies the 1989 C standard as amended in 19141994, which is the current default; the value @code{199901L} signifies 1915the 1999 revision of the C standard. Support for the 1999 revision is 1916not yet complete. 1917 1918This macro is not defined if the @option{-traditional-cpp} option is 1919used, nor when compiling C++ or Objective-C@. 1920 1921@item __STDC_HOSTED__ 1922This macro is defined, with value 1, if the compiler's target is a 1923@dfn{hosted environment}. A hosted environment has the complete 1924facilities of the standard C library available. 1925 1926@item __cplusplus 1927This macro is defined when the C++ compiler is in use. You can use 1928@code{__cplusplus} to test whether a header is compiled by a C compiler 1929or a C++ compiler. This macro is similar to @code{__STDC_VERSION__}, in 1930that it expands to a version number. Depending on the language standard 1931selected, the value of the macro is @code{199711L}, as mandated by the 19321998 C++ standard; @code{201103L}, per the 2011 C++ standard; an 1933unspecified value strictly larger than @code{201103L} for the experimental 1934languages enabled by @option{-std=c++1y} and @option{-std=gnu++1y}. 1935 1936@item __OBJC__ 1937This macro is defined, with value 1, when the Objective-C compiler is in 1938use. You can use @code{__OBJC__} to test whether a header is compiled 1939by a C compiler or an Objective-C compiler. 1940 1941@item __ASSEMBLER__ 1942This macro is defined with value 1 when preprocessing assembly 1943language. 1944 1945@end table 1946 1947@node Common Predefined Macros 1948@subsection Common Predefined Macros 1949@cindex common predefined macros 1950 1951The common predefined macros are GNU C extensions. They are available 1952with the same meanings regardless of the machine or operating system on 1953which you are using GNU C or GNU Fortran. Their names all start with 1954double underscores. 1955 1956@table @code 1957 1958@item __COUNTER__ 1959This macro expands to sequential integral values starting from 0. In 1960conjunction with the @code{##} operator, this provides a convenient means to 1961generate unique identifiers. Care must be taken to ensure that 1962@code{__COUNTER__} is not expanded prior to inclusion of precompiled headers 1963which use it. Otherwise, the precompiled headers will not be used. 1964 1965@item __GFORTRAN__ 1966The GNU Fortran compiler defines this. 1967 1968@item __GNUC__ 1969@itemx __GNUC_MINOR__ 1970@itemx __GNUC_PATCHLEVEL__ 1971These macros are defined by all GNU compilers that use the C 1972preprocessor: C, C++, Objective-C and Fortran. Their values are the major 1973version, minor version, and patch level of the compiler, as integer 1974constants. For example, GCC 3.2.1 will define @code{__GNUC__} to 3, 1975@code{__GNUC_MINOR__} to 2, and @code{__GNUC_PATCHLEVEL__} to 1. These 1976macros are also defined if you invoke the preprocessor directly. 1977 1978@code{__GNUC_PATCHLEVEL__} is new to GCC 3.0; it is also present in the 1979widely-used development snapshots leading up to 3.0 (which identify 1980themselves as GCC 2.96 or 2.97, depending on which snapshot you have). 1981 1982If all you need to know is whether or not your program is being compiled 1983by GCC, or a non-GCC compiler that claims to accept the GNU C dialects, 1984you can simply test @code{__GNUC__}. If you need to write code 1985which depends on a specific version, you must be more careful. Each 1986time the minor version is increased, the patch level is reset to zero; 1987each time the major version is increased (which happens rarely), the 1988minor version and patch level are reset. If you wish to use the 1989predefined macros directly in the conditional, you will need to write it 1990like this: 1991 1992@smallexample 1993/* @r{Test for GCC > 3.2.0} */ 1994#if __GNUC__ > 3 || \ 1995 (__GNUC__ == 3 && (__GNUC_MINOR__ > 2 || \ 1996 (__GNUC_MINOR__ == 2 && \ 1997 __GNUC_PATCHLEVEL__ > 0)) 1998@end smallexample 1999 2000@noindent 2001Another approach is to use the predefined macros to 2002calculate a single number, then compare that against a threshold: 2003 2004@smallexample 2005#define GCC_VERSION (__GNUC__ * 10000 \ 2006 + __GNUC_MINOR__ * 100 \ 2007 + __GNUC_PATCHLEVEL__) 2008@dots{} 2009/* @r{Test for GCC > 3.2.0} */ 2010#if GCC_VERSION > 30200 2011@end smallexample 2012 2013@noindent 2014Many people find this form easier to understand. 2015 2016@item __GNUG__ 2017The GNU C++ compiler defines this. Testing it is equivalent to 2018testing @code{@w{(__GNUC__ && __cplusplus)}}. 2019 2020@item __STRICT_ANSI__ 2021GCC defines this macro if and only if the @option{-ansi} switch, or a 2022@option{-std} switch specifying strict conformance to some version of ISO C 2023or ISO C++, was specified when GCC was invoked. It is defined to @samp{1}. 2024This macro exists primarily to direct GNU libc's header files to 2025restrict their definitions to the minimal set found in the 1989 C 2026standard. 2027 2028@item __BASE_FILE__ 2029This macro expands to the name of the main input file, in the form 2030of a C string constant. This is the source file that was specified 2031on the command line of the preprocessor or C compiler. 2032 2033@item __INCLUDE_LEVEL__ 2034This macro expands to a decimal integer constant that represents the 2035depth of nesting in include files. The value of this macro is 2036incremented on every @samp{#include} directive and decremented at the 2037end of every included file. It starts out at 0, its value within the 2038base file specified on the command line. 2039 2040@item __ELF__ 2041This macro is defined if the target uses the ELF object format. 2042 2043@item __VERSION__ 2044This macro expands to a string constant which describes the version of 2045the compiler in use. You should not rely on its contents having any 2046particular form, but it can be counted on to contain at least the 2047release number. 2048 2049@item __OPTIMIZE__ 2050@itemx __OPTIMIZE_SIZE__ 2051@itemx __NO_INLINE__ 2052These macros describe the compilation mode. @code{__OPTIMIZE__} is 2053defined in all optimizing compilations. @code{__OPTIMIZE_SIZE__} is 2054defined if the compiler is optimizing for size, not speed. 2055@code{__NO_INLINE__} is defined if no functions will be inlined into 2056their callers (when not optimizing, or when inlining has been 2057specifically disabled by @option{-fno-inline}). 2058 2059These macros cause certain GNU header files to provide optimized 2060definitions, using macros or inline functions, of system library 2061functions. You should not use these macros in any way unless you make 2062sure that programs will execute with the same effect whether or not they 2063are defined. If they are defined, their value is 1. 2064 2065@item __GNUC_GNU_INLINE__ 2066GCC defines this macro if functions declared @code{inline} will be 2067handled in GCC's traditional gnu90 mode. Object files will contain 2068externally visible definitions of all functions declared @code{inline} 2069without @code{extern} or @code{static}. They will not contain any 2070definitions of any functions declared @code{extern inline}. 2071 2072@item __GNUC_STDC_INLINE__ 2073GCC defines this macro if functions declared @code{inline} will be 2074handled according to the ISO C99 standard. Object files will contain 2075externally visible definitions of all functions declared @code{extern 2076inline}. They will not contain definitions of any functions declared 2077@code{inline} without @code{extern}. 2078 2079If this macro is defined, GCC supports the @code{gnu_inline} function 2080attribute as a way to always get the gnu90 behavior. Support for 2081this and @code{__GNUC_GNU_INLINE__} was added in GCC 4.1.3. If 2082neither macro is defined, an older version of GCC is being used: 2083@code{inline} functions will be compiled in gnu90 mode, and the 2084@code{gnu_inline} function attribute will not be recognized. 2085 2086@item __CHAR_UNSIGNED__ 2087GCC defines this macro if and only if the data type @code{char} is 2088unsigned on the target machine. It exists to cause the standard header 2089file @file{limits.h} to work correctly. You should not use this macro 2090yourself; instead, refer to the standard macros defined in @file{limits.h}. 2091 2092@item __WCHAR_UNSIGNED__ 2093Like @code{__CHAR_UNSIGNED__}, this macro is defined if and only if the 2094data type @code{wchar_t} is unsigned and the front-end is in C++ mode. 2095 2096@item __REGISTER_PREFIX__ 2097This macro expands to a single token (not a string constant) which is 2098the prefix applied to CPU register names in assembly language for this 2099target. You can use it to write assembly that is usable in multiple 2100environments. For example, in the @code{m68k-aout} environment it 2101expands to nothing, but in the @code{m68k-coff} environment it expands 2102to a single @samp{%}. 2103 2104@item __USER_LABEL_PREFIX__ 2105This macro expands to a single token which is the prefix applied to 2106user labels (symbols visible to C code) in assembly. For example, in 2107the @code{m68k-aout} environment it expands to an @samp{_}, but in the 2108@code{m68k-coff} environment it expands to nothing. 2109 2110This macro will have the correct definition even if 2111@option{-f(no-)underscores} is in use, but it will not be correct if 2112target-specific options that adjust this prefix are used (e.g.@: the 2113OSF/rose @option{-mno-underscores} option). 2114 2115@item __SIZE_TYPE__ 2116@itemx __PTRDIFF_TYPE__ 2117@itemx __WCHAR_TYPE__ 2118@itemx __WINT_TYPE__ 2119@itemx __INTMAX_TYPE__ 2120@itemx __UINTMAX_TYPE__ 2121@itemx __SIG_ATOMIC_TYPE__ 2122@itemx __INT8_TYPE__ 2123@itemx __INT16_TYPE__ 2124@itemx __INT32_TYPE__ 2125@itemx __INT64_TYPE__ 2126@itemx __UINT8_TYPE__ 2127@itemx __UINT16_TYPE__ 2128@itemx __UINT32_TYPE__ 2129@itemx __UINT64_TYPE__ 2130@itemx __INT_LEAST8_TYPE__ 2131@itemx __INT_LEAST16_TYPE__ 2132@itemx __INT_LEAST32_TYPE__ 2133@itemx __INT_LEAST64_TYPE__ 2134@itemx __UINT_LEAST8_TYPE__ 2135@itemx __UINT_LEAST16_TYPE__ 2136@itemx __UINT_LEAST32_TYPE__ 2137@itemx __UINT_LEAST64_TYPE__ 2138@itemx __INT_FAST8_TYPE__ 2139@itemx __INT_FAST16_TYPE__ 2140@itemx __INT_FAST32_TYPE__ 2141@itemx __INT_FAST64_TYPE__ 2142@itemx __UINT_FAST8_TYPE__ 2143@itemx __UINT_FAST16_TYPE__ 2144@itemx __UINT_FAST32_TYPE__ 2145@itemx __UINT_FAST64_TYPE__ 2146@itemx __INTPTR_TYPE__ 2147@itemx __UINTPTR_TYPE__ 2148These macros are defined to the correct underlying types for the 2149@code{size_t}, @code{ptrdiff_t}, @code{wchar_t}, @code{wint_t}, 2150@code{intmax_t}, @code{uintmax_t}, @code{sig_atomic_t}, @code{int8_t}, 2151@code{int16_t}, @code{int32_t}, @code{int64_t}, @code{uint8_t}, 2152@code{uint16_t}, @code{uint32_t}, @code{uint64_t}, 2153@code{int_least8_t}, @code{int_least16_t}, @code{int_least32_t}, 2154@code{int_least64_t}, @code{uint_least8_t}, @code{uint_least16_t}, 2155@code{uint_least32_t}, @code{uint_least64_t}, @code{int_fast8_t}, 2156@code{int_fast16_t}, @code{int_fast32_t}, @code{int_fast64_t}, 2157@code{uint_fast8_t}, @code{uint_fast16_t}, @code{uint_fast32_t}, 2158@code{uint_fast64_t}, @code{intptr_t}, and @code{uintptr_t} typedefs, 2159respectively. They exist to make the standard header files 2160@file{stddef.h}, @file{stdint.h}, and @file{wchar.h} work correctly. 2161You should not use these macros directly; instead, include the 2162appropriate headers and use the typedefs. Some of these macros may 2163not be defined on particular systems if GCC does not provide a 2164@file{stdint.h} header on those systems. 2165 2166@item __CHAR_BIT__ 2167Defined to the number of bits used in the representation of the 2168@code{char} data type. It exists to make the standard header given 2169numerical limits work correctly. You should not use 2170this macro directly; instead, include the appropriate headers. 2171 2172@item __SCHAR_MAX__ 2173@itemx __WCHAR_MAX__ 2174@itemx __SHRT_MAX__ 2175@itemx __INT_MAX__ 2176@itemx __LONG_MAX__ 2177@itemx __LONG_LONG_MAX__ 2178@itemx __WINT_MAX__ 2179@itemx __SIZE_MAX__ 2180@itemx __PTRDIFF_MAX__ 2181@itemx __INTMAX_MAX__ 2182@itemx __UINTMAX_MAX__ 2183@itemx __SIG_ATOMIC_MAX__ 2184@itemx __INT8_MAX__ 2185@itemx __INT16_MAX__ 2186@itemx __INT32_MAX__ 2187@itemx __INT64_MAX__ 2188@itemx __UINT8_MAX__ 2189@itemx __UINT16_MAX__ 2190@itemx __UINT32_MAX__ 2191@itemx __UINT64_MAX__ 2192@itemx __INT_LEAST8_MAX__ 2193@itemx __INT_LEAST16_MAX__ 2194@itemx __INT_LEAST32_MAX__ 2195@itemx __INT_LEAST64_MAX__ 2196@itemx __UINT_LEAST8_MAX__ 2197@itemx __UINT_LEAST16_MAX__ 2198@itemx __UINT_LEAST32_MAX__ 2199@itemx __UINT_LEAST64_MAX__ 2200@itemx __INT_FAST8_MAX__ 2201@itemx __INT_FAST16_MAX__ 2202@itemx __INT_FAST32_MAX__ 2203@itemx __INT_FAST64_MAX__ 2204@itemx __UINT_FAST8_MAX__ 2205@itemx __UINT_FAST16_MAX__ 2206@itemx __UINT_FAST32_MAX__ 2207@itemx __UINT_FAST64_MAX__ 2208@itemx __INTPTR_MAX__ 2209@itemx __UINTPTR_MAX__ 2210@itemx __WCHAR_MIN__ 2211@itemx __WINT_MIN__ 2212@itemx __SIG_ATOMIC_MIN__ 2213Defined to the maximum value of the @code{signed char}, @code{wchar_t}, 2214@code{signed short}, 2215@code{signed int}, @code{signed long}, @code{signed long long}, 2216@code{wint_t}, @code{size_t}, @code{ptrdiff_t}, 2217@code{intmax_t}, @code{uintmax_t}, @code{sig_atomic_t}, @code{int8_t}, 2218@code{int16_t}, @code{int32_t}, @code{int64_t}, @code{uint8_t}, 2219@code{uint16_t}, @code{uint32_t}, @code{uint64_t}, 2220@code{int_least8_t}, @code{int_least16_t}, @code{int_least32_t}, 2221@code{int_least64_t}, @code{uint_least8_t}, @code{uint_least16_t}, 2222@code{uint_least32_t}, @code{uint_least64_t}, @code{int_fast8_t}, 2223@code{int_fast16_t}, @code{int_fast32_t}, @code{int_fast64_t}, 2224@code{uint_fast8_t}, @code{uint_fast16_t}, @code{uint_fast32_t}, 2225@code{uint_fast64_t}, @code{intptr_t}, and @code{uintptr_t} types and 2226to the minimum value of the @code{wchar_t}, @code{wint_t}, and 2227@code{sig_atomic_t} types respectively. They exist to make the 2228standard header given numerical limits work correctly. You should not 2229use these macros directly; instead, include the appropriate headers. 2230Some of these macros may not be defined on particular systems if GCC 2231does not provide a @file{stdint.h} header on those systems. 2232 2233@item __INT8_C 2234@itemx __INT16_C 2235@itemx __INT32_C 2236@itemx __INT64_C 2237@itemx __UINT8_C 2238@itemx __UINT16_C 2239@itemx __UINT32_C 2240@itemx __UINT64_C 2241@itemx __INTMAX_C 2242@itemx __UINTMAX_C 2243Defined to implementations of the standard @file{stdint.h} macros with 2244the same names without the leading @code{__}. They exist the make the 2245implementation of that header work correctly. You should not use 2246these macros directly; instead, include the appropriate headers. Some 2247of these macros may not be defined on particular systems if GCC does 2248not provide a @file{stdint.h} header on those systems. 2249 2250@item __SIZEOF_INT__ 2251@itemx __SIZEOF_LONG__ 2252@itemx __SIZEOF_LONG_LONG__ 2253@itemx __SIZEOF_SHORT__ 2254@itemx __SIZEOF_POINTER__ 2255@itemx __SIZEOF_FLOAT__ 2256@itemx __SIZEOF_DOUBLE__ 2257@itemx __SIZEOF_LONG_DOUBLE__ 2258@itemx __SIZEOF_SIZE_T__ 2259@itemx __SIZEOF_WCHAR_T__ 2260@itemx __SIZEOF_WINT_T__ 2261@itemx __SIZEOF_PTRDIFF_T__ 2262Defined to the number of bytes of the C standard data types: @code{int}, 2263@code{long}, @code{long long}, @code{short}, @code{void *}, @code{float}, 2264@code{double}, @code{long double}, @code{size_t}, @code{wchar_t}, @code{wint_t} 2265and @code{ptrdiff_t}. 2266 2267@item __BYTE_ORDER__ 2268@itemx __ORDER_LITTLE_ENDIAN__ 2269@itemx __ORDER_BIG_ENDIAN__ 2270@itemx __ORDER_PDP_ENDIAN__ 2271@code{__BYTE_ORDER__} is defined to one of the values 2272@code{__ORDER_LITTLE_ENDIAN__}, @code{__ORDER_BIG_ENDIAN__}, or 2273@code{__ORDER_PDP_ENDIAN__} to reflect the layout of multi-byte and 2274multi-word quantities in memory. If @code{__BYTE_ORDER__} is equal to 2275@code{__ORDER_LITTLE_ENDIAN__} or @code{__ORDER_BIG_ENDIAN__}, then 2276multi-byte and multi-word quantities are laid out identically: the 2277byte (word) at the lowest address is the least significant or most 2278significant byte (word) of the quantity, respectively. If 2279@code{__BYTE_ORDER__} is equal to @code{__ORDER_PDP_ENDIAN__}, then 2280bytes in 16-bit words are laid out in a little-endian fashion, whereas 2281the 16-bit subwords of a 32-bit quantity are laid out in big-endian 2282fashion. 2283 2284You should use these macros for testing like this: 2285 2286@smallexample 2287/* @r{Test for a little-endian machine} */ 2288#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ 2289@end smallexample 2290 2291@item __FLOAT_WORD_ORDER__ 2292@code{__FLOAT_WORD_ORDER__} is defined to one of the values 2293@code{__ORDER_LITTLE_ENDIAN__} or @code{__ORDER_BIG_ENDIAN__} to reflect 2294the layout of the words of multi-word floating-point quantities. 2295 2296@item __DEPRECATED 2297This macro is defined, with value 1, when compiling a C++ source file 2298with warnings about deprecated constructs enabled. These warnings are 2299enabled by default, but can be disabled with @option{-Wno-deprecated}. 2300 2301@item __EXCEPTIONS 2302This macro is defined, with value 1, when compiling a C++ source file 2303with exceptions enabled. If @option{-fno-exceptions} is used when 2304compiling the file, then this macro is not defined. 2305 2306@item __GXX_RTTI 2307This macro is defined, with value 1, when compiling a C++ source file 2308with runtime type identification enabled. If @option{-fno-rtti} is 2309used when compiling the file, then this macro is not defined. 2310 2311@item __USING_SJLJ_EXCEPTIONS__ 2312This macro is defined, with value 1, if the compiler uses the old 2313mechanism based on @code{setjmp} and @code{longjmp} for exception 2314handling. 2315 2316@item __GXX_EXPERIMENTAL_CXX0X__ 2317This macro is defined when compiling a C++ source file with the option 2318@option{-std=c++0x} or @option{-std=gnu++0x}. It indicates that some 2319features likely to be included in C++0x are available. Note that these 2320features are experimental, and may change or be removed in future 2321versions of GCC. 2322 2323@item __GXX_WEAK__ 2324This macro is defined when compiling a C++ source file. It has the 2325value 1 if the compiler will use weak symbols, COMDAT sections, or 2326other similar techniques to collapse symbols with ``vague linkage'' 2327that are defined in multiple translation units. If the compiler will 2328not collapse such symbols, this macro is defined with value 0. In 2329general, user code should not need to make use of this macro; the 2330purpose of this macro is to ease implementation of the C++ runtime 2331library provided with G++. 2332 2333@item __NEXT_RUNTIME__ 2334This macro is defined, with value 1, if (and only if) the NeXT runtime 2335(as in @option{-fnext-runtime}) is in use for Objective-C@. If the GNU 2336runtime is used, this macro is not defined, so that you can use this 2337macro to determine which runtime (NeXT or GNU) is being used. 2338 2339@item __LP64__ 2340@itemx _LP64 2341These macros are defined, with value 1, if (and only if) the compilation 2342is for a target where @code{long int} and pointer both use 64-bits and 2343@code{int} uses 32-bit. 2344 2345@item __SSP__ 2346This macro is defined, with value 1, when @option{-fstack-protector} is in 2347use. 2348 2349@item __SSP_ALL__ 2350This macro is defined, with value 2, when @option{-fstack-protector-all} is 2351in use. 2352 2353@item __SSP_STRONG__ 2354This macro is defined, with value 3, when @option{-fstack-protector-strong} is 2355in use. 2356 2357@item __SSP_EXPLICIT__ 2358This macro is defined, with value 4, when @option{-fstack-protector-explicit} is 2359in use. 2360 2361@item __SANITIZE_ADDRESS__ 2362This macro is defined, with value 1, when @option{-fsanitize=address} 2363or @option{-fsanitize=kernel-address} are in use. 2364 2365@item __TIMESTAMP__ 2366This macro expands to a string constant that describes the date and time 2367of the last modification of the current source file. The string constant 2368contains abbreviated day of the week, month, day of the month, time in 2369hh:mm:ss form, year and looks like @code{@w{"Sun Sep 16 01:03:52 1973"}}. 2370If the day of the month is less than 10, it is padded with a space on the left. 2371 2372If GCC cannot determine the current date, it will emit a warning message 2373(once per compilation) and @code{__TIMESTAMP__} will expand to 2374@code{@w{"??? ??? ?? ??:??:?? ????"}}. 2375 2376@item __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1 2377@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2 2378@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 2379@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8 2380@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 2381These macros are defined when the target processor supports atomic compare 2382and swap operations on operands 1, 2, 4, 8 or 16 bytes in length, respectively. 2383 2384@item __GCC_HAVE_DWARF2_CFI_ASM 2385This macro is defined when the compiler is emitting Dwarf2 CFI directives 2386to the assembler. When this is defined, it is possible to emit those same 2387directives in inline assembly. 2388 2389@item __FP_FAST_FMA 2390@itemx __FP_FAST_FMAF 2391@itemx __FP_FAST_FMAL 2392These macros are defined with value 1 if the backend supports the 2393@code{fma}, @code{fmaf}, and @code{fmal} builtin functions, so that 2394the include file @file{math.h} can define the macros 2395@code{FP_FAST_FMA}, @code{FP_FAST_FMAF}, and @code{FP_FAST_FMAL} 2396for compatibility with the 1999 C standard. 2397 2398@item __GCC_IEC_559 2399This macro is defined to indicate the intended level of support for 2400IEEE 754 (IEC 60559) floating-point arithmetic. It expands to a 2401nonnegative integer value. If 0, it indicates that the combination of 2402the compiler configuration and the command-line options is not 2403intended to support IEEE 754 arithmetic for @code{float} and 2404@code{double} as defined in C99 and C11 Annex F (for example, that the 2405standard rounding modes and exceptions are not supported, or that 2406optimizations are enabled that conflict with IEEE 754 semantics). If 24071, it indicates that IEEE 754 arithmetic is intended to be supported; 2408this does not mean that all relevant language features are supported 2409by GCC. If 2 or more, it additionally indicates support for IEEE 2410754-2008 (in particular, that the binary encodings for quiet and 2411signaling NaNs are as specified in IEEE 754-2008). 2412 2413This macro does not indicate the default state of command-line options 2414that control optimizations that C99 and C11 permit to be controlled by 2415standard pragmas, where those standards do not require a particular 2416default state. It does not indicate whether optimizations respect 2417signaling NaN semantics (the macro for that is 2418@code{__SUPPORT_SNAN__}). It does not indicate support for decimal 2419floating point or the IEEE 754 binary16 and binary128 types. 2420 2421@item __GCC_IEC_559_COMPLEX 2422This macro is defined to indicate the intended level of support for 2423IEEE 754 (IEC 60559) floating-point arithmetic for complex numbers, as 2424defined in C99 and C11 Annex G. It expands to a nonnegative integer 2425value. If 0, it indicates that the combination of the compiler 2426configuration and the command-line options is not intended to support 2427Annex G requirements (for example, because @option{-fcx-limited-range} 2428was used). If 1 or more, it indicates that it is intended to support 2429those requirements; this does not mean that all relevant language 2430features are supported by GCC. 2431 2432@item __NO_MATH_ERRNO__ 2433This macro is defined if @option{-fno-math-errno} is used, or enabled 2434by another option such as @option{-ffast-math} or by default. 2435@end table 2436 2437@node System-specific Predefined Macros 2438@subsection System-specific Predefined Macros 2439 2440@cindex system-specific predefined macros 2441@cindex predefined macros, system-specific 2442@cindex reserved namespace 2443 2444The C preprocessor normally predefines several macros that indicate what 2445type of system and machine is in use. They are obviously different on 2446each target supported by GCC@. This manual, being for all systems and 2447machines, cannot tell you what their names are, but you can use 2448@command{cpp -dM} to see them all. @xref{Invocation}. All system-specific 2449predefined macros expand to a constant value, so you can test them with 2450either @samp{#ifdef} or @samp{#if}. 2451 2452The C standard requires that all system-specific macros be part of the 2453@dfn{reserved namespace}. All names which begin with two underscores, 2454or an underscore and a capital letter, are reserved for the compiler and 2455library to use as they wish. However, historically system-specific 2456macros have had names with no special prefix; for instance, it is common 2457to find @code{unix} defined on Unix systems. For all such macros, GCC 2458provides a parallel macro with two underscores added at the beginning 2459and the end. If @code{unix} is defined, @code{__unix__} will be defined 2460too. There will never be more than two underscores; the parallel of 2461@code{_mips} is @code{__mips__}. 2462 2463When the @option{-ansi} option, or any @option{-std} option that 2464requests strict conformance, is given to the compiler, all the 2465system-specific predefined macros outside the reserved namespace are 2466suppressed. The parallel macros, inside the reserved namespace, remain 2467defined. 2468 2469We are slowly phasing out all predefined macros which are outside the 2470reserved namespace. You should never use them in new programs, and we 2471encourage you to correct older code to use the parallel macros whenever 2472you find it. We don't recommend you use the system-specific macros that 2473are in the reserved namespace, either. It is better in the long run to 2474check specifically for features you need, using a tool such as 2475@command{autoconf}. 2476 2477@node C++ Named Operators 2478@subsection C++ Named Operators 2479@cindex named operators 2480@cindex C++ named operators 2481@cindex @file{iso646.h} 2482 2483In C++, there are eleven keywords which are simply alternate spellings 2484of operators normally written with punctuation. These keywords are 2485treated as such even in the preprocessor. They function as operators in 2486@samp{#if}, and they cannot be defined as macros or poisoned. In C, you 2487can request that those keywords take their C++ meaning by including 2488@file{iso646.h}. That header defines each one as a normal object-like 2489macro expanding to the appropriate punctuator. 2490 2491These are the named operators and their corresponding punctuators: 2492 2493@multitable {Named Operator} {Punctuator} 2494@item Named Operator @tab Punctuator 2495@item @code{and} @tab @code{&&} 2496@item @code{and_eq} @tab @code{&=} 2497@item @code{bitand} @tab @code{&} 2498@item @code{bitor} @tab @code{|} 2499@item @code{compl} @tab @code{~} 2500@item @code{not} @tab @code{!} 2501@item @code{not_eq} @tab @code{!=} 2502@item @code{or} @tab @code{||} 2503@item @code{or_eq} @tab @code{|=} 2504@item @code{xor} @tab @code{^} 2505@item @code{xor_eq} @tab @code{^=} 2506@end multitable 2507 2508@node Undefining and Redefining Macros 2509@section Undefining and Redefining Macros 2510@cindex undefining macros 2511@cindex redefining macros 2512@findex #undef 2513 2514If a macro ceases to be useful, it may be @dfn{undefined} with the 2515@samp{#undef} directive. @samp{#undef} takes a single argument, the 2516name of the macro to undefine. You use the bare macro name, even if the 2517macro is function-like. It is an error if anything appears on the line 2518after the macro name. @samp{#undef} has no effect if the name is not a 2519macro. 2520 2521@smallexample 2522#define FOO 4 2523x = FOO; @expansion{} x = 4; 2524#undef FOO 2525x = FOO; @expansion{} x = FOO; 2526@end smallexample 2527 2528Once a macro has been undefined, that identifier may be @dfn{redefined} 2529as a macro by a subsequent @samp{#define} directive. The new definition 2530need not have any resemblance to the old definition. 2531 2532However, if an identifier which is currently a macro is redefined, then 2533the new definition must be @dfn{effectively the same} as the old one. 2534Two macro definitions are effectively the same if: 2535@itemize @bullet 2536@item Both are the same type of macro (object- or function-like). 2537@item All the tokens of the replacement list are the same. 2538@item If there are any parameters, they are the same. 2539@item Whitespace appears in the same places in both. It need not be 2540exactly the same amount of whitespace, though. Remember that comments 2541count as whitespace. 2542@end itemize 2543 2544@noindent 2545These definitions are effectively the same: 2546@smallexample 2547#define FOUR (2 + 2) 2548#define FOUR (2 + 2) 2549#define FOUR (2 /* @r{two} */ + 2) 2550@end smallexample 2551@noindent 2552but these are not: 2553@smallexample 2554#define FOUR (2 + 2) 2555#define FOUR ( 2+2 ) 2556#define FOUR (2 * 2) 2557#define FOUR(score,and,seven,years,ago) (2 + 2) 2558@end smallexample 2559 2560If a macro is redefined with a definition that is not effectively the 2561same as the old one, the preprocessor issues a warning and changes the 2562macro to use the new definition. If the new definition is effectively 2563the same, the redefinition is silently ignored. This allows, for 2564instance, two different headers to define a common macro. The 2565preprocessor will only complain if the definitions do not match. 2566 2567@node Directives Within Macro Arguments 2568@section Directives Within Macro Arguments 2569@cindex macro arguments and directives 2570 2571Occasionally it is convenient to use preprocessor directives within 2572the arguments of a macro. The C and C++ standards declare that 2573behavior in these cases is undefined. 2574 2575Versions of CPP prior to 3.2 would reject such constructs with an 2576error message. This was the only syntactic difference between normal 2577functions and function-like macros, so it seemed attractive to remove 2578this limitation, and people would often be surprised that they could 2579not use macros in this way. Moreover, sometimes people would use 2580conditional compilation in the argument list to a normal library 2581function like @samp{printf}, only to find that after a library upgrade 2582@samp{printf} had changed to be a function-like macro, and their code 2583would no longer compile. So from version 3.2 we changed CPP to 2584successfully process arbitrary directives within macro arguments in 2585exactly the same way as it would have processed the directive were the 2586function-like macro invocation not present. 2587 2588If, within a macro invocation, that macro is redefined, then the new 2589definition takes effect in time for argument pre-expansion, but the 2590original definition is still used for argument replacement. Here is a 2591pathological example: 2592 2593@smallexample 2594#define f(x) x x 2595f (1 2596#undef f 2597#define f 2 2598f) 2599@end smallexample 2600 2601@noindent 2602which expands to 2603 2604@smallexample 26051 2 1 2 2606@end smallexample 2607 2608@noindent 2609with the semantics described above. 2610 2611@node Macro Pitfalls 2612@section Macro Pitfalls 2613@cindex problems with macros 2614@cindex pitfalls of macros 2615 2616In this section we describe some special rules that apply to macros and 2617macro expansion, and point out certain cases in which the rules have 2618counter-intuitive consequences that you must watch out for. 2619 2620@menu 2621* Misnesting:: 2622* Operator Precedence Problems:: 2623* Swallowing the Semicolon:: 2624* Duplication of Side Effects:: 2625* Self-Referential Macros:: 2626* Argument Prescan:: 2627* Newlines in Arguments:: 2628@end menu 2629 2630@node Misnesting 2631@subsection Misnesting 2632 2633When a macro is called with arguments, the arguments are substituted 2634into the macro body and the result is checked, together with the rest of 2635the input file, for more macro calls. It is possible to piece together 2636a macro call coming partially from the macro body and partially from the 2637arguments. For example, 2638 2639@smallexample 2640#define twice(x) (2*(x)) 2641#define call_with_1(x) x(1) 2642call_with_1 (twice) 2643 @expansion{} twice(1) 2644 @expansion{} (2*(1)) 2645@end smallexample 2646 2647Macro definitions do not have to have balanced parentheses. By writing 2648an unbalanced open parenthesis in a macro body, it is possible to create 2649a macro call that begins inside the macro body but ends outside of it. 2650For example, 2651 2652@smallexample 2653#define strange(file) fprintf (file, "%s %d", 2654@dots{} 2655strange(stderr) p, 35) 2656 @expansion{} fprintf (stderr, "%s %d", p, 35) 2657@end smallexample 2658 2659The ability to piece together a macro call can be useful, but the use of 2660unbalanced open parentheses in a macro body is just confusing, and 2661should be avoided. 2662 2663@node Operator Precedence Problems 2664@subsection Operator Precedence Problems 2665@cindex parentheses in macro bodies 2666 2667You may have noticed that in most of the macro definition examples shown 2668above, each occurrence of a macro argument name had parentheses around 2669it. In addition, another pair of parentheses usually surround the 2670entire macro definition. Here is why it is best to write macros that 2671way. 2672 2673Suppose you define a macro as follows, 2674 2675@smallexample 2676#define ceil_div(x, y) (x + y - 1) / y 2677@end smallexample 2678 2679@noindent 2680whose purpose is to divide, rounding up. (One use for this operation is 2681to compute how many @code{int} objects are needed to hold a certain 2682number of @code{char} objects.) Then suppose it is used as follows: 2683 2684@smallexample 2685a = ceil_div (b & c, sizeof (int)); 2686 @expansion{} a = (b & c + sizeof (int) - 1) / sizeof (int); 2687@end smallexample 2688 2689@noindent 2690This does not do what is intended. The operator-precedence rules of 2691C make it equivalent to this: 2692 2693@smallexample 2694a = (b & (c + sizeof (int) - 1)) / sizeof (int); 2695@end smallexample 2696 2697@noindent 2698What we want is this: 2699 2700@smallexample 2701a = ((b & c) + sizeof (int) - 1)) / sizeof (int); 2702@end smallexample 2703 2704@noindent 2705Defining the macro as 2706 2707@smallexample 2708#define ceil_div(x, y) ((x) + (y) - 1) / (y) 2709@end smallexample 2710 2711@noindent 2712provides the desired result. 2713 2714Unintended grouping can result in another way. Consider @code{sizeof 2715ceil_div(1, 2)}. That has the appearance of a C expression that would 2716compute the size of the type of @code{ceil_div (1, 2)}, but in fact it 2717means something very different. Here is what it expands to: 2718 2719@smallexample 2720sizeof ((1) + (2) - 1) / (2) 2721@end smallexample 2722 2723@noindent 2724This would take the size of an integer and divide it by two. The 2725precedence rules have put the division outside the @code{sizeof} when it 2726was intended to be inside. 2727 2728Parentheses around the entire macro definition prevent such problems. 2729Here, then, is the recommended way to define @code{ceil_div}: 2730 2731@smallexample 2732#define ceil_div(x, y) (((x) + (y) - 1) / (y)) 2733@end smallexample 2734 2735@node Swallowing the Semicolon 2736@subsection Swallowing the Semicolon 2737@cindex semicolons (after macro calls) 2738 2739Often it is desirable to define a macro that expands into a compound 2740statement. Consider, for example, the following macro, that advances a 2741pointer (the argument @code{p} says where to find it) across whitespace 2742characters: 2743 2744@smallexample 2745#define SKIP_SPACES(p, limit) \ 2746@{ char *lim = (limit); \ 2747 while (p < lim) @{ \ 2748 if (*p++ != ' ') @{ \ 2749 p--; break; @}@}@} 2750@end smallexample 2751 2752@noindent 2753Here backslash-newline is used to split the macro definition, which must 2754be a single logical line, so that it resembles the way such code would 2755be laid out if not part of a macro definition. 2756 2757A call to this macro might be @code{SKIP_SPACES (p, lim)}. Strictly 2758speaking, the call expands to a compound statement, which is a complete 2759statement with no need for a semicolon to end it. However, since it 2760looks like a function call, it minimizes confusion if you can use it 2761like a function call, writing a semicolon afterward, as in 2762@code{SKIP_SPACES (p, lim);} 2763 2764This can cause trouble before @code{else} statements, because the 2765semicolon is actually a null statement. Suppose you write 2766 2767@smallexample 2768if (*p != 0) 2769 SKIP_SPACES (p, lim); 2770else @dots{} 2771@end smallexample 2772 2773@noindent 2774The presence of two statements---the compound statement and a null 2775statement---in between the @code{if} condition and the @code{else} 2776makes invalid C code. 2777 2778The definition of the macro @code{SKIP_SPACES} can be altered to solve 2779this problem, using a @code{do @dots{} while} statement. Here is how: 2780 2781@smallexample 2782#define SKIP_SPACES(p, limit) \ 2783do @{ char *lim = (limit); \ 2784 while (p < lim) @{ \ 2785 if (*p++ != ' ') @{ \ 2786 p--; break; @}@}@} \ 2787while (0) 2788@end smallexample 2789 2790Now @code{SKIP_SPACES (p, lim);} expands into 2791 2792@smallexample 2793do @{@dots{}@} while (0); 2794@end smallexample 2795 2796@noindent 2797which is one statement. The loop executes exactly once; most compilers 2798generate no extra code for it. 2799 2800@node Duplication of Side Effects 2801@subsection Duplication of Side Effects 2802 2803@cindex side effects (in macro arguments) 2804@cindex unsafe macros 2805Many C programs define a macro @code{min}, for ``minimum'', like this: 2806 2807@smallexample 2808#define min(X, Y) ((X) < (Y) ? (X) : (Y)) 2809@end smallexample 2810 2811When you use this macro with an argument containing a side effect, 2812as shown here, 2813 2814@smallexample 2815next = min (x + y, foo (z)); 2816@end smallexample 2817 2818@noindent 2819it expands as follows: 2820 2821@smallexample 2822next = ((x + y) < (foo (z)) ? (x + y) : (foo (z))); 2823@end smallexample 2824 2825@noindent 2826where @code{x + y} has been substituted for @code{X} and @code{foo (z)} 2827for @code{Y}. 2828 2829The function @code{foo} is used only once in the statement as it appears 2830in the program, but the expression @code{foo (z)} has been substituted 2831twice into the macro expansion. As a result, @code{foo} might be called 2832two times when the statement is executed. If it has side effects or if 2833it takes a long time to compute, the results might not be what you 2834intended. We say that @code{min} is an @dfn{unsafe} macro. 2835 2836The best solution to this problem is to define @code{min} in a way that 2837computes the value of @code{foo (z)} only once. The C language offers 2838no standard way to do this, but it can be done with GNU extensions as 2839follows: 2840 2841@smallexample 2842#define min(X, Y) \ 2843(@{ typeof (X) x_ = (X); \ 2844 typeof (Y) y_ = (Y); \ 2845 (x_ < y_) ? x_ : y_; @}) 2846@end smallexample 2847 2848The @samp{(@{ @dots{} @})} notation produces a compound statement that 2849acts as an expression. Its value is the value of its last statement. 2850This permits us to define local variables and assign each argument to 2851one. The local variables have underscores after their names to reduce 2852the risk of conflict with an identifier of wider scope (it is impossible 2853to avoid this entirely). Now each argument is evaluated exactly once. 2854 2855If you do not wish to use GNU C extensions, the only solution is to be 2856careful when @emph{using} the macro @code{min}. For example, you can 2857calculate the value of @code{foo (z)}, save it in a variable, and use 2858that variable in @code{min}: 2859 2860@smallexample 2861@group 2862#define min(X, Y) ((X) < (Y) ? (X) : (Y)) 2863@dots{} 2864@{ 2865 int tem = foo (z); 2866 next = min (x + y, tem); 2867@} 2868@end group 2869@end smallexample 2870 2871@noindent 2872(where we assume that @code{foo} returns type @code{int}). 2873 2874@node Self-Referential Macros 2875@subsection Self-Referential Macros 2876@cindex self-reference 2877 2878A @dfn{self-referential} macro is one whose name appears in its 2879definition. Recall that all macro definitions are rescanned for more 2880macros to replace. If the self-reference were considered a use of the 2881macro, it would produce an infinitely large expansion. To prevent this, 2882the self-reference is not considered a macro call. It is passed into 2883the preprocessor output unchanged. Consider an example: 2884 2885@smallexample 2886#define foo (4 + foo) 2887@end smallexample 2888 2889@noindent 2890where @code{foo} is also a variable in your program. 2891 2892Following the ordinary rules, each reference to @code{foo} will expand 2893into @code{(4 + foo)}; then this will be rescanned and will expand into 2894@code{(4 + (4 + foo))}; and so on until the computer runs out of memory. 2895 2896The self-reference rule cuts this process short after one step, at 2897@code{(4 + foo)}. Therefore, this macro definition has the possibly 2898useful effect of causing the program to add 4 to the value of @code{foo} 2899wherever @code{foo} is referred to. 2900 2901In most cases, it is a bad idea to take advantage of this feature. A 2902person reading the program who sees that @code{foo} is a variable will 2903not expect that it is a macro as well. The reader will come across the 2904identifier @code{foo} in the program and think its value should be that 2905of the variable @code{foo}, whereas in fact the value is four greater. 2906 2907One common, useful use of self-reference is to create a macro which 2908expands to itself. If you write 2909 2910@smallexample 2911#define EPERM EPERM 2912@end smallexample 2913 2914@noindent 2915then the macro @code{EPERM} expands to @code{EPERM}. Effectively, it is 2916left alone by the preprocessor whenever it's used in running text. You 2917can tell that it's a macro with @samp{#ifdef}. You might do this if you 2918want to define numeric constants with an @code{enum}, but have 2919@samp{#ifdef} be true for each constant. 2920 2921If a macro @code{x} expands to use a macro @code{y}, and the expansion of 2922@code{y} refers to the macro @code{x}, that is an @dfn{indirect 2923self-reference} of @code{x}. @code{x} is not expanded in this case 2924either. Thus, if we have 2925 2926@smallexample 2927#define x (4 + y) 2928#define y (2 * x) 2929@end smallexample 2930 2931@noindent 2932then @code{x} and @code{y} expand as follows: 2933 2934@smallexample 2935@group 2936x @expansion{} (4 + y) 2937 @expansion{} (4 + (2 * x)) 2938 2939y @expansion{} (2 * x) 2940 @expansion{} (2 * (4 + y)) 2941@end group 2942@end smallexample 2943 2944@noindent 2945Each macro is expanded when it appears in the definition of the other 2946macro, but not when it indirectly appears in its own definition. 2947 2948@node Argument Prescan 2949@subsection Argument Prescan 2950@cindex expansion of arguments 2951@cindex macro argument expansion 2952@cindex prescan of macro arguments 2953 2954Macro arguments are completely macro-expanded before they are 2955substituted into a macro body, unless they are stringified or pasted 2956with other tokens. After substitution, the entire macro body, including 2957the substituted arguments, is scanned again for macros to be expanded. 2958The result is that the arguments are scanned @emph{twice} to expand 2959macro calls in them. 2960 2961Most of the time, this has no effect. If the argument contained any 2962macro calls, they are expanded during the first scan. The result 2963therefore contains no macro calls, so the second scan does not change 2964it. If the argument were substituted as given, with no prescan, the 2965single remaining scan would find the same macro calls and produce the 2966same results. 2967 2968You might expect the double scan to change the results when a 2969self-referential macro is used in an argument of another macro 2970(@pxref{Self-Referential Macros}): the self-referential macro would be 2971expanded once in the first scan, and a second time in the second scan. 2972However, this is not what happens. The self-references that do not 2973expand in the first scan are marked so that they will not expand in the 2974second scan either. 2975 2976You might wonder, ``Why mention the prescan, if it makes no difference? 2977And why not skip it and make the preprocessor faster?'' The answer is 2978that the prescan does make a difference in three special cases: 2979 2980@itemize @bullet 2981@item 2982Nested calls to a macro. 2983 2984We say that @dfn{nested} calls to a macro occur when a macro's argument 2985contains a call to that very macro. For example, if @code{f} is a macro 2986that expects one argument, @code{f (f (1))} is a nested pair of calls to 2987@code{f}. The desired expansion is made by expanding @code{f (1)} and 2988substituting that into the definition of @code{f}. The prescan causes 2989the expected result to happen. Without the prescan, @code{f (1)} itself 2990would be substituted as an argument, and the inner use of @code{f} would 2991appear during the main scan as an indirect self-reference and would not 2992be expanded. 2993 2994@item 2995Macros that call other macros that stringify or concatenate. 2996 2997If an argument is stringified or concatenated, the prescan does not 2998occur. If you @emph{want} to expand a macro, then stringify or 2999concatenate its expansion, you can do that by causing one macro to call 3000another macro that does the stringification or concatenation. For 3001instance, if you have 3002 3003@smallexample 3004#define AFTERX(x) X_ ## x 3005#define XAFTERX(x) AFTERX(x) 3006#define TABLESIZE 1024 3007#define BUFSIZE TABLESIZE 3008@end smallexample 3009 3010then @code{AFTERX(BUFSIZE)} expands to @code{X_BUFSIZE}, and 3011@code{XAFTERX(BUFSIZE)} expands to @code{X_1024}. (Not to 3012@code{X_TABLESIZE}. Prescan always does a complete expansion.) 3013 3014@item 3015Macros used in arguments, whose expansions contain unshielded commas. 3016 3017This can cause a macro expanded on the second scan to be called with the 3018wrong number of arguments. Here is an example: 3019 3020@smallexample 3021#define foo a,b 3022#define bar(x) lose(x) 3023#define lose(x) (1 + (x)) 3024@end smallexample 3025 3026We would like @code{bar(foo)} to turn into @code{(1 + (foo))}, which 3027would then turn into @code{(1 + (a,b))}. Instead, @code{bar(foo)} 3028expands into @code{lose(a,b)}, and you get an error because @code{lose} 3029requires a single argument. In this case, the problem is easily solved 3030by the same parentheses that ought to be used to prevent misnesting of 3031arithmetic operations: 3032 3033@smallexample 3034#define foo (a,b) 3035@exdent or 3036#define bar(x) lose((x)) 3037@end smallexample 3038 3039The extra pair of parentheses prevents the comma in @code{foo}'s 3040definition from being interpreted as an argument separator. 3041 3042@end itemize 3043 3044@node Newlines in Arguments 3045@subsection Newlines in Arguments 3046@cindex newlines in macro arguments 3047 3048The invocation of a function-like macro can extend over many logical 3049lines. However, in the present implementation, the entire expansion 3050comes out on one line. Thus line numbers emitted by the compiler or 3051debugger refer to the line the invocation started on, which might be 3052different to the line containing the argument causing the problem. 3053 3054Here is an example illustrating this: 3055 3056@smallexample 3057#define ignore_second_arg(a,b,c) a; c 3058 3059ignore_second_arg (foo (), 3060 ignored (), 3061 syntax error); 3062@end smallexample 3063 3064@noindent 3065The syntax error triggered by the tokens @code{syntax error} results in 3066an error message citing line three---the line of ignore_second_arg--- 3067even though the problematic code comes from line five. 3068 3069We consider this a bug, and intend to fix it in the near future. 3070 3071@node Conditionals 3072@chapter Conditionals 3073@cindex conditionals 3074 3075A @dfn{conditional} is a directive that instructs the preprocessor to 3076select whether or not to include a chunk of code in the final token 3077stream passed to the compiler. Preprocessor conditionals can test 3078arithmetic expressions, or whether a name is defined as a macro, or both 3079simultaneously using the special @code{defined} operator. 3080 3081A conditional in the C preprocessor resembles in some ways an @code{if} 3082statement in C, but it is important to understand the difference between 3083them. The condition in an @code{if} statement is tested during the 3084execution of your program. Its purpose is to allow your program to 3085behave differently from run to run, depending on the data it is 3086operating on. The condition in a preprocessing conditional directive is 3087tested when your program is compiled. Its purpose is to allow different 3088code to be included in the program depending on the situation at the 3089time of compilation. 3090 3091However, the distinction is becoming less clear. Modern compilers often 3092do test @code{if} statements when a program is compiled, if their 3093conditions are known not to vary at run time, and eliminate code which 3094can never be executed. If you can count on your compiler to do this, 3095you may find that your program is more readable if you use @code{if} 3096statements with constant conditions (perhaps determined by macros). Of 3097course, you can only use this to exclude code, not type definitions or 3098other preprocessing directives, and you can only do it if the code 3099remains syntactically valid when it is not to be used. 3100 3101GCC version 3 eliminates this kind of never-executed code even when 3102not optimizing. Older versions did it only when optimizing. 3103 3104@menu 3105* Conditional Uses:: 3106* Conditional Syntax:: 3107* Deleted Code:: 3108@end menu 3109 3110@node Conditional Uses 3111@section Conditional Uses 3112 3113There are three general reasons to use a conditional. 3114 3115@itemize @bullet 3116@item 3117A program may need to use different code depending on the machine or 3118operating system it is to run on. In some cases the code for one 3119operating system may be erroneous on another operating system; for 3120example, it might refer to data types or constants that do not exist on 3121the other system. When this happens, it is not enough to avoid 3122executing the invalid code. Its mere presence will cause the compiler 3123to reject the program. With a preprocessing conditional, the offending 3124code can be effectively excised from the program when it is not valid. 3125 3126@item 3127You may want to be able to compile the same source file into two 3128different programs. One version might make frequent time-consuming 3129consistency checks on its intermediate data, or print the values of 3130those data for debugging, and the other not. 3131 3132@item 3133A conditional whose condition is always false is one way to exclude code 3134from the program but keep it as a sort of comment for future reference. 3135@end itemize 3136 3137Simple programs that do not need system-specific logic or complex 3138debugging hooks generally will not need to use preprocessing 3139conditionals. 3140 3141@node Conditional Syntax 3142@section Conditional Syntax 3143 3144@findex #if 3145A conditional in the C preprocessor begins with a @dfn{conditional 3146directive}: @samp{#if}, @samp{#ifdef} or @samp{#ifndef}. 3147 3148@menu 3149* Ifdef:: 3150* If:: 3151* Defined:: 3152* Else:: 3153* Elif:: 3154@end menu 3155 3156@node Ifdef 3157@subsection Ifdef 3158@findex #ifdef 3159@findex #endif 3160 3161The simplest sort of conditional is 3162 3163@smallexample 3164@group 3165#ifdef @var{MACRO} 3166 3167@var{controlled text} 3168 3169#endif /* @var{MACRO} */ 3170@end group 3171@end smallexample 3172 3173@cindex conditional group 3174This block is called a @dfn{conditional group}. @var{controlled text} 3175will be included in the output of the preprocessor if and only if 3176@var{MACRO} is defined. We say that the conditional @dfn{succeeds} if 3177@var{MACRO} is defined, @dfn{fails} if it is not. 3178 3179The @var{controlled text} inside of a conditional can include 3180preprocessing directives. They are executed only if the conditional 3181succeeds. You can nest conditional groups inside other conditional 3182groups, but they must be completely nested. In other words, 3183@samp{#endif} always matches the nearest @samp{#ifdef} (or 3184@samp{#ifndef}, or @samp{#if}). Also, you cannot start a conditional 3185group in one file and end it in another. 3186 3187Even if a conditional fails, the @var{controlled text} inside it is 3188still run through initial transformations and tokenization. Therefore, 3189it must all be lexically valid C@. Normally the only way this matters is 3190that all comments and string literals inside a failing conditional group 3191must still be properly ended. 3192 3193The comment following the @samp{#endif} is not required, but it is a 3194good practice if there is a lot of @var{controlled text}, because it 3195helps people match the @samp{#endif} to the corresponding @samp{#ifdef}. 3196Older programs sometimes put @var{MACRO} directly after the 3197@samp{#endif} without enclosing it in a comment. This is invalid code 3198according to the C standard. CPP accepts it with a warning. It 3199never affects which @samp{#ifndef} the @samp{#endif} matches. 3200 3201@findex #ifndef 3202Sometimes you wish to use some code if a macro is @emph{not} defined. 3203You can do this by writing @samp{#ifndef} instead of @samp{#ifdef}. 3204One common use of @samp{#ifndef} is to include code only the first 3205time a header file is included. @xref{Once-Only Headers}. 3206 3207Macro definitions can vary between compilations for several reasons. 3208Here are some samples. 3209 3210@itemize @bullet 3211@item 3212Some macros are predefined on each kind of machine 3213(@pxref{System-specific Predefined Macros}). This allows you to provide 3214code specially tuned for a particular machine. 3215 3216@item 3217System header files define more macros, associated with the features 3218they implement. You can test these macros with conditionals to avoid 3219using a system feature on a machine where it is not implemented. 3220 3221@item 3222Macros can be defined or undefined with the @option{-D} and @option{-U} 3223command-line options when you compile the program. You can arrange to 3224compile the same source file into two different programs by choosing a 3225macro name to specify which program you want, writing conditionals to 3226test whether or how this macro is defined, and then controlling the 3227state of the macro with command-line options, perhaps set in the 3228Makefile. @xref{Invocation}. 3229 3230@item 3231Your program might have a special header file (often called 3232@file{config.h}) that is adjusted when the program is compiled. It can 3233define or not define macros depending on the features of the system and 3234the desired capabilities of the program. The adjustment can be 3235automated by a tool such as @command{autoconf}, or done by hand. 3236@end itemize 3237 3238@node If 3239@subsection If 3240 3241The @samp{#if} directive allows you to test the value of an arithmetic 3242expression, rather than the mere existence of one macro. Its syntax is 3243 3244@smallexample 3245@group 3246#if @var{expression} 3247 3248@var{controlled text} 3249 3250#endif /* @var{expression} */ 3251@end group 3252@end smallexample 3253 3254@var{expression} is a C expression of integer type, subject to stringent 3255restrictions. It may contain 3256 3257@itemize @bullet 3258@item 3259Integer constants. 3260 3261@item 3262Character constants, which are interpreted as they would be in normal 3263code. 3264 3265@item 3266Arithmetic operators for addition, subtraction, multiplication, 3267division, bitwise operations, shifts, comparisons, and logical 3268operations (@code{&&} and @code{||}). The latter two obey the usual 3269short-circuiting rules of standard C@. 3270 3271@item 3272Macros. All macros in the expression are expanded before actual 3273computation of the expression's value begins. 3274 3275@item 3276Uses of the @code{defined} operator, which lets you check whether macros 3277are defined in the middle of an @samp{#if}. 3278 3279@item 3280Identifiers that are not macros, which are all considered to be the 3281number zero. This allows you to write @code{@w{#if MACRO}} instead of 3282@code{@w{#ifdef MACRO}}, if you know that MACRO, when defined, will 3283always have a nonzero value. Function-like macros used without their 3284function call parentheses are also treated as zero. 3285 3286In some contexts this shortcut is undesirable. The @option{-Wundef} 3287option causes GCC to warn whenever it encounters an identifier which is 3288not a macro in an @samp{#if}. 3289@end itemize 3290 3291The preprocessor does not know anything about types in the language. 3292Therefore, @code{sizeof} operators are not recognized in @samp{#if}, and 3293neither are @code{enum} constants. They will be taken as identifiers 3294which are not macros, and replaced by zero. In the case of 3295@code{sizeof}, this is likely to cause the expression to be invalid. 3296 3297The preprocessor calculates the value of @var{expression}. It carries 3298out all calculations in the widest integer type known to the compiler; 3299on most machines supported by GCC this is 64 bits. This is not the same 3300rule as the compiler uses to calculate the value of a constant 3301expression, and may give different results in some cases. If the value 3302comes out to be nonzero, the @samp{#if} succeeds and the @var{controlled 3303text} is included; otherwise it is skipped. 3304 3305@node Defined 3306@subsection Defined 3307 3308@cindex @code{defined} 3309The special operator @code{defined} is used in @samp{#if} and 3310@samp{#elif} expressions to test whether a certain name is defined as a 3311macro. @code{defined @var{name}} and @code{defined (@var{name})} are 3312both expressions whose value is 1 if @var{name} is defined as a macro at 3313the current point in the program, and 0 otherwise. Thus, @code{@w{#if 3314defined MACRO}} is precisely equivalent to @code{@w{#ifdef MACRO}}. 3315 3316@code{defined} is useful when you wish to test more than one macro for 3317existence at once. For example, 3318 3319@smallexample 3320#if defined (__vax__) || defined (__ns16000__) 3321@end smallexample 3322 3323@noindent 3324would succeed if either of the names @code{__vax__} or 3325@code{__ns16000__} is defined as a macro. 3326 3327Conditionals written like this: 3328 3329@smallexample 3330#if defined BUFSIZE && BUFSIZE >= 1024 3331@end smallexample 3332 3333@noindent 3334can generally be simplified to just @code{@w{#if BUFSIZE >= 1024}}, 3335since if @code{BUFSIZE} is not defined, it will be interpreted as having 3336the value zero. 3337 3338If the @code{defined} operator appears as a result of a macro expansion, 3339the C standard says the behavior is undefined. GNU cpp treats it as a 3340genuine @code{defined} operator and evaluates it normally. It will warn 3341wherever your code uses this feature if you use the command-line option 3342@option{-pedantic}, since other compilers may handle it differently. 3343 3344@node Else 3345@subsection Else 3346 3347@findex #else 3348The @samp{#else} directive can be added to a conditional to provide 3349alternative text to be used if the condition fails. This is what it 3350looks like: 3351 3352@smallexample 3353@group 3354#if @var{expression} 3355@var{text-if-true} 3356#else /* Not @var{expression} */ 3357@var{text-if-false} 3358#endif /* Not @var{expression} */ 3359@end group 3360@end smallexample 3361 3362@noindent 3363If @var{expression} is nonzero, the @var{text-if-true} is included and 3364the @var{text-if-false} is skipped. If @var{expression} is zero, the 3365opposite happens. 3366 3367You can use @samp{#else} with @samp{#ifdef} and @samp{#ifndef}, too. 3368 3369@node Elif 3370@subsection Elif 3371 3372@findex #elif 3373One common case of nested conditionals is used to check for more than two 3374possible alternatives. For example, you might have 3375 3376@smallexample 3377#if X == 1 3378@dots{} 3379#else /* X != 1 */ 3380#if X == 2 3381@dots{} 3382#else /* X != 2 */ 3383@dots{} 3384#endif /* X != 2 */ 3385#endif /* X != 1 */ 3386@end smallexample 3387 3388Another conditional directive, @samp{#elif}, allows this to be 3389abbreviated as follows: 3390 3391@smallexample 3392#if X == 1 3393@dots{} 3394#elif X == 2 3395@dots{} 3396#else /* X != 2 and X != 1*/ 3397@dots{} 3398#endif /* X != 2 and X != 1*/ 3399@end smallexample 3400 3401@samp{#elif} stands for ``else if''. Like @samp{#else}, it goes in the 3402middle of a conditional group and subdivides it; it does not require a 3403matching @samp{#endif} of its own. Like @samp{#if}, the @samp{#elif} 3404directive includes an expression to be tested. The text following the 3405@samp{#elif} is processed only if the original @samp{#if}-condition 3406failed and the @samp{#elif} condition succeeds. 3407 3408More than one @samp{#elif} can go in the same conditional group. Then 3409the text after each @samp{#elif} is processed only if the @samp{#elif} 3410condition succeeds after the original @samp{#if} and all previous 3411@samp{#elif} directives within it have failed. 3412 3413@samp{#else} is allowed after any number of @samp{#elif} directives, but 3414@samp{#elif} may not follow @samp{#else}. 3415 3416@node Deleted Code 3417@section Deleted Code 3418@cindex commenting out code 3419 3420If you replace or delete a part of the program but want to keep the old 3421code around for future reference, you often cannot simply comment it 3422out. Block comments do not nest, so the first comment inside the old 3423code will end the commenting-out. The probable result is a flood of 3424syntax errors. 3425 3426One way to avoid this problem is to use an always-false conditional 3427instead. For instance, put @code{#if 0} before the deleted code and 3428@code{#endif} after it. This works even if the code being turned 3429off contains conditionals, but they must be entire conditionals 3430(balanced @samp{#if} and @samp{#endif}). 3431 3432Some people use @code{#ifdef notdef} instead. This is risky, because 3433@code{notdef} might be accidentally defined as a macro, and then the 3434conditional would succeed. @code{#if 0} can be counted on to fail. 3435 3436Do not use @code{#if 0} for comments which are not C code. Use a real 3437comment, instead. The interior of @code{#if 0} must consist of complete 3438tokens; in particular, single-quote characters must balance. Comments 3439often contain unbalanced single-quote characters (known in English as 3440apostrophes). These confuse @code{#if 0}. They don't confuse 3441@samp{/*}. 3442 3443@node Diagnostics 3444@chapter Diagnostics 3445@cindex diagnostic 3446@cindex reporting errors 3447@cindex reporting warnings 3448 3449@findex #error 3450The directive @samp{#error} causes the preprocessor to report a fatal 3451error. The tokens forming the rest of the line following @samp{#error} 3452are used as the error message. 3453 3454You would use @samp{#error} inside of a conditional that detects a 3455combination of parameters which you know the program does not properly 3456support. For example, if you know that the program will not run 3457properly on a VAX, you might write 3458 3459@smallexample 3460@group 3461#ifdef __vax__ 3462#error "Won't work on VAXen. See comments at get_last_object." 3463#endif 3464@end group 3465@end smallexample 3466 3467If you have several configuration parameters that must be set up by 3468the installation in a consistent way, you can use conditionals to detect 3469an inconsistency and report it with @samp{#error}. For example, 3470 3471@smallexample 3472#if !defined(FOO) && defined(BAR) 3473#error "BAR requires FOO." 3474#endif 3475@end smallexample 3476 3477@findex #warning 3478The directive @samp{#warning} is like @samp{#error}, but causes the 3479preprocessor to issue a warning and continue preprocessing. The tokens 3480following @samp{#warning} are used as the warning message. 3481 3482You might use @samp{#warning} in obsolete header files, with a message 3483directing the user to the header file which should be used instead. 3484 3485Neither @samp{#error} nor @samp{#warning} macro-expands its argument. 3486Internal whitespace sequences are each replaced with a single space. 3487The line must consist of complete tokens. It is wisest to make the 3488argument of these directives be a single string constant; this avoids 3489problems with apostrophes and the like. 3490 3491@node Line Control 3492@chapter Line Control 3493@cindex line control 3494 3495The C preprocessor informs the C compiler of the location in your source 3496code where each token came from. Presently, this is just the file name 3497and line number. All the tokens resulting from macro expansion are 3498reported as having appeared on the line of the source file where the 3499outermost macro was used. We intend to be more accurate in the future. 3500 3501If you write a program which generates source code, such as the 3502@command{bison} parser generator, you may want to adjust the preprocessor's 3503notion of the current file name and line number by hand. Parts of the 3504output from @command{bison} are generated from scratch, other parts come 3505from a standard parser file. The rest are copied verbatim from 3506@command{bison}'s input. You would like compiler error messages and 3507symbolic debuggers to be able to refer to @code{bison}'s input file. 3508 3509@findex #line 3510@command{bison} or any such program can arrange this by writing 3511@samp{#line} directives into the output file. @samp{#line} is a 3512directive that specifies the original line number and source file name 3513for subsequent input in the current preprocessor input file. 3514@samp{#line} has three variants: 3515 3516@table @code 3517@item #line @var{linenum} 3518@var{linenum} is a non-negative decimal integer constant. It specifies 3519the line number which should be reported for the following line of 3520input. Subsequent lines are counted from @var{linenum}. 3521 3522@item #line @var{linenum} @var{filename} 3523@var{linenum} is the same as for the first form, and has the same 3524effect. In addition, @var{filename} is a string constant. The 3525following line and all subsequent lines are reported to come from the 3526file it specifies, until something else happens to change that. 3527@var{filename} is interpreted according to the normal rules for a string 3528constant: backslash escapes are interpreted. This is different from 3529@samp{#include}. 3530 3531Previous versions of CPP did not interpret escapes in @samp{#line}; 3532we have changed it because the standard requires they be interpreted, 3533and most other compilers do. 3534 3535@item #line @var{anything else} 3536@var{anything else} is checked for macro calls, which are expanded. 3537The result should match one of the above two forms. 3538@end table 3539 3540@samp{#line} directives alter the results of the @code{__FILE__} and 3541@code{__LINE__} predefined macros from that point on. @xref{Standard 3542Predefined Macros}. They do not have any effect on @samp{#include}'s 3543idea of the directory containing the current file. This is a change 3544from GCC 2.95. Previously, a file reading 3545 3546@smallexample 3547#line 1 "../src/gram.y" 3548#include "gram.h" 3549@end smallexample 3550 3551would search for @file{gram.h} in @file{../src}, then the @option{-I} 3552chain; the directory containing the physical source file would not be 3553searched. In GCC 3.0 and later, the @samp{#include} is not affected by 3554the presence of a @samp{#line} referring to a different directory. 3555 3556We made this change because the old behavior caused problems when 3557generated source files were transported between machines. For instance, 3558it is common practice to ship generated parsers with a source release, 3559so that people building the distribution do not need to have yacc or 3560Bison installed. These files frequently have @samp{#line} directives 3561referring to the directory tree of the system where the distribution was 3562created. If GCC tries to search for headers in those directories, the 3563build is likely to fail. 3564 3565The new behavior can cause failures too, if the generated file is not 3566in the same directory as its source and it attempts to include a header 3567which would be visible searching from the directory containing the 3568source file. However, this problem is easily solved with an additional 3569@option{-I} switch on the command line. The failures caused by the old 3570semantics could sometimes be corrected only by editing the generated 3571files, which is difficult and error-prone. 3572 3573@node Pragmas 3574@chapter Pragmas 3575 3576The @samp{#pragma} directive is the method specified by the C standard 3577for providing additional information to the compiler, beyond what is 3578conveyed in the language itself. Three forms of this directive 3579(commonly known as @dfn{pragmas}) are specified by the 1999 C standard. 3580A C compiler is free to attach any meaning it likes to other pragmas. 3581 3582GCC has historically preferred to use extensions to the syntax of the 3583language, such as @code{__attribute__}, for this purpose. However, GCC 3584does define a few pragmas of its own. These mostly have effects on the 3585entire translation unit or source file. 3586 3587In GCC version 3, all GNU-defined, supported pragmas have been given a 3588@code{GCC} prefix. This is in line with the @code{STDC} prefix on all 3589pragmas defined by C99. For backward compatibility, pragmas which were 3590recognized by previous versions are still recognized without the 3591@code{GCC} prefix, but that usage is deprecated. Some older pragmas are 3592deprecated in their entirety. They are not recognized with the 3593@code{GCC} prefix. @xref{Obsolete Features}. 3594 3595@cindex @code{_Pragma} 3596C99 introduces the @code{@w{_Pragma}} operator. This feature addresses a 3597major problem with @samp{#pragma}: being a directive, it cannot be 3598produced as the result of macro expansion. @code{@w{_Pragma}} is an 3599operator, much like @code{sizeof} or @code{defined}, and can be embedded 3600in a macro. 3601 3602Its syntax is @code{@w{_Pragma (@var{string-literal})}}, where 3603@var{string-literal} can be either a normal or wide-character string 3604literal. It is destringized, by replacing all @samp{\\} with a single 3605@samp{\} and all @samp{\"} with a @samp{"}. The result is then 3606processed as if it had appeared as the right hand side of a 3607@samp{#pragma} directive. For example, 3608 3609@smallexample 3610_Pragma ("GCC dependency \"parse.y\"") 3611@end smallexample 3612 3613@noindent 3614has the same effect as @code{#pragma GCC dependency "parse.y"}. The 3615same effect could be achieved using macros, for example 3616 3617@smallexample 3618#define DO_PRAGMA(x) _Pragma (#x) 3619DO_PRAGMA (GCC dependency "parse.y") 3620@end smallexample 3621 3622The standard is unclear on where a @code{_Pragma} operator can appear. 3623The preprocessor does not accept it within a preprocessing conditional 3624directive like @samp{#if}. To be safe, you are probably best keeping it 3625out of directives other than @samp{#define}, and putting it on a line of 3626its own. 3627 3628This manual documents the pragmas which are meaningful to the 3629preprocessor itself. Other pragmas are meaningful to the C or C++ 3630compilers. They are documented in the GCC manual. 3631 3632GCC plugins may provide their own pragmas. 3633 3634@ftable @code 3635@item #pragma GCC dependency 3636@code{#pragma GCC dependency} allows you to check the relative dates of 3637the current file and another file. If the other file is more recent than 3638the current file, a warning is issued. This is useful if the current 3639file is derived from the other file, and should be regenerated. The 3640other file is searched for using the normal include search path. 3641Optional trailing text can be used to give more information in the 3642warning message. 3643 3644@smallexample 3645#pragma GCC dependency "parse.y" 3646#pragma GCC dependency "/usr/include/time.h" rerun fixincludes 3647@end smallexample 3648 3649@item #pragma GCC poison 3650Sometimes, there is an identifier that you want to remove completely 3651from your program, and make sure that it never creeps back in. To 3652enforce this, you can @dfn{poison} the identifier with this pragma. 3653@code{#pragma GCC poison} is followed by a list of identifiers to 3654poison. If any of those identifiers appears anywhere in the source 3655after the directive, it is a hard error. For example, 3656 3657@smallexample 3658#pragma GCC poison printf sprintf fprintf 3659sprintf(some_string, "hello"); 3660@end smallexample 3661 3662@noindent 3663will produce an error. 3664 3665If a poisoned identifier appears as part of the expansion of a macro 3666which was defined before the identifier was poisoned, it will @emph{not} 3667cause an error. This lets you poison an identifier without worrying 3668about system headers defining macros that use it. 3669 3670For example, 3671 3672@smallexample 3673#define strrchr rindex 3674#pragma GCC poison rindex 3675strrchr(some_string, 'h'); 3676@end smallexample 3677 3678@noindent 3679will not produce an error. 3680 3681@item #pragma GCC system_header 3682This pragma takes no arguments. It causes the rest of the code in the 3683current file to be treated as if it came from a system header. 3684@xref{System Headers}. 3685 3686@item #pragma GCC warning 3687@itemx #pragma GCC error 3688@code{#pragma GCC warning "message"} causes the preprocessor to issue 3689a warning diagnostic with the text @samp{message}. The message 3690contained in the pragma must be a single string literal. Similarly, 3691@code{#pragma GCC error "message"} issues an error message. Unlike 3692the @samp{#warning} and @samp{#error} directives, these pragmas can be 3693embedded in preprocessor macros using @samp{_Pragma}. 3694 3695@end ftable 3696 3697@node Other Directives 3698@chapter Other Directives 3699 3700@findex #ident 3701@findex #sccs 3702The @samp{#ident} directive takes one argument, a string constant. On 3703some systems, that string constant is copied into a special segment of 3704the object file. On other systems, the directive is ignored. The 3705@samp{#sccs} directive is a synonym for @samp{#ident}. 3706 3707These directives are not part of the C standard, but they are not 3708official GNU extensions either. What historical information we have 3709been able to find, suggests they originated with System V@. 3710 3711@cindex null directive 3712The @dfn{null directive} consists of a @samp{#} followed by a newline, 3713with only whitespace (including comments) in between. A null directive 3714is understood as a preprocessing directive but has no effect on the 3715preprocessor output. The primary significance of the existence of the 3716null directive is that an input line consisting of just a @samp{#} will 3717produce no output, rather than a line of output containing just a 3718@samp{#}. Supposedly some old C programs contain such lines. 3719 3720@node Preprocessor Output 3721@chapter Preprocessor Output 3722 3723When the C preprocessor is used with the C, C++, or Objective-C 3724compilers, it is integrated into the compiler and communicates a stream 3725of binary tokens directly to the compiler's parser. However, it can 3726also be used in the more conventional standalone mode, where it produces 3727textual output. 3728@c FIXME: Document the library interface. 3729 3730@cindex output format 3731The output from the C preprocessor looks much like the input, except 3732that all preprocessing directive lines have been replaced with blank 3733lines and all comments with spaces. Long runs of blank lines are 3734discarded. 3735 3736The ISO standard specifies that it is implementation defined whether a 3737preprocessor preserves whitespace between tokens, or replaces it with 3738e.g.@: a single space. In GNU CPP, whitespace between tokens is collapsed 3739to become a single space, with the exception that the first token on a 3740non-directive line is preceded with sufficient spaces that it appears in 3741the same column in the preprocessed output that it appeared in the 3742original source file. This is so the output is easy to read. 3743@xref{Differences from previous versions}. CPP does not insert any 3744whitespace where there was none in the original source, except where 3745necessary to prevent an accidental token paste. 3746 3747@cindex linemarkers 3748Source file name and line number information is conveyed by lines 3749of the form 3750 3751@smallexample 3752# @var{linenum} @var{filename} @var{flags} 3753@end smallexample 3754 3755@noindent 3756These are called @dfn{linemarkers}. They are inserted as needed into 3757the output (but never within a string or character constant). They mean 3758that the following line originated in file @var{filename} at line 3759@var{linenum}. @var{filename} will never contain any non-printing 3760characters; they are replaced with octal escape sequences. 3761 3762After the file name comes zero or more flags, which are @samp{1}, 3763@samp{2}, @samp{3}, or @samp{4}. If there are multiple flags, spaces 3764separate them. Here is what the flags mean: 3765 3766@table @samp 3767@item 1 3768This indicates the start of a new file. 3769@item 2 3770This indicates returning to a file (after having included another file). 3771@item 3 3772This indicates that the following text comes from a system header file, 3773so certain warnings should be suppressed. 3774@item 4 3775This indicates that the following text should be treated as being 3776wrapped in an implicit @code{extern "C"} block. 3777@c maybe cross reference NO_IMPLICIT_EXTERN_C 3778@end table 3779 3780As an extension, the preprocessor accepts linemarkers in non-assembler 3781input files. They are treated like the corresponding @samp{#line} 3782directive, (@pxref{Line Control}), except that trailing flags are 3783permitted, and are interpreted with the meanings described above. If 3784multiple flags are given, they must be in ascending order. 3785 3786Some directives may be duplicated in the output of the preprocessor. 3787These are @samp{#ident} (always), @samp{#pragma} (only if the 3788preprocessor does not handle the pragma itself), and @samp{#define} and 3789@samp{#undef} (with certain debugging options). If this happens, the 3790@samp{#} of the directive will always be in the first column, and there 3791will be no space between the @samp{#} and the directive name. If macro 3792expansion happens to generate tokens which might be mistaken for a 3793duplicated directive, a space will be inserted between the @samp{#} and 3794the directive name. 3795 3796@node Traditional Mode 3797@chapter Traditional Mode 3798 3799Traditional (pre-standard) C preprocessing is rather different from 3800the preprocessing specified by the standard. When GCC is given the 3801@option{-traditional-cpp} option, it attempts to emulate a traditional 3802preprocessor. 3803 3804GCC versions 3.2 and later only support traditional mode semantics in 3805the preprocessor, and not in the compiler front ends. This chapter 3806outlines the traditional preprocessor semantics we implemented. 3807 3808The implementation does not correspond precisely to the behavior of 3809earlier versions of GCC, nor to any true traditional preprocessor. 3810After all, inconsistencies among traditional implementations were a 3811major motivation for C standardization. However, we intend that it 3812should be compatible with true traditional preprocessors in all ways 3813that actually matter. 3814 3815@menu 3816* Traditional lexical analysis:: 3817* Traditional macros:: 3818* Traditional miscellany:: 3819* Traditional warnings:: 3820@end menu 3821 3822@node Traditional lexical analysis 3823@section Traditional lexical analysis 3824 3825The traditional preprocessor does not decompose its input into tokens 3826the same way a standards-conforming preprocessor does. The input is 3827simply treated as a stream of text with minimal internal form. 3828 3829This implementation does not treat trigraphs (@pxref{trigraphs}) 3830specially since they were an invention of the standards committee. It 3831handles arbitrarily-positioned escaped newlines properly and splices 3832the lines as you would expect; many traditional preprocessors did not 3833do this. 3834 3835The form of horizontal whitespace in the input file is preserved in 3836the output. In particular, hard tabs remain hard tabs. This can be 3837useful if, for example, you are preprocessing a Makefile. 3838 3839Traditional CPP only recognizes C-style block comments, and treats the 3840@samp{/*} sequence as introducing a comment only if it lies outside 3841quoted text. Quoted text is introduced by the usual single and double 3842quotes, and also by an initial @samp{<} in a @code{#include} 3843directive. 3844 3845Traditionally, comments are completely removed and are not replaced 3846with a space. Since a traditional compiler does its own tokenization 3847of the output of the preprocessor, this means that comments can 3848effectively be used as token paste operators. However, comments 3849behave like separators for text handled by the preprocessor itself, 3850since it doesn't re-lex its input. For example, in 3851 3852@smallexample 3853#if foo/**/bar 3854@end smallexample 3855 3856@noindent 3857@samp{foo} and @samp{bar} are distinct identifiers and expanded 3858separately if they happen to be macros. In other words, this 3859directive is equivalent to 3860 3861@smallexample 3862#if foo bar 3863@end smallexample 3864 3865@noindent 3866rather than 3867 3868@smallexample 3869#if foobar 3870@end smallexample 3871 3872Generally speaking, in traditional mode an opening quote need not have 3873a matching closing quote. In particular, a macro may be defined with 3874replacement text that contains an unmatched quote. Of course, if you 3875attempt to compile preprocessed output containing an unmatched quote 3876you will get a syntax error. 3877 3878However, all preprocessing directives other than @code{#define} 3879require matching quotes. For example: 3880 3881@smallexample 3882#define m This macro's fine and has an unmatched quote 3883"/* This is not a comment. */ 3884/* @r{This is a comment. The following #include directive 3885 is ill-formed.} */ 3886#include <stdio.h 3887@end smallexample 3888 3889Just as for the ISO preprocessor, what would be a closing quote can be 3890escaped with a backslash to prevent the quoted text from closing. 3891 3892@node Traditional macros 3893@section Traditional macros 3894 3895The major difference between traditional and ISO macros is that the 3896former expand to text rather than to a token sequence. CPP removes 3897all leading and trailing horizontal whitespace from a macro's 3898replacement text before storing it, but preserves the form of internal 3899whitespace. 3900 3901One consequence is that it is legitimate for the replacement text to 3902contain an unmatched quote (@pxref{Traditional lexical analysis}). An 3903unclosed string or character constant continues into the text 3904following the macro call. Similarly, the text at the end of a macro's 3905expansion can run together with the text after the macro invocation to 3906produce a single token. 3907 3908Normally comments are removed from the replacement text after the 3909macro is expanded, but if the @option{-CC} option is passed on the 3910command-line comments are preserved. (In fact, the current 3911implementation removes comments even before saving the macro 3912replacement text, but it careful to do it in such a way that the 3913observed effect is identical even in the function-like macro case.) 3914 3915The ISO stringification operator @samp{#} and token paste operator 3916@samp{##} have no special meaning. As explained later, an effect 3917similar to these operators can be obtained in a different way. Macro 3918names that are embedded in quotes, either from the main file or after 3919macro replacement, do not expand. 3920 3921CPP replaces an unquoted object-like macro name with its replacement 3922text, and then rescans it for further macros to replace. Unlike 3923standard macro expansion, traditional macro expansion has no provision 3924to prevent recursion. If an object-like macro appears unquoted in its 3925replacement text, it will be replaced again during the rescan pass, 3926and so on @emph{ad infinitum}. GCC detects when it is expanding 3927recursive macros, emits an error message, and continues after the 3928offending macro invocation. 3929 3930@smallexample 3931#define PLUS + 3932#define INC(x) PLUS+x 3933INC(foo); 3934 @expansion{} ++foo; 3935@end smallexample 3936 3937Function-like macros are similar in form but quite different in 3938behavior to their ISO counterparts. Their arguments are contained 3939within parentheses, are comma-separated, and can cross physical lines. 3940Commas within nested parentheses are not treated as argument 3941separators. Similarly, a quote in an argument cannot be left 3942unclosed; a following comma or parenthesis that comes before the 3943closing quote is treated like any other character. There is no 3944facility for handling variadic macros. 3945 3946This implementation removes all comments from macro arguments, unless 3947the @option{-C} option is given. The form of all other horizontal 3948whitespace in arguments is preserved, including leading and trailing 3949whitespace. In particular 3950 3951@smallexample 3952f( ) 3953@end smallexample 3954 3955@noindent 3956is treated as an invocation of the macro @samp{f} with a single 3957argument consisting of a single space. If you want to invoke a 3958function-like macro that takes no arguments, you must not leave any 3959whitespace between the parentheses. 3960 3961If a macro argument crosses a new line, the new line is replaced with 3962a space when forming the argument. If the previous line contained an 3963unterminated quote, the following line inherits the quoted state. 3964 3965Traditional preprocessors replace parameters in the replacement text 3966with their arguments regardless of whether the parameters are within 3967quotes or not. This provides a way to stringize arguments. For 3968example 3969 3970@smallexample 3971#define str(x) "x" 3972str(/* @r{A comment} */some text ) 3973 @expansion{} "some text " 3974@end smallexample 3975 3976@noindent 3977Note that the comment is removed, but that the trailing space is 3978preserved. Here is an example of using a comment to effect token 3979pasting. 3980 3981@smallexample 3982#define suffix(x) foo_/**/x 3983suffix(bar) 3984 @expansion{} foo_bar 3985@end smallexample 3986 3987@node Traditional miscellany 3988@section Traditional miscellany 3989 3990Here are some things to be aware of when using the traditional 3991preprocessor. 3992 3993@itemize @bullet 3994@item 3995Preprocessing directives are recognized only when their leading 3996@samp{#} appears in the first column. There can be no whitespace 3997between the beginning of the line and the @samp{#}, but whitespace can 3998follow the @samp{#}. 3999 4000@item 4001A true traditional C preprocessor does not recognize @samp{#error} or 4002@samp{#pragma}, and may not recognize @samp{#elif}. CPP supports all 4003the directives in traditional mode that it supports in ISO mode, 4004including extensions, with the exception that the effects of 4005@samp{#pragma GCC poison} are undefined. 4006 4007@item 4008__STDC__ is not defined. 4009 4010@item 4011If you use digraphs the behavior is undefined. 4012 4013@item 4014If a line that looks like a directive appears within macro arguments, 4015the behavior is undefined. 4016 4017@end itemize 4018 4019@node Traditional warnings 4020@section Traditional warnings 4021You can request warnings about features that did not exist, or worked 4022differently, in traditional C with the @option{-Wtraditional} option. 4023GCC does not warn about features of ISO C which you must use when you 4024are using a conforming compiler, such as the @samp{#} and @samp{##} 4025operators. 4026 4027Presently @option{-Wtraditional} warns about: 4028 4029@itemize @bullet 4030@item 4031Macro parameters that appear within string literals in the macro body. 4032In traditional C macro replacement takes place within string literals, 4033but does not in ISO C@. 4034 4035@item 4036In traditional C, some preprocessor directives did not exist. 4037Traditional preprocessors would only consider a line to be a directive 4038if the @samp{#} appeared in column 1 on the line. Therefore 4039@option{-Wtraditional} warns about directives that traditional C 4040understands but would ignore because the @samp{#} does not appear as the 4041first character on the line. It also suggests you hide directives like 4042@samp{#pragma} not understood by traditional C by indenting them. Some 4043traditional implementations would not recognize @samp{#elif}, so it 4044suggests avoiding it altogether. 4045 4046@item 4047A function-like macro that appears without an argument list. In some 4048traditional preprocessors this was an error. In ISO C it merely means 4049that the macro is not expanded. 4050 4051@item 4052The unary plus operator. This did not exist in traditional C@. 4053 4054@item 4055The @samp{U} and @samp{LL} integer constant suffixes, which were not 4056available in traditional C@. (Traditional C does support the @samp{L} 4057suffix for simple long integer constants.) You are not warned about 4058uses of these suffixes in macros defined in system headers. For 4059instance, @code{UINT_MAX} may well be defined as @code{4294967295U}, but 4060you will not be warned if you use @code{UINT_MAX}. 4061 4062You can usually avoid the warning, and the related warning about 4063constants which are so large that they are unsigned, by writing the 4064integer constant in question in hexadecimal, with no U suffix. Take 4065care, though, because this gives the wrong result in exotic cases. 4066@end itemize 4067 4068@node Implementation Details 4069@chapter Implementation Details 4070 4071Here we document details of how the preprocessor's implementation 4072affects its user-visible behavior. You should try to avoid undue 4073reliance on behavior described here, as it is possible that it will 4074change subtly in future implementations. 4075 4076Also documented here are obsolete features and changes from previous 4077versions of CPP@. 4078 4079@menu 4080* Implementation-defined behavior:: 4081* Implementation limits:: 4082* Obsolete Features:: 4083* Differences from previous versions:: 4084@end menu 4085 4086@node Implementation-defined behavior 4087@section Implementation-defined behavior 4088@cindex implementation-defined behavior 4089 4090This is how CPP behaves in all the cases which the C standard 4091describes as @dfn{implementation-defined}. This term means that the 4092implementation is free to do what it likes, but must document its choice 4093and stick to it. 4094@c FIXME: Check the C++ standard for more implementation-defined stuff. 4095 4096@itemize @bullet 4097@need 1000 4098@item The mapping of physical source file multi-byte characters to the 4099execution character set. 4100 4101The input character set can be specified using the 4102@option{-finput-charset} option, while the execution character set may 4103be controlled using the @option{-fexec-charset} and 4104@option{-fwide-exec-charset} options. 4105 4106@item Identifier characters. 4107@anchor{Identifier characters} 4108 4109The C and C++ standards allow identifiers to be composed of @samp{_} 4110and the alphanumeric characters. C++ and C99 also allow universal 4111character names, and C99 further permits implementation-defined 4112characters. 4113 4114GCC allows the @samp{$} character in identifiers as an extension for 4115most targets. This is true regardless of the @option{std=} switch, 4116since this extension cannot conflict with standards-conforming 4117programs. When preprocessing assembler, however, dollars are not 4118identifier characters by default. 4119 4120Currently the targets that by default do not permit @samp{$} are AVR, 4121IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC targets for the AIX 4122operating system. 4123 4124You can override the default with @option{-fdollars-in-identifiers} or 4125@option{fno-dollars-in-identifiers}. @xref{fdollars-in-identifiers}. 4126 4127@item Non-empty sequences of whitespace characters. 4128 4129In textual output, each whitespace sequence is collapsed to a single 4130space. For aesthetic reasons, the first token on each non-directive 4131line of output is preceded with sufficient spaces that it appears in the 4132same column as it did in the original source file. 4133 4134@item The numeric value of character constants in preprocessor expressions. 4135 4136The preprocessor and compiler interpret character constants in the 4137same way; i.e.@: escape sequences such as @samp{\a} are given the 4138values they would have on the target machine. 4139 4140The compiler evaluates a multi-character character constant a character 4141at a time, shifting the previous value left by the number of bits per 4142target character, and then or-ing in the bit-pattern of the new 4143character truncated to the width of a target character. The final 4144bit-pattern is given type @code{int}, and is therefore signed, 4145regardless of whether single characters are signed or not (a slight 4146change from versions 3.1 and earlier of GCC)@. If there are more 4147characters in the constant than would fit in the target @code{int} the 4148compiler issues a warning, and the excess leading characters are 4149ignored. 4150 4151For example, @code{'ab'} for a target with an 8-bit @code{char} would be 4152interpreted as @w{@samp{(int) ((unsigned char) 'a' * 256 + (unsigned char) 4153'b')}}, and @code{'\234a'} as @w{@samp{(int) ((unsigned char) '\234' * 4154256 + (unsigned char) 'a')}}. 4155 4156@item Source file inclusion. 4157 4158For a discussion on how the preprocessor locates header files, 4159@ref{Include Operation}. 4160 4161@item Interpretation of the filename resulting from a macro-expanded 4162@samp{#include} directive. 4163 4164@xref{Computed Includes}. 4165 4166@item Treatment of a @samp{#pragma} directive that after macro-expansion 4167results in a standard pragma. 4168 4169No macro expansion occurs on any @samp{#pragma} directive line, so the 4170question does not arise. 4171 4172Note that GCC does not yet implement any of the standard 4173pragmas. 4174 4175@end itemize 4176 4177@node Implementation limits 4178@section Implementation limits 4179@cindex implementation limits 4180 4181CPP has a small number of internal limits. This section lists the 4182limits which the C standard requires to be no lower than some minimum, 4183and all the others known. It is intended that there should be as few limits 4184as possible. If you encounter an undocumented or inconvenient limit, 4185please report that as a bug. @xref{Bugs, , Reporting Bugs, gcc, Using 4186the GNU Compiler Collection (GCC)}. 4187 4188Where we say something is limited @dfn{only by available memory}, that 4189means that internal data structures impose no intrinsic limit, and space 4190is allocated with @code{malloc} or equivalent. The actual limit will 4191therefore depend on many things, such as the size of other things 4192allocated by the compiler at the same time, the amount of memory 4193consumed by other processes on the same computer, etc. 4194 4195@itemize @bullet 4196 4197@item Nesting levels of @samp{#include} files. 4198 4199We impose an arbitrary limit of 200 levels, to avoid runaway recursion. 4200The standard requires at least 15 levels. 4201 4202@item Nesting levels of conditional inclusion. 4203 4204The C standard mandates this be at least 63. CPP is limited only by 4205available memory. 4206 4207@item Levels of parenthesized expressions within a full expression. 4208 4209The C standard requires this to be at least 63. In preprocessor 4210conditional expressions, it is limited only by available memory. 4211 4212@item Significant initial characters in an identifier or macro name. 4213 4214The preprocessor treats all characters as significant. The C standard 4215requires only that the first 63 be significant. 4216 4217@item Number of macros simultaneously defined in a single translation unit. 4218 4219The standard requires at least 4095 be possible. CPP is limited only 4220by available memory. 4221 4222@item Number of parameters in a macro definition and arguments in a macro call. 4223 4224We allow @code{USHRT_MAX}, which is no smaller than 65,535. The minimum 4225required by the standard is 127. 4226 4227@item Number of characters on a logical source line. 4228 4229The C standard requires a minimum of 4096 be permitted. CPP places 4230no limits on this, but you may get incorrect column numbers reported in 4231diagnostics for lines longer than 65,535 characters. 4232 4233@item Maximum size of a source file. 4234 4235The standard does not specify any lower limit on the maximum size of a 4236source file. GNU cpp maps files into memory, so it is limited by the 4237available address space. This is generally at least two gigabytes. 4238Depending on the operating system, the size of physical memory may or 4239may not be a limitation. 4240 4241@end itemize 4242 4243@node Obsolete Features 4244@section Obsolete Features 4245 4246CPP has some features which are present mainly for compatibility with 4247older programs. We discourage their use in new code. In some cases, 4248we plan to remove the feature in a future version of GCC@. 4249 4250@subsection Assertions 4251@cindex assertions 4252 4253@dfn{Assertions} are a deprecated alternative to macros in writing 4254conditionals to test what sort of computer or system the compiled 4255program will run on. Assertions are usually predefined, but you can 4256define them with preprocessing directives or command-line options. 4257 4258Assertions were intended to provide a more systematic way to describe 4259the compiler's target system and we added them for compatibility with 4260existing compilers. In practice they are just as unpredictable as the 4261system-specific predefined macros. In addition, they are not part of 4262any standard, and only a few compilers support them. 4263Therefore, the use of assertions is @strong{less} portable than the use 4264of system-specific predefined macros. We recommend you do not use them at 4265all. 4266 4267@cindex predicates 4268An assertion looks like this: 4269 4270@smallexample 4271#@var{predicate} (@var{answer}) 4272@end smallexample 4273 4274@noindent 4275@var{predicate} must be a single identifier. @var{answer} can be any 4276sequence of tokens; all characters are significant except for leading 4277and trailing whitespace, and differences in internal whitespace 4278sequences are ignored. (This is similar to the rules governing macro 4279redefinition.) Thus, @code{(x + y)} is different from @code{(x+y)} but 4280equivalent to @code{@w{( x + y )}}. Parentheses do not nest inside an 4281answer. 4282 4283@cindex testing predicates 4284To test an assertion, you write it in an @samp{#if}. For example, this 4285conditional succeeds if either @code{vax} or @code{ns16000} has been 4286asserted as an answer for @code{machine}. 4287 4288@smallexample 4289#if #machine (vax) || #machine (ns16000) 4290@end smallexample 4291 4292@noindent 4293You can test whether @emph{any} answer is asserted for a predicate by 4294omitting the answer in the conditional: 4295 4296@smallexample 4297#if #machine 4298@end smallexample 4299 4300@findex #assert 4301Assertions are made with the @samp{#assert} directive. Its sole 4302argument is the assertion to make, without the leading @samp{#} that 4303identifies assertions in conditionals. 4304 4305@smallexample 4306#assert @var{predicate} (@var{answer}) 4307@end smallexample 4308 4309@noindent 4310You may make several assertions with the same predicate and different 4311answers. Subsequent assertions do not override previous ones for the 4312same predicate. All the answers for any given predicate are 4313simultaneously true. 4314 4315@cindex assertions, canceling 4316@findex #unassert 4317Assertions can be canceled with the @samp{#unassert} directive. It 4318has the same syntax as @samp{#assert}. In that form it cancels only the 4319answer which was specified on the @samp{#unassert} line; other answers 4320for that predicate remain true. You can cancel an entire predicate by 4321leaving out the answer: 4322 4323@smallexample 4324#unassert @var{predicate} 4325@end smallexample 4326 4327@noindent 4328In either form, if no such assertion has been made, @samp{#unassert} has 4329no effect. 4330 4331You can also make or cancel assertions using command-line options. 4332@xref{Invocation}. 4333 4334@node Differences from previous versions 4335@section Differences from previous versions 4336@cindex differences from previous versions 4337 4338This section details behavior which has changed from previous versions 4339of CPP@. We do not plan to change it again in the near future, but 4340we do not promise not to, either. 4341 4342The ``previous versions'' discussed here are 2.95 and before. The 4343behavior of GCC 3.0 is mostly the same as the behavior of the widely 4344used 2.96 and 2.97 development snapshots. Where there are differences, 4345they generally represent bugs in the snapshots. 4346 4347@itemize @bullet 4348 4349@item -I- deprecated 4350 4351This option has been deprecated in 4.0. @option{-iquote} is meant to 4352replace the need for this option. 4353 4354@item Order of evaluation of @samp{#} and @samp{##} operators 4355 4356The standard does not specify the order of evaluation of a chain of 4357@samp{##} operators, nor whether @samp{#} is evaluated before, after, or 4358at the same time as @samp{##}. You should therefore not write any code 4359which depends on any specific ordering. It is possible to guarantee an 4360ordering, if you need one, by suitable use of nested macros. 4361 4362An example of where this might matter is pasting the arguments @samp{1}, 4363@samp{e} and @samp{-2}. This would be fine for left-to-right pasting, 4364but right-to-left pasting would produce an invalid token @samp{e-2}. 4365 4366GCC 3.0 evaluates @samp{#} and @samp{##} at the same time and strictly 4367left to right. Older versions evaluated all @samp{#} operators first, 4368then all @samp{##} operators, in an unreliable order. 4369 4370@item The form of whitespace between tokens in preprocessor output 4371 4372@xref{Preprocessor Output}, for the current textual format. This is 4373also the format used by stringification. Normally, the preprocessor 4374communicates tokens directly to the compiler's parser, and whitespace 4375does not come up at all. 4376 4377Older versions of GCC preserved all whitespace provided by the user and 4378inserted lots more whitespace of their own, because they could not 4379accurately predict when extra spaces were needed to prevent accidental 4380token pasting. 4381 4382@item Optional argument when invoking rest argument macros 4383 4384As an extension, GCC permits you to omit the variable arguments entirely 4385when you use a variable argument macro. This is forbidden by the 1999 C 4386standard, and will provoke a pedantic warning with GCC 3.0. Previous 4387versions accepted it silently. 4388 4389@item @samp{##} swallowing preceding text in rest argument macros 4390 4391Formerly, in a macro expansion, if @samp{##} appeared before a variable 4392arguments parameter, and the set of tokens specified for that argument 4393in the macro invocation was empty, previous versions of CPP would 4394back up and remove the preceding sequence of non-whitespace characters 4395(@strong{not} the preceding token). This extension is in direct 4396conflict with the 1999 C standard and has been drastically pared back. 4397 4398In the current version of the preprocessor, if @samp{##} appears between 4399a comma and a variable arguments parameter, and the variable argument is 4400omitted entirely, the comma will be removed from the expansion. If the 4401variable argument is empty, or the token before @samp{##} is not a 4402comma, then @samp{##} behaves as a normal token paste. 4403 4404@item @samp{#line} and @samp{#include} 4405 4406The @samp{#line} directive used to change GCC's notion of the 4407``directory containing the current file'', used by @samp{#include} with 4408a double-quoted header file name. In 3.0 and later, it does not. 4409@xref{Line Control}, for further explanation. 4410 4411@item Syntax of @samp{#line} 4412 4413In GCC 2.95 and previous, the string constant argument to @samp{#line} 4414was treated the same way as the argument to @samp{#include}: backslash 4415escapes were not honored, and the string ended at the second @samp{"}. 4416This is not compliant with the C standard. In GCC 3.0, an attempt was 4417made to correct the behavior, so that the string was treated as a real 4418string constant, but it turned out to be buggy. In 3.1, the bugs have 4419been fixed. (We are not fixing the bugs in 3.0 because they affect 4420relatively few people and the fix is quite invasive.) 4421 4422@end itemize 4423 4424@node Invocation 4425@chapter Invocation 4426@cindex invocation 4427@cindex command line 4428 4429Most often when you use the C preprocessor you will not have to invoke it 4430explicitly: the C compiler will do so automatically. However, the 4431preprocessor is sometimes useful on its own. All the options listed 4432here are also acceptable to the C compiler and have the same meaning, 4433except that the C compiler has different rules for specifying the output 4434file. 4435 4436@emph{Note:} Whether you use the preprocessor by way of @command{gcc} 4437or @command{cpp}, the @dfn{compiler driver} is run first. This 4438program's purpose is to translate your command into invocations of the 4439programs that do the actual work. Their command-line interfaces are 4440similar but not identical to the documented interface, and may change 4441without notice. 4442 4443@ignore 4444@c man begin SYNOPSIS 4445cpp [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}] 4446 [@option{-I}@var{dir}@dots{}] [@option{-iquote}@var{dir}@dots{}] 4447 [@option{-iremap}@var{src}:@var{dst}] 4448 [@option{-W}@var{warn}@dots{}] 4449 [@option{-M}|@option{-MM}] [@option{-MG}] [@option{-MF} @var{filename}] 4450 [@option{-MP}] [@option{-MQ} @var{target}@dots{}] 4451 [@option{-MT} @var{target}@dots{}] 4452 [@option{-P}] [@option{-fno-working-directory}] 4453 [@option{-x} @var{language}] [@option{-std=}@var{standard}] 4454 @var{infile} @var{outfile} 4455 4456Only the most useful options are listed here; see below for the remainder. 4457@c man end 4458@c man begin SEEALSO 4459gpl(7), gfdl(7), fsf-funding(7), 4460gcc(1), as(1), ld(1), and the Info entries for @file{cpp}, @file{gcc}, and 4461@file{binutils}. 4462@c man end 4463@end ignore 4464 4465@c man begin OPTIONS 4466The C preprocessor expects two file names as arguments, @var{infile} and 4467@var{outfile}. The preprocessor reads @var{infile} together with any 4468other files it specifies with @samp{#include}. All the output generated 4469by the combined input files is written in @var{outfile}. 4470 4471Either @var{infile} or @var{outfile} may be @option{-}, which as 4472@var{infile} means to read from standard input and as @var{outfile} 4473means to write to standard output. Also, if either file is omitted, it 4474means the same as if @option{-} had been specified for that file. 4475 4476Unless otherwise noted, or the option ends in @samp{=}, all options 4477which take an argument may have that argument appear either immediately 4478after the option, or with a space between option and argument: 4479@option{-Ifoo} and @option{-I foo} have the same effect. 4480 4481@cindex grouping options 4482@cindex options, grouping 4483Many options have multi-letter names; therefore multiple single-letter 4484options may @emph{not} be grouped: @option{-dM} is very different from 4485@w{@samp{-d -M}}. 4486 4487@cindex options 4488@include cppopts.texi 4489@c man end 4490 4491@node Environment Variables 4492@chapter Environment Variables 4493@cindex environment variables 4494@c man begin ENVIRONMENT 4495 4496This section describes the environment variables that affect how CPP 4497operates. You can use them to specify directories or prefixes to use 4498when searching for include files, or to control dependency output. 4499 4500Note that you can also specify places to search using options such as 4501@option{-I}, and control dependency output with options like 4502@option{-M} (@pxref{Invocation}). These take precedence over 4503environment variables, which in turn take precedence over the 4504configuration of GCC@. 4505 4506@include cppenv.texi 4507@c man end 4508 4509@page 4510@include fdl.texi 4511 4512@page 4513@node Index of Directives 4514@unnumbered Index of Directives 4515@printindex fn 4516 4517@node Option Index 4518@unnumbered Option Index 4519@noindent 4520CPP's command-line options and environment variables are indexed here 4521without any initial @samp{-} or @samp{--}. 4522@printindex op 4523 4524@page 4525@node Concept Index 4526@unnumbered Concept Index 4527@printindex cp 4528 4529@bye 4530