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, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 141997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 152008, 2009, 2010, 2011 16Free Software Foundation, Inc. 17 18Permission is granted to copy, distribute and/or modify this document 19under the terms of the GNU Free Documentation License, Version 1.2 or 20any later version published by the Free Software Foundation. A copy of 21the license is included in the 22@c man end 23section entitled ``GNU Free Documentation License''. 24@ignore 25@c man begin COPYRIGHT 26man page gfdl(7). 27@c man end 28@end ignore 29 30@c man begin COPYRIGHT 31This manual contains no Invariant Sections. The Front-Cover Texts are 32(a) (see below), and the Back-Cover Texts are (b) (see below). 33 34(a) The FSF's Front-Cover Text is: 35 36 A GNU Manual 37 38(b) The FSF's Back-Cover Text is: 39 40 You have freedom to copy and modify this GNU Manual, like GNU 41 software. Copies published by the Free Software Foundation raise 42 funds for GNU development. 43@c man end 44@end copying 45 46@c Create a separate index for command line options. 47@defcodeindex op 48@syncodeindex vr op 49 50@c Used in cppopts.texi and cppenv.texi. 51@set cppmanual 52 53@ifinfo 54@dircategory Software development 55@direntry 56* Cpp: (cpp). The GNU C preprocessor. 57@end direntry 58@end ifinfo 59 60@titlepage 61@title The C Preprocessor 62@versionsubtitle 63@author Richard M. Stallman, Zachary Weinberg 64@page 65@c There is a fill at the bottom of the page, so we need a filll to 66@c override it. 67@vskip 0pt plus 1filll 68@insertcopying 69@end titlepage 70@contents 71@page 72 73@ifnottex 74@node Top 75@top 76The C preprocessor implements the macro language used to transform C, 77C++, and Objective-C programs before they are compiled. It can also be 78useful on its own. 79 80@menu 81* Overview:: 82* Header Files:: 83* Macros:: 84* Conditionals:: 85* Diagnostics:: 86* Line Control:: 87* Pragmas:: 88* Other Directives:: 89* Preprocessor Output:: 90* Traditional Mode:: 91* Implementation Details:: 92* Invocation:: 93* Environment Variables:: 94* GNU Free Documentation License:: 95* Index of Directives:: 96* Option Index:: 97* Concept Index:: 98 99@detailmenu 100 --- The Detailed Node Listing --- 101 102Overview 103 104* Character sets:: 105* Initial processing:: 106* Tokenization:: 107* The preprocessing language:: 108 109Header Files 110 111* Include Syntax:: 112* Include Operation:: 113* Search Path:: 114* Once-Only Headers:: 115* Alternatives to Wrapper #ifndef:: 116* Computed Includes:: 117* Wrapper Headers:: 118* System Headers:: 119 120Macros 121 122* Object-like Macros:: 123* Function-like Macros:: 124* Macro Arguments:: 125* Stringification:: 126* Concatenation:: 127* Variadic Macros:: 128* Predefined Macros:: 129* Undefining and Redefining Macros:: 130* Directives Within Macro Arguments:: 131* Macro Pitfalls:: 132 133Predefined Macros 134 135* Standard Predefined Macros:: 136* Common Predefined Macros:: 137* System-specific Predefined Macros:: 138* C++ Named Operators:: 139 140Macro Pitfalls 141 142* Misnesting:: 143* Operator Precedence Problems:: 144* Swallowing the Semicolon:: 145* Duplication of Side Effects:: 146* Self-Referential Macros:: 147* Argument Prescan:: 148* Newlines in Arguments:: 149 150Conditionals 151 152* Conditional Uses:: 153* Conditional Syntax:: 154* Deleted Code:: 155 156Conditional Syntax 157 158* Ifdef:: 159* If:: 160* Defined:: 161* Else:: 162* Elif:: 163 164Implementation Details 165 166* Implementation-defined behavior:: 167* Implementation limits:: 168* Obsolete Features:: 169* Differences from previous versions:: 170 171Obsolete Features 172 173* Obsolete Features:: 174 175@end detailmenu 176@end menu 177 178@insertcopying 179@end ifnottex 180 181@node Overview 182@chapter Overview 183@c man begin DESCRIPTION 184The C preprocessor, often known as @dfn{cpp}, is a @dfn{macro processor} 185that is used automatically by the C compiler to transform your program 186before compilation. It is called a macro processor because it allows 187you to define @dfn{macros}, which are brief abbreviations for longer 188constructs. 189 190The C preprocessor is intended to be used only with C, C++, and 191Objective-C source code. In the past, it has been abused as a general 192text processor. It will choke on input which does not obey C's lexical 193rules. For example, apostrophes will be interpreted as the beginning of 194character constants, and cause errors. Also, you cannot rely on it 195preserving characteristics of the input which are not significant to 196C-family languages. If a Makefile is preprocessed, all the hard tabs 197will be removed, and the Makefile will not work. 198 199Having said that, you can often get away with using cpp on things which 200are not C@. Other Algol-ish programming languages are often safe 201(Pascal, Ada, etc.) So is assembly, with caution. @option{-traditional-cpp} 202mode preserves more white space, and is otherwise more permissive. Many 203of the problems can be avoided by writing C or C++ style comments 204instead of native language comments, and keeping macros simple. 205 206Wherever possible, you should use a preprocessor geared to the language 207you are writing in. Modern versions of the GNU assembler have macro 208facilities. Most high level programming languages have their own 209conditional compilation and inclusion mechanism. If all else fails, 210try a true general text processor, such as GNU M4. 211 212C preprocessors vary in some details. This manual discusses the GNU C 213preprocessor, which provides a small superset of the features of ISO 214Standard C@. In its default mode, the GNU C preprocessor does not do a 215few things required by the standard. These are features which are 216rarely, if ever, used, and may cause surprising changes to the meaning 217of a program which does not expect them. To get strict ISO Standard C, 218you should use the @option{-std=c90} or @option{-std=c99} options, depending 219on which version of the standard you want. To get all the mandatory 220diagnostics, you must also use @option{-pedantic}. @xref{Invocation}. 221 222This manual describes the behavior of the ISO preprocessor. To 223minimize gratuitous differences, where the ISO preprocessor's 224behavior does not conflict with traditional semantics, the 225traditional preprocessor should behave the same way. The various 226differences that do exist are detailed in the section @ref{Traditional 227Mode}. 228 229For clarity, unless noted otherwise, references to @samp{CPP} in this 230manual refer to GNU CPP@. 231@c man end 232 233@menu 234* Character sets:: 235* Initial processing:: 236* Tokenization:: 237* The preprocessing language:: 238@end menu 239 240@node Character sets 241@section Character sets 242 243Source code character set processing in C and related languages is 244rather complicated. The C standard discusses two character sets, but 245there are really at least four. 246 247The files input to CPP might be in any character set at all. CPP's 248very first action, before it even looks for line boundaries, is to 249convert the file into the character set it uses for internal 250processing. That set is what the C standard calls the @dfn{source} 251character set. It must be isomorphic with ISO 10646, also known as 252Unicode. CPP uses the UTF-8 encoding of Unicode. 253 254The character sets of the input files are specified using the 255@option{-finput-charset=} option. 256 257All preprocessing work (the subject of the rest of this manual) is 258carried out in the source character set. If you request textual 259output from the preprocessor with the @option{-E} option, it will be 260in UTF-8. 261 262After preprocessing is complete, string and character constants are 263converted again, into the @dfn{execution} character set. This 264character set is under control of the user; the default is UTF-8, 265matching the source character set. Wide string and character 266constants have their own character set, which is not called out 267specifically in the standard. Again, it is under control of the user. 268The default is UTF-16 or UTF-32, whichever fits in the target's 269@code{wchar_t} type, in the target machine's byte 270order.@footnote{UTF-16 does not meet the requirements of the C 271standard for a wide character set, but the choice of 16-bit 272@code{wchar_t} is enshrined in some system ABIs so we cannot fix 273this.} Octal and hexadecimal escape sequences do not undergo 274conversion; @t{'\x12'} has the value 0x12 regardless of the currently 275selected execution character set. All other escapes are replaced by 276the character in the source character set that they represent, then 277converted to the execution character set, just like unescaped 278characters. 279 280Unless the experimental @option{-fextended-identifiers} option is used, 281GCC does not permit the use of characters outside the ASCII range, nor 282@samp{\u} and @samp{\U} escapes, in identifiers. Even with that 283option, characters outside the ASCII range can only be specified with 284the @samp{\u} and @samp{\U} escapes, not used directly in identifiers. 285 286@node Initial processing 287@section Initial processing 288 289The preprocessor performs a series of textual transformations on its 290input. These happen before all other processing. Conceptually, they 291happen in a rigid order, and the entire file is run through each 292transformation before the next one begins. CPP actually does them 293all at once, for performance reasons. These transformations correspond 294roughly to the first three ``phases of translation'' described in the C 295standard. 296 297@enumerate 298@item 299@cindex line endings 300The input file is read into memory and broken into lines. 301 302Different systems use different conventions to indicate the end of a 303line. GCC accepts the ASCII control sequences @kbd{LF}, @kbd{@w{CR 304LF}} and @kbd{CR} as end-of-line markers. These are the canonical 305sequences used by Unix, DOS and VMS, and the classic Mac OS (before 306OSX) respectively. You may therefore safely copy source code written 307on any of those systems to a different one and use it without 308conversion. (GCC may lose track of the current line number if a file 309doesn't consistently use one convention, as sometimes happens when it 310is edited on computers with different conventions that share a network 311file system.) 312 313If the last line of any input file lacks an end-of-line marker, the end 314of the file is considered to implicitly supply one. The C standard says 315that this condition provokes undefined behavior, so GCC will emit a 316warning message. 317 318@item 319@cindex trigraphs 320@anchor{trigraphs}If trigraphs are enabled, they are replaced by their 321corresponding single characters. By default GCC ignores trigraphs, 322but if you request a strictly conforming mode with the @option{-std} 323option, or you specify the @option{-trigraphs} option, then it 324converts them. 325 326These are nine three-character sequences, all starting with @samp{??}, 327that are defined by ISO C to stand for single characters. They permit 328obsolete systems that lack some of C's punctuation to use C@. For 329example, @samp{??/} stands for @samp{\}, so @t{'??/n'} is a character 330constant for a newline. 331 332Trigraphs are not popular and many compilers implement them 333incorrectly. Portable code should not rely on trigraphs being either 334converted or ignored. With @option{-Wtrigraphs} GCC will warn you 335when a trigraph may change the meaning of your program if it were 336converted. @xref{Wtrigraphs}. 337 338In a string constant, you can prevent a sequence of question marks 339from being confused with a trigraph by inserting a backslash between 340the question marks, or by separating the string literal at the 341trigraph and making use of string literal concatenation. @t{"(??\?)"} 342is the string @samp{(???)}, not @samp{(?]}. Traditional C compilers 343do not recognize these idioms. 344 345The nine trigraphs and their replacements are 346 347@smallexample 348Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??- 349Replacement: [ ] @{ @} # \ ^ | ~ 350@end smallexample 351 352@item 353@cindex continued lines 354@cindex backslash-newline 355Continued lines are merged into one long line. 356 357A continued line is a line which ends with a backslash, @samp{\}. The 358backslash is removed and the following line is joined with the current 359one. No space is inserted, so you may split a line anywhere, even in 360the middle of a word. (It is generally more readable to split lines 361only at white space.) 362 363The trailing backslash on a continued line is commonly referred to as a 364@dfn{backslash-newline}. 365 366If there is white space between a backslash and the end of a line, that 367is still a continued line. However, as this is usually the result of an 368editing mistake, and many compilers will not accept it as a continued 369line, GCC will warn you about it. 370 371@item 372@cindex comments 373@cindex line comments 374@cindex block comments 375All comments are replaced with single spaces. 376 377There are two kinds of comments. @dfn{Block comments} begin with 378@samp{/*} and continue until the next @samp{*/}. Block comments do not 379nest: 380 381@smallexample 382/* @r{this is} /* @r{one comment} */ @r{text outside comment} 383@end smallexample 384 385@dfn{Line comments} begin with @samp{//} and continue to the end of the 386current line. Line comments do not nest either, but it does not matter, 387because they would end in the same place anyway. 388 389@smallexample 390// @r{this is} // @r{one comment} 391@r{text outside comment} 392@end smallexample 393@end enumerate 394 395It is safe to put line comments inside block comments, or vice versa. 396 397@smallexample 398@group 399/* @r{block comment} 400 // @r{contains line comment} 401 @r{yet more comment} 402 */ @r{outside comment} 403 404// @r{line comment} /* @r{contains block comment} */ 405@end group 406@end smallexample 407 408But beware of commenting out one end of a block comment with a line 409comment. 410 411@smallexample 412@group 413 // @r{l.c.} /* @r{block comment begins} 414 @r{oops! this isn't a comment anymore} */ 415@end group 416@end smallexample 417 418Comments are not recognized within string literals. 419@t{@w{"/* blah */"}} is the string constant @samp{@w{/* blah */}}, not 420an empty string. 421 422Line comments are not in the 1989 edition of the C standard, but they 423are recognized by GCC as an extension. In C++ and in the 1999 edition 424of the C standard, they are an official part of the language. 425 426Since these transformations happen before all other processing, you can 427split a line mechanically with backslash-newline anywhere. You can 428comment out the end of a line. You can continue a line comment onto the 429next line with backslash-newline. You can even split @samp{/*}, 430@samp{*/}, and @samp{//} onto multiple lines with backslash-newline. 431For example: 432 433@smallexample 434@group 435/\ 436* 437*/ # /* 438*/ defi\ 439ne FO\ 440O 10\ 44120 442@end group 443@end smallexample 444 445@noindent 446is equivalent to @code{@w{#define FOO 1020}}. All these tricks are 447extremely confusing and should not be used in code intended to be 448readable. 449 450There is no way to prevent a backslash at the end of a line from being 451interpreted as a backslash-newline. This cannot affect any correct 452program, however. 453 454@node Tokenization 455@section Tokenization 456 457@cindex tokens 458@cindex preprocessing tokens 459After the textual transformations are finished, the input file is 460converted into a sequence of @dfn{preprocessing tokens}. These mostly 461correspond to the syntactic tokens used by the C compiler, but there are 462a few differences. White space separates tokens; it is not itself a 463token of any kind. Tokens do not have to be separated by white space, 464but it is often necessary to avoid ambiguities. 465 466When faced with a sequence of characters that has more than one possible 467tokenization, the preprocessor is greedy. It always makes each token, 468starting from the left, as big as possible before moving on to the next 469token. For instance, @code{a+++++b} is interpreted as 470@code{@w{a ++ ++ + b}}, not as @code{@w{a ++ + ++ b}}, even though the 471latter tokenization could be part of a valid C program and the former 472could not. 473 474Once the input file is broken into tokens, the token boundaries never 475change, except when the @samp{##} preprocessing operator is used to paste 476tokens together. @xref{Concatenation}. For example, 477 478@smallexample 479@group 480#define foo() bar 481foo()baz 482 @expansion{} bar baz 483@emph{not} 484 @expansion{} barbaz 485@end group 486@end smallexample 487 488The compiler does not re-tokenize the preprocessor's output. Each 489preprocessing token becomes one compiler token. 490 491@cindex identifiers 492Preprocessing tokens fall into five broad classes: identifiers, 493preprocessing numbers, string literals, punctuators, and other. An 494@dfn{identifier} is the same as an identifier in C: any sequence of 495letters, digits, or underscores, which begins with a letter or 496underscore. Keywords of C have no significance to the preprocessor; 497they are ordinary identifiers. You can define a macro whose name is a 498keyword, for instance. The only identifier which can be considered a 499preprocessing keyword is @code{defined}. @xref{Defined}. 500 501This is mostly true of other languages which use the C preprocessor. 502However, a few of the keywords of C++ are significant even in the 503preprocessor. @xref{C++ Named Operators}. 504 505In the 1999 C standard, identifiers may contain letters which are not 506part of the ``basic source character set'', at the implementation's 507discretion (such as accented Latin letters, Greek letters, or Chinese 508ideograms). This may be done with an extended character set, or the 509@samp{\u} and @samp{\U} escape sequences. The implementation of this 510feature in GCC is experimental; such characters are only accepted in 511the @samp{\u} and @samp{\U} forms and only if 512@option{-fextended-identifiers} is used. 513 514As an extension, GCC treats @samp{$} as a letter. This is for 515compatibility with some systems, such as VMS, where @samp{$} is commonly 516used in system-defined function and object names. @samp{$} is not a 517letter in strictly conforming mode, or if you specify the @option{-$} 518option. @xref{Invocation}. 519 520@cindex numbers 521@cindex preprocessing numbers 522A @dfn{preprocessing number} has a rather bizarre definition. The 523category includes all the normal integer and floating point constants 524one expects of C, but also a number of other things one might not 525initially recognize as a number. Formally, preprocessing numbers begin 526with an optional period, a required decimal digit, and then continue 527with any sequence of letters, digits, underscores, periods, and 528exponents. Exponents are the two-character sequences @samp{e+}, 529@samp{e-}, @samp{E+}, @samp{E-}, @samp{p+}, @samp{p-}, @samp{P+}, and 530@samp{P-}. (The exponents that begin with @samp{p} or @samp{P} are new 531to C99. They are used for hexadecimal floating-point constants.) 532 533The purpose of this unusual definition is to isolate the preprocessor 534from the full complexity of numeric constants. It does not have to 535distinguish between lexically valid and invalid floating-point numbers, 536which is complicated. The definition also permits you to split an 537identifier at any position and get exactly two tokens, which can then be 538pasted back together with the @samp{##} operator. 539 540It's possible for preprocessing numbers to cause programs to be 541misinterpreted. For example, @code{0xE+12} is a preprocessing number 542which does not translate to any valid numeric constant, therefore a 543syntax error. It does not mean @code{@w{0xE + 12}}, which is what you 544might have intended. 545 546@cindex string literals 547@cindex string constants 548@cindex character constants 549@cindex header file names 550@c the @: prevents makeinfo from turning '' into ". 551@dfn{String literals} are string constants, character constants, and 552header file names (the argument of @samp{#include}).@footnote{The C 553standard uses the term @dfn{string literal} to refer only to what we are 554calling @dfn{string constants}.} String constants and character 555constants are straightforward: @t{"@dots{}"} or @t{'@dots{}'}. In 556either case embedded quotes should be escaped with a backslash: 557@t{'\'@:'} is the character constant for @samp{'}. There is no limit on 558the length of a character constant, but the value of a character 559constant that contains more than one character is 560implementation-defined. @xref{Implementation Details}. 561 562Header file names either look like string constants, @t{"@dots{}"}, or are 563written with angle brackets instead, @t{<@dots{}>}. In either case, 564backslash is an ordinary character. There is no way to escape the 565closing quote or angle bracket. The preprocessor looks for the header 566file in different places depending on which form you use. @xref{Include 567Operation}. 568 569No string literal may extend past the end of a line. Older versions 570of GCC accepted multi-line string constants. You may use continued 571lines instead, or string constant concatenation. @xref{Differences 572from previous versions}. 573 574@cindex punctuators 575@cindex digraphs 576@cindex alternative tokens 577@dfn{Punctuators} are all the usual bits of punctuation which are 578meaningful to C and C++. All but three of the punctuation characters in 579ASCII are C punctuators. The exceptions are @samp{@@}, @samp{$}, and 580@samp{`}. In addition, all the two- and three-character operators are 581punctuators. There are also six @dfn{digraphs}, which the C++ standard 582calls @dfn{alternative tokens}, which are merely alternate ways to spell 583other punctuators. This is a second attempt to work around missing 584punctuation in obsolete systems. It has no negative side effects, 585unlike trigraphs, but does not cover as much ground. The digraphs and 586their corresponding normal punctuators are: 587 588@smallexample 589Digraph: <% %> <: :> %: %:%: 590Punctuator: @{ @} [ ] # ## 591@end smallexample 592 593@cindex other tokens 594Any other single character is considered ``other''. It is passed on to 595the preprocessor's output unmolested. The C compiler will almost 596certainly reject source code containing ``other'' tokens. In ASCII, the 597only other characters are @samp{@@}, @samp{$}, @samp{`}, and control 598characters other than NUL (all bits zero). (Note that @samp{$} is 599normally considered a letter.) All characters with the high bit set 600(numeric range 0x7F--0xFF) are also ``other'' in the present 601implementation. This will change when proper support for international 602character sets is added to GCC@. 603 604NUL is a special case because of the high probability that its 605appearance is accidental, and because it may be invisible to the user 606(many terminals do not display NUL at all). Within comments, NULs are 607silently ignored, just as any other character would be. In running 608text, NUL is considered white space. For example, these two directives 609have the same meaning. 610 611@smallexample 612#define X^@@1 613#define X 1 614@end smallexample 615 616@noindent 617(where @samp{^@@} is ASCII NUL)@. Within string or character constants, 618NULs are preserved. In the latter two cases the preprocessor emits a 619warning message. 620 621@node The preprocessing language 622@section The preprocessing language 623@cindex directives 624@cindex preprocessing directives 625@cindex directive line 626@cindex directive name 627 628After tokenization, the stream of tokens may simply be passed straight 629to the compiler's parser. However, if it contains any operations in the 630@dfn{preprocessing language}, it will be transformed first. This stage 631corresponds roughly to the standard's ``translation phase 4'' and is 632what most people think of as the preprocessor's job. 633 634The preprocessing language consists of @dfn{directives} to be executed 635and @dfn{macros} to be expanded. Its primary capabilities are: 636 637@itemize @bullet 638@item 639Inclusion of header files. These are files of declarations that can be 640substituted into your program. 641 642@item 643Macro expansion. You can define @dfn{macros}, which are abbreviations 644for arbitrary fragments of C code. The preprocessor will replace the 645macros with their definitions throughout the program. Some macros are 646automatically defined for you. 647 648@item 649Conditional compilation. You can include or exclude parts of the 650program according to various conditions. 651 652@item 653Line control. If you use a program to combine or rearrange source files 654into an intermediate file which is then compiled, you can use line 655control to inform the compiler where each source line originally came 656from. 657 658@item 659Diagnostics. You can detect problems at compile time and issue errors 660or warnings. 661@end itemize 662 663There are a few more, less useful, features. 664 665Except for expansion of predefined macros, all these operations are 666triggered with @dfn{preprocessing directives}. Preprocessing directives 667are lines in your program that start with @samp{#}. Whitespace is 668allowed before and after the @samp{#}. The @samp{#} is followed by an 669identifier, the @dfn{directive name}. It specifies the operation to 670perform. Directives are commonly referred to as @samp{#@var{name}} 671where @var{name} is the directive name. For example, @samp{#define} is 672the directive that defines a macro. 673 674The @samp{#} which begins a directive cannot come from a macro 675expansion. Also, the directive name is not macro expanded. Thus, if 676@code{foo} is defined as a macro expanding to @code{define}, that does 677not make @samp{#foo} a valid preprocessing directive. 678 679The set of valid directive names is fixed. Programs cannot define new 680preprocessing directives. 681 682Some directives require arguments; these make up the rest of the 683directive line and must be separated from the directive name by 684whitespace. For example, @samp{#define} must be followed by a macro 685name and the intended expansion of the macro. 686 687A preprocessing directive cannot cover more than one line. The line 688may, however, be continued with backslash-newline, or by a block comment 689which extends past the end of the line. In either case, when the 690directive is processed, the continuations have already been merged with 691the first line to make one long line. 692 693@node Header Files 694@chapter Header Files 695 696@cindex header file 697A header file is a file containing C declarations and macro definitions 698(@pxref{Macros}) to be shared between several source files. You request 699the use of a header file in your program by @dfn{including} it, with the 700C preprocessing directive @samp{#include}. 701 702Header files serve two purposes. 703 704@itemize @bullet 705@item 706@cindex system header files 707System header files declare the interfaces to parts of the operating 708system. You include them in your program to supply the definitions and 709declarations you need to invoke system calls and libraries. 710 711@item 712Your own header files contain declarations for interfaces between the 713source files of your program. Each time you have a group of related 714declarations and macro definitions all or most of which are needed in 715several different source files, it is a good idea to create a header 716file for them. 717@end itemize 718 719Including a header file produces the same results as copying the header 720file into each source file that needs it. Such copying would be 721time-consuming and error-prone. With a header file, the related 722declarations appear in only one place. If they need to be changed, they 723can be changed in one place, and programs that include the header file 724will automatically use the new version when next recompiled. The header 725file eliminates the labor of finding and changing all the copies as well 726as the risk that a failure to find one copy will result in 727inconsistencies within a program. 728 729In C, the usual convention is to give header files names that end with 730@file{.h}. It is most portable to use only letters, digits, dashes, and 731underscores in header file names, and at most one dot. 732 733@menu 734* Include Syntax:: 735* Include Operation:: 736* Search Path:: 737* Once-Only Headers:: 738* Alternatives to Wrapper #ifndef:: 739* Computed Includes:: 740* Wrapper Headers:: 741* System Headers:: 742@end menu 743 744@node Include Syntax 745@section Include Syntax 746 747@findex #include 748Both user and system header files are included using the preprocessing 749directive @samp{#include}. It has two variants: 750 751@table @code 752@item #include <@var{file}> 753This variant is used for system header files. It searches for a file 754named @var{file} in a standard list of system directories. You can prepend 755directories to this list with the @option{-I} option (@pxref{Invocation}). 756 757@item #include "@var{file}" 758This variant is used for header files of your own program. It 759searches for a file named @var{file} first in the directory containing 760the current file, then in the quote directories and then the same 761directories used for @code{<@var{file}>}. You can prepend directories 762to the list of quote directories with the @option{-iquote} option. 763@end table 764 765The argument of @samp{#include}, whether delimited with quote marks or 766angle brackets, behaves like a string constant in that comments are not 767recognized, and macro names are not expanded. Thus, @code{@w{#include 768<x/*y>}} specifies inclusion of a system header file named @file{x/*y}. 769 770However, if backslashes occur within @var{file}, they are considered 771ordinary text characters, not escape characters. None of the character 772escape sequences appropriate to string constants in C are processed. 773Thus, @code{@w{#include "x\n\\y"}} specifies a filename containing three 774backslashes. (Some systems interpret @samp{\} as a pathname separator. 775All of these also interpret @samp{/} the same way. It is most portable 776to use only @samp{/}.) 777 778It is an error if there is anything (other than comments) on the line 779after the file name. 780 781@node Include Operation 782@section Include Operation 783 784The @samp{#include} directive works by directing the C preprocessor to 785scan the specified file as input before continuing with the rest of the 786current file. The output from the preprocessor contains the output 787already generated, followed by the output resulting from the included 788file, followed by the output that comes from the text after the 789@samp{#include} directive. For example, if you have a header file 790@file{header.h} as follows, 791 792@smallexample 793char *test (void); 794@end smallexample 795 796@noindent 797and a main program called @file{program.c} that uses the header file, 798like this, 799 800@smallexample 801int x; 802#include "header.h" 803 804int 805main (void) 806@{ 807 puts (test ()); 808@} 809@end smallexample 810 811@noindent 812the compiler will see the same token stream as it would if 813@file{program.c} read 814 815@smallexample 816int x; 817char *test (void); 818 819int 820main (void) 821@{ 822 puts (test ()); 823@} 824@end smallexample 825 826Included files are not limited to declarations and macro definitions; 827those are merely the typical uses. Any fragment of a C program can be 828included from another file. The include file could even contain the 829beginning of a statement that is concluded in the containing file, or 830the end of a statement that was started in the including file. However, 831an included file must consist of complete tokens. Comments and string 832literals which have not been closed by the end of an included file are 833invalid. For error recovery, they are considered to end at the end of 834the file. 835 836To avoid confusion, it is best if header files contain only complete 837syntactic units---function declarations or definitions, type 838declarations, etc. 839 840The line following the @samp{#include} directive is always treated as a 841separate line by the C preprocessor, even if the included file lacks a 842final newline. 843 844@node Search Path 845@section Search Path 846 847GCC looks in several different places for headers. On a normal Unix 848system, if you do not instruct it otherwise, it will look for headers 849requested with @code{@w{#include <@var{file}>}} in: 850 851@smallexample 852/usr/local/include 853@var{libdir}/gcc/@var{target}/@var{version}/include 854/usr/@var{target}/include 855/usr/include 856@end smallexample 857 858For C++ programs, it will also look in @file{/usr/include/g++-v3}, 859first. In the above, @var{target} is the canonical name of the system 860GCC was configured to compile code for; often but not always the same as 861the canonical name of the system it runs on. @var{version} is the 862version of GCC in use. 863 864You can add to this list with the @option{-I@var{dir}} command line 865option. All the directories named by @option{-I} are searched, in 866left-to-right order, @emph{before} the default directories. The only 867exception is when @file{dir} is already searched by default. In 868this case, the option is ignored and the search order for system 869directories remains unchanged. 870 871Duplicate directories are removed from the quote and bracket search 872chains before the two chains are merged to make the final search chain. 873Thus, it is possible for a directory to occur twice in the final search 874chain if it was specified in both the quote and bracket chains. 875 876You can prevent GCC from searching any of the default directories with 877the @option{-nostdinc} option. This is useful when you are compiling an 878operating system kernel or some other program that does not use the 879standard C library facilities, or the standard C library itself. 880@option{-I} options are not ignored as described above when 881@option{-nostdinc} is in effect. 882 883GCC looks for headers requested with @code{@w{#include "@var{file}"}} 884first in the directory containing the current file, then in the 885directories as specified by @option{-iquote} options, then in the same 886places it would have looked for a header requested with angle 887brackets. For example, if @file{/usr/include/sys/stat.h} contains 888@code{@w{#include "types.h"}}, GCC looks for @file{types.h} first in 889@file{/usr/include/sys}, then in its usual search path. 890 891@samp{#line} (@pxref{Line Control}) does not change GCC's idea of the 892directory containing the current file. 893 894You may put @option{-I-} at any point in your list of @option{-I} options. 895This has two effects. First, directories appearing before the 896@option{-I-} in the list are searched only for headers requested with 897quote marks. Directories after @option{-I-} are searched for all 898headers. Second, the directory containing the current file is not 899searched for anything, unless it happens to be one of the directories 900named by an @option{-I} switch. @option{-I-} is deprecated, @option{-iquote} 901should be used instead. 902 903@option{-I. -I-} is not the same as no @option{-I} options at all, and does 904not cause the same behavior for @samp{<>} includes that @samp{""} 905includes get with no special options. @option{-I.} searches the 906compiler's current working directory for header files. That may or may 907not be the same as the directory containing the current file. 908 909If you need to look for headers in a directory named @file{-}, write 910@option{-I./-}. 911 912There are several more ways to adjust the header search path. They are 913generally less useful. @xref{Invocation}. 914 915@node Once-Only Headers 916@section Once-Only Headers 917@cindex repeated inclusion 918@cindex including just once 919@cindex wrapper @code{#ifndef} 920 921If a header file happens to be included twice, the compiler will process 922its contents twice. This is very likely to cause an error, e.g.@: when the 923compiler sees the same structure definition twice. Even if it does not, 924it will certainly waste time. 925 926The standard way to prevent this is to enclose the entire real contents 927of the file in a conditional, like this: 928 929@smallexample 930@group 931/* File foo. */ 932#ifndef FILE_FOO_SEEN 933#define FILE_FOO_SEEN 934 935@var{the entire file} 936 937#endif /* !FILE_FOO_SEEN */ 938@end group 939@end smallexample 940 941This construct is commonly known as a @dfn{wrapper #ifndef}. 942When the header is included again, the conditional will be false, 943because @code{FILE_FOO_SEEN} is defined. The preprocessor will skip 944over the entire contents of the file, and the compiler will not see it 945twice. 946 947CPP optimizes even further. It remembers when a header file has a 948wrapper @samp{#ifndef}. If a subsequent @samp{#include} specifies that 949header, and the macro in the @samp{#ifndef} is still defined, it does 950not bother to rescan the file at all. 951 952You can put comments outside the wrapper. They will not interfere with 953this optimization. 954 955@cindex controlling macro 956@cindex guard macro 957The macro @code{FILE_FOO_SEEN} is called the @dfn{controlling macro} or 958@dfn{guard macro}. In a user header file, the macro name should not 959begin with @samp{_}. In a system header file, it should begin with 960@samp{__} to avoid conflicts with user programs. In any kind of header 961file, the macro name should contain the name of the file and some 962additional text, to avoid conflicts with other header files. 963 964@node Alternatives to Wrapper #ifndef 965@section Alternatives to Wrapper #ifndef 966 967CPP supports two more ways of indicating that a header file should be 968read only once. Neither one is as portable as a wrapper @samp{#ifndef} 969and we recommend you do not use them in new programs, with the caveat 970that @samp{#import} is standard practice in Objective-C. 971 972@findex #import 973CPP supports a variant of @samp{#include} called @samp{#import} which 974includes a file, but does so at most once. If you use @samp{#import} 975instead of @samp{#include}, then you don't need the conditionals 976inside the header file to prevent multiple inclusion of the contents. 977@samp{#import} is standard in Objective-C, but is considered a 978deprecated extension in C and C++. 979 980@samp{#import} is not a well designed feature. It requires the users of 981a header file to know that it should only be included once. It is much 982better for the header file's implementor to write the file so that users 983don't need to know this. Using a wrapper @samp{#ifndef} accomplishes 984this goal. 985 986In the present implementation, a single use of @samp{#import} will 987prevent the file from ever being read again, by either @samp{#import} or 988@samp{#include}. You should not rely on this; do not use both 989@samp{#import} and @samp{#include} to refer to the same header file. 990 991Another way to prevent a header file from being included more than once 992is with the @samp{#pragma once} directive. If @samp{#pragma once} is 993seen when scanning a header file, that file will never be read again, no 994matter what. 995 996@samp{#pragma once} does not have the problems that @samp{#import} does, 997but it is not recognized by all preprocessors, so you cannot rely on it 998in a portable program. 999 1000@node Computed Includes 1001@section Computed Includes 1002@cindex computed includes 1003@cindex macros in include 1004 1005Sometimes it is necessary to select one of several different header 1006files to be included into your program. They might specify 1007configuration parameters to be used on different sorts of operating 1008systems, for instance. You could do this with a series of conditionals, 1009 1010@smallexample 1011#if SYSTEM_1 1012# include "system_1.h" 1013#elif SYSTEM_2 1014# include "system_2.h" 1015#elif SYSTEM_3 1016@dots{} 1017#endif 1018@end smallexample 1019 1020That rapidly becomes tedious. Instead, the preprocessor offers the 1021ability to use a macro for the header name. This is called a 1022@dfn{computed include}. Instead of writing a header name as the direct 1023argument of @samp{#include}, you simply put a macro name there instead: 1024 1025@smallexample 1026#define SYSTEM_H "system_1.h" 1027@dots{} 1028#include SYSTEM_H 1029@end smallexample 1030 1031@noindent 1032@code{SYSTEM_H} will be expanded, and the preprocessor will look for 1033@file{system_1.h} as if the @samp{#include} had been written that way 1034originally. @code{SYSTEM_H} could be defined by your Makefile with a 1035@option{-D} option. 1036 1037You must be careful when you define the macro. @samp{#define} saves 1038tokens, not text. The preprocessor has no way of knowing that the macro 1039will be used as the argument of @samp{#include}, so it generates 1040ordinary tokens, not a header name. This is unlikely to cause problems 1041if you use double-quote includes, which are close enough to string 1042constants. If you use angle brackets, however, you may have trouble. 1043 1044The syntax of a computed include is actually a bit more general than the 1045above. If the first non-whitespace character after @samp{#include} is 1046not @samp{"} or @samp{<}, then the entire line is macro-expanded 1047like running text would be. 1048 1049If the line expands to a single string constant, the contents of that 1050string constant are the file to be included. CPP does not re-examine the 1051string for embedded quotes, but neither does it process backslash 1052escapes in the string. Therefore 1053 1054@smallexample 1055#define HEADER "a\"b" 1056#include HEADER 1057@end smallexample 1058 1059@noindent 1060looks for a file named @file{a\"b}. CPP searches for the file according 1061to the rules for double-quoted includes. 1062 1063If the line expands to a token stream beginning with a @samp{<} token 1064and including a @samp{>} token, then the tokens between the @samp{<} and 1065the first @samp{>} are combined to form the filename to be included. 1066Any whitespace between tokens is reduced to a single space; then any 1067space after the initial @samp{<} is retained, but a trailing space 1068before the closing @samp{>} is ignored. CPP searches for the file 1069according to the rules for angle-bracket includes. 1070 1071In either case, if there are any tokens on the line after the file name, 1072an error occurs and the directive is not processed. It is also an error 1073if the result of expansion does not match either of the two expected 1074forms. 1075 1076These rules are implementation-defined behavior according to the C 1077standard. To minimize the risk of different compilers interpreting your 1078computed includes differently, we recommend you use only a single 1079object-like macro which expands to a string constant. This will also 1080minimize confusion for people reading your program. 1081 1082@node Wrapper Headers 1083@section Wrapper Headers 1084@cindex wrapper headers 1085@cindex overriding a header file 1086@findex #include_next 1087 1088Sometimes it is necessary to adjust the contents of a system-provided 1089header file without editing it directly. GCC's @command{fixincludes} 1090operation does this, for example. One way to do that would be to create 1091a new header file with the same name and insert it in the search path 1092before the original header. That works fine as long as you're willing 1093to replace the old header entirely. But what if you want to refer to 1094the old header from the new one? 1095 1096You cannot simply include the old header with @samp{#include}. That 1097will start from the beginning, and find your new header again. If your 1098header is not protected from multiple inclusion (@pxref{Once-Only 1099Headers}), it will recurse infinitely and cause a fatal error. 1100 1101You could include the old header with an absolute pathname: 1102@smallexample 1103#include "/usr/include/old-header.h" 1104@end smallexample 1105@noindent 1106This works, but is not clean; should the system headers ever move, you 1107would have to edit the new headers to match. 1108 1109There is no way to solve this problem within the C standard, but you can 1110use the GNU extension @samp{#include_next}. It means, ``Include the 1111@emph{next} file with this name''. This directive works like 1112@samp{#include} except in searching for the specified file: it starts 1113searching the list of header file directories @emph{after} the directory 1114in which the current file was found. 1115 1116Suppose you specify @option{-I /usr/local/include}, and the list of 1117directories to search also includes @file{/usr/include}; and suppose 1118both directories contain @file{signal.h}. Ordinary @code{@w{#include 1119<signal.h>}} finds the file under @file{/usr/local/include}. If that 1120file contains @code{@w{#include_next <signal.h>}}, it starts searching 1121after that directory, and finds the file in @file{/usr/include}. 1122 1123@samp{#include_next} does not distinguish between @code{<@var{file}>} 1124and @code{"@var{file}"} inclusion, nor does it check that the file you 1125specify has the same name as the current file. It simply looks for the 1126file named, starting with the directory in the search path after the one 1127where the current file was found. 1128 1129The use of @samp{#include_next} can lead to great confusion. We 1130recommend it be used only when there is no other alternative. In 1131particular, it should not be used in the headers belonging to a specific 1132program; it should be used only to make global corrections along the 1133lines of @command{fixincludes}. 1134 1135@node System Headers 1136@section System Headers 1137@cindex system header files 1138 1139The header files declaring interfaces to the operating system and 1140runtime libraries often cannot be written in strictly conforming C@. 1141Therefore, GCC gives code found in @dfn{system headers} special 1142treatment. All warnings, other than those generated by @samp{#warning} 1143(@pxref{Diagnostics}), are suppressed while GCC is processing a system 1144header. Macros defined in a system header are immune to a few warnings 1145wherever they are expanded. This immunity is granted on an ad-hoc 1146basis, when we find that a warning generates lots of false positives 1147because of code in macros defined in system headers. 1148 1149Normally, only the headers found in specific directories are considered 1150system headers. These directories are determined when GCC is compiled. 1151There are, however, two ways to make normal headers into system headers. 1152 1153The @option{-isystem} command line option adds its argument to the list of 1154directories to search for headers, just like @option{-I}. Any headers 1155found in that directory will be considered system headers. 1156 1157All directories named by @option{-isystem} are searched @emph{after} all 1158directories named by @option{-I}, no matter what their order was on the 1159command line. If the same directory is named by both @option{-I} and 1160@option{-isystem}, the @option{-I} option is ignored. GCC provides an 1161informative message when this occurs if @option{-v} is used. 1162 1163The @option{-cxx-isystem} command line option adds its argument to the 1164list of C++ system headers, similar to @option{-isystem} for C headers. 1165 1166@findex #pragma GCC system_header 1167There is also a directive, @code{@w{#pragma GCC system_header}}, which 1168tells GCC to consider the rest of the current include file a system 1169header, no matter where it was found. Code that comes before the 1170@samp{#pragma} in the file will not be affected. @code{@w{#pragma GCC 1171system_header}} has no effect in the primary source file. 1172 1173On very old systems, some of the pre-defined system header directories 1174get even more special treatment. GNU C++ considers code in headers 1175found in those directories to be surrounded by an @code{@w{extern "C"}} 1176block. There is no way to request this behavior with a @samp{#pragma}, 1177or from the command line. 1178 1179@node Macros 1180@chapter Macros 1181 1182A @dfn{macro} is a fragment of code which has been given a name. 1183Whenever the name is used, it is replaced by the contents of the macro. 1184There are two kinds of macros. They differ mostly in what they look 1185like when they are used. @dfn{Object-like} macros resemble data objects 1186when used, @dfn{function-like} macros resemble function calls. 1187 1188You may define any valid identifier as a macro, even if it is a C 1189keyword. The preprocessor does not know anything about keywords. This 1190can be useful if you wish to hide a keyword such as @code{const} from an 1191older compiler that does not understand it. However, the preprocessor 1192operator @code{defined} (@pxref{Defined}) can never be defined as a 1193macro, and C++'s named operators (@pxref{C++ Named Operators}) cannot be 1194macros when you are compiling C++. 1195 1196@menu 1197* Object-like Macros:: 1198* Function-like Macros:: 1199* Macro Arguments:: 1200* Stringification:: 1201* Concatenation:: 1202* Variadic Macros:: 1203* Predefined Macros:: 1204* Undefining and Redefining Macros:: 1205* Directives Within Macro Arguments:: 1206* Macro Pitfalls:: 1207@end menu 1208 1209@node Object-like Macros 1210@section Object-like Macros 1211@cindex object-like macro 1212@cindex symbolic constants 1213@cindex manifest constants 1214 1215An @dfn{object-like macro} is a simple identifier which will be replaced 1216by a code fragment. It is called object-like because it looks like a 1217data object in code that uses it. They are most commonly used to give 1218symbolic names to numeric constants. 1219 1220@findex #define 1221You create macros with the @samp{#define} directive. @samp{#define} is 1222followed by the name of the macro and then the token sequence it should 1223be an abbreviation for, which is variously referred to as the macro's 1224@dfn{body}, @dfn{expansion} or @dfn{replacement list}. For example, 1225 1226@smallexample 1227#define BUFFER_SIZE 1024 1228@end smallexample 1229 1230@noindent 1231defines a macro named @code{BUFFER_SIZE} as an abbreviation for the 1232token @code{1024}. If somewhere after this @samp{#define} directive 1233there comes a C statement of the form 1234 1235@smallexample 1236foo = (char *) malloc (BUFFER_SIZE); 1237@end smallexample 1238 1239@noindent 1240then the C preprocessor will recognize and @dfn{expand} the macro 1241@code{BUFFER_SIZE}. The C compiler will see the same tokens as it would 1242if you had written 1243 1244@smallexample 1245foo = (char *) malloc (1024); 1246@end smallexample 1247 1248By convention, macro names are written in uppercase. Programs are 1249easier to read when it is possible to tell at a glance which names are 1250macros. 1251 1252The macro's body ends at the end of the @samp{#define} line. You may 1253continue the definition onto multiple lines, if necessary, using 1254backslash-newline. When the macro is expanded, however, it will all 1255come out on one line. For example, 1256 1257@smallexample 1258#define NUMBERS 1, \ 1259 2, \ 1260 3 1261int x[] = @{ NUMBERS @}; 1262 @expansion{} int x[] = @{ 1, 2, 3 @}; 1263@end smallexample 1264 1265@noindent 1266The most common visible consequence of this is surprising line numbers 1267in error messages. 1268 1269There is no restriction on what can go in a macro body provided it 1270decomposes into valid preprocessing tokens. Parentheses need not 1271balance, and the body need not resemble valid C code. (If it does not, 1272you may get error messages from the C compiler when you use the macro.) 1273 1274The C preprocessor scans your program sequentially. Macro definitions 1275take effect at the place you write them. Therefore, the following input 1276to the C preprocessor 1277 1278@smallexample 1279foo = X; 1280#define X 4 1281bar = X; 1282@end smallexample 1283 1284@noindent 1285produces 1286 1287@smallexample 1288foo = X; 1289bar = 4; 1290@end smallexample 1291 1292When the preprocessor expands a macro name, the macro's expansion 1293replaces the macro invocation, then the expansion is examined for more 1294macros to expand. For example, 1295 1296@smallexample 1297@group 1298#define TABLESIZE BUFSIZE 1299#define BUFSIZE 1024 1300TABLESIZE 1301 @expansion{} BUFSIZE 1302 @expansion{} 1024 1303@end group 1304@end smallexample 1305 1306@noindent 1307@code{TABLESIZE} is expanded first to produce @code{BUFSIZE}, then that 1308macro is expanded to produce the final result, @code{1024}. 1309 1310Notice that @code{BUFSIZE} was not defined when @code{TABLESIZE} was 1311defined. The @samp{#define} for @code{TABLESIZE} uses exactly the 1312expansion you specify---in this case, @code{BUFSIZE}---and does not 1313check to see whether it too contains macro names. Only when you 1314@emph{use} @code{TABLESIZE} is the result of its expansion scanned for 1315more macro names. 1316 1317This makes a difference if you change the definition of @code{BUFSIZE} 1318at some point in the source file. @code{TABLESIZE}, defined as shown, 1319will always expand using the definition of @code{BUFSIZE} that is 1320currently in effect: 1321 1322@smallexample 1323#define BUFSIZE 1020 1324#define TABLESIZE BUFSIZE 1325#undef BUFSIZE 1326#define BUFSIZE 37 1327@end smallexample 1328 1329@noindent 1330Now @code{TABLESIZE} expands (in two stages) to @code{37}. 1331 1332If the expansion of a macro contains its own name, either directly or 1333via intermediate macros, it is not expanded again when the expansion is 1334examined for more macros. This prevents infinite recursion. 1335@xref{Self-Referential Macros}, for the precise details. 1336 1337@node Function-like Macros 1338@section Function-like Macros 1339@cindex function-like macros 1340 1341You can also define macros whose use looks like a function call. These 1342are called @dfn{function-like macros}. To define a function-like macro, 1343you use the same @samp{#define} directive, but you put a pair of 1344parentheses immediately after the macro name. For example, 1345 1346@smallexample 1347#define lang_init() c_init() 1348lang_init() 1349 @expansion{} c_init() 1350@end smallexample 1351 1352A function-like macro is only expanded if its name appears with a pair 1353of parentheses after it. If you write just the name, it is left alone. 1354This can be useful when you have a function and a macro of the same 1355name, and you wish to use the function sometimes. 1356 1357@smallexample 1358extern void foo(void); 1359#define foo() /* @r{optimized inline version} */ 1360@dots{} 1361 foo(); 1362 funcptr = foo; 1363@end smallexample 1364 1365Here the call to @code{foo()} will use the macro, but the function 1366pointer will get the address of the real function. If the macro were to 1367be expanded, it would cause a syntax error. 1368 1369If you put spaces between the macro name and the parentheses in the 1370macro definition, that does not define a function-like macro, it defines 1371an object-like macro whose expansion happens to begin with a pair of 1372parentheses. 1373 1374@smallexample 1375#define lang_init () c_init() 1376lang_init() 1377 @expansion{} () c_init()() 1378@end smallexample 1379 1380The first two pairs of parentheses in this expansion come from the 1381macro. The third is the pair that was originally after the macro 1382invocation. Since @code{lang_init} is an object-like macro, it does not 1383consume those parentheses. 1384 1385@node Macro Arguments 1386@section Macro Arguments 1387@cindex arguments 1388@cindex macros with arguments 1389@cindex arguments in macro definitions 1390 1391Function-like macros can take @dfn{arguments}, just like true functions. 1392To define a macro that uses arguments, you insert @dfn{parameters} 1393between the pair of parentheses in the macro definition that make the 1394macro function-like. The parameters must be valid C identifiers, 1395separated by commas and optionally whitespace. 1396 1397To invoke a macro that takes arguments, you write the name of the macro 1398followed by a list of @dfn{actual arguments} in parentheses, separated 1399by commas. The invocation of the macro need not be restricted to a 1400single logical line---it can cross as many lines in the source file as 1401you wish. The number of arguments you give must match the number of 1402parameters in the macro definition. When the macro is expanded, each 1403use of a parameter in its body is replaced by the tokens of the 1404corresponding argument. (You need not use all of the parameters in the 1405macro body.) 1406 1407As an example, here is a macro that computes the minimum of two numeric 1408values, as it is defined in many C programs, and some uses. 1409 1410@smallexample 1411#define min(X, Y) ((X) < (Y) ? (X) : (Y)) 1412 x = min(a, b); @expansion{} x = ((a) < (b) ? (a) : (b)); 1413 y = min(1, 2); @expansion{} y = ((1) < (2) ? (1) : (2)); 1414 z = min(a + 28, *p); @expansion{} z = ((a + 28) < (*p) ? (a + 28) : (*p)); 1415@end smallexample 1416 1417@noindent 1418(In this small example you can already see several of the dangers of 1419macro arguments. @xref{Macro Pitfalls}, for detailed explanations.) 1420 1421Leading and trailing whitespace in each argument is dropped, and all 1422whitespace between the tokens of an argument is reduced to a single 1423space. Parentheses within each argument must balance; a comma within 1424such parentheses does not end the argument. However, there is no 1425requirement for square brackets or braces to balance, and they do not 1426prevent a comma from separating arguments. Thus, 1427 1428@smallexample 1429macro (array[x = y, x + 1]) 1430@end smallexample 1431 1432@noindent 1433passes two arguments to @code{macro}: @code{array[x = y} and @code{x + 14341]}. If you want to supply @code{array[x = y, x + 1]} as an argument, 1435you can write it as @code{array[(x = y, x + 1)]}, which is equivalent C 1436code. 1437 1438All arguments to a macro are completely macro-expanded before they are 1439substituted into the macro body. After substitution, the complete text 1440is scanned again for macros to expand, including the arguments. This rule 1441may seem strange, but it is carefully designed so you need not worry 1442about whether any function call is actually a macro invocation. You can 1443run into trouble if you try to be too clever, though. @xref{Argument 1444Prescan}, for detailed discussion. 1445 1446For example, @code{min (min (a, b), c)} is first expanded to 1447 1448@smallexample 1449 min (((a) < (b) ? (a) : (b)), (c)) 1450@end smallexample 1451 1452@noindent 1453and then to 1454 1455@smallexample 1456@group 1457((((a) < (b) ? (a) : (b))) < (c) 1458 ? (((a) < (b) ? (a) : (b))) 1459 : (c)) 1460@end group 1461@end smallexample 1462 1463@noindent 1464(Line breaks shown here for clarity would not actually be generated.) 1465 1466@cindex empty macro arguments 1467You can leave macro arguments empty; this is not an error to the 1468preprocessor (but many macros will then expand to invalid code). 1469You cannot leave out arguments entirely; if a macro takes two arguments, 1470there must be exactly one comma at the top level of its argument list. 1471Here are some silly examples using @code{min}: 1472 1473@smallexample 1474min(, b) @expansion{} (( ) < (b) ? ( ) : (b)) 1475min(a, ) @expansion{} ((a ) < ( ) ? (a ) : ( )) 1476min(,) @expansion{} (( ) < ( ) ? ( ) : ( )) 1477min((,),) @expansion{} (((,)) < ( ) ? ((,)) : ( )) 1478 1479min() @error{} macro "min" requires 2 arguments, but only 1 given 1480min(,,) @error{} macro "min" passed 3 arguments, but takes just 2 1481@end smallexample 1482 1483Whitespace is not a preprocessing token, so if a macro @code{foo} takes 1484one argument, @code{@w{foo ()}} and @code{@w{foo ( )}} both supply it an 1485empty argument. Previous GNU preprocessor implementations and 1486documentation were incorrect on this point, insisting that a 1487function-like macro that takes a single argument be passed a space if an 1488empty argument was required. 1489 1490Macro parameters appearing inside string literals are not replaced by 1491their corresponding actual arguments. 1492 1493@smallexample 1494#define foo(x) x, "x" 1495foo(bar) @expansion{} bar, "x" 1496@end smallexample 1497 1498@node Stringification 1499@section Stringification 1500@cindex stringification 1501@cindex @samp{#} operator 1502 1503Sometimes you may want to convert a macro argument into a string 1504constant. Parameters are not replaced inside string constants, but you 1505can use the @samp{#} preprocessing operator instead. When a macro 1506parameter is used with a leading @samp{#}, the preprocessor replaces it 1507with the literal text of the actual argument, converted to a string 1508constant. Unlike normal parameter replacement, the argument is not 1509macro-expanded first. This is called @dfn{stringification}. 1510 1511There is no way to combine an argument with surrounding text and 1512stringify it all together. Instead, you can write a series of adjacent 1513string constants and stringified arguments. The preprocessor will 1514replace the stringified arguments with string constants. The C 1515compiler will then combine all the adjacent string constants into one 1516long string. 1517 1518Here is an example of a macro definition that uses stringification: 1519 1520@smallexample 1521@group 1522#define WARN_IF(EXP) \ 1523do @{ if (EXP) \ 1524 fprintf (stderr, "Warning: " #EXP "\n"); @} \ 1525while (0) 1526WARN_IF (x == 0); 1527 @expansion{} do @{ if (x == 0) 1528 fprintf (stderr, "Warning: " "x == 0" "\n"); @} while (0); 1529@end group 1530@end smallexample 1531 1532@noindent 1533The argument for @code{EXP} is substituted once, as-is, into the 1534@code{if} statement, and once, stringified, into the argument to 1535@code{fprintf}. If @code{x} were a macro, it would be expanded in the 1536@code{if} statement, but not in the string. 1537 1538The @code{do} and @code{while (0)} are a kludge to make it possible to 1539write @code{WARN_IF (@var{arg});}, which the resemblance of 1540@code{WARN_IF} to a function would make C programmers want to do; see 1541@ref{Swallowing the Semicolon}. 1542 1543Stringification in C involves more than putting double-quote characters 1544around the fragment. The preprocessor backslash-escapes the quotes 1545surrounding embedded string constants, and all backslashes within string and 1546character constants, in order to get a valid C string constant with the 1547proper contents. Thus, stringifying @code{@w{p = "foo\n";}} results in 1548@t{@w{"p = \"foo\\n\";"}}. However, backslashes that are not inside string 1549or character constants are not duplicated: @samp{\n} by itself 1550stringifies to @t{"\n"}. 1551 1552All leading and trailing whitespace in text being stringified is 1553ignored. Any sequence of whitespace in the middle of the text is 1554converted to a single space in the stringified result. Comments are 1555replaced by whitespace long before stringification happens, so they 1556never appear in stringified text. 1557 1558There is no way to convert a macro argument into a character constant. 1559 1560If you want to stringify the result of expansion of a macro argument, 1561you have to use two levels of macros. 1562 1563@smallexample 1564#define xstr(s) str(s) 1565#define str(s) #s 1566#define foo 4 1567str (foo) 1568 @expansion{} "foo" 1569xstr (foo) 1570 @expansion{} xstr (4) 1571 @expansion{} str (4) 1572 @expansion{} "4" 1573@end smallexample 1574 1575@code{s} is stringified when it is used in @code{str}, so it is not 1576macro-expanded first. But @code{s} is an ordinary argument to 1577@code{xstr}, so it is completely macro-expanded before @code{xstr} 1578itself is expanded (@pxref{Argument Prescan}). Therefore, by the time 1579@code{str} gets to its argument, it has already been macro-expanded. 1580 1581@node Concatenation 1582@section Concatenation 1583@cindex concatenation 1584@cindex token pasting 1585@cindex token concatenation 1586@cindex @samp{##} operator 1587 1588It is often useful to merge two tokens into one while expanding macros. 1589This is called @dfn{token pasting} or @dfn{token concatenation}. The 1590@samp{##} preprocessing operator performs token pasting. When a macro 1591is expanded, the two tokens on either side of each @samp{##} operator 1592are combined into a single token, which then replaces the @samp{##} and 1593the two original tokens in the macro expansion. Usually both will be 1594identifiers, or one will be an identifier and the other a preprocessing 1595number. When pasted, they make a longer identifier. This isn't the 1596only valid case. It is also possible to concatenate two numbers (or a 1597number and a name, such as @code{1.5} and @code{e3}) into a number. 1598Also, multi-character operators such as @code{+=} can be formed by 1599token pasting. 1600 1601However, two tokens that don't together form a valid token cannot be 1602pasted together. For example, you cannot concatenate @code{x} with 1603@code{+} in either order. If you try, the preprocessor issues a warning 1604and emits the two tokens. Whether it puts white space between the 1605tokens is undefined. It is common to find unnecessary uses of @samp{##} 1606in complex macros. If you get this warning, it is likely that you can 1607simply remove the @samp{##}. 1608 1609Both the tokens combined by @samp{##} could come from the macro body, 1610but you could just as well write them as one token in the first place. 1611Token pasting is most useful when one or both of the tokens comes from a 1612macro argument. If either of the tokens next to an @samp{##} is a 1613parameter name, it is replaced by its actual argument before @samp{##} 1614executes. As with stringification, the actual argument is not 1615macro-expanded first. If the argument is empty, that @samp{##} has no 1616effect. 1617 1618Keep in mind that the C preprocessor converts comments to whitespace 1619before macros are even considered. Therefore, you cannot create a 1620comment by concatenating @samp{/} and @samp{*}. You can put as much 1621whitespace between @samp{##} and its operands as you like, including 1622comments, and you can put comments in arguments that will be 1623concatenated. However, it is an error if @samp{##} appears at either 1624end of a macro body. 1625 1626Consider a C program that interprets named commands. There probably 1627needs to be a table of commands, perhaps an array of structures declared 1628as follows: 1629 1630@smallexample 1631@group 1632struct command 1633@{ 1634 char *name; 1635 void (*function) (void); 1636@}; 1637@end group 1638 1639@group 1640struct command commands[] = 1641@{ 1642 @{ "quit", quit_command @}, 1643 @{ "help", help_command @}, 1644 @dots{} 1645@}; 1646@end group 1647@end smallexample 1648 1649It would be cleaner not to have to give each command name twice, once in 1650the string constant and once in the function name. A macro which takes the 1651name of a command as an argument can make this unnecessary. The string 1652constant can be created with stringification, and the function name by 1653concatenating the argument with @samp{_command}. Here is how it is done: 1654 1655@smallexample 1656#define COMMAND(NAME) @{ #NAME, NAME ## _command @} 1657 1658struct command commands[] = 1659@{ 1660 COMMAND (quit), 1661 COMMAND (help), 1662 @dots{} 1663@}; 1664@end smallexample 1665 1666@node Variadic Macros 1667@section Variadic Macros 1668@cindex variable number of arguments 1669@cindex macros with variable arguments 1670@cindex variadic macros 1671 1672A macro can be declared to accept a variable number of arguments much as 1673a function can. The syntax for defining the macro is similar to that of 1674a function. Here is an example: 1675 1676@smallexample 1677#define eprintf(@dots{}) fprintf (stderr, __VA_ARGS__) 1678@end smallexample 1679 1680This kind of macro is called @dfn{variadic}. When the macro is invoked, 1681all the tokens in its argument list after the last named argument (this 1682macro has none), including any commas, become the @dfn{variable 1683argument}. This sequence of tokens replaces the identifier 1684@code{@w{__VA_ARGS__}} in the macro body wherever it appears. Thus, we 1685have this expansion: 1686 1687@smallexample 1688eprintf ("%s:%d: ", input_file, lineno) 1689 @expansion{} fprintf (stderr, "%s:%d: ", input_file, lineno) 1690@end smallexample 1691 1692The variable argument is completely macro-expanded before it is inserted 1693into the macro expansion, just like an ordinary argument. You may use 1694the @samp{#} and @samp{##} operators to stringify the variable argument 1695or to paste its leading or trailing token with another token. (But see 1696below for an important special case for @samp{##}.) 1697 1698If your macro is complicated, you may want a more descriptive name for 1699the variable argument than @code{@w{__VA_ARGS__}}. CPP permits 1700this, as an extension. You may write an argument name immediately 1701before the @samp{@dots{}}; that name is used for the variable argument. 1702The @code{eprintf} macro above could be written 1703 1704@smallexample 1705#define eprintf(args@dots{}) fprintf (stderr, args) 1706@end smallexample 1707 1708@noindent 1709using this extension. You cannot use @code{@w{__VA_ARGS__}} and this 1710extension in the same macro. 1711 1712You can have named arguments as well as variable arguments in a variadic 1713macro. We could define @code{eprintf} like this, instead: 1714 1715@smallexample 1716#define eprintf(format, @dots{}) fprintf (stderr, format, __VA_ARGS__) 1717@end smallexample 1718 1719@noindent 1720This formulation looks more descriptive, but unfortunately it is less 1721flexible: you must now supply at least one argument after the format 1722string. In standard C, you cannot omit the comma separating the named 1723argument from the variable arguments. Furthermore, if you leave the 1724variable argument empty, you will get a syntax error, because 1725there will be an extra comma after the format string. 1726 1727@smallexample 1728eprintf("success!\n", ); 1729 @expansion{} fprintf(stderr, "success!\n", ); 1730@end smallexample 1731 1732GNU CPP has a pair of extensions which deal with this problem. First, 1733you are allowed to leave the variable argument out entirely: 1734 1735@smallexample 1736eprintf ("success!\n") 1737 @expansion{} fprintf(stderr, "success!\n", ); 1738@end smallexample 1739 1740@noindent 1741Second, the @samp{##} token paste operator has a special meaning when 1742placed between a comma and a variable argument. If you write 1743 1744@smallexample 1745#define eprintf(format, @dots{}) fprintf (stderr, format, ##__VA_ARGS__) 1746@end smallexample 1747 1748@noindent 1749and the variable argument is left out when the @code{eprintf} macro is 1750used, then the comma before the @samp{##} will be deleted. This does 1751@emph{not} happen if you pass an empty argument, nor does it happen if 1752the token preceding @samp{##} is anything other than a comma. 1753 1754@smallexample 1755eprintf ("success!\n") 1756 @expansion{} fprintf(stderr, "success!\n"); 1757@end smallexample 1758 1759@noindent 1760The above explanation is ambiguous about the case where the only macro 1761parameter is a variable arguments parameter, as it is meaningless to 1762try to distinguish whether no argument at all is an empty argument or 1763a missing argument. In this case the C99 standard is clear that the 1764comma must remain, however the existing GCC extension used to swallow 1765the comma. So CPP retains the comma when conforming to a specific C 1766standard, and drops it otherwise. 1767 1768C99 mandates that the only place the identifier @code{@w{__VA_ARGS__}} 1769can appear is in the replacement list of a variadic macro. It may not 1770be used as a macro name, macro argument name, or within a different type 1771of macro. It may also be forbidden in open text; the standard is 1772ambiguous. We recommend you avoid using it except for its defined 1773purpose. 1774 1775Variadic macros are a new feature in C99. GNU CPP has supported them 1776for a long time, but only with a named variable argument 1777(@samp{args@dots{}}, not @samp{@dots{}} and @code{@w{__VA_ARGS__}}). If you are 1778concerned with portability to previous versions of GCC, you should use 1779only named variable arguments. On the other hand, if you are concerned 1780with portability to other conforming implementations of C99, you should 1781use only @code{@w{__VA_ARGS__}}. 1782 1783Previous versions of CPP implemented the comma-deletion extension 1784much more generally. We have restricted it in this release to minimize 1785the differences from C99. To get the same effect with both this and 1786previous versions of GCC, the token preceding the special @samp{##} must 1787be a comma, and there must be white space between that comma and 1788whatever comes immediately before it: 1789 1790@smallexample 1791#define eprintf(format, args@dots{}) fprintf (stderr, format , ##args) 1792@end smallexample 1793 1794@noindent 1795@xref{Differences from previous versions}, for the gory details. 1796 1797@node Predefined Macros 1798@section Predefined Macros 1799 1800@cindex predefined macros 1801Several object-like macros are predefined; you use them without 1802supplying their definitions. They fall into three classes: standard, 1803common, and system-specific. 1804 1805In C++, there is a fourth category, the named operators. They act like 1806predefined macros, but you cannot undefine them. 1807 1808@menu 1809* Standard Predefined Macros:: 1810* Common Predefined Macros:: 1811* System-specific Predefined Macros:: 1812* C++ Named Operators:: 1813@end menu 1814 1815@node Standard Predefined Macros 1816@subsection Standard Predefined Macros 1817@cindex standard predefined macros. 1818 1819The standard predefined macros are specified by the relevant 1820language standards, so they are available with all compilers that 1821implement those standards. Older compilers may not provide all of 1822them. Their names all start with double underscores. 1823 1824@table @code 1825@item __FILE__ 1826This macro expands to the name of the current input file, in the form of 1827a C string constant. This is the path by which the preprocessor opened 1828the file, not the short name specified in @samp{#include} or as the 1829input file name argument. For example, 1830@code{"/usr/local/include/myheader.h"} is a possible expansion of this 1831macro. 1832 1833@item __LINE__ 1834This macro expands to the current input line number, in the form of a 1835decimal integer constant. While we call it a predefined macro, it's 1836a pretty strange macro, since its ``definition'' changes with each 1837new line of source code. 1838@end table 1839 1840@code{__FILE__} and @code{__LINE__} are useful in generating an error 1841message to report an inconsistency detected by the program; the message 1842can state the source line at which the inconsistency was detected. For 1843example, 1844 1845@smallexample 1846fprintf (stderr, "Internal error: " 1847 "negative string length " 1848 "%d at %s, line %d.", 1849 length, __FILE__, __LINE__); 1850@end smallexample 1851 1852An @samp{#include} directive changes the expansions of @code{__FILE__} 1853and @code{__LINE__} to correspond to the included file. At the end of 1854that file, when processing resumes on the input file that contained 1855the @samp{#include} directive, the expansions of @code{__FILE__} and 1856@code{__LINE__} revert to the values they had before the 1857@samp{#include} (but @code{__LINE__} is then incremented by one as 1858processing moves to the line after the @samp{#include}). 1859 1860A @samp{#line} directive changes @code{__LINE__}, and may change 1861@code{__FILE__} as well. @xref{Line Control}. 1862 1863C99 introduces @code{__func__}, and GCC has provided @code{__FUNCTION__} 1864for a long time. Both of these are strings containing the name of the 1865current function (there are slight semantic differences; see the GCC 1866manual). Neither of them is a macro; the preprocessor does not know the 1867name of the current function. They tend to be useful in conjunction 1868with @code{__FILE__} and @code{__LINE__}, though. 1869 1870@table @code 1871 1872@item __DATE__ 1873This macro expands to a string constant that describes the date on which 1874the preprocessor is being run. The string constant contains eleven 1875characters and looks like @code{@w{"Feb 12 1996"}}. If the day of the 1876month is less than 10, it is padded with a space on the left. 1877 1878If GCC cannot determine the current date, it will emit a warning message 1879(once per compilation) and @code{__DATE__} will expand to 1880@code{@w{"??? ?? ????"}}. 1881 1882@item __TIME__ 1883This macro expands to a string constant that describes the time at 1884which the preprocessor is being run. The string constant contains 1885eight characters and looks like @code{"23:59:01"}. 1886 1887If GCC cannot determine the current time, it will emit a warning message 1888(once per compilation) and @code{__TIME__} will expand to 1889@code{"??:??:??"}. 1890 1891@item __STDC__ 1892In normal operation, this macro expands to the constant 1, to signify 1893that this compiler conforms to ISO Standard C@. If GNU CPP is used with 1894a compiler other than GCC, this is not necessarily true; however, the 1895preprocessor always conforms to the standard unless the 1896@option{-traditional-cpp} option is used. 1897 1898This macro is not defined if the @option{-traditional-cpp} option is used. 1899 1900On some hosts, the system compiler uses a different convention, where 1901@code{__STDC__} is normally 0, but is 1 if the user specifies strict 1902conformance to the C Standard. CPP follows the host convention when 1903processing system header files, but when processing user files 1904@code{__STDC__} is always 1. This has been reported to cause problems; 1905for instance, some versions of Solaris provide X Windows headers that 1906expect @code{__STDC__} to be either undefined or 1. @xref{Invocation}. 1907 1908@item __STDC_VERSION__ 1909This macro expands to the C Standard's version number, a long integer 1910constant of the form @code{@var{yyyy}@var{mm}L} where @var{yyyy} and 1911@var{mm} are the year and month of the Standard version. This signifies 1912which version of the C Standard the compiler conforms to. Like 1913@code{__STDC__}, this is not necessarily accurate for the entire 1914implementation, unless GNU CPP is being used with GCC@. 1915 1916The value @code{199409L} signifies the 1989 C standard as amended in 19171994, which is the current default; the value @code{199901L} signifies 1918the 1999 revision of the C standard. Support for the 1999 revision is 1919not yet complete. 1920 1921This macro is not defined if the @option{-traditional-cpp} option is 1922used, nor when compiling C++ or Objective-C@. 1923 1924@item __STDC_HOSTED__ 1925This macro is defined, with value 1, if the compiler's target is a 1926@dfn{hosted environment}. A hosted environment has the complete 1927facilities of the standard C library available. 1928 1929@item __cplusplus 1930This macro is defined when the C++ compiler is in use. You can use 1931@code{__cplusplus} to test whether a header is compiled by a C compiler 1932or a C++ compiler. This macro is similar to @code{__STDC_VERSION__}, in 1933that it expands to a version number. A fully conforming implementation 1934of the 1998 C++ standard will define this macro to @code{199711L}. The 1935GNU C++ compiler is not yet fully conforming, so it uses @code{1} 1936instead. It is hoped to complete the implementation of standard C++ 1937in the near future. 1938 1939@item __OBJC__ 1940This macro is defined, with value 1, when the Objective-C compiler is in 1941use. You can use @code{__OBJC__} to test whether a header is compiled 1942by a C compiler or an Objective-C compiler. 1943 1944@item __ASSEMBLER__ 1945This macro is defined with value 1 when preprocessing assembly 1946language. 1947 1948@end table 1949 1950@node Common Predefined Macros 1951@subsection Common Predefined Macros 1952@cindex common predefined macros 1953 1954The common predefined macros are GNU C extensions. They are available 1955with the same meanings regardless of the machine or operating system on 1956which you are using GNU C or GNU Fortran. Their names all start with 1957double underscores. 1958 1959@table @code 1960 1961@item __COUNTER__ 1962This macro expands to sequential integral values starting from 0. In 1963conjunction with the @code{##} operator, this provides a convenient means to 1964generate unique identifiers. Care must be taken to ensure that 1965@code{__COUNTER__} is not expanded prior to inclusion of precompiled headers 1966which use it. Otherwise, the precompiled headers will not be used. 1967 1968@item __GFORTRAN__ 1969The GNU Fortran compiler defines this. 1970 1971@item __GNUC__ 1972@itemx __GNUC_MINOR__ 1973@itemx __GNUC_PATCHLEVEL__ 1974These macros are defined by all GNU compilers that use the C 1975preprocessor: C, C++, Objective-C and Fortran. Their values are the major 1976version, minor version, and patch level of the compiler, as integer 1977constants. For example, GCC 3.2.1 will define @code{__GNUC__} to 3, 1978@code{__GNUC_MINOR__} to 2, and @code{__GNUC_PATCHLEVEL__} to 1. These 1979macros are also defined if you invoke the preprocessor directly. 1980 1981@code{__GNUC_PATCHLEVEL__} is new to GCC 3.0; it is also present in the 1982widely-used development snapshots leading up to 3.0 (which identify 1983themselves as GCC 2.96 or 2.97, depending on which snapshot you have). 1984 1985If all you need to know is whether or not your program is being compiled 1986by GCC, or a non-GCC compiler that claims to accept the GNU C dialects, 1987you can simply test @code{__GNUC__}. If you need to write code 1988which depends on a specific version, you must be more careful. Each 1989time the minor version is increased, the patch level is reset to zero; 1990each time the major version is increased (which happens rarely), the 1991minor version and patch level are reset. If you wish to use the 1992predefined macros directly in the conditional, you will need to write it 1993like this: 1994 1995@smallexample 1996/* @r{Test for GCC > 3.2.0} */ 1997#if __GNUC__ > 3 || \ 1998 (__GNUC__ == 3 && (__GNUC_MINOR__ > 2 || \ 1999 (__GNUC_MINOR__ == 2 && \ 2000 __GNUC_PATCHLEVEL__ > 0)) 2001@end smallexample 2002 2003@noindent 2004Another approach is to use the predefined macros to 2005calculate a single number, then compare that against a threshold: 2006 2007@smallexample 2008#define GCC_VERSION (__GNUC__ * 10000 \ 2009 + __GNUC_MINOR__ * 100 \ 2010 + __GNUC_PATCHLEVEL__) 2011@dots{} 2012/* @r{Test for GCC > 3.2.0} */ 2013#if GCC_VERSION > 30200 2014@end smallexample 2015 2016@noindent 2017Many people find this form easier to understand. 2018 2019@item __GNUG__ 2020The GNU C++ compiler defines this. Testing it is equivalent to 2021testing @code{@w{(__GNUC__ && __cplusplus)}}. 2022 2023@item __STRICT_ANSI__ 2024GCC defines this macro if and only if the @option{-ansi} switch, or a 2025@option{-std} switch specifying strict conformance to some version of ISO C, 2026was specified when GCC was invoked. It is defined to @samp{1}. 2027This macro exists primarily to direct GNU libc's header files to 2028restrict their definitions to the minimal set found in the 1989 C 2029standard. 2030 2031@item __BASE_FILE__ 2032This macro expands to the name of the main input file, in the form 2033of a C string constant. This is the source file that was specified 2034on the command line of the preprocessor or C compiler. 2035 2036@item __INCLUDE_LEVEL__ 2037This macro expands to a decimal integer constant that represents the 2038depth of nesting in include files. The value of this macro is 2039incremented on every @samp{#include} directive and decremented at the 2040end of every included file. It starts out at 0, its value within the 2041base file specified on the command line. 2042 2043@item __ELF__ 2044This macro is defined if the target uses the ELF object format. 2045 2046@item __VERSION__ 2047This macro expands to a string constant which describes the version of 2048the compiler in use. You should not rely on its contents having any 2049particular form, but it can be counted on to contain at least the 2050release number. 2051 2052@item __OPTIMIZE__ 2053@itemx __OPTIMIZE_SIZE__ 2054@itemx __NO_INLINE__ 2055These macros describe the compilation mode. @code{__OPTIMIZE__} is 2056defined in all optimizing compilations. @code{__OPTIMIZE_SIZE__} is 2057defined if the compiler is optimizing for size, not speed. 2058@code{__NO_INLINE__} is defined if no functions will be inlined into 2059their callers (when not optimizing, or when inlining has been 2060specifically disabled by @option{-fno-inline}). 2061 2062These macros cause certain GNU header files to provide optimized 2063definitions, using macros or inline functions, of system library 2064functions. You should not use these macros in any way unless you make 2065sure that programs will execute with the same effect whether or not they 2066are defined. If they are defined, their value is 1. 2067 2068@item __GNUC_GNU_INLINE__ 2069GCC defines this macro if functions declared @code{inline} will be 2070handled in GCC's traditional gnu90 mode. Object files will contain 2071externally visible definitions of all functions declared @code{inline} 2072without @code{extern} or @code{static}. They will not contain any 2073definitions of any functions declared @code{extern inline}. 2074 2075@item __GNUC_STDC_INLINE__ 2076GCC defines this macro if functions declared @code{inline} will be 2077handled according to the ISO C99 standard. Object files will contain 2078externally visible definitions of all functions declared @code{extern 2079inline}. They will not contain definitions of any functions declared 2080@code{inline} without @code{extern}. 2081 2082If this macro is defined, GCC supports the @code{gnu_inline} function 2083attribute as a way to always get the gnu90 behavior. Support for 2084this and @code{__GNUC_GNU_INLINE__} was added in GCC 4.1.3. If 2085neither macro is defined, an older version of GCC is being used: 2086@code{inline} functions will be compiled in gnu90 mode, and the 2087@code{gnu_inline} function attribute will not be recognized. 2088 2089@item __CHAR_UNSIGNED__ 2090GCC defines this macro if and only if the data type @code{char} is 2091unsigned on the target machine. It exists to cause the standard header 2092file @file{limits.h} to work correctly. You should not use this macro 2093yourself; instead, refer to the standard macros defined in @file{limits.h}. 2094 2095@item __WCHAR_UNSIGNED__ 2096Like @code{__CHAR_UNSIGNED__}, this macro is defined if and only if the 2097data type @code{wchar_t} is unsigned and the front-end is in C++ mode. 2098 2099@item __REGISTER_PREFIX__ 2100This macro expands to a single token (not a string constant) which is 2101the prefix applied to CPU register names in assembly language for this 2102target. You can use it to write assembly that is usable in multiple 2103environments. For example, in the @code{m68k-aout} environment it 2104expands to nothing, but in the @code{m68k-coff} environment it expands 2105to a single @samp{%}. 2106 2107@item __USER_LABEL_PREFIX__ 2108This macro expands to a single token which is the prefix applied to 2109user labels (symbols visible to C code) in assembly. For example, in 2110the @code{m68k-aout} environment it expands to an @samp{_}, but in the 2111@code{m68k-coff} environment it expands to nothing. 2112 2113This macro will have the correct definition even if 2114@option{-f(no-)underscores} is in use, but it will not be correct if 2115target-specific options that adjust this prefix are used (e.g.@: the 2116OSF/rose @option{-mno-underscores} option). 2117 2118@item __SIZE_TYPE__ 2119@itemx __PTRDIFF_TYPE__ 2120@itemx __WCHAR_TYPE__ 2121@itemx __WINT_TYPE__ 2122@itemx __INTMAX_TYPE__ 2123@itemx __UINTMAX_TYPE__ 2124@itemx __SIG_ATOMIC_TYPE__ 2125@itemx __INT8_TYPE__ 2126@itemx __INT16_TYPE__ 2127@itemx __INT32_TYPE__ 2128@itemx __INT64_TYPE__ 2129@itemx __UINT8_TYPE__ 2130@itemx __UINT16_TYPE__ 2131@itemx __UINT32_TYPE__ 2132@itemx __UINT64_TYPE__ 2133@itemx __INT_LEAST8_TYPE__ 2134@itemx __INT_LEAST16_TYPE__ 2135@itemx __INT_LEAST32_TYPE__ 2136@itemx __INT_LEAST64_TYPE__ 2137@itemx __UINT_LEAST8_TYPE__ 2138@itemx __UINT_LEAST16_TYPE__ 2139@itemx __UINT_LEAST32_TYPE__ 2140@itemx __UINT_LEAST64_TYPE__ 2141@itemx __INT_FAST8_TYPE__ 2142@itemx __INT_FAST16_TYPE__ 2143@itemx __INT_FAST32_TYPE__ 2144@itemx __INT_FAST64_TYPE__ 2145@itemx __UINT_FAST8_TYPE__ 2146@itemx __UINT_FAST16_TYPE__ 2147@itemx __UINT_FAST32_TYPE__ 2148@itemx __UINT_FAST64_TYPE__ 2149@itemx __INTPTR_TYPE__ 2150@itemx __UINTPTR_TYPE__ 2151These macros are defined to the correct underlying types for the 2152@code{size_t}, @code{ptrdiff_t}, @code{wchar_t}, @code{wint_t}, 2153@code{intmax_t}, @code{uintmax_t}, @code{sig_atomic_t}, @code{int8_t}, 2154@code{int16_t}, @code{int32_t}, @code{int64_t}, @code{uint8_t}, 2155@code{uint16_t}, @code{uint32_t}, @code{uint64_t}, 2156@code{int_least8_t}, @code{int_least16_t}, @code{int_least32_t}, 2157@code{int_least64_t}, @code{uint_least8_t}, @code{uint_least16_t}, 2158@code{uint_least32_t}, @code{uint_least64_t}, @code{int_fast8_t}, 2159@code{int_fast16_t}, @code{int_fast32_t}, @code{int_fast64_t}, 2160@code{uint_fast8_t}, @code{uint_fast16_t}, @code{uint_fast32_t}, 2161@code{uint_fast64_t}, @code{intptr_t}, and @code{uintptr_t} typedefs, 2162respectively. They exist to make the standard header files 2163@file{stddef.h}, @file{stdint.h}, and @file{wchar.h} work correctly. 2164You should not use these macros directly; instead, include the 2165appropriate headers and use the typedefs. Some of these macros may 2166not be defined on particular systems if GCC does not provide a 2167@file{stdint.h} header on those systems. 2168 2169@item __CHAR_BIT__ 2170Defined to the number of bits used in the representation of the 2171@code{char} data type. It exists to make the standard header given 2172numerical limits work correctly. You should not use 2173this macro directly; instead, include the appropriate headers. 2174 2175@item __SCHAR_MAX__ 2176@itemx __WCHAR_MAX__ 2177@itemx __SHRT_MAX__ 2178@itemx __INT_MAX__ 2179@itemx __LONG_MAX__ 2180@itemx __LONG_LONG_MAX__ 2181@itemx __WINT_MAX__ 2182@itemx __SIZE_MAX__ 2183@itemx __PTRDIFF_MAX__ 2184@itemx __INTMAX_MAX__ 2185@itemx __UINTMAX_MAX__ 2186@itemx __SIG_ATOMIC_MAX__ 2187@itemx __INT8_MAX__ 2188@itemx __INT16_MAX__ 2189@itemx __INT32_MAX__ 2190@itemx __INT64_MAX__ 2191@itemx __UINT8_MAX__ 2192@itemx __UINT16_MAX__ 2193@itemx __UINT32_MAX__ 2194@itemx __UINT64_MAX__ 2195@itemx __INT_LEAST8_MAX__ 2196@itemx __INT_LEAST16_MAX__ 2197@itemx __INT_LEAST32_MAX__ 2198@itemx __INT_LEAST64_MAX__ 2199@itemx __UINT_LEAST8_MAX__ 2200@itemx __UINT_LEAST16_MAX__ 2201@itemx __UINT_LEAST32_MAX__ 2202@itemx __UINT_LEAST64_MAX__ 2203@itemx __INT_FAST8_MAX__ 2204@itemx __INT_FAST16_MAX__ 2205@itemx __INT_FAST32_MAX__ 2206@itemx __INT_FAST64_MAX__ 2207@itemx __UINT_FAST8_MAX__ 2208@itemx __UINT_FAST16_MAX__ 2209@itemx __UINT_FAST32_MAX__ 2210@itemx __UINT_FAST64_MAX__ 2211@itemx __INTPTR_MAX__ 2212@itemx __UINTPTR_MAX__ 2213@itemx __WCHAR_MIN__ 2214@itemx __WINT_MIN__ 2215@itemx __SIG_ATOMIC_MIN__ 2216Defined to the maximum value of the @code{signed char}, @code{wchar_t}, 2217@code{signed short}, 2218@code{signed int}, @code{signed long}, @code{signed long long}, 2219@code{wint_t}, @code{size_t}, @code{ptrdiff_t}, 2220@code{intmax_t}, @code{uintmax_t}, @code{sig_atomic_t}, @code{int8_t}, 2221@code{int16_t}, @code{int32_t}, @code{int64_t}, @code{uint8_t}, 2222@code{uint16_t}, @code{uint32_t}, @code{uint64_t}, 2223@code{int_least8_t}, @code{int_least16_t}, @code{int_least32_t}, 2224@code{int_least64_t}, @code{uint_least8_t}, @code{uint_least16_t}, 2225@code{uint_least32_t}, @code{uint_least64_t}, @code{int_fast8_t}, 2226@code{int_fast16_t}, @code{int_fast32_t}, @code{int_fast64_t}, 2227@code{uint_fast8_t}, @code{uint_fast16_t}, @code{uint_fast32_t}, 2228@code{uint_fast64_t}, @code{intptr_t}, and @code{uintptr_t} types and 2229to the minimum value of the @code{wchar_t}, @code{wint_t}, and 2230@code{sig_atomic_t} types respectively. They exist to make the 2231standard header given numerical limits work correctly. You should not 2232use these macros directly; instead, include the appropriate headers. 2233Some of these macros may not be defined on particular systems if GCC 2234does not provide a @file{stdint.h} header on those systems. 2235 2236@item __INT8_C 2237@itemx __INT16_C 2238@itemx __INT32_C 2239@itemx __INT64_C 2240@itemx __UINT8_C 2241@itemx __UINT16_C 2242@itemx __UINT32_C 2243@itemx __UINT64_C 2244@itemx __INTMAX_C 2245@itemx __UINTMAX_C 2246Defined to implementations of the standard @file{stdint.h} macros with 2247the same names without the leading @code{__}. They exist the make the 2248implementation of that header work correctly. You should not use 2249these macros directly; instead, include the appropriate headers. Some 2250of these macros may not be defined on particular systems if GCC does 2251not provide a @file{stdint.h} header on those systems. 2252 2253@item __SIZEOF_INT__ 2254@itemx __SIZEOF_LONG__ 2255@itemx __SIZEOF_LONG_LONG__ 2256@itemx __SIZEOF_SHORT__ 2257@itemx __SIZEOF_POINTER__ 2258@itemx __SIZEOF_FLOAT__ 2259@itemx __SIZEOF_DOUBLE__ 2260@itemx __SIZEOF_LONG_DOUBLE__ 2261@itemx __SIZEOF_SIZE_T__ 2262@itemx __SIZEOF_WCHAR_T__ 2263@itemx __SIZEOF_WINT_T__ 2264@itemx __SIZEOF_PTRDIFF_T__ 2265Defined to the number of bytes of the C standard data types: @code{int}, 2266@code{long}, @code{long long}, @code{short}, @code{void *}, @code{float}, 2267@code{double}, @code{long double}, @code{size_t}, @code{wchar_t}, @code{wint_t} 2268and @code{ptrdiff_t}. 2269 2270@item __DEPRECATED 2271This macro is defined, with value 1, when compiling a C++ source file 2272with warnings about deprecated constructs enabled. These warnings are 2273enabled by default, but can be disabled with @option{-Wno-deprecated}. 2274 2275@item __EXCEPTIONS 2276This macro is defined, with value 1, when compiling a C++ source file 2277with exceptions enabled. If @option{-fno-exceptions} is used when 2278compiling the file, then this macro is not defined. 2279 2280@item __GXX_RTTI 2281This macro is defined, with value 1, when compiling a C++ source file 2282with runtime type identification enabled. If @option{-fno-rtti} is 2283used when compiling the file, then this macro is not defined. 2284 2285@item __USING_SJLJ_EXCEPTIONS__ 2286This macro is defined, with value 1, if the compiler uses the old 2287mechanism based on @code{setjmp} and @code{longjmp} for exception 2288handling. 2289 2290@item __GXX_EXPERIMENTAL_CXX0X__ 2291This macro is defined when compiling a C++ source file with the option 2292@option{-std=c++0x} or @option{-std=gnu++0x}. It indicates that some 2293features likely to be included in C++0x are available. Note that these 2294features are experimental, and may change or be removed in future 2295versions of GCC. 2296 2297@item __GXX_WEAK__ 2298This macro is defined when compiling a C++ source file. It has the 2299value 1 if the compiler will use weak symbols, COMDAT sections, or 2300other similar techniques to collapse symbols with ``vague linkage'' 2301that are defined in multiple translation units. If the compiler will 2302not collapse such symbols, this macro is defined with value 0. In 2303general, user code should not need to make use of this macro; the 2304purpose of this macro is to ease implementation of the C++ runtime 2305library provided with G++. 2306 2307@item __NEXT_RUNTIME__ 2308This macro is defined, with value 1, if (and only if) the NeXT runtime 2309(as in @option{-fnext-runtime}) is in use for Objective-C@. If the GNU 2310runtime is used, this macro is not defined, so that you can use this 2311macro to determine which runtime (NeXT or GNU) is being used. 2312 2313@item __LP64__ 2314@itemx _LP64 2315These macros are defined, with value 1, if (and only if) the compilation 2316is for a target where @code{long int} and pointer both use 64-bits and 2317@code{int} uses 32-bit. 2318 2319@item __SSP__ 2320This macro is defined, with value 1, when @option{-fstack-protector} is in 2321use. 2322 2323@item __SSP_ALL__ 2324This macro is defined, with value 2, when @option{-fstack-protector-all} is 2325in use. 2326 2327@item __TIMESTAMP__ 2328This macro expands to a string constant that describes the date and time 2329of the last modification of the current source file. The string constant 2330contains abbreviated day of the week, month, day of the month, time in 2331hh:mm:ss form, year and looks like @code{@w{"Sun Sep 16 01:03:52 1973"}}. 2332If the day of the month is less than 10, it is padded with a space on the left. 2333 2334If GCC cannot determine the current date, it will emit a warning message 2335(once per compilation) and @code{__TIMESTAMP__} will expand to 2336@code{@w{"??? ??? ?? ??:??:?? ????"}}. 2337 2338@item __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1 2339@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2 2340@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 2341@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8 2342@itemx __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 2343These macros are defined when the target processor supports atomic compare 2344and swap operations on operands 1, 2, 4, 8 or 16 bytes in length, respectively. 2345 2346@item __GCC_HAVE_DWARF2_CFI_ASM 2347This macro is defined when the compiler is emitting Dwarf2 CFI directives 2348to the assembler. When this is defined, it is possible to emit those same 2349directives in inline assembly. 2350@end table 2351 2352@node System-specific Predefined Macros 2353@subsection System-specific Predefined Macros 2354 2355@cindex system-specific predefined macros 2356@cindex predefined macros, system-specific 2357@cindex reserved namespace 2358 2359The C preprocessor normally predefines several macros that indicate what 2360type of system and machine is in use. They are obviously different on 2361each target supported by GCC@. This manual, being for all systems and 2362machines, cannot tell you what their names are, but you can use 2363@command{cpp -dM} to see them all. @xref{Invocation}. All system-specific 2364predefined macros expand to the constant 1, so you can test them with 2365either @samp{#ifdef} or @samp{#if}. 2366 2367The C standard requires that all system-specific macros be part of the 2368@dfn{reserved namespace}. All names which begin with two underscores, 2369or an underscore and a capital letter, are reserved for the compiler and 2370library to use as they wish. However, historically system-specific 2371macros have had names with no special prefix; for instance, it is common 2372to find @code{unix} defined on Unix systems. For all such macros, GCC 2373provides a parallel macro with two underscores added at the beginning 2374and the end. If @code{unix} is defined, @code{__unix__} will be defined 2375too. There will never be more than two underscores; the parallel of 2376@code{_mips} is @code{__mips__}. 2377 2378When the @option{-ansi} option, or any @option{-std} option that 2379requests strict conformance, is given to the compiler, all the 2380system-specific predefined macros outside the reserved namespace are 2381suppressed. The parallel macros, inside the reserved namespace, remain 2382defined. 2383 2384We are slowly phasing out all predefined macros which are outside the 2385reserved namespace. You should never use them in new programs, and we 2386encourage you to correct older code to use the parallel macros whenever 2387you find it. We don't recommend you use the system-specific macros that 2388are in the reserved namespace, either. It is better in the long run to 2389check specifically for features you need, using a tool such as 2390@command{autoconf}. 2391 2392@node C++ Named Operators 2393@subsection C++ Named Operators 2394@cindex named operators 2395@cindex C++ named operators 2396@cindex iso646.h 2397 2398In C++, there are eleven keywords which are simply alternate spellings 2399of operators normally written with punctuation. These keywords are 2400treated as such even in the preprocessor. They function as operators in 2401@samp{#if}, and they cannot be defined as macros or poisoned. In C, you 2402can request that those keywords take their C++ meaning by including 2403@file{iso646.h}. That header defines each one as a normal object-like 2404macro expanding to the appropriate punctuator. 2405 2406These are the named operators and their corresponding punctuators: 2407 2408@multitable {Named Operator} {Punctuator} 2409@item Named Operator @tab Punctuator 2410@item @code{and} @tab @code{&&} 2411@item @code{and_eq} @tab @code{&=} 2412@item @code{bitand} @tab @code{&} 2413@item @code{bitor} @tab @code{|} 2414@item @code{compl} @tab @code{~} 2415@item @code{not} @tab @code{!} 2416@item @code{not_eq} @tab @code{!=} 2417@item @code{or} @tab @code{||} 2418@item @code{or_eq} @tab @code{|=} 2419@item @code{xor} @tab @code{^} 2420@item @code{xor_eq} @tab @code{^=} 2421@end multitable 2422 2423@node Undefining and Redefining Macros 2424@section Undefining and Redefining Macros 2425@cindex undefining macros 2426@cindex redefining macros 2427@findex #undef 2428 2429If a macro ceases to be useful, it may be @dfn{undefined} with the 2430@samp{#undef} directive. @samp{#undef} takes a single argument, the 2431name of the macro to undefine. You use the bare macro name, even if the 2432macro is function-like. It is an error if anything appears on the line 2433after the macro name. @samp{#undef} has no effect if the name is not a 2434macro. 2435 2436@smallexample 2437#define FOO 4 2438x = FOO; @expansion{} x = 4; 2439#undef FOO 2440x = FOO; @expansion{} x = FOO; 2441@end smallexample 2442 2443Once a macro has been undefined, that identifier may be @dfn{redefined} 2444as a macro by a subsequent @samp{#define} directive. The new definition 2445need not have any resemblance to the old definition. 2446 2447However, if an identifier which is currently a macro is redefined, then 2448the new definition must be @dfn{effectively the same} as the old one. 2449Two macro definitions are effectively the same if: 2450@itemize @bullet 2451@item Both are the same type of macro (object- or function-like). 2452@item All the tokens of the replacement list are the same. 2453@item If there are any parameters, they are the same. 2454@item Whitespace appears in the same places in both. It need not be 2455exactly the same amount of whitespace, though. Remember that comments 2456count as whitespace. 2457@end itemize 2458 2459@noindent 2460These definitions are effectively the same: 2461@smallexample 2462#define FOUR (2 + 2) 2463#define FOUR (2 + 2) 2464#define FOUR (2 /* @r{two} */ + 2) 2465@end smallexample 2466@noindent 2467but these are not: 2468@smallexample 2469#define FOUR (2 + 2) 2470#define FOUR ( 2+2 ) 2471#define FOUR (2 * 2) 2472#define FOUR(score,and,seven,years,ago) (2 + 2) 2473@end smallexample 2474 2475If a macro is redefined with a definition that is not effectively the 2476same as the old one, the preprocessor issues a warning and changes the 2477macro to use the new definition. If the new definition is effectively 2478the same, the redefinition is silently ignored. This allows, for 2479instance, two different headers to define a common macro. The 2480preprocessor will only complain if the definitions do not match. 2481 2482@node Directives Within Macro Arguments 2483@section Directives Within Macro Arguments 2484@cindex macro arguments and directives 2485 2486Occasionally it is convenient to use preprocessor directives within 2487the arguments of a macro. The C and C++ standards declare that 2488behavior in these cases is undefined. 2489 2490Versions of CPP prior to 3.2 would reject such constructs with an 2491error message. This was the only syntactic difference between normal 2492functions and function-like macros, so it seemed attractive to remove 2493this limitation, and people would often be surprised that they could 2494not use macros in this way. Moreover, sometimes people would use 2495conditional compilation in the argument list to a normal library 2496function like @samp{printf}, only to find that after a library upgrade 2497@samp{printf} had changed to be a function-like macro, and their code 2498would no longer compile. So from version 3.2 we changed CPP to 2499successfully process arbitrary directives within macro arguments in 2500exactly the same way as it would have processed the directive were the 2501function-like macro invocation not present. 2502 2503If, within a macro invocation, that macro is redefined, then the new 2504definition takes effect in time for argument pre-expansion, but the 2505original definition is still used for argument replacement. Here is a 2506pathological example: 2507 2508@smallexample 2509#define f(x) x x 2510f (1 2511#undef f 2512#define f 2 2513f) 2514@end smallexample 2515 2516@noindent 2517which expands to 2518 2519@smallexample 25201 2 1 2 2521@end smallexample 2522 2523@noindent 2524with the semantics described above. 2525 2526@node Macro Pitfalls 2527@section Macro Pitfalls 2528@cindex problems with macros 2529@cindex pitfalls of macros 2530 2531In this section we describe some special rules that apply to macros and 2532macro expansion, and point out certain cases in which the rules have 2533counter-intuitive consequences that you must watch out for. 2534 2535@menu 2536* Misnesting:: 2537* Operator Precedence Problems:: 2538* Swallowing the Semicolon:: 2539* Duplication of Side Effects:: 2540* Self-Referential Macros:: 2541* Argument Prescan:: 2542* Newlines in Arguments:: 2543@end menu 2544 2545@node Misnesting 2546@subsection Misnesting 2547 2548When a macro is called with arguments, the arguments are substituted 2549into the macro body and the result is checked, together with the rest of 2550the input file, for more macro calls. It is possible to piece together 2551a macro call coming partially from the macro body and partially from the 2552arguments. For example, 2553 2554@smallexample 2555#define twice(x) (2*(x)) 2556#define call_with_1(x) x(1) 2557call_with_1 (twice) 2558 @expansion{} twice(1) 2559 @expansion{} (2*(1)) 2560@end smallexample 2561 2562Macro definitions do not have to have balanced parentheses. By writing 2563an unbalanced open parenthesis in a macro body, it is possible to create 2564a macro call that begins inside the macro body but ends outside of it. 2565For example, 2566 2567@smallexample 2568#define strange(file) fprintf (file, "%s %d", 2569@dots{} 2570strange(stderr) p, 35) 2571 @expansion{} fprintf (stderr, "%s %d", p, 35) 2572@end smallexample 2573 2574The ability to piece together a macro call can be useful, but the use of 2575unbalanced open parentheses in a macro body is just confusing, and 2576should be avoided. 2577 2578@node Operator Precedence Problems 2579@subsection Operator Precedence Problems 2580@cindex parentheses in macro bodies 2581 2582You may have noticed that in most of the macro definition examples shown 2583above, each occurrence of a macro argument name had parentheses around 2584it. In addition, another pair of parentheses usually surround the 2585entire macro definition. Here is why it is best to write macros that 2586way. 2587 2588Suppose you define a macro as follows, 2589 2590@smallexample 2591#define ceil_div(x, y) (x + y - 1) / y 2592@end smallexample 2593 2594@noindent 2595whose purpose is to divide, rounding up. (One use for this operation is 2596to compute how many @code{int} objects are needed to hold a certain 2597number of @code{char} objects.) Then suppose it is used as follows: 2598 2599@smallexample 2600a = ceil_div (b & c, sizeof (int)); 2601 @expansion{} a = (b & c + sizeof (int) - 1) / sizeof (int); 2602@end smallexample 2603 2604@noindent 2605This does not do what is intended. The operator-precedence rules of 2606C make it equivalent to this: 2607 2608@smallexample 2609a = (b & (c + sizeof (int) - 1)) / sizeof (int); 2610@end smallexample 2611 2612@noindent 2613What we want is this: 2614 2615@smallexample 2616a = ((b & c) + sizeof (int) - 1)) / sizeof (int); 2617@end smallexample 2618 2619@noindent 2620Defining the macro as 2621 2622@smallexample 2623#define ceil_div(x, y) ((x) + (y) - 1) / (y) 2624@end smallexample 2625 2626@noindent 2627provides the desired result. 2628 2629Unintended grouping can result in another way. Consider @code{sizeof 2630ceil_div(1, 2)}. That has the appearance of a C expression that would 2631compute the size of the type of @code{ceil_div (1, 2)}, but in fact it 2632means something very different. Here is what it expands to: 2633 2634@smallexample 2635sizeof ((1) + (2) - 1) / (2) 2636@end smallexample 2637 2638@noindent 2639This would take the size of an integer and divide it by two. The 2640precedence rules have put the division outside the @code{sizeof} when it 2641was intended to be inside. 2642 2643Parentheses around the entire macro definition prevent such problems. 2644Here, then, is the recommended way to define @code{ceil_div}: 2645 2646@smallexample 2647#define ceil_div(x, y) (((x) + (y) - 1) / (y)) 2648@end smallexample 2649 2650@node Swallowing the Semicolon 2651@subsection Swallowing the Semicolon 2652@cindex semicolons (after macro calls) 2653 2654Often it is desirable to define a macro that expands into a compound 2655statement. Consider, for example, the following macro, that advances a 2656pointer (the argument @code{p} says where to find it) across whitespace 2657characters: 2658 2659@smallexample 2660#define SKIP_SPACES(p, limit) \ 2661@{ char *lim = (limit); \ 2662 while (p < lim) @{ \ 2663 if (*p++ != ' ') @{ \ 2664 p--; break; @}@}@} 2665@end smallexample 2666 2667@noindent 2668Here backslash-newline is used to split the macro definition, which must 2669be a single logical line, so that it resembles the way such code would 2670be laid out if not part of a macro definition. 2671 2672A call to this macro might be @code{SKIP_SPACES (p, lim)}. Strictly 2673speaking, the call expands to a compound statement, which is a complete 2674statement with no need for a semicolon to end it. However, since it 2675looks like a function call, it minimizes confusion if you can use it 2676like a function call, writing a semicolon afterward, as in 2677@code{SKIP_SPACES (p, lim);} 2678 2679This can cause trouble before @code{else} statements, because the 2680semicolon is actually a null statement. Suppose you write 2681 2682@smallexample 2683if (*p != 0) 2684 SKIP_SPACES (p, lim); 2685else @dots{} 2686@end smallexample 2687 2688@noindent 2689The presence of two statements---the compound statement and a null 2690statement---in between the @code{if} condition and the @code{else} 2691makes invalid C code. 2692 2693The definition of the macro @code{SKIP_SPACES} can be altered to solve 2694this problem, using a @code{do @dots{} while} statement. Here is how: 2695 2696@smallexample 2697#define SKIP_SPACES(p, limit) \ 2698do @{ char *lim = (limit); \ 2699 while (p < lim) @{ \ 2700 if (*p++ != ' ') @{ \ 2701 p--; break; @}@}@} \ 2702while (0) 2703@end smallexample 2704 2705Now @code{SKIP_SPACES (p, lim);} expands into 2706 2707@smallexample 2708do @{@dots{}@} while (0); 2709@end smallexample 2710 2711@noindent 2712which is one statement. The loop executes exactly once; most compilers 2713generate no extra code for it. 2714 2715@node Duplication of Side Effects 2716@subsection Duplication of Side Effects 2717 2718@cindex side effects (in macro arguments) 2719@cindex unsafe macros 2720Many C programs define a macro @code{min}, for ``minimum'', like this: 2721 2722@smallexample 2723#define min(X, Y) ((X) < (Y) ? (X) : (Y)) 2724@end smallexample 2725 2726When you use this macro with an argument containing a side effect, 2727as shown here, 2728 2729@smallexample 2730next = min (x + y, foo (z)); 2731@end smallexample 2732 2733@noindent 2734it expands as follows: 2735 2736@smallexample 2737next = ((x + y) < (foo (z)) ? (x + y) : (foo (z))); 2738@end smallexample 2739 2740@noindent 2741where @code{x + y} has been substituted for @code{X} and @code{foo (z)} 2742for @code{Y}. 2743 2744The function @code{foo} is used only once in the statement as it appears 2745in the program, but the expression @code{foo (z)} has been substituted 2746twice into the macro expansion. As a result, @code{foo} might be called 2747two times when the statement is executed. If it has side effects or if 2748it takes a long time to compute, the results might not be what you 2749intended. We say that @code{min} is an @dfn{unsafe} macro. 2750 2751The best solution to this problem is to define @code{min} in a way that 2752computes the value of @code{foo (z)} only once. The C language offers 2753no standard way to do this, but it can be done with GNU extensions as 2754follows: 2755 2756@smallexample 2757#define min(X, Y) \ 2758(@{ typeof (X) x_ = (X); \ 2759 typeof (Y) y_ = (Y); \ 2760 (x_ < y_) ? x_ : y_; @}) 2761@end smallexample 2762 2763The @samp{(@{ @dots{} @})} notation produces a compound statement that 2764acts as an expression. Its value is the value of its last statement. 2765This permits us to define local variables and assign each argument to 2766one. The local variables have underscores after their names to reduce 2767the risk of conflict with an identifier of wider scope (it is impossible 2768to avoid this entirely). Now each argument is evaluated exactly once. 2769 2770If you do not wish to use GNU C extensions, the only solution is to be 2771careful when @emph{using} the macro @code{min}. For example, you can 2772calculate the value of @code{foo (z)}, save it in a variable, and use 2773that variable in @code{min}: 2774 2775@smallexample 2776@group 2777#define min(X, Y) ((X) < (Y) ? (X) : (Y)) 2778@dots{} 2779@{ 2780 int tem = foo (z); 2781 next = min (x + y, tem); 2782@} 2783@end group 2784@end smallexample 2785 2786@noindent 2787(where we assume that @code{foo} returns type @code{int}). 2788 2789@node Self-Referential Macros 2790@subsection Self-Referential Macros 2791@cindex self-reference 2792 2793A @dfn{self-referential} macro is one whose name appears in its 2794definition. Recall that all macro definitions are rescanned for more 2795macros to replace. If the self-reference were considered a use of the 2796macro, it would produce an infinitely large expansion. To prevent this, 2797the self-reference is not considered a macro call. It is passed into 2798the preprocessor output unchanged. Consider an example: 2799 2800@smallexample 2801#define foo (4 + foo) 2802@end smallexample 2803 2804@noindent 2805where @code{foo} is also a variable in your program. 2806 2807Following the ordinary rules, each reference to @code{foo} will expand 2808into @code{(4 + foo)}; then this will be rescanned and will expand into 2809@code{(4 + (4 + foo))}; and so on until the computer runs out of memory. 2810 2811The self-reference rule cuts this process short after one step, at 2812@code{(4 + foo)}. Therefore, this macro definition has the possibly 2813useful effect of causing the program to add 4 to the value of @code{foo} 2814wherever @code{foo} is referred to. 2815 2816In most cases, it is a bad idea to take advantage of this feature. A 2817person reading the program who sees that @code{foo} is a variable will 2818not expect that it is a macro as well. The reader will come across the 2819identifier @code{foo} in the program and think its value should be that 2820of the variable @code{foo}, whereas in fact the value is four greater. 2821 2822One common, useful use of self-reference is to create a macro which 2823expands to itself. If you write 2824 2825@smallexample 2826#define EPERM EPERM 2827@end smallexample 2828 2829@noindent 2830then the macro @code{EPERM} expands to @code{EPERM}. Effectively, it is 2831left alone by the preprocessor whenever it's used in running text. You 2832can tell that it's a macro with @samp{#ifdef}. You might do this if you 2833want to define numeric constants with an @code{enum}, but have 2834@samp{#ifdef} be true for each constant. 2835 2836If a macro @code{x} expands to use a macro @code{y}, and the expansion of 2837@code{y} refers to the macro @code{x}, that is an @dfn{indirect 2838self-reference} of @code{x}. @code{x} is not expanded in this case 2839either. Thus, if we have 2840 2841@smallexample 2842#define x (4 + y) 2843#define y (2 * x) 2844@end smallexample 2845 2846@noindent 2847then @code{x} and @code{y} expand as follows: 2848 2849@smallexample 2850@group 2851x @expansion{} (4 + y) 2852 @expansion{} (4 + (2 * x)) 2853 2854y @expansion{} (2 * x) 2855 @expansion{} (2 * (4 + y)) 2856@end group 2857@end smallexample 2858 2859@noindent 2860Each macro is expanded when it appears in the definition of the other 2861macro, but not when it indirectly appears in its own definition. 2862 2863@node Argument Prescan 2864@subsection Argument Prescan 2865@cindex expansion of arguments 2866@cindex macro argument expansion 2867@cindex prescan of macro arguments 2868 2869Macro arguments are completely macro-expanded before they are 2870substituted into a macro body, unless they are stringified or pasted 2871with other tokens. After substitution, the entire macro body, including 2872the substituted arguments, is scanned again for macros to be expanded. 2873The result is that the arguments are scanned @emph{twice} to expand 2874macro calls in them. 2875 2876Most of the time, this has no effect. If the argument contained any 2877macro calls, they are expanded during the first scan. The result 2878therefore contains no macro calls, so the second scan does not change 2879it. If the argument were substituted as given, with no prescan, the 2880single remaining scan would find the same macro calls and produce the 2881same results. 2882 2883You might expect the double scan to change the results when a 2884self-referential macro is used in an argument of another macro 2885(@pxref{Self-Referential Macros}): the self-referential macro would be 2886expanded once in the first scan, and a second time in the second scan. 2887However, this is not what happens. The self-references that do not 2888expand in the first scan are marked so that they will not expand in the 2889second scan either. 2890 2891You might wonder, ``Why mention the prescan, if it makes no difference? 2892And why not skip it and make the preprocessor faster?'' The answer is 2893that the prescan does make a difference in three special cases: 2894 2895@itemize @bullet 2896@item 2897Nested calls to a macro. 2898 2899We say that @dfn{nested} calls to a macro occur when a macro's argument 2900contains a call to that very macro. For example, if @code{f} is a macro 2901that expects one argument, @code{f (f (1))} is a nested pair of calls to 2902@code{f}. The desired expansion is made by expanding @code{f (1)} and 2903substituting that into the definition of @code{f}. The prescan causes 2904the expected result to happen. Without the prescan, @code{f (1)} itself 2905would be substituted as an argument, and the inner use of @code{f} would 2906appear during the main scan as an indirect self-reference and would not 2907be expanded. 2908 2909@item 2910Macros that call other macros that stringify or concatenate. 2911 2912If an argument is stringified or concatenated, the prescan does not 2913occur. If you @emph{want} to expand a macro, then stringify or 2914concatenate its expansion, you can do that by causing one macro to call 2915another macro that does the stringification or concatenation. For 2916instance, if you have 2917 2918@smallexample 2919#define AFTERX(x) X_ ## x 2920#define XAFTERX(x) AFTERX(x) 2921#define TABLESIZE 1024 2922#define BUFSIZE TABLESIZE 2923@end smallexample 2924 2925then @code{AFTERX(BUFSIZE)} expands to @code{X_BUFSIZE}, and 2926@code{XAFTERX(BUFSIZE)} expands to @code{X_1024}. (Not to 2927@code{X_TABLESIZE}. Prescan always does a complete expansion.) 2928 2929@item 2930Macros used in arguments, whose expansions contain unshielded commas. 2931 2932This can cause a macro expanded on the second scan to be called with the 2933wrong number of arguments. Here is an example: 2934 2935@smallexample 2936#define foo a,b 2937#define bar(x) lose(x) 2938#define lose(x) (1 + (x)) 2939@end smallexample 2940 2941We would like @code{bar(foo)} to turn into @code{(1 + (foo))}, which 2942would then turn into @code{(1 + (a,b))}. Instead, @code{bar(foo)} 2943expands into @code{lose(a,b)}, and you get an error because @code{lose} 2944requires a single argument. In this case, the problem is easily solved 2945by the same parentheses that ought to be used to prevent misnesting of 2946arithmetic operations: 2947 2948@smallexample 2949#define foo (a,b) 2950@exdent or 2951#define bar(x) lose((x)) 2952@end smallexample 2953 2954The extra pair of parentheses prevents the comma in @code{foo}'s 2955definition from being interpreted as an argument separator. 2956 2957@end itemize 2958 2959@node Newlines in Arguments 2960@subsection Newlines in Arguments 2961@cindex newlines in macro arguments 2962 2963The invocation of a function-like macro can extend over many logical 2964lines. However, in the present implementation, the entire expansion 2965comes out on one line. Thus line numbers emitted by the compiler or 2966debugger refer to the line the invocation started on, which might be 2967different to the line containing the argument causing the problem. 2968 2969Here is an example illustrating this: 2970 2971@smallexample 2972#define ignore_second_arg(a,b,c) a; c 2973 2974ignore_second_arg (foo (), 2975 ignored (), 2976 syntax error); 2977@end smallexample 2978 2979@noindent 2980The syntax error triggered by the tokens @code{syntax error} results in 2981an error message citing line three---the line of ignore_second_arg--- 2982even though the problematic code comes from line five. 2983 2984We consider this a bug, and intend to fix it in the near future. 2985 2986@node Conditionals 2987@chapter Conditionals 2988@cindex conditionals 2989 2990A @dfn{conditional} is a directive that instructs the preprocessor to 2991select whether or not to include a chunk of code in the final token 2992stream passed to the compiler. Preprocessor conditionals can test 2993arithmetic expressions, or whether a name is defined as a macro, or both 2994simultaneously using the special @code{defined} operator. 2995 2996A conditional in the C preprocessor resembles in some ways an @code{if} 2997statement in C, but it is important to understand the difference between 2998them. The condition in an @code{if} statement is tested during the 2999execution of your program. Its purpose is to allow your program to 3000behave differently from run to run, depending on the data it is 3001operating on. The condition in a preprocessing conditional directive is 3002tested when your program is compiled. Its purpose is to allow different 3003code to be included in the program depending on the situation at the 3004time of compilation. 3005 3006However, the distinction is becoming less clear. Modern compilers often 3007do test @code{if} statements when a program is compiled, if their 3008conditions are known not to vary at run time, and eliminate code which 3009can never be executed. If you can count on your compiler to do this, 3010you may find that your program is more readable if you use @code{if} 3011statements with constant conditions (perhaps determined by macros). Of 3012course, you can only use this to exclude code, not type definitions or 3013other preprocessing directives, and you can only do it if the code 3014remains syntactically valid when it is not to be used. 3015 3016GCC version 3 eliminates this kind of never-executed code even when 3017not optimizing. Older versions did it only when optimizing. 3018 3019@menu 3020* Conditional Uses:: 3021* Conditional Syntax:: 3022* Deleted Code:: 3023@end menu 3024 3025@node Conditional Uses 3026@section Conditional Uses 3027 3028There are three general reasons to use a conditional. 3029 3030@itemize @bullet 3031@item 3032A program may need to use different code depending on the machine or 3033operating system it is to run on. In some cases the code for one 3034operating system may be erroneous on another operating system; for 3035example, it might refer to data types or constants that do not exist on 3036the other system. When this happens, it is not enough to avoid 3037executing the invalid code. Its mere presence will cause the compiler 3038to reject the program. With a preprocessing conditional, the offending 3039code can be effectively excised from the program when it is not valid. 3040 3041@item 3042You may want to be able to compile the same source file into two 3043different programs. One version might make frequent time-consuming 3044consistency checks on its intermediate data, or print the values of 3045those data for debugging, and the other not. 3046 3047@item 3048A conditional whose condition is always false is one way to exclude code 3049from the program but keep it as a sort of comment for future reference. 3050@end itemize 3051 3052Simple programs that do not need system-specific logic or complex 3053debugging hooks generally will not need to use preprocessing 3054conditionals. 3055 3056@node Conditional Syntax 3057@section Conditional Syntax 3058 3059@findex #if 3060A conditional in the C preprocessor begins with a @dfn{conditional 3061directive}: @samp{#if}, @samp{#ifdef} or @samp{#ifndef}. 3062 3063@menu 3064* Ifdef:: 3065* If:: 3066* Defined:: 3067* Else:: 3068* Elif:: 3069@end menu 3070 3071@node Ifdef 3072@subsection Ifdef 3073@findex #ifdef 3074@findex #endif 3075 3076The simplest sort of conditional is 3077 3078@smallexample 3079@group 3080#ifdef @var{MACRO} 3081 3082@var{controlled text} 3083 3084#endif /* @var{MACRO} */ 3085@end group 3086@end smallexample 3087 3088@cindex conditional group 3089This block is called a @dfn{conditional group}. @var{controlled text} 3090will be included in the output of the preprocessor if and only if 3091@var{MACRO} is defined. We say that the conditional @dfn{succeeds} if 3092@var{MACRO} is defined, @dfn{fails} if it is not. 3093 3094The @var{controlled text} inside of a conditional can include 3095preprocessing directives. They are executed only if the conditional 3096succeeds. You can nest conditional groups inside other conditional 3097groups, but they must be completely nested. In other words, 3098@samp{#endif} always matches the nearest @samp{#ifdef} (or 3099@samp{#ifndef}, or @samp{#if}). Also, you cannot start a conditional 3100group in one file and end it in another. 3101 3102Even if a conditional fails, the @var{controlled text} inside it is 3103still run through initial transformations and tokenization. Therefore, 3104it must all be lexically valid C@. Normally the only way this matters is 3105that all comments and string literals inside a failing conditional group 3106must still be properly ended. 3107 3108The comment following the @samp{#endif} is not required, but it is a 3109good practice if there is a lot of @var{controlled text}, because it 3110helps people match the @samp{#endif} to the corresponding @samp{#ifdef}. 3111Older programs sometimes put @var{MACRO} directly after the 3112@samp{#endif} without enclosing it in a comment. This is invalid code 3113according to the C standard. CPP accepts it with a warning. It 3114never affects which @samp{#ifndef} the @samp{#endif} matches. 3115 3116@findex #ifndef 3117Sometimes you wish to use some code if a macro is @emph{not} defined. 3118You can do this by writing @samp{#ifndef} instead of @samp{#ifdef}. 3119One common use of @samp{#ifndef} is to include code only the first 3120time a header file is included. @xref{Once-Only Headers}. 3121 3122Macro definitions can vary between compilations for several reasons. 3123Here are some samples. 3124 3125@itemize @bullet 3126@item 3127Some macros are predefined on each kind of machine 3128(@pxref{System-specific Predefined Macros}). This allows you to provide 3129code specially tuned for a particular machine. 3130 3131@item 3132System header files define more macros, associated with the features 3133they implement. You can test these macros with conditionals to avoid 3134using a system feature on a machine where it is not implemented. 3135 3136@item 3137Macros can be defined or undefined with the @option{-D} and @option{-U} 3138command line options when you compile the program. You can arrange to 3139compile the same source file into two different programs by choosing a 3140macro name to specify which program you want, writing conditionals to 3141test whether or how this macro is defined, and then controlling the 3142state of the macro with command line options, perhaps set in the 3143Makefile. @xref{Invocation}. 3144 3145@item 3146Your program might have a special header file (often called 3147@file{config.h}) that is adjusted when the program is compiled. It can 3148define or not define macros depending on the features of the system and 3149the desired capabilities of the program. The adjustment can be 3150automated by a tool such as @command{autoconf}, or done by hand. 3151@end itemize 3152 3153@node If 3154@subsection If 3155 3156The @samp{#if} directive allows you to test the value of an arithmetic 3157expression, rather than the mere existence of one macro. Its syntax is 3158 3159@smallexample 3160@group 3161#if @var{expression} 3162 3163@var{controlled text} 3164 3165#endif /* @var{expression} */ 3166@end group 3167@end smallexample 3168 3169@var{expression} is a C expression of integer type, subject to stringent 3170restrictions. It may contain 3171 3172@itemize @bullet 3173@item 3174Integer constants. 3175 3176@item 3177Character constants, which are interpreted as they would be in normal 3178code. 3179 3180@item 3181Arithmetic operators for addition, subtraction, multiplication, 3182division, bitwise operations, shifts, comparisons, and logical 3183operations (@code{&&} and @code{||}). The latter two obey the usual 3184short-circuiting rules of standard C@. 3185 3186@item 3187Macros. All macros in the expression are expanded before actual 3188computation of the expression's value begins. 3189 3190@item 3191Uses of the @code{defined} operator, which lets you check whether macros 3192are defined in the middle of an @samp{#if}. 3193 3194@item 3195Identifiers that are not macros, which are all considered to be the 3196number zero. This allows you to write @code{@w{#if MACRO}} instead of 3197@code{@w{#ifdef MACRO}}, if you know that MACRO, when defined, will 3198always have a nonzero value. Function-like macros used without their 3199function call parentheses are also treated as zero. 3200 3201In some contexts this shortcut is undesirable. The @option{-Wundef} 3202option causes GCC to warn whenever it encounters an identifier which is 3203not a macro in an @samp{#if}. 3204@end itemize 3205 3206The preprocessor does not know anything about types in the language. 3207Therefore, @code{sizeof} operators are not recognized in @samp{#if}, and 3208neither are @code{enum} constants. They will be taken as identifiers 3209which are not macros, and replaced by zero. In the case of 3210@code{sizeof}, this is likely to cause the expression to be invalid. 3211 3212The preprocessor calculates the value of @var{expression}. It carries 3213out all calculations in the widest integer type known to the compiler; 3214on most machines supported by GCC this is 64 bits. This is not the same 3215rule as the compiler uses to calculate the value of a constant 3216expression, and may give different results in some cases. If the value 3217comes out to be nonzero, the @samp{#if} succeeds and the @var{controlled 3218text} is included; otherwise it is skipped. 3219 3220@node Defined 3221@subsection Defined 3222 3223@cindex @code{defined} 3224The special operator @code{defined} is used in @samp{#if} and 3225@samp{#elif} expressions to test whether a certain name is defined as a 3226macro. @code{defined @var{name}} and @code{defined (@var{name})} are 3227both expressions whose value is 1 if @var{name} is defined as a macro at 3228the current point in the program, and 0 otherwise. Thus, @code{@w{#if 3229defined MACRO}} is precisely equivalent to @code{@w{#ifdef MACRO}}. 3230 3231@code{defined} is useful when you wish to test more than one macro for 3232existence at once. For example, 3233 3234@smallexample 3235#if defined (__vax__) || defined (__ns16000__) 3236@end smallexample 3237 3238@noindent 3239would succeed if either of the names @code{__vax__} or 3240@code{__ns16000__} is defined as a macro. 3241 3242Conditionals written like this: 3243 3244@smallexample 3245#if defined BUFSIZE && BUFSIZE >= 1024 3246@end smallexample 3247 3248@noindent 3249can generally be simplified to just @code{@w{#if BUFSIZE >= 1024}}, 3250since if @code{BUFSIZE} is not defined, it will be interpreted as having 3251the value zero. 3252 3253If the @code{defined} operator appears as a result of a macro expansion, 3254the C standard says the behavior is undefined. GNU cpp treats it as a 3255genuine @code{defined} operator and evaluates it normally. It will warn 3256wherever your code uses this feature if you use the command-line option 3257@option{-pedantic}, since other compilers may handle it differently. 3258 3259@node Else 3260@subsection Else 3261 3262@findex #else 3263The @samp{#else} directive can be added to a conditional to provide 3264alternative text to be used if the condition fails. This is what it 3265looks like: 3266 3267@smallexample 3268@group 3269#if @var{expression} 3270@var{text-if-true} 3271#else /* Not @var{expression} */ 3272@var{text-if-false} 3273#endif /* Not @var{expression} */ 3274@end group 3275@end smallexample 3276 3277@noindent 3278If @var{expression} is nonzero, the @var{text-if-true} is included and 3279the @var{text-if-false} is skipped. If @var{expression} is zero, the 3280opposite happens. 3281 3282You can use @samp{#else} with @samp{#ifdef} and @samp{#ifndef}, too. 3283 3284@node Elif 3285@subsection Elif 3286 3287@findex #elif 3288One common case of nested conditionals is used to check for more than two 3289possible alternatives. For example, you might have 3290 3291@smallexample 3292#if X == 1 3293@dots{} 3294#else /* X != 1 */ 3295#if X == 2 3296@dots{} 3297#else /* X != 2 */ 3298@dots{} 3299#endif /* X != 2 */ 3300#endif /* X != 1 */ 3301@end smallexample 3302 3303Another conditional directive, @samp{#elif}, allows this to be 3304abbreviated as follows: 3305 3306@smallexample 3307#if X == 1 3308@dots{} 3309#elif X == 2 3310@dots{} 3311#else /* X != 2 and X != 1*/ 3312@dots{} 3313#endif /* X != 2 and X != 1*/ 3314@end smallexample 3315 3316@samp{#elif} stands for ``else if''. Like @samp{#else}, it goes in the 3317middle of a conditional group and subdivides it; it does not require a 3318matching @samp{#endif} of its own. Like @samp{#if}, the @samp{#elif} 3319directive includes an expression to be tested. The text following the 3320@samp{#elif} is processed only if the original @samp{#if}-condition 3321failed and the @samp{#elif} condition succeeds. 3322 3323More than one @samp{#elif} can go in the same conditional group. Then 3324the text after each @samp{#elif} is processed only if the @samp{#elif} 3325condition succeeds after the original @samp{#if} and all previous 3326@samp{#elif} directives within it have failed. 3327 3328@samp{#else} is allowed after any number of @samp{#elif} directives, but 3329@samp{#elif} may not follow @samp{#else}. 3330 3331@node Deleted Code 3332@section Deleted Code 3333@cindex commenting out code 3334 3335If you replace or delete a part of the program but want to keep the old 3336code around for future reference, you often cannot simply comment it 3337out. Block comments do not nest, so the first comment inside the old 3338code will end the commenting-out. The probable result is a flood of 3339syntax errors. 3340 3341One way to avoid this problem is to use an always-false conditional 3342instead. For instance, put @code{#if 0} before the deleted code and 3343@code{#endif} after it. This works even if the code being turned 3344off contains conditionals, but they must be entire conditionals 3345(balanced @samp{#if} and @samp{#endif}). 3346 3347Some people use @code{#ifdef notdef} instead. This is risky, because 3348@code{notdef} might be accidentally defined as a macro, and then the 3349conditional would succeed. @code{#if 0} can be counted on to fail. 3350 3351Do not use @code{#if 0} for comments which are not C code. Use a real 3352comment, instead. The interior of @code{#if 0} must consist of complete 3353tokens; in particular, single-quote characters must balance. Comments 3354often contain unbalanced single-quote characters (known in English as 3355apostrophes). These confuse @code{#if 0}. They don't confuse 3356@samp{/*}. 3357 3358@node Diagnostics 3359@chapter Diagnostics 3360@cindex diagnostic 3361@cindex reporting errors 3362@cindex reporting warnings 3363 3364@findex #error 3365The directive @samp{#error} causes the preprocessor to report a fatal 3366error. The tokens forming the rest of the line following @samp{#error} 3367are used as the error message. 3368 3369You would use @samp{#error} inside of a conditional that detects a 3370combination of parameters which you know the program does not properly 3371support. For example, if you know that the program will not run 3372properly on a VAX, you might write 3373 3374@smallexample 3375@group 3376#ifdef __vax__ 3377#error "Won't work on VAXen. See comments at get_last_object." 3378#endif 3379@end group 3380@end smallexample 3381 3382If you have several configuration parameters that must be set up by 3383the installation in a consistent way, you can use conditionals to detect 3384an inconsistency and report it with @samp{#error}. For example, 3385 3386@smallexample 3387#if !defined(UNALIGNED_INT_ASM_OP) && defined(DWARF2_DEBUGGING_INFO) 3388#error "DWARF2_DEBUGGING_INFO requires UNALIGNED_INT_ASM_OP." 3389#endif 3390@end smallexample 3391 3392@findex #warning 3393The directive @samp{#warning} is like @samp{#error}, but causes the 3394preprocessor to issue a warning and continue preprocessing. The tokens 3395following @samp{#warning} are used as the warning message. 3396 3397You might use @samp{#warning} in obsolete header files, with a message 3398directing the user to the header file which should be used instead. 3399 3400Neither @samp{#error} nor @samp{#warning} macro-expands its argument. 3401Internal whitespace sequences are each replaced with a single space. 3402The line must consist of complete tokens. It is wisest to make the 3403argument of these directives be a single string constant; this avoids 3404problems with apostrophes and the like. 3405 3406@node Line Control 3407@chapter Line Control 3408@cindex line control 3409 3410The C preprocessor informs the C compiler of the location in your source 3411code where each token came from. Presently, this is just the file name 3412and line number. All the tokens resulting from macro expansion are 3413reported as having appeared on the line of the source file where the 3414outermost macro was used. We intend to be more accurate in the future. 3415 3416If you write a program which generates source code, such as the 3417@command{bison} parser generator, you may want to adjust the preprocessor's 3418notion of the current file name and line number by hand. Parts of the 3419output from @command{bison} are generated from scratch, other parts come 3420from a standard parser file. The rest are copied verbatim from 3421@command{bison}'s input. You would like compiler error messages and 3422symbolic debuggers to be able to refer to @code{bison}'s input file. 3423 3424@findex #line 3425@command{bison} or any such program can arrange this by writing 3426@samp{#line} directives into the output file. @samp{#line} is a 3427directive that specifies the original line number and source file name 3428for subsequent input in the current preprocessor input file. 3429@samp{#line} has three variants: 3430 3431@table @code 3432@item #line @var{linenum} 3433@var{linenum} is a non-negative decimal integer constant. It specifies 3434the line number which should be reported for the following line of 3435input. Subsequent lines are counted from @var{linenum}. 3436 3437@item #line @var{linenum} @var{filename} 3438@var{linenum} is the same as for the first form, and has the same 3439effect. In addition, @var{filename} is a string constant. The 3440following line and all subsequent lines are reported to come from the 3441file it specifies, until something else happens to change that. 3442@var{filename} is interpreted according to the normal rules for a string 3443constant: backslash escapes are interpreted. This is different from 3444@samp{#include}. 3445 3446Previous versions of CPP did not interpret escapes in @samp{#line}; 3447we have changed it because the standard requires they be interpreted, 3448and most other compilers do. 3449 3450@item #line @var{anything else} 3451@var{anything else} is checked for macro calls, which are expanded. 3452The result should match one of the above two forms. 3453@end table 3454 3455@samp{#line} directives alter the results of the @code{__FILE__} and 3456@code{__LINE__} predefined macros from that point on. @xref{Standard 3457Predefined Macros}. They do not have any effect on @samp{#include}'s 3458idea of the directory containing the current file. This is a change 3459from GCC 2.95. Previously, a file reading 3460 3461@smallexample 3462#line 1 "../src/gram.y" 3463#include "gram.h" 3464@end smallexample 3465 3466would search for @file{gram.h} in @file{../src}, then the @option{-I} 3467chain; the directory containing the physical source file would not be 3468searched. In GCC 3.0 and later, the @samp{#include} is not affected by 3469the presence of a @samp{#line} referring to a different directory. 3470 3471We made this change because the old behavior caused problems when 3472generated source files were transported between machines. For instance, 3473it is common practice to ship generated parsers with a source release, 3474so that people building the distribution do not need to have yacc or 3475Bison installed. These files frequently have @samp{#line} directives 3476referring to the directory tree of the system where the distribution was 3477created. If GCC tries to search for headers in those directories, the 3478build is likely to fail. 3479 3480The new behavior can cause failures too, if the generated file is not 3481in the same directory as its source and it attempts to include a header 3482which would be visible searching from the directory containing the 3483source file. However, this problem is easily solved with an additional 3484@option{-I} switch on the command line. The failures caused by the old 3485semantics could sometimes be corrected only by editing the generated 3486files, which is difficult and error-prone. 3487 3488@node Pragmas 3489@chapter Pragmas 3490 3491The @samp{#pragma} directive is the method specified by the C standard 3492for providing additional information to the compiler, beyond what is 3493conveyed in the language itself. Three forms of this directive 3494(commonly known as @dfn{pragmas}) are specified by the 1999 C standard. 3495A C compiler is free to attach any meaning it likes to other pragmas. 3496 3497GCC has historically preferred to use extensions to the syntax of the 3498language, such as @code{__attribute__}, for this purpose. However, GCC 3499does define a few pragmas of its own. These mostly have effects on the 3500entire translation unit or source file. 3501 3502In GCC version 3, all GNU-defined, supported pragmas have been given a 3503@code{GCC} prefix. This is in line with the @code{STDC} prefix on all 3504pragmas defined by C99. For backward compatibility, pragmas which were 3505recognized by previous versions are still recognized without the 3506@code{GCC} prefix, but that usage is deprecated. Some older pragmas are 3507deprecated in their entirety. They are not recognized with the 3508@code{GCC} prefix. @xref{Obsolete Features}. 3509 3510@cindex @code{_Pragma} 3511C99 introduces the @code{@w{_Pragma}} operator. This feature addresses a 3512major problem with @samp{#pragma}: being a directive, it cannot be 3513produced as the result of macro expansion. @code{@w{_Pragma}} is an 3514operator, much like @code{sizeof} or @code{defined}, and can be embedded 3515in a macro. 3516 3517Its syntax is @code{@w{_Pragma (@var{string-literal})}}, where 3518@var{string-literal} can be either a normal or wide-character string 3519literal. It is destringized, by replacing all @samp{\\} with a single 3520@samp{\} and all @samp{\"} with a @samp{"}. The result is then 3521processed as if it had appeared as the right hand side of a 3522@samp{#pragma} directive. For example, 3523 3524@smallexample 3525_Pragma ("GCC dependency \"parse.y\"") 3526@end smallexample 3527 3528@noindent 3529has the same effect as @code{#pragma GCC dependency "parse.y"}. The 3530same effect could be achieved using macros, for example 3531 3532@smallexample 3533#define DO_PRAGMA(x) _Pragma (#x) 3534DO_PRAGMA (GCC dependency "parse.y") 3535@end smallexample 3536 3537The standard is unclear on where a @code{_Pragma} operator can appear. 3538The preprocessor does not accept it within a preprocessing conditional 3539directive like @samp{#if}. To be safe, you are probably best keeping it 3540out of directives other than @samp{#define}, and putting it on a line of 3541its own. 3542 3543This manual documents the pragmas which are meaningful to the 3544preprocessor itself. Other pragmas are meaningful to the C or C++ 3545compilers. They are documented in the GCC manual. 3546 3547GCC plugins may provide their own pragmas. 3548 3549@ftable @code 3550@item #pragma GCC dependency 3551@code{#pragma GCC dependency} allows you to check the relative dates of 3552the current file and another file. If the other file is more recent than 3553the current file, a warning is issued. This is useful if the current 3554file is derived from the other file, and should be regenerated. The 3555other file is searched for using the normal include search path. 3556Optional trailing text can be used to give more information in the 3557warning message. 3558 3559@smallexample 3560#pragma GCC dependency "parse.y" 3561#pragma GCC dependency "/usr/include/time.h" rerun fixincludes 3562@end smallexample 3563 3564@item #pragma GCC poison 3565Sometimes, there is an identifier that you want to remove completely 3566from your program, and make sure that it never creeps back in. To 3567enforce this, you can @dfn{poison} the identifier with this pragma. 3568@code{#pragma GCC poison} is followed by a list of identifiers to 3569poison. If any of those identifiers appears anywhere in the source 3570after the directive, it is a hard error. For example, 3571 3572@smallexample 3573#pragma GCC poison printf sprintf fprintf 3574sprintf(some_string, "hello"); 3575@end smallexample 3576 3577@noindent 3578will produce an error. 3579 3580If a poisoned identifier appears as part of the expansion of a macro 3581which was defined before the identifier was poisoned, it will @emph{not} 3582cause an error. This lets you poison an identifier without worrying 3583about system headers defining macros that use it. 3584 3585For example, 3586 3587@smallexample 3588#define strrchr rindex 3589#pragma GCC poison rindex 3590strrchr(some_string, 'h'); 3591@end smallexample 3592 3593@noindent 3594will not produce an error. 3595 3596@item #pragma GCC system_header 3597This pragma takes no arguments. It causes the rest of the code in the 3598current file to be treated as if it came from a system header. 3599@xref{System Headers}. 3600 3601@end ftable 3602 3603@node Other Directives 3604@chapter Other Directives 3605 3606@findex #ident 3607@findex #sccs 3608The @samp{#ident} directive takes one argument, a string constant. On 3609some systems, that string constant is copied into a special segment of 3610the object file. On other systems, the directive is ignored. The 3611@samp{#sccs} directive is a synonym for @samp{#ident}. 3612 3613These directives are not part of the C standard, but they are not 3614official GNU extensions either. What historical information we have 3615been able to find, suggests they originated with System V@. 3616 3617@cindex null directive 3618The @dfn{null directive} consists of a @samp{#} followed by a newline, 3619with only whitespace (including comments) in between. A null directive 3620is understood as a preprocessing directive but has no effect on the 3621preprocessor output. The primary significance of the existence of the 3622null directive is that an input line consisting of just a @samp{#} will 3623produce no output, rather than a line of output containing just a 3624@samp{#}. Supposedly some old C programs contain such lines. 3625 3626@node Preprocessor Output 3627@chapter Preprocessor Output 3628 3629When the C preprocessor is used with the C, C++, or Objective-C 3630compilers, it is integrated into the compiler and communicates a stream 3631of binary tokens directly to the compiler's parser. However, it can 3632also be used in the more conventional standalone mode, where it produces 3633textual output. 3634@c FIXME: Document the library interface. 3635 3636@cindex output format 3637The output from the C preprocessor looks much like the input, except 3638that all preprocessing directive lines have been replaced with blank 3639lines and all comments with spaces. Long runs of blank lines are 3640discarded. 3641 3642The ISO standard specifies that it is implementation defined whether a 3643preprocessor preserves whitespace between tokens, or replaces it with 3644e.g.@: a single space. In GNU CPP, whitespace between tokens is collapsed 3645to become a single space, with the exception that the first token on a 3646non-directive line is preceded with sufficient spaces that it appears in 3647the same column in the preprocessed output that it appeared in the 3648original source file. This is so the output is easy to read. 3649@xref{Differences from previous versions}. CPP does not insert any 3650whitespace where there was none in the original source, except where 3651necessary to prevent an accidental token paste. 3652 3653@cindex linemarkers 3654Source file name and line number information is conveyed by lines 3655of the form 3656 3657@smallexample 3658# @var{linenum} @var{filename} @var{flags} 3659@end smallexample 3660 3661@noindent 3662These are called @dfn{linemarkers}. They are inserted as needed into 3663the output (but never within a string or character constant). They mean 3664that the following line originated in file @var{filename} at line 3665@var{linenum}. @var{filename} will never contain any non-printing 3666characters; they are replaced with octal escape sequences. 3667 3668After the file name comes zero or more flags, which are @samp{1}, 3669@samp{2}, @samp{3}, or @samp{4}. If there are multiple flags, spaces 3670separate them. Here is what the flags mean: 3671 3672@table @samp 3673@item 1 3674This indicates the start of a new file. 3675@item 2 3676This indicates returning to a file (after having included another file). 3677@item 3 3678This indicates that the following text comes from a system header file, 3679so certain warnings should be suppressed. 3680@item 4 3681This indicates that the following text should be treated as being 3682wrapped in an implicit @code{extern "C"} block. 3683@c maybe cross reference NO_IMPLICIT_EXTERN_C 3684@end table 3685 3686As an extension, the preprocessor accepts linemarkers in non-assembler 3687input files. They are treated like the corresponding @samp{#line} 3688directive, (@pxref{Line Control}), except that trailing flags are 3689permitted, and are interpreted with the meanings described above. If 3690multiple flags are given, they must be in ascending order. 3691 3692Some directives may be duplicated in the output of the preprocessor. 3693These are @samp{#ident} (always), @samp{#pragma} (only if the 3694preprocessor does not handle the pragma itself), and @samp{#define} and 3695@samp{#undef} (with certain debugging options). If this happens, the 3696@samp{#} of the directive will always be in the first column, and there 3697will be no space between the @samp{#} and the directive name. If macro 3698expansion happens to generate tokens which might be mistaken for a 3699duplicated directive, a space will be inserted between the @samp{#} and 3700the directive name. 3701 3702@node Traditional Mode 3703@chapter Traditional Mode 3704 3705Traditional (pre-standard) C preprocessing is rather different from 3706the preprocessing specified by the standard. When GCC is given the 3707@option{-traditional-cpp} option, it attempts to emulate a traditional 3708preprocessor. 3709 3710GCC versions 3.2 and later only support traditional mode semantics in 3711the preprocessor, and not in the compiler front ends. This chapter 3712outlines the traditional preprocessor semantics we implemented. 3713 3714The implementation does not correspond precisely to the behavior of 3715earlier versions of GCC, nor to any true traditional preprocessor. 3716After all, inconsistencies among traditional implementations were a 3717major motivation for C standardization. However, we intend that it 3718should be compatible with true traditional preprocessors in all ways 3719that actually matter. 3720 3721@menu 3722* Traditional lexical analysis:: 3723* Traditional macros:: 3724* Traditional miscellany:: 3725* Traditional warnings:: 3726@end menu 3727 3728@node Traditional lexical analysis 3729@section Traditional lexical analysis 3730 3731The traditional preprocessor does not decompose its input into tokens 3732the same way a standards-conforming preprocessor does. The input is 3733simply treated as a stream of text with minimal internal form. 3734 3735This implementation does not treat trigraphs (@pxref{trigraphs}) 3736specially since they were an invention of the standards committee. It 3737handles arbitrarily-positioned escaped newlines properly and splices 3738the lines as you would expect; many traditional preprocessors did not 3739do this. 3740 3741The form of horizontal whitespace in the input file is preserved in 3742the output. In particular, hard tabs remain hard tabs. This can be 3743useful if, for example, you are preprocessing a Makefile. 3744 3745Traditional CPP only recognizes C-style block comments, and treats the 3746@samp{/*} sequence as introducing a comment only if it lies outside 3747quoted text. Quoted text is introduced by the usual single and double 3748quotes, and also by an initial @samp{<} in a @code{#include} 3749directive. 3750 3751Traditionally, comments are completely removed and are not replaced 3752with a space. Since a traditional compiler does its own tokenization 3753of the output of the preprocessor, this means that comments can 3754effectively be used as token paste operators. However, comments 3755behave like separators for text handled by the preprocessor itself, 3756since it doesn't re-lex its input. For example, in 3757 3758@smallexample 3759#if foo/**/bar 3760@end smallexample 3761 3762@noindent 3763@samp{foo} and @samp{bar} are distinct identifiers and expanded 3764separately if they happen to be macros. In other words, this 3765directive is equivalent to 3766 3767@smallexample 3768#if foo bar 3769@end smallexample 3770 3771@noindent 3772rather than 3773 3774@smallexample 3775#if foobar 3776@end smallexample 3777 3778Generally speaking, in traditional mode an opening quote need not have 3779a matching closing quote. In particular, a macro may be defined with 3780replacement text that contains an unmatched quote. Of course, if you 3781attempt to compile preprocessed output containing an unmatched quote 3782you will get a syntax error. 3783 3784However, all preprocessing directives other than @code{#define} 3785require matching quotes. For example: 3786 3787@smallexample 3788#define m This macro's fine and has an unmatched quote 3789"/* This is not a comment. */ 3790/* @r{This is a comment. The following #include directive 3791 is ill-formed.} */ 3792#include <stdio.h 3793@end smallexample 3794 3795Just as for the ISO preprocessor, what would be a closing quote can be 3796escaped with a backslash to prevent the quoted text from closing. 3797 3798@node Traditional macros 3799@section Traditional macros 3800 3801The major difference between traditional and ISO macros is that the 3802former expand to text rather than to a token sequence. CPP removes 3803all leading and trailing horizontal whitespace from a macro's 3804replacement text before storing it, but preserves the form of internal 3805whitespace. 3806 3807One consequence is that it is legitimate for the replacement text to 3808contain an unmatched quote (@pxref{Traditional lexical analysis}). An 3809unclosed string or character constant continues into the text 3810following the macro call. Similarly, the text at the end of a macro's 3811expansion can run together with the text after the macro invocation to 3812produce a single token. 3813 3814Normally comments are removed from the replacement text after the 3815macro is expanded, but if the @option{-CC} option is passed on the 3816command line comments are preserved. (In fact, the current 3817implementation removes comments even before saving the macro 3818replacement text, but it careful to do it in such a way that the 3819observed effect is identical even in the function-like macro case.) 3820 3821The ISO stringification operator @samp{#} and token paste operator 3822@samp{##} have no special meaning. As explained later, an effect 3823similar to these operators can be obtained in a different way. Macro 3824names that are embedded in quotes, either from the main file or after 3825macro replacement, do not expand. 3826 3827CPP replaces an unquoted object-like macro name with its replacement 3828text, and then rescans it for further macros to replace. Unlike 3829standard macro expansion, traditional macro expansion has no provision 3830to prevent recursion. If an object-like macro appears unquoted in its 3831replacement text, it will be replaced again during the rescan pass, 3832and so on @emph{ad infinitum}. GCC detects when it is expanding 3833recursive macros, emits an error message, and continues after the 3834offending macro invocation. 3835 3836@smallexample 3837#define PLUS + 3838#define INC(x) PLUS+x 3839INC(foo); 3840 @expansion{} ++foo; 3841@end smallexample 3842 3843Function-like macros are similar in form but quite different in 3844behavior to their ISO counterparts. Their arguments are contained 3845within parentheses, are comma-separated, and can cross physical lines. 3846Commas within nested parentheses are not treated as argument 3847separators. Similarly, a quote in an argument cannot be left 3848unclosed; a following comma or parenthesis that comes before the 3849closing quote is treated like any other character. There is no 3850facility for handling variadic macros. 3851 3852This implementation removes all comments from macro arguments, unless 3853the @option{-C} option is given. The form of all other horizontal 3854whitespace in arguments is preserved, including leading and trailing 3855whitespace. In particular 3856 3857@smallexample 3858f( ) 3859@end smallexample 3860 3861@noindent 3862is treated as an invocation of the macro @samp{f} with a single 3863argument consisting of a single space. If you want to invoke a 3864function-like macro that takes no arguments, you must not leave any 3865whitespace between the parentheses. 3866 3867If a macro argument crosses a new line, the new line is replaced with 3868a space when forming the argument. If the previous line contained an 3869unterminated quote, the following line inherits the quoted state. 3870 3871Traditional preprocessors replace parameters in the replacement text 3872with their arguments regardless of whether the parameters are within 3873quotes or not. This provides a way to stringize arguments. For 3874example 3875 3876@smallexample 3877#define str(x) "x" 3878str(/* @r{A comment} */some text ) 3879 @expansion{} "some text " 3880@end smallexample 3881 3882@noindent 3883Note that the comment is removed, but that the trailing space is 3884preserved. Here is an example of using a comment to effect token 3885pasting. 3886 3887@smallexample 3888#define suffix(x) foo_/**/x 3889suffix(bar) 3890 @expansion{} foo_bar 3891@end smallexample 3892 3893@node Traditional miscellany 3894@section Traditional miscellany 3895 3896Here are some things to be aware of when using the traditional 3897preprocessor. 3898 3899@itemize @bullet 3900@item 3901Preprocessing directives are recognized only when their leading 3902@samp{#} appears in the first column. There can be no whitespace 3903between the beginning of the line and the @samp{#}, but whitespace can 3904follow the @samp{#}. 3905 3906@item 3907A true traditional C preprocessor does not recognize @samp{#error} or 3908@samp{#pragma}, and may not recognize @samp{#elif}. CPP supports all 3909the directives in traditional mode that it supports in ISO mode, 3910including extensions, with the exception that the effects of 3911@samp{#pragma GCC poison} are undefined. 3912 3913@item 3914__STDC__ is not defined. 3915 3916@item 3917If you use digraphs the behavior is undefined. 3918 3919@item 3920If a line that looks like a directive appears within macro arguments, 3921the behavior is undefined. 3922 3923@end itemize 3924 3925@node Traditional warnings 3926@section Traditional warnings 3927You can request warnings about features that did not exist, or worked 3928differently, in traditional C with the @option{-Wtraditional} option. 3929GCC does not warn about features of ISO C which you must use when you 3930are using a conforming compiler, such as the @samp{#} and @samp{##} 3931operators. 3932 3933Presently @option{-Wtraditional} warns about: 3934 3935@itemize @bullet 3936@item 3937Macro parameters that appear within string literals in the macro body. 3938In traditional C macro replacement takes place within string literals, 3939but does not in ISO C@. 3940 3941@item 3942In traditional C, some preprocessor directives did not exist. 3943Traditional preprocessors would only consider a line to be a directive 3944if the @samp{#} appeared in column 1 on the line. Therefore 3945@option{-Wtraditional} warns about directives that traditional C 3946understands but would ignore because the @samp{#} does not appear as the 3947first character on the line. It also suggests you hide directives like 3948@samp{#pragma} not understood by traditional C by indenting them. Some 3949traditional implementations would not recognize @samp{#elif}, so it 3950suggests avoiding it altogether. 3951 3952@item 3953A function-like macro that appears without an argument list. In some 3954traditional preprocessors this was an error. In ISO C it merely means 3955that the macro is not expanded. 3956 3957@item 3958The unary plus operator. This did not exist in traditional C@. 3959 3960@item 3961The @samp{U} and @samp{LL} integer constant suffixes, which were not 3962available in traditional C@. (Traditional C does support the @samp{L} 3963suffix for simple long integer constants.) You are not warned about 3964uses of these suffixes in macros defined in system headers. For 3965instance, @code{UINT_MAX} may well be defined as @code{4294967295U}, but 3966you will not be warned if you use @code{UINT_MAX}. 3967 3968You can usually avoid the warning, and the related warning about 3969constants which are so large that they are unsigned, by writing the 3970integer constant in question in hexadecimal, with no U suffix. Take 3971care, though, because this gives the wrong result in exotic cases. 3972@end itemize 3973 3974@node Implementation Details 3975@chapter Implementation Details 3976 3977Here we document details of how the preprocessor's implementation 3978affects its user-visible behavior. You should try to avoid undue 3979reliance on behavior described here, as it is possible that it will 3980change subtly in future implementations. 3981 3982Also documented here are obsolete features and changes from previous 3983versions of CPP@. 3984 3985@menu 3986* Implementation-defined behavior:: 3987* Implementation limits:: 3988* Obsolete Features:: 3989* Differences from previous versions:: 3990@end menu 3991 3992@node Implementation-defined behavior 3993@section Implementation-defined behavior 3994@cindex implementation-defined behavior 3995 3996This is how CPP behaves in all the cases which the C standard 3997describes as @dfn{implementation-defined}. This term means that the 3998implementation is free to do what it likes, but must document its choice 3999and stick to it. 4000@c FIXME: Check the C++ standard for more implementation-defined stuff. 4001 4002@itemize @bullet 4003@need 1000 4004@item The mapping of physical source file multi-byte characters to the 4005execution character set. 4006 4007The input character set can be specified using the 4008@option{-finput-charset} option, while the execution character set may 4009be controlled using the @option{-fexec-charset} and 4010@option{-fwide-exec-charset} options. 4011 4012@item Identifier characters. 4013@anchor{Identifier characters} 4014 4015The C and C++ standards allow identifiers to be composed of @samp{_} 4016and the alphanumeric characters. C++ and C99 also allow universal 4017character names, and C99 further permits implementation-defined 4018characters. GCC currently only permits universal character names if 4019@option{-fextended-identifiers} is used, because the implementation of 4020universal character names in identifiers is experimental. 4021 4022GCC allows the @samp{$} character in identifiers as an extension for 4023most targets. This is true regardless of the @option{std=} switch, 4024since this extension cannot conflict with standards-conforming 4025programs. When preprocessing assembler, however, dollars are not 4026identifier characters by default. 4027 4028Currently the targets that by default do not permit @samp{$} are AVR, 4029IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC targets for the AIX 4030operating system. 4031 4032You can override the default with @option{-fdollars-in-identifiers} or 4033@option{fno-dollars-in-identifiers}. @xref{fdollars-in-identifiers}. 4034 4035@item Non-empty sequences of whitespace characters. 4036 4037In textual output, each whitespace sequence is collapsed to a single 4038space. For aesthetic reasons, the first token on each non-directive 4039line of output is preceded with sufficient spaces that it appears in the 4040same column as it did in the original source file. 4041 4042@item The numeric value of character constants in preprocessor expressions. 4043 4044The preprocessor and compiler interpret character constants in the 4045same way; i.e.@: escape sequences such as @samp{\a} are given the 4046values they would have on the target machine. 4047 4048The compiler evaluates a multi-character character constant a character 4049at a time, shifting the previous value left by the number of bits per 4050target character, and then or-ing in the bit-pattern of the new 4051character truncated to the width of a target character. The final 4052bit-pattern is given type @code{int}, and is therefore signed, 4053regardless of whether single characters are signed or not (a slight 4054change from versions 3.1 and earlier of GCC)@. If there are more 4055characters in the constant than would fit in the target @code{int} the 4056compiler issues a warning, and the excess leading characters are 4057ignored. 4058 4059For example, @code{'ab'} for a target with an 8-bit @code{char} would be 4060interpreted as @w{@samp{(int) ((unsigned char) 'a' * 256 + (unsigned char) 4061'b')}}, and @code{'\234a'} as @w{@samp{(int) ((unsigned char) '\234' * 4062256 + (unsigned char) 'a')}}. 4063 4064@item Source file inclusion. 4065 4066For a discussion on how the preprocessor locates header files, 4067@ref{Include Operation}. 4068 4069@item Interpretation of the filename resulting from a macro-expanded 4070@samp{#include} directive. 4071 4072@xref{Computed Includes}. 4073 4074@item Treatment of a @samp{#pragma} directive that after macro-expansion 4075results in a standard pragma. 4076 4077No macro expansion occurs on any @samp{#pragma} directive line, so the 4078question does not arise. 4079 4080Note that GCC does not yet implement any of the standard 4081pragmas. 4082 4083@end itemize 4084 4085@node Implementation limits 4086@section Implementation limits 4087@cindex implementation limits 4088 4089CPP has a small number of internal limits. This section lists the 4090limits which the C standard requires to be no lower than some minimum, 4091and all the others known. It is intended that there should be as few limits 4092as possible. If you encounter an undocumented or inconvenient limit, 4093please report that as a bug. @xref{Bugs, , Reporting Bugs, gcc, Using 4094the GNU Compiler Collection (GCC)}. 4095 4096Where we say something is limited @dfn{only by available memory}, that 4097means that internal data structures impose no intrinsic limit, and space 4098is allocated with @code{malloc} or equivalent. The actual limit will 4099therefore depend on many things, such as the size of other things 4100allocated by the compiler at the same time, the amount of memory 4101consumed by other processes on the same computer, etc. 4102 4103@itemize @bullet 4104 4105@item Nesting levels of @samp{#include} files. 4106 4107We impose an arbitrary limit of 200 levels, to avoid runaway recursion. 4108The standard requires at least 15 levels. 4109 4110@item Nesting levels of conditional inclusion. 4111 4112The C standard mandates this be at least 63. CPP is limited only by 4113available memory. 4114 4115@item Levels of parenthesized expressions within a full expression. 4116 4117The C standard requires this to be at least 63. In preprocessor 4118conditional expressions, it is limited only by available memory. 4119 4120@item Significant initial characters in an identifier or macro name. 4121 4122The preprocessor treats all characters as significant. The C standard 4123requires only that the first 63 be significant. 4124 4125@item Number of macros simultaneously defined in a single translation unit. 4126 4127The standard requires at least 4095 be possible. CPP is limited only 4128by available memory. 4129 4130@item Number of parameters in a macro definition and arguments in a macro call. 4131 4132We allow @code{USHRT_MAX}, which is no smaller than 65,535. The minimum 4133required by the standard is 127. 4134 4135@item Number of characters on a logical source line. 4136 4137The C standard requires a minimum of 4096 be permitted. CPP places 4138no limits on this, but you may get incorrect column numbers reported in 4139diagnostics for lines longer than 65,535 characters. 4140 4141@item Maximum size of a source file. 4142 4143The standard does not specify any lower limit on the maximum size of a 4144source file. GNU cpp maps files into memory, so it is limited by the 4145available address space. This is generally at least two gigabytes. 4146Depending on the operating system, the size of physical memory may or 4147may not be a limitation. 4148 4149@end itemize 4150 4151@node Obsolete Features 4152@section Obsolete Features 4153 4154CPP has some features which are present mainly for compatibility with 4155older programs. We discourage their use in new code. In some cases, 4156we plan to remove the feature in a future version of GCC@. 4157 4158@subsection Assertions 4159@cindex assertions 4160 4161@dfn{Assertions} are a deprecated alternative to macros in writing 4162conditionals to test what sort of computer or system the compiled 4163program will run on. Assertions are usually predefined, but you can 4164define them with preprocessing directives or command-line options. 4165 4166Assertions were intended to provide a more systematic way to describe 4167the compiler's target system and we added them for compatibility with 4168existing compilers. In practice they are just as unpredictable as the 4169system-specific predefined macros. In addition, they are not part of 4170any standard, and only a few compilers support them. 4171Therefore, the use of assertions is @strong{less} portable than the use 4172of system-specific predefined macros. We recommend you do not use them at 4173all. 4174 4175@cindex predicates 4176An assertion looks like this: 4177 4178@smallexample 4179#@var{predicate} (@var{answer}) 4180@end smallexample 4181 4182@noindent 4183@var{predicate} must be a single identifier. @var{answer} can be any 4184sequence of tokens; all characters are significant except for leading 4185and trailing whitespace, and differences in internal whitespace 4186sequences are ignored. (This is similar to the rules governing macro 4187redefinition.) Thus, @code{(x + y)} is different from @code{(x+y)} but 4188equivalent to @code{@w{( x + y )}}. Parentheses do not nest inside an 4189answer. 4190 4191@cindex testing predicates 4192To test an assertion, you write it in an @samp{#if}. For example, this 4193conditional succeeds if either @code{vax} or @code{ns16000} has been 4194asserted as an answer for @code{machine}. 4195 4196@smallexample 4197#if #machine (vax) || #machine (ns16000) 4198@end smallexample 4199 4200@noindent 4201You can test whether @emph{any} answer is asserted for a predicate by 4202omitting the answer in the conditional: 4203 4204@smallexample 4205#if #machine 4206@end smallexample 4207 4208@findex #assert 4209Assertions are made with the @samp{#assert} directive. Its sole 4210argument is the assertion to make, without the leading @samp{#} that 4211identifies assertions in conditionals. 4212 4213@smallexample 4214#assert @var{predicate} (@var{answer}) 4215@end smallexample 4216 4217@noindent 4218You may make several assertions with the same predicate and different 4219answers. Subsequent assertions do not override previous ones for the 4220same predicate. All the answers for any given predicate are 4221simultaneously true. 4222 4223@cindex assertions, canceling 4224@findex #unassert 4225Assertions can be canceled with the @samp{#unassert} directive. It 4226has the same syntax as @samp{#assert}. In that form it cancels only the 4227answer which was specified on the @samp{#unassert} line; other answers 4228for that predicate remain true. You can cancel an entire predicate by 4229leaving out the answer: 4230 4231@smallexample 4232#unassert @var{predicate} 4233@end smallexample 4234 4235@noindent 4236In either form, if no such assertion has been made, @samp{#unassert} has 4237no effect. 4238 4239You can also make or cancel assertions using command line options. 4240@xref{Invocation}. 4241 4242@node Differences from previous versions 4243@section Differences from previous versions 4244@cindex differences from previous versions 4245 4246This section details behavior which has changed from previous versions 4247of CPP@. We do not plan to change it again in the near future, but 4248we do not promise not to, either. 4249 4250The ``previous versions'' discussed here are 2.95 and before. The 4251behavior of GCC 3.0 is mostly the same as the behavior of the widely 4252used 2.96 and 2.97 development snapshots. Where there are differences, 4253they generally represent bugs in the snapshots. 4254 4255@itemize @bullet 4256 4257@item -I- deprecated 4258 4259This option has been deprecated in 4.0. @option{-iquote} is meant to 4260replace the need for this option. 4261 4262@item Order of evaluation of @samp{#} and @samp{##} operators 4263 4264The standard does not specify the order of evaluation of a chain of 4265@samp{##} operators, nor whether @samp{#} is evaluated before, after, or 4266at the same time as @samp{##}. You should therefore not write any code 4267which depends on any specific ordering. It is possible to guarantee an 4268ordering, if you need one, by suitable use of nested macros. 4269 4270An example of where this might matter is pasting the arguments @samp{1}, 4271@samp{e} and @samp{-2}. This would be fine for left-to-right pasting, 4272but right-to-left pasting would produce an invalid token @samp{e-2}. 4273 4274GCC 3.0 evaluates @samp{#} and @samp{##} at the same time and strictly 4275left to right. Older versions evaluated all @samp{#} operators first, 4276then all @samp{##} operators, in an unreliable order. 4277 4278@item The form of whitespace between tokens in preprocessor output 4279 4280@xref{Preprocessor Output}, for the current textual format. This is 4281also the format used by stringification. Normally, the preprocessor 4282communicates tokens directly to the compiler's parser, and whitespace 4283does not come up at all. 4284 4285Older versions of GCC preserved all whitespace provided by the user and 4286inserted lots more whitespace of their own, because they could not 4287accurately predict when extra spaces were needed to prevent accidental 4288token pasting. 4289 4290@item Optional argument when invoking rest argument macros 4291 4292As an extension, GCC permits you to omit the variable arguments entirely 4293when you use a variable argument macro. This is forbidden by the 1999 C 4294standard, and will provoke a pedantic warning with GCC 3.0. Previous 4295versions accepted it silently. 4296 4297@item @samp{##} swallowing preceding text in rest argument macros 4298 4299Formerly, in a macro expansion, if @samp{##} appeared before a variable 4300arguments parameter, and the set of tokens specified for that argument 4301in the macro invocation was empty, previous versions of CPP would 4302back up and remove the preceding sequence of non-whitespace characters 4303(@strong{not} the preceding token). This extension is in direct 4304conflict with the 1999 C standard and has been drastically pared back. 4305 4306In the current version of the preprocessor, if @samp{##} appears between 4307a comma and a variable arguments parameter, and the variable argument is 4308omitted entirely, the comma will be removed from the expansion. If the 4309variable argument is empty, or the token before @samp{##} is not a 4310comma, then @samp{##} behaves as a normal token paste. 4311 4312@item @samp{#line} and @samp{#include} 4313 4314The @samp{#line} directive used to change GCC's notion of the 4315``directory containing the current file'', used by @samp{#include} with 4316a double-quoted header file name. In 3.0 and later, it does not. 4317@xref{Line Control}, for further explanation. 4318 4319@item Syntax of @samp{#line} 4320 4321In GCC 2.95 and previous, the string constant argument to @samp{#line} 4322was treated the same way as the argument to @samp{#include}: backslash 4323escapes were not honored, and the string ended at the second @samp{"}. 4324This is not compliant with the C standard. In GCC 3.0, an attempt was 4325made to correct the behavior, so that the string was treated as a real 4326string constant, but it turned out to be buggy. In 3.1, the bugs have 4327been fixed. (We are not fixing the bugs in 3.0 because they affect 4328relatively few people and the fix is quite invasive.) 4329 4330@end itemize 4331 4332@node Invocation 4333@chapter Invocation 4334@cindex invocation 4335@cindex command line 4336 4337Most often when you use the C preprocessor you will not have to invoke it 4338explicitly: the C compiler will do so automatically. However, the 4339preprocessor is sometimes useful on its own. All the options listed 4340here are also acceptable to the C compiler and have the same meaning, 4341except that the C compiler has different rules for specifying the output 4342file. 4343 4344@emph{Note:} Whether you use the preprocessor by way of @command{gcc} 4345or @command{cpp}, the @dfn{compiler driver} is run first. This 4346program's purpose is to translate your command into invocations of the 4347programs that do the actual work. Their command line interfaces are 4348similar but not identical to the documented interface, and may change 4349without notice. 4350 4351@ignore 4352@c man begin SYNOPSIS 4353cpp [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}] 4354 [@option{-I}@var{dir}@dots{}] [@option{-iquote}@var{dir}@dots{}] 4355 [@option{-iremap}@var{src}:@var{dst}] 4356 [@option{-W}@var{warn}@dots{}] 4357 [@option{-M}|@option{-MM}] [@option{-MG}] [@option{-MF} @var{filename}] 4358 [@option{-MP}] [@option{-MQ} @var{target}@dots{}] 4359 [@option{-MT} @var{target}@dots{}] 4360 [@option{-P}] [@option{-fno-working-directory}] 4361 [@option{-x} @var{language}] [@option{-std=}@var{standard}] 4362 @var{infile} @var{outfile} 4363 4364Only the most useful options are listed here; see below for the remainder. 4365@c man end 4366@c man begin SEEALSO 4367gpl(7), gfdl(7), fsf-funding(7), 4368gcc(1), as(1), ld(1), and the Info entries for @file{cpp}, @file{gcc}, and 4369@file{binutils}. 4370@c man end 4371@end ignore 4372 4373@c man begin OPTIONS 4374The C preprocessor expects two file names as arguments, @var{infile} and 4375@var{outfile}. The preprocessor reads @var{infile} together with any 4376other files it specifies with @samp{#include}. All the output generated 4377by the combined input files is written in @var{outfile}. 4378 4379Either @var{infile} or @var{outfile} may be @option{-}, which as 4380@var{infile} means to read from standard input and as @var{outfile} 4381means to write to standard output. Also, if either file is omitted, it 4382means the same as if @option{-} had been specified for that file. 4383 4384Unless otherwise noted, or the option ends in @samp{=}, all options 4385which take an argument may have that argument appear either immediately 4386after the option, or with a space between option and argument: 4387@option{-Ifoo} and @option{-I foo} have the same effect. 4388 4389@cindex grouping options 4390@cindex options, grouping 4391Many options have multi-letter names; therefore multiple single-letter 4392options may @emph{not} be grouped: @option{-dM} is very different from 4393@w{@samp{-d -M}}. 4394 4395@cindex options 4396@include cppopts.texi 4397@c man end 4398 4399@node Environment Variables 4400@chapter Environment Variables 4401@cindex environment variables 4402@c man begin ENVIRONMENT 4403 4404This section describes the environment variables that affect how CPP 4405operates. You can use them to specify directories or prefixes to use 4406when searching for include files, or to control dependency output. 4407 4408Note that you can also specify places to search using options such as 4409@option{-I}, and control dependency output with options like 4410@option{-M} (@pxref{Invocation}). These take precedence over 4411environment variables, which in turn take precedence over the 4412configuration of GCC@. 4413 4414@include cppenv.texi 4415@c man end 4416 4417@page 4418@include fdl.texi 4419 4420@page 4421@node Index of Directives 4422@unnumbered Index of Directives 4423@printindex fn 4424 4425@node Option Index 4426@unnumbered Option Index 4427@noindent 4428CPP's command line options and environment variables are indexed here 4429without any initial @samp{-} or @samp{--}. 4430@printindex op 4431 4432@page 4433@node Concept Index 4434@unnumbered Concept Index 4435@printindex cp 4436 4437@bye 4438