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      3 
      4    Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997,
      5 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software
      6 Foundation, Inc.
      7 
      8    Permission is granted to copy, distribute and/or modify this document
      9 under the terms of the GNU Free Documentation License, Version 1.1 or
     10 any later version published by the Free Software Foundation.  A copy of
     11 the license is included in the section entitled "GNU Free Documentation
     12 License".
     13 
     14    This manual contains no Invariant Sections.  The Front-Cover Texts
     15 are (a) (see below), and the Back-Cover Texts are (b) (see below).
     16 
     17    (a) The FSF's Front-Cover Text is:
     18 
     19    A GNU Manual
     20 
     21    (b) The FSF's Back-Cover Text is:
     22 
     23    You have freedom to copy and modify this GNU Manual, like GNU
     24 software.  Copies published by the Free Software Foundation raise
     25 funds for GNU development.
     26 
     27 INFO-DIR-SECTION Software development
     28 START-INFO-DIR-ENTRY
     29 * Cpp: (cpp).		       The GNU C preprocessor.
     30 END-INFO-DIR-ENTRY
     31 
     32 
     33 File: cpp.info,  Node: Top,  Next: Overview,  Up: (dir)
     34 
     35 The C Preprocessor
     36 ******************
     37 
     38 The C preprocessor implements the macro language used to transform C,
     39 C++, and Objective-C programs before they are compiled.  It can also be
     40 useful on its own.
     41 
     42 * Menu:
     43 
     44 * Overview::
     45 * Header Files::
     46 * Macros::
     47 * Conditionals::
     48 * Diagnostics::
     49 * Line Control::
     50 * Pragmas::
     51 * Other Directives::
     52 * Preprocessor Output::
     53 * Traditional Mode::
     54 * Implementation Details::
     55 * Invocation::
     56 * Environment Variables::
     57 * GNU Free Documentation License::
     58 * Index of Directives::
     59 * Option Index::
     60 * Concept Index::
     61 
     62  --- The Detailed Node Listing ---
     63 
     64 Overview
     65 
     66 * Character sets::
     67 * Initial processing::
     68 * Tokenization::
     69 * The preprocessing language::
     70 
     71 Header Files
     72 
     73 * Include Syntax::
     74 * Include Operation::
     75 * Search Path::
     76 * Once-Only Headers::
     77 * Computed Includes::
     78 * Wrapper Headers::
     79 * System Headers::
     80 
     81 Macros
     82 
     83 * Object-like Macros::
     84 * Function-like Macros::
     85 * Macro Arguments::
     86 * Stringification::
     87 * Concatenation::
     88 * Variadic Macros::
     89 * Predefined Macros::
     90 * Undefining and Redefining Macros::
     91 * Directives Within Macro Arguments::
     92 * Macro Pitfalls::
     93 
     94 Predefined Macros
     95 
     96 * Standard Predefined Macros::
     97 * Common Predefined Macros::
     98 * System-specific Predefined Macros::
     99 * C++ Named Operators::
    100 
    101 Macro Pitfalls
    102 
    103 * Misnesting::
    104 * Operator Precedence Problems::
    105 * Swallowing the Semicolon::
    106 * Duplication of Side Effects::
    107 * Self-Referential Macros::
    108 * Argument Prescan::
    109 * Newlines in Arguments::
    110 
    111 Conditionals
    112 
    113 * Conditional Uses::
    114 * Conditional Syntax::
    115 * Deleted Code::
    116 
    117 Conditional Syntax
    118 
    119 * Ifdef::
    120 * If::
    121 * Defined::
    122 * Else::
    123 * Elif::
    124 
    125 Implementation Details
    126 
    127 * Implementation-defined behavior::
    128 * Implementation limits::
    129 * Obsolete Features::
    130 * Differences from previous versions::
    131 
    132 Obsolete Features
    133 
    134 * Assertions::
    135 * Obsolete once-only headers::
    136 
    137    Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997,
    138 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software
    139 Foundation, Inc.
    140 
    141    Permission is granted to copy, distribute and/or modify this document
    142 under the terms of the GNU Free Documentation License, Version 1.1 or
    143 any later version published by the Free Software Foundation.  A copy of
    144 the license is included in the section entitled "GNU Free Documentation
    145 License".
    146 
    147    This manual contains no Invariant Sections.  The Front-Cover Texts
    148 are (a) (see below), and the Back-Cover Texts are (b) (see below).
    149 
    150    (a) The FSF's Front-Cover Text is:
    151 
    152    A GNU Manual
    153 
    154    (b) The FSF's Back-Cover Text is:
    155 
    156    You have freedom to copy and modify this GNU Manual, like GNU
    157 software.  Copies published by the Free Software Foundation raise
    158 funds for GNU development.
    159 
    160 
    161 File: cpp.info,  Node: Overview,  Next: Header Files,  Prev: Top,  Up: Top
    162 
    163 1 Overview
    164 **********
    165 
    166 The C preprocessor, often known as "cpp", is a "macro processor" that
    167 is used automatically by the C compiler to transform your program
    168 before compilation.  It is called a macro processor because it allows
    169 you to define "macros", which are brief abbreviations for longer
    170 constructs.
    171 
    172    The C preprocessor is intended to be used only with C, C++, and
    173 Objective-C source code.  In the past, it has been abused as a general
    174 text processor.  It will choke on input which does not obey C's lexical
    175 rules.  For example, apostrophes will be interpreted as the beginning of
    176 character constants, and cause errors.  Also, you cannot rely on it
    177 preserving characteristics of the input which are not significant to
    178 C-family languages.  If a Makefile is preprocessed, all the hard tabs
    179 will be removed, and the Makefile will not work.
    180 
    181    Having said that, you can often get away with using cpp on things
    182 which are not C.  Other Algol-ish programming languages are often safe
    183 (Pascal, Ada, etc.) So is assembly, with caution.  `-traditional-cpp'
    184 mode preserves more white space, and is otherwise more permissive.  Many
    185 of the problems can be avoided by writing C or C++ style comments
    186 instead of native language comments, and keeping macros simple.
    187 
    188    Wherever possible, you should use a preprocessor geared to the
    189 language you are writing in.  Modern versions of the GNU assembler have
    190 macro facilities.  Most high level programming languages have their own
    191 conditional compilation and inclusion mechanism.  If all else fails,
    192 try a true general text processor, such as GNU M4.
    193 
    194    C preprocessors vary in some details.  This manual discusses the GNU
    195 C preprocessor, which provides a small superset of the features of ISO
    196 Standard C.  In its default mode, the GNU C preprocessor does not do a
    197 few things required by the standard.  These are features which are
    198 rarely, if ever, used, and may cause surprising changes to the meaning
    199 of a program which does not expect them.  To get strict ISO Standard C,
    200 you should use the `-std=c89' or `-std=c99' options, depending on which
    201 version of the standard you want.  To get all the mandatory
    202 diagnostics, you must also use `-pedantic'.  *Note Invocation::.
    203 
    204    This manual describes the behavior of the ISO preprocessor.  To
    205 minimize gratuitous differences, where the ISO preprocessor's behavior
    206 does not conflict with traditional semantics, the traditional
    207 preprocessor should behave the same way.  The various differences that
    208 do exist are detailed in the section *Note Traditional Mode::.
    209 
    210    For clarity, unless noted otherwise, references to `CPP' in this
    211 manual refer to GNU CPP.
    212 
    213 * Menu:
    214 
    215 * Character sets::
    216 * Initial processing::
    217 * Tokenization::
    218 * The preprocessing language::
    219 
    220 
    221 File: cpp.info,  Node: Character sets,  Next: Initial processing,  Up: Overview
    222 
    223 1.1 Character sets
    224 ==================
    225 
    226 Source code character set processing in C and related languages is
    227 rather complicated.  The C standard discusses two character sets, but
    228 there are really at least four.
    229 
    230    The files input to CPP might be in any character set at all.  CPP's
    231 very first action, before it even looks for line boundaries, is to
    232 convert the file into the character set it uses for internal
    233 processing.  That set is what the C standard calls the "source"
    234 character set.  It must be isomorphic with ISO 10646, also known as
    235 Unicode.  CPP uses the UTF-8 encoding of Unicode.
    236 
    237    The character sets of the input files are specified using the
    238 `-finput-charset=' option.
    239 
    240    All preprocessing work (the subject of the rest of this manual) is
    241 carried out in the source character set.  If you request textual output
    242 from the preprocessor with the `-E' option, it will be in UTF-8.
    243 
    244    After preprocessing is complete, string and character constants are
    245 converted again, into the "execution" character set.  This character
    246 set is under control of the user; the default is UTF-8, matching the
    247 source character set.  Wide string and character constants have their
    248 own character set, which is not called out specifically in the
    249 standard.  Again, it is under control of the user.  The default is
    250 UTF-16 or UTF-32, whichever fits in the target's `wchar_t' type, in the
    251 target machine's byte order.(1)  Octal and hexadecimal escape sequences
    252 do not undergo conversion; '\x12' has the value 0x12 regardless of the
    253 currently selected execution character set.  All other escapes are
    254 replaced by the character in the source character set that they
    255 represent, then converted to the execution character set, just like
    256 unescaped characters.
    257 
    258    Unless the experimental `-fextended-identifiers' option is used, GCC
    259 does not permit the use of characters outside the ASCII range, nor `\u'
    260 and `\U' escapes, in identifiers.  Even with that option, characters
    261 outside the ASCII range can only be specified with the `\u' and `\U'
    262 escapes, not used directly in identifiers.
    263 
    264    ---------- Footnotes ----------
    265 
    266    (1) UTF-16 does not meet the requirements of the C standard for a
    267 wide character set, but the choice of 16-bit `wchar_t' is enshrined in
    268 some system ABIs so we cannot fix this.
    269 
    270 
    271 File: cpp.info,  Node: Initial processing,  Next: Tokenization,  Prev: Character sets,  Up: Overview
    272 
    273 1.2 Initial processing
    274 ======================
    275 
    276 The preprocessor performs a series of textual transformations on its
    277 input.  These happen before all other processing.  Conceptually, they
    278 happen in a rigid order, and the entire file is run through each
    279 transformation before the next one begins.  CPP actually does them all
    280 at once, for performance reasons.  These transformations correspond
    281 roughly to the first three "phases of translation" described in the C
    282 standard.
    283 
    284   1. The input file is read into memory and broken into lines.
    285 
    286      Different systems use different conventions to indicate the end of
    287      a line.  GCC accepts the ASCII control sequences `LF', `CR LF' and
    288      `CR' as end-of-line markers.  These are the canonical sequences
    289      used by Unix, DOS and VMS, and the classic Mac OS (before OSX)
    290      respectively.  You may therefore safely copy source code written
    291      on any of those systems to a different one and use it without
    292      conversion.  (GCC may lose track of the current line number if a
    293      file doesn't consistently use one convention, as sometimes happens
    294      when it is edited on computers with different conventions that
    295      share a network file system.)
    296 
    297      If the last line of any input file lacks an end-of-line marker,
    298      the end of the file is considered to implicitly supply one.  The C
    299      standard says that this condition provokes undefined behavior, so
    300      GCC will emit a warning message.
    301 
    302   2. If trigraphs are enabled, they are replaced by their corresponding
    303      single characters.  By default GCC ignores trigraphs, but if you
    304      request a strictly conforming mode with the `-std' option, or you
    305      specify the `-trigraphs' option, then it converts them.
    306 
    307      These are nine three-character sequences, all starting with `??',
    308      that are defined by ISO C to stand for single characters.  They
    309      permit obsolete systems that lack some of C's punctuation to use
    310      C.  For example, `??/' stands for `\', so '??/n' is a character
    311      constant for a newline.
    312 
    313      Trigraphs are not popular and many compilers implement them
    314      incorrectly.  Portable code should not rely on trigraphs being
    315      either converted or ignored.  With `-Wtrigraphs' GCC will warn you
    316      when a trigraph may change the meaning of your program if it were
    317      converted.  *Note Wtrigraphs::.
    318 
    319      In a string constant, you can prevent a sequence of question marks
    320      from being confused with a trigraph by inserting a backslash
    321      between the question marks, or by separating the string literal at
    322      the trigraph and making use of string literal concatenation.
    323      "(??\?)"  is the string `(???)', not `(?]'.  Traditional C
    324      compilers do not recognize these idioms.
    325 
    326      The nine trigraphs and their replacements are
    327 
    328           Trigraph:       ??(  ??)  ??<  ??>  ??=  ??/  ??'  ??!  ??-
    329           Replacement:      [    ]    {    }    #    \    ^    |    ~
    330 
    331   3. Continued lines are merged into one long line.
    332 
    333      A continued line is a line which ends with a backslash, `\'.  The
    334      backslash is removed and the following line is joined with the
    335      current one.  No space is inserted, so you may split a line
    336      anywhere, even in the middle of a word.  (It is generally more
    337      readable to split lines only at white space.)
    338 
    339      The trailing backslash on a continued line is commonly referred to
    340      as a "backslash-newline".
    341 
    342      If there is white space between a backslash and the end of a line,
    343      that is still a continued line.  However, as this is usually the
    344      result of an editing mistake, and many compilers will not accept
    345      it as a continued line, GCC will warn you about it.
    346 
    347   4. All comments are replaced with single spaces.
    348 
    349      There are two kinds of comments.  "Block comments" begin with `/*'
    350      and continue until the next `*/'.  Block comments do not nest:
    351 
    352           /* this is /* one comment */ text outside comment
    353 
    354      "Line comments" begin with `//' and continue to the end of the
    355      current line.  Line comments do not nest either, but it does not
    356      matter, because they would end in the same place anyway.
    357 
    358           // this is // one comment
    359           text outside comment
    360 
    361    It is safe to put line comments inside block comments, or vice versa.
    362 
    363      /* block comment
    364         // contains line comment
    365         yet more comment
    366       */ outside comment
    367 
    368      // line comment /* contains block comment */
    369 
    370    But beware of commenting out one end of a block comment with a line
    371 comment.
    372 
    373       // l.c.  /* block comment begins
    374          oops! this isn't a comment anymore */
    375 
    376    Comments are not recognized within string literals.  "/* blah */" is
    377 the string constant `/* blah */', not an empty string.
    378 
    379    Line comments are not in the 1989 edition of the C standard, but they
    380 are recognized by GCC as an extension.  In C++ and in the 1999 edition
    381 of the C standard, they are an official part of the language.
    382 
    383    Since these transformations happen before all other processing, you
    384 can split a line mechanically with backslash-newline anywhere.  You can
    385 comment out the end of a line.  You can continue a line comment onto the
    386 next line with backslash-newline.  You can even split `/*', `*/', and
    387 `//' onto multiple lines with backslash-newline.  For example:
    388 
    389      /\
    390      *
    391      */ # /*
    392      */ defi\
    393      ne FO\
    394      O 10\
    395      20
    396 
    397 is equivalent to `#define FOO 1020'.  All these tricks are extremely
    398 confusing and should not be used in code intended to be readable.
    399 
    400    There is no way to prevent a backslash at the end of a line from
    401 being interpreted as a backslash-newline.  This cannot affect any
    402 correct program, however.
    403 
    404 
    405 File: cpp.info,  Node: Tokenization,  Next: The preprocessing language,  Prev: Initial processing,  Up: Overview
    406 
    407 1.3 Tokenization
    408 ================
    409 
    410 After the textual transformations are finished, the input file is
    411 converted into a sequence of "preprocessing tokens".  These mostly
    412 correspond to the syntactic tokens used by the C compiler, but there are
    413 a few differences.  White space separates tokens; it is not itself a
    414 token of any kind.  Tokens do not have to be separated by white space,
    415 but it is often necessary to avoid ambiguities.
    416 
    417    When faced with a sequence of characters that has more than one
    418 possible tokenization, the preprocessor is greedy.  It always makes
    419 each token, starting from the left, as big as possible before moving on
    420 to the next token.  For instance, `a+++++b' is interpreted as
    421 `a ++ ++ + b', not as `a ++ + ++ b', even though the latter
    422 tokenization could be part of a valid C program and the former could
    423 not.
    424 
    425    Once the input file is broken into tokens, the token boundaries never
    426 change, except when the `##' preprocessing operator is used to paste
    427 tokens together.  *Note Concatenation::.  For example,
    428 
    429      #define foo() bar
    430      foo()baz
    431           ==> bar baz
    432      _not_
    433           ==> barbaz
    434 
    435    The compiler does not re-tokenize the preprocessor's output.  Each
    436 preprocessing token becomes one compiler token.
    437 
    438    Preprocessing tokens fall into five broad classes: identifiers,
    439 preprocessing numbers, string literals, punctuators, and other.  An
    440 "identifier" is the same as an identifier in C: any sequence of
    441 letters, digits, or underscores, which begins with a letter or
    442 underscore.  Keywords of C have no significance to the preprocessor;
    443 they are ordinary identifiers.  You can define a macro whose name is a
    444 keyword, for instance.  The only identifier which can be considered a
    445 preprocessing keyword is `defined'.  *Note Defined::.
    446 
    447    This is mostly true of other languages which use the C preprocessor.
    448 However, a few of the keywords of C++ are significant even in the
    449 preprocessor.  *Note C++ Named Operators::.
    450 
    451    In the 1999 C standard, identifiers may contain letters which are not
    452 part of the "basic source character set", at the implementation's
    453 discretion (such as accented Latin letters, Greek letters, or Chinese
    454 ideograms).  This may be done with an extended character set, or the
    455 `\u' and `\U' escape sequences.  The implementation of this feature in
    456 GCC is experimental; such characters are only accepted in the `\u' and
    457 `\U' forms and only if `-fextended-identifiers' is used.
    458 
    459    As an extension, GCC treats `$' as a letter.  This is for
    460 compatibility with some systems, such as VMS, where `$' is commonly
    461 used in system-defined function and object names.  `$' is not a letter
    462 in strictly conforming mode, or if you specify the `-$' option.  *Note
    463 Invocation::.
    464 
    465    A "preprocessing number" has a rather bizarre definition.  The
    466 category includes all the normal integer and floating point constants
    467 one expects of C, but also a number of other things one might not
    468 initially recognize as a number.  Formally, preprocessing numbers begin
    469 with an optional period, a required decimal digit, and then continue
    470 with any sequence of letters, digits, underscores, periods, and
    471 exponents.  Exponents are the two-character sequences `e+', `e-', `E+',
    472 `E-', `p+', `p-', `P+', and `P-'.  (The exponents that begin with `p'
    473 or `P' are new to C99.  They are used for hexadecimal floating-point
    474 constants.)
    475 
    476    The purpose of this unusual definition is to isolate the preprocessor
    477 from the full complexity of numeric constants.  It does not have to
    478 distinguish between lexically valid and invalid floating-point numbers,
    479 which is complicated.  The definition also permits you to split an
    480 identifier at any position and get exactly two tokens, which can then be
    481 pasted back together with the `##' operator.
    482 
    483    It's possible for preprocessing numbers to cause programs to be
    484 misinterpreted.  For example, `0xE+12' is a preprocessing number which
    485 does not translate to any valid numeric constant, therefore a syntax
    486 error.  It does not mean `0xE + 12', which is what you might have
    487 intended.
    488 
    489    "String literals" are string constants, character constants, and
    490 header file names (the argument of `#include').(1)  String constants
    491 and character constants are straightforward: "..." or '...'.  In either
    492 case embedded quotes should be escaped with a backslash: '\'' is the
    493 character constant for `''.  There is no limit on the length of a
    494 character constant, but the value of a character constant that contains
    495 more than one character is implementation-defined.  *Note
    496 Implementation Details::.
    497 
    498    Header file names either look like string constants, "...", or are
    499 written with angle brackets instead, <...>.  In either case, backslash
    500 is an ordinary character.  There is no way to escape the closing quote
    501 or angle bracket.  The preprocessor looks for the header file in
    502 different places depending on which form you use.  *Note Include
    503 Operation::.
    504 
    505    No string literal may extend past the end of a line.  Older versions
    506 of GCC accepted multi-line string constants.  You may use continued
    507 lines instead, or string constant concatenation.  *Note Differences
    508 from previous versions::.
    509 
    510    "Punctuators" are all the usual bits of punctuation which are
    511 meaningful to C and C++.  All but three of the punctuation characters in
    512 ASCII are C punctuators.  The exceptions are `@', `$', and ``'.  In
    513 addition, all the two- and three-character operators are punctuators.
    514 There are also six "digraphs", which the C++ standard calls
    515 "alternative tokens", which are merely alternate ways to spell other
    516 punctuators.  This is a second attempt to work around missing
    517 punctuation in obsolete systems.  It has no negative side effects,
    518 unlike trigraphs, but does not cover as much ground.  The digraphs and
    519 their corresponding normal punctuators are:
    520 
    521      Digraph:        <%  %>  <:  :>  %:  %:%:
    522      Punctuator:      {   }   [   ]   #    ##
    523 
    524    Any other single character is considered "other".  It is passed on to
    525 the preprocessor's output unmolested.  The C compiler will almost
    526 certainly reject source code containing "other" tokens.  In ASCII, the
    527 only other characters are `@', `$', ``', and control characters other
    528 than NUL (all bits zero).  (Note that `$' is normally considered a
    529 letter.)  All characters with the high bit set (numeric range
    530 0x7F-0xFF) are also "other" in the present implementation.  This will
    531 change when proper support for international character sets is added to
    532 GCC.
    533 
    534    NUL is a special case because of the high probability that its
    535 appearance is accidental, and because it may be invisible to the user
    536 (many terminals do not display NUL at all).  Within comments, NULs are
    537 silently ignored, just as any other character would be.  In running
    538 text, NUL is considered white space.  For example, these two directives
    539 have the same meaning.
    540 
    541      #define X^@1
    542      #define X 1
    543 
    544 (where `^@' is ASCII NUL).  Within string or character constants, NULs
    545 are preserved.  In the latter two cases the preprocessor emits a
    546 warning message.
    547 
    548    ---------- Footnotes ----------
    549 
    550    (1) The C standard uses the term "string literal" to refer only to
    551 what we are calling "string constants".
    552 
    553 
    554 File: cpp.info,  Node: The preprocessing language,  Prev: Tokenization,  Up: Overview
    555 
    556 1.4 The preprocessing language
    557 ==============================
    558 
    559 After tokenization, the stream of tokens may simply be passed straight
    560 to the compiler's parser.  However, if it contains any operations in the
    561 "preprocessing language", it will be transformed first.  This stage
    562 corresponds roughly to the standard's "translation phase 4" and is what
    563 most people think of as the preprocessor's job.
    564 
    565    The preprocessing language consists of "directives" to be executed
    566 and "macros" to be expanded.  Its primary capabilities are:
    567 
    568    * Inclusion of header files.  These are files of declarations that
    569      can be substituted into your program.
    570 
    571    * Macro expansion.  You can define "macros", which are abbreviations
    572      for arbitrary fragments of C code.  The preprocessor will replace
    573      the macros with their definitions throughout the program.  Some
    574      macros are automatically defined for you.
    575 
    576    * Conditional compilation.  You can include or exclude parts of the
    577      program according to various conditions.
    578 
    579    * Line control.  If you use a program to combine or rearrange source
    580      files into an intermediate file which is then compiled, you can
    581      use line control to inform the compiler where each source line
    582      originally came from.
    583 
    584    * Diagnostics.  You can detect problems at compile time and issue
    585      errors or warnings.
    586 
    587    There are a few more, less useful, features.
    588 
    589    Except for expansion of predefined macros, all these operations are
    590 triggered with "preprocessing directives".  Preprocessing directives
    591 are lines in your program that start with `#'.  Whitespace is allowed
    592 before and after the `#'.  The `#' is followed by an identifier, the
    593 "directive name".  It specifies the operation to perform.  Directives
    594 are commonly referred to as `#NAME' where NAME is the directive name.
    595 For example, `#define' is the directive that defines a macro.
    596 
    597    The `#' which begins a directive cannot come from a macro expansion.
    598 Also, the directive name is not macro expanded.  Thus, if `foo' is
    599 defined as a macro expanding to `define', that does not make `#foo' a
    600 valid preprocessing directive.
    601 
    602    The set of valid directive names is fixed.  Programs cannot define
    603 new preprocessing directives.
    604 
    605    Some directives require arguments; these make up the rest of the
    606 directive line and must be separated from the directive name by
    607 whitespace.  For example, `#define' must be followed by a macro name
    608 and the intended expansion of the macro.
    609 
    610    A preprocessing directive cannot cover more than one line.  The line
    611 may, however, be continued with backslash-newline, or by a block comment
    612 which extends past the end of the line.  In either case, when the
    613 directive is processed, the continuations have already been merged with
    614 the first line to make one long line.
    615 
    616 
    617 File: cpp.info,  Node: Header Files,  Next: Macros,  Prev: Overview,  Up: Top
    618 
    619 2 Header Files
    620 **************
    621 
    622 A header file is a file containing C declarations and macro definitions
    623 (*note Macros::) to be shared between several source files.  You request
    624 the use of a header file in your program by "including" it, with the C
    625 preprocessing directive `#include'.
    626 
    627    Header files serve two purposes.
    628 
    629    * System header files declare the interfaces to parts of the
    630      operating system.  You include them in your program to supply the
    631      definitions and declarations you need to invoke system calls and
    632      libraries.
    633 
    634    * Your own header files contain declarations for interfaces between
    635      the source files of your program.  Each time you have a group of
    636      related declarations and macro definitions all or most of which
    637      are needed in several different source files, it is a good idea to
    638      create a header file for them.
    639 
    640    Including a header file produces the same results as copying the
    641 header file into each source file that needs it.  Such copying would be
    642 time-consuming and error-prone.  With a header file, the related
    643 declarations appear in only one place.  If they need to be changed, they
    644 can be changed in one place, and programs that include the header file
    645 will automatically use the new version when next recompiled.  The header
    646 file eliminates the labor of finding and changing all the copies as well
    647 as the risk that a failure to find one copy will result in
    648 inconsistencies within a program.
    649 
    650    In C, the usual convention is to give header files names that end
    651 with `.h'.  It is most portable to use only letters, digits, dashes, and
    652 underscores in header file names, and at most one dot.
    653 
    654 * Menu:
    655 
    656 * Include Syntax::
    657 * Include Operation::
    658 * Search Path::
    659 * Once-Only Headers::
    660 * Computed Includes::
    661 * Wrapper Headers::
    662 * System Headers::
    663 
    664 
    665 File: cpp.info,  Node: Include Syntax,  Next: Include Operation,  Up: Header Files
    666 
    667 2.1 Include Syntax
    668 ==================
    669 
    670 Both user and system header files are included using the preprocessing
    671 directive `#include'.  It has two variants:
    672 
    673 `#include <FILE>'
    674      This variant is used for system header files.  It searches for a
    675      file named FILE in a standard list of system directories.  You can
    676      prepend directories to this list with the `-I' option (*note
    677      Invocation::).
    678 
    679 `#include "FILE"'
    680      This variant is used for header files of your own program.  It
    681      searches for a file named FILE first in the directory containing
    682      the current file, then in the quote directories and then the same
    683      directories used for `<FILE>'.  You can prepend directories to the
    684      list of quote directories with the `-iquote' option.
    685 
    686    The argument of `#include', whether delimited with quote marks or
    687 angle brackets, behaves like a string constant in that comments are not
    688 recognized, and macro names are not expanded.  Thus, `#include <x/*y>'
    689 specifies inclusion of a system header file named `x/*y'.
    690 
    691    However, if backslashes occur within FILE, they are considered
    692 ordinary text characters, not escape characters.  None of the character
    693 escape sequences appropriate to string constants in C are processed.
    694 Thus, `#include "x\n\\y"' specifies a filename containing three
    695 backslashes.  (Some systems interpret `\' as a pathname separator.  All
    696 of these also interpret `/' the same way.  It is most portable to use
    697 only `/'.)
    698 
    699    It is an error if there is anything (other than comments) on the line
    700 after the file name.
    701 
    702 
    703 File: cpp.info,  Node: Include Operation,  Next: Search Path,  Prev: Include Syntax,  Up: Header Files
    704 
    705 2.2 Include Operation
    706 =====================
    707 
    708 The `#include' directive works by directing the C preprocessor to scan
    709 the specified file as input before continuing with the rest of the
    710 current file.  The output from the preprocessor contains the output
    711 already generated, followed by the output resulting from the included
    712 file, followed by the output that comes from the text after the
    713 `#include' directive.  For example, if you have a header file
    714 `header.h' as follows,
    715 
    716      char *test (void);
    717 
    718 and a main program called `program.c' that uses the header file, like
    719 this,
    720 
    721      int x;
    722      #include "header.h"
    723 
    724      int
    725      main (void)
    726      {
    727        puts (test ());
    728      }
    729 
    730 the compiler will see the same token stream as it would if `program.c'
    731 read
    732 
    733      int x;
    734      char *test (void);
    735 
    736      int
    737      main (void)
    738      {
    739        puts (test ());
    740      }
    741 
    742    Included files are not limited to declarations and macro definitions;
    743 those are merely the typical uses.  Any fragment of a C program can be
    744 included from another file.  The include file could even contain the
    745 beginning of a statement that is concluded in the containing file, or
    746 the end of a statement that was started in the including file.  However,
    747 an included file must consist of complete tokens.  Comments and string
    748 literals which have not been closed by the end of an included file are
    749 invalid.  For error recovery, they are considered to end at the end of
    750 the file.
    751 
    752    To avoid confusion, it is best if header files contain only complete
    753 syntactic units--function declarations or definitions, type
    754 declarations, etc.
    755 
    756    The line following the `#include' directive is always treated as a
    757 separate line by the C preprocessor, even if the included file lacks a
    758 final newline.
    759 
    760 
    761 File: cpp.info,  Node: Search Path,  Next: Once-Only Headers,  Prev: Include Operation,  Up: Header Files
    762 
    763 2.3 Search Path
    764 ===============
    765 
    766 GCC looks in several different places for headers.  On a normal Unix
    767 system, if you do not instruct it otherwise, it will look for headers
    768 requested with `#include <FILE>' in:
    769 
    770      /usr/local/include
    771      LIBDIR/gcc/TARGET/VERSION/include
    772      /usr/TARGET/include
    773      /usr/include
    774 
    775    For C++ programs, it will also look in `/usr/include/g++-v3', first.
    776 In the above, TARGET is the canonical name of the system GCC was
    777 configured to compile code for; often but not always the same as the
    778 canonical name of the system it runs on.  VERSION is the version of GCC
    779 in use.
    780 
    781    You can add to this list with the `-IDIR' command line option.  All
    782 the directories named by `-I' are searched, in left-to-right order,
    783 _before_ the default directories.  The only exception is when `dir' is
    784 already searched by default.  In this case, the option is ignored and
    785 the search order for system directories remains unchanged.
    786 
    787    Duplicate directories are removed from the quote and bracket search
    788 chains before the two chains are merged to make the final search chain.
    789 Thus, it is possible for a directory to occur twice in the final search
    790 chain if it was specified in both the quote and bracket chains.
    791 
    792    You can prevent GCC from searching any of the default directories
    793 with the `-nostdinc' option.  This is useful when you are compiling an
    794 operating system kernel or some other program that does not use the
    795 standard C library facilities, or the standard C library itself.  `-I'
    796 options are not ignored as described above when `-nostdinc' is in
    797 effect.
    798 
    799    GCC looks for headers requested with `#include "FILE"' first in the
    800 directory containing the current file, then in the directories as
    801 specified by `-iquote' options, then in the same places it would have
    802 looked for a header requested with angle brackets.  For example, if
    803 `/usr/include/sys/stat.h' contains `#include "types.h"', GCC looks for
    804 `types.h' first in `/usr/include/sys', then in its usual search path.
    805 
    806    `#line' (*note Line Control::) does not change GCC's idea of the
    807 directory containing the current file.
    808 
    809    You may put `-I-' at any point in your list of `-I' options.  This
    810 has two effects.  First, directories appearing before the `-I-' in the
    811 list are searched only for headers requested with quote marks.
    812 Directories after `-I-' are searched for all headers.  Second, the
    813 directory containing the current file is not searched for anything,
    814 unless it happens to be one of the directories named by an `-I' switch.
    815 `-I-' is deprecated, `-iquote' should be used instead.
    816 
    817    `-I. -I-' is not the same as no `-I' options at all, and does not
    818 cause the same behavior for `<>' includes that `""' includes get with
    819 no special options.  `-I.' searches the compiler's current working
    820 directory for header files.  That may or may not be the same as the
    821 directory containing the current file.
    822 
    823    If you need to look for headers in a directory named `-', write
    824 `-I./-'.
    825 
    826    There are several more ways to adjust the header search path.  They
    827 are generally less useful.  *Note Invocation::.
    828 
    829 
    830 File: cpp.info,  Node: Once-Only Headers,  Next: Computed Includes,  Prev: Search Path,  Up: Header Files
    831 
    832 2.4 Once-Only Headers
    833 =====================
    834 
    835 If a header file happens to be included twice, the compiler will process
    836 its contents twice.  This is very likely to cause an error, e.g. when
    837 the compiler sees the same structure definition twice.  Even if it does
    838 not, it will certainly waste time.
    839 
    840    The standard way to prevent this is to enclose the entire real
    841 contents of the file in a conditional, like this:
    842 
    843      /* File foo.  */
    844      #ifndef FILE_FOO_SEEN
    845      #define FILE_FOO_SEEN
    846 
    847      THE ENTIRE FILE
    848 
    849      #endif /* !FILE_FOO_SEEN */
    850 
    851    This construct is commonly known as a "wrapper #ifndef".  When the
    852 header is included again, the conditional will be false, because
    853 `FILE_FOO_SEEN' is defined.  The preprocessor will skip over the entire
    854 contents of the file, and the compiler will not see it twice.
    855 
    856    CPP optimizes even further.  It remembers when a header file has a
    857 wrapper `#ifndef'.  If a subsequent `#include' specifies that header,
    858 and the macro in the `#ifndef' is still defined, it does not bother to
    859 rescan the file at all.
    860 
    861    You can put comments outside the wrapper.  They will not interfere
    862 with this optimization.
    863 
    864    The macro `FILE_FOO_SEEN' is called the "controlling macro" or
    865 "guard macro".  In a user header file, the macro name should not begin
    866 with `_'.  In a system header file, it should begin with `__' to avoid
    867 conflicts with user programs.  In any kind of header file, the macro
    868 name should contain the name of the file and some additional text, to
    869 avoid conflicts with other header files.
    870 
    871 
    872 File: cpp.info,  Node: Computed Includes,  Next: Wrapper Headers,  Prev: Once-Only Headers,  Up: Header Files
    873 
    874 2.5 Computed Includes
    875 =====================
    876 
    877 Sometimes it is necessary to select one of several different header
    878 files to be included into your program.  They might specify
    879 configuration parameters to be used on different sorts of operating
    880 systems, for instance.  You could do this with a series of conditionals,
    881 
    882      #if SYSTEM_1
    883      # include "system_1.h"
    884      #elif SYSTEM_2
    885      # include "system_2.h"
    886      #elif SYSTEM_3
    887      ...
    888      #endif
    889 
    890    That rapidly becomes tedious.  Instead, the preprocessor offers the
    891 ability to use a macro for the header name.  This is called a "computed
    892 include".  Instead of writing a header name as the direct argument of
    893 `#include', you simply put a macro name there instead:
    894 
    895      #define SYSTEM_H "system_1.h"
    896      ...
    897      #include SYSTEM_H
    898 
    899 `SYSTEM_H' will be expanded, and the preprocessor will look for
    900 `system_1.h' as if the `#include' had been written that way originally.
    901 `SYSTEM_H' could be defined by your Makefile with a `-D' option.
    902 
    903    You must be careful when you define the macro.  `#define' saves
    904 tokens, not text.  The preprocessor has no way of knowing that the macro
    905 will be used as the argument of `#include', so it generates ordinary
    906 tokens, not a header name.  This is unlikely to cause problems if you
    907 use double-quote includes, which are close enough to string constants.
    908 If you use angle brackets, however, you may have trouble.
    909 
    910    The syntax of a computed include is actually a bit more general than
    911 the above.  If the first non-whitespace character after `#include' is
    912 not `"' or `<', then the entire line is macro-expanded like running
    913 text would be.
    914 
    915    If the line expands to a single string constant, the contents of that
    916 string constant are the file to be included.  CPP does not re-examine
    917 the string for embedded quotes, but neither does it process backslash
    918 escapes in the string.  Therefore
    919 
    920      #define HEADER "a\"b"
    921      #include HEADER
    922 
    923 looks for a file named `a\"b'.  CPP searches for the file according to
    924 the rules for double-quoted includes.
    925 
    926    If the line expands to a token stream beginning with a `<' token and
    927 including a `>' token, then the tokens between the `<' and the first
    928 `>' are combined to form the filename to be included.  Any whitespace
    929 between tokens is reduced to a single space; then any space after the
    930 initial `<' is retained, but a trailing space before the closing `>' is
    931 ignored.  CPP searches for the file according to the rules for
    932 angle-bracket includes.
    933 
    934    In either case, if there are any tokens on the line after the file
    935 name, an error occurs and the directive is not processed.  It is also
    936 an error if the result of expansion does not match either of the two
    937 expected forms.
    938 
    939    These rules are implementation-defined behavior according to the C
    940 standard.  To minimize the risk of different compilers interpreting your
    941 computed includes differently, we recommend you use only a single
    942 object-like macro which expands to a string constant.  This will also
    943 minimize confusion for people reading your program.
    944 
    945 
    946 File: cpp.info,  Node: Wrapper Headers,  Next: System Headers,  Prev: Computed Includes,  Up: Header Files
    947 
    948 2.6 Wrapper Headers
    949 ===================
    950 
    951 Sometimes it is necessary to adjust the contents of a system-provided
    952 header file without editing it directly.  GCC's `fixincludes' operation
    953 does this, for example.  One way to do that would be to create a new
    954 header file with the same name and insert it in the search path before
    955 the original header.  That works fine as long as you're willing to
    956 replace the old header entirely.  But what if you want to refer to the
    957 old header from the new one?
    958 
    959    You cannot simply include the old header with `#include'.  That will
    960 start from the beginning, and find your new header again.  If your
    961 header is not protected from multiple inclusion (*note Once-Only
    962 Headers::), it will recurse infinitely and cause a fatal error.
    963 
    964    You could include the old header with an absolute pathname:
    965      #include "/usr/include/old-header.h"
    966    This works, but is not clean; should the system headers ever move,
    967 you would have to edit the new headers to match.
    968 
    969    There is no way to solve this problem within the C standard, but you
    970 can use the GNU extension `#include_next'.  It means, "Include the
    971 _next_ file with this name".  This directive works like `#include'
    972 except in searching for the specified file: it starts searching the
    973 list of header file directories _after_ the directory in which the
    974 current file was found.
    975 
    976    Suppose you specify `-I /usr/local/include', and the list of
    977 directories to search also includes `/usr/include'; and suppose both
    978 directories contain `signal.h'.  Ordinary `#include <signal.h>' finds
    979 the file under `/usr/local/include'.  If that file contains
    980 `#include_next <signal.h>', it starts searching after that directory,
    981 and finds the file in `/usr/include'.
    982 
    983    `#include_next' does not distinguish between `<FILE>' and `"FILE"'
    984 inclusion, nor does it check that the file you specify has the same
    985 name as the current file.  It simply looks for the file named, starting
    986 with the directory in the search path after the one where the current
    987 file was found.
    988 
    989    The use of `#include_next' can lead to great confusion.  We
    990 recommend it be used only when there is no other alternative.  In
    991 particular, it should not be used in the headers belonging to a specific
    992 program; it should be used only to make global corrections along the
    993 lines of `fixincludes'.
    994 
    995 
    996 File: cpp.info,  Node: System Headers,  Prev: Wrapper Headers,  Up: Header Files
    997 
    998 2.7 System Headers
    999 ==================
   1000 
   1001 The header files declaring interfaces to the operating system and
   1002 runtime libraries often cannot be written in strictly conforming C.
   1003 Therefore, GCC gives code found in "system headers" special treatment.
   1004 All warnings, other than those generated by `#warning' (*note
   1005 Diagnostics::), are suppressed while GCC is processing a system header.
   1006 Macros defined in a system header are immune to a few warnings
   1007 wherever they are expanded.  This immunity is granted on an ad-hoc
   1008 basis, when we find that a warning generates lots of false positives
   1009 because of code in macros defined in system headers.
   1010 
   1011    Normally, only the headers found in specific directories are
   1012 considered system headers.  These directories are determined when GCC
   1013 is compiled.  There are, however, two ways to make normal headers into
   1014 system headers.
   1015 
   1016    The `-isystem' command line option adds its argument to the list of
   1017 directories to search for headers, just like `-I'.  Any headers found
   1018 in that directory will be considered system headers.
   1019 
   1020    All directories named by `-isystem' are searched _after_ all
   1021 directories named by `-I', no matter what their order was on the
   1022 command line.  If the same directory is named by both `-I' and
   1023 `-isystem', the `-I' option is ignored.  GCC provides an informative
   1024 message when this occurs if `-v' is used.
   1025 
   1026    There is also a directive, `#pragma GCC system_header', which tells
   1027 GCC to consider the rest of the current include file a system header,
   1028 no matter where it was found.  Code that comes before the `#pragma' in
   1029 the file will not be affected.  `#pragma GCC system_header' has no
   1030 effect in the primary source file.
   1031 
   1032    On very old systems, some of the pre-defined system header
   1033 directories get even more special treatment.  GNU C++ considers code in
   1034 headers found in those directories to be surrounded by an `extern "C"'
   1035 block.  There is no way to request this behavior with a `#pragma', or
   1036 from the command line.
   1037 
   1038 
   1039 File: cpp.info,  Node: Macros,  Next: Conditionals,  Prev: Header Files,  Up: Top
   1040 
   1041 3 Macros
   1042 ********
   1043 
   1044 A "macro" is a fragment of code which has been given a name.  Whenever
   1045 the name is used, it is replaced by the contents of the macro.  There
   1046 are two kinds of macros.  They differ mostly in what they look like
   1047 when they are used.  "Object-like" macros resemble data objects when
   1048 used, "function-like" macros resemble function calls.
   1049 
   1050    You may define any valid identifier as a macro, even if it is a C
   1051 keyword.  The preprocessor does not know anything about keywords.  This
   1052 can be useful if you wish to hide a keyword such as `const' from an
   1053 older compiler that does not understand it.  However, the preprocessor
   1054 operator `defined' (*note Defined::) can never be defined as a macro,
   1055 and C++'s named operators (*note C++ Named Operators::) cannot be
   1056 macros when you are compiling C++.
   1057 
   1058 * Menu:
   1059 
   1060 * Object-like Macros::
   1061 * Function-like Macros::
   1062 * Macro Arguments::
   1063 * Stringification::
   1064 * Concatenation::
   1065 * Variadic Macros::
   1066 * Predefined Macros::
   1067 * Undefining and Redefining Macros::
   1068 * Directives Within Macro Arguments::
   1069 * Macro Pitfalls::
   1070 
   1071 
   1072 File: cpp.info,  Node: Object-like Macros,  Next: Function-like Macros,  Up: Macros
   1073 
   1074 3.1 Object-like Macros
   1075 ======================
   1076 
   1077 An "object-like macro" is a simple identifier which will be replaced by
   1078 a code fragment.  It is called object-like because it looks like a data
   1079 object in code that uses it.  They are most commonly used to give
   1080 symbolic names to numeric constants.
   1081 
   1082    You create macros with the `#define' directive.  `#define' is
   1083 followed by the name of the macro and then the token sequence it should
   1084 be an abbreviation for, which is variously referred to as the macro's
   1085 "body", "expansion" or "replacement list".  For example,
   1086 
   1087      #define BUFFER_SIZE 1024
   1088 
   1089 defines a macro named `BUFFER_SIZE' as an abbreviation for the token
   1090 `1024'.  If somewhere after this `#define' directive there comes a C
   1091 statement of the form
   1092 
   1093      foo = (char *) malloc (BUFFER_SIZE);
   1094 
   1095 then the C preprocessor will recognize and "expand" the macro
   1096 `BUFFER_SIZE'.  The C compiler will see the same tokens as it would if
   1097 you had written
   1098 
   1099      foo = (char *) malloc (1024);
   1100 
   1101    By convention, macro names are written in uppercase.  Programs are
   1102 easier to read when it is possible to tell at a glance which names are
   1103 macros.
   1104 
   1105    The macro's body ends at the end of the `#define' line.  You may
   1106 continue the definition onto multiple lines, if necessary, using
   1107 backslash-newline.  When the macro is expanded, however, it will all
   1108 come out on one line.  For example,
   1109 
   1110      #define NUMBERS 1, \
   1111                      2, \
   1112                      3
   1113      int x[] = { NUMBERS };
   1114           ==> int x[] = { 1, 2, 3 };
   1115 
   1116 The most common visible consequence of this is surprising line numbers
   1117 in error messages.
   1118 
   1119    There is no restriction on what can go in a macro body provided it
   1120 decomposes into valid preprocessing tokens.  Parentheses need not
   1121 balance, and the body need not resemble valid C code.  (If it does not,
   1122 you may get error messages from the C compiler when you use the macro.)
   1123 
   1124    The C preprocessor scans your program sequentially.  Macro
   1125 definitions take effect at the place you write them.  Therefore, the
   1126 following input to the C preprocessor
   1127 
   1128      foo = X;
   1129      #define X 4
   1130      bar = X;
   1131 
   1132 produces
   1133 
   1134      foo = X;
   1135      bar = 4;
   1136 
   1137    When the preprocessor expands a macro name, the macro's expansion
   1138 replaces the macro invocation, then the expansion is examined for more
   1139 macros to expand.  For example,
   1140 
   1141      #define TABLESIZE BUFSIZE
   1142      #define BUFSIZE 1024
   1143      TABLESIZE
   1144           ==> BUFSIZE
   1145           ==> 1024
   1146 
   1147 `TABLESIZE' is expanded first to produce `BUFSIZE', then that macro is
   1148 expanded to produce the final result, `1024'.
   1149 
   1150    Notice that `BUFSIZE' was not defined when `TABLESIZE' was defined.
   1151 The `#define' for `TABLESIZE' uses exactly the expansion you
   1152 specify--in this case, `BUFSIZE'--and does not check to see whether it
   1153 too contains macro names.  Only when you _use_ `TABLESIZE' is the
   1154 result of its expansion scanned for more macro names.
   1155 
   1156    This makes a difference if you change the definition of `BUFSIZE' at
   1157 some point in the source file.  `TABLESIZE', defined as shown, will
   1158 always expand using the definition of `BUFSIZE' that is currently in
   1159 effect:
   1160 
   1161      #define BUFSIZE 1020
   1162      #define TABLESIZE BUFSIZE
   1163      #undef BUFSIZE
   1164      #define BUFSIZE 37
   1165 
   1166 Now `TABLESIZE' expands (in two stages) to `37'.
   1167 
   1168    If the expansion of a macro contains its own name, either directly or
   1169 via intermediate macros, it is not expanded again when the expansion is
   1170 examined for more macros.  This prevents infinite recursion.  *Note
   1171 Self-Referential Macros::, for the precise details.
   1172 
   1173 
   1174 File: cpp.info,  Node: Function-like Macros,  Next: Macro Arguments,  Prev: Object-like Macros,  Up: Macros
   1175 
   1176 3.2 Function-like Macros
   1177 ========================
   1178 
   1179 You can also define macros whose use looks like a function call.  These
   1180 are called "function-like macros".  To define a function-like macro,
   1181 you use the same `#define' directive, but you put a pair of parentheses
   1182 immediately after the macro name.  For example,
   1183 
   1184      #define lang_init()  c_init()
   1185      lang_init()
   1186           ==> c_init()
   1187 
   1188    A function-like macro is only expanded if its name appears with a
   1189 pair of parentheses after it.  If you write just the name, it is left
   1190 alone.  This can be useful when you have a function and a macro of the
   1191 same name, and you wish to use the function sometimes.
   1192 
   1193      extern void foo(void);
   1194      #define foo() /* optimized inline version */
   1195      ...
   1196        foo();
   1197        funcptr = foo;
   1198 
   1199    Here the call to `foo()' will use the macro, but the function
   1200 pointer will get the address of the real function.  If the macro were to
   1201 be expanded, it would cause a syntax error.
   1202 
   1203    If you put spaces between the macro name and the parentheses in the
   1204 macro definition, that does not define a function-like macro, it defines
   1205 an object-like macro whose expansion happens to begin with a pair of
   1206 parentheses.
   1207 
   1208      #define lang_init ()    c_init()
   1209      lang_init()
   1210           ==> () c_init()()
   1211 
   1212    The first two pairs of parentheses in this expansion come from the
   1213 macro.  The third is the pair that was originally after the macro
   1214 invocation.  Since `lang_init' is an object-like macro, it does not
   1215 consume those parentheses.
   1216 
   1217 
   1218 File: cpp.info,  Node: Macro Arguments,  Next: Stringification,  Prev: Function-like Macros,  Up: Macros
   1219 
   1220 3.3 Macro Arguments
   1221 ===================
   1222 
   1223 Function-like macros can take "arguments", just like true functions.
   1224 To define a macro that uses arguments, you insert "parameters" between
   1225 the pair of parentheses in the macro definition that make the macro
   1226 function-like.  The parameters must be valid C identifiers, separated
   1227 by commas and optionally whitespace.
   1228 
   1229    To invoke a macro that takes arguments, you write the name of the
   1230 macro followed by a list of "actual arguments" in parentheses, separated
   1231 by commas.  The invocation of the macro need not be restricted to a
   1232 single logical line--it can cross as many lines in the source file as
   1233 you wish.  The number of arguments you give must match the number of
   1234 parameters in the macro definition.  When the macro is expanded, each
   1235 use of a parameter in its body is replaced by the tokens of the
   1236 corresponding argument.  (You need not use all of the parameters in the
   1237 macro body.)
   1238 
   1239    As an example, here is a macro that computes the minimum of two
   1240 numeric values, as it is defined in many C programs, and some uses.
   1241 
   1242      #define min(X, Y)  ((X) < (Y) ? (X) : (Y))
   1243        x = min(a, b);          ==>  x = ((a) < (b) ? (a) : (b));
   1244        y = min(1, 2);          ==>  y = ((1) < (2) ? (1) : (2));
   1245        z = min(a + 28, *p);    ==>  z = ((a + 28) < (*p) ? (a + 28) : (*p));
   1246 
   1247 (In this small example you can already see several of the dangers of
   1248 macro arguments.  *Note Macro Pitfalls::, for detailed explanations.)
   1249 
   1250    Leading and trailing whitespace in each argument is dropped, and all
   1251 whitespace between the tokens of an argument is reduced to a single
   1252 space.  Parentheses within each argument must balance; a comma within
   1253 such parentheses does not end the argument.  However, there is no
   1254 requirement for square brackets or braces to balance, and they do not
   1255 prevent a comma from separating arguments.  Thus,
   1256 
   1257      macro (array[x = y, x + 1])
   1258 
   1259 passes two arguments to `macro': `array[x = y' and `x + 1]'.  If you
   1260 want to supply `array[x = y, x + 1]' as an argument, you can write it
   1261 as `array[(x = y, x + 1)]', which is equivalent C code.
   1262 
   1263    All arguments to a macro are completely macro-expanded before they
   1264 are substituted into the macro body.  After substitution, the complete
   1265 text is scanned again for macros to expand, including the arguments.
   1266 This rule may seem strange, but it is carefully designed so you need
   1267 not worry about whether any function call is actually a macro
   1268 invocation.  You can run into trouble if you try to be too clever,
   1269 though.  *Note Argument Prescan::, for detailed discussion.
   1270 
   1271    For example, `min (min (a, b), c)' is first expanded to
   1272 
   1273        min (((a) < (b) ? (a) : (b)), (c))
   1274 
   1275 and then to
   1276 
   1277      ((((a) < (b) ? (a) : (b))) < (c)
   1278       ? (((a) < (b) ? (a) : (b)))
   1279       : (c))
   1280 
   1281 (Line breaks shown here for clarity would not actually be generated.)
   1282 
   1283    You can leave macro arguments empty; this is not an error to the
   1284 preprocessor (but many macros will then expand to invalid code).  You
   1285 cannot leave out arguments entirely; if a macro takes two arguments,
   1286 there must be exactly one comma at the top level of its argument list.
   1287 Here are some silly examples using `min':
   1288 
   1289      min(, b)        ==> ((   ) < (b) ? (   ) : (b))
   1290      min(a, )        ==> ((a  ) < ( ) ? (a  ) : ( ))
   1291      min(,)          ==> ((   ) < ( ) ? (   ) : ( ))
   1292      min((,),)       ==> (((,)) < ( ) ? ((,)) : ( ))
   1293 
   1294      min()      error--> macro "min" requires 2 arguments, but only 1 given
   1295      min(,,)    error--> macro "min" passed 3 arguments, but takes just 2
   1296 
   1297    Whitespace is not a preprocessing token, so if a macro `foo' takes
   1298 one argument, `foo ()' and `foo ( )' both supply it an empty argument.
   1299 Previous GNU preprocessor implementations and documentation were
   1300 incorrect on this point, insisting that a function-like macro that
   1301 takes a single argument be passed a space if an empty argument was
   1302 required.
   1303 
   1304    Macro parameters appearing inside string literals are not replaced by
   1305 their corresponding actual arguments.
   1306 
   1307      #define foo(x) x, "x"
   1308      foo(bar)        ==> bar, "x"
   1309 
   1310 
   1311 File: cpp.info,  Node: Stringification,  Next: Concatenation,  Prev: Macro Arguments,  Up: Macros
   1312 
   1313 3.4 Stringification
   1314 ===================
   1315 
   1316 Sometimes you may want to convert a macro argument into a string
   1317 constant.  Parameters are not replaced inside string constants, but you
   1318 can use the `#' preprocessing operator instead.  When a macro parameter
   1319 is used with a leading `#', the preprocessor replaces it with the
   1320 literal text of the actual argument, converted to a string constant.
   1321 Unlike normal parameter replacement, the argument is not macro-expanded
   1322 first.  This is called "stringification".
   1323 
   1324    There is no way to combine an argument with surrounding text and
   1325 stringify it all together.  Instead, you can write a series of adjacent
   1326 string constants and stringified arguments.  The preprocessor will
   1327 replace the stringified arguments with string constants.  The C
   1328 compiler will then combine all the adjacent string constants into one
   1329 long string.
   1330 
   1331    Here is an example of a macro definition that uses stringification:
   1332 
   1333      #define WARN_IF(EXP) \
   1334      do { if (EXP) \
   1335              fprintf (stderr, "Warning: " #EXP "\n"); } \
   1336      while (0)
   1337      WARN_IF (x == 0);
   1338           ==> do { if (x == 0)
   1339                 fprintf (stderr, "Warning: " "x == 0" "\n"); } while (0);
   1340 
   1341 The argument for `EXP' is substituted once, as-is, into the `if'
   1342 statement, and once, stringified, into the argument to `fprintf'.  If
   1343 `x' were a macro, it would be expanded in the `if' statement, but not
   1344 in the string.
   1345 
   1346    The `do' and `while (0)' are a kludge to make it possible to write
   1347 `WARN_IF (ARG);', which the resemblance of `WARN_IF' to a function
   1348 would make C programmers want to do; see *Note Swallowing the
   1349 Semicolon::.
   1350 
   1351    Stringification in C involves more than putting double-quote
   1352 characters around the fragment.  The preprocessor backslash-escapes the
   1353 quotes surrounding embedded string constants, and all backslashes
   1354 within string and character constants, in order to get a valid C string
   1355 constant with the proper contents.  Thus, stringifying `p = "foo\n";'
   1356 results in "p = \"foo\\n\";".  However, backslashes that are not inside
   1357 string or character constants are not duplicated: `\n' by itself
   1358 stringifies to "\n".
   1359 
   1360    All leading and trailing whitespace in text being stringified is
   1361 ignored.  Any sequence of whitespace in the middle of the text is
   1362 converted to a single space in the stringified result.  Comments are
   1363 replaced by whitespace long before stringification happens, so they
   1364 never appear in stringified text.
   1365 
   1366    There is no way to convert a macro argument into a character
   1367 constant.
   1368 
   1369    If you want to stringify the result of expansion of a macro argument,
   1370 you have to use two levels of macros.
   1371 
   1372      #define xstr(s) str(s)
   1373      #define str(s) #s
   1374      #define foo 4
   1375      str (foo)
   1376           ==> "foo"
   1377      xstr (foo)
   1378           ==> xstr (4)
   1379           ==> str (4)
   1380           ==> "4"
   1381 
   1382    `s' is stringified when it is used in `str', so it is not
   1383 macro-expanded first.  But `s' is an ordinary argument to `xstr', so it
   1384 is completely macro-expanded before `xstr' itself is expanded (*note
   1385 Argument Prescan::).  Therefore, by the time `str' gets to its
   1386 argument, it has already been macro-expanded.
   1387 
   1388 
   1389 File: cpp.info,  Node: Concatenation,  Next: Variadic Macros,  Prev: Stringification,  Up: Macros
   1390 
   1391 3.5 Concatenation
   1392 =================
   1393 
   1394 It is often useful to merge two tokens into one while expanding macros.
   1395 This is called "token pasting" or "token concatenation".  The `##'
   1396 preprocessing operator performs token pasting.  When a macro is
   1397 expanded, the two tokens on either side of each `##' operator are
   1398 combined into a single token, which then replaces the `##' and the two
   1399 original tokens in the macro expansion.  Usually both will be
   1400 identifiers, or one will be an identifier and the other a preprocessing
   1401 number.  When pasted, they make a longer identifier.  This isn't the
   1402 only valid case.  It is also possible to concatenate two numbers (or a
   1403 number and a name, such as `1.5' and `e3') into a number.  Also,
   1404 multi-character operators such as `+=' can be formed by token pasting.
   1405 
   1406    However, two tokens that don't together form a valid token cannot be
   1407 pasted together.  For example, you cannot concatenate `x' with `+' in
   1408 either order.  If you try, the preprocessor issues a warning and emits
   1409 the two tokens.  Whether it puts white space between the tokens is
   1410 undefined.  It is common to find unnecessary uses of `##' in complex
   1411 macros.  If you get this warning, it is likely that you can simply
   1412 remove the `##'.
   1413 
   1414    Both the tokens combined by `##' could come from the macro body, but
   1415 you could just as well write them as one token in the first place.
   1416 Token pasting is most useful when one or both of the tokens comes from a
   1417 macro argument.  If either of the tokens next to an `##' is a parameter
   1418 name, it is replaced by its actual argument before `##' executes.  As
   1419 with stringification, the actual argument is not macro-expanded first.
   1420 If the argument is empty, that `##' has no effect.
   1421 
   1422    Keep in mind that the C preprocessor converts comments to whitespace
   1423 before macros are even considered.  Therefore, you cannot create a
   1424 comment by concatenating `/' and `*'.  You can put as much whitespace
   1425 between `##' and its operands as you like, including comments, and you
   1426 can put comments in arguments that will be concatenated.  However, it
   1427 is an error if `##' appears at either end of a macro body.
   1428 
   1429    Consider a C program that interprets named commands.  There probably
   1430 needs to be a table of commands, perhaps an array of structures declared
   1431 as follows:
   1432 
   1433      struct command
   1434      {
   1435        char *name;
   1436        void (*function) (void);
   1437      };
   1438 
   1439      struct command commands[] =
   1440      {
   1441        { "quit", quit_command },
   1442        { "help", help_command },
   1443        ...
   1444      };
   1445 
   1446    It would be cleaner not to have to give each command name twice,
   1447 once in the string constant and once in the function name.  A macro
   1448 which takes the name of a command as an argument can make this
   1449 unnecessary.  The string constant can be created with stringification,
   1450 and the function name by concatenating the argument with `_command'.
   1451 Here is how it is done:
   1452 
   1453      #define COMMAND(NAME)  { #NAME, NAME ## _command }
   1454 
   1455      struct command commands[] =
   1456      {
   1457        COMMAND (quit),
   1458        COMMAND (help),
   1459        ...
   1460      };
   1461 
   1462 
   1463 File: cpp.info,  Node: Variadic Macros,  Next: Predefined Macros,  Prev: Concatenation,  Up: Macros
   1464 
   1465 3.6 Variadic Macros
   1466 ===================
   1467 
   1468 A macro can be declared to accept a variable number of arguments much as
   1469 a function can.  The syntax for defining the macro is similar to that of
   1470 a function.  Here is an example:
   1471 
   1472      #define eprintf(...) fprintf (stderr, __VA_ARGS__)
   1473 
   1474    This kind of macro is called "variadic".  When the macro is invoked,
   1475 all the tokens in its argument list after the last named argument (this
   1476 macro has none), including any commas, become the "variable argument".
   1477 This sequence of tokens replaces the identifier `__VA_ARGS__' in the
   1478 macro body wherever it appears.  Thus, we have this expansion:
   1479 
   1480      eprintf ("%s:%d: ", input_file, lineno)
   1481           ==>  fprintf (stderr, "%s:%d: ", input_file, lineno)
   1482 
   1483    The variable argument is completely macro-expanded before it is
   1484 inserted into the macro expansion, just like an ordinary argument.  You
   1485 may use the `#' and `##' operators to stringify the variable argument
   1486 or to paste its leading or trailing token with another token.  (But see
   1487 below for an important special case for `##'.)
   1488 
   1489    If your macro is complicated, you may want a more descriptive name
   1490 for the variable argument than `__VA_ARGS__'.  CPP permits this, as an
   1491 extension.  You may write an argument name immediately before the
   1492 `...'; that name is used for the variable argument.  The `eprintf'
   1493 macro above could be written
   1494 
   1495      #define eprintf(args...) fprintf (stderr, args)
   1496 
   1497 using this extension.  You cannot use `__VA_ARGS__' and this extension
   1498 in the same macro.
   1499 
   1500    You can have named arguments as well as variable arguments in a
   1501 variadic macro.  We could define `eprintf' like this, instead:
   1502 
   1503      #define eprintf(format, ...) fprintf (stderr, format, __VA_ARGS__)
   1504 
   1505 This formulation looks more descriptive, but unfortunately it is less
   1506 flexible: you must now supply at least one argument after the format
   1507 string.  In standard C, you cannot omit the comma separating the named
   1508 argument from the variable arguments.  Furthermore, if you leave the
   1509 variable argument empty, you will get a syntax error, because there
   1510 will be an extra comma after the format string.
   1511 
   1512      eprintf("success!\n", );
   1513           ==> fprintf(stderr, "success!\n", );
   1514 
   1515    GNU CPP has a pair of extensions which deal with this problem.
   1516 First, you are allowed to leave the variable argument out entirely:
   1517 
   1518      eprintf ("success!\n")
   1519           ==> fprintf(stderr, "success!\n", );
   1520 
   1521 Second, the `##' token paste operator has a special meaning when placed
   1522 between a comma and a variable argument.  If you write
   1523 
   1524      #define eprintf(format, ...) fprintf (stderr, format, ##__VA_ARGS__)
   1525 
   1526 and the variable argument is left out when the `eprintf' macro is used,
   1527 then the comma before the `##' will be deleted.  This does _not_ happen
   1528 if you pass an empty argument, nor does it happen if the token
   1529 preceding `##' is anything other than a comma.
   1530 
   1531      eprintf ("success!\n")
   1532           ==> fprintf(stderr, "success!\n");
   1533 
   1534 The above explanation is ambiguous about the case where the only macro
   1535 parameter is a variable arguments parameter, as it is meaningless to
   1536 try to distinguish whether no argument at all is an empty argument or a
   1537 missing argument.  In this case the C99 standard is clear that the
   1538 comma must remain, however the existing GCC extension used to swallow
   1539 the comma.  So CPP retains the comma when conforming to a specific C
   1540 standard, and drops it otherwise.
   1541 
   1542    C99 mandates that the only place the identifier `__VA_ARGS__' can
   1543 appear is in the replacement list of a variadic macro.  It may not be
   1544 used as a macro name, macro argument name, or within a different type
   1545 of macro.  It may also be forbidden in open text; the standard is
   1546 ambiguous.  We recommend you avoid using it except for its defined
   1547 purpose.
   1548 
   1549    Variadic macros are a new feature in C99.  GNU CPP has supported them
   1550 for a long time, but only with a named variable argument (`args...',
   1551 not `...' and `__VA_ARGS__').  If you are concerned with portability to
   1552 previous versions of GCC, you should use only named variable arguments.
   1553 On the other hand, if you are concerned with portability to other
   1554 conforming implementations of C99, you should use only `__VA_ARGS__'.
   1555 
   1556    Previous versions of CPP implemented the comma-deletion extension
   1557 much more generally.  We have restricted it in this release to minimize
   1558 the differences from C99.  To get the same effect with both this and
   1559 previous versions of GCC, the token preceding the special `##' must be
   1560 a comma, and there must be white space between that comma and whatever
   1561 comes immediately before it:
   1562 
   1563      #define eprintf(format, args...) fprintf (stderr, format , ##args)
   1564 
   1565 *Note Differences from previous versions::, for the gory details.
   1566 
   1567 
   1568 File: cpp.info,  Node: Predefined Macros,  Next: Undefining and Redefining Macros,  Prev: Variadic Macros,  Up: Macros
   1569 
   1570 3.7 Predefined Macros
   1571 =====================
   1572 
   1573 Several object-like macros are predefined; you use them without
   1574 supplying their definitions.  They fall into three classes: standard,
   1575 common, and system-specific.
   1576 
   1577    In C++, there is a fourth category, the named operators.  They act
   1578 like predefined macros, but you cannot undefine them.
   1579 
   1580 * Menu:
   1581 
   1582 * Standard Predefined Macros::
   1583 * Common Predefined Macros::
   1584 * System-specific Predefined Macros::
   1585 * C++ Named Operators::
   1586 
   1587 
   1588 File: cpp.info,  Node: Standard Predefined Macros,  Next: Common Predefined Macros,  Up: Predefined Macros
   1589 
   1590 3.7.1 Standard Predefined Macros
   1591 --------------------------------
   1592 
   1593 The standard predefined macros are specified by the relevant language
   1594 standards, so they are available with all compilers that implement
   1595 those standards.  Older compilers may not provide all of them.  Their
   1596 names all start with double underscores.
   1597 
   1598 `__FILE__'
   1599      This macro expands to the name of the current input file, in the
   1600      form of a C string constant.  This is the path by which the
   1601      preprocessor opened the file, not the short name specified in
   1602      `#include' or as the input file name argument.  For example,
   1603      `"/usr/local/include/myheader.h"' is a possible expansion of this
   1604      macro.
   1605 
   1606 `__LINE__'
   1607      This macro expands to the current input line number, in the form
   1608      of a decimal integer constant.  While we call it a predefined
   1609      macro, it's a pretty strange macro, since its "definition" changes
   1610      with each new line of source code.
   1611 
   1612    `__FILE__' and `__LINE__' are useful in generating an error message
   1613 to report an inconsistency detected by the program; the message can
   1614 state the source line at which the inconsistency was detected.  For
   1615 example,
   1616 
   1617      fprintf (stderr, "Internal error: "
   1618                       "negative string length "
   1619                       "%d at %s, line %d.",
   1620               length, __FILE__, __LINE__);
   1621 
   1622    An `#include' directive changes the expansions of `__FILE__' and
   1623 `__LINE__' to correspond to the included file.  At the end of that
   1624 file, when processing resumes on the input file that contained the
   1625 `#include' directive, the expansions of `__FILE__' and `__LINE__'
   1626 revert to the values they had before the `#include' (but `__LINE__' is
   1627 then incremented by one as processing moves to the line after the
   1628 `#include').
   1629 
   1630    A `#line' directive changes `__LINE__', and may change `__FILE__' as
   1631 well.  *Note Line Control::.
   1632 
   1633    C99 introduces `__func__', and GCC has provided `__FUNCTION__' for a
   1634 long time.  Both of these are strings containing the name of the
   1635 current function (there are slight semantic differences; see the GCC
   1636 manual).  Neither of them is a macro; the preprocessor does not know the
   1637 name of the current function.  They tend to be useful in conjunction
   1638 with `__FILE__' and `__LINE__', though.
   1639 
   1640 `__DATE__'
   1641      This macro expands to a string constant that describes the date on
   1642      which the preprocessor is being run.  The string constant contains
   1643      eleven characters and looks like `"Feb 12 1996"'.  If the day of
   1644      the month is less than 10, it is padded with a space on the left.
   1645 
   1646      If GCC cannot determine the current date, it will emit a warning
   1647      message (once per compilation) and `__DATE__' will expand to
   1648      `"??? ?? ????"'.
   1649 
   1650 `__TIME__'
   1651      This macro expands to a string constant that describes the time at
   1652      which the preprocessor is being run.  The string constant contains
   1653      eight characters and looks like `"23:59:01"'.
   1654 
   1655      If GCC cannot determine the current time, it will emit a warning
   1656      message (once per compilation) and `__TIME__' will expand to
   1657      `"??:??:??"'.
   1658 
   1659 `__STDC__'
   1660      In normal operation, this macro expands to the constant 1, to
   1661      signify that this compiler conforms to ISO Standard C.  If GNU CPP
   1662      is used with a compiler other than GCC, this is not necessarily
   1663      true; however, the preprocessor always conforms to the standard
   1664      unless the `-traditional-cpp' option is used.
   1665 
   1666      This macro is not defined if the `-traditional-cpp' option is used.
   1667 
   1668      On some hosts, the system compiler uses a different convention,
   1669      where `__STDC__' is normally 0, but is 1 if the user specifies
   1670      strict conformance to the C Standard.  CPP follows the host
   1671      convention when processing system header files, but when
   1672      processing user files `__STDC__' is always 1.  This has been
   1673      reported to cause problems; for instance, some versions of Solaris
   1674      provide X Windows headers that expect `__STDC__' to be either
   1675      undefined or 1.  *Note Invocation::.
   1676 
   1677 `__STDC_VERSION__'
   1678      This macro expands to the C Standard's version number, a long
   1679      integer constant of the form `YYYYMML' where YYYY and MM are the
   1680      year and month of the Standard version.  This signifies which
   1681      version of the C Standard the compiler conforms to.  Like
   1682      `__STDC__', this is not necessarily accurate for the entire
   1683      implementation, unless GNU CPP is being used with GCC.
   1684 
   1685      The value `199409L' signifies the 1989 C standard as amended in
   1686      1994, which is the current default; the value `199901L' signifies
   1687      the 1999 revision of the C standard.  Support for the 1999
   1688      revision is not yet complete.
   1689 
   1690      This macro is not defined if the `-traditional-cpp' option is
   1691      used, nor when compiling C++ or Objective-C.
   1692 
   1693 `__STDC_HOSTED__'
   1694      This macro is defined, with value 1, if the compiler's target is a
   1695      "hosted environment".  A hosted environment has the complete
   1696      facilities of the standard C library available.
   1697 
   1698 `__cplusplus'
   1699      This macro is defined when the C++ compiler is in use.  You can use
   1700      `__cplusplus' to test whether a header is compiled by a C compiler
   1701      or a C++ compiler.  This macro is similar to `__STDC_VERSION__', in
   1702      that it expands to a version number.  A fully conforming
   1703      implementation of the 1998 C++ standard will define this macro to
   1704      `199711L'.  The GNU C++ compiler is not yet fully conforming, so
   1705      it uses `1' instead.  It is hoped to complete the implementation
   1706      of standard C++ in the near future.
   1707 
   1708 `__OBJC__'
   1709      This macro is defined, with value 1, when the Objective-C compiler
   1710      is in use.  You can use `__OBJC__' to test whether a header is
   1711      compiled by a C compiler or a Objective-C compiler.
   1712 
   1713 `__ASSEMBLER__'
   1714      This macro is defined with value 1 when preprocessing assembly
   1715      language.
   1716 
   1717 
   1718 
   1719 File: cpp.info,  Node: Common Predefined Macros,  Next: System-specific Predefined Macros,  Prev: Standard Predefined Macros,  Up: Predefined Macros
   1720 
   1721 3.7.2 Common Predefined Macros
   1722 ------------------------------
   1723 
   1724 The common predefined macros are GNU C extensions.  They are available
   1725 with the same meanings regardless of the machine or operating system on
   1726 which you are using GNU C.  Their names all start with double
   1727 underscores.
   1728 
   1729 `__COUNTER__'
   1730      This macro expands to sequential integral values starting from 0.
   1731      In conjuction with the `##' operator, this provides a convenient
   1732      means to generate unique identifiers.  Care must be taken to
   1733      ensure that `__COUNTER__' is not expanded prior to inclusion of
   1734      precompiled headers which use it.  Otherwise, the precompiled
   1735      headers will not be used.
   1736 
   1737 `__GNUC__'
   1738 `__GNUC_MINOR__'
   1739 `__GNUC_PATCHLEVEL__'
   1740      These macros are defined by all GNU compilers that use the C
   1741      preprocessor: C, C++, and Objective-C.  Their values are the major
   1742      version, minor version, and patch level of the compiler, as integer
   1743      constants.  For example, GCC 3.2.1 will define `__GNUC__' to 3,
   1744      `__GNUC_MINOR__' to 2, and `__GNUC_PATCHLEVEL__' to 1.  These
   1745      macros are also defined if you invoke the preprocessor directly.
   1746 
   1747      `__GNUC_PATCHLEVEL__' is new to GCC 3.0; it is also present in the
   1748      widely-used development snapshots leading up to 3.0 (which identify
   1749      themselves as GCC 2.96 or 2.97, depending on which snapshot you
   1750      have).
   1751 
   1752      If all you need to know is whether or not your program is being
   1753      compiled by GCC, or a non-GCC compiler that claims to accept the
   1754      GNU C dialects, you can simply test `__GNUC__'.  If you need to
   1755      write code which depends on a specific version, you must be more
   1756      careful.  Each time the minor version is increased, the patch
   1757      level is reset to zero; each time the major version is increased
   1758      (which happens rarely), the minor version and patch level are
   1759      reset.  If you wish to use the predefined macros directly in the
   1760      conditional, you will need to write it like this:
   1761 
   1762           /* Test for GCC > 3.2.0 */
   1763           #if __GNUC__ > 3 || \
   1764               (__GNUC__ == 3 && (__GNUC_MINOR__ > 2 || \
   1765                                  (__GNUC_MINOR__ == 2 && \
   1766                                   __GNUC_PATCHLEVEL__ > 0))
   1767 
   1768      Another approach is to use the predefined macros to calculate a
   1769      single number, then compare that against a threshold:
   1770 
   1771           #define GCC_VERSION (__GNUC__ * 10000 \
   1772                                + __GNUC_MINOR__ * 100 \
   1773                                + __GNUC_PATCHLEVEL__)
   1774           ...
   1775           /* Test for GCC > 3.2.0 */
   1776           #if GCC_VERSION > 30200
   1777 
   1778      Many people find this form easier to understand.
   1779 
   1780 `__GNUG__'
   1781      The GNU C++ compiler defines this.  Testing it is equivalent to
   1782      testing `(__GNUC__ && __cplusplus)'.
   1783 
   1784 `__STRICT_ANSI__'
   1785      GCC defines this macro if and only if the `-ansi' switch, or a
   1786      `-std' switch specifying strict conformance to some version of ISO
   1787      C, was specified when GCC was invoked.  It is defined to `1'.
   1788      This macro exists primarily to direct GNU libc's header files to
   1789      restrict their definitions to the minimal set found in the 1989 C
   1790      standard.
   1791 
   1792 `__BASE_FILE__'
   1793      This macro expands to the name of the main input file, in the form
   1794      of a C string constant.  This is the source file that was specified
   1795      on the command line of the preprocessor or C compiler.
   1796 
   1797 `__INCLUDE_LEVEL__'
   1798      This macro expands to a decimal integer constant that represents
   1799      the depth of nesting in include files.  The value of this macro is
   1800      incremented on every `#include' directive and decremented at the
   1801      end of every included file.  It starts out at 0, it's value within
   1802      the base file specified on the command line.
   1803 
   1804 `__ELF__'
   1805      This macro is defined if the target uses the ELF object format.
   1806 
   1807 `__VERSION__'
   1808      This macro expands to a string constant which describes the
   1809      version of the compiler in use.  You should not rely on its
   1810      contents having any particular form, but it can be counted on to
   1811      contain at least the release number.
   1812 
   1813 `__OPTIMIZE__'
   1814 `__OPTIMIZE_SIZE__'
   1815 `__NO_INLINE__'
   1816      These macros describe the compilation mode.  `__OPTIMIZE__' is
   1817      defined in all optimizing compilations.  `__OPTIMIZE_SIZE__' is
   1818      defined if the compiler is optimizing for size, not speed.
   1819      `__NO_INLINE__' is defined if no functions will be inlined into
   1820      their callers (when not optimizing, or when inlining has been
   1821      specifically disabled by `-fno-inline').
   1822 
   1823      These macros cause certain GNU header files to provide optimized
   1824      definitions, using macros or inline functions, of system library
   1825      functions.  You should not use these macros in any way unless you
   1826      make sure that programs will execute with the same effect whether
   1827      or not they are defined.  If they are defined, their value is 1.
   1828 
   1829 `__GNUC_GNU_INLINE__'
   1830      GCC defines this macro if functions declared `inline' will be
   1831      handled in GCC's traditional gnu89 mode.  In this mode an `extern
   1832      inline' function will never be compiled as a standalone function,
   1833      and an `inline' function which is neither `extern' nor `static'
   1834      will always be compiled as a standalone function.
   1835 
   1836 `__GNUC_STDC_INLINE__'
   1837      GCC defines this macro if functions declared `inline' will be
   1838      handled according to the ISO C99 standard.  In this mode an
   1839      `extern inline' function will always be compiled as a standalone
   1840      externally visible function, and an `inline' function which is
   1841      neither `extern' nor `static' will never be compiled as a
   1842      standalone function.
   1843 
   1844      If this macro is defined, GCC supports the `gnu_inline' function
   1845      attribute as a way to always get the gnu89 behaviour.  Support for
   1846      this and `__GNUC_GNU_INLINE__' was added in GCC 4.1.3.  If neither
   1847      macro is defined, an older version of GCC is being used: `inline'
   1848      functions will be compiled in gnu89 mode, and the `gnu_inline'
   1849      function attribute will not be recognized.
   1850 
   1851 `__CHAR_UNSIGNED__'
   1852      GCC defines this macro if and only if the data type `char' is
   1853      unsigned on the target machine.  It exists to cause the standard
   1854      header file `limits.h' to work correctly.  You should not use this
   1855      macro yourself; instead, refer to the standard macros defined in
   1856      `limits.h'.
   1857 
   1858 `__WCHAR_UNSIGNED__'
   1859      Like `__CHAR_UNSIGNED__', this macro is defined if and only if the
   1860      data type `wchar_t' is unsigned and the front-end is in C++ mode.
   1861 
   1862 `__REGISTER_PREFIX__'
   1863      This macro expands to a single token (not a string constant) which
   1864      is the prefix applied to CPU register names in assembly language
   1865      for this target.  You can use it to write assembly that is usable
   1866      in multiple environments.  For example, in the `m68k-aout'
   1867      environment it expands to nothing, but in the `m68k-coff'
   1868      environment it expands to a single `%'.
   1869 
   1870 `__USER_LABEL_PREFIX__'
   1871      This macro expands to a single token which is the prefix applied to
   1872      user labels (symbols visible to C code) in assembly.  For example,
   1873      in the `m68k-aout' environment it expands to an `_', but in the
   1874      `m68k-coff' environment it expands to nothing.
   1875 
   1876      This macro will have the correct definition even if
   1877      `-f(no-)underscores' is in use, but it will not be correct if
   1878      target-specific options that adjust this prefix are used (e.g. the
   1879      OSF/rose `-mno-underscores' option).
   1880 
   1881 `__SIZE_TYPE__'
   1882 `__PTRDIFF_TYPE__'
   1883 `__WCHAR_TYPE__'
   1884 `__WINT_TYPE__'
   1885 `__INTMAX_TYPE__'
   1886 `__UINTMAX_TYPE__'
   1887      These macros are defined to the correct underlying types for the
   1888      `size_t', `ptrdiff_t', `wchar_t', `wint_t', `intmax_t', and
   1889      `uintmax_t' typedefs, respectively.  They exist to make the
   1890      standard header files `stddef.h' and `wchar.h' work correctly.
   1891      You should not use these macros directly; instead, include the
   1892      appropriate headers and use the typedefs.
   1893 
   1894 `__CHAR_BIT__'
   1895      Defined to the number of bits used in the representation of the
   1896      `char' data type.  It exists to make the standard header given
   1897      numerical limits work correctly.  You should not use this macro
   1898      directly; instead, include the appropriate headers.
   1899 
   1900 `__SCHAR_MAX__'
   1901 `__WCHAR_MAX__'
   1902 `__SHRT_MAX__'
   1903 `__INT_MAX__'
   1904 `__LONG_MAX__'
   1905 `__LONG_LONG_MAX__'
   1906 `__INTMAX_MAX__'
   1907      Defined to the maximum value of the `signed char', `wchar_t',
   1908      `signed short', `signed int', `signed long', `signed long long',
   1909      and `intmax_t' types respectively.  They exist to make the
   1910      standard header given numerical limits work correctly.  You should
   1911      not use these macros directly; instead, include the appropriate
   1912      headers.
   1913 
   1914 `__DEPRECATED'
   1915      This macro is defined, with value 1, when compiling a C++ source
   1916      file with warnings about deprecated constructs enabled.  These
   1917      warnings are enabled by default, but can be disabled with
   1918      `-Wno-deprecated'.
   1919 
   1920 `__EXCEPTIONS'
   1921      This macro is defined, with value 1, when compiling a C++ source
   1922      file with exceptions enabled.  If `-fno-exceptions' was used when
   1923      compiling the file, then this macro will not be defined.
   1924 
   1925 `__USING_SJLJ_EXCEPTIONS__'
   1926      This macro is defined, with value 1, if the compiler uses the old
   1927      mechanism based on `setjmp' and `longjmp' for exception handling.
   1928 
   1929 `__GXX_WEAK__'
   1930      This macro is defined when compiling a C++ source file.  It has the
   1931      value 1 if the compiler will use weak symbols, COMDAT sections, or
   1932      other similar techniques to collapse symbols with "vague linkage"
   1933      that are defined in multiple translation units.  If the compiler
   1934      will not collapse such symbols, this macro is defined with value
   1935      0.  In general, user code should not need to make use of this
   1936      macro; the purpose of this macro is to ease implementation of the
   1937      C++ runtime library provided with G++.
   1938 
   1939 `__NEXT_RUNTIME__'
   1940      This macro is defined, with value 1, if (and only if) the NeXT
   1941      runtime (as in `-fnext-runtime') is in use for Objective-C.  If
   1942      the GNU runtime is used, this macro is not defined, so that you
   1943      can use this macro to determine which runtime (NeXT or GNU) is
   1944      being used.
   1945 
   1946 `__LP64__'
   1947 `_LP64'
   1948      These macros are defined, with value 1, if (and only if) the
   1949      compilation is for a target where `long int' and pointer both use
   1950      64-bits and `int' uses 32-bit.
   1951 
   1952 `__SSP__'
   1953      This macro is defined, with value 1, when `-fstack-protector' is in
   1954      use.
   1955 
   1956 `__SSP_ALL__'
   1957      This macro is defined, with value 2, when `-fstack-protector-all'
   1958      is in use.
   1959 
   1960 `__TIMESTAMP__'
   1961      This macro expands to a string constant that describes the date
   1962      and time of the last modification of the current source file. The
   1963      string constant contains abbreviated day of the week, month, day
   1964      of the month, time in hh:mm:ss form, year and looks like
   1965      `"Sun Sep 16 01:03:52 1973"'.  If the day of the month is less
   1966      than 10, it is padded with a space on the left.
   1967 
   1968      If GCC cannot determine the current date, it will emit a warning
   1969      message (once per compilation) and `__TIMESTAMP__' will expand to
   1970      `"??? ??? ?? ??:??:?? ????"'.
   1971 
   1972 
   1973 
   1974 File: cpp.info,  Node: System-specific Predefined Macros,  Next: C++ Named Operators,  Prev: Common Predefined Macros,  Up: Predefined Macros
   1975 
   1976 3.7.3 System-specific Predefined Macros
   1977 ---------------------------------------
   1978 
   1979 The C preprocessor normally predefines several macros that indicate what
   1980 type of system and machine is in use.  They are obviously different on
   1981 each target supported by GCC.  This manual, being for all systems and
   1982 machines, cannot tell you what their names are, but you can use `cpp
   1983 -dM' to see them all.  *Note Invocation::.  All system-specific
   1984 predefined macros expand to the constant 1, so you can test them with
   1985 either `#ifdef' or `#if'.
   1986 
   1987    The C standard requires that all system-specific macros be part of
   1988 the "reserved namespace".  All names which begin with two underscores,
   1989 or an underscore and a capital letter, are reserved for the compiler and
   1990 library to use as they wish.  However, historically system-specific
   1991 macros have had names with no special prefix; for instance, it is common
   1992 to find `unix' defined on Unix systems.  For all such macros, GCC
   1993 provides a parallel macro with two underscores added at the beginning
   1994 and the end.  If `unix' is defined, `__unix__' will be defined too.
   1995 There will never be more than two underscores; the parallel of `_mips'
   1996 is `__mips__'.
   1997 
   1998    When the `-ansi' option, or any `-std' option that requests strict
   1999 conformance, is given to the compiler, all the system-specific
   2000 predefined macros outside the reserved namespace are suppressed.  The
   2001 parallel macros, inside the reserved namespace, remain defined.
   2002 
   2003    We are slowly phasing out all predefined macros which are outside the
   2004 reserved namespace.  You should never use them in new programs, and we
   2005 encourage you to correct older code to use the parallel macros whenever
   2006 you find it.  We don't recommend you use the system-specific macros that
   2007 are in the reserved namespace, either.  It is better in the long run to
   2008 check specifically for features you need, using a tool such as
   2009 `autoconf'.
   2010 
   2011 
   2012 File: cpp.info,  Node: C++ Named Operators,  Prev: System-specific Predefined Macros,  Up: Predefined Macros
   2013 
   2014 3.7.4 C++ Named Operators
   2015 -------------------------
   2016 
   2017 In C++, there are eleven keywords which are simply alternate spellings
   2018 of operators normally written with punctuation.  These keywords are
   2019 treated as such even in the preprocessor.  They function as operators in
   2020 `#if', and they cannot be defined as macros or poisoned.  In C, you can
   2021 request that those keywords take their C++ meaning by including
   2022 `iso646.h'.  That header defines each one as a normal object-like macro
   2023 expanding to the appropriate punctuator.
   2024 
   2025    These are the named operators and their corresponding punctuators:
   2026 
   2027 Named Operator   Punctuator
   2028 `and'            `&&'
   2029 `and_eq'         `&='
   2030 `bitand'         `&'
   2031 `bitor'          `|'
   2032 `compl'          `~'
   2033 `not'            `!'
   2034 `not_eq'         `!='
   2035 `or'             `||'
   2036 `or_eq'          `|='
   2037 `xor'            `^'
   2038 `xor_eq'         `^='
   2039 
   2040 
   2041 File: cpp.info,  Node: Undefining and Redefining Macros,  Next: Directives Within Macro Arguments,  Prev: Predefined Macros,  Up: Macros
   2042 
   2043 3.8 Undefining and Redefining Macros
   2044 ====================================
   2045 
   2046 If a macro ceases to be useful, it may be "undefined" with the `#undef'
   2047 directive.  `#undef' takes a single argument, the name of the macro to
   2048 undefine.  You use the bare macro name, even if the macro is
   2049 function-like.  It is an error if anything appears on the line after
   2050 the macro name.  `#undef' has no effect if the name is not a macro.
   2051 
   2052      #define FOO 4
   2053      x = FOO;        ==> x = 4;
   2054      #undef FOO
   2055      x = FOO;        ==> x = FOO;
   2056 
   2057    Once a macro has been undefined, that identifier may be "redefined"
   2058 as a macro by a subsequent `#define' directive.  The new definition
   2059 need not have any resemblance to the old definition.
   2060 
   2061    However, if an identifier which is currently a macro is redefined,
   2062 then the new definition must be "effectively the same" as the old one.
   2063 Two macro definitions are effectively the same if:
   2064    * Both are the same type of macro (object- or function-like).
   2065 
   2066    * All the tokens of the replacement list are the same.
   2067 
   2068    * If there are any parameters, they are the same.
   2069 
   2070    * Whitespace appears in the same places in both.  It need not be
   2071      exactly the same amount of whitespace, though.  Remember that
   2072      comments count as whitespace.
   2073 
   2074 These definitions are effectively the same:
   2075      #define FOUR (2 + 2)
   2076      #define FOUR         (2    +    2)
   2077      #define FOUR (2 /* two */ + 2)
   2078    but these are not:
   2079      #define FOUR (2 + 2)
   2080      #define FOUR ( 2+2 )
   2081      #define FOUR (2 * 2)
   2082      #define FOUR(score,and,seven,years,ago) (2 + 2)
   2083 
   2084    If a macro is redefined with a definition that is not effectively the
   2085 same as the old one, the preprocessor issues a warning and changes the
   2086 macro to use the new definition.  If the new definition is effectively
   2087 the same, the redefinition is silently ignored.  This allows, for
   2088 instance, two different headers to define a common macro.  The
   2089 preprocessor will only complain if the definitions do not match.
   2090 
   2091 
   2092 File: cpp.info,  Node: Directives Within Macro Arguments,  Next: Macro Pitfalls,  Prev: Undefining and Redefining Macros,  Up: Macros
   2093 
   2094 3.9 Directives Within Macro Arguments
   2095 =====================================
   2096 
   2097 Occasionally it is convenient to use preprocessor directives within the
   2098 arguments of a macro.  The C and C++ standards declare that behavior in
   2099 these cases is undefined.
   2100 
   2101    Versions of CPP prior to 3.2 would reject such constructs with an
   2102 error message.  This was the only syntactic difference between normal
   2103 functions and function-like macros, so it seemed attractive to remove
   2104 this limitation, and people would often be surprised that they could
   2105 not use macros in this way.  Moreover, sometimes people would use
   2106 conditional compilation in the argument list to a normal library
   2107 function like `printf', only to find that after a library upgrade
   2108 `printf' had changed to be a function-like macro, and their code would
   2109 no longer compile.  So from version 3.2 we changed CPP to successfully
   2110 process arbitrary directives within macro arguments in exactly the same
   2111 way as it would have processed the directive were the function-like
   2112 macro invocation not present.
   2113 
   2114    If, within a macro invocation, that macro is redefined, then the new
   2115 definition takes effect in time for argument pre-expansion, but the
   2116 original definition is still used for argument replacement.  Here is a
   2117 pathological example:
   2118 
   2119      #define f(x) x x
   2120      f (1
   2121      #undef f
   2122      #define f 2
   2123      f)
   2124 
   2125 which expands to
   2126 
   2127      1 2 1 2
   2128 
   2129 with the semantics described above.
   2130 
   2131 
   2132 File: cpp.info,  Node: Macro Pitfalls,  Prev: Directives Within Macro Arguments,  Up: Macros
   2133 
   2134 3.10 Macro Pitfalls
   2135 ===================
   2136 
   2137 In this section we describe some special rules that apply to macros and
   2138 macro expansion, and point out certain cases in which the rules have
   2139 counter-intuitive consequences that you must watch out for.
   2140 
   2141 * Menu:
   2142 
   2143 * Misnesting::
   2144 * Operator Precedence Problems::
   2145 * Swallowing the Semicolon::
   2146 * Duplication of Side Effects::
   2147 * Self-Referential Macros::
   2148 * Argument Prescan::
   2149 * Newlines in Arguments::
   2150 
   2151 
   2152 File: cpp.info,  Node: Misnesting,  Next: Operator Precedence Problems,  Up: Macro Pitfalls
   2153 
   2154 3.10.1 Misnesting
   2155 -----------------
   2156 
   2157 When a macro is called with arguments, the arguments are substituted
   2158 into the macro body and the result is checked, together with the rest of
   2159 the input file, for more macro calls.  It is possible to piece together
   2160 a macro call coming partially from the macro body and partially from the
   2161 arguments.  For example,
   2162 
   2163      #define twice(x) (2*(x))
   2164      #define call_with_1(x) x(1)
   2165      call_with_1 (twice)
   2166           ==> twice(1)
   2167           ==> (2*(1))
   2168 
   2169    Macro definitions do not have to have balanced parentheses.  By
   2170 writing an unbalanced open parenthesis in a macro body, it is possible
   2171 to create a macro call that begins inside the macro body but ends
   2172 outside of it.  For example,
   2173 
   2174      #define strange(file) fprintf (file, "%s %d",
   2175      ...
   2176      strange(stderr) p, 35)
   2177           ==> fprintf (stderr, "%s %d", p, 35)
   2178 
   2179    The ability to piece together a macro call can be useful, but the
   2180 use of unbalanced open parentheses in a macro body is just confusing,
   2181 and should be avoided.
   2182 
   2183 
   2184 File: cpp.info,  Node: Operator Precedence Problems,  Next: Swallowing the Semicolon,  Prev: Misnesting,  Up: Macro Pitfalls
   2185 
   2186 3.10.2 Operator Precedence Problems
   2187 -----------------------------------
   2188 
   2189 You may have noticed that in most of the macro definition examples shown
   2190 above, each occurrence of a macro argument name had parentheses around
   2191 it.  In addition, another pair of parentheses usually surround the
   2192 entire macro definition.  Here is why it is best to write macros that
   2193 way.
   2194 
   2195    Suppose you define a macro as follows,
   2196 
   2197      #define ceil_div(x, y) (x + y - 1) / y
   2198 
   2199 whose purpose is to divide, rounding up.  (One use for this operation is
   2200 to compute how many `int' objects are needed to hold a certain number
   2201 of `char' objects.)  Then suppose it is used as follows:
   2202 
   2203      a = ceil_div (b & c, sizeof (int));
   2204           ==> a = (b & c + sizeof (int) - 1) / sizeof (int);
   2205 
   2206 This does not do what is intended.  The operator-precedence rules of C
   2207 make it equivalent to this:
   2208 
   2209      a = (b & (c + sizeof (int) - 1)) / sizeof (int);
   2210 
   2211 What we want is this:
   2212 
   2213      a = ((b & c) + sizeof (int) - 1)) / sizeof (int);
   2214 
   2215 Defining the macro as
   2216 
   2217      #define ceil_div(x, y) ((x) + (y) - 1) / (y)
   2218 
   2219 provides the desired result.
   2220 
   2221    Unintended grouping can result in another way.  Consider `sizeof
   2222 ceil_div(1, 2)'.  That has the appearance of a C expression that would
   2223 compute the size of the type of `ceil_div (1, 2)', but in fact it means
   2224 something very different.  Here is what it expands to:
   2225 
   2226      sizeof ((1) + (2) - 1) / (2)
   2227 
   2228 This would take the size of an integer and divide it by two.  The
   2229 precedence rules have put the division outside the `sizeof' when it was
   2230 intended to be inside.
   2231 
   2232    Parentheses around the entire macro definition prevent such problems.
   2233 Here, then, is the recommended way to define `ceil_div':
   2234 
   2235      #define ceil_div(x, y) (((x) + (y) - 1) / (y))
   2236 
   2237 
   2238 File: cpp.info,  Node: Swallowing the Semicolon,  Next: Duplication of Side Effects,  Prev: Operator Precedence Problems,  Up: Macro Pitfalls
   2239 
   2240 3.10.3 Swallowing the Semicolon
   2241 -------------------------------
   2242 
   2243 Often it is desirable to define a macro that expands into a compound
   2244 statement.  Consider, for example, the following macro, that advances a
   2245 pointer (the argument `p' says where to find it) across whitespace
   2246 characters:
   2247 
   2248      #define SKIP_SPACES(p, limit)  \
   2249      { char *lim = (limit);         \
   2250        while (p < lim) {            \
   2251          if (*p++ != ' ') {         \
   2252            p--; break; }}}
   2253 
   2254 Here backslash-newline is used to split the macro definition, which must
   2255 be a single logical line, so that it resembles the way such code would
   2256 be laid out if not part of a macro definition.
   2257 
   2258    A call to this macro might be `SKIP_SPACES (p, lim)'.  Strictly
   2259 speaking, the call expands to a compound statement, which is a complete
   2260 statement with no need for a semicolon to end it.  However, since it
   2261 looks like a function call, it minimizes confusion if you can use it
   2262 like a function call, writing a semicolon afterward, as in `SKIP_SPACES
   2263 (p, lim);'
   2264 
   2265    This can cause trouble before `else' statements, because the
   2266 semicolon is actually a null statement.  Suppose you write
   2267 
   2268      if (*p != 0)
   2269        SKIP_SPACES (p, lim);
   2270      else ...
   2271 
   2272 The presence of two statements--the compound statement and a null
   2273 statement--in between the `if' condition and the `else' makes invalid C
   2274 code.
   2275 
   2276    The definition of the macro `SKIP_SPACES' can be altered to solve
   2277 this problem, using a `do ... while' statement.  Here is how:
   2278 
   2279      #define SKIP_SPACES(p, limit)     \
   2280      do { char *lim = (limit);         \
   2281           while (p < lim) {            \
   2282             if (*p++ != ' ') {         \
   2283               p--; break; }}}          \
   2284      while (0)
   2285 
   2286    Now `SKIP_SPACES (p, lim);' expands into
   2287 
   2288      do {...} while (0);
   2289 
   2290 which is one statement.  The loop executes exactly once; most compilers
   2291 generate no extra code for it.
   2292 
   2293 
   2294 File: cpp.info,  Node: Duplication of Side Effects,  Next: Self-Referential Macros,  Prev: Swallowing the Semicolon,  Up: Macro Pitfalls
   2295 
   2296 3.10.4 Duplication of Side Effects
   2297 ----------------------------------
   2298 
   2299 Many C programs define a macro `min', for "minimum", like this:
   2300 
   2301      #define min(X, Y)  ((X) < (Y) ? (X) : (Y))
   2302 
   2303    When you use this macro with an argument containing a side effect,
   2304 as shown here,
   2305 
   2306      next = min (x + y, foo (z));
   2307 
   2308 it expands as follows:
   2309 
   2310      next = ((x + y) < (foo (z)) ? (x + y) : (foo (z)));
   2311 
   2312 where `x + y' has been substituted for `X' and `foo (z)' for `Y'.
   2313 
   2314    The function `foo' is used only once in the statement as it appears
   2315 in the program, but the expression `foo (z)' has been substituted twice
   2316 into the macro expansion.  As a result, `foo' might be called two times
   2317 when the statement is executed.  If it has side effects or if it takes
   2318 a long time to compute, the results might not be what you intended.  We
   2319 say that `min' is an "unsafe" macro.
   2320 
   2321    The best solution to this problem is to define `min' in a way that
   2322 computes the value of `foo (z)' only once.  The C language offers no
   2323 standard way to do this, but it can be done with GNU extensions as
   2324 follows:
   2325 
   2326      #define min(X, Y)                \
   2327      ({ typeof (X) x_ = (X);          \
   2328         typeof (Y) y_ = (Y);          \
   2329         (x_ < y_) ? x_ : y_; })
   2330 
   2331    The `({ ... })' notation produces a compound statement that acts as
   2332 an expression.  Its value is the value of its last statement.  This
   2333 permits us to define local variables and assign each argument to one.
   2334 The local variables have underscores after their names to reduce the
   2335 risk of conflict with an identifier of wider scope (it is impossible to
   2336 avoid this entirely).  Now each argument is evaluated exactly once.
   2337 
   2338    If you do not wish to use GNU C extensions, the only solution is to
   2339 be careful when _using_ the macro `min'.  For example, you can
   2340 calculate the value of `foo (z)', save it in a variable, and use that
   2341 variable in `min':
   2342 
   2343      #define min(X, Y)  ((X) < (Y) ? (X) : (Y))
   2344      ...
   2345      {
   2346        int tem = foo (z);
   2347        next = min (x + y, tem);
   2348      }
   2349 
   2350 (where we assume that `foo' returns type `int').
   2351 
   2352 
   2353 File: cpp.info,  Node: Self-Referential Macros,  Next: Argument Prescan,  Prev: Duplication of Side Effects,  Up: Macro Pitfalls
   2354 
   2355 3.10.5 Self-Referential Macros
   2356 ------------------------------
   2357 
   2358 A "self-referential" macro is one whose name appears in its definition.
   2359 Recall that all macro definitions are rescanned for more macros to
   2360 replace.  If the self-reference were considered a use of the macro, it
   2361 would produce an infinitely large expansion.  To prevent this, the
   2362 self-reference is not considered a macro call.  It is passed into the
   2363 preprocessor output unchanged.  Consider an example:
   2364 
   2365      #define foo (4 + foo)
   2366 
   2367 where `foo' is also a variable in your program.
   2368 
   2369    Following the ordinary rules, each reference to `foo' will expand
   2370 into `(4 + foo)'; then this will be rescanned and will expand into `(4
   2371 + (4 + foo))'; and so on until the computer runs out of memory.
   2372 
   2373    The self-reference rule cuts this process short after one step, at
   2374 `(4 + foo)'.  Therefore, this macro definition has the possibly useful
   2375 effect of causing the program to add 4 to the value of `foo' wherever
   2376 `foo' is referred to.
   2377 
   2378    In most cases, it is a bad idea to take advantage of this feature.  A
   2379 person reading the program who sees that `foo' is a variable will not
   2380 expect that it is a macro as well.  The reader will come across the
   2381 identifier `foo' in the program and think its value should be that of
   2382 the variable `foo', whereas in fact the value is four greater.
   2383 
   2384    One common, useful use of self-reference is to create a macro which
   2385 expands to itself.  If you write
   2386 
   2387      #define EPERM EPERM
   2388 
   2389 then the macro `EPERM' expands to `EPERM'.  Effectively, it is left
   2390 alone by the preprocessor whenever it's used in running text.  You can
   2391 tell that it's a macro with `#ifdef'.  You might do this if you want to
   2392 define numeric constants with an `enum', but have `#ifdef' be true for
   2393 each constant.
   2394 
   2395    If a macro `x' expands to use a macro `y', and the expansion of `y'
   2396 refers to the macro `x', that is an "indirect self-reference" of `x'.
   2397 `x' is not expanded in this case either.  Thus, if we have
   2398 
   2399      #define x (4 + y)
   2400      #define y (2 * x)
   2401 
   2402 then `x' and `y' expand as follows:
   2403 
   2404      x    ==> (4 + y)
   2405           ==> (4 + (2 * x))
   2406 
   2407      y    ==> (2 * x)
   2408           ==> (2 * (4 + y))
   2409 
   2410 Each macro is expanded when it appears in the definition of the other
   2411 macro, but not when it indirectly appears in its own definition.
   2412 
   2413 
   2414 File: cpp.info,  Node: Argument Prescan,  Next: Newlines in Arguments,  Prev: Self-Referential Macros,  Up: Macro Pitfalls
   2415 
   2416 3.10.6 Argument Prescan
   2417 -----------------------
   2418 
   2419 Macro arguments are completely macro-expanded before they are
   2420 substituted into a macro body, unless they are stringified or pasted
   2421 with other tokens.  After substitution, the entire macro body, including
   2422 the substituted arguments, is scanned again for macros to be expanded.
   2423 The result is that the arguments are scanned _twice_ to expand macro
   2424 calls in them.
   2425 
   2426    Most of the time, this has no effect.  If the argument contained any
   2427 macro calls, they are expanded during the first scan.  The result
   2428 therefore contains no macro calls, so the second scan does not change
   2429 it.  If the argument were substituted as given, with no prescan, the
   2430 single remaining scan would find the same macro calls and produce the
   2431 same results.
   2432 
   2433    You might expect the double scan to change the results when a
   2434 self-referential macro is used in an argument of another macro (*note
   2435 Self-Referential Macros::): the self-referential macro would be
   2436 expanded once in the first scan, and a second time in the second scan.
   2437 However, this is not what happens.  The self-references that do not
   2438 expand in the first scan are marked so that they will not expand in the
   2439 second scan either.
   2440 
   2441    You might wonder, "Why mention the prescan, if it makes no
   2442 difference?  And why not skip it and make the preprocessor faster?"
   2443 The answer is that the prescan does make a difference in three special
   2444 cases:
   2445 
   2446    * Nested calls to a macro.
   2447 
   2448      We say that "nested" calls to a macro occur when a macro's argument
   2449      contains a call to that very macro.  For example, if `f' is a macro
   2450      that expects one argument, `f (f (1))' is a nested pair of calls to
   2451      `f'.  The desired expansion is made by expanding `f (1)' and
   2452      substituting that into the definition of `f'.  The prescan causes
   2453      the expected result to happen.  Without the prescan, `f (1)' itself
   2454      would be substituted as an argument, and the inner use of `f' would
   2455      appear during the main scan as an indirect self-reference and
   2456      would not be expanded.
   2457 
   2458    * Macros that call other macros that stringify or concatenate.
   2459 
   2460      If an argument is stringified or concatenated, the prescan does not
   2461      occur.  If you _want_ to expand a macro, then stringify or
   2462      concatenate its expansion, you can do that by causing one macro to
   2463      call another macro that does the stringification or concatenation.
   2464      For instance, if you have
   2465 
   2466           #define AFTERX(x) X_ ## x
   2467           #define XAFTERX(x) AFTERX(x)
   2468           #define TABLESIZE 1024
   2469           #define BUFSIZE TABLESIZE
   2470 
   2471      then `AFTERX(BUFSIZE)' expands to `X_BUFSIZE', and
   2472      `XAFTERX(BUFSIZE)' expands to `X_1024'.  (Not to `X_TABLESIZE'.
   2473      Prescan always does a complete expansion.)
   2474 
   2475    * Macros used in arguments, whose expansions contain unshielded
   2476      commas.
   2477 
   2478      This can cause a macro expanded on the second scan to be called
   2479      with the wrong number of arguments.  Here is an example:
   2480 
   2481           #define foo  a,b
   2482           #define bar(x) lose(x)
   2483           #define lose(x) (1 + (x))
   2484 
   2485      We would like `bar(foo)' to turn into `(1 + (foo))', which would
   2486      then turn into `(1 + (a,b))'.  Instead, `bar(foo)' expands into
   2487      `lose(a,b)', and you get an error because `lose' requires a single
   2488      argument.  In this case, the problem is easily solved by the same
   2489      parentheses that ought to be used to prevent misnesting of
   2490      arithmetic operations:
   2491 
   2492           #define foo (a,b)
   2493      or
   2494           #define bar(x) lose((x))
   2495 
   2496      The extra pair of parentheses prevents the comma in `foo''s
   2497      definition from being interpreted as an argument separator.
   2498 
   2499 
   2500 
   2501 File: cpp.info,  Node: Newlines in Arguments,  Prev: Argument Prescan,  Up: Macro Pitfalls
   2502 
   2503 3.10.7 Newlines in Arguments
   2504 ----------------------------
   2505 
   2506 The invocation of a function-like macro can extend over many logical
   2507 lines.  However, in the present implementation, the entire expansion
   2508 comes out on one line.  Thus line numbers emitted by the compiler or
   2509 debugger refer to the line the invocation started on, which might be
   2510 different to the line containing the argument causing the problem.
   2511 
   2512    Here is an example illustrating this:
   2513 
   2514      #define ignore_second_arg(a,b,c) a; c
   2515 
   2516      ignore_second_arg (foo (),
   2517                         ignored (),
   2518                         syntax error);
   2519 
   2520 The syntax error triggered by the tokens `syntax error' results in an
   2521 error message citing line three--the line of ignore_second_arg-- even
   2522 though the problematic code comes from line five.
   2523 
   2524    We consider this a bug, and intend to fix it in the near future.
   2525 
   2526 
   2527 File: cpp.info,  Node: Conditionals,  Next: Diagnostics,  Prev: Macros,  Up: Top
   2528 
   2529 4 Conditionals
   2530 **************
   2531 
   2532 A "conditional" is a directive that instructs the preprocessor to
   2533 select whether or not to include a chunk of code in the final token
   2534 stream passed to the compiler.  Preprocessor conditionals can test
   2535 arithmetic expressions, or whether a name is defined as a macro, or both
   2536 simultaneously using the special `defined' operator.
   2537 
   2538    A conditional in the C preprocessor resembles in some ways an `if'
   2539 statement in C, but it is important to understand the difference between
   2540 them.  The condition in an `if' statement is tested during the
   2541 execution of your program.  Its purpose is to allow your program to
   2542 behave differently from run to run, depending on the data it is
   2543 operating on.  The condition in a preprocessing conditional directive is
   2544 tested when your program is compiled.  Its purpose is to allow different
   2545 code to be included in the program depending on the situation at the
   2546 time of compilation.
   2547 
   2548    However, the distinction is becoming less clear.  Modern compilers
   2549 often do test `if' statements when a program is compiled, if their
   2550 conditions are known not to vary at run time, and eliminate code which
   2551 can never be executed.  If you can count on your compiler to do this,
   2552 you may find that your program is more readable if you use `if'
   2553 statements with constant conditions (perhaps determined by macros).  Of
   2554 course, you can only use this to exclude code, not type definitions or
   2555 other preprocessing directives, and you can only do it if the code
   2556 remains syntactically valid when it is not to be used.
   2557 
   2558    GCC version 3 eliminates this kind of never-executed code even when
   2559 not optimizing.  Older versions did it only when optimizing.
   2560 
   2561 * Menu:
   2562 
   2563 * Conditional Uses::
   2564 * Conditional Syntax::
   2565 * Deleted Code::
   2566 
   2567 
   2568 File: cpp.info,  Node: Conditional Uses,  Next: Conditional Syntax,  Up: Conditionals
   2569 
   2570 4.1 Conditional Uses
   2571 ====================
   2572 
   2573 There are three general reasons to use a conditional.
   2574 
   2575    * A program may need to use different code depending on the machine
   2576      or operating system it is to run on.  In some cases the code for
   2577      one operating system may be erroneous on another operating system;
   2578      for example, it might refer to data types or constants that do not
   2579      exist on the other system.  When this happens, it is not enough to
   2580      avoid executing the invalid code.  Its mere presence will cause
   2581      the compiler to reject the program.  With a preprocessing
   2582      conditional, the offending code can be effectively excised from
   2583      the program when it is not valid.
   2584 
   2585    * You may want to be able to compile the same source file into two
   2586      different programs.  One version might make frequent time-consuming
   2587      consistency checks on its intermediate data, or print the values of
   2588      those data for debugging, and the other not.
   2589 
   2590    * A conditional whose condition is always false is one way to
   2591      exclude code from the program but keep it as a sort of comment for
   2592      future reference.
   2593 
   2594    Simple programs that do not need system-specific logic or complex
   2595 debugging hooks generally will not need to use preprocessing
   2596 conditionals.
   2597 
   2598 
   2599 File: cpp.info,  Node: Conditional Syntax,  Next: Deleted Code,  Prev: Conditional Uses,  Up: Conditionals
   2600 
   2601 4.2 Conditional Syntax
   2602 ======================
   2603 
   2604 A conditional in the C preprocessor begins with a "conditional
   2605 directive": `#if', `#ifdef' or `#ifndef'.
   2606 
   2607 * Menu:
   2608 
   2609 * Ifdef::
   2610 * If::
   2611 * Defined::
   2612 * Else::
   2613 * Elif::
   2614 
   2615 
   2616 File: cpp.info,  Node: Ifdef,  Next: If,  Up: Conditional Syntax
   2617 
   2618 4.2.1 Ifdef
   2619 -----------
   2620 
   2621 The simplest sort of conditional is
   2622 
   2623      #ifdef MACRO
   2624 
   2625      CONTROLLED TEXT
   2626 
   2627      #endif /* MACRO */
   2628 
   2629    This block is called a "conditional group".  CONTROLLED TEXT will be
   2630 included in the output of the preprocessor if and only if MACRO is
   2631 defined.  We say that the conditional "succeeds" if MACRO is defined,
   2632 "fails" if it is not.
   2633 
   2634    The CONTROLLED TEXT inside of a conditional can include
   2635 preprocessing directives.  They are executed only if the conditional
   2636 succeeds.  You can nest conditional groups inside other conditional
   2637 groups, but they must be completely nested.  In other words, `#endif'
   2638 always matches the nearest `#ifdef' (or `#ifndef', or `#if').  Also,
   2639 you cannot start a conditional group in one file and end it in another.
   2640 
   2641    Even if a conditional fails, the CONTROLLED TEXT inside it is still
   2642 run through initial transformations and tokenization.  Therefore, it
   2643 must all be lexically valid C.  Normally the only way this matters is
   2644 that all comments and string literals inside a failing conditional group
   2645 must still be properly ended.
   2646 
   2647    The comment following the `#endif' is not required, but it is a good
   2648 practice if there is a lot of CONTROLLED TEXT, because it helps people
   2649 match the `#endif' to the corresponding `#ifdef'.  Older programs
   2650 sometimes put MACRO directly after the `#endif' without enclosing it in
   2651 a comment.  This is invalid code according to the C standard.  CPP
   2652 accepts it with a warning.  It never affects which `#ifndef' the
   2653 `#endif' matches.
   2654 
   2655    Sometimes you wish to use some code if a macro is _not_ defined.
   2656 You can do this by writing `#ifndef' instead of `#ifdef'.  One common
   2657 use of `#ifndef' is to include code only the first time a header file
   2658 is included.  *Note Once-Only Headers::.
   2659 
   2660    Macro definitions can vary between compilations for several reasons.
   2661 Here are some samples.
   2662 
   2663    * Some macros are predefined on each kind of machine (*note
   2664      System-specific Predefined Macros::).  This allows you to provide
   2665      code specially tuned for a particular machine.
   2666 
   2667    * System header files define more macros, associated with the
   2668      features they implement.  You can test these macros with
   2669      conditionals to avoid using a system feature on a machine where it
   2670      is not implemented.
   2671 
   2672    * Macros can be defined or undefined with the `-D' and `-U' command
   2673      line options when you compile the program.  You can arrange to
   2674      compile the same source file into two different programs by
   2675      choosing a macro name to specify which program you want, writing
   2676      conditionals to test whether or how this macro is defined, and
   2677      then controlling the state of the macro with command line options,
   2678      perhaps set in the Makefile.  *Note Invocation::.
   2679 
   2680    * Your program might have a special header file (often called
   2681      `config.h') that is adjusted when the program is compiled.  It can
   2682      define or not define macros depending on the features of the
   2683      system and the desired capabilities of the program.  The
   2684      adjustment can be automated by a tool such as `autoconf', or done
   2685      by hand.
   2686 
   2687 
   2688 File: cpp.info,  Node: If,  Next: Defined,  Prev: Ifdef,  Up: Conditional Syntax
   2689 
   2690 4.2.2 If
   2691 --------
   2692 
   2693 The `#if' directive allows you to test the value of an arithmetic
   2694 expression, rather than the mere existence of one macro.  Its syntax is
   2695 
   2696      #if EXPRESSION
   2697 
   2698      CONTROLLED TEXT
   2699 
   2700      #endif /* EXPRESSION */
   2701 
   2702    EXPRESSION is a C expression of integer type, subject to stringent
   2703 restrictions.  It may contain
   2704 
   2705    * Integer constants.
   2706 
   2707    * Character constants, which are interpreted as they would be in
   2708      normal code.
   2709 
   2710    * Arithmetic operators for addition, subtraction, multiplication,
   2711      division, bitwise operations, shifts, comparisons, and logical
   2712      operations (`&&' and `||').  The latter two obey the usual
   2713      short-circuiting rules of standard C.
   2714 
   2715    * Macros.  All macros in the expression are expanded before actual
   2716      computation of the expression's value begins.
   2717 
   2718    * Uses of the `defined' operator, which lets you check whether macros
   2719      are defined in the middle of an `#if'.
   2720 
   2721    * Identifiers that are not macros, which are all considered to be the
   2722      number zero.  This allows you to write `#if MACRO' instead of
   2723      `#ifdef MACRO', if you know that MACRO, when defined, will always
   2724      have a nonzero value.  Function-like macros used without their
   2725      function call parentheses are also treated as zero.
   2726 
   2727      In some contexts this shortcut is undesirable.  The `-Wundef'
   2728      option causes GCC to warn whenever it encounters an identifier
   2729      which is not a macro in an `#if'.
   2730 
   2731    The preprocessor does not know anything about types in the language.
   2732 Therefore, `sizeof' operators are not recognized in `#if', and neither
   2733 are `enum' constants.  They will be taken as identifiers which are not
   2734 macros, and replaced by zero.  In the case of `sizeof', this is likely
   2735 to cause the expression to be invalid.
   2736 
   2737    The preprocessor calculates the value of EXPRESSION.  It carries out
   2738 all calculations in the widest integer type known to the compiler; on
   2739 most machines supported by GCC this is 64 bits.  This is not the same
   2740 rule as the compiler uses to calculate the value of a constant
   2741 expression, and may give different results in some cases.  If the value
   2742 comes out to be nonzero, the `#if' succeeds and the CONTROLLED TEXT is
   2743 included; otherwise it is skipped.
   2744 
   2745 
   2746 File: cpp.info,  Node: Defined,  Next: Else,  Prev: If,  Up: Conditional Syntax
   2747 
   2748 4.2.3 Defined
   2749 -------------
   2750 
   2751 The special operator `defined' is used in `#if' and `#elif' expressions
   2752 to test whether a certain name is defined as a macro.  `defined NAME'
   2753 and `defined (NAME)' are both expressions whose value is 1 if NAME is
   2754 defined as a macro at the current point in the program, and 0
   2755 otherwise.  Thus,  `#if defined MACRO' is precisely equivalent to
   2756 `#ifdef MACRO'.
   2757 
   2758    `defined' is useful when you wish to test more than one macro for
   2759 existence at once.  For example,
   2760 
   2761      #if defined (__vax__) || defined (__ns16000__)
   2762 
   2763 would succeed if either of the names `__vax__' or `__ns16000__' is
   2764 defined as a macro.
   2765 
   2766    Conditionals written like this:
   2767 
   2768      #if defined BUFSIZE && BUFSIZE >= 1024
   2769 
   2770 can generally be simplified to just `#if BUFSIZE >= 1024', since if
   2771 `BUFSIZE' is not defined, it will be interpreted as having the value
   2772 zero.
   2773 
   2774    If the `defined' operator appears as a result of a macro expansion,
   2775 the C standard says the behavior is undefined.  GNU cpp treats it as a
   2776 genuine `defined' operator and evaluates it normally.  It will warn
   2777 wherever your code uses this feature if you use the command-line option
   2778 `-pedantic', since other compilers may handle it differently.
   2779 
   2780 
   2781 File: cpp.info,  Node: Else,  Next: Elif,  Prev: Defined,  Up: Conditional Syntax
   2782 
   2783 4.2.4 Else
   2784 ----------
   2785 
   2786 The `#else' directive can be added to a conditional to provide
   2787 alternative text to be used if the condition fails.  This is what it
   2788 looks like:
   2789 
   2790      #if EXPRESSION
   2791      TEXT-IF-TRUE
   2792      #else /* Not EXPRESSION */
   2793      TEXT-IF-FALSE
   2794      #endif /* Not EXPRESSION */
   2795 
   2796 If EXPRESSION is nonzero, the TEXT-IF-TRUE is included and the
   2797 TEXT-IF-FALSE is skipped.  If EXPRESSION is zero, the opposite happens.
   2798 
   2799    You can use `#else' with `#ifdef' and `#ifndef', too.
   2800 
   2801 
   2802 File: cpp.info,  Node: Elif,  Prev: Else,  Up: Conditional Syntax
   2803 
   2804 4.2.5 Elif
   2805 ----------
   2806 
   2807 One common case of nested conditionals is used to check for more than
   2808 two possible alternatives.  For example, you might have
   2809 
   2810      #if X == 1
   2811      ...
   2812      #else /* X != 1 */
   2813      #if X == 2
   2814      ...
   2815      #else /* X != 2 */
   2816      ...
   2817      #endif /* X != 2 */
   2818      #endif /* X != 1 */
   2819 
   2820    Another conditional directive, `#elif', allows this to be
   2821 abbreviated as follows:
   2822 
   2823      #if X == 1
   2824      ...
   2825      #elif X == 2
   2826      ...
   2827      #else /* X != 2 and X != 1*/
   2828      ...
   2829      #endif /* X != 2 and X != 1*/
   2830 
   2831    `#elif' stands for "else if".  Like `#else', it goes in the middle
   2832 of a conditional group and subdivides it; it does not require a
   2833 matching `#endif' of its own.  Like `#if', the `#elif' directive
   2834 includes an expression to be tested.  The text following the `#elif' is
   2835 processed only if the original `#if'-condition failed and the `#elif'
   2836 condition succeeds.
   2837 
   2838    More than one `#elif' can go in the same conditional group.  Then
   2839 the text after each `#elif' is processed only if the `#elif' condition
   2840 succeeds after the original `#if' and all previous `#elif' directives
   2841 within it have failed.
   2842 
   2843    `#else' is allowed after any number of `#elif' directives, but
   2844 `#elif' may not follow `#else'.
   2845 
   2846 
   2847 File: cpp.info,  Node: Deleted Code,  Prev: Conditional Syntax,  Up: Conditionals
   2848 
   2849 4.3 Deleted Code
   2850 ================
   2851 
   2852 If you replace or delete a part of the program but want to keep the old
   2853 code around for future reference, you often cannot simply comment it
   2854 out.  Block comments do not nest, so the first comment inside the old
   2855 code will end the commenting-out.  The probable result is a flood of
   2856 syntax errors.
   2857 
   2858    One way to avoid this problem is to use an always-false conditional
   2859 instead.  For instance, put `#if 0' before the deleted code and
   2860 `#endif' after it.  This works even if the code being turned off
   2861 contains conditionals, but they must be entire conditionals (balanced
   2862 `#if' and `#endif').
   2863 
   2864    Some people use `#ifdef notdef' instead.  This is risky, because
   2865 `notdef' might be accidentally defined as a macro, and then the
   2866 conditional would succeed.  `#if 0' can be counted on to fail.
   2867 
   2868    Do not use `#if 0' for comments which are not C code.  Use a real
   2869 comment, instead.  The interior of `#if 0' must consist of complete
   2870 tokens; in particular, single-quote characters must balance.  Comments
   2871 often contain unbalanced single-quote characters (known in English as
   2872 apostrophes).  These confuse `#if 0'.  They don't confuse `/*'.
   2873 
   2874 
   2875 File: cpp.info,  Node: Diagnostics,  Next: Line Control,  Prev: Conditionals,  Up: Top
   2876 
   2877 5 Diagnostics
   2878 *************
   2879 
   2880 The directive `#error' causes the preprocessor to report a fatal error.
   2881 The tokens forming the rest of the line following `#error' are used as
   2882 the error message.
   2883 
   2884    You would use `#error' inside of a conditional that detects a
   2885 combination of parameters which you know the program does not properly
   2886 support.  For example, if you know that the program will not run
   2887 properly on a VAX, you might write
   2888 
   2889      #ifdef __vax__
   2890      #error "Won't work on VAXen.  See comments at get_last_object."
   2891      #endif
   2892 
   2893    If you have several configuration parameters that must be set up by
   2894 the installation in a consistent way, you can use conditionals to detect
   2895 an inconsistency and report it with `#error'.  For example,
   2896 
   2897      #if !defined(UNALIGNED_INT_ASM_OP) && defined(DWARF2_DEBUGGING_INFO)
   2898      #error "DWARF2_DEBUGGING_INFO requires UNALIGNED_INT_ASM_OP."
   2899      #endif
   2900 
   2901    The directive `#warning' is like `#error', but causes the
   2902 preprocessor to issue a warning and continue preprocessing.  The tokens
   2903 following `#warning' are used as the warning message.
   2904 
   2905    You might use `#warning' in obsolete header files, with a message
   2906 directing the user to the header file which should be used instead.
   2907 
   2908    Neither `#error' nor `#warning' macro-expands its argument.
   2909 Internal whitespace sequences are each replaced with a single space.
   2910 The line must consist of complete tokens.  It is wisest to make the
   2911 argument of these directives be a single string constant; this avoids
   2912 problems with apostrophes and the like.
   2913 
   2914 
   2915 File: cpp.info,  Node: Line Control,  Next: Pragmas,  Prev: Diagnostics,  Up: Top
   2916 
   2917 6 Line Control
   2918 **************
   2919 
   2920 The C preprocessor informs the C compiler of the location in your source
   2921 code where each token came from.  Presently, this is just the file name
   2922 and line number.  All the tokens resulting from macro expansion are
   2923 reported as having appeared on the line of the source file where the
   2924 outermost macro was used.  We intend to be more accurate in the future.
   2925 
   2926    If you write a program which generates source code, such as the
   2927 `bison' parser generator, you may want to adjust the preprocessor's
   2928 notion of the current file name and line number by hand.  Parts of the
   2929 output from `bison' are generated from scratch, other parts come from a
   2930 standard parser file.  The rest are copied verbatim from `bison''s
   2931 input.  You would like compiler error messages and symbolic debuggers
   2932 to be able to refer to `bison''s input file.
   2933 
   2934    `bison' or any such program can arrange this by writing `#line'
   2935 directives into the output file.  `#line' is a directive that specifies
   2936 the original line number and source file name for subsequent input in
   2937 the current preprocessor input file.  `#line' has three variants:
   2938 
   2939 `#line LINENUM'
   2940      LINENUM is a non-negative decimal integer constant.  It specifies
   2941      the line number which should be reported for the following line of
   2942      input.  Subsequent lines are counted from LINENUM.
   2943 
   2944 `#line LINENUM FILENAME'
   2945      LINENUM is the same as for the first form, and has the same
   2946      effect.  In addition, FILENAME is a string constant.  The
   2947      following line and all subsequent lines are reported to come from
   2948      the file it specifies, until something else happens to change that.
   2949      FILENAME is interpreted according to the normal rules for a string
   2950      constant: backslash escapes are interpreted.  This is different
   2951      from `#include'.
   2952 
   2953      Previous versions of CPP did not interpret escapes in `#line'; we
   2954      have changed it because the standard requires they be interpreted,
   2955      and most other compilers do.
   2956 
   2957 `#line ANYTHING ELSE'
   2958      ANYTHING ELSE is checked for macro calls, which are expanded.  The
   2959      result should match one of the above two forms.
   2960 
   2961    `#line' directives alter the results of the `__FILE__' and
   2962 `__LINE__' predefined macros from that point on.  *Note Standard
   2963 Predefined Macros::.  They do not have any effect on `#include''s idea
   2964 of the directory containing the current file.  This is a change from
   2965 GCC 2.95.  Previously, a file reading
   2966 
   2967      #line 1 "../src/gram.y"
   2968      #include "gram.h"
   2969 
   2970    would search for `gram.h' in `../src', then the `-I' chain; the
   2971 directory containing the physical source file would not be searched.
   2972 In GCC 3.0 and later, the `#include' is not affected by the presence of
   2973 a `#line' referring to a different directory.
   2974 
   2975    We made this change because the old behavior caused problems when
   2976 generated source files were transported between machines.  For instance,
   2977 it is common practice to ship generated parsers with a source release,
   2978 so that people building the distribution do not need to have yacc or
   2979 Bison installed.  These files frequently have `#line' directives
   2980 referring to the directory tree of the system where the distribution was
   2981 created.  If GCC tries to search for headers in those directories, the
   2982 build is likely to fail.
   2983 
   2984    The new behavior can cause failures too, if the generated file is not
   2985 in the same directory as its source and it attempts to include a header
   2986 which would be visible searching from the directory containing the
   2987 source file.  However, this problem is easily solved with an additional
   2988 `-I' switch on the command line.  The failures caused by the old
   2989 semantics could sometimes be corrected only by editing the generated
   2990 files, which is difficult and error-prone.
   2991 
   2992 
   2993 File: cpp.info,  Node: Pragmas,  Next: Other Directives,  Prev: Line Control,  Up: Top
   2994 
   2995 7 Pragmas
   2996 *********
   2997 
   2998 The `#pragma' directive is the method specified by the C standard for
   2999 providing additional information to the compiler, beyond what is
   3000 conveyed in the language itself.  Three forms of this directive
   3001 (commonly known as "pragmas") are specified by the 1999 C standard.  A
   3002 C compiler is free to attach any meaning it likes to other pragmas.
   3003 
   3004    GCC has historically preferred to use extensions to the syntax of the
   3005 language, such as `__attribute__', for this purpose.  However, GCC does
   3006 define a few pragmas of its own.  These mostly have effects on the
   3007 entire translation unit or source file.
   3008 
   3009    In GCC version 3, all GNU-defined, supported pragmas have been given
   3010 a `GCC' prefix.  This is in line with the `STDC' prefix on all pragmas
   3011 defined by C99.  For backward compatibility, pragmas which were
   3012 recognized by previous versions are still recognized without the `GCC'
   3013 prefix, but that usage is deprecated.  Some older pragmas are
   3014 deprecated in their entirety.  They are not recognized with the `GCC'
   3015 prefix.  *Note Obsolete Features::.
   3016 
   3017    C99 introduces the `_Pragma' operator.  This feature addresses a
   3018 major problem with `#pragma': being a directive, it cannot be produced
   3019 as the result of macro expansion.  `_Pragma' is an operator, much like
   3020 `sizeof' or `defined', and can be embedded in a macro.
   3021 
   3022    Its syntax is `_Pragma (STRING-LITERAL)', where STRING-LITERAL can
   3023 be either a normal or wide-character string literal.  It is
   3024 destringized, by replacing all `\\' with a single `\' and all `\"' with
   3025 a `"'.  The result is then processed as if it had appeared as the right
   3026 hand side of a `#pragma' directive.  For example,
   3027 
   3028      _Pragma ("GCC dependency \"parse.y\"")
   3029 
   3030 has the same effect as `#pragma GCC dependency "parse.y"'.  The same
   3031 effect could be achieved using macros, for example
   3032 
   3033      #define DO_PRAGMA(x) _Pragma (#x)
   3034      DO_PRAGMA (GCC dependency "parse.y")
   3035 
   3036    The standard is unclear on where a `_Pragma' operator can appear.
   3037 The preprocessor does not accept it within a preprocessing conditional
   3038 directive like `#if'.  To be safe, you are probably best keeping it out
   3039 of directives other than `#define', and putting it on a line of its own.
   3040 
   3041    This manual documents the pragmas which are meaningful to the
   3042 preprocessor itself.  Other pragmas are meaningful to the C or C++
   3043 compilers.  They are documented in the GCC manual.
   3044 
   3045 `#pragma GCC dependency'
   3046      `#pragma GCC dependency' allows you to check the relative dates of
   3047      the current file and another file.  If the other file is more
   3048      recent than the current file, a warning is issued.  This is useful
   3049      if the current file is derived from the other file, and should be
   3050      regenerated.  The other file is searched for using the normal
   3051      include search path.  Optional trailing text can be used to give
   3052      more information in the warning message.
   3053 
   3054           #pragma GCC dependency "parse.y"
   3055           #pragma GCC dependency "/usr/include/time.h" rerun fixincludes
   3056 
   3057 `#pragma GCC poison'
   3058      Sometimes, there is an identifier that you want to remove
   3059      completely from your program, and make sure that it never creeps
   3060      back in.  To enforce this, you can "poison" the identifier with
   3061      this pragma.  `#pragma GCC poison' is followed by a list of
   3062      identifiers to poison.  If any of those identifiers appears
   3063      anywhere in the source after the directive, it is a hard error.
   3064      For example,
   3065 
   3066           #pragma GCC poison printf sprintf fprintf
   3067           sprintf(some_string, "hello");
   3068 
   3069      will produce an error.
   3070 
   3071      If a poisoned identifier appears as part of the expansion of a
   3072      macro which was defined before the identifier was poisoned, it
   3073      will _not_ cause an error.  This lets you poison an identifier
   3074      without worrying about system headers defining macros that use it.
   3075 
   3076      For example,
   3077 
   3078           #define strrchr rindex
   3079           #pragma GCC poison rindex
   3080           strrchr(some_string, 'h');
   3081 
   3082      will not produce an error.
   3083 
   3084 `#pragma GCC system_header'
   3085      This pragma takes no arguments.  It causes the rest of the code in
   3086      the current file to be treated as if it came from a system header.
   3087      *Note System Headers::.
   3088 
   3089 
   3090 
   3091 File: cpp.info,  Node: Other Directives,  Next: Preprocessor Output,  Prev: Pragmas,  Up: Top
   3092 
   3093 8 Other Directives
   3094 ******************
   3095 
   3096 The `#ident' directive takes one argument, a string constant.  On some
   3097 systems, that string constant is copied into a special segment of the
   3098 object file.  On other systems, the directive is ignored.  The `#sccs'
   3099 directive is a synonym for `#ident'.
   3100 
   3101    These directives are not part of the C standard, but they are not
   3102 official GNU extensions either.  What historical information we have
   3103 been able to find, suggests they originated with System V.
   3104 
   3105    The "null directive" consists of a `#' followed by a newline, with
   3106 only whitespace (including comments) in between.  A null directive is
   3107 understood as a preprocessing directive but has no effect on the
   3108 preprocessor output.  The primary significance of the existence of the
   3109 null directive is that an input line consisting of just a `#' will
   3110 produce no output, rather than a line of output containing just a `#'.
   3111 Supposedly some old C programs contain such lines.
   3112 
   3113 
   3114 File: cpp.info,  Node: Preprocessor Output,  Next: Traditional Mode,  Prev: Other Directives,  Up: Top
   3115 
   3116 9 Preprocessor Output
   3117 *********************
   3118 
   3119 When the C preprocessor is used with the C, C++, or Objective-C
   3120 compilers, it is integrated into the compiler and communicates a stream
   3121 of binary tokens directly to the compiler's parser.  However, it can
   3122 also be used in the more conventional standalone mode, where it produces
   3123 textual output.
   3124 
   3125    The output from the C preprocessor looks much like the input, except
   3126 that all preprocessing directive lines have been replaced with blank
   3127 lines and all comments with spaces.  Long runs of blank lines are
   3128 discarded.
   3129 
   3130    The ISO standard specifies that it is implementation defined whether
   3131 a preprocessor preserves whitespace between tokens, or replaces it with
   3132 e.g. a single space.  In GNU CPP, whitespace between tokens is collapsed
   3133 to become a single space, with the exception that the first token on a
   3134 non-directive line is preceded with sufficient spaces that it appears in
   3135 the same column in the preprocessed output that it appeared in the
   3136 original source file.  This is so the output is easy to read.  *Note
   3137 Differences from previous versions::.  CPP does not insert any
   3138 whitespace where there was none in the original source, except where
   3139 necessary to prevent an accidental token paste.
   3140 
   3141    Source file name and line number information is conveyed by lines of
   3142 the form
   3143 
   3144      # LINENUM FILENAME FLAGS
   3145 
   3146 These are called "linemarkers".  They are inserted as needed into the
   3147 output (but never within a string or character constant).  They mean
   3148 that the following line originated in file FILENAME at line LINENUM.
   3149 FILENAME will never contain any non-printing characters; they are
   3150 replaced with octal escape sequences.
   3151 
   3152    After the file name comes zero or more flags, which are `1', `2',
   3153 `3', or `4'.  If there are multiple flags, spaces separate them.  Here
   3154 is what the flags mean:
   3155 
   3156 `1'
   3157      This indicates the start of a new file.
   3158 
   3159 `2'
   3160      This indicates returning to a file (after having included another
   3161      file).
   3162 
   3163 `3'
   3164      This indicates that the following text comes from a system header
   3165      file, so certain warnings should be suppressed.
   3166 
   3167 `4'
   3168      This indicates that the following text should be treated as being
   3169      wrapped in an implicit `extern "C"' block.
   3170 
   3171    As an extension, the preprocessor accepts linemarkers in
   3172 non-assembler input files.  They are treated like the corresponding
   3173 `#line' directive, (*note Line Control::), except that trailing flags
   3174 are permitted, and are interpreted with the meanings described above.
   3175 If multiple flags are given, they must be in ascending order.
   3176 
   3177    Some directives may be duplicated in the output of the preprocessor.
   3178 These are `#ident' (always), `#pragma' (only if the preprocessor does
   3179 not handle the pragma itself), and `#define' and `#undef' (with certain
   3180 debugging options).  If this happens, the `#' of the directive will
   3181 always be in the first column, and there will be no space between the
   3182 `#' and the directive name.  If macro expansion happens to generate
   3183 tokens which might be mistaken for a duplicated directive, a space will
   3184 be inserted between the `#' and the directive name.
   3185 
   3186 
   3187 File: cpp.info,  Node: Traditional Mode,  Next: Implementation Details,  Prev: Preprocessor Output,  Up: Top
   3188 
   3189 10 Traditional Mode
   3190 *******************
   3191 
   3192 Traditional (pre-standard) C preprocessing is rather different from the
   3193 preprocessing specified by the standard.  When GCC is given the
   3194 `-traditional-cpp' option, it attempts to emulate a traditional
   3195 preprocessor.
   3196 
   3197    GCC versions 3.2 and later only support traditional mode semantics in
   3198 the preprocessor, and not in the compiler front ends.  This chapter
   3199 outlines the traditional preprocessor semantics we implemented.
   3200 
   3201    The implementation does not correspond precisely to the behavior of
   3202 earlier versions of GCC, nor to any true traditional preprocessor.
   3203 After all, inconsistencies among traditional implementations were a
   3204 major motivation for C standardization.  However, we intend that it
   3205 should be compatible with true traditional preprocessors in all ways
   3206 that actually matter.
   3207 
   3208 * Menu:
   3209 
   3210 * Traditional lexical analysis::
   3211 * Traditional macros::
   3212 * Traditional miscellany::
   3213 * Traditional warnings::
   3214 
   3215 
   3216 File: cpp.info,  Node: Traditional lexical analysis,  Next: Traditional macros,  Up: Traditional Mode
   3217 
   3218 10.1 Traditional lexical analysis
   3219 =================================
   3220 
   3221 The traditional preprocessor does not decompose its input into tokens
   3222 the same way a standards-conforming preprocessor does.  The input is
   3223 simply treated as a stream of text with minimal internal form.
   3224 
   3225    This implementation does not treat trigraphs (*note trigraphs::)
   3226 specially since they were an invention of the standards committee.  It
   3227 handles arbitrarily-positioned escaped newlines properly and splices
   3228 the lines as you would expect; many traditional preprocessors did not
   3229 do this.
   3230 
   3231    The form of horizontal whitespace in the input file is preserved in
   3232 the output.  In particular, hard tabs remain hard tabs.  This can be
   3233 useful if, for example, you are preprocessing a Makefile.
   3234 
   3235    Traditional CPP only recognizes C-style block comments, and treats
   3236 the `/*' sequence as introducing a comment only if it lies outside
   3237 quoted text.  Quoted text is introduced by the usual single and double
   3238 quotes, and also by an initial `<' in a `#include' directive.
   3239 
   3240    Traditionally, comments are completely removed and are not replaced
   3241 with a space.  Since a traditional compiler does its own tokenization
   3242 of the output of the preprocessor, this means that comments can
   3243 effectively be used as token paste operators.  However, comments behave
   3244 like separators for text handled by the preprocessor itself, since it
   3245 doesn't re-lex its input.  For example, in
   3246 
   3247      #if foo/**/bar
   3248 
   3249 `foo' and `bar' are distinct identifiers and expanded separately if
   3250 they happen to be macros.  In other words, this directive is equivalent
   3251 to
   3252 
   3253      #if foo bar
   3254 
   3255 rather than
   3256 
   3257      #if foobar
   3258 
   3259    Generally speaking, in traditional mode an opening quote need not
   3260 have a matching closing quote.  In particular, a macro may be defined
   3261 with replacement text that contains an unmatched quote.  Of course, if
   3262 you attempt to compile preprocessed output containing an unmatched quote
   3263 you will get a syntax error.
   3264 
   3265    However, all preprocessing directives other than `#define' require
   3266 matching quotes.  For example:
   3267 
   3268      #define m This macro's fine and has an unmatched quote
   3269      "/* This is not a comment.  */
   3270      /* This is a comment.  The following #include directive
   3271         is ill-formed.  */
   3272      #include <stdio.h
   3273 
   3274    Just as for the ISO preprocessor, what would be a closing quote can
   3275 be escaped with a backslash to prevent the quoted text from closing.
   3276 
   3277 
   3278 File: cpp.info,  Node: Traditional macros,  Next: Traditional miscellany,  Prev: Traditional lexical analysis,  Up: Traditional Mode
   3279 
   3280 10.2 Traditional macros
   3281 =======================
   3282 
   3283 The major difference between traditional and ISO macros is that the
   3284 former expand to text rather than to a token sequence.  CPP removes all
   3285 leading and trailing horizontal whitespace from a macro's replacement
   3286 text before storing it, but preserves the form of internal whitespace.
   3287 
   3288    One consequence is that it is legitimate for the replacement text to
   3289 contain an unmatched quote (*note Traditional lexical analysis::).  An
   3290 unclosed string or character constant continues into the text following
   3291 the macro call.  Similarly, the text at the end of a macro's expansion
   3292 can run together with the text after the macro invocation to produce a
   3293 single token.
   3294 
   3295    Normally comments are removed from the replacement text after the
   3296 macro is expanded, but if the `-CC' option is passed on the command
   3297 line comments are preserved.  (In fact, the current implementation
   3298 removes comments even before saving the macro replacement text, but it
   3299 careful to do it in such a way that the observed effect is identical
   3300 even in the function-like macro case.)
   3301 
   3302    The ISO stringification operator `#' and token paste operator `##'
   3303 have no special meaning.  As explained later, an effect similar to
   3304 these operators can be obtained in a different way.  Macro names that
   3305 are embedded in quotes, either from the main file or after macro
   3306 replacement, do not expand.
   3307 
   3308    CPP replaces an unquoted object-like macro name with its replacement
   3309 text, and then rescans it for further macros to replace.  Unlike
   3310 standard macro expansion, traditional macro expansion has no provision
   3311 to prevent recursion.  If an object-like macro appears unquoted in its
   3312 replacement text, it will be replaced again during the rescan pass, and
   3313 so on _ad infinitum_.  GCC detects when it is expanding recursive
   3314 macros, emits an error message, and continues after the offending macro
   3315 invocation.
   3316 
   3317      #define PLUS +
   3318      #define INC(x) PLUS+x
   3319      INC(foo);
   3320           ==> ++foo;
   3321 
   3322    Function-like macros are similar in form but quite different in
   3323 behavior to their ISO counterparts.  Their arguments are contained
   3324 within parentheses, are comma-separated, and can cross physical lines.
   3325 Commas within nested parentheses are not treated as argument
   3326 separators.  Similarly, a quote in an argument cannot be left unclosed;
   3327 a following comma or parenthesis that comes before the closing quote is
   3328 treated like any other character.  There is no facility for handling
   3329 variadic macros.
   3330 
   3331    This implementation removes all comments from macro arguments, unless
   3332 the `-C' option is given.  The form of all other horizontal whitespace
   3333 in arguments is preserved, including leading and trailing whitespace.
   3334 In particular
   3335 
   3336      f( )
   3337 
   3338 is treated as an invocation of the macro `f' with a single argument
   3339 consisting of a single space.  If you want to invoke a function-like
   3340 macro that takes no arguments, you must not leave any whitespace
   3341 between the parentheses.
   3342 
   3343    If a macro argument crosses a new line, the new line is replaced with
   3344 a space when forming the argument.  If the previous line contained an
   3345 unterminated quote, the following line inherits the quoted state.
   3346 
   3347    Traditional preprocessors replace parameters in the replacement text
   3348 with their arguments regardless of whether the parameters are within
   3349 quotes or not.  This provides a way to stringize arguments.  For example
   3350 
   3351      #define str(x) "x"
   3352      str(/* A comment */some text )
   3353           ==> "some text "
   3354 
   3355 Note that the comment is removed, but that the trailing space is
   3356 preserved.  Here is an example of using a comment to effect token
   3357 pasting.
   3358 
   3359      #define suffix(x) foo_/**/x
   3360      suffix(bar)
   3361           ==> foo_bar
   3362 
   3363 
   3364 File: cpp.info,  Node: Traditional miscellany,  Next: Traditional warnings,  Prev: Traditional macros,  Up: Traditional Mode
   3365 
   3366 10.3 Traditional miscellany
   3367 ===========================
   3368 
   3369 Here are some things to be aware of when using the traditional
   3370 preprocessor.
   3371 
   3372    * Preprocessing directives are recognized only when their leading
   3373      `#' appears in the first column.  There can be no whitespace
   3374      between the beginning of the line and the `#', but whitespace can
   3375      follow the `#'.
   3376 
   3377    * A true traditional C preprocessor does not recognize `#error' or
   3378      `#pragma', and may not recognize `#elif'.  CPP supports all the
   3379      directives in traditional mode that it supports in ISO mode,
   3380      including extensions, with the exception that the effects of
   3381      `#pragma GCC poison' are undefined.
   3382 
   3383    * __STDC__ is not defined.
   3384 
   3385    * If you use digraphs the behavior is undefined.
   3386 
   3387    * If a line that looks like a directive appears within macro
   3388      arguments, the behavior is undefined.
   3389 
   3390 
   3391 
   3392 File: cpp.info,  Node: Traditional warnings,  Prev: Traditional miscellany,  Up: Traditional Mode
   3393 
   3394 10.4 Traditional warnings
   3395 =========================
   3396 
   3397 You can request warnings about features that did not exist, or worked
   3398 differently, in traditional C with the `-Wtraditional' option.  GCC
   3399 does not warn about features of ISO C which you must use when you are
   3400 using a conforming compiler, such as the `#' and `##' operators.
   3401 
   3402    Presently `-Wtraditional' warns about:
   3403 
   3404    * Macro parameters that appear within string literals in the macro
   3405      body.  In traditional C macro replacement takes place within
   3406      string literals, but does not in ISO C.
   3407 
   3408    * In traditional C, some preprocessor directives did not exist.
   3409      Traditional preprocessors would only consider a line to be a
   3410      directive if the `#' appeared in column 1 on the line.  Therefore
   3411      `-Wtraditional' warns about directives that traditional C
   3412      understands but would ignore because the `#' does not appear as the
   3413      first character on the line.  It also suggests you hide directives
   3414      like `#pragma' not understood by traditional C by indenting them.
   3415      Some traditional implementations would not recognize `#elif', so it
   3416      suggests avoiding it altogether.
   3417 
   3418    * A function-like macro that appears without an argument list.  In
   3419      some traditional preprocessors this was an error.  In ISO C it
   3420      merely means that the macro is not expanded.
   3421 
   3422    * The unary plus operator.  This did not exist in traditional C.
   3423 
   3424    * The `U' and `LL' integer constant suffixes, which were not
   3425      available in traditional C.  (Traditional C does support the `L'
   3426      suffix for simple long integer constants.)  You are not warned
   3427      about uses of these suffixes in macros defined in system headers.
   3428      For instance, `UINT_MAX' may well be defined as `4294967295U', but
   3429      you will not be warned if you use `UINT_MAX'.
   3430 
   3431      You can usually avoid the warning, and the related warning about
   3432      constants which are so large that they are unsigned, by writing the
   3433      integer constant in question in hexadecimal, with no U suffix.
   3434      Take care, though, because this gives the wrong result in exotic
   3435      cases.
   3436 
   3437 
   3438 File: cpp.info,  Node: Implementation Details,  Next: Invocation,  Prev: Traditional Mode,  Up: Top
   3439 
   3440 11 Implementation Details
   3441 *************************
   3442 
   3443 Here we document details of how the preprocessor's implementation
   3444 affects its user-visible behavior.  You should try to avoid undue
   3445 reliance on behavior described here, as it is possible that it will
   3446 change subtly in future implementations.
   3447 
   3448    Also documented here are obsolete features and changes from previous
   3449 versions of CPP.
   3450 
   3451 * Menu:
   3452 
   3453 * Implementation-defined behavior::
   3454 * Implementation limits::
   3455 * Obsolete Features::
   3456 * Differences from previous versions::
   3457 
   3458 
   3459 File: cpp.info,  Node: Implementation-defined behavior,  Next: Implementation limits,  Up: Implementation Details
   3460 
   3461 11.1 Implementation-defined behavior
   3462 ====================================
   3463 
   3464 This is how CPP behaves in all the cases which the C standard describes
   3465 as "implementation-defined".  This term means that the implementation
   3466 is free to do what it likes, but must document its choice and stick to
   3467 it.
   3468 
   3469    * The mapping of physical source file multi-byte characters to the
   3470      execution character set.
   3471 
   3472      Currently, CPP requires its input to be ASCII or UTF-8.  The
   3473      execution character set may be controlled by the user, with the
   3474      `-fexec-charset' and `-fwide-exec-charset' options.
   3475 
   3476    * Identifier characters.
   3477 
   3478      The C and C++ standards allow identifiers to be composed of `_'
   3479      and the alphanumeric characters.  C++ and C99 also allow universal
   3480      character names, and C99 further permits implementation-defined
   3481      characters.  GCC currently only permits universal character names
   3482      if `-fextended-identifiers' is used, because the implementation of
   3483      universal character names in identifiers is experimental.
   3484 
   3485      GCC allows the `$' character in identifiers as an extension for
   3486      most targets.  This is true regardless of the `std=' switch, since
   3487      this extension cannot conflict with standards-conforming programs.
   3488      When preprocessing assembler, however, dollars are not identifier
   3489      characters by default.
   3490 
   3491      Currently the targets that by default do not permit `$' are AVR,
   3492      IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC targets for the AIX
   3493      and BeOS operating systems.
   3494 
   3495      You can override the default with `-fdollars-in-identifiers' or
   3496      `fno-dollars-in-identifiers'.  *Note fdollars-in-identifiers::.
   3497 
   3498    * Non-empty sequences of whitespace characters.
   3499 
   3500      In textual output, each whitespace sequence is collapsed to a
   3501      single space.  For aesthetic reasons, the first token on each
   3502      non-directive line of output is preceded with sufficient spaces
   3503      that it appears in the same column as it did in the original
   3504      source file.
   3505 
   3506    * The numeric value of character constants in preprocessor
   3507      expressions.
   3508 
   3509      The preprocessor and compiler interpret character constants in the
   3510      same way; i.e. escape sequences such as `\a' are given the values
   3511      they would have on the target machine.
   3512 
   3513      The compiler values a multi-character character constant a
   3514      character at a time, shifting the previous value left by the
   3515      number of bits per target character, and then or-ing in the
   3516      bit-pattern of the new character truncated to the width of a
   3517      target character.  The final bit-pattern is given type `int', and
   3518      is therefore signed, regardless of whether single characters are
   3519      signed or not (a slight change from versions 3.1 and earlier of
   3520      GCC).  If there are more characters in the constant than would fit
   3521      in the target `int' the compiler issues a warning, and the excess
   3522      leading characters are ignored.
   3523 
   3524      For example, `'ab'' for a target with an 8-bit `char' would be
   3525      interpreted as
   3526      `(int) ((unsigned char) 'a' * 256 + (unsigned char) 'b')', and
   3527      `'\234a'' as
   3528      `(int) ((unsigned char) '\234' * 256 + (unsigned char) 'a')'.
   3529 
   3530    * Source file inclusion.
   3531 
   3532      For a discussion on how the preprocessor locates header files,
   3533      *Note Include Operation::.
   3534 
   3535    * Interpretation of the filename resulting from a macro-expanded
   3536      `#include' directive.
   3537 
   3538      *Note Computed Includes::.
   3539 
   3540    * Treatment of a `#pragma' directive that after macro-expansion
   3541      results in a standard pragma.
   3542 
   3543      No macro expansion occurs on any `#pragma' directive line, so the
   3544      question does not arise.
   3545 
   3546      Note that GCC does not yet implement any of the standard pragmas.
   3547 
   3548 
   3549 
   3550 File: cpp.info,  Node: Implementation limits,  Next: Obsolete Features,  Prev: Implementation-defined behavior,  Up: Implementation Details
   3551 
   3552 11.2 Implementation limits
   3553 ==========================
   3554 
   3555 CPP has a small number of internal limits.  This section lists the
   3556 limits which the C standard requires to be no lower than some minimum,
   3557 and all the others known.  It is intended that there should be as few
   3558 limits as possible.  If you encounter an undocumented or inconvenient
   3559 limit, please report that as a bug.  *Note Reporting Bugs: (gcc)Bugs.
   3560 
   3561    Where we say something is limited "only by available memory", that
   3562 means that internal data structures impose no intrinsic limit, and space
   3563 is allocated with `malloc' or equivalent.  The actual limit will
   3564 therefore depend on many things, such as the size of other things
   3565 allocated by the compiler at the same time, the amount of memory
   3566 consumed by other processes on the same computer, etc.
   3567 
   3568    * Nesting levels of `#include' files.
   3569 
   3570      We impose an arbitrary limit of 200 levels, to avoid runaway
   3571      recursion.  The standard requires at least 15 levels.
   3572 
   3573    * Nesting levels of conditional inclusion.
   3574 
   3575      The C standard mandates this be at least 63.  CPP is limited only
   3576      by available memory.
   3577 
   3578    * Levels of parenthesized expressions within a full expression.
   3579 
   3580      The C standard requires this to be at least 63.  In preprocessor
   3581      conditional expressions, it is limited only by available memory.
   3582 
   3583    * Significant initial characters in an identifier or macro name.
   3584 
   3585      The preprocessor treats all characters as significant.  The C
   3586      standard requires only that the first 63 be significant.
   3587 
   3588    * Number of macros simultaneously defined in a single translation
   3589      unit.
   3590 
   3591      The standard requires at least 4095 be possible.  CPP is limited
   3592      only by available memory.
   3593 
   3594    * Number of parameters in a macro definition and arguments in a
   3595      macro call.
   3596 
   3597      We allow `USHRT_MAX', which is no smaller than 65,535.  The minimum
   3598      required by the standard is 127.
   3599 
   3600    * Number of characters on a logical source line.
   3601 
   3602      The C standard requires a minimum of 4096 be permitted.  CPP places
   3603      no limits on this, but you may get incorrect column numbers
   3604      reported in diagnostics for lines longer than 65,535 characters.
   3605 
   3606    * Maximum size of a source file.
   3607 
   3608      The standard does not specify any lower limit on the maximum size
   3609      of a source file.  GNU cpp maps files into memory, so it is
   3610      limited by the available address space.  This is generally at
   3611      least two gigabytes.  Depending on the operating system, the size
   3612      of physical memory may or may not be a limitation.
   3613 
   3614 
   3615 
   3616 File: cpp.info,  Node: Obsolete Features,  Next: Differences from previous versions,  Prev: Implementation limits,  Up: Implementation Details
   3617 
   3618 11.3 Obsolete Features
   3619 ======================
   3620 
   3621 CPP has a number of features which are present mainly for compatibility
   3622 with older programs.  We discourage their use in new code.  In some
   3623 cases, we plan to remove the feature in a future version of GCC.
   3624 
   3625 * Menu:
   3626 
   3627 * Assertions::
   3628 * Obsolete once-only headers::
   3629 
   3630 
   3631 File: cpp.info,  Node: Assertions,  Next: Obsolete once-only headers,  Up: Obsolete Features
   3632 
   3633 11.3.1 Assertions
   3634 -----------------
   3635 
   3636 "Assertions" are a deprecated alternative to macros in writing
   3637 conditionals to test what sort of computer or system the compiled
   3638 program will run on.  Assertions are usually predefined, but you can
   3639 define them with preprocessing directives or command-line options.
   3640 
   3641    Assertions were intended to provide a more systematic way to describe
   3642 the compiler's target system.  However, in practice they are just as
   3643 unpredictable as the system-specific predefined macros.  In addition,
   3644 they are not part of any standard, and only a few compilers support
   3645 them.  Therefore, the use of assertions is *less* portable than the use
   3646 of system-specific predefined macros.  We recommend you do not use them
   3647 at all.
   3648 
   3649    An assertion looks like this:
   3650 
   3651      #PREDICATE (ANSWER)
   3652 
   3653 PREDICATE must be a single identifier.  ANSWER can be any sequence of
   3654 tokens; all characters are significant except for leading and trailing
   3655 whitespace, and differences in internal whitespace sequences are
   3656 ignored.  (This is similar to the rules governing macro redefinition.)
   3657 Thus, `(x + y)' is different from `(x+y)' but equivalent to
   3658 `( x + y )'.  Parentheses do not nest inside an answer.
   3659 
   3660    To test an assertion, you write it in an `#if'.  For example, this
   3661 conditional succeeds if either `vax' or `ns16000' has been asserted as
   3662 an answer for `machine'.
   3663 
   3664      #if #machine (vax) || #machine (ns16000)
   3665 
   3666 You can test whether _any_ answer is asserted for a predicate by
   3667 omitting the answer in the conditional:
   3668 
   3669      #if #machine
   3670 
   3671    Assertions are made with the `#assert' directive.  Its sole argument
   3672 is the assertion to make, without the leading `#' that identifies
   3673 assertions in conditionals.
   3674 
   3675      #assert PREDICATE (ANSWER)
   3676 
   3677 You may make several assertions with the same predicate and different
   3678 answers.  Subsequent assertions do not override previous ones for the
   3679 same predicate.  All the answers for any given predicate are
   3680 simultaneously true.
   3681 
   3682    Assertions can be canceled with the `#unassert' directive.  It has
   3683 the same syntax as `#assert'.  In that form it cancels only the answer
   3684 which was specified on the `#unassert' line; other answers for that
   3685 predicate remain true.  You can cancel an entire predicate by leaving
   3686 out the answer:
   3687 
   3688      #unassert PREDICATE
   3689 
   3690 In either form, if no such assertion has been made, `#unassert' has no
   3691 effect.
   3692 
   3693    You can also make or cancel assertions using command line options.
   3694 *Note Invocation::.
   3695 
   3696 
   3697 File: cpp.info,  Node: Obsolete once-only headers,  Prev: Assertions,  Up: Obsolete Features
   3698 
   3699 11.3.2 Obsolete once-only headers
   3700 ---------------------------------
   3701 
   3702 CPP supports two more ways of indicating that a header file should be
   3703 read only once.  Neither one is as portable as a wrapper `#ifndef', and
   3704 we recommend you do not use them in new programs.
   3705 
   3706    In the Objective-C language, there is a variant of `#include' called
   3707 `#import' which includes a file, but does so at most once.  If you use
   3708 `#import' instead of `#include', then you don't need the conditionals
   3709 inside the header file to prevent multiple inclusion of the contents.
   3710 GCC permits the use of `#import' in C and C++ as well as Objective-C.
   3711 However, it is not in standard C or C++ and should therefore not be
   3712 used by portable programs.
   3713 
   3714    `#import' is not a well designed feature.  It requires the users of
   3715 a header file to know that it should only be included once.  It is much
   3716 better for the header file's implementor to write the file so that users
   3717 don't need to know this.  Using a wrapper `#ifndef' accomplishes this
   3718 goal.
   3719 
   3720    In the present implementation, a single use of `#import' will
   3721 prevent the file from ever being read again, by either `#import' or
   3722 `#include'.  You should not rely on this; do not use both `#import' and
   3723 `#include' to refer to the same header file.
   3724 
   3725    Another way to prevent a header file from being included more than
   3726 once is with the `#pragma once' directive.  If `#pragma once' is seen
   3727 when scanning a header file, that file will never be read again, no
   3728 matter what.
   3729 
   3730    `#pragma once' does not have the problems that `#import' does, but
   3731 it is not recognized by all preprocessors, so you cannot rely on it in
   3732 a portable program.
   3733 
   3734 
   3735 File: cpp.info,  Node: Differences from previous versions,  Prev: Obsolete Features,  Up: Implementation Details
   3736 
   3737 11.4 Differences from previous versions
   3738 =======================================
   3739 
   3740 This section details behavior which has changed from previous versions
   3741 of CPP.  We do not plan to change it again in the near future, but we
   3742 do not promise not to, either.
   3743 
   3744    The "previous versions" discussed here are 2.95 and before.  The
   3745 behavior of GCC 3.0 is mostly the same as the behavior of the widely
   3746 used 2.96 and 2.97 development snapshots.  Where there are differences,
   3747 they generally represent bugs in the snapshots.
   3748 
   3749    * -I- deprecated
   3750 
   3751      This option has been deprecated in 4.0.  `-iquote' is meant to
   3752      replace the need for this option.
   3753 
   3754    * Order of evaluation of `#' and `##' operators
   3755 
   3756      The standard does not specify the order of evaluation of a chain of
   3757      `##' operators, nor whether `#' is evaluated before, after, or at
   3758      the same time as `##'.  You should therefore not write any code
   3759      which depends on any specific ordering.  It is possible to
   3760      guarantee an ordering, if you need one, by suitable use of nested
   3761      macros.
   3762 
   3763      An example of where this might matter is pasting the arguments `1',
   3764      `e' and `-2'.  This would be fine for left-to-right pasting, but
   3765      right-to-left pasting would produce an invalid token `e-2'.
   3766 
   3767      GCC 3.0 evaluates `#' and `##' at the same time and strictly left
   3768      to right.  Older versions evaluated all `#' operators first, then
   3769      all `##' operators, in an unreliable order.
   3770 
   3771    * The form of whitespace between tokens in preprocessor output
   3772 
   3773      *Note Preprocessor Output::, for the current textual format.  This
   3774      is also the format used by stringification.  Normally, the
   3775      preprocessor communicates tokens directly to the compiler's
   3776      parser, and whitespace does not come up at all.
   3777 
   3778      Older versions of GCC preserved all whitespace provided by the
   3779      user and inserted lots more whitespace of their own, because they
   3780      could not accurately predict when extra spaces were needed to
   3781      prevent accidental token pasting.
   3782 
   3783    * Optional argument when invoking rest argument macros
   3784 
   3785      As an extension, GCC permits you to omit the variable arguments
   3786      entirely when you use a variable argument macro.  This is
   3787      forbidden by the 1999 C standard, and will provoke a pedantic
   3788      warning with GCC 3.0.  Previous versions accepted it silently.
   3789 
   3790    * `##' swallowing preceding text in rest argument macros
   3791 
   3792      Formerly, in a macro expansion, if `##' appeared before a variable
   3793      arguments parameter, and the set of tokens specified for that
   3794      argument in the macro invocation was empty, previous versions of
   3795      CPP would back up and remove the preceding sequence of
   3796      non-whitespace characters (*not* the preceding token).  This
   3797      extension is in direct conflict with the 1999 C standard and has
   3798      been drastically pared back.
   3799 
   3800      In the current version of the preprocessor, if `##' appears between
   3801      a comma and a variable arguments parameter, and the variable
   3802      argument is omitted entirely, the comma will be removed from the
   3803      expansion.  If the variable argument is empty, or the token before
   3804      `##' is not a comma, then `##' behaves as a normal token paste.
   3805 
   3806    * `#line' and `#include'
   3807 
   3808      The `#line' directive used to change GCC's notion of the
   3809      "directory containing the current file", used by `#include' with a
   3810      double-quoted header file name.  In 3.0 and later, it does not.
   3811      *Note Line Control::, for further explanation.
   3812 
   3813    * Syntax of `#line'
   3814 
   3815      In GCC 2.95 and previous, the string constant argument to `#line'
   3816      was treated the same way as the argument to `#include': backslash
   3817      escapes were not honored, and the string ended at the second `"'.
   3818      This is not compliant with the C standard.  In GCC 3.0, an attempt
   3819      was made to correct the behavior, so that the string was treated
   3820      as a real string constant, but it turned out to be buggy.  In 3.1,
   3821      the bugs have been fixed.  (We are not fixing the bugs in 3.0
   3822      because they affect relatively few people and the fix is quite
   3823      invasive.)
   3824 
   3825 
   3826 
   3827 File: cpp.info,  Node: Invocation,  Next: Environment Variables,  Prev: Implementation Details,  Up: Top
   3828 
   3829 12 Invocation
   3830 *************
   3831 
   3832 Most often when you use the C preprocessor you will not have to invoke
   3833 it explicitly: the C compiler will do so automatically.  However, the
   3834 preprocessor is sometimes useful on its own.  All the options listed
   3835 here are also acceptable to the C compiler and have the same meaning,
   3836 except that the C compiler has different rules for specifying the output
   3837 file.
   3838 
   3839    _Note:_ Whether you use the preprocessor by way of `gcc' or `cpp',
   3840 the "compiler driver" is run first.  This program's purpose is to
   3841 translate your command into invocations of the programs that do the
   3842 actual work.  Their command line interfaces are similar but not
   3843 identical to the documented interface, and may change without notice.
   3844 
   3845    The C preprocessor expects two file names as arguments, INFILE and
   3846 OUTFILE.  The preprocessor reads INFILE together with any other files
   3847 it specifies with `#include'.  All the output generated by the combined
   3848 input files is written in OUTFILE.
   3849 
   3850    Either INFILE or OUTFILE may be `-', which as INFILE means to read
   3851 from standard input and as OUTFILE means to write to standard output.
   3852 Also, if either file is omitted, it means the same as if `-' had been
   3853 specified for that file.
   3854 
   3855    Unless otherwise noted, or the option ends in `=', all options which
   3856 take an argument may have that argument appear either immediately after
   3857 the option, or with a space between option and argument: `-Ifoo' and
   3858 `-I foo' have the same effect.
   3859 
   3860    Many options have multi-letter names; therefore multiple
   3861 single-letter options may _not_ be grouped: `-dM' is very different from
   3862 `-d -M'.
   3863 
   3864 `-D NAME'
   3865      Predefine NAME as a macro, with definition `1'.
   3866 
   3867 `-D NAME=DEFINITION'
   3868      The contents of DEFINITION are tokenized and processed as if they
   3869      appeared during translation phase three in a `#define' directive.
   3870      In particular, the definition will be truncated by embedded
   3871      newline characters.
   3872 
   3873      If you are invoking the preprocessor from a shell or shell-like
   3874      program you may need to use the shell's quoting syntax to protect
   3875      characters such as spaces that have a meaning in the shell syntax.
   3876 
   3877      If you wish to define a function-like macro on the command line,
   3878      write its argument list with surrounding parentheses before the
   3879      equals sign (if any).  Parentheses are meaningful to most shells,
   3880      so you will need to quote the option.  With `sh' and `csh',
   3881      `-D'NAME(ARGS...)=DEFINITION'' works.
   3882 
   3883      `-D' and `-U' options are processed in the order they are given on
   3884      the command line.  All `-imacros FILE' and `-include FILE' options
   3885      are processed after all `-D' and `-U' options.
   3886 
   3887 `-U NAME'
   3888      Cancel any previous definition of NAME, either built in or
   3889      provided with a `-D' option.
   3890 
   3891 `-undef'
   3892      Do not predefine any system-specific or GCC-specific macros.  The
   3893      standard predefined macros remain defined.  *Note Standard
   3894      Predefined Macros::.
   3895 
   3896 `-I DIR'
   3897      Add the directory DIR to the list of directories to be searched
   3898      for header files.  *Note Search Path::.  Directories named by `-I'
   3899      are searched before the standard system include directories.  If
   3900      the directory DIR is a standard system include directory, the
   3901      option is ignored to ensure that the default search order for
   3902      system directories and the special treatment of system headers are
   3903      not defeated (*note System Headers::) .
   3904 
   3905 `-o FILE'
   3906      Write output to FILE.  This is the same as specifying FILE as the
   3907      second non-option argument to `cpp'.  `gcc' has a different
   3908      interpretation of a second non-option argument, so you must use
   3909      `-o' to specify the output file.
   3910 
   3911 `-Wall'
   3912      Turns on all optional warnings which are desirable for normal code.
   3913      At present this is `-Wcomment', `-Wtrigraphs', `-Wmultichar' and a
   3914      warning about integer promotion causing a change of sign in `#if'
   3915      expressions.  Note that many of the preprocessor's warnings are on
   3916      by default and have no options to control them.
   3917 
   3918 `-Wcomment'
   3919 `-Wcomments'
   3920      Warn whenever a comment-start sequence `/*' appears in a `/*'
   3921      comment, or whenever a backslash-newline appears in a `//' comment.
   3922      (Both forms have the same effect.)
   3923 
   3924 `-Wtrigraphs'
   3925      Most trigraphs in comments cannot affect the meaning of the
   3926      program.  However, a trigraph that would form an escaped newline
   3927      (`??/' at the end of a line) can, by changing where the comment
   3928      begins or ends.  Therefore, only trigraphs that would form escaped
   3929      newlines produce warnings inside a comment.
   3930 
   3931      This option is implied by `-Wall'.  If `-Wall' is not given, this
   3932      option is still enabled unless trigraphs are enabled.  To get
   3933      trigraph conversion without warnings, but get the other `-Wall'
   3934      warnings, use `-trigraphs -Wall -Wno-trigraphs'.
   3935 
   3936 `-Wtraditional'
   3937      Warn about certain constructs that behave differently in
   3938      traditional and ISO C.  Also warn about ISO C constructs that have
   3939      no traditional C equivalent, and problematic constructs which
   3940      should be avoided.  *Note Traditional Mode::.
   3941 
   3942 `-Wimport'
   3943      Warn the first time `#import' is used.
   3944 
   3945 `-Wundef'
   3946      Warn whenever an identifier which is not a macro is encountered in
   3947      an `#if' directive, outside of `defined'.  Such identifiers are
   3948      replaced with zero.
   3949 
   3950 `-Wunused-macros'
   3951      Warn about macros defined in the main file that are unused.  A
   3952      macro is "used" if it is expanded or tested for existence at least
   3953      once.  The preprocessor will also warn if the macro has not been
   3954      used at the time it is redefined or undefined.
   3955 
   3956      Built-in macros, macros defined on the command line, and macros
   3957      defined in include files are not warned about.
   3958 
   3959      _Note:_ If a macro is actually used, but only used in skipped
   3960      conditional blocks, then CPP will report it as unused.  To avoid
   3961      the warning in such a case, you might improve the scope of the
   3962      macro's definition by, for example, moving it into the first
   3963      skipped block.  Alternatively, you could provide a dummy use with
   3964      something like:
   3965 
   3966           #if defined the_macro_causing_the_warning
   3967           #endif
   3968 
   3969 `-Wendif-labels'
   3970      Warn whenever an `#else' or an `#endif' are followed by text.
   3971      This usually happens in code of the form
   3972 
   3973           #if FOO
   3974           ...
   3975           #else FOO
   3976           ...
   3977           #endif FOO
   3978 
   3979      The second and third `FOO' should be in comments, but often are not
   3980      in older programs.  This warning is on by default.
   3981 
   3982 `-Werror'
   3983      Make all warnings into hard errors.  Source code which triggers
   3984      warnings will be rejected.
   3985 
   3986 `-Wsystem-headers'
   3987      Issue warnings for code in system headers.  These are normally
   3988      unhelpful in finding bugs in your own code, therefore suppressed.
   3989      If you are responsible for the system library, you may want to see
   3990      them.
   3991 
   3992 `-w'
   3993      Suppress all warnings, including those which GNU CPP issues by
   3994      default.
   3995 
   3996 `-pedantic'
   3997      Issue all the mandatory diagnostics listed in the C standard.
   3998      Some of them are left out by default, since they trigger
   3999      frequently on harmless code.
   4000 
   4001 `-pedantic-errors'
   4002      Issue all the mandatory diagnostics, and make all mandatory
   4003      diagnostics into errors.  This includes mandatory diagnostics that
   4004      GCC issues without `-pedantic' but treats as warnings.
   4005 
   4006 `-M'
   4007      Instead of outputting the result of preprocessing, output a rule
   4008      suitable for `make' describing the dependencies of the main source
   4009      file.  The preprocessor outputs one `make' rule containing the
   4010      object file name for that source file, a colon, and the names of
   4011      all the included files, including those coming from `-include' or
   4012      `-imacros' command line options.
   4013 
   4014      Unless specified explicitly (with `-MT' or `-MQ'), the object file
   4015      name consists of the basename of the source file with any suffix
   4016      replaced with object file suffix.  If there are many included
   4017      files then the rule is split into several lines using `\'-newline.
   4018      The rule has no commands.
   4019 
   4020      This option does not suppress the preprocessor's debug output,
   4021      such as `-dM'.  To avoid mixing such debug output with the
   4022      dependency rules you should explicitly specify the dependency
   4023      output file with `-MF', or use an environment variable like
   4024      `DEPENDENCIES_OUTPUT' (*note Environment Variables::).  Debug
   4025      output will still be sent to the regular output stream as normal.
   4026 
   4027      Passing `-M' to the driver implies `-E', and suppresses warnings
   4028      with an implicit `-w'.
   4029 
   4030 `-MM'
   4031      Like `-M' but do not mention header files that are found in system
   4032      header directories, nor header files that are included, directly
   4033      or indirectly, from such a header.
   4034 
   4035      This implies that the choice of angle brackets or double quotes in
   4036      an `#include' directive does not in itself determine whether that
   4037      header will appear in `-MM' dependency output.  This is a slight
   4038      change in semantics from GCC versions 3.0 and earlier.
   4039 
   4040 `-MF FILE'
   4041      When used with `-M' or `-MM', specifies a file to write the
   4042      dependencies to.  If no `-MF' switch is given the preprocessor
   4043      sends the rules to the same place it would have sent preprocessed
   4044      output.
   4045 
   4046      When used with the driver options `-MD' or `-MMD', `-MF' overrides
   4047      the default dependency output file.
   4048 
   4049 `-MG'
   4050      In conjunction with an option such as `-M' requesting dependency
   4051      generation, `-MG' assumes missing header files are generated files
   4052      and adds them to the dependency list without raising an error.
   4053      The dependency filename is taken directly from the `#include'
   4054      directive without prepending any path.  `-MG' also suppresses
   4055      preprocessed output, as a missing header file renders this useless.
   4056 
   4057      This feature is used in automatic updating of makefiles.
   4058 
   4059 `-MP'
   4060      This option instructs CPP to add a phony target for each dependency
   4061      other than the main file, causing each to depend on nothing.  These
   4062      dummy rules work around errors `make' gives if you remove header
   4063      files without updating the `Makefile' to match.
   4064 
   4065      This is typical output:
   4066 
   4067           test.o: test.c test.h
   4068 
   4069           test.h:
   4070 
   4071 `-MT TARGET'
   4072      Change the target of the rule emitted by dependency generation.  By
   4073      default CPP takes the name of the main input file, including any
   4074      path, deletes any file suffix such as `.c', and appends the
   4075      platform's usual object suffix.  The result is the target.
   4076 
   4077      An `-MT' option will set the target to be exactly the string you
   4078      specify.  If you want multiple targets, you can specify them as a
   4079      single argument to `-MT', or use multiple `-MT' options.
   4080 
   4081      For example, `-MT '$(objpfx)foo.o'' might give
   4082 
   4083           $(objpfx)foo.o: foo.c
   4084 
   4085 `-MQ TARGET'
   4086      Same as `-MT', but it quotes any characters which are special to
   4087      Make.  `-MQ '$(objpfx)foo.o'' gives
   4088 
   4089           $$(objpfx)foo.o: foo.c
   4090 
   4091      The default target is automatically quoted, as if it were given
   4092      with `-MQ'.
   4093 
   4094 `-MD'
   4095      `-MD' is equivalent to `-M -MF FILE', except that `-E' is not
   4096      implied.  The driver determines FILE based on whether an `-o'
   4097      option is given.  If it is, the driver uses its argument but with
   4098      a suffix of `.d', otherwise it take the basename of the input file
   4099      and applies a `.d' suffix.
   4100 
   4101      If `-MD' is used in conjunction with `-E', any `-o' switch is
   4102      understood to specify the dependency output file (*note -MF:
   4103      dashMF.), but if used without `-E', each `-o' is understood to
   4104      specify a target object file.
   4105 
   4106      Since `-E' is not implied, `-MD' can be used to generate a
   4107      dependency output file as a side-effect of the compilation process.
   4108 
   4109 `-MMD'
   4110      Like `-MD' except mention only user header files, not system
   4111      header files.
   4112 
   4113 `-x c'
   4114 `-x c++'
   4115 `-x objective-c'
   4116 `-x assembler-with-cpp'
   4117      Specify the source language: C, C++, Objective-C, or assembly.
   4118      This has nothing to do with standards conformance or extensions;
   4119      it merely selects which base syntax to expect.  If you give none
   4120      of these options, cpp will deduce the language from the extension
   4121      of the source file: `.c', `.cc', `.m', or `.S'.  Some other common
   4122      extensions for C++ and assembly are also recognized.  If cpp does
   4123      not recognize the extension, it will treat the file as C; this is
   4124      the most generic mode.
   4125 
   4126      _Note:_ Previous versions of cpp accepted a `-lang' option which
   4127      selected both the language and the standards conformance level.
   4128      This option has been removed, because it conflicts with the `-l'
   4129      option.
   4130 
   4131 `-std=STANDARD'
   4132 `-ansi'
   4133      Specify the standard to which the code should conform.  Currently
   4134      CPP knows about C and C++ standards; others may be added in the
   4135      future.
   4136 
   4137      STANDARD may be one of:
   4138     `iso9899:1990'
   4139     `c89'
   4140           The ISO C standard from 1990.  `c89' is the customary
   4141           shorthand for this version of the standard.
   4142 
   4143           The `-ansi' option is equivalent to `-std=c89'.
   4144 
   4145     `iso9899:199409'
   4146           The 1990 C standard, as amended in 1994.
   4147 
   4148     `iso9899:1999'
   4149     `c99'
   4150     `iso9899:199x'
   4151     `c9x'
   4152           The revised ISO C standard, published in December 1999.
   4153           Before publication, this was known as C9X.
   4154 
   4155     `gnu89'
   4156           The 1990 C standard plus GNU extensions.  This is the default.
   4157 
   4158     `gnu99'
   4159     `gnu9x'
   4160           The 1999 C standard plus GNU extensions.
   4161 
   4162     `c++98'
   4163           The 1998 ISO C++ standard plus amendments.
   4164 
   4165     `gnu++98'
   4166           The same as `-std=c++98' plus GNU extensions.  This is the
   4167           default for C++ code.
   4168 
   4169 `-I-'
   4170      Split the include path.  Any directories specified with `-I'
   4171      options before `-I-' are searched only for headers requested with
   4172      `#include "FILE"'; they are not searched for `#include <FILE>'.
   4173      If additional directories are specified with `-I' options after
   4174      the `-I-', those directories are searched for all `#include'
   4175      directives.
   4176 
   4177      In addition, `-I-' inhibits the use of the directory of the current
   4178      file directory as the first search directory for `#include "FILE"'.
   4179      *Note Search Path::.  This option has been deprecated.
   4180 
   4181 `-nostdinc'
   4182      Do not search the standard system directories for header files.
   4183      Only the directories you have specified with `-I' options (and the
   4184      directory of the current file, if appropriate) are searched.
   4185 
   4186 `-nostdinc++'
   4187      Do not search for header files in the C++-specific standard
   4188      directories, but do still search the other standard directories.
   4189      (This option is used when building the C++ library.)
   4190 
   4191 `-include FILE'
   4192      Process FILE as if `#include "file"' appeared as the first line of
   4193      the primary source file.  However, the first directory searched
   4194      for FILE is the preprocessor's working directory _instead of_ the
   4195      directory containing the main source file.  If not found there, it
   4196      is searched for in the remainder of the `#include "..."' search
   4197      chain as normal.
   4198 
   4199      If multiple `-include' options are given, the files are included
   4200      in the order they appear on the command line.
   4201 
   4202 `-imacros FILE'
   4203      Exactly like `-include', except that any output produced by
   4204      scanning FILE is thrown away.  Macros it defines remain defined.
   4205      This allows you to acquire all the macros from a header without
   4206      also processing its declarations.
   4207 
   4208      All files specified by `-imacros' are processed before all files
   4209      specified by `-include'.
   4210 
   4211 `-idirafter DIR'
   4212      Search DIR for header files, but do it _after_ all directories
   4213      specified with `-I' and the standard system directories have been
   4214      exhausted.  DIR is treated as a system include directory.
   4215 
   4216 `-iprefix PREFIX'
   4217      Specify PREFIX as the prefix for subsequent `-iwithprefix'
   4218      options.  If the prefix represents a directory, you should include
   4219      the final `/'.
   4220 
   4221 `-iwithprefix DIR'
   4222 `-iwithprefixbefore DIR'
   4223      Append DIR to the prefix specified previously with `-iprefix', and
   4224      add the resulting directory to the include search path.
   4225      `-iwithprefixbefore' puts it in the same place `-I' would;
   4226      `-iwithprefix' puts it where `-idirafter' would.
   4227 
   4228 `-isysroot DIR'
   4229      This option is like the `--sysroot' option, but applies only to
   4230      header files.  See the `--sysroot' option for more information.
   4231 
   4232 `-imultilib DIR'
   4233      Use DIR as a subdirectory of the directory containing
   4234      target-specific C++ headers.
   4235 
   4236 `-isystem DIR'
   4237      Search DIR for header files, after all directories specified by
   4238      `-I' but before the standard system directories.  Mark it as a
   4239      system directory, so that it gets the same special treatment as is
   4240      applied to the standard system directories.  *Note System
   4241      Headers::.
   4242 
   4243 `-iquote DIR'
   4244      Search DIR only for header files requested with `#include "FILE"';
   4245      they are not searched for `#include <FILE>', before all
   4246      directories specified by `-I' and before the standard system
   4247      directories.  *Note Search Path::.
   4248 
   4249 `-fdirectives-only'
   4250      This option provides a simplified preprocessor to improve the
   4251      performance of distributed build systems such as distcc.  It's
   4252      behavior depends on a number of other flags.
   4253 
   4254      If the `-E' option is enabled, it suppresses things like macro
   4255      expansion, trigraph conversion, and escaped newline splicing
   4256      outside of directives.  All directives are processed normally,
   4257      except that macro definitions are output similar to the `-dD'
   4258      option.
   4259 
   4260      If the `-fpreprocessed' option is enabled, it suppresses
   4261      predefinition of most builtin and command line macros.  This
   4262      prevents duplicate definition of macros output with the `-E'
   4263      option.
   4264 
   4265 `-fdollars-in-identifiers'
   4266      Accept `$' in identifiers.    *Note Identifier characters::.
   4267 
   4268 `-fextended-identifiers'
   4269      Accept universal character names in identifiers.  This option is
   4270      experimental; in a future version of GCC, it will be enabled by
   4271      default for C99 and C++.
   4272 
   4273 `-fpreprocessed'
   4274      Indicate to the preprocessor that the input file has already been
   4275      preprocessed.  This suppresses things like macro expansion,
   4276      trigraph conversion, escaped newline splicing, and processing of
   4277      most directives.  The preprocessor still recognizes and removes
   4278      comments, so that you can pass a file preprocessed with `-C' to
   4279      the compiler without problems.  In this mode the integrated
   4280      preprocessor is little more than a tokenizer for the front ends.
   4281 
   4282      `-fpreprocessed' is implicit if the input file has one of the
   4283      extensions `.i', `.ii' or `.mi'.  These are the extensions that
   4284      GCC uses for preprocessed files created by `-save-temps'.
   4285 
   4286 `-ftabstop=WIDTH'
   4287      Set the distance between tab stops.  This helps the preprocessor
   4288      report correct column numbers in warnings or errors, even if tabs
   4289      appear on the line.  If the value is less than 1 or greater than
   4290      100, the option is ignored.  The default is 8.
   4291 
   4292 `-fexec-charset=CHARSET'
   4293      Set the execution character set, used for string and character
   4294      constants.  The default is UTF-8.  CHARSET can be any encoding
   4295      supported by the system's `iconv' library routine.
   4296 
   4297 `-fwide-exec-charset=CHARSET'
   4298      Set the wide execution character set, used for wide string and
   4299      character constants.  The default is UTF-32 or UTF-16, whichever
   4300      corresponds to the width of `wchar_t'.  As with `-fexec-charset',
   4301      CHARSET can be any encoding supported by the system's `iconv'
   4302      library routine; however, you will have problems with encodings
   4303      that do not fit exactly in `wchar_t'.
   4304 
   4305 `-finput-charset=CHARSET'
   4306      Set the input character set, used for translation from the
   4307      character set of the input file to the source character set used
   4308      by GCC.  If the locale does not specify, or GCC cannot get this
   4309      information from the locale, the default is UTF-8.  This can be
   4310      overridden by either the locale or this command line option.
   4311      Currently the command line option takes precedence if there's a
   4312      conflict.  CHARSET can be any encoding supported by the system's
   4313      `iconv' library routine.
   4314 
   4315 `-fworking-directory'
   4316      Enable generation of linemarkers in the preprocessor output that
   4317      will let the compiler know the current working directory at the
   4318      time of preprocessing.  When this option is enabled, the
   4319      preprocessor will emit, after the initial linemarker, a second
   4320      linemarker with the current working directory followed by two
   4321      slashes.  GCC will use this directory, when it's present in the
   4322      preprocessed input, as the directory emitted as the current
   4323      working directory in some debugging information formats.  This
   4324      option is implicitly enabled if debugging information is enabled,
   4325      but this can be inhibited with the negated form
   4326      `-fno-working-directory'.  If the `-P' flag is present in the
   4327      command line, this option has no effect, since no `#line'
   4328      directives are emitted whatsoever.
   4329 
   4330 `-fno-show-column'
   4331      Do not print column numbers in diagnostics.  This may be necessary
   4332      if diagnostics are being scanned by a program that does not
   4333      understand the column numbers, such as `dejagnu'.
   4334 
   4335 `-A PREDICATE=ANSWER'
   4336      Make an assertion with the predicate PREDICATE and answer ANSWER.
   4337      This form is preferred to the older form `-A PREDICATE(ANSWER)',
   4338      which is still supported, because it does not use shell special
   4339      characters.  *Note Assertions::.
   4340 
   4341 `-A -PREDICATE=ANSWER'
   4342      Cancel an assertion with the predicate PREDICATE and answer ANSWER.
   4343 
   4344 `-dCHARS'
   4345      CHARS is a sequence of one or more of the following characters,
   4346      and must not be preceded by a space.  Other characters are
   4347      interpreted by the compiler proper, or reserved for future
   4348      versions of GCC, and so are silently ignored.  If you specify
   4349      characters whose behavior conflicts, the result is undefined.
   4350 
   4351     `M'
   4352           Instead of the normal output, generate a list of `#define'
   4353           directives for all the macros defined during the execution of
   4354           the preprocessor, including predefined macros.  This gives
   4355           you a way of finding out what is predefined in your version
   4356           of the preprocessor.  Assuming you have no file `foo.h', the
   4357           command
   4358 
   4359                touch foo.h; cpp -dM foo.h
   4360 
   4361           will show all the predefined macros.
   4362 
   4363     `D'
   4364           Like `M' except in two respects: it does _not_ include the
   4365           predefined macros, and it outputs _both_ the `#define'
   4366           directives and the result of preprocessing.  Both kinds of
   4367           output go to the standard output file.
   4368 
   4369     `N'
   4370           Like `D', but emit only the macro names, not their expansions.
   4371 
   4372     `I'
   4373           Output `#include' directives in addition to the result of
   4374           preprocessing.
   4375 
   4376 `-P'
   4377      Inhibit generation of linemarkers in the output from the
   4378      preprocessor.  This might be useful when running the preprocessor
   4379      on something that is not C code, and will be sent to a program
   4380      which might be confused by the linemarkers.  *Note Preprocessor
   4381      Output::.
   4382 
   4383 `-C'
   4384      Do not discard comments.  All comments are passed through to the
   4385      output file, except for comments in processed directives, which
   4386      are deleted along with the directive.
   4387 
   4388      You should be prepared for side effects when using `-C'; it causes
   4389      the preprocessor to treat comments as tokens in their own right.
   4390      For example, comments appearing at the start of what would be a
   4391      directive line have the effect of turning that line into an
   4392      ordinary source line, since the first token on the line is no
   4393      longer a `#'.
   4394 
   4395 `-CC'
   4396      Do not discard comments, including during macro expansion.  This is
   4397      like `-C', except that comments contained within macros are also
   4398      passed through to the output file where the macro is expanded.
   4399 
   4400      In addition to the side-effects of the `-C' option, the `-CC'
   4401      option causes all C++-style comments inside a macro to be
   4402      converted to C-style comments.  This is to prevent later use of
   4403      that macro from inadvertently commenting out the remainder of the
   4404      source line.
   4405 
   4406      The `-CC' option is generally used to support lint comments.
   4407 
   4408 `-traditional-cpp'
   4409      Try to imitate the behavior of old-fashioned C preprocessors, as
   4410      opposed to ISO C preprocessors.  *Note Traditional Mode::.
   4411 
   4412 `-trigraphs'
   4413      Process trigraph sequences.  *Note Initial processing::.
   4414 
   4415 `-remap'
   4416      Enable special code to work around file systems which only permit
   4417      very short file names, such as MS-DOS.
   4418 
   4419 `--help'
   4420 `--target-help'
   4421      Print text describing all the command line options instead of
   4422      preprocessing anything.
   4423 
   4424 `-v'
   4425      Verbose mode.  Print out GNU CPP's version number at the beginning
   4426      of execution, and report the final form of the include path.
   4427 
   4428 `-H'
   4429      Print the name of each header file used, in addition to other
   4430      normal activities.  Each name is indented to show how deep in the
   4431      `#include' stack it is.  Precompiled header files are also
   4432      printed, even if they are found to be invalid; an invalid
   4433      precompiled header file is printed with `...x' and a valid one
   4434      with `...!' .
   4435 
   4436 `-version'
   4437 `--version'
   4438      Print out GNU CPP's version number.  With one dash, proceed to
   4439      preprocess as normal.  With two dashes, exit immediately.
   4440 
   4441 
   4442 File: cpp.info,  Node: Environment Variables,  Next: GNU Free Documentation License,  Prev: Invocation,  Up: Top
   4443 
   4444 13 Environment Variables
   4445 ************************
   4446 
   4447 This section describes the environment variables that affect how CPP
   4448 operates.  You can use them to specify directories or prefixes to use
   4449 when searching for include files, or to control dependency output.
   4450 
   4451    Note that you can also specify places to search using options such as
   4452 `-I', and control dependency output with options like `-M' (*note
   4453 Invocation::).  These take precedence over environment variables, which
   4454 in turn take precedence over the configuration of GCC.
   4455 
   4456 `CPATH'
   4457 `C_INCLUDE_PATH'
   4458 `CPLUS_INCLUDE_PATH'
   4459 `OBJC_INCLUDE_PATH'
   4460      Each variable's value is a list of directories separated by a
   4461      special character, much like `PATH', in which to look for header
   4462      files.  The special character, `PATH_SEPARATOR', is
   4463      target-dependent and determined at GCC build time.  For Microsoft
   4464      Windows-based targets it is a semicolon, and for almost all other
   4465      targets it is a colon.
   4466 
   4467      `CPATH' specifies a list of directories to be searched as if
   4468      specified with `-I', but after any paths given with `-I' options
   4469      on the command line.  This environment variable is used regardless
   4470      of which language is being preprocessed.
   4471 
   4472      The remaining environment variables apply only when preprocessing
   4473      the particular language indicated.  Each specifies a list of
   4474      directories to be searched as if specified with `-isystem', but
   4475      after any paths given with `-isystem' options on the command line.
   4476 
   4477      In all these variables, an empty element instructs the compiler to
   4478      search its current working directory.  Empty elements can appear
   4479      at the beginning or end of a path.  For instance, if the value of
   4480      `CPATH' is `:/special/include', that has the same effect as
   4481      `-I. -I/special/include'.
   4482 
   4483      See also *Note Search Path::.
   4484 
   4485 `DEPENDENCIES_OUTPUT'
   4486      If this variable is set, its value specifies how to output
   4487      dependencies for Make based on the non-system header files
   4488      processed by the compiler.  System header files are ignored in the
   4489      dependency output.
   4490 
   4491      The value of `DEPENDENCIES_OUTPUT' can be just a file name, in
   4492      which case the Make rules are written to that file, guessing the
   4493      target name from the source file name.  Or the value can have the
   4494      form `FILE TARGET', in which case the rules are written to file
   4495      FILE using TARGET as the target name.
   4496 
   4497      In other words, this environment variable is equivalent to
   4498      combining the options `-MM' and `-MF' (*note Invocation::), with
   4499      an optional `-MT' switch too.
   4500 
   4501 `SUNPRO_DEPENDENCIES'
   4502      This variable is the same as `DEPENDENCIES_OUTPUT' (see above),
   4503      except that system header files are not ignored, so it implies
   4504      `-M' rather than `-MM'.  However, the dependence on the main input
   4505      file is omitted.  *Note Invocation::.
   4506 
   4507 
   4508 File: cpp.info,  Node: GNU Free Documentation License,  Next: Index of Directives,  Prev: Environment Variables,  Up: Top
   4509 
   4510 GNU Free Documentation License
   4511 ******************************
   4512 
   4513                       Version 1.2, November 2002
   4514 
   4515      Copyright (C) 2000,2001,2002 Free Software Foundation, Inc.
   4516      51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
   4517 
   4518      Everyone is permitted to copy and distribute verbatim copies
   4519      of this license document, but changing it is not allowed.
   4520 
   4521   0. PREAMBLE
   4522 
   4523      The purpose of this License is to make a manual, textbook, or other
   4524      functional and useful document "free" in the sense of freedom: to
   4525      assure everyone the effective freedom to copy and redistribute it,
   4526      with or without modifying it, either commercially or
   4527      noncommercially.  Secondarily, this License preserves for the
   4528      author and publisher a way to get credit for their work, while not
   4529      being considered responsible for modifications made by others.
   4530 
   4531      This License is a kind of "copyleft", which means that derivative
   4532      works of the document must themselves be free in the same sense.
   4533      It complements the GNU General Public License, which is a copyleft
   4534      license designed for free software.
   4535 
   4536      We have designed this License in order to use it for manuals for
   4537      free software, because free software needs free documentation: a
   4538      free program should come with manuals providing the same freedoms
   4539      that the software does.  But this License is not limited to
   4540      software manuals; it can be used for any textual work, regardless
   4541      of subject matter or whether it is published as a printed book.
   4542      We recommend this License principally for works whose purpose is
   4543      instruction or reference.
   4544 
   4545   1. APPLICABILITY AND DEFINITIONS
   4546 
   4547      This License applies to any manual or other work, in any medium,
   4548      that contains a notice placed by the copyright holder saying it
   4549      can be distributed under the terms of this License.  Such a notice
   4550      grants a world-wide, royalty-free license, unlimited in duration,
   4551      to use that work under the conditions stated herein.  The
   4552      "Document", below, refers to any such manual or work.  Any member
   4553      of the public is a licensee, and is addressed as "you".  You
   4554      accept the license if you copy, modify or distribute the work in a
   4555      way requiring permission under copyright law.
   4556 
   4557      A "Modified Version" of the Document means any work containing the
   4558      Document or a portion of it, either copied verbatim, or with
   4559      modifications and/or translated into another language.
   4560 
   4561      A "Secondary Section" is a named appendix or a front-matter section
   4562      of the Document that deals exclusively with the relationship of the
   4563      publishers or authors of the Document to the Document's overall
   4564      subject (or to related matters) and contains nothing that could
   4565      fall directly within that overall subject.  (Thus, if the Document
   4566      is in part a textbook of mathematics, a Secondary Section may not
   4567      explain any mathematics.)  The relationship could be a matter of
   4568      historical connection with the subject or with related matters, or
   4569      of legal, commercial, philosophical, ethical or political position
   4570      regarding them.
   4571 
   4572      The "Invariant Sections" are certain Secondary Sections whose
   4573      titles are designated, as being those of Invariant Sections, in
   4574      the notice that says that the Document is released under this
   4575      License.  If a section does not fit the above definition of
   4576      Secondary then it is not allowed to be designated as Invariant.
   4577      The Document may contain zero Invariant Sections.  If the Document
   4578      does not identify any Invariant Sections then there are none.
   4579 
   4580      The "Cover Texts" are certain short passages of text that are
   4581      listed, as Front-Cover Texts or Back-Cover Texts, in the notice
   4582      that says that the Document is released under this License.  A
   4583      Front-Cover Text may be at most 5 words, and a Back-Cover Text may
   4584      be at most 25 words.
   4585 
   4586      A "Transparent" copy of the Document means a machine-readable copy,
   4587      represented in a format whose specification is available to the
   4588      general public, that is suitable for revising the document
   4589      straightforwardly with generic text editors or (for images
   4590      composed of pixels) generic paint programs or (for drawings) some
   4591      widely available drawing editor, and that is suitable for input to
   4592      text formatters or for automatic translation to a variety of
   4593      formats suitable for input to text formatters.  A copy made in an
   4594      otherwise Transparent file format whose markup, or absence of
   4595      markup, has been arranged to thwart or discourage subsequent
   4596      modification by readers is not Transparent.  An image format is
   4597      not Transparent if used for any substantial amount of text.  A
   4598      copy that is not "Transparent" is called "Opaque".
   4599 
   4600      Examples of suitable formats for Transparent copies include plain
   4601      ASCII without markup, Texinfo input format, LaTeX input format,
   4602      SGML or XML using a publicly available DTD, and
   4603      standard-conforming simple HTML, PostScript or PDF designed for
   4604      human modification.  Examples of transparent image formats include
   4605      PNG, XCF and JPG.  Opaque formats include proprietary formats that
   4606      can be read and edited only by proprietary word processors, SGML or
   4607      XML for which the DTD and/or processing tools are not generally
   4608      available, and the machine-generated HTML, PostScript or PDF
   4609      produced by some word processors for output purposes only.
   4610 
   4611      The "Title Page" means, for a printed book, the title page itself,
   4612      plus such following pages as are needed to hold, legibly, the
   4613      material this License requires to appear in the title page.  For
   4614      works in formats which do not have any title page as such, "Title
   4615      Page" means the text near the most prominent appearance of the
   4616      work's title, preceding the beginning of the body of the text.
   4617 
   4618      A section "Entitled XYZ" means a named subunit of the Document
   4619      whose title either is precisely XYZ or contains XYZ in parentheses
   4620      following text that translates XYZ in another language.  (Here XYZ
   4621      stands for a specific section name mentioned below, such as
   4622      "Acknowledgements", "Dedications", "Endorsements", or "History".)
   4623      To "Preserve the Title" of such a section when you modify the
   4624      Document means that it remains a section "Entitled XYZ" according
   4625      to this definition.
   4626 
   4627      The Document may include Warranty Disclaimers next to the notice
   4628      which states that this License applies to the Document.  These
   4629      Warranty Disclaimers are considered to be included by reference in
   4630      this License, but only as regards disclaiming warranties: any other
   4631      implication that these Warranty Disclaimers may have is void and
   4632      has no effect on the meaning of this License.
   4633 
   4634   2. VERBATIM COPYING
   4635 
   4636      You may copy and distribute the Document in any medium, either
   4637      commercially or noncommercially, provided that this License, the
   4638      copyright notices, and the license notice saying this License
   4639      applies to the Document are reproduced in all copies, and that you
   4640      add no other conditions whatsoever to those of this License.  You
   4641      may not use technical measures to obstruct or control the reading
   4642      or further copying of the copies you make or distribute.  However,
   4643      you may accept compensation in exchange for copies.  If you
   4644      distribute a large enough number of copies you must also follow
   4645      the conditions in section 3.
   4646 
   4647      You may also lend copies, under the same conditions stated above,
   4648      and you may publicly display copies.
   4649 
   4650   3. COPYING IN QUANTITY
   4651 
   4652      If you publish printed copies (or copies in media that commonly
   4653      have printed covers) of the Document, numbering more than 100, and
   4654      the Document's license notice requires Cover Texts, you must
   4655      enclose the copies in covers that carry, clearly and legibly, all
   4656      these Cover Texts: Front-Cover Texts on the front cover, and
   4657      Back-Cover Texts on the back cover.  Both covers must also clearly
   4658      and legibly identify you as the publisher of these copies.  The
   4659      front cover must present the full title with all words of the
   4660      title equally prominent and visible.  You may add other material
   4661      on the covers in addition.  Copying with changes limited to the
   4662      covers, as long as they preserve the title of the Document and
   4663      satisfy these conditions, can be treated as verbatim copying in
   4664      other respects.
   4665 
   4666      If the required texts for either cover are too voluminous to fit
   4667      legibly, you should put the first ones listed (as many as fit
   4668      reasonably) on the actual cover, and continue the rest onto
   4669      adjacent pages.
   4670 
   4671      If you publish or distribute Opaque copies of the Document
   4672      numbering more than 100, you must either include a
   4673      machine-readable Transparent copy along with each Opaque copy, or
   4674      state in or with each Opaque copy a computer-network location from
   4675      which the general network-using public has access to download
   4676      using public-standard network protocols a complete Transparent
   4677      copy of the Document, free of added material.  If you use the
   4678      latter option, you must take reasonably prudent steps, when you
   4679      begin distribution of Opaque copies in quantity, to ensure that
   4680      this Transparent copy will remain thus accessible at the stated
   4681      location until at least one year after the last time you
   4682      distribute an Opaque copy (directly or through your agents or
   4683      retailers) of that edition to the public.
   4684 
   4685      It is requested, but not required, that you contact the authors of
   4686      the Document well before redistributing any large number of
   4687      copies, to give them a chance to provide you with an updated
   4688      version of the Document.
   4689 
   4690   4. MODIFICATIONS
   4691 
   4692      You may copy and distribute a Modified Version of the Document
   4693      under the conditions of sections 2 and 3 above, provided that you
   4694      release the Modified Version under precisely this License, with
   4695      the Modified Version filling the role of the Document, thus
   4696      licensing distribution and modification of the Modified Version to
   4697      whoever possesses a copy of it.  In addition, you must do these
   4698      things in the Modified Version:
   4699 
   4700        A. Use in the Title Page (and on the covers, if any) a title
   4701           distinct from that of the Document, and from those of
   4702           previous versions (which should, if there were any, be listed
   4703           in the History section of the Document).  You may use the
   4704           same title as a previous version if the original publisher of
   4705           that version gives permission.
   4706 
   4707        B. List on the Title Page, as authors, one or more persons or
   4708           entities responsible for authorship of the modifications in
   4709           the Modified Version, together with at least five of the
   4710           principal authors of the Document (all of its principal
   4711           authors, if it has fewer than five), unless they release you
   4712           from this requirement.
   4713 
   4714        C. State on the Title page the name of the publisher of the
   4715           Modified Version, as the publisher.
   4716 
   4717        D. Preserve all the copyright notices of the Document.
   4718 
   4719        E. Add an appropriate copyright notice for your modifications
   4720           adjacent to the other copyright notices.
   4721 
   4722        F. Include, immediately after the copyright notices, a license
   4723           notice giving the public permission to use the Modified
   4724           Version under the terms of this License, in the form shown in
   4725           the Addendum below.
   4726 
   4727        G. Preserve in that license notice the full lists of Invariant
   4728           Sections and required Cover Texts given in the Document's
   4729           license notice.
   4730 
   4731        H. Include an unaltered copy of this License.
   4732 
   4733        I. Preserve the section Entitled "History", Preserve its Title,
   4734           and add to it an item stating at least the title, year, new
   4735           authors, and publisher of the Modified Version as given on
   4736           the Title Page.  If there is no section Entitled "History" in
   4737           the Document, create one stating the title, year, authors,
   4738           and publisher of the Document as given on its Title Page,
   4739           then add an item describing the Modified Version as stated in
   4740           the previous sentence.
   4741 
   4742        J. Preserve the network location, if any, given in the Document
   4743           for public access to a Transparent copy of the Document, and
   4744           likewise the network locations given in the Document for
   4745           previous versions it was based on.  These may be placed in
   4746           the "History" section.  You may omit a network location for a
   4747           work that was published at least four years before the
   4748           Document itself, or if the original publisher of the version
   4749           it refers to gives permission.
   4750 
   4751        K. For any section Entitled "Acknowledgements" or "Dedications",
   4752           Preserve the Title of the section, and preserve in the
   4753           section all the substance and tone of each of the contributor
   4754           acknowledgements and/or dedications given therein.
   4755 
   4756        L. Preserve all the Invariant Sections of the Document,
   4757           unaltered in their text and in their titles.  Section numbers
   4758           or the equivalent are not considered part of the section
   4759           titles.
   4760 
   4761        M. Delete any section Entitled "Endorsements".  Such a section
   4762           may not be included in the Modified Version.
   4763 
   4764        N. Do not retitle any existing section to be Entitled
   4765           "Endorsements" or to conflict in title with any Invariant
   4766           Section.
   4767 
   4768        O. Preserve any Warranty Disclaimers.
   4769 
   4770      If the Modified Version includes new front-matter sections or
   4771      appendices that qualify as Secondary Sections and contain no
   4772      material copied from the Document, you may at your option
   4773      designate some or all of these sections as invariant.  To do this,
   4774      add their titles to the list of Invariant Sections in the Modified
   4775      Version's license notice.  These titles must be distinct from any
   4776      other section titles.
   4777 
   4778      You may add a section Entitled "Endorsements", provided it contains
   4779      nothing but endorsements of your Modified Version by various
   4780      parties--for example, statements of peer review or that the text
   4781      has been approved by an organization as the authoritative
   4782      definition of a standard.
   4783 
   4784      You may add a passage of up to five words as a Front-Cover Text,
   4785      and a passage of up to 25 words as a Back-Cover Text, to the end
   4786      of the list of Cover Texts in the Modified Version.  Only one
   4787      passage of Front-Cover Text and one of Back-Cover Text may be
   4788      added by (or through arrangements made by) any one entity.  If the
   4789      Document already includes a cover text for the same cover,
   4790      previously added by you or by arrangement made by the same entity
   4791      you are acting on behalf of, you may not add another; but you may
   4792      replace the old one, on explicit permission from the previous
   4793      publisher that added the old one.
   4794 
   4795      The author(s) and publisher(s) of the Document do not by this
   4796      License give permission to use their names for publicity for or to
   4797      assert or imply endorsement of any Modified Version.
   4798 
   4799   5. COMBINING DOCUMENTS
   4800 
   4801      You may combine the Document with other documents released under
   4802      this License, under the terms defined in section 4 above for
   4803      modified versions, provided that you include in the combination
   4804      all of the Invariant Sections of all of the original documents,
   4805      unmodified, and list them all as Invariant Sections of your
   4806      combined work in its license notice, and that you preserve all
   4807      their Warranty Disclaimers.
   4808 
   4809      The combined work need only contain one copy of this License, and
   4810      multiple identical Invariant Sections may be replaced with a single
   4811      copy.  If there are multiple Invariant Sections with the same name
   4812      but different contents, make the title of each such section unique
   4813      by adding at the end of it, in parentheses, the name of the
   4814      original author or publisher of that section if known, or else a
   4815      unique number.  Make the same adjustment to the section titles in
   4816      the list of Invariant Sections in the license notice of the
   4817      combined work.
   4818 
   4819      In the combination, you must combine any sections Entitled
   4820      "History" in the various original documents, forming one section
   4821      Entitled "History"; likewise combine any sections Entitled
   4822      "Acknowledgements", and any sections Entitled "Dedications".  You
   4823      must delete all sections Entitled "Endorsements."
   4824 
   4825   6. COLLECTIONS OF DOCUMENTS
   4826 
   4827      You may make a collection consisting of the Document and other
   4828      documents released under this License, and replace the individual
   4829      copies of this License in the various documents with a single copy
   4830      that is included in the collection, provided that you follow the
   4831      rules of this License for verbatim copying of each of the
   4832      documents in all other respects.
   4833 
   4834      You may extract a single document from such a collection, and
   4835      distribute it individually under this License, provided you insert
   4836      a copy of this License into the extracted document, and follow
   4837      this License in all other respects regarding verbatim copying of
   4838      that document.
   4839 
   4840   7. AGGREGATION WITH INDEPENDENT WORKS
   4841 
   4842      A compilation of the Document or its derivatives with other
   4843      separate and independent documents or works, in or on a volume of
   4844      a storage or distribution medium, is called an "aggregate" if the
   4845      copyright resulting from the compilation is not used to limit the
   4846      legal rights of the compilation's users beyond what the individual
   4847      works permit.  When the Document is included in an aggregate, this
   4848      License does not apply to the other works in the aggregate which
   4849      are not themselves derivative works of the Document.
   4850 
   4851      If the Cover Text requirement of section 3 is applicable to these
   4852      copies of the Document, then if the Document is less than one half
   4853      of the entire aggregate, the Document's Cover Texts may be placed
   4854      on covers that bracket the Document within the aggregate, or the
   4855      electronic equivalent of covers if the Document is in electronic
   4856      form.  Otherwise they must appear on printed covers that bracket
   4857      the whole aggregate.
   4858 
   4859   8. TRANSLATION
   4860 
   4861      Translation is considered a kind of modification, so you may
   4862      distribute translations of the Document under the terms of section
   4863      4.  Replacing Invariant Sections with translations requires special
   4864      permission from their copyright holders, but you may include
   4865      translations of some or all Invariant Sections in addition to the
   4866      original versions of these Invariant Sections.  You may include a
   4867      translation of this License, and all the license notices in the
   4868      Document, and any Warranty Disclaimers, provided that you also
   4869      include the original English version of this License and the
   4870      original versions of those notices and disclaimers.  In case of a
   4871      disagreement between the translation and the original version of
   4872      this License or a notice or disclaimer, the original version will
   4873      prevail.
   4874 
   4875      If a section in the Document is Entitled "Acknowledgements",
   4876      "Dedications", or "History", the requirement (section 4) to
   4877      Preserve its Title (section 1) will typically require changing the
   4878      actual title.
   4879 
   4880   9. TERMINATION
   4881 
   4882      You may not copy, modify, sublicense, or distribute the Document
   4883      except as expressly provided for under this License.  Any other
   4884      attempt to copy, modify, sublicense or distribute the Document is
   4885      void, and will automatically terminate your rights under this
   4886      License.  However, parties who have received copies, or rights,
   4887      from you under this License will not have their licenses
   4888      terminated so long as such parties remain in full compliance.
   4889 
   4890  10. FUTURE REVISIONS OF THIS LICENSE
   4891 
   4892      The Free Software Foundation may publish new, revised versions of
   4893      the GNU Free Documentation License from time to time.  Such new
   4894      versions will be similar in spirit to the present version, but may
   4895      differ in detail to address new problems or concerns.  See
   4896      `http://www.gnu.org/copyleft/'.
   4897 
   4898      Each version of the License is given a distinguishing version
   4899      number.  If the Document specifies that a particular numbered
   4900      version of this License "or any later version" applies to it, you
   4901      have the option of following the terms and conditions either of
   4902      that specified version or of any later version that has been
   4903      published (not as a draft) by the Free Software Foundation.  If
   4904      the Document does not specify a version number of this License,
   4905      you may choose any version ever published (not as a draft) by the
   4906      Free Software Foundation.
   4907 
   4908 ADDENDUM: How to use this License for your documents
   4909 ====================================================
   4910 
   4911 To use this License in a document you have written, include a copy of
   4912 the License in the document and put the following copyright and license
   4913 notices just after the title page:
   4914 
   4915        Copyright (C)  YEAR  YOUR NAME.
   4916        Permission is granted to copy, distribute and/or modify this document
   4917        under the terms of the GNU Free Documentation License, Version 1.2
   4918        or any later version published by the Free Software Foundation;
   4919        with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
   4920        Texts.  A copy of the license is included in the section entitled ``GNU
   4921        Free Documentation License''.
   4922 
   4923    If you have Invariant Sections, Front-Cover Texts and Back-Cover
   4924 Texts, replace the "with...Texts." line with this:
   4925 
   4926          with the Invariant Sections being LIST THEIR TITLES, with
   4927          the Front-Cover Texts being LIST, and with the Back-Cover Texts
   4928          being LIST.
   4929 
   4930    If you have Invariant Sections without Cover Texts, or some other
   4931 combination of the three, merge those two alternatives to suit the
   4932 situation.
   4933 
   4934    If your document contains nontrivial examples of program code, we
   4935 recommend releasing these examples in parallel under your choice of
   4936 free software license, such as the GNU General Public License, to
   4937 permit their use in free software.
   4938 
   4939 
   4940 File: cpp.info,  Node: Index of Directives,  Next: Option Index,  Prev: GNU Free Documentation License,  Up: Top
   4941 
   4942 Index of Directives
   4943 *******************
   4944 
   4945 [index]
   4946 * Menu:
   4947 
   4948 * #assert:                               Assertions.           (line 41)
   4949 * #define:                               Object-like Macros.   (line 11)
   4950 * #elif:                                 Elif.                 (line  6)
   4951 * #else:                                 Else.                 (line  6)
   4952 * #endif:                                Ifdef.                (line  6)
   4953 * #error:                                Diagnostics.          (line  6)
   4954 * #ident:                                Other Directives.     (line  6)
   4955 * #if:                                   Conditional Syntax.   (line  6)
   4956 * #ifdef:                                Ifdef.                (line  6)
   4957 * #ifndef:                               Ifdef.                (line 40)
   4958 * #import:                               Obsolete once-only headers.
   4959                                                                (line 10)
   4960 * #include:                              Include Syntax.       (line  6)
   4961 * #include_next:                         Wrapper Headers.      (line  6)
   4962 * #line:                                 Line Control.         (line 20)
   4963 * #pragma GCC dependency:                Pragmas.              (line 53)
   4964 * #pragma GCC poison:                    Pragmas.              (line 65)
   4965 * #pragma GCC system_header <1>:         Pragmas.              (line 92)
   4966 * #pragma GCC system_header:             System Headers.       (line 31)
   4967 * #sccs:                                 Other Directives.     (line  6)
   4968 * #unassert:                             Assertions.           (line 52)
   4969 * #undef:                                Undefining and Redefining Macros.
   4970                                                                (line  6)
   4971 * #warning:                              Diagnostics.          (line 27)
   4972 
   4973 
   4974 File: cpp.info,  Node: Option Index,  Next: Concept Index,  Prev: Index of Directives,  Up: Top
   4975 
   4976 Option Index
   4977 ************
   4978 
   4979 CPP's command line options and environment variables are indexed here
   4980 without any initial `-' or `--'.
   4981 
   4982 [index]
   4983 * Menu:
   4984 
   4985 * A:                                     Invocation.          (line 510)
   4986 * ansi:                                  Invocation.          (line 307)
   4987 * C:                                     Invocation.          (line 558)
   4988 * C_INCLUDE_PATH:                        Environment Variables.
   4989                                                               (line  16)
   4990 * CPATH:                                 Environment Variables.
   4991                                                               (line  15)
   4992 * CPLUS_INCLUDE_PATH:                    Environment Variables.
   4993                                                               (line  17)
   4994 * D:                                     Invocation.          (line  39)
   4995 * dD:                                    Invocation.          (line 538)
   4996 * DEPENDENCIES_OUTPUT:                   Environment Variables.
   4997                                                               (line  44)
   4998 * dI:                                    Invocation.          (line 547)
   4999 * dM:                                    Invocation.          (line 526)
   5000 * dN:                                    Invocation.          (line 544)
   5001 * fdirectives-only:                      Invocation.          (line 424)
   5002 * fdollars-in-identifiers:               Invocation.          (line 440)
   5003 * fexec-charset:                         Invocation.          (line 467)
   5004 * fextended-identifiers:                 Invocation.          (line 443)
   5005 * finput-charset:                        Invocation.          (line 480)
   5006 * fno-show-column:                       Invocation.          (line 505)
   5007 * fno-working-directory:                 Invocation.          (line 490)
   5008 * fpreprocessed:                         Invocation.          (line 448)
   5009 * ftabstop:                              Invocation.          (line 461)
   5010 * fwide-exec-charset:                    Invocation.          (line 472)
   5011 * fworking-directory:                    Invocation.          (line 490)
   5012 * H:                                     Invocation.          (line 603)
   5013 * help:                                  Invocation.          (line 595)
   5014 * I:                                     Invocation.          (line  71)
   5015 * I-:                                    Invocation.          (line 344)
   5016 * idirafter:                             Invocation.          (line 386)
   5017 * imacros:                               Invocation.          (line 377)
   5018 * imultilib:                             Invocation.          (line 407)
   5019 * include:                               Invocation.          (line 366)
   5020 * iprefix:                               Invocation.          (line 391)
   5021 * iquote:                                Invocation.          (line 418)
   5022 * isysroot:                              Invocation.          (line 403)
   5023 * isystem:                               Invocation.          (line 411)
   5024 * iwithprefix:                           Invocation.          (line 397)
   5025 * iwithprefixbefore:                     Invocation.          (line 397)
   5026 * M:                                     Invocation.          (line 181)
   5027 * MD:                                    Invocation.          (line 269)
   5028 * MF:                                    Invocation.          (line 215)
   5029 * MG:                                    Invocation.          (line 224)
   5030 * MM:                                    Invocation.          (line 205)
   5031 * MMD:                                   Invocation.          (line 284)
   5032 * MP:                                    Invocation.          (line 234)
   5033 * MQ:                                    Invocation.          (line 260)
   5034 * MT:                                    Invocation.          (line 246)
   5035 * nostdinc:                              Invocation.          (line 356)
   5036 * nostdinc++:                            Invocation.          (line 361)
   5037 * o:                                     Invocation.          (line  80)
   5038 * OBJC_INCLUDE_PATH:                     Environment Variables.
   5039                                                               (line  18)
   5040 * P:                                     Invocation.          (line 551)
   5041 * pedantic:                              Invocation.          (line 171)
   5042 * pedantic-errors:                       Invocation.          (line 176)
   5043 * remap:                                 Invocation.          (line 590)
   5044 * std=:                                  Invocation.          (line 307)
   5045 * SUNPRO_DEPENDENCIES:                   Environment Variables.
   5046                                                               (line  60)
   5047 * target-help:                           Invocation.          (line 595)
   5048 * traditional-cpp:                       Invocation.          (line 583)
   5049 * trigraphs:                             Invocation.          (line 587)
   5050 * U:                                     Invocation.          (line  62)
   5051 * undef:                                 Invocation.          (line  66)
   5052 * v:                                     Invocation.          (line 599)
   5053 * version:                               Invocation.          (line 612)
   5054 * w:                                     Invocation.          (line 167)
   5055 * Wall:                                  Invocation.          (line  86)
   5056 * Wcomment:                              Invocation.          (line  94)
   5057 * Wcomments:                             Invocation.          (line  94)
   5058 * Wendif-labels:                         Invocation.          (line 144)
   5059 * Werror:                                Invocation.          (line 157)
   5060 * Wimport:                               Invocation.          (line 117)
   5061 * Wsystem-headers:                       Invocation.          (line 161)
   5062 * Wtraditional:                          Invocation.          (line 111)
   5063 * Wtrigraphs:                            Invocation.          (line  99)
   5064 * Wundef:                                Invocation.          (line 120)
   5065 * Wunused-macros:                        Invocation.          (line 125)
   5066 * x:                                     Invocation.          (line 291)
   5067 
   5068 
   5069 File: cpp.info,  Node: Concept Index,  Prev: Option Index,  Up: Top
   5070 
   5071 Concept Index
   5072 *************
   5073 
   5074 [index]
   5075 * Menu:
   5076 
   5077 * # operator:                            Stringification.     (line   6)
   5078 * ## operator:                           Concatenation.       (line   6)
   5079 * _Pragma:                               Pragmas.             (line  25)
   5080 * alternative tokens:                    Tokenization.        (line 106)
   5081 * arguments:                             Macro Arguments.     (line   6)
   5082 * arguments in macro definitions:        Macro Arguments.     (line   6)
   5083 * assertions:                            Assertions.          (line   6)
   5084 * assertions, canceling:                 Assertions.          (line  52)
   5085 * backslash-newline:                     Initial processing.  (line  61)
   5086 * block comments:                        Initial processing.  (line  77)
   5087 * C++ named operators:                   C++ Named Operators. (line   6)
   5088 * character constants:                   Tokenization.        (line  85)
   5089 * character set, execution:              Invocation.          (line 467)
   5090 * character set, input:                  Invocation.          (line 480)
   5091 * character set, wide execution:         Invocation.          (line 472)
   5092 * command line:                          Invocation.          (line   6)
   5093 * commenting out code:                   Deleted Code.        (line   6)
   5094 * comments:                              Initial processing.  (line  77)
   5095 * common predefined macros:              Common Predefined Macros.
   5096                                                               (line   6)
   5097 * computed includes:                     Computed Includes.   (line   6)
   5098 * concatenation:                         Concatenation.       (line   6)
   5099 * conditional group:                     Ifdef.               (line  14)
   5100 * conditionals:                          Conditionals.        (line   6)
   5101 * continued lines:                       Initial processing.  (line  61)
   5102 * controlling macro:                     Once-Only Headers.   (line  35)
   5103 * defined:                               Defined.             (line   6)
   5104 * dependencies for make as output:       Environment Variables.
   5105                                                               (line  45)
   5106 * dependencies, make:                    Invocation.          (line 181)
   5107 * diagnostic:                            Diagnostics.         (line   6)
   5108 * differences from previous versions:    Differences from previous versions.
   5109                                                               (line   6)
   5110 * digraphs:                              Tokenization.        (line 106)
   5111 * directive line:                        The preprocessing language.
   5112                                                               (line   6)
   5113 * directive name:                        The preprocessing language.
   5114                                                               (line   6)
   5115 * directives:                            The preprocessing language.
   5116                                                               (line   6)
   5117 * empty macro arguments:                 Macro Arguments.     (line  66)
   5118 * environment variables:                 Environment Variables.
   5119                                                               (line   6)
   5120 * expansion of arguments:                Argument Prescan.    (line   6)
   5121 * FDL, GNU Free Documentation License:   GNU Free Documentation License.
   5122                                                               (line   6)
   5123 * function-like macros:                  Function-like Macros.
   5124                                                               (line   6)
   5125 * grouping options:                      Invocation.          (line  34)
   5126 * guard macro:                           Once-Only Headers.   (line  35)
   5127 * header file:                           Header Files.        (line   6)
   5128 * header file names:                     Tokenization.        (line  85)
   5129 * identifiers:                           Tokenization.        (line  34)
   5130 * implementation limits:                 Implementation limits.
   5131                                                               (line   6)
   5132 * implementation-defined behavior:       Implementation-defined behavior.
   5133                                                               (line   6)
   5134 * including just once:                   Once-Only Headers.   (line   6)
   5135 * invocation:                            Invocation.          (line   6)
   5136 * iso646.h:                              C++ Named Operators. (line   6)
   5137 * line comments:                         Initial processing.  (line  77)
   5138 * line control:                          Line Control.        (line   6)
   5139 * line endings:                          Initial processing.  (line  14)
   5140 * linemarkers:                           Preprocessor Output. (line  28)
   5141 * macro argument expansion:              Argument Prescan.    (line   6)
   5142 * macro arguments and directives:        Directives Within Macro Arguments.
   5143                                                               (line   6)
   5144 * macros in include:                     Computed Includes.   (line   6)
   5145 * macros with arguments:                 Macro Arguments.     (line   6)
   5146 * macros with variable arguments:        Variadic Macros.     (line   6)
   5147 * make:                                  Invocation.          (line 181)
   5148 * manifest constants:                    Object-like Macros.  (line   6)
   5149 * named operators:                       C++ Named Operators. (line   6)
   5150 * newlines in macro arguments:           Newlines in Arguments.
   5151                                                               (line   6)
   5152 * null directive:                        Other Directives.    (line  15)
   5153 * numbers:                               Tokenization.        (line  61)
   5154 * object-like macro:                     Object-like Macros.  (line   6)
   5155 * options:                               Invocation.          (line  38)
   5156 * options, grouping:                     Invocation.          (line  34)
   5157 * other tokens:                          Tokenization.        (line 120)
   5158 * output format:                         Preprocessor Output. (line  12)
   5159 * overriding a header file:              Wrapper Headers.     (line   6)
   5160 * parentheses in macro bodies:           Operator Precedence Problems.
   5161                                                               (line   6)
   5162 * pitfalls of macros:                    Macro Pitfalls.      (line   6)
   5163 * predefined macros:                     Predefined Macros.   (line   6)
   5164 * predefined macros, system-specific:    System-specific Predefined Macros.
   5165                                                               (line   6)
   5166 * predicates:                            Assertions.          (line  19)
   5167 * preprocessing directives:              The preprocessing language.
   5168                                                               (line   6)
   5169 * preprocessing numbers:                 Tokenization.        (line  61)
   5170 * preprocessing tokens:                  Tokenization.        (line   6)
   5171 * prescan of macro arguments:            Argument Prescan.    (line   6)
   5172 * problems with macros:                  Macro Pitfalls.      (line   6)
   5173 * punctuators:                           Tokenization.        (line 106)
   5174 * redefining macros:                     Undefining and Redefining Macros.
   5175                                                               (line   6)
   5176 * repeated inclusion:                    Once-Only Headers.   (line   6)
   5177 * reporting errors:                      Diagnostics.         (line   6)
   5178 * reporting warnings:                    Diagnostics.         (line   6)
   5179 * reserved namespace:                    System-specific Predefined Macros.
   5180                                                               (line   6)
   5181 * self-reference:                        Self-Referential Macros.
   5182                                                               (line   6)
   5183 * semicolons (after macro calls):        Swallowing the Semicolon.
   5184                                                               (line   6)
   5185 * side effects (in macro arguments):     Duplication of Side Effects.
   5186                                                               (line   6)
   5187 * standard predefined macros.:           Standard Predefined Macros.
   5188                                                               (line   6)
   5189 * string constants:                      Tokenization.        (line  85)
   5190 * string literals:                       Tokenization.        (line  85)
   5191 * stringification:                       Stringification.     (line   6)
   5192 * symbolic constants:                    Object-like Macros.  (line   6)
   5193 * system header files <1>:               System Headers.      (line   6)
   5194 * system header files:                   Header Files.        (line  13)
   5195 * system-specific predefined macros:     System-specific Predefined Macros.
   5196                                                               (line   6)
   5197 * testing predicates:                    Assertions.          (line  30)
   5198 * token concatenation:                   Concatenation.       (line   6)
   5199 * token pasting:                         Concatenation.       (line   6)
   5200 * tokens:                                Tokenization.        (line   6)
   5201 * trigraphs:                             Initial processing.  (line  32)
   5202 * undefining macros:                     Undefining and Redefining Macros.
   5203                                                               (line   6)
   5204 * unsafe macros:                         Duplication of Side Effects.
   5205                                                               (line   6)
   5206 * variable number of arguments:          Variadic Macros.     (line   6)
   5207 * variadic macros:                       Variadic Macros.     (line   6)
   5208 * wrapper #ifndef:                       Once-Only Headers.   (line   6)
   5209 * wrapper headers:                       Wrapper Headers.     (line   6)
   5210 
   5211 
   5212 
   5213 Tag Table:
   5214 Node: Top1090
   5215 Node: Overview3774
   5216 Node: Character sets6595
   5217 Ref: Character sets-Footnote-18778
   5218 Node: Initial processing8959
   5219 Ref: trigraphs10518
   5220 Node: Tokenization14720
   5221 Ref: Tokenization-Footnote-121856
   5222 Node: The preprocessing language21967
   5223 Node: Header Files24845
   5224 Node: Include Syntax26725
   5225 Node: Include Operation28362
   5226 Node: Search Path30210
   5227 Node: Once-Only Headers33400
   5228 Node: Computed Includes35045
   5229 Node: Wrapper Headers38189
   5230 Node: System Headers40615
   5231 Node: Macros42665
   5232 Node: Object-like Macros43806
   5233 Node: Function-like Macros47396
   5234 Node: Macro Arguments49012
   5235 Node: Stringification53157
   5236 Node: Concatenation56363
   5237 Node: Variadic Macros59471
   5238 Node: Predefined Macros64258
   5239 Node: Standard Predefined Macros64846
   5240 Node: Common Predefined Macros70782
   5241 Node: System-specific Predefined Macros82063
   5242 Node: C++ Named Operators84084
   5243 Node: Undefining and Redefining Macros85048
   5244 Node: Directives Within Macro Arguments87152
   5245 Node: Macro Pitfalls88700
   5246 Node: Misnesting89233
   5247 Node: Operator Precedence Problems90345
   5248 Node: Swallowing the Semicolon92211
   5249 Node: Duplication of Side Effects94234
   5250 Node: Self-Referential Macros96417
   5251 Node: Argument Prescan98826
   5252 Node: Newlines in Arguments102580
   5253 Node: Conditionals103531
   5254 Node: Conditional Uses105361
   5255 Node: Conditional Syntax106719
   5256 Node: Ifdef107039
   5257 Node: If110200
   5258 Node: Defined112504
   5259 Node: Else113787
   5260 Node: Elif114357
   5261 Node: Deleted Code115646
   5262 Node: Diagnostics116893
   5263 Node: Line Control118510
   5264 Node: Pragmas122314
   5265 Node: Other Directives126584
   5266 Node: Preprocessor Output127634
   5267 Node: Traditional Mode130835
   5268 Node: Traditional lexical analysis131893
   5269 Node: Traditional macros134396
   5270 Node: Traditional miscellany138198
   5271 Node: Traditional warnings139195
   5272 Node: Implementation Details141392
   5273 Node: Implementation-defined behavior142013
   5274 Ref: Identifier characters142746
   5275 Node: Implementation limits145831
   5276 Node: Obsolete Features148505
   5277 Node: Assertions148960
   5278 Node: Obsolete once-only headers151501
   5279 Node: Differences from previous versions153236
   5280 Node: Invocation157444
   5281 Ref: Wtrigraphs161775
   5282 Ref: dashMF166559
   5283 Ref: fdollars-in-identifiers175278
   5284 Node: Environment Variables182931
   5285 Node: GNU Free Documentation License185897
   5286 Node: Index of Directives208330
   5287 Node: Option Index210259
   5288 Node: Concept Index216443
   5289 
   5290 End Tag Table
   5291