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      4    Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997,
      5 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007 Free
      6 Software 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, 2006, 2007 Free
    139 Software 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 or GNU Fortran.  Their names all start with
   1727 double underscores.
   1728 
   1729 `__COUNTER__'
   1730      This macro expands to sequential integral values starting from 0.
   1731      In conjunction 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 `__GFORTRAN__'
   1738      The GNU Fortran compiler defines this.
   1739 
   1740 `__GNUC__'
   1741 `__GNUC_MINOR__'
   1742 `__GNUC_PATCHLEVEL__'
   1743      These macros are defined by all GNU compilers that use the C
   1744      preprocessor: C, C++, Objective-C and Fortran.  Their values are
   1745      the major version, minor version, and patch level of the compiler,
   1746      as integer constants.  For example, GCC 3.2.1 will define
   1747      `__GNUC__' to 3, `__GNUC_MINOR__' to 2, and `__GNUC_PATCHLEVEL__'
   1748      to 1.  These macros are also defined if you invoke the
   1749      preprocessor directly.
   1750 
   1751      `__GNUC_PATCHLEVEL__' is new to GCC 3.0; it is also present in the
   1752      widely-used development snapshots leading up to 3.0 (which identify
   1753      themselves as GCC 2.96 or 2.97, depending on which snapshot you
   1754      have).
   1755 
   1756      If all you need to know is whether or not your program is being
   1757      compiled by GCC, or a non-GCC compiler that claims to accept the
   1758      GNU C dialects, you can simply test `__GNUC__'.  If you need to
   1759      write code which depends on a specific version, you must be more
   1760      careful.  Each time the minor version is increased, the patch
   1761      level is reset to zero; each time the major version is increased
   1762      (which happens rarely), the minor version and patch level are
   1763      reset.  If you wish to use the predefined macros directly in the
   1764      conditional, you will need to write it like this:
   1765 
   1766           /* Test for GCC > 3.2.0 */
   1767           #if __GNUC__ > 3 || \
   1768               (__GNUC__ == 3 && (__GNUC_MINOR__ > 2 || \
   1769                                  (__GNUC_MINOR__ == 2 && \
   1770                                   __GNUC_PATCHLEVEL__ > 0))
   1771 
   1772      Another approach is to use the predefined macros to calculate a
   1773      single number, then compare that against a threshold:
   1774 
   1775           #define GCC_VERSION (__GNUC__ * 10000 \
   1776                                + __GNUC_MINOR__ * 100 \
   1777                                + __GNUC_PATCHLEVEL__)
   1778           ...
   1779           /* Test for GCC > 3.2.0 */
   1780           #if GCC_VERSION > 30200
   1781 
   1782      Many people find this form easier to understand.
   1783 
   1784 `__GNUG__'
   1785      The GNU C++ compiler defines this.  Testing it is equivalent to
   1786      testing `(__GNUC__ && __cplusplus)'.
   1787 
   1788 `__STRICT_ANSI__'
   1789      GCC defines this macro if and only if the `-ansi' switch, or a
   1790      `-std' switch specifying strict conformance to some version of ISO
   1791      C, was specified when GCC was invoked.  It is defined to `1'.
   1792      This macro exists primarily to direct GNU libc's header files to
   1793      restrict their definitions to the minimal set found in the 1989 C
   1794      standard.
   1795 
   1796 `__BASE_FILE__'
   1797      This macro expands to the name of the main input file, in the form
   1798      of a C string constant.  This is the source file that was specified
   1799      on the command line of the preprocessor or C compiler.
   1800 
   1801 `__INCLUDE_LEVEL__'
   1802      This macro expands to a decimal integer constant that represents
   1803      the depth of nesting in include files.  The value of this macro is
   1804      incremented on every `#include' directive and decremented at the
   1805      end of every included file.  It starts out at 0, its value within
   1806      the base file specified on the command line.
   1807 
   1808 `__ELF__'
   1809      This macro is defined if the target uses the ELF object format.
   1810 
   1811 `__VERSION__'
   1812      This macro expands to a string constant which describes the
   1813      version of the compiler in use.  You should not rely on its
   1814      contents having any particular form, but it can be counted on to
   1815      contain at least the release number.
   1816 
   1817 `__OPTIMIZE__'
   1818 `__OPTIMIZE_SIZE__'
   1819 `__NO_INLINE__'
   1820      These macros describe the compilation mode.  `__OPTIMIZE__' is
   1821      defined in all optimizing compilations.  `__OPTIMIZE_SIZE__' is
   1822      defined if the compiler is optimizing for size, not speed.
   1823      `__NO_INLINE__' is defined if no functions will be inlined into
   1824      their callers (when not optimizing, or when inlining has been
   1825      specifically disabled by `-fno-inline').
   1826 
   1827      These macros cause certain GNU header files to provide optimized
   1828      definitions, using macros or inline functions, of system library
   1829      functions.  You should not use these macros in any way unless you
   1830      make sure that programs will execute with the same effect whether
   1831      or not they are defined.  If they are defined, their value is 1.
   1832 
   1833 `__GNUC_GNU_INLINE__'
   1834      GCC defines this macro if functions declared `inline' will be
   1835      handled in GCC's traditional gnu89 mode.  Object files will contain
   1836      externally visible definitions of all functions declared `inline'
   1837      without `extern' or `static'.  They will not contain any
   1838      definitions of any functions declared `extern inline'.
   1839 
   1840 `__GNUC_STDC_INLINE__'
   1841      GCC defines this macro if functions declared `inline' will be
   1842      handled according to the ISO C99 standard.  Object files will
   1843      contain externally visible definitions of all functions declared
   1844      `extern inline'.  They will not contain definitions of any
   1845      functions declared `inline' without `extern'.
   1846 
   1847      If this macro is defined, GCC supports the `gnu_inline' function
   1848      attribute as a way to always get the gnu89 behavior.  Support for
   1849      this and `__GNUC_GNU_INLINE__' was added in GCC 4.1.3.  If neither
   1850      macro is defined, an older version of GCC is being used: `inline'
   1851      functions will be compiled in gnu89 mode, and the `gnu_inline'
   1852      function attribute will not be recognized.
   1853 
   1854 `__CHAR_UNSIGNED__'
   1855      GCC defines this macro if and only if the data type `char' is
   1856      unsigned on the target machine.  It exists to cause the standard
   1857      header file `limits.h' to work correctly.  You should not use this
   1858      macro yourself; instead, refer to the standard macros defined in
   1859      `limits.h'.
   1860 
   1861 `__WCHAR_UNSIGNED__'
   1862      Like `__CHAR_UNSIGNED__', this macro is defined if and only if the
   1863      data type `wchar_t' is unsigned and the front-end is in C++ mode.
   1864 
   1865 `__REGISTER_PREFIX__'
   1866      This macro expands to a single token (not a string constant) which
   1867      is the prefix applied to CPU register names in assembly language
   1868      for this target.  You can use it to write assembly that is usable
   1869      in multiple environments.  For example, in the `m68k-aout'
   1870      environment it expands to nothing, but in the `m68k-coff'
   1871      environment it expands to a single `%'.
   1872 
   1873 `__USER_LABEL_PREFIX__'
   1874      This macro expands to a single token which is the prefix applied to
   1875      user labels (symbols visible to C code) in assembly.  For example,
   1876      in the `m68k-aout' environment it expands to an `_', but in the
   1877      `m68k-coff' environment it expands to nothing.
   1878 
   1879      This macro will have the correct definition even if
   1880      `-f(no-)underscores' is in use, but it will not be correct if
   1881      target-specific options that adjust this prefix are used (e.g. the
   1882      OSF/rose `-mno-underscores' option).
   1883 
   1884 `__SIZE_TYPE__'
   1885 `__PTRDIFF_TYPE__'
   1886 `__WCHAR_TYPE__'
   1887 `__WINT_TYPE__'
   1888 `__INTMAX_TYPE__'
   1889 `__UINTMAX_TYPE__'
   1890      These macros are defined to the correct underlying types for the
   1891      `size_t', `ptrdiff_t', `wchar_t', `wint_t', `intmax_t', and
   1892      `uintmax_t' typedefs, respectively.  They exist to make the
   1893      standard header files `stddef.h' and `wchar.h' work correctly.
   1894      You should not use these macros directly; instead, include the
   1895      appropriate headers and use the typedefs.
   1896 
   1897 `__CHAR_BIT__'
   1898      Defined to the number of bits used in the representation of the
   1899      `char' data type.  It exists to make the standard header given
   1900      numerical limits work correctly.  You should not use this macro
   1901      directly; instead, include the appropriate headers.
   1902 
   1903 `__SCHAR_MAX__'
   1904 `__WCHAR_MAX__'
   1905 `__SHRT_MAX__'
   1906 `__INT_MAX__'
   1907 `__LONG_MAX__'
   1908 `__LONG_LONG_MAX__'
   1909 `__INTMAX_MAX__'
   1910      Defined to the maximum value of the `signed char', `wchar_t',
   1911      `signed short', `signed int', `signed long', `signed long long',
   1912      and `intmax_t' types respectively.  They exist to make the
   1913      standard header given numerical limits work correctly.  You should
   1914      not use these macros directly; instead, include the appropriate
   1915      headers.
   1916 
   1917 `__SIZEOF_INT__'
   1918 `__SIZEOF_LONG__'
   1919 `__SIZEOF_LONG_LONG__'
   1920 `__SIZEOF_SHORT__'
   1921 `__SIZEOF_POINTER__'
   1922 `__SIZEOF_FLOAT__'
   1923 `__SIZEOF_DOUBLE__'
   1924 `__SIZEOF_LONG_DOUBLE__'
   1925 `__SIZEOF_SIZE_T__'
   1926 `__SIZEOF_WCHAR_T__'
   1927 `__SIZEOF_WINT_T__'
   1928 `__SIZEOF_PTRDIFF_T__'
   1929      Defined to the number of bytes of the C standard data types: `int',
   1930      `long', `long long', `short', `void *', `float', `double', `long
   1931      double', `size_t', `wchar_t', `wint_t' and `ptrdiff_t'.
   1932 
   1933 `__DEPRECATED'
   1934      This macro is defined, with value 1, when compiling a C++ source
   1935      file with warnings about deprecated constructs enabled.  These
   1936      warnings are enabled by default, but can be disabled with
   1937      `-Wno-deprecated'.
   1938 
   1939 `__EXCEPTIONS'
   1940      This macro is defined, with value 1, when compiling a C++ source
   1941      file with exceptions enabled.  If `-fno-exceptions' is used when
   1942      compiling the file, then this macro is not defined.
   1943 
   1944 `__GXX_RTTI'
   1945      This macro is defined, with value 1, when compiling a C++ source
   1946      file with runtime type identification enabled.  If `-fno-rtti' is
   1947      used when compiling the file, then this macro is not defined.
   1948 
   1949 `__USING_SJLJ_EXCEPTIONS__'
   1950      This macro is defined, with value 1, if the compiler uses the old
   1951      mechanism based on `setjmp' and `longjmp' for exception handling.
   1952 
   1953 `__GXX_EXPERIMENTAL_CXX0X__'
   1954      This macro is defined when compiling a C++ source file with the
   1955      option `-std=c++0x' or `-std=gnu++0x'. It indicates that some
   1956      features likely to be included in C++0x are available. Note that
   1957      these features are experimental, and may change or be removed in
   1958      future versions of GCC.
   1959 
   1960 `__GXX_WEAK__'
   1961      This macro is defined when compiling a C++ source file.  It has the
   1962      value 1 if the compiler will use weak symbols, COMDAT sections, or
   1963      other similar techniques to collapse symbols with "vague linkage"
   1964      that are defined in multiple translation units.  If the compiler
   1965      will not collapse such symbols, this macro is defined with value
   1966      0.  In general, user code should not need to make use of this
   1967      macro; the purpose of this macro is to ease implementation of the
   1968      C++ runtime library provided with G++.
   1969 
   1970 `__NEXT_RUNTIME__'
   1971      This macro is defined, with value 1, if (and only if) the NeXT
   1972      runtime (as in `-fnext-runtime') is in use for Objective-C.  If
   1973      the GNU runtime is used, this macro is not defined, so that you
   1974      can use this macro to determine which runtime (NeXT or GNU) is
   1975      being used.
   1976 
   1977 `__LP64__'
   1978 `_LP64'
   1979      These macros are defined, with value 1, if (and only if) the
   1980      compilation is for a target where `long int' and pointer both use
   1981      64-bits and `int' uses 32-bit.
   1982 
   1983 `__SSP__'
   1984      This macro is defined, with value 1, when `-fstack-protector' is in
   1985      use.
   1986 
   1987 `__SSP_ALL__'
   1988      This macro is defined, with value 2, when `-fstack-protector-all'
   1989      is in use.
   1990 
   1991 `__TIMESTAMP__'
   1992      This macro expands to a string constant that describes the date
   1993      and time of the last modification of the current source file. The
   1994      string constant contains abbreviated day of the week, month, day
   1995      of the month, time in hh:mm:ss form, year and looks like
   1996      `"Sun Sep 16 01:03:52 1973"'.  If the day of the month is less
   1997      than 10, it is padded with a space on the left.
   1998 
   1999      If GCC cannot determine the current date, it will emit a warning
   2000      message (once per compilation) and `__TIMESTAMP__' will expand to
   2001      `"??? ??? ?? ??:??:?? ????"'.
   2002 
   2003 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_1'
   2004 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_2'
   2005 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4'
   2006 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_8'
   2007 `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16'
   2008      These macros are defined when the target processor supports atomic
   2009      compare and swap operations on operands 1, 2, 4, 8 or 16 bytes in
   2010      length, respectively.
   2011 
   2012 
   2013 
   2014 File: cpp.info,  Node: System-specific Predefined Macros,  Next: C++ Named Operators,  Prev: Common Predefined Macros,  Up: Predefined Macros
   2015 
   2016 3.7.3 System-specific Predefined Macros
   2017 ---------------------------------------
   2018 
   2019 The C preprocessor normally predefines several macros that indicate what
   2020 type of system and machine is in use.  They are obviously different on
   2021 each target supported by GCC.  This manual, being for all systems and
   2022 machines, cannot tell you what their names are, but you can use `cpp
   2023 -dM' to see them all.  *Note Invocation::.  All system-specific
   2024 predefined macros expand to the constant 1, so you can test them with
   2025 either `#ifdef' or `#if'.
   2026 
   2027    The C standard requires that all system-specific macros be part of
   2028 the "reserved namespace".  All names which begin with two underscores,
   2029 or an underscore and a capital letter, are reserved for the compiler and
   2030 library to use as they wish.  However, historically system-specific
   2031 macros have had names with no special prefix; for instance, it is common
   2032 to find `unix' defined on Unix systems.  For all such macros, GCC
   2033 provides a parallel macro with two underscores added at the beginning
   2034 and the end.  If `unix' is defined, `__unix__' will be defined too.
   2035 There will never be more than two underscores; the parallel of `_mips'
   2036 is `__mips__'.
   2037 
   2038    When the `-ansi' option, or any `-std' option that requests strict
   2039 conformance, is given to the compiler, all the system-specific
   2040 predefined macros outside the reserved namespace are suppressed.  The
   2041 parallel macros, inside the reserved namespace, remain defined.
   2042 
   2043    We are slowly phasing out all predefined macros which are outside the
   2044 reserved namespace.  You should never use them in new programs, and we
   2045 encourage you to correct older code to use the parallel macros whenever
   2046 you find it.  We don't recommend you use the system-specific macros that
   2047 are in the reserved namespace, either.  It is better in the long run to
   2048 check specifically for features you need, using a tool such as
   2049 `autoconf'.
   2050 
   2051 
   2052 File: cpp.info,  Node: C++ Named Operators,  Prev: System-specific Predefined Macros,  Up: Predefined Macros
   2053 
   2054 3.7.4 C++ Named Operators
   2055 -------------------------
   2056 
   2057 In C++, there are eleven keywords which are simply alternate spellings
   2058 of operators normally written with punctuation.  These keywords are
   2059 treated as such even in the preprocessor.  They function as operators in
   2060 `#if', and they cannot be defined as macros or poisoned.  In C, you can
   2061 request that those keywords take their C++ meaning by including
   2062 `iso646.h'.  That header defines each one as a normal object-like macro
   2063 expanding to the appropriate punctuator.
   2064 
   2065    These are the named operators and their corresponding punctuators:
   2066 
   2067 Named Operator   Punctuator
   2068 `and'            `&&'
   2069 `and_eq'         `&='
   2070 `bitand'         `&'
   2071 `bitor'          `|'
   2072 `compl'          `~'
   2073 `not'            `!'
   2074 `not_eq'         `!='
   2075 `or'             `||'
   2076 `or_eq'          `|='
   2077 `xor'            `^'
   2078 `xor_eq'         `^='
   2079 
   2080 
   2081 File: cpp.info,  Node: Undefining and Redefining Macros,  Next: Directives Within Macro Arguments,  Prev: Predefined Macros,  Up: Macros
   2082 
   2083 3.8 Undefining and Redefining Macros
   2084 ====================================
   2085 
   2086 If a macro ceases to be useful, it may be "undefined" with the `#undef'
   2087 directive.  `#undef' takes a single argument, the name of the macro to
   2088 undefine.  You use the bare macro name, even if the macro is
   2089 function-like.  It is an error if anything appears on the line after
   2090 the macro name.  `#undef' has no effect if the name is not a macro.
   2091 
   2092      #define FOO 4
   2093      x = FOO;        ==> x = 4;
   2094      #undef FOO
   2095      x = FOO;        ==> x = FOO;
   2096 
   2097    Once a macro has been undefined, that identifier may be "redefined"
   2098 as a macro by a subsequent `#define' directive.  The new definition
   2099 need not have any resemblance to the old definition.
   2100 
   2101    However, if an identifier which is currently a macro is redefined,
   2102 then the new definition must be "effectively the same" as the old one.
   2103 Two macro definitions are effectively the same if:
   2104    * Both are the same type of macro (object- or function-like).
   2105 
   2106    * All the tokens of the replacement list are the same.
   2107 
   2108    * If there are any parameters, they are the same.
   2109 
   2110    * Whitespace appears in the same places in both.  It need not be
   2111      exactly the same amount of whitespace, though.  Remember that
   2112      comments count as whitespace.
   2113 
   2114 These definitions are effectively the same:
   2115      #define FOUR (2 + 2)
   2116      #define FOUR         (2    +    2)
   2117      #define FOUR (2 /* two */ + 2)
   2118    but these are not:
   2119      #define FOUR (2 + 2)
   2120      #define FOUR ( 2+2 )
   2121      #define FOUR (2 * 2)
   2122      #define FOUR(score,and,seven,years,ago) (2 + 2)
   2123 
   2124    If a macro is redefined with a definition that is not effectively the
   2125 same as the old one, the preprocessor issues a warning and changes the
   2126 macro to use the new definition.  If the new definition is effectively
   2127 the same, the redefinition is silently ignored.  This allows, for
   2128 instance, two different headers to define a common macro.  The
   2129 preprocessor will only complain if the definitions do not match.
   2130 
   2131 
   2132 File: cpp.info,  Node: Directives Within Macro Arguments,  Next: Macro Pitfalls,  Prev: Undefining and Redefining Macros,  Up: Macros
   2133 
   2134 3.9 Directives Within Macro Arguments
   2135 =====================================
   2136 
   2137 Occasionally it is convenient to use preprocessor directives within the
   2138 arguments of a macro.  The C and C++ standards declare that behavior in
   2139 these cases is undefined.
   2140 
   2141    Versions of CPP prior to 3.2 would reject such constructs with an
   2142 error message.  This was the only syntactic difference between normal
   2143 functions and function-like macros, so it seemed attractive to remove
   2144 this limitation, and people would often be surprised that they could
   2145 not use macros in this way.  Moreover, sometimes people would use
   2146 conditional compilation in the argument list to a normal library
   2147 function like `printf', only to find that after a library upgrade
   2148 `printf' had changed to be a function-like macro, and their code would
   2149 no longer compile.  So from version 3.2 we changed CPP to successfully
   2150 process arbitrary directives within macro arguments in exactly the same
   2151 way as it would have processed the directive were the function-like
   2152 macro invocation not present.
   2153 
   2154    If, within a macro invocation, that macro is redefined, then the new
   2155 definition takes effect in time for argument pre-expansion, but the
   2156 original definition is still used for argument replacement.  Here is a
   2157 pathological example:
   2158 
   2159      #define f(x) x x
   2160      f (1
   2161      #undef f
   2162      #define f 2
   2163      f)
   2164 
   2165 which expands to
   2166 
   2167      1 2 1 2
   2168 
   2169 with the semantics described above.
   2170 
   2171 
   2172 File: cpp.info,  Node: Macro Pitfalls,  Prev: Directives Within Macro Arguments,  Up: Macros
   2173 
   2174 3.10 Macro Pitfalls
   2175 ===================
   2176 
   2177 In this section we describe some special rules that apply to macros and
   2178 macro expansion, and point out certain cases in which the rules have
   2179 counter-intuitive consequences that you must watch out for.
   2180 
   2181 * Menu:
   2182 
   2183 * Misnesting::
   2184 * Operator Precedence Problems::
   2185 * Swallowing the Semicolon::
   2186 * Duplication of Side Effects::
   2187 * Self-Referential Macros::
   2188 * Argument Prescan::
   2189 * Newlines in Arguments::
   2190 
   2191 
   2192 File: cpp.info,  Node: Misnesting,  Next: Operator Precedence Problems,  Up: Macro Pitfalls
   2193 
   2194 3.10.1 Misnesting
   2195 -----------------
   2196 
   2197 When a macro is called with arguments, the arguments are substituted
   2198 into the macro body and the result is checked, together with the rest of
   2199 the input file, for more macro calls.  It is possible to piece together
   2200 a macro call coming partially from the macro body and partially from the
   2201 arguments.  For example,
   2202 
   2203      #define twice(x) (2*(x))
   2204      #define call_with_1(x) x(1)
   2205      call_with_1 (twice)
   2206           ==> twice(1)
   2207           ==> (2*(1))
   2208 
   2209    Macro definitions do not have to have balanced parentheses.  By
   2210 writing an unbalanced open parenthesis in a macro body, it is possible
   2211 to create a macro call that begins inside the macro body but ends
   2212 outside of it.  For example,
   2213 
   2214      #define strange(file) fprintf (file, "%s %d",
   2215      ...
   2216      strange(stderr) p, 35)
   2217           ==> fprintf (stderr, "%s %d", p, 35)
   2218 
   2219    The ability to piece together a macro call can be useful, but the
   2220 use of unbalanced open parentheses in a macro body is just confusing,
   2221 and should be avoided.
   2222 
   2223 
   2224 File: cpp.info,  Node: Operator Precedence Problems,  Next: Swallowing the Semicolon,  Prev: Misnesting,  Up: Macro Pitfalls
   2225 
   2226 3.10.2 Operator Precedence Problems
   2227 -----------------------------------
   2228 
   2229 You may have noticed that in most of the macro definition examples shown
   2230 above, each occurrence of a macro argument name had parentheses around
   2231 it.  In addition, another pair of parentheses usually surround the
   2232 entire macro definition.  Here is why it is best to write macros that
   2233 way.
   2234 
   2235    Suppose you define a macro as follows,
   2236 
   2237      #define ceil_div(x, y) (x + y - 1) / y
   2238 
   2239 whose purpose is to divide, rounding up.  (One use for this operation is
   2240 to compute how many `int' objects are needed to hold a certain number
   2241 of `char' objects.)  Then suppose it is used as follows:
   2242 
   2243      a = ceil_div (b & c, sizeof (int));
   2244           ==> a = (b & c + sizeof (int) - 1) / sizeof (int);
   2245 
   2246 This does not do what is intended.  The operator-precedence rules of C
   2247 make it equivalent to this:
   2248 
   2249      a = (b & (c + sizeof (int) - 1)) / sizeof (int);
   2250 
   2251 What we want is this:
   2252 
   2253      a = ((b & c) + sizeof (int) - 1)) / sizeof (int);
   2254 
   2255 Defining the macro as
   2256 
   2257      #define ceil_div(x, y) ((x) + (y) - 1) / (y)
   2258 
   2259 provides the desired result.
   2260 
   2261    Unintended grouping can result in another way.  Consider `sizeof
   2262 ceil_div(1, 2)'.  That has the appearance of a C expression that would
   2263 compute the size of the type of `ceil_div (1, 2)', but in fact it means
   2264 something very different.  Here is what it expands to:
   2265 
   2266      sizeof ((1) + (2) - 1) / (2)
   2267 
   2268 This would take the size of an integer and divide it by two.  The
   2269 precedence rules have put the division outside the `sizeof' when it was
   2270 intended to be inside.
   2271 
   2272    Parentheses around the entire macro definition prevent such problems.
   2273 Here, then, is the recommended way to define `ceil_div':
   2274 
   2275      #define ceil_div(x, y) (((x) + (y) - 1) / (y))
   2276 
   2277 
   2278 File: cpp.info,  Node: Swallowing the Semicolon,  Next: Duplication of Side Effects,  Prev: Operator Precedence Problems,  Up: Macro Pitfalls
   2279 
   2280 3.10.3 Swallowing the Semicolon
   2281 -------------------------------
   2282 
   2283 Often it is desirable to define a macro that expands into a compound
   2284 statement.  Consider, for example, the following macro, that advances a
   2285 pointer (the argument `p' says where to find it) across whitespace
   2286 characters:
   2287 
   2288      #define SKIP_SPACES(p, limit)  \
   2289      { char *lim = (limit);         \
   2290        while (p < lim) {            \
   2291          if (*p++ != ' ') {         \
   2292            p--; break; }}}
   2293 
   2294 Here backslash-newline is used to split the macro definition, which must
   2295 be a single logical line, so that it resembles the way such code would
   2296 be laid out if not part of a macro definition.
   2297 
   2298    A call to this macro might be `SKIP_SPACES (p, lim)'.  Strictly
   2299 speaking, the call expands to a compound statement, which is a complete
   2300 statement with no need for a semicolon to end it.  However, since it
   2301 looks like a function call, it minimizes confusion if you can use it
   2302 like a function call, writing a semicolon afterward, as in `SKIP_SPACES
   2303 (p, lim);'
   2304 
   2305    This can cause trouble before `else' statements, because the
   2306 semicolon is actually a null statement.  Suppose you write
   2307 
   2308      if (*p != 0)
   2309        SKIP_SPACES (p, lim);
   2310      else ...
   2311 
   2312 The presence of two statements--the compound statement and a null
   2313 statement--in between the `if' condition and the `else' makes invalid C
   2314 code.
   2315 
   2316    The definition of the macro `SKIP_SPACES' can be altered to solve
   2317 this problem, using a `do ... while' statement.  Here is how:
   2318 
   2319      #define SKIP_SPACES(p, limit)     \
   2320      do { char *lim = (limit);         \
   2321           while (p < lim) {            \
   2322             if (*p++ != ' ') {         \
   2323               p--; break; }}}          \
   2324      while (0)
   2325 
   2326    Now `SKIP_SPACES (p, lim);' expands into
   2327 
   2328      do {...} while (0);
   2329 
   2330 which is one statement.  The loop executes exactly once; most compilers
   2331 generate no extra code for it.
   2332 
   2333 
   2334 File: cpp.info,  Node: Duplication of Side Effects,  Next: Self-Referential Macros,  Prev: Swallowing the Semicolon,  Up: Macro Pitfalls
   2335 
   2336 3.10.4 Duplication of Side Effects
   2337 ----------------------------------
   2338 
   2339 Many C programs define a macro `min', for "minimum", like this:
   2340 
   2341      #define min(X, Y)  ((X) < (Y) ? (X) : (Y))
   2342 
   2343    When you use this macro with an argument containing a side effect,
   2344 as shown here,
   2345 
   2346      next = min (x + y, foo (z));
   2347 
   2348 it expands as follows:
   2349 
   2350      next = ((x + y) < (foo (z)) ? (x + y) : (foo (z)));
   2351 
   2352 where `x + y' has been substituted for `X' and `foo (z)' for `Y'.
   2353 
   2354    The function `foo' is used only once in the statement as it appears
   2355 in the program, but the expression `foo (z)' has been substituted twice
   2356 into the macro expansion.  As a result, `foo' might be called two times
   2357 when the statement is executed.  If it has side effects or if it takes
   2358 a long time to compute, the results might not be what you intended.  We
   2359 say that `min' is an "unsafe" macro.
   2360 
   2361    The best solution to this problem is to define `min' in a way that
   2362 computes the value of `foo (z)' only once.  The C language offers no
   2363 standard way to do this, but it can be done with GNU extensions as
   2364 follows:
   2365 
   2366      #define min(X, Y)                \
   2367      ({ typeof (X) x_ = (X);          \
   2368         typeof (Y) y_ = (Y);          \
   2369         (x_ < y_) ? x_ : y_; })
   2370 
   2371    The `({ ... })' notation produces a compound statement that acts as
   2372 an expression.  Its value is the value of its last statement.  This
   2373 permits us to define local variables and assign each argument to one.
   2374 The local variables have underscores after their names to reduce the
   2375 risk of conflict with an identifier of wider scope (it is impossible to
   2376 avoid this entirely).  Now each argument is evaluated exactly once.
   2377 
   2378    If you do not wish to use GNU C extensions, the only solution is to
   2379 be careful when _using_ the macro `min'.  For example, you can
   2380 calculate the value of `foo (z)', save it in a variable, and use that
   2381 variable in `min':
   2382 
   2383      #define min(X, Y)  ((X) < (Y) ? (X) : (Y))
   2384      ...
   2385      {
   2386        int tem = foo (z);
   2387        next = min (x + y, tem);
   2388      }
   2389 
   2390 (where we assume that `foo' returns type `int').
   2391 
   2392 
   2393 File: cpp.info,  Node: Self-Referential Macros,  Next: Argument Prescan,  Prev: Duplication of Side Effects,  Up: Macro Pitfalls
   2394 
   2395 3.10.5 Self-Referential Macros
   2396 ------------------------------
   2397 
   2398 A "self-referential" macro is one whose name appears in its definition.
   2399 Recall that all macro definitions are rescanned for more macros to
   2400 replace.  If the self-reference were considered a use of the macro, it
   2401 would produce an infinitely large expansion.  To prevent this, the
   2402 self-reference is not considered a macro call.  It is passed into the
   2403 preprocessor output unchanged.  Consider an example:
   2404 
   2405      #define foo (4 + foo)
   2406 
   2407 where `foo' is also a variable in your program.
   2408 
   2409    Following the ordinary rules, each reference to `foo' will expand
   2410 into `(4 + foo)'; then this will be rescanned and will expand into `(4
   2411 + (4 + foo))'; and so on until the computer runs out of memory.
   2412 
   2413    The self-reference rule cuts this process short after one step, at
   2414 `(4 + foo)'.  Therefore, this macro definition has the possibly useful
   2415 effect of causing the program to add 4 to the value of `foo' wherever
   2416 `foo' is referred to.
   2417 
   2418    In most cases, it is a bad idea to take advantage of this feature.  A
   2419 person reading the program who sees that `foo' is a variable will not
   2420 expect that it is a macro as well.  The reader will come across the
   2421 identifier `foo' in the program and think its value should be that of
   2422 the variable `foo', whereas in fact the value is four greater.
   2423 
   2424    One common, useful use of self-reference is to create a macro which
   2425 expands to itself.  If you write
   2426 
   2427      #define EPERM EPERM
   2428 
   2429 then the macro `EPERM' expands to `EPERM'.  Effectively, it is left
   2430 alone by the preprocessor whenever it's used in running text.  You can
   2431 tell that it's a macro with `#ifdef'.  You might do this if you want to
   2432 define numeric constants with an `enum', but have `#ifdef' be true for
   2433 each constant.
   2434 
   2435    If a macro `x' expands to use a macro `y', and the expansion of `y'
   2436 refers to the macro `x', that is an "indirect self-reference" of `x'.
   2437 `x' is not expanded in this case either.  Thus, if we have
   2438 
   2439      #define x (4 + y)
   2440      #define y (2 * x)
   2441 
   2442 then `x' and `y' expand as follows:
   2443 
   2444      x    ==> (4 + y)
   2445           ==> (4 + (2 * x))
   2446 
   2447      y    ==> (2 * x)
   2448           ==> (2 * (4 + y))
   2449 
   2450 Each macro is expanded when it appears in the definition of the other
   2451 macro, but not when it indirectly appears in its own definition.
   2452 
   2453 
   2454 File: cpp.info,  Node: Argument Prescan,  Next: Newlines in Arguments,  Prev: Self-Referential Macros,  Up: Macro Pitfalls
   2455 
   2456 3.10.6 Argument Prescan
   2457 -----------------------
   2458 
   2459 Macro arguments are completely macro-expanded before they are
   2460 substituted into a macro body, unless they are stringified or pasted
   2461 with other tokens.  After substitution, the entire macro body, including
   2462 the substituted arguments, is scanned again for macros to be expanded.
   2463 The result is that the arguments are scanned _twice_ to expand macro
   2464 calls in them.
   2465 
   2466    Most of the time, this has no effect.  If the argument contained any
   2467 macro calls, they are expanded during the first scan.  The result
   2468 therefore contains no macro calls, so the second scan does not change
   2469 it.  If the argument were substituted as given, with no prescan, the
   2470 single remaining scan would find the same macro calls and produce the
   2471 same results.
   2472 
   2473    You might expect the double scan to change the results when a
   2474 self-referential macro is used in an argument of another macro (*note
   2475 Self-Referential Macros::): the self-referential macro would be
   2476 expanded once in the first scan, and a second time in the second scan.
   2477 However, this is not what happens.  The self-references that do not
   2478 expand in the first scan are marked so that they will not expand in the
   2479 second scan either.
   2480 
   2481    You might wonder, "Why mention the prescan, if it makes no
   2482 difference?  And why not skip it and make the preprocessor faster?"
   2483 The answer is that the prescan does make a difference in three special
   2484 cases:
   2485 
   2486    * Nested calls to a macro.
   2487 
   2488      We say that "nested" calls to a macro occur when a macro's argument
   2489      contains a call to that very macro.  For example, if `f' is a macro
   2490      that expects one argument, `f (f (1))' is a nested pair of calls to
   2491      `f'.  The desired expansion is made by expanding `f (1)' and
   2492      substituting that into the definition of `f'.  The prescan causes
   2493      the expected result to happen.  Without the prescan, `f (1)' itself
   2494      would be substituted as an argument, and the inner use of `f' would
   2495      appear during the main scan as an indirect self-reference and
   2496      would not be expanded.
   2497 
   2498    * Macros that call other macros that stringify or concatenate.
   2499 
   2500      If an argument is stringified or concatenated, the prescan does not
   2501      occur.  If you _want_ to expand a macro, then stringify or
   2502      concatenate its expansion, you can do that by causing one macro to
   2503      call another macro that does the stringification or concatenation.
   2504      For instance, if you have
   2505 
   2506           #define AFTERX(x) X_ ## x
   2507           #define XAFTERX(x) AFTERX(x)
   2508           #define TABLESIZE 1024
   2509           #define BUFSIZE TABLESIZE
   2510 
   2511      then `AFTERX(BUFSIZE)' expands to `X_BUFSIZE', and
   2512      `XAFTERX(BUFSIZE)' expands to `X_1024'.  (Not to `X_TABLESIZE'.
   2513      Prescan always does a complete expansion.)
   2514 
   2515    * Macros used in arguments, whose expansions contain unshielded
   2516      commas.
   2517 
   2518      This can cause a macro expanded on the second scan to be called
   2519      with the wrong number of arguments.  Here is an example:
   2520 
   2521           #define foo  a,b
   2522           #define bar(x) lose(x)
   2523           #define lose(x) (1 + (x))
   2524 
   2525      We would like `bar(foo)' to turn into `(1 + (foo))', which would
   2526      then turn into `(1 + (a,b))'.  Instead, `bar(foo)' expands into
   2527      `lose(a,b)', and you get an error because `lose' requires a single
   2528      argument.  In this case, the problem is easily solved by the same
   2529      parentheses that ought to be used to prevent misnesting of
   2530      arithmetic operations:
   2531 
   2532           #define foo (a,b)
   2533      or
   2534           #define bar(x) lose((x))
   2535 
   2536      The extra pair of parentheses prevents the comma in `foo''s
   2537      definition from being interpreted as an argument separator.
   2538 
   2539 
   2540 
   2541 File: cpp.info,  Node: Newlines in Arguments,  Prev: Argument Prescan,  Up: Macro Pitfalls
   2542 
   2543 3.10.7 Newlines in Arguments
   2544 ----------------------------
   2545 
   2546 The invocation of a function-like macro can extend over many logical
   2547 lines.  However, in the present implementation, the entire expansion
   2548 comes out on one line.  Thus line numbers emitted by the compiler or
   2549 debugger refer to the line the invocation started on, which might be
   2550 different to the line containing the argument causing the problem.
   2551 
   2552    Here is an example illustrating this:
   2553 
   2554      #define ignore_second_arg(a,b,c) a; c
   2555 
   2556      ignore_second_arg (foo (),
   2557                         ignored (),
   2558                         syntax error);
   2559 
   2560 The syntax error triggered by the tokens `syntax error' results in an
   2561 error message citing line three--the line of ignore_second_arg-- even
   2562 though the problematic code comes from line five.
   2563 
   2564    We consider this a bug, and intend to fix it in the near future.
   2565 
   2566 
   2567 File: cpp.info,  Node: Conditionals,  Next: Diagnostics,  Prev: Macros,  Up: Top
   2568 
   2569 4 Conditionals
   2570 **************
   2571 
   2572 A "conditional" is a directive that instructs the preprocessor to
   2573 select whether or not to include a chunk of code in the final token
   2574 stream passed to the compiler.  Preprocessor conditionals can test
   2575 arithmetic expressions, or whether a name is defined as a macro, or both
   2576 simultaneously using the special `defined' operator.
   2577 
   2578    A conditional in the C preprocessor resembles in some ways an `if'
   2579 statement in C, but it is important to understand the difference between
   2580 them.  The condition in an `if' statement is tested during the
   2581 execution of your program.  Its purpose is to allow your program to
   2582 behave differently from run to run, depending on the data it is
   2583 operating on.  The condition in a preprocessing conditional directive is
   2584 tested when your program is compiled.  Its purpose is to allow different
   2585 code to be included in the program depending on the situation at the
   2586 time of compilation.
   2587 
   2588    However, the distinction is becoming less clear.  Modern compilers
   2589 often do test `if' statements when a program is compiled, if their
   2590 conditions are known not to vary at run time, and eliminate code which
   2591 can never be executed.  If you can count on your compiler to do this,
   2592 you may find that your program is more readable if you use `if'
   2593 statements with constant conditions (perhaps determined by macros).  Of
   2594 course, you can only use this to exclude code, not type definitions or
   2595 other preprocessing directives, and you can only do it if the code
   2596 remains syntactically valid when it is not to be used.
   2597 
   2598    GCC version 3 eliminates this kind of never-executed code even when
   2599 not optimizing.  Older versions did it only when optimizing.
   2600 
   2601 * Menu:
   2602 
   2603 * Conditional Uses::
   2604 * Conditional Syntax::
   2605 * Deleted Code::
   2606 
   2607 
   2608 File: cpp.info,  Node: Conditional Uses,  Next: Conditional Syntax,  Up: Conditionals
   2609 
   2610 4.1 Conditional Uses
   2611 ====================
   2612 
   2613 There are three general reasons to use a conditional.
   2614 
   2615    * A program may need to use different code depending on the machine
   2616      or operating system it is to run on.  In some cases the code for
   2617      one operating system may be erroneous on another operating system;
   2618      for example, it might refer to data types or constants that do not
   2619      exist on the other system.  When this happens, it is not enough to
   2620      avoid executing the invalid code.  Its mere presence will cause
   2621      the compiler to reject the program.  With a preprocessing
   2622      conditional, the offending code can be effectively excised from
   2623      the program when it is not valid.
   2624 
   2625    * You may want to be able to compile the same source file into two
   2626      different programs.  One version might make frequent time-consuming
   2627      consistency checks on its intermediate data, or print the values of
   2628      those data for debugging, and the other not.
   2629 
   2630    * A conditional whose condition is always false is one way to
   2631      exclude code from the program but keep it as a sort of comment for
   2632      future reference.
   2633 
   2634    Simple programs that do not need system-specific logic or complex
   2635 debugging hooks generally will not need to use preprocessing
   2636 conditionals.
   2637 
   2638 
   2639 File: cpp.info,  Node: Conditional Syntax,  Next: Deleted Code,  Prev: Conditional Uses,  Up: Conditionals
   2640 
   2641 4.2 Conditional Syntax
   2642 ======================
   2643 
   2644 A conditional in the C preprocessor begins with a "conditional
   2645 directive": `#if', `#ifdef' or `#ifndef'.
   2646 
   2647 * Menu:
   2648 
   2649 * Ifdef::
   2650 * If::
   2651 * Defined::
   2652 * Else::
   2653 * Elif::
   2654 
   2655 
   2656 File: cpp.info,  Node: Ifdef,  Next: If,  Up: Conditional Syntax
   2657 
   2658 4.2.1 Ifdef
   2659 -----------
   2660 
   2661 The simplest sort of conditional is
   2662 
   2663      #ifdef MACRO
   2664 
   2665      CONTROLLED TEXT
   2666 
   2667      #endif /* MACRO */
   2668 
   2669    This block is called a "conditional group".  CONTROLLED TEXT will be
   2670 included in the output of the preprocessor if and only if MACRO is
   2671 defined.  We say that the conditional "succeeds" if MACRO is defined,
   2672 "fails" if it is not.
   2673 
   2674    The CONTROLLED TEXT inside of a conditional can include
   2675 preprocessing directives.  They are executed only if the conditional
   2676 succeeds.  You can nest conditional groups inside other conditional
   2677 groups, but they must be completely nested.  In other words, `#endif'
   2678 always matches the nearest `#ifdef' (or `#ifndef', or `#if').  Also,
   2679 you cannot start a conditional group in one file and end it in another.
   2680 
   2681    Even if a conditional fails, the CONTROLLED TEXT inside it is still
   2682 run through initial transformations and tokenization.  Therefore, it
   2683 must all be lexically valid C.  Normally the only way this matters is
   2684 that all comments and string literals inside a failing conditional group
   2685 must still be properly ended.
   2686 
   2687    The comment following the `#endif' is not required, but it is a good
   2688 practice if there is a lot of CONTROLLED TEXT, because it helps people
   2689 match the `#endif' to the corresponding `#ifdef'.  Older programs
   2690 sometimes put MACRO directly after the `#endif' without enclosing it in
   2691 a comment.  This is invalid code according to the C standard.  CPP
   2692 accepts it with a warning.  It never affects which `#ifndef' the
   2693 `#endif' matches.
   2694 
   2695    Sometimes you wish to use some code if a macro is _not_ defined.
   2696 You can do this by writing `#ifndef' instead of `#ifdef'.  One common
   2697 use of `#ifndef' is to include code only the first time a header file
   2698 is included.  *Note Once-Only Headers::.
   2699 
   2700    Macro definitions can vary between compilations for several reasons.
   2701 Here are some samples.
   2702 
   2703    * Some macros are predefined on each kind of machine (*note
   2704      System-specific Predefined Macros::).  This allows you to provide
   2705      code specially tuned for a particular machine.
   2706 
   2707    * System header files define more macros, associated with the
   2708      features they implement.  You can test these macros with
   2709      conditionals to avoid using a system feature on a machine where it
   2710      is not implemented.
   2711 
   2712    * Macros can be defined or undefined with the `-D' and `-U' command
   2713      line options when you compile the program.  You can arrange to
   2714      compile the same source file into two different programs by
   2715      choosing a macro name to specify which program you want, writing
   2716      conditionals to test whether or how this macro is defined, and
   2717      then controlling the state of the macro with command line options,
   2718      perhaps set in the Makefile.  *Note Invocation::.
   2719 
   2720    * Your program might have a special header file (often called
   2721      `config.h') that is adjusted when the program is compiled.  It can
   2722      define or not define macros depending on the features of the
   2723      system and the desired capabilities of the program.  The
   2724      adjustment can be automated by a tool such as `autoconf', or done
   2725      by hand.
   2726 
   2727 
   2728 File: cpp.info,  Node: If,  Next: Defined,  Prev: Ifdef,  Up: Conditional Syntax
   2729 
   2730 4.2.2 If
   2731 --------
   2732 
   2733 The `#if' directive allows you to test the value of an arithmetic
   2734 expression, rather than the mere existence of one macro.  Its syntax is
   2735 
   2736      #if EXPRESSION
   2737 
   2738      CONTROLLED TEXT
   2739 
   2740      #endif /* EXPRESSION */
   2741 
   2742    EXPRESSION is a C expression of integer type, subject to stringent
   2743 restrictions.  It may contain
   2744 
   2745    * Integer constants.
   2746 
   2747    * Character constants, which are interpreted as they would be in
   2748      normal code.
   2749 
   2750    * Arithmetic operators for addition, subtraction, multiplication,
   2751      division, bitwise operations, shifts, comparisons, and logical
   2752      operations (`&&' and `||').  The latter two obey the usual
   2753      short-circuiting rules of standard C.
   2754 
   2755    * Macros.  All macros in the expression are expanded before actual
   2756      computation of the expression's value begins.
   2757 
   2758    * Uses of the `defined' operator, which lets you check whether macros
   2759      are defined in the middle of an `#if'.
   2760 
   2761    * Identifiers that are not macros, which are all considered to be the
   2762      number zero.  This allows you to write `#if MACRO' instead of
   2763      `#ifdef MACRO', if you know that MACRO, when defined, will always
   2764      have a nonzero value.  Function-like macros used without their
   2765      function call parentheses are also treated as zero.
   2766 
   2767      In some contexts this shortcut is undesirable.  The `-Wundef'
   2768      option causes GCC to warn whenever it encounters an identifier
   2769      which is not a macro in an `#if'.
   2770 
   2771    The preprocessor does not know anything about types in the language.
   2772 Therefore, `sizeof' operators are not recognized in `#if', and neither
   2773 are `enum' constants.  They will be taken as identifiers which are not
   2774 macros, and replaced by zero.  In the case of `sizeof', this is likely
   2775 to cause the expression to be invalid.
   2776 
   2777    The preprocessor calculates the value of EXPRESSION.  It carries out
   2778 all calculations in the widest integer type known to the compiler; on
   2779 most machines supported by GCC this is 64 bits.  This is not the same
   2780 rule as the compiler uses to calculate the value of a constant
   2781 expression, and may give different results in some cases.  If the value
   2782 comes out to be nonzero, the `#if' succeeds and the CONTROLLED TEXT is
   2783 included; otherwise it is skipped.
   2784 
   2785 
   2786 File: cpp.info,  Node: Defined,  Next: Else,  Prev: If,  Up: Conditional Syntax
   2787 
   2788 4.2.3 Defined
   2789 -------------
   2790 
   2791 The special operator `defined' is used in `#if' and `#elif' expressions
   2792 to test whether a certain name is defined as a macro.  `defined NAME'
   2793 and `defined (NAME)' are both expressions whose value is 1 if NAME is
   2794 defined as a macro at the current point in the program, and 0
   2795 otherwise.  Thus,  `#if defined MACRO' is precisely equivalent to
   2796 `#ifdef MACRO'.
   2797 
   2798    `defined' is useful when you wish to test more than one macro for
   2799 existence at once.  For example,
   2800 
   2801      #if defined (__vax__) || defined (__ns16000__)
   2802 
   2803 would succeed if either of the names `__vax__' or `__ns16000__' is
   2804 defined as a macro.
   2805 
   2806    Conditionals written like this:
   2807 
   2808      #if defined BUFSIZE && BUFSIZE >= 1024
   2809 
   2810 can generally be simplified to just `#if BUFSIZE >= 1024', since if
   2811 `BUFSIZE' is not defined, it will be interpreted as having the value
   2812 zero.
   2813 
   2814    If the `defined' operator appears as a result of a macro expansion,
   2815 the C standard says the behavior is undefined.  GNU cpp treats it as a
   2816 genuine `defined' operator and evaluates it normally.  It will warn
   2817 wherever your code uses this feature if you use the command-line option
   2818 `-pedantic', since other compilers may handle it differently.
   2819 
   2820 
   2821 File: cpp.info,  Node: Else,  Next: Elif,  Prev: Defined,  Up: Conditional Syntax
   2822 
   2823 4.2.4 Else
   2824 ----------
   2825 
   2826 The `#else' directive can be added to a conditional to provide
   2827 alternative text to be used if the condition fails.  This is what it
   2828 looks like:
   2829 
   2830      #if EXPRESSION
   2831      TEXT-IF-TRUE
   2832      #else /* Not EXPRESSION */
   2833      TEXT-IF-FALSE
   2834      #endif /* Not EXPRESSION */
   2835 
   2836 If EXPRESSION is nonzero, the TEXT-IF-TRUE is included and the
   2837 TEXT-IF-FALSE is skipped.  If EXPRESSION is zero, the opposite happens.
   2838 
   2839    You can use `#else' with `#ifdef' and `#ifndef', too.
   2840 
   2841 
   2842 File: cpp.info,  Node: Elif,  Prev: Else,  Up: Conditional Syntax
   2843 
   2844 4.2.5 Elif
   2845 ----------
   2846 
   2847 One common case of nested conditionals is used to check for more than
   2848 two possible alternatives.  For example, you might have
   2849 
   2850      #if X == 1
   2851      ...
   2852      #else /* X != 1 */
   2853      #if X == 2
   2854      ...
   2855      #else /* X != 2 */
   2856      ...
   2857      #endif /* X != 2 */
   2858      #endif /* X != 1 */
   2859 
   2860    Another conditional directive, `#elif', allows this to be
   2861 abbreviated as follows:
   2862 
   2863      #if X == 1
   2864      ...
   2865      #elif X == 2
   2866      ...
   2867      #else /* X != 2 and X != 1*/
   2868      ...
   2869      #endif /* X != 2 and X != 1*/
   2870 
   2871    `#elif' stands for "else if".  Like `#else', it goes in the middle
   2872 of a conditional group and subdivides it; it does not require a
   2873 matching `#endif' of its own.  Like `#if', the `#elif' directive
   2874 includes an expression to be tested.  The text following the `#elif' is
   2875 processed only if the original `#if'-condition failed and the `#elif'
   2876 condition succeeds.
   2877 
   2878    More than one `#elif' can go in the same conditional group.  Then
   2879 the text after each `#elif' is processed only if the `#elif' condition
   2880 succeeds after the original `#if' and all previous `#elif' directives
   2881 within it have failed.
   2882 
   2883    `#else' is allowed after any number of `#elif' directives, but
   2884 `#elif' may not follow `#else'.
   2885 
   2886 
   2887 File: cpp.info,  Node: Deleted Code,  Prev: Conditional Syntax,  Up: Conditionals
   2888 
   2889 4.3 Deleted Code
   2890 ================
   2891 
   2892 If you replace or delete a part of the program but want to keep the old
   2893 code around for future reference, you often cannot simply comment it
   2894 out.  Block comments do not nest, so the first comment inside the old
   2895 code will end the commenting-out.  The probable result is a flood of
   2896 syntax errors.
   2897 
   2898    One way to avoid this problem is to use an always-false conditional
   2899 instead.  For instance, put `#if 0' before the deleted code and
   2900 `#endif' after it.  This works even if the code being turned off
   2901 contains conditionals, but they must be entire conditionals (balanced
   2902 `#if' and `#endif').
   2903 
   2904    Some people use `#ifdef notdef' instead.  This is risky, because
   2905 `notdef' might be accidentally defined as a macro, and then the
   2906 conditional would succeed.  `#if 0' can be counted on to fail.
   2907 
   2908    Do not use `#if 0' for comments which are not C code.  Use a real
   2909 comment, instead.  The interior of `#if 0' must consist of complete
   2910 tokens; in particular, single-quote characters must balance.  Comments
   2911 often contain unbalanced single-quote characters (known in English as
   2912 apostrophes).  These confuse `#if 0'.  They don't confuse `/*'.
   2913 
   2914 
   2915 File: cpp.info,  Node: Diagnostics,  Next: Line Control,  Prev: Conditionals,  Up: Top
   2916 
   2917 5 Diagnostics
   2918 *************
   2919 
   2920 The directive `#error' causes the preprocessor to report a fatal error.
   2921 The tokens forming the rest of the line following `#error' are used as
   2922 the error message.
   2923 
   2924    You would use `#error' inside of a conditional that detects a
   2925 combination of parameters which you know the program does not properly
   2926 support.  For example, if you know that the program will not run
   2927 properly on a VAX, you might write
   2928 
   2929      #ifdef __vax__
   2930      #error "Won't work on VAXen.  See comments at get_last_object."
   2931      #endif
   2932 
   2933    If you have several configuration parameters that must be set up by
   2934 the installation in a consistent way, you can use conditionals to detect
   2935 an inconsistency and report it with `#error'.  For example,
   2936 
   2937      #if !defined(UNALIGNED_INT_ASM_OP) && defined(DWARF2_DEBUGGING_INFO)
   2938      #error "DWARF2_DEBUGGING_INFO requires UNALIGNED_INT_ASM_OP."
   2939      #endif
   2940 
   2941    The directive `#warning' is like `#error', but causes the
   2942 preprocessor to issue a warning and continue preprocessing.  The tokens
   2943 following `#warning' are used as the warning message.
   2944 
   2945    You might use `#warning' in obsolete header files, with a message
   2946 directing the user to the header file which should be used instead.
   2947 
   2948    Neither `#error' nor `#warning' macro-expands its argument.
   2949 Internal whitespace sequences are each replaced with a single space.
   2950 The line must consist of complete tokens.  It is wisest to make the
   2951 argument of these directives be a single string constant; this avoids
   2952 problems with apostrophes and the like.
   2953 
   2954 
   2955 File: cpp.info,  Node: Line Control,  Next: Pragmas,  Prev: Diagnostics,  Up: Top
   2956 
   2957 6 Line Control
   2958 **************
   2959 
   2960 The C preprocessor informs the C compiler of the location in your source
   2961 code where each token came from.  Presently, this is just the file name
   2962 and line number.  All the tokens resulting from macro expansion are
   2963 reported as having appeared on the line of the source file where the
   2964 outermost macro was used.  We intend to be more accurate in the future.
   2965 
   2966    If you write a program which generates source code, such as the
   2967 `bison' parser generator, you may want to adjust the preprocessor's
   2968 notion of the current file name and line number by hand.  Parts of the
   2969 output from `bison' are generated from scratch, other parts come from a
   2970 standard parser file.  The rest are copied verbatim from `bison''s
   2971 input.  You would like compiler error messages and symbolic debuggers
   2972 to be able to refer to `bison''s input file.
   2973 
   2974    `bison' or any such program can arrange this by writing `#line'
   2975 directives into the output file.  `#line' is a directive that specifies
   2976 the original line number and source file name for subsequent input in
   2977 the current preprocessor input file.  `#line' has three variants:
   2978 
   2979 `#line LINENUM'
   2980      LINENUM is a non-negative decimal integer constant.  It specifies
   2981      the line number which should be reported for the following line of
   2982      input.  Subsequent lines are counted from LINENUM.
   2983 
   2984 `#line LINENUM FILENAME'
   2985      LINENUM is the same as for the first form, and has the same
   2986      effect.  In addition, FILENAME is a string constant.  The
   2987      following line and all subsequent lines are reported to come from
   2988      the file it specifies, until something else happens to change that.
   2989      FILENAME is interpreted according to the normal rules for a string
   2990      constant: backslash escapes are interpreted.  This is different
   2991      from `#include'.
   2992 
   2993      Previous versions of CPP did not interpret escapes in `#line'; we
   2994      have changed it because the standard requires they be interpreted,
   2995      and most other compilers do.
   2996 
   2997 `#line ANYTHING ELSE'
   2998      ANYTHING ELSE is checked for macro calls, which are expanded.  The
   2999      result should match one of the above two forms.
   3000 
   3001    `#line' directives alter the results of the `__FILE__' and
   3002 `__LINE__' predefined macros from that point on.  *Note Standard
   3003 Predefined Macros::.  They do not have any effect on `#include''s idea
   3004 of the directory containing the current file.  This is a change from
   3005 GCC 2.95.  Previously, a file reading
   3006 
   3007      #line 1 "../src/gram.y"
   3008      #include "gram.h"
   3009 
   3010    would search for `gram.h' in `../src', then the `-I' chain; the
   3011 directory containing the physical source file would not be searched.
   3012 In GCC 3.0 and later, the `#include' is not affected by the presence of
   3013 a `#line' referring to a different directory.
   3014 
   3015    We made this change because the old behavior caused problems when
   3016 generated source files were transported between machines.  For instance,
   3017 it is common practice to ship generated parsers with a source release,
   3018 so that people building the distribution do not need to have yacc or
   3019 Bison installed.  These files frequently have `#line' directives
   3020 referring to the directory tree of the system where the distribution was
   3021 created.  If GCC tries to search for headers in those directories, the
   3022 build is likely to fail.
   3023 
   3024    The new behavior can cause failures too, if the generated file is not
   3025 in the same directory as its source and it attempts to include a header
   3026 which would be visible searching from the directory containing the
   3027 source file.  However, this problem is easily solved with an additional
   3028 `-I' switch on the command line.  The failures caused by the old
   3029 semantics could sometimes be corrected only by editing the generated
   3030 files, which is difficult and error-prone.
   3031 
   3032 
   3033 File: cpp.info,  Node: Pragmas,  Next: Other Directives,  Prev: Line Control,  Up: Top
   3034 
   3035 7 Pragmas
   3036 *********
   3037 
   3038 The `#pragma' directive is the method specified by the C standard for
   3039 providing additional information to the compiler, beyond what is
   3040 conveyed in the language itself.  Three forms of this directive
   3041 (commonly known as "pragmas") are specified by the 1999 C standard.  A
   3042 C compiler is free to attach any meaning it likes to other pragmas.
   3043 
   3044    GCC has historically preferred to use extensions to the syntax of the
   3045 language, such as `__attribute__', for this purpose.  However, GCC does
   3046 define a few pragmas of its own.  These mostly have effects on the
   3047 entire translation unit or source file.
   3048 
   3049    In GCC version 3, all GNU-defined, supported pragmas have been given
   3050 a `GCC' prefix.  This is in line with the `STDC' prefix on all pragmas
   3051 defined by C99.  For backward compatibility, pragmas which were
   3052 recognized by previous versions are still recognized without the `GCC'
   3053 prefix, but that usage is deprecated.  Some older pragmas are
   3054 deprecated in their entirety.  They are not recognized with the `GCC'
   3055 prefix.  *Note Obsolete Features::.
   3056 
   3057    C99 introduces the `_Pragma' operator.  This feature addresses a
   3058 major problem with `#pragma': being a directive, it cannot be produced
   3059 as the result of macro expansion.  `_Pragma' is an operator, much like
   3060 `sizeof' or `defined', and can be embedded in a macro.
   3061 
   3062    Its syntax is `_Pragma (STRING-LITERAL)', where STRING-LITERAL can
   3063 be either a normal or wide-character string literal.  It is
   3064 destringized, by replacing all `\\' with a single `\' and all `\"' with
   3065 a `"'.  The result is then processed as if it had appeared as the right
   3066 hand side of a `#pragma' directive.  For example,
   3067 
   3068      _Pragma ("GCC dependency \"parse.y\"")
   3069 
   3070 has the same effect as `#pragma GCC dependency "parse.y"'.  The same
   3071 effect could be achieved using macros, for example
   3072 
   3073      #define DO_PRAGMA(x) _Pragma (#x)
   3074      DO_PRAGMA (GCC dependency "parse.y")
   3075 
   3076    The standard is unclear on where a `_Pragma' operator can appear.
   3077 The preprocessor does not accept it within a preprocessing conditional
   3078 directive like `#if'.  To be safe, you are probably best keeping it out
   3079 of directives other than `#define', and putting it on a line of its own.
   3080 
   3081    This manual documents the pragmas which are meaningful to the
   3082 preprocessor itself.  Other pragmas are meaningful to the C or C++
   3083 compilers.  They are documented in the GCC manual.
   3084 
   3085 `#pragma GCC dependency'
   3086      `#pragma GCC dependency' allows you to check the relative dates of
   3087      the current file and another file.  If the other file is more
   3088      recent than the current file, a warning is issued.  This is useful
   3089      if the current file is derived from the other file, and should be
   3090      regenerated.  The other file is searched for using the normal
   3091      include search path.  Optional trailing text can be used to give
   3092      more information in the warning message.
   3093 
   3094           #pragma GCC dependency "parse.y"
   3095           #pragma GCC dependency "/usr/include/time.h" rerun fixincludes
   3096 
   3097 `#pragma GCC poison'
   3098      Sometimes, there is an identifier that you want to remove
   3099      completely from your program, and make sure that it never creeps
   3100      back in.  To enforce this, you can "poison" the identifier with
   3101      this pragma.  `#pragma GCC poison' is followed by a list of
   3102      identifiers to poison.  If any of those identifiers appears
   3103      anywhere in the source after the directive, it is a hard error.
   3104      For example,
   3105 
   3106           #pragma GCC poison printf sprintf fprintf
   3107           sprintf(some_string, "hello");
   3108 
   3109      will produce an error.
   3110 
   3111      If a poisoned identifier appears as part of the expansion of a
   3112      macro which was defined before the identifier was poisoned, it
   3113      will _not_ cause an error.  This lets you poison an identifier
   3114      without worrying about system headers defining macros that use it.
   3115 
   3116      For example,
   3117 
   3118           #define strrchr rindex
   3119           #pragma GCC poison rindex
   3120           strrchr(some_string, 'h');
   3121 
   3122      will not produce an error.
   3123 
   3124 `#pragma GCC system_header'
   3125      This pragma takes no arguments.  It causes the rest of the code in
   3126      the current file to be treated as if it came from a system header.
   3127      *Note System Headers::.
   3128 
   3129 
   3130 
   3131 File: cpp.info,  Node: Other Directives,  Next: Preprocessor Output,  Prev: Pragmas,  Up: Top
   3132 
   3133 8 Other Directives
   3134 ******************
   3135 
   3136 The `#ident' directive takes one argument, a string constant.  On some
   3137 systems, that string constant is copied into a special segment of the
   3138 object file.  On other systems, the directive is ignored.  The `#sccs'
   3139 directive is a synonym for `#ident'.
   3140 
   3141    These directives are not part of the C standard, but they are not
   3142 official GNU extensions either.  What historical information we have
   3143 been able to find, suggests they originated with System V.
   3144 
   3145    The "null directive" consists of a `#' followed by a newline, with
   3146 only whitespace (including comments) in between.  A null directive is
   3147 understood as a preprocessing directive but has no effect on the
   3148 preprocessor output.  The primary significance of the existence of the
   3149 null directive is that an input line consisting of just a `#' will
   3150 produce no output, rather than a line of output containing just a `#'.
   3151 Supposedly some old C programs contain such lines.
   3152 
   3153 
   3154 File: cpp.info,  Node: Preprocessor Output,  Next: Traditional Mode,  Prev: Other Directives,  Up: Top
   3155 
   3156 9 Preprocessor Output
   3157 *********************
   3158 
   3159 When the C preprocessor is used with the C, C++, or Objective-C
   3160 compilers, it is integrated into the compiler and communicates a stream
   3161 of binary tokens directly to the compiler's parser.  However, it can
   3162 also be used in the more conventional standalone mode, where it produces
   3163 textual output.
   3164 
   3165    The output from the C preprocessor looks much like the input, except
   3166 that all preprocessing directive lines have been replaced with blank
   3167 lines and all comments with spaces.  Long runs of blank lines are
   3168 discarded.
   3169 
   3170    The ISO standard specifies that it is implementation defined whether
   3171 a preprocessor preserves whitespace between tokens, or replaces it with
   3172 e.g. a single space.  In GNU CPP, whitespace between tokens is collapsed
   3173 to become a single space, with the exception that the first token on a
   3174 non-directive line is preceded with sufficient spaces that it appears in
   3175 the same column in the preprocessed output that it appeared in the
   3176 original source file.  This is so the output is easy to read.  *Note
   3177 Differences from previous versions::.  CPP does not insert any
   3178 whitespace where there was none in the original source, except where
   3179 necessary to prevent an accidental token paste.
   3180 
   3181    Source file name and line number information is conveyed by lines of
   3182 the form
   3183 
   3184      # LINENUM FILENAME FLAGS
   3185 
   3186 These are called "linemarkers".  They are inserted as needed into the
   3187 output (but never within a string or character constant).  They mean
   3188 that the following line originated in file FILENAME at line LINENUM.
   3189 FILENAME will never contain any non-printing characters; they are
   3190 replaced with octal escape sequences.
   3191 
   3192    After the file name comes zero or more flags, which are `1', `2',
   3193 `3', or `4'.  If there are multiple flags, spaces separate them.  Here
   3194 is what the flags mean:
   3195 
   3196 `1'
   3197      This indicates the start of a new file.
   3198 
   3199 `2'
   3200      This indicates returning to a file (after having included another
   3201      file).
   3202 
   3203 `3'
   3204      This indicates that the following text comes from a system header
   3205      file, so certain warnings should be suppressed.
   3206 
   3207 `4'
   3208      This indicates that the following text should be treated as being
   3209      wrapped in an implicit `extern "C"' block.
   3210 
   3211    As an extension, the preprocessor accepts linemarkers in
   3212 non-assembler input files.  They are treated like the corresponding
   3213 `#line' directive, (*note Line Control::), except that trailing flags
   3214 are permitted, and are interpreted with the meanings described above.
   3215 If multiple flags are given, they must be in ascending order.
   3216 
   3217    Some directives may be duplicated in the output of the preprocessor.
   3218 These are `#ident' (always), `#pragma' (only if the preprocessor does
   3219 not handle the pragma itself), and `#define' and `#undef' (with certain
   3220 debugging options).  If this happens, the `#' of the directive will
   3221 always be in the first column, and there will be no space between the
   3222 `#' and the directive name.  If macro expansion happens to generate
   3223 tokens which might be mistaken for a duplicated directive, a space will
   3224 be inserted between the `#' and the directive name.
   3225 
   3226 
   3227 File: cpp.info,  Node: Traditional Mode,  Next: Implementation Details,  Prev: Preprocessor Output,  Up: Top
   3228 
   3229 10 Traditional Mode
   3230 *******************
   3231 
   3232 Traditional (pre-standard) C preprocessing is rather different from the
   3233 preprocessing specified by the standard.  When GCC is given the
   3234 `-traditional-cpp' option, it attempts to emulate a traditional
   3235 preprocessor.
   3236 
   3237    GCC versions 3.2 and later only support traditional mode semantics in
   3238 the preprocessor, and not in the compiler front ends.  This chapter
   3239 outlines the traditional preprocessor semantics we implemented.
   3240 
   3241    The implementation does not correspond precisely to the behavior of
   3242 earlier versions of GCC, nor to any true traditional preprocessor.
   3243 After all, inconsistencies among traditional implementations were a
   3244 major motivation for C standardization.  However, we intend that it
   3245 should be compatible with true traditional preprocessors in all ways
   3246 that actually matter.
   3247 
   3248 * Menu:
   3249 
   3250 * Traditional lexical analysis::
   3251 * Traditional macros::
   3252 * Traditional miscellany::
   3253 * Traditional warnings::
   3254 
   3255 
   3256 File: cpp.info,  Node: Traditional lexical analysis,  Next: Traditional macros,  Up: Traditional Mode
   3257 
   3258 10.1 Traditional lexical analysis
   3259 =================================
   3260 
   3261 The traditional preprocessor does not decompose its input into tokens
   3262 the same way a standards-conforming preprocessor does.  The input is
   3263 simply treated as a stream of text with minimal internal form.
   3264 
   3265    This implementation does not treat trigraphs (*note trigraphs::)
   3266 specially since they were an invention of the standards committee.  It
   3267 handles arbitrarily-positioned escaped newlines properly and splices
   3268 the lines as you would expect; many traditional preprocessors did not
   3269 do this.
   3270 
   3271    The form of horizontal whitespace in the input file is preserved in
   3272 the output.  In particular, hard tabs remain hard tabs.  This can be
   3273 useful if, for example, you are preprocessing a Makefile.
   3274 
   3275    Traditional CPP only recognizes C-style block comments, and treats
   3276 the `/*' sequence as introducing a comment only if it lies outside
   3277 quoted text.  Quoted text is introduced by the usual single and double
   3278 quotes, and also by an initial `<' in a `#include' directive.
   3279 
   3280    Traditionally, comments are completely removed and are not replaced
   3281 with a space.  Since a traditional compiler does its own tokenization
   3282 of the output of the preprocessor, this means that comments can
   3283 effectively be used as token paste operators.  However, comments behave
   3284 like separators for text handled by the preprocessor itself, since it
   3285 doesn't re-lex its input.  For example, in
   3286 
   3287      #if foo/**/bar
   3288 
   3289 `foo' and `bar' are distinct identifiers and expanded separately if
   3290 they happen to be macros.  In other words, this directive is equivalent
   3291 to
   3292 
   3293      #if foo bar
   3294 
   3295 rather than
   3296 
   3297      #if foobar
   3298 
   3299    Generally speaking, in traditional mode an opening quote need not
   3300 have a matching closing quote.  In particular, a macro may be defined
   3301 with replacement text that contains an unmatched quote.  Of course, if
   3302 you attempt to compile preprocessed output containing an unmatched quote
   3303 you will get a syntax error.
   3304 
   3305    However, all preprocessing directives other than `#define' require
   3306 matching quotes.  For example:
   3307 
   3308      #define m This macro's fine and has an unmatched quote
   3309      "/* This is not a comment.  */
   3310      /* This is a comment.  The following #include directive
   3311         is ill-formed.  */
   3312      #include <stdio.h
   3313 
   3314    Just as for the ISO preprocessor, what would be a closing quote can
   3315 be escaped with a backslash to prevent the quoted text from closing.
   3316 
   3317 
   3318 File: cpp.info,  Node: Traditional macros,  Next: Traditional miscellany,  Prev: Traditional lexical analysis,  Up: Traditional Mode
   3319 
   3320 10.2 Traditional macros
   3321 =======================
   3322 
   3323 The major difference between traditional and ISO macros is that the
   3324 former expand to text rather than to a token sequence.  CPP removes all
   3325 leading and trailing horizontal whitespace from a macro's replacement
   3326 text before storing it, but preserves the form of internal whitespace.
   3327 
   3328    One consequence is that it is legitimate for the replacement text to
   3329 contain an unmatched quote (*note Traditional lexical analysis::).  An
   3330 unclosed string or character constant continues into the text following
   3331 the macro call.  Similarly, the text at the end of a macro's expansion
   3332 can run together with the text after the macro invocation to produce a
   3333 single token.
   3334 
   3335    Normally comments are removed from the replacement text after the
   3336 macro is expanded, but if the `-CC' option is passed on the command
   3337 line comments are preserved.  (In fact, the current implementation
   3338 removes comments even before saving the macro replacement text, but it
   3339 careful to do it in such a way that the observed effect is identical
   3340 even in the function-like macro case.)
   3341 
   3342    The ISO stringification operator `#' and token paste operator `##'
   3343 have no special meaning.  As explained later, an effect similar to
   3344 these operators can be obtained in a different way.  Macro names that
   3345 are embedded in quotes, either from the main file or after macro
   3346 replacement, do not expand.
   3347 
   3348    CPP replaces an unquoted object-like macro name with its replacement
   3349 text, and then rescans it for further macros to replace.  Unlike
   3350 standard macro expansion, traditional macro expansion has no provision
   3351 to prevent recursion.  If an object-like macro appears unquoted in its
   3352 replacement text, it will be replaced again during the rescan pass, and
   3353 so on _ad infinitum_.  GCC detects when it is expanding recursive
   3354 macros, emits an error message, and continues after the offending macro
   3355 invocation.
   3356 
   3357      #define PLUS +
   3358      #define INC(x) PLUS+x
   3359      INC(foo);
   3360           ==> ++foo;
   3361 
   3362    Function-like macros are similar in form but quite different in
   3363 behavior to their ISO counterparts.  Their arguments are contained
   3364 within parentheses, are comma-separated, and can cross physical lines.
   3365 Commas within nested parentheses are not treated as argument
   3366 separators.  Similarly, a quote in an argument cannot be left unclosed;
   3367 a following comma or parenthesis that comes before the closing quote is
   3368 treated like any other character.  There is no facility for handling
   3369 variadic macros.
   3370 
   3371    This implementation removes all comments from macro arguments, unless
   3372 the `-C' option is given.  The form of all other horizontal whitespace
   3373 in arguments is preserved, including leading and trailing whitespace.
   3374 In particular
   3375 
   3376      f( )
   3377 
   3378 is treated as an invocation of the macro `f' with a single argument
   3379 consisting of a single space.  If you want to invoke a function-like
   3380 macro that takes no arguments, you must not leave any whitespace
   3381 between the parentheses.
   3382 
   3383    If a macro argument crosses a new line, the new line is replaced with
   3384 a space when forming the argument.  If the previous line contained an
   3385 unterminated quote, the following line inherits the quoted state.
   3386 
   3387    Traditional preprocessors replace parameters in the replacement text
   3388 with their arguments regardless of whether the parameters are within
   3389 quotes or not.  This provides a way to stringize arguments.  For example
   3390 
   3391      #define str(x) "x"
   3392      str(/* A comment */some text )
   3393           ==> "some text "
   3394 
   3395 Note that the comment is removed, but that the trailing space is
   3396 preserved.  Here is an example of using a comment to effect token
   3397 pasting.
   3398 
   3399      #define suffix(x) foo_/**/x
   3400      suffix(bar)
   3401           ==> foo_bar
   3402 
   3403 
   3404 File: cpp.info,  Node: Traditional miscellany,  Next: Traditional warnings,  Prev: Traditional macros,  Up: Traditional Mode
   3405 
   3406 10.3 Traditional miscellany
   3407 ===========================
   3408 
   3409 Here are some things to be aware of when using the traditional
   3410 preprocessor.
   3411 
   3412    * Preprocessing directives are recognized only when their leading
   3413      `#' appears in the first column.  There can be no whitespace
   3414      between the beginning of the line and the `#', but whitespace can
   3415      follow the `#'.
   3416 
   3417    * A true traditional C preprocessor does not recognize `#error' or
   3418      `#pragma', and may not recognize `#elif'.  CPP supports all the
   3419      directives in traditional mode that it supports in ISO mode,
   3420      including extensions, with the exception that the effects of
   3421      `#pragma GCC poison' are undefined.
   3422 
   3423    * __STDC__ is not defined.
   3424 
   3425    * If you use digraphs the behavior is undefined.
   3426 
   3427    * If a line that looks like a directive appears within macro
   3428      arguments, the behavior is undefined.
   3429 
   3430 
   3431 
   3432 File: cpp.info,  Node: Traditional warnings,  Prev: Traditional miscellany,  Up: Traditional Mode
   3433 
   3434 10.4 Traditional warnings
   3435 =========================
   3436 
   3437 You can request warnings about features that did not exist, or worked
   3438 differently, in traditional C with the `-Wtraditional' option.  GCC
   3439 does not warn about features of ISO C which you must use when you are
   3440 using a conforming compiler, such as the `#' and `##' operators.
   3441 
   3442    Presently `-Wtraditional' warns about:
   3443 
   3444    * Macro parameters that appear within string literals in the macro
   3445      body.  In traditional C macro replacement takes place within
   3446      string literals, but does not in ISO C.
   3447 
   3448    * In traditional C, some preprocessor directives did not exist.
   3449      Traditional preprocessors would only consider a line to be a
   3450      directive if the `#' appeared in column 1 on the line.  Therefore
   3451      `-Wtraditional' warns about directives that traditional C
   3452      understands but would ignore because the `#' does not appear as the
   3453      first character on the line.  It also suggests you hide directives
   3454      like `#pragma' not understood by traditional C by indenting them.
   3455      Some traditional implementations would not recognize `#elif', so it
   3456      suggests avoiding it altogether.
   3457 
   3458    * A function-like macro that appears without an argument list.  In
   3459      some traditional preprocessors this was an error.  In ISO C it
   3460      merely means that the macro is not expanded.
   3461 
   3462    * The unary plus operator.  This did not exist in traditional C.
   3463 
   3464    * The `U' and `LL' integer constant suffixes, which were not
   3465      available in traditional C.  (Traditional C does support the `L'
   3466      suffix for simple long integer constants.)  You are not warned
   3467      about uses of these suffixes in macros defined in system headers.
   3468      For instance, `UINT_MAX' may well be defined as `4294967295U', but
   3469      you will not be warned if you use `UINT_MAX'.
   3470 
   3471      You can usually avoid the warning, and the related warning about
   3472      constants which are so large that they are unsigned, by writing the
   3473      integer constant in question in hexadecimal, with no U suffix.
   3474      Take care, though, because this gives the wrong result in exotic
   3475      cases.
   3476 
   3477 
   3478 File: cpp.info,  Node: Implementation Details,  Next: Invocation,  Prev: Traditional Mode,  Up: Top
   3479 
   3480 11 Implementation Details
   3481 *************************
   3482 
   3483 Here we document details of how the preprocessor's implementation
   3484 affects its user-visible behavior.  You should try to avoid undue
   3485 reliance on behavior described here, as it is possible that it will
   3486 change subtly in future implementations.
   3487 
   3488    Also documented here are obsolete features and changes from previous
   3489 versions of CPP.
   3490 
   3491 * Menu:
   3492 
   3493 * Implementation-defined behavior::
   3494 * Implementation limits::
   3495 * Obsolete Features::
   3496 * Differences from previous versions::
   3497 
   3498 
   3499 File: cpp.info,  Node: Implementation-defined behavior,  Next: Implementation limits,  Up: Implementation Details
   3500 
   3501 11.1 Implementation-defined behavior
   3502 ====================================
   3503 
   3504 This is how CPP behaves in all the cases which the C standard describes
   3505 as "implementation-defined".  This term means that the implementation
   3506 is free to do what it likes, but must document its choice and stick to
   3507 it.
   3508 
   3509    * The mapping of physical source file multi-byte characters to the
   3510      execution character set.
   3511 
   3512      Currently, CPP requires its input to be ASCII or UTF-8.  The
   3513      execution character set may be controlled by the user, with the
   3514      `-fexec-charset' and `-fwide-exec-charset' options.
   3515 
   3516    * Identifier characters.
   3517 
   3518      The C and C++ standards allow identifiers to be composed of `_'
   3519      and the alphanumeric characters.  C++ and C99 also allow universal
   3520      character names, and C99 further permits implementation-defined
   3521      characters.  GCC currently only permits universal character names
   3522      if `-fextended-identifiers' is used, because the implementation of
   3523      universal character names in identifiers is experimental.
   3524 
   3525      GCC allows the `$' character in identifiers as an extension for
   3526      most targets.  This is true regardless of the `std=' switch, since
   3527      this extension cannot conflict with standards-conforming programs.
   3528      When preprocessing assembler, however, dollars are not identifier
   3529      characters by default.
   3530 
   3531      Currently the targets that by default do not permit `$' are AVR,
   3532      IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC targets for the AIX
   3533      and BeOS operating systems.
   3534 
   3535      You can override the default with `-fdollars-in-identifiers' or
   3536      `fno-dollars-in-identifiers'.  *Note fdollars-in-identifiers::.
   3537 
   3538    * Non-empty sequences of whitespace characters.
   3539 
   3540      In textual output, each whitespace sequence is collapsed to a
   3541      single space.  For aesthetic reasons, the first token on each
   3542      non-directive line of output is preceded with sufficient spaces
   3543      that it appears in the same column as it did in the original
   3544      source file.
   3545 
   3546    * The numeric value of character constants in preprocessor
   3547      expressions.
   3548 
   3549      The preprocessor and compiler interpret character constants in the
   3550      same way; i.e. escape sequences such as `\a' are given the values
   3551      they would have on the target machine.
   3552 
   3553      The compiler values a multi-character character constant a
   3554      character at a time, shifting the previous value left by the
   3555      number of bits per target character, and then or-ing in the
   3556      bit-pattern of the new character truncated to the width of a
   3557      target character.  The final bit-pattern is given type `int', and
   3558      is therefore signed, regardless of whether single characters are
   3559      signed or not (a slight change from versions 3.1 and earlier of
   3560      GCC).  If there are more characters in the constant than would fit
   3561      in the target `int' the compiler issues a warning, and the excess
   3562      leading characters are ignored.
   3563 
   3564      For example, `'ab'' for a target with an 8-bit `char' would be
   3565      interpreted as
   3566      `(int) ((unsigned char) 'a' * 256 + (unsigned char) 'b')', and
   3567      `'\234a'' as
   3568      `(int) ((unsigned char) '\234' * 256 + (unsigned char) 'a')'.
   3569 
   3570    * Source file inclusion.
   3571 
   3572      For a discussion on how the preprocessor locates header files,
   3573      *Note Include Operation::.
   3574 
   3575    * Interpretation of the filename resulting from a macro-expanded
   3576      `#include' directive.
   3577 
   3578      *Note Computed Includes::.
   3579 
   3580    * Treatment of a `#pragma' directive that after macro-expansion
   3581      results in a standard pragma.
   3582 
   3583      No macro expansion occurs on any `#pragma' directive line, so the
   3584      question does not arise.
   3585 
   3586      Note that GCC does not yet implement any of the standard pragmas.
   3587 
   3588 
   3589 
   3590 File: cpp.info,  Node: Implementation limits,  Next: Obsolete Features,  Prev: Implementation-defined behavior,  Up: Implementation Details
   3591 
   3592 11.2 Implementation limits
   3593 ==========================
   3594 
   3595 CPP has a small number of internal limits.  This section lists the
   3596 limits which the C standard requires to be no lower than some minimum,
   3597 and all the others known.  It is intended that there should be as few
   3598 limits as possible.  If you encounter an undocumented or inconvenient
   3599 limit, please report that as a bug.  *Note Reporting Bugs: (gcc)Bugs.
   3600 
   3601    Where we say something is limited "only by available memory", that
   3602 means that internal data structures impose no intrinsic limit, and space
   3603 is allocated with `malloc' or equivalent.  The actual limit will
   3604 therefore depend on many things, such as the size of other things
   3605 allocated by the compiler at the same time, the amount of memory
   3606 consumed by other processes on the same computer, etc.
   3607 
   3608    * Nesting levels of `#include' files.
   3609 
   3610      We impose an arbitrary limit of 200 levels, to avoid runaway
   3611      recursion.  The standard requires at least 15 levels.
   3612 
   3613    * Nesting levels of conditional inclusion.
   3614 
   3615      The C standard mandates this be at least 63.  CPP is limited only
   3616      by available memory.
   3617 
   3618    * Levels of parenthesized expressions within a full expression.
   3619 
   3620      The C standard requires this to be at least 63.  In preprocessor
   3621      conditional expressions, it is limited only by available memory.
   3622 
   3623    * Significant initial characters in an identifier or macro name.
   3624 
   3625      The preprocessor treats all characters as significant.  The C
   3626      standard requires only that the first 63 be significant.
   3627 
   3628    * Number of macros simultaneously defined in a single translation
   3629      unit.
   3630 
   3631      The standard requires at least 4095 be possible.  CPP is limited
   3632      only by available memory.
   3633 
   3634    * Number of parameters in a macro definition and arguments in a
   3635      macro call.
   3636 
   3637      We allow `USHRT_MAX', which is no smaller than 65,535.  The minimum
   3638      required by the standard is 127.
   3639 
   3640    * Number of characters on a logical source line.
   3641 
   3642      The C standard requires a minimum of 4096 be permitted.  CPP places
   3643      no limits on this, but you may get incorrect column numbers
   3644      reported in diagnostics for lines longer than 65,535 characters.
   3645 
   3646    * Maximum size of a source file.
   3647 
   3648      The standard does not specify any lower limit on the maximum size
   3649      of a source file.  GNU cpp maps files into memory, so it is
   3650      limited by the available address space.  This is generally at
   3651      least two gigabytes.  Depending on the operating system, the size
   3652      of physical memory may or may not be a limitation.
   3653 
   3654 
   3655 
   3656 File: cpp.info,  Node: Obsolete Features,  Next: Differences from previous versions,  Prev: Implementation limits,  Up: Implementation Details
   3657 
   3658 11.3 Obsolete Features
   3659 ======================
   3660 
   3661 CPP has a number of features which are present mainly for compatibility
   3662 with older programs.  We discourage their use in new code.  In some
   3663 cases, we plan to remove the feature in a future version of GCC.
   3664 
   3665 * Menu:
   3666 
   3667 * Assertions::
   3668 * Obsolete once-only headers::
   3669 
   3670 
   3671 File: cpp.info,  Node: Assertions,  Next: Obsolete once-only headers,  Up: Obsolete Features
   3672 
   3673 11.3.1 Assertions
   3674 -----------------
   3675 
   3676 "Assertions" are a deprecated alternative to macros in writing
   3677 conditionals to test what sort of computer or system the compiled
   3678 program will run on.  Assertions are usually predefined, but you can
   3679 define them with preprocessing directives or command-line options.
   3680 
   3681    Assertions were intended to provide a more systematic way to describe
   3682 the compiler's target system.  However, in practice they are just as
   3683 unpredictable as the system-specific predefined macros.  In addition,
   3684 they are not part of any standard, and only a few compilers support
   3685 them.  Therefore, the use of assertions is *less* portable than the use
   3686 of system-specific predefined macros.  We recommend you do not use them
   3687 at all.
   3688 
   3689    An assertion looks like this:
   3690 
   3691      #PREDICATE (ANSWER)
   3692 
   3693 PREDICATE must be a single identifier.  ANSWER can be any sequence of
   3694 tokens; all characters are significant except for leading and trailing
   3695 whitespace, and differences in internal whitespace sequences are
   3696 ignored.  (This is similar to the rules governing macro redefinition.)
   3697 Thus, `(x + y)' is different from `(x+y)' but equivalent to
   3698 `( x + y )'.  Parentheses do not nest inside an answer.
   3699 
   3700    To test an assertion, you write it in an `#if'.  For example, this
   3701 conditional succeeds if either `vax' or `ns16000' has been asserted as
   3702 an answer for `machine'.
   3703 
   3704      #if #machine (vax) || #machine (ns16000)
   3705 
   3706 You can test whether _any_ answer is asserted for a predicate by
   3707 omitting the answer in the conditional:
   3708 
   3709      #if #machine
   3710 
   3711    Assertions are made with the `#assert' directive.  Its sole argument
   3712 is the assertion to make, without the leading `#' that identifies
   3713 assertions in conditionals.
   3714 
   3715      #assert PREDICATE (ANSWER)
   3716 
   3717 You may make several assertions with the same predicate and different
   3718 answers.  Subsequent assertions do not override previous ones for the
   3719 same predicate.  All the answers for any given predicate are
   3720 simultaneously true.
   3721 
   3722    Assertions can be canceled with the `#unassert' directive.  It has
   3723 the same syntax as `#assert'.  In that form it cancels only the answer
   3724 which was specified on the `#unassert' line; other answers for that
   3725 predicate remain true.  You can cancel an entire predicate by leaving
   3726 out the answer:
   3727 
   3728      #unassert PREDICATE
   3729 
   3730 In either form, if no such assertion has been made, `#unassert' has no
   3731 effect.
   3732 
   3733    You can also make or cancel assertions using command line options.
   3734 *Note Invocation::.
   3735 
   3736 
   3737 File: cpp.info,  Node: Obsolete once-only headers,  Prev: Assertions,  Up: Obsolete Features
   3738 
   3739 11.3.2 Obsolete once-only headers
   3740 ---------------------------------
   3741 
   3742 CPP supports two more ways of indicating that a header file should be
   3743 read only once.  Neither one is as portable as a wrapper `#ifndef', and
   3744 we recommend you do not use them in new programs.
   3745 
   3746    In the Objective-C language, there is a variant of `#include' called
   3747 `#import' which includes a file, but does so at most once.  If you use
   3748 `#import' instead of `#include', then you don't need the conditionals
   3749 inside the header file to prevent multiple inclusion of the contents.
   3750 GCC permits the use of `#import' in C and C++ as well as Objective-C.
   3751 However, it is not in standard C or C++ and should therefore not be
   3752 used by portable programs.
   3753 
   3754    `#import' is not a well designed feature.  It requires the users of
   3755 a header file to know that it should only be included once.  It is much
   3756 better for the header file's implementor to write the file so that users
   3757 don't need to know this.  Using a wrapper `#ifndef' accomplishes this
   3758 goal.
   3759 
   3760    In the present implementation, a single use of `#import' will
   3761 prevent the file from ever being read again, by either `#import' or
   3762 `#include'.  You should not rely on this; do not use both `#import' and
   3763 `#include' to refer to the same header file.
   3764 
   3765    Another way to prevent a header file from being included more than
   3766 once is with the `#pragma once' directive.  If `#pragma once' is seen
   3767 when scanning a header file, that file will never be read again, no
   3768 matter what.
   3769 
   3770    `#pragma once' does not have the problems that `#import' does, but
   3771 it is not recognized by all preprocessors, so you cannot rely on it in
   3772 a portable program.
   3773 
   3774 
   3775 File: cpp.info,  Node: Differences from previous versions,  Prev: Obsolete Features,  Up: Implementation Details
   3776 
   3777 11.4 Differences from previous versions
   3778 =======================================
   3779 
   3780 This section details behavior which has changed from previous versions
   3781 of CPP.  We do not plan to change it again in the near future, but we
   3782 do not promise not to, either.
   3783 
   3784    The "previous versions" discussed here are 2.95 and before.  The
   3785 behavior of GCC 3.0 is mostly the same as the behavior of the widely
   3786 used 2.96 and 2.97 development snapshots.  Where there are differences,
   3787 they generally represent bugs in the snapshots.
   3788 
   3789    * -I- deprecated
   3790 
   3791      This option has been deprecated in 4.0.  `-iquote' is meant to
   3792      replace the need for this option.
   3793 
   3794    * Order of evaluation of `#' and `##' operators
   3795 
   3796      The standard does not specify the order of evaluation of a chain of
   3797      `##' operators, nor whether `#' is evaluated before, after, or at
   3798      the same time as `##'.  You should therefore not write any code
   3799      which depends on any specific ordering.  It is possible to
   3800      guarantee an ordering, if you need one, by suitable use of nested
   3801      macros.
   3802 
   3803      An example of where this might matter is pasting the arguments `1',
   3804      `e' and `-2'.  This would be fine for left-to-right pasting, but
   3805      right-to-left pasting would produce an invalid token `e-2'.
   3806 
   3807      GCC 3.0 evaluates `#' and `##' at the same time and strictly left
   3808      to right.  Older versions evaluated all `#' operators first, then
   3809      all `##' operators, in an unreliable order.
   3810 
   3811    * The form of whitespace between tokens in preprocessor output
   3812 
   3813      *Note Preprocessor Output::, for the current textual format.  This
   3814      is also the format used by stringification.  Normally, the
   3815      preprocessor communicates tokens directly to the compiler's
   3816      parser, and whitespace does not come up at all.
   3817 
   3818      Older versions of GCC preserved all whitespace provided by the
   3819      user and inserted lots more whitespace of their own, because they
   3820      could not accurately predict when extra spaces were needed to
   3821      prevent accidental token pasting.
   3822 
   3823    * Optional argument when invoking rest argument macros
   3824 
   3825      As an extension, GCC permits you to omit the variable arguments
   3826      entirely when you use a variable argument macro.  This is
   3827      forbidden by the 1999 C standard, and will provoke a pedantic
   3828      warning with GCC 3.0.  Previous versions accepted it silently.
   3829 
   3830    * `##' swallowing preceding text in rest argument macros
   3831 
   3832      Formerly, in a macro expansion, if `##' appeared before a variable
   3833      arguments parameter, and the set of tokens specified for that
   3834      argument in the macro invocation was empty, previous versions of
   3835      CPP would back up and remove the preceding sequence of
   3836      non-whitespace characters (*not* the preceding token).  This
   3837      extension is in direct conflict with the 1999 C standard and has
   3838      been drastically pared back.
   3839 
   3840      In the current version of the preprocessor, if `##' appears between
   3841      a comma and a variable arguments parameter, and the variable
   3842      argument is omitted entirely, the comma will be removed from the
   3843      expansion.  If the variable argument is empty, or the token before
   3844      `##' is not a comma, then `##' behaves as a normal token paste.
   3845 
   3846    * `#line' and `#include'
   3847 
   3848      The `#line' directive used to change GCC's notion of the
   3849      "directory containing the current file", used by `#include' with a
   3850      double-quoted header file name.  In 3.0 and later, it does not.
   3851      *Note Line Control::, for further explanation.
   3852 
   3853    * Syntax of `#line'
   3854 
   3855      In GCC 2.95 and previous, the string constant argument to `#line'
   3856      was treated the same way as the argument to `#include': backslash
   3857      escapes were not honored, and the string ended at the second `"'.
   3858      This is not compliant with the C standard.  In GCC 3.0, an attempt
   3859      was made to correct the behavior, so that the string was treated
   3860      as a real string constant, but it turned out to be buggy.  In 3.1,
   3861      the bugs have been fixed.  (We are not fixing the bugs in 3.0
   3862      because they affect relatively few people and the fix is quite
   3863      invasive.)
   3864 
   3865 
   3866 
   3867 File: cpp.info,  Node: Invocation,  Next: Environment Variables,  Prev: Implementation Details,  Up: Top
   3868 
   3869 12 Invocation
   3870 *************
   3871 
   3872 Most often when you use the C preprocessor you will not have to invoke
   3873 it explicitly: the C compiler will do so automatically.  However, the
   3874 preprocessor is sometimes useful on its own.  All the options listed
   3875 here are also acceptable to the C compiler and have the same meaning,
   3876 except that the C compiler has different rules for specifying the output
   3877 file.
   3878 
   3879    _Note:_ Whether you use the preprocessor by way of `gcc' or `cpp',
   3880 the "compiler driver" is run first.  This program's purpose is to
   3881 translate your command into invocations of the programs that do the
   3882 actual work.  Their command line interfaces are similar but not
   3883 identical to the documented interface, and may change without notice.
   3884 
   3885    The C preprocessor expects two file names as arguments, INFILE and
   3886 OUTFILE.  The preprocessor reads INFILE together with any other files
   3887 it specifies with `#include'.  All the output generated by the combined
   3888 input files is written in OUTFILE.
   3889 
   3890    Either INFILE or OUTFILE may be `-', which as INFILE means to read
   3891 from standard input and as OUTFILE means to write to standard output.
   3892 Also, if either file is omitted, it means the same as if `-' had been
   3893 specified for that file.
   3894 
   3895    Unless otherwise noted, or the option ends in `=', all options which
   3896 take an argument may have that argument appear either immediately after
   3897 the option, or with a space between option and argument: `-Ifoo' and
   3898 `-I foo' have the same effect.
   3899 
   3900    Many options have multi-letter names; therefore multiple
   3901 single-letter options may _not_ be grouped: `-dM' is very different from
   3902 `-d -M'.
   3903 
   3904 `-D NAME'
   3905      Predefine NAME as a macro, with definition `1'.
   3906 
   3907 `-D NAME=DEFINITION'
   3908      The contents of DEFINITION are tokenized and processed as if they
   3909      appeared during translation phase three in a `#define' directive.
   3910      In particular, the definition will be truncated by embedded
   3911      newline characters.
   3912 
   3913      If you are invoking the preprocessor from a shell or shell-like
   3914      program you may need to use the shell's quoting syntax to protect
   3915      characters such as spaces that have a meaning in the shell syntax.
   3916 
   3917      If you wish to define a function-like macro on the command line,
   3918      write its argument list with surrounding parentheses before the
   3919      equals sign (if any).  Parentheses are meaningful to most shells,
   3920      so you will need to quote the option.  With `sh' and `csh',
   3921      `-D'NAME(ARGS...)=DEFINITION'' works.
   3922 
   3923      `-D' and `-U' options are processed in the order they are given on
   3924      the command line.  All `-imacros FILE' and `-include FILE' options
   3925      are processed after all `-D' and `-U' options.
   3926 
   3927 `-U NAME'
   3928      Cancel any previous definition of NAME, either built in or
   3929      provided with a `-D' option.
   3930 
   3931 `-undef'
   3932      Do not predefine any system-specific or GCC-specific macros.  The
   3933      standard predefined macros remain defined.  *Note Standard
   3934      Predefined Macros::.
   3935 
   3936 `-I DIR'
   3937      Add the directory DIR to the list of directories to be searched
   3938      for header files.  *Note Search Path::.  Directories named by `-I'
   3939      are searched before the standard system include directories.  If
   3940      the directory DIR is a standard system include directory, the
   3941      option is ignored to ensure that the default search order for
   3942      system directories and the special treatment of system headers are
   3943      not defeated (*note System Headers::) .  If DIR begins with `=',
   3944      then the `=' will be replaced by the sysroot prefix; see
   3945      `--sysroot' and `-isysroot'.
   3946 
   3947 `-o FILE'
   3948      Write output to FILE.  This is the same as specifying FILE as the
   3949      second non-option argument to `cpp'.  `gcc' has a different
   3950      interpretation of a second non-option argument, so you must use
   3951      `-o' to specify the output file.
   3952 
   3953 `-Wall'
   3954      Turns on all optional warnings which are desirable for normal code.
   3955      At present this is `-Wcomment', `-Wtrigraphs', `-Wmultichar' and a
   3956      warning about integer promotion causing a change of sign in `#if'
   3957      expressions.  Note that many of the preprocessor's warnings are on
   3958      by default and have no options to control them.
   3959 
   3960 `-Wcomment'
   3961 `-Wcomments'
   3962      Warn whenever a comment-start sequence `/*' appears in a `/*'
   3963      comment, or whenever a backslash-newline appears in a `//' comment.
   3964      (Both forms have the same effect.)
   3965 
   3966 `-Wtrigraphs'
   3967      Most trigraphs in comments cannot affect the meaning of the
   3968      program.  However, a trigraph that would form an escaped newline
   3969      (`??/' at the end of a line) can, by changing where the comment
   3970      begins or ends.  Therefore, only trigraphs that would form escaped
   3971      newlines produce warnings inside a comment.
   3972 
   3973      This option is implied by `-Wall'.  If `-Wall' is not given, this
   3974      option is still enabled unless trigraphs are enabled.  To get
   3975      trigraph conversion without warnings, but get the other `-Wall'
   3976      warnings, use `-trigraphs -Wall -Wno-trigraphs'.
   3977 
   3978 `-Wtraditional'
   3979      Warn about certain constructs that behave differently in
   3980      traditional and ISO C.  Also warn about ISO C constructs that have
   3981      no traditional C equivalent, and problematic constructs which
   3982      should be avoided.  *Note Traditional Mode::.
   3983 
   3984 `-Wimport'
   3985      Warn the first time `#import' is used.
   3986 
   3987 `-Wundef'
   3988      Warn whenever an identifier which is not a macro is encountered in
   3989      an `#if' directive, outside of `defined'.  Such identifiers are
   3990      replaced with zero.
   3991 
   3992 `-Wunused-macros'
   3993      Warn about macros defined in the main file that are unused.  A
   3994      macro is "used" if it is expanded or tested for existence at least
   3995      once.  The preprocessor will also warn if the macro has not been
   3996      used at the time it is redefined or undefined.
   3997 
   3998      Built-in macros, macros defined on the command line, and macros
   3999      defined in include files are not warned about.
   4000 
   4001      _Note:_ If a macro is actually used, but only used in skipped
   4002      conditional blocks, then CPP will report it as unused.  To avoid
   4003      the warning in such a case, you might improve the scope of the
   4004      macro's definition by, for example, moving it into the first
   4005      skipped block.  Alternatively, you could provide a dummy use with
   4006      something like:
   4007 
   4008           #if defined the_macro_causing_the_warning
   4009           #endif
   4010 
   4011 `-Wendif-labels'
   4012      Warn whenever an `#else' or an `#endif' are followed by text.
   4013      This usually happens in code of the form
   4014 
   4015           #if FOO
   4016           ...
   4017           #else FOO
   4018           ...
   4019           #endif FOO
   4020 
   4021      The second and third `FOO' should be in comments, but often are not
   4022      in older programs.  This warning is on by default.
   4023 
   4024 `-Werror'
   4025      Make all warnings into hard errors.  Source code which triggers
   4026      warnings will be rejected.
   4027 
   4028 `-Wsystem-headers'
   4029      Issue warnings for code in system headers.  These are normally
   4030      unhelpful in finding bugs in your own code, therefore suppressed.
   4031      If you are responsible for the system library, you may want to see
   4032      them.
   4033 
   4034 `-w'
   4035      Suppress all warnings, including those which GNU CPP issues by
   4036      default.
   4037 
   4038 `-pedantic'
   4039      Issue all the mandatory diagnostics listed in the C standard.
   4040      Some of them are left out by default, since they trigger
   4041      frequently on harmless code.
   4042 
   4043 `-pedantic-errors'
   4044      Issue all the mandatory diagnostics, and make all mandatory
   4045      diagnostics into errors.  This includes mandatory diagnostics that
   4046      GCC issues without `-pedantic' but treats as warnings.
   4047 
   4048 `-M'
   4049      Instead of outputting the result of preprocessing, output a rule
   4050      suitable for `make' describing the dependencies of the main source
   4051      file.  The preprocessor outputs one `make' rule containing the
   4052      object file name for that source file, a colon, and the names of
   4053      all the included files, including those coming from `-include' or
   4054      `-imacros' command line options.
   4055 
   4056      Unless specified explicitly (with `-MT' or `-MQ'), the object file
   4057      name consists of the name of the source file with any suffix
   4058      replaced with object file suffix and with any leading directory
   4059      parts removed.  If there are many included files then the rule is
   4060      split into several lines using `\'-newline.  The rule has no
   4061      commands.
   4062 
   4063      This option does not suppress the preprocessor's debug output,
   4064      such as `-dM'.  To avoid mixing such debug output with the
   4065      dependency rules you should explicitly specify the dependency
   4066      output file with `-MF', or use an environment variable like
   4067      `DEPENDENCIES_OUTPUT' (*note Environment Variables::).  Debug
   4068      output will still be sent to the regular output stream as normal.
   4069 
   4070      Passing `-M' to the driver implies `-E', and suppresses warnings
   4071      with an implicit `-w'.
   4072 
   4073 `-MM'
   4074      Like `-M' but do not mention header files that are found in system
   4075      header directories, nor header files that are included, directly
   4076      or indirectly, from such a header.
   4077 
   4078      This implies that the choice of angle brackets or double quotes in
   4079      an `#include' directive does not in itself determine whether that
   4080      header will appear in `-MM' dependency output.  This is a slight
   4081      change in semantics from GCC versions 3.0 and earlier.
   4082 
   4083 `-MF FILE'
   4084      When used with `-M' or `-MM', specifies a file to write the
   4085      dependencies to.  If no `-MF' switch is given the preprocessor
   4086      sends the rules to the same place it would have sent preprocessed
   4087      output.
   4088 
   4089      When used with the driver options `-MD' or `-MMD', `-MF' overrides
   4090      the default dependency output file.
   4091 
   4092 `-MG'
   4093      In conjunction with an option such as `-M' requesting dependency
   4094      generation, `-MG' assumes missing header files are generated files
   4095      and adds them to the dependency list without raising an error.
   4096      The dependency filename is taken directly from the `#include'
   4097      directive without prepending any path.  `-MG' also suppresses
   4098      preprocessed output, as a missing header file renders this useless.
   4099 
   4100      This feature is used in automatic updating of makefiles.
   4101 
   4102 `-MP'
   4103      This option instructs CPP to add a phony target for each dependency
   4104      other than the main file, causing each to depend on nothing.  These
   4105      dummy rules work around errors `make' gives if you remove header
   4106      files without updating the `Makefile' to match.
   4107 
   4108      This is typical output:
   4109 
   4110           test.o: test.c test.h
   4111 
   4112           test.h:
   4113 
   4114 `-MT TARGET'
   4115      Change the target of the rule emitted by dependency generation.  By
   4116      default CPP takes the name of the main input file, deletes any
   4117      directory components and any file suffix such as `.c', and appends
   4118      the platform's usual object suffix.  The result is the target.
   4119 
   4120      An `-MT' option will set the target to be exactly the string you
   4121      specify.  If you want multiple targets, you can specify them as a
   4122      single argument to `-MT', or use multiple `-MT' options.
   4123 
   4124      For example, `-MT '$(objpfx)foo.o'' might give
   4125 
   4126           $(objpfx)foo.o: foo.c
   4127 
   4128 `-MQ TARGET'
   4129      Same as `-MT', but it quotes any characters which are special to
   4130      Make.  `-MQ '$(objpfx)foo.o'' gives
   4131 
   4132           $$(objpfx)foo.o: foo.c
   4133 
   4134      The default target is automatically quoted, as if it were given
   4135      with `-MQ'.
   4136 
   4137 `-MD'
   4138      `-MD' is equivalent to `-M -MF FILE', except that `-E' is not
   4139      implied.  The driver determines FILE based on whether an `-o'
   4140      option is given.  If it is, the driver uses its argument but with
   4141      a suffix of `.d', otherwise it takes the name of the input file,
   4142      removes any directory components and suffix, and applies a `.d'
   4143      suffix.
   4144 
   4145      If `-MD' is used in conjunction with `-E', any `-o' switch is
   4146      understood to specify the dependency output file (*note -MF:
   4147      dashMF.), but if used without `-E', each `-o' is understood to
   4148      specify a target object file.
   4149 
   4150      Since `-E' is not implied, `-MD' can be used to generate a
   4151      dependency output file as a side-effect of the compilation process.
   4152 
   4153 `-MMD'
   4154      Like `-MD' except mention only user header files, not system
   4155      header files.
   4156 
   4157 `-x c'
   4158 `-x c++'
   4159 `-x objective-c'
   4160 `-x assembler-with-cpp'
   4161      Specify the source language: C, C++, Objective-C, or assembly.
   4162      This has nothing to do with standards conformance or extensions;
   4163      it merely selects which base syntax to expect.  If you give none
   4164      of these options, cpp will deduce the language from the extension
   4165      of the source file: `.c', `.cc', `.m', or `.S'.  Some other common
   4166      extensions for C++ and assembly are also recognized.  If cpp does
   4167      not recognize the extension, it will treat the file as C; this is
   4168      the most generic mode.
   4169 
   4170      _Note:_ Previous versions of cpp accepted a `-lang' option which
   4171      selected both the language and the standards conformance level.
   4172      This option has been removed, because it conflicts with the `-l'
   4173      option.
   4174 
   4175 `-std=STANDARD'
   4176 `-ansi'
   4177      Specify the standard to which the code should conform.  Currently
   4178      CPP knows about C and C++ standards; others may be added in the
   4179      future.
   4180 
   4181      STANDARD may be one of:
   4182     `iso9899:1990'
   4183     `c89'
   4184           The ISO C standard from 1990.  `c89' is the customary
   4185           shorthand for this version of the standard.
   4186 
   4187           The `-ansi' option is equivalent to `-std=c89'.
   4188 
   4189     `iso9899:199409'
   4190           The 1990 C standard, as amended in 1994.
   4191 
   4192     `iso9899:1999'
   4193     `c99'
   4194     `iso9899:199x'
   4195     `c9x'
   4196           The revised ISO C standard, published in December 1999.
   4197           Before publication, this was known as C9X.
   4198 
   4199     `gnu89'
   4200           The 1990 C standard plus GNU extensions.  This is the default.
   4201 
   4202     `gnu99'
   4203     `gnu9x'
   4204           The 1999 C standard plus GNU extensions.
   4205 
   4206     `c++98'
   4207           The 1998 ISO C++ standard plus amendments.
   4208 
   4209     `gnu++98'
   4210           The same as `-std=c++98' plus GNU extensions.  This is the
   4211           default for C++ code.
   4212 
   4213 `-I-'
   4214      Split the include path.  Any directories specified with `-I'
   4215      options before `-I-' are searched only for headers requested with
   4216      `#include "FILE"'; they are not searched for `#include <FILE>'.
   4217      If additional directories are specified with `-I' options after
   4218      the `-I-', those directories are searched for all `#include'
   4219      directives.
   4220 
   4221      In addition, `-I-' inhibits the use of the directory of the current
   4222      file directory as the first search directory for `#include "FILE"'.
   4223      *Note Search Path::.  This option has been deprecated.
   4224 
   4225 `-nostdinc'
   4226      Do not search the standard system directories for header files.
   4227      Only the directories you have specified with `-I' options (and the
   4228      directory of the current file, if appropriate) are searched.
   4229 
   4230 `-nostdinc++'
   4231      Do not search for header files in the C++-specific standard
   4232      directories, but do still search the other standard directories.
   4233      (This option is used when building the C++ library.)
   4234 
   4235 `-include FILE'
   4236      Process FILE as if `#include "file"' appeared as the first line of
   4237      the primary source file.  However, the first directory searched
   4238      for FILE is the preprocessor's working directory _instead of_ the
   4239      directory containing the main source file.  If not found there, it
   4240      is searched for in the remainder of the `#include "..."' search
   4241      chain as normal.
   4242 
   4243      If multiple `-include' options are given, the files are included
   4244      in the order they appear on the command line.
   4245 
   4246 `-imacros FILE'
   4247      Exactly like `-include', except that any output produced by
   4248      scanning FILE is thrown away.  Macros it defines remain defined.
   4249      This allows you to acquire all the macros from a header without
   4250      also processing its declarations.
   4251 
   4252      All files specified by `-imacros' are processed before all files
   4253      specified by `-include'.
   4254 
   4255 `-idirafter DIR'
   4256      Search DIR for header files, but do it _after_ all directories
   4257      specified with `-I' and the standard system directories have been
   4258      exhausted.  DIR is treated as a system include directory.  If DIR
   4259      begins with `=', then the `=' will be replaced by the sysroot
   4260      prefix; see `--sysroot' and `-isysroot'.
   4261 
   4262 `-iprefix PREFIX'
   4263      Specify PREFIX as the prefix for subsequent `-iwithprefix'
   4264      options.  If the prefix represents a directory, you should include
   4265      the final `/'.
   4266 
   4267 `-iwithprefix DIR'
   4268 `-iwithprefixbefore DIR'
   4269      Append DIR to the prefix specified previously with `-iprefix', and
   4270      add the resulting directory to the include search path.
   4271      `-iwithprefixbefore' puts it in the same place `-I' would;
   4272      `-iwithprefix' puts it where `-idirafter' would.
   4273 
   4274 `-isysroot DIR'
   4275      This option is like the `--sysroot' option, but applies only to
   4276      header files.  See the `--sysroot' option for more information.
   4277 
   4278 `-imultilib DIR'
   4279      Use DIR as a subdirectory of the directory containing
   4280      target-specific C++ headers.
   4281 
   4282 `-isystem DIR'
   4283      Search DIR for header files, after all directories specified by
   4284      `-I' but before the standard system directories.  Mark it as a
   4285      system directory, so that it gets the same special treatment as is
   4286      applied to the standard system directories.  *Note System
   4287      Headers::.  If DIR begins with `=', then the `=' will be replaced
   4288      by the sysroot prefix; see `--sysroot' and `-isysroot'.
   4289 
   4290 `-iquote DIR'
   4291      Search DIR only for header files requested with `#include "FILE"';
   4292      they are not searched for `#include <FILE>', before all
   4293      directories specified by `-I' and before the standard system
   4294      directories.  *Note Search Path::.  If DIR begins with `=', then
   4295      the `=' will be replaced by the sysroot prefix; see `--sysroot'
   4296      and `-isysroot'.
   4297 
   4298 `-fdirectives-only'
   4299      When preprocessing, handle directives, but do not expand macros.
   4300 
   4301      The option's behavior depends on the `-E' and `-fpreprocessed'
   4302      options.
   4303 
   4304      With `-E', preprocessing is limited to the handling of directives
   4305      such as `#define', `#ifdef', and `#error'.  Other preprocessor
   4306      operations, such as macro expansion and trigraph conversion are
   4307      not performed.  In addition, the `-dD' option is implicitly
   4308      enabled.
   4309 
   4310      With `-fpreprocessed', predefinition of command line and most
   4311      builtin macros is disabled.  Macros such as `__LINE__', which are
   4312      contextually dependent, are handled normally.  This enables
   4313      compilation of files previously preprocessed with `-E
   4314      -fdirectives-only'.
   4315 
   4316      With both `-E' and `-fpreprocessed', the rules for
   4317      `-fpreprocessed' take precedence.  This enables full preprocessing
   4318      of files previously preprocessed with `-E -fdirectives-only'.
   4319 
   4320 `-fdollars-in-identifiers'
   4321      Accept `$' in identifiers.    *Note Identifier characters::.
   4322 
   4323 `-fextended-identifiers'
   4324      Accept universal character names in identifiers.  This option is
   4325      experimental; in a future version of GCC, it will be enabled by
   4326      default for C99 and C++.
   4327 
   4328 `-fpreprocessed'
   4329      Indicate to the preprocessor that the input file has already been
   4330      preprocessed.  This suppresses things like macro expansion,
   4331      trigraph conversion, escaped newline splicing, and processing of
   4332      most directives.  The preprocessor still recognizes and removes
   4333      comments, so that you can pass a file preprocessed with `-C' to
   4334      the compiler without problems.  In this mode the integrated
   4335      preprocessor is little more than a tokenizer for the front ends.
   4336 
   4337      `-fpreprocessed' is implicit if the input file has one of the
   4338      extensions `.i', `.ii' or `.mi'.  These are the extensions that
   4339      GCC uses for preprocessed files created by `-save-temps'.
   4340 
   4341 `-ftabstop=WIDTH'
   4342      Set the distance between tab stops.  This helps the preprocessor
   4343      report correct column numbers in warnings or errors, even if tabs
   4344      appear on the line.  If the value is less than 1 or greater than
   4345      100, the option is ignored.  The default is 8.
   4346 
   4347 `-fexec-charset=CHARSET'
   4348      Set the execution character set, used for string and character
   4349      constants.  The default is UTF-8.  CHARSET can be any encoding
   4350      supported by the system's `iconv' library routine.
   4351 
   4352 `-fwide-exec-charset=CHARSET'
   4353      Set the wide execution character set, used for wide string and
   4354      character constants.  The default is UTF-32 or UTF-16, whichever
   4355      corresponds to the width of `wchar_t'.  As with `-fexec-charset',
   4356      CHARSET can be any encoding supported by the system's `iconv'
   4357      library routine; however, you will have problems with encodings
   4358      that do not fit exactly in `wchar_t'.
   4359 
   4360 `-finput-charset=CHARSET'
   4361      Set the input character set, used for translation from the
   4362      character set of the input file to the source character set used
   4363      by GCC.  If the locale does not specify, or GCC cannot get this
   4364      information from the locale, the default is UTF-8.  This can be
   4365      overridden by either the locale or this command line option.
   4366      Currently the command line option takes precedence if there's a
   4367      conflict.  CHARSET can be any encoding supported by the system's
   4368      `iconv' library routine.
   4369 
   4370 `-fworking-directory'
   4371      Enable generation of linemarkers in the preprocessor output that
   4372      will let the compiler know the current working directory at the
   4373      time of preprocessing.  When this option is enabled, the
   4374      preprocessor will emit, after the initial linemarker, a second
   4375      linemarker with the current working directory followed by two
   4376      slashes.  GCC will use this directory, when it's present in the
   4377      preprocessed input, as the directory emitted as the current
   4378      working directory in some debugging information formats.  This
   4379      option is implicitly enabled if debugging information is enabled,
   4380      but this can be inhibited with the negated form
   4381      `-fno-working-directory'.  If the `-P' flag is present in the
   4382      command line, this option has no effect, since no `#line'
   4383      directives are emitted whatsoever.
   4384 
   4385 `-fno-show-column'
   4386      Do not print column numbers in diagnostics.  This may be necessary
   4387      if diagnostics are being scanned by a program that does not
   4388      understand the column numbers, such as `dejagnu'.
   4389 
   4390 `-A PREDICATE=ANSWER'
   4391      Make an assertion with the predicate PREDICATE and answer ANSWER.
   4392      This form is preferred to the older form `-A PREDICATE(ANSWER)',
   4393      which is still supported, because it does not use shell special
   4394      characters.  *Note Assertions::.
   4395 
   4396 `-A -PREDICATE=ANSWER'
   4397      Cancel an assertion with the predicate PREDICATE and answer ANSWER.
   4398 
   4399 `-dCHARS'
   4400      CHARS is a sequence of one or more of the following characters,
   4401      and must not be preceded by a space.  Other characters are
   4402      interpreted by the compiler proper, or reserved for future
   4403      versions of GCC, and so are silently ignored.  If you specify
   4404      characters whose behavior conflicts, the result is undefined.
   4405 
   4406     `M'
   4407           Instead of the normal output, generate a list of `#define'
   4408           directives for all the macros defined during the execution of
   4409           the preprocessor, including predefined macros.  This gives
   4410           you a way of finding out what is predefined in your version
   4411           of the preprocessor.  Assuming you have no file `foo.h', the
   4412           command
   4413 
   4414                touch foo.h; cpp -dM foo.h
   4415 
   4416           will show all the predefined macros.
   4417 
   4418           If you use `-dM' without the `-E' option, `-dM' is
   4419           interpreted as a synonym for `-fdump-rtl-mach'.  *Note
   4420           Debugging Options: (gcc)Debugging Options.
   4421 
   4422     `D'
   4423           Like `M' except in two respects: it does _not_ include the
   4424           predefined macros, and it outputs _both_ the `#define'
   4425           directives and the result of preprocessing.  Both kinds of
   4426           output go to the standard output file.
   4427 
   4428     `N'
   4429           Like `D', but emit only the macro names, not their expansions.
   4430 
   4431     `I'
   4432           Output `#include' directives in addition to the result of
   4433           preprocessing.
   4434 
   4435 `-P'
   4436      Inhibit generation of linemarkers in the output from the
   4437      preprocessor.  This might be useful when running the preprocessor
   4438      on something that is not C code, and will be sent to a program
   4439      which might be confused by the linemarkers.  *Note Preprocessor
   4440      Output::.
   4441 
   4442 `-C'
   4443      Do not discard comments.  All comments are passed through to the
   4444      output file, except for comments in processed directives, which
   4445      are deleted along with the directive.
   4446 
   4447      You should be prepared for side effects when using `-C'; it causes
   4448      the preprocessor to treat comments as tokens in their own right.
   4449      For example, comments appearing at the start of what would be a
   4450      directive line have the effect of turning that line into an
   4451      ordinary source line, since the first token on the line is no
   4452      longer a `#'.
   4453 
   4454 `-CC'
   4455      Do not discard comments, including during macro expansion.  This is
   4456      like `-C', except that comments contained within macros are also
   4457      passed through to the output file where the macro is expanded.
   4458 
   4459      In addition to the side-effects of the `-C' option, the `-CC'
   4460      option causes all C++-style comments inside a macro to be
   4461      converted to C-style comments.  This is to prevent later use of
   4462      that macro from inadvertently commenting out the remainder of the
   4463      source line.
   4464 
   4465      The `-CC' option is generally used to support lint comments.
   4466 
   4467 `-traditional-cpp'
   4468      Try to imitate the behavior of old-fashioned C preprocessors, as
   4469      opposed to ISO C preprocessors.  *Note Traditional Mode::.
   4470 
   4471 `-trigraphs'
   4472      Process trigraph sequences.  *Note Initial processing::.
   4473 
   4474 `-remap'
   4475      Enable special code to work around file systems which only permit
   4476      very short file names, such as MS-DOS.
   4477 
   4478 `--help'
   4479 `--target-help'
   4480      Print text describing all the command line options instead of
   4481      preprocessing anything.
   4482 
   4483 `-v'
   4484      Verbose mode.  Print out GNU CPP's version number at the beginning
   4485      of execution, and report the final form of the include path.
   4486 
   4487 `-H'
   4488      Print the name of each header file used, in addition to other
   4489      normal activities.  Each name is indented to show how deep in the
   4490      `#include' stack it is.  Precompiled header files are also
   4491      printed, even if they are found to be invalid; an invalid
   4492      precompiled header file is printed with `...x' and a valid one
   4493      with `...!' .
   4494 
   4495 `-version'
   4496 `--version'
   4497      Print out GNU CPP's version number.  With one dash, proceed to
   4498      preprocess as normal.  With two dashes, exit immediately.
   4499 
   4500 
   4501 File: cpp.info,  Node: Environment Variables,  Next: GNU Free Documentation License,  Prev: Invocation,  Up: Top
   4502 
   4503 13 Environment Variables
   4504 ************************
   4505 
   4506 This section describes the environment variables that affect how CPP
   4507 operates.  You can use them to specify directories or prefixes to use
   4508 when searching for include files, or to control dependency output.
   4509 
   4510    Note that you can also specify places to search using options such as
   4511 `-I', and control dependency output with options like `-M' (*note
   4512 Invocation::).  These take precedence over environment variables, which
   4513 in turn take precedence over the configuration of GCC.
   4514 
   4515 `CPATH'
   4516 `C_INCLUDE_PATH'
   4517 `CPLUS_INCLUDE_PATH'
   4518 `OBJC_INCLUDE_PATH'
   4519      Each variable's value is a list of directories separated by a
   4520      special character, much like `PATH', in which to look for header
   4521      files.  The special character, `PATH_SEPARATOR', is
   4522      target-dependent and determined at GCC build time.  For Microsoft
   4523      Windows-based targets it is a semicolon, and for almost all other
   4524      targets it is a colon.
   4525 
   4526      `CPATH' specifies a list of directories to be searched as if
   4527      specified with `-I', but after any paths given with `-I' options
   4528      on the command line.  This environment variable is used regardless
   4529      of which language is being preprocessed.
   4530 
   4531      The remaining environment variables apply only when preprocessing
   4532      the particular language indicated.  Each specifies a list of
   4533      directories to be searched as if specified with `-isystem', but
   4534      after any paths given with `-isystem' options on the command line.
   4535 
   4536      In all these variables, an empty element instructs the compiler to
   4537      search its current working directory.  Empty elements can appear
   4538      at the beginning or end of a path.  For instance, if the value of
   4539      `CPATH' is `:/special/include', that has the same effect as
   4540      `-I. -I/special/include'.
   4541 
   4542      See also *Note Search Path::.
   4543 
   4544 `DEPENDENCIES_OUTPUT'
   4545      If this variable is set, its value specifies how to output
   4546      dependencies for Make based on the non-system header files
   4547      processed by the compiler.  System header files are ignored in the
   4548      dependency output.
   4549 
   4550      The value of `DEPENDENCIES_OUTPUT' can be just a file name, in
   4551      which case the Make rules are written to that file, guessing the
   4552      target name from the source file name.  Or the value can have the
   4553      form `FILE TARGET', in which case the rules are written to file
   4554      FILE using TARGET as the target name.
   4555 
   4556      In other words, this environment variable is equivalent to
   4557      combining the options `-MM' and `-MF' (*note Invocation::), with
   4558      an optional `-MT' switch too.
   4559 
   4560 `SUNPRO_DEPENDENCIES'
   4561      This variable is the same as `DEPENDENCIES_OUTPUT' (see above),
   4562      except that system header files are not ignored, so it implies
   4563      `-M' rather than `-MM'.  However, the dependence on the main input
   4564      file is omitted.  *Note Invocation::.
   4565 
   4566 
   4567 File: cpp.info,  Node: GNU Free Documentation License,  Next: Index of Directives,  Prev: Environment Variables,  Up: Top
   4568 
   4569 GNU Free Documentation License
   4570 ******************************
   4571 
   4572                       Version 1.2, November 2002
   4573 
   4574      Copyright (C) 2000,2001,2002 Free Software Foundation, Inc.
   4575      51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
   4576 
   4577      Everyone is permitted to copy and distribute verbatim copies
   4578      of this license document, but changing it is not allowed.
   4579 
   4580   0. PREAMBLE
   4581 
   4582      The purpose of this License is to make a manual, textbook, or other
   4583      functional and useful document "free" in the sense of freedom: to
   4584      assure everyone the effective freedom to copy and redistribute it,
   4585      with or without modifying it, either commercially or
   4586      noncommercially.  Secondarily, this License preserves for the
   4587      author and publisher a way to get credit for their work, while not
   4588      being considered responsible for modifications made by others.
   4589 
   4590      This License is a kind of "copyleft", which means that derivative
   4591      works of the document must themselves be free in the same sense.
   4592      It complements the GNU General Public License, which is a copyleft
   4593      license designed for free software.
   4594 
   4595      We have designed this License in order to use it for manuals for
   4596      free software, because free software needs free documentation: a
   4597      free program should come with manuals providing the same freedoms
   4598      that the software does.  But this License is not limited to
   4599      software manuals; it can be used for any textual work, regardless
   4600      of subject matter or whether it is published as a printed book.
   4601      We recommend this License principally for works whose purpose is
   4602      instruction or reference.
   4603 
   4604   1. APPLICABILITY AND DEFINITIONS
   4605 
   4606      This License applies to any manual or other work, in any medium,
   4607      that contains a notice placed by the copyright holder saying it
   4608      can be distributed under the terms of this License.  Such a notice
   4609      grants a world-wide, royalty-free license, unlimited in duration,
   4610      to use that work under the conditions stated herein.  The
   4611      "Document", below, refers to any such manual or work.  Any member
   4612      of the public is a licensee, and is addressed as "you".  You
   4613      accept the license if you copy, modify or distribute the work in a
   4614      way requiring permission under copyright law.
   4615 
   4616      A "Modified Version" of the Document means any work containing the
   4617      Document or a portion of it, either copied verbatim, or with
   4618      modifications and/or translated into another language.
   4619 
   4620      A "Secondary Section" is a named appendix or a front-matter section
   4621      of the Document that deals exclusively with the relationship of the
   4622      publishers or authors of the Document to the Document's overall
   4623      subject (or to related matters) and contains nothing that could
   4624      fall directly within that overall subject.  (Thus, if the Document
   4625      is in part a textbook of mathematics, a Secondary Section may not
   4626      explain any mathematics.)  The relationship could be a matter of
   4627      historical connection with the subject or with related matters, or
   4628      of legal, commercial, philosophical, ethical or political position
   4629      regarding them.
   4630 
   4631      The "Invariant Sections" are certain Secondary Sections whose
   4632      titles are designated, as being those of Invariant Sections, in
   4633      the notice that says that the Document is released under this
   4634      License.  If a section does not fit the above definition of
   4635      Secondary then it is not allowed to be designated as Invariant.
   4636      The Document may contain zero Invariant Sections.  If the Document
   4637      does not identify any Invariant Sections then there are none.
   4638 
   4639      The "Cover Texts" are certain short passages of text that are
   4640      listed, as Front-Cover Texts or Back-Cover Texts, in the notice
   4641      that says that the Document is released under this License.  A
   4642      Front-Cover Text may be at most 5 words, and a Back-Cover Text may
   4643      be at most 25 words.
   4644 
   4645      A "Transparent" copy of the Document means a machine-readable copy,
   4646      represented in a format whose specification is available to the
   4647      general public, that is suitable for revising the document
   4648      straightforwardly with generic text editors or (for images
   4649      composed of pixels) generic paint programs or (for drawings) some
   4650      widely available drawing editor, and that is suitable for input to
   4651      text formatters or for automatic translation to a variety of
   4652      formats suitable for input to text formatters.  A copy made in an
   4653      otherwise Transparent file format whose markup, or absence of
   4654      markup, has been arranged to thwart or discourage subsequent
   4655      modification by readers is not Transparent.  An image format is
   4656      not Transparent if used for any substantial amount of text.  A
   4657      copy that is not "Transparent" is called "Opaque".
   4658 
   4659      Examples of suitable formats for Transparent copies include plain
   4660      ASCII without markup, Texinfo input format, LaTeX input format,
   4661      SGML or XML using a publicly available DTD, and
   4662      standard-conforming simple HTML, PostScript or PDF designed for
   4663      human modification.  Examples of transparent image formats include
   4664      PNG, XCF and JPG.  Opaque formats include proprietary formats that
   4665      can be read and edited only by proprietary word processors, SGML or
   4666      XML for which the DTD and/or processing tools are not generally
   4667      available, and the machine-generated HTML, PostScript or PDF
   4668      produced by some word processors for output purposes only.
   4669 
   4670      The "Title Page" means, for a printed book, the title page itself,
   4671      plus such following pages as are needed to hold, legibly, the
   4672      material this License requires to appear in the title page.  For
   4673      works in formats which do not have any title page as such, "Title
   4674      Page" means the text near the most prominent appearance of the
   4675      work's title, preceding the beginning of the body of the text.
   4676 
   4677      A section "Entitled XYZ" means a named subunit of the Document
   4678      whose title either is precisely XYZ or contains XYZ in parentheses
   4679      following text that translates XYZ in another language.  (Here XYZ
   4680      stands for a specific section name mentioned below, such as
   4681      "Acknowledgements", "Dedications", "Endorsements", or "History".)
   4682      To "Preserve the Title" of such a section when you modify the
   4683      Document means that it remains a section "Entitled XYZ" according
   4684      to this definition.
   4685 
   4686      The Document may include Warranty Disclaimers next to the notice
   4687      which states that this License applies to the Document.  These
   4688      Warranty Disclaimers are considered to be included by reference in
   4689      this License, but only as regards disclaiming warranties: any other
   4690      implication that these Warranty Disclaimers may have is void and
   4691      has no effect on the meaning of this License.
   4692 
   4693   2. VERBATIM COPYING
   4694 
   4695      You may copy and distribute the Document in any medium, either
   4696      commercially or noncommercially, provided that this License, the
   4697      copyright notices, and the license notice saying this License
   4698      applies to the Document are reproduced in all copies, and that you
   4699      add no other conditions whatsoever to those of this License.  You
   4700      may not use technical measures to obstruct or control the reading
   4701      or further copying of the copies you make or distribute.  However,
   4702      you may accept compensation in exchange for copies.  If you
   4703      distribute a large enough number of copies you must also follow
   4704      the conditions in section 3.
   4705 
   4706      You may also lend copies, under the same conditions stated above,
   4707      and you may publicly display copies.
   4708 
   4709   3. COPYING IN QUANTITY
   4710 
   4711      If you publish printed copies (or copies in media that commonly
   4712      have printed covers) of the Document, numbering more than 100, and
   4713      the Document's license notice requires Cover Texts, you must
   4714      enclose the copies in covers that carry, clearly and legibly, all
   4715      these Cover Texts: Front-Cover Texts on the front cover, and
   4716      Back-Cover Texts on the back cover.  Both covers must also clearly
   4717      and legibly identify you as the publisher of these copies.  The
   4718      front cover must present the full title with all words of the
   4719      title equally prominent and visible.  You may add other material
   4720      on the covers in addition.  Copying with changes limited to the
   4721      covers, as long as they preserve the title of the Document and
   4722      satisfy these conditions, can be treated as verbatim copying in
   4723      other respects.
   4724 
   4725      If the required texts for either cover are too voluminous to fit
   4726      legibly, you should put the first ones listed (as many as fit
   4727      reasonably) on the actual cover, and continue the rest onto
   4728      adjacent pages.
   4729 
   4730      If you publish or distribute Opaque copies of the Document
   4731      numbering more than 100, you must either include a
   4732      machine-readable Transparent copy along with each Opaque copy, or
   4733      state in or with each Opaque copy a computer-network location from
   4734      which the general network-using public has access to download
   4735      using public-standard network protocols a complete Transparent
   4736      copy of the Document, free of added material.  If you use the
   4737      latter option, you must take reasonably prudent steps, when you
   4738      begin distribution of Opaque copies in quantity, to ensure that
   4739      this Transparent copy will remain thus accessible at the stated
   4740      location until at least one year after the last time you
   4741      distribute an Opaque copy (directly or through your agents or
   4742      retailers) of that edition to the public.
   4743 
   4744      It is requested, but not required, that you contact the authors of
   4745      the Document well before redistributing any large number of
   4746      copies, to give them a chance to provide you with an updated
   4747      version of the Document.
   4748 
   4749   4. MODIFICATIONS
   4750 
   4751      You may copy and distribute a Modified Version of the Document
   4752      under the conditions of sections 2 and 3 above, provided that you
   4753      release the Modified Version under precisely this License, with
   4754      the Modified Version filling the role of the Document, thus
   4755      licensing distribution and modification of the Modified Version to
   4756      whoever possesses a copy of it.  In addition, you must do these
   4757      things in the Modified Version:
   4758 
   4759        A. Use in the Title Page (and on the covers, if any) a title
   4760           distinct from that of the Document, and from those of
   4761           previous versions (which should, if there were any, be listed
   4762           in the History section of the Document).  You may use the
   4763           same title as a previous version if the original publisher of
   4764           that version gives permission.
   4765 
   4766        B. List on the Title Page, as authors, one or more persons or
   4767           entities responsible for authorship of the modifications in
   4768           the Modified Version, together with at least five of the
   4769           principal authors of the Document (all of its principal
   4770           authors, if it has fewer than five), unless they release you
   4771           from this requirement.
   4772 
   4773        C. State on the Title page the name of the publisher of the
   4774           Modified Version, as the publisher.
   4775 
   4776        D. Preserve all the copyright notices of the Document.
   4777 
   4778        E. Add an appropriate copyright notice for your modifications
   4779           adjacent to the other copyright notices.
   4780 
   4781        F. Include, immediately after the copyright notices, a license
   4782           notice giving the public permission to use the Modified
   4783           Version under the terms of this License, in the form shown in
   4784           the Addendum below.
   4785 
   4786        G. Preserve in that license notice the full lists of Invariant
   4787           Sections and required Cover Texts given in the Document's
   4788           license notice.
   4789 
   4790        H. Include an unaltered copy of this License.
   4791 
   4792        I. Preserve the section Entitled "History", Preserve its Title,
   4793           and add to it an item stating at least the title, year, new
   4794           authors, and publisher of the Modified Version as given on
   4795           the Title Page.  If there is no section Entitled "History" in
   4796           the Document, create one stating the title, year, authors,
   4797           and publisher of the Document as given on its Title Page,
   4798           then add an item describing the Modified Version as stated in
   4799           the previous sentence.
   4800 
   4801        J. Preserve the network location, if any, given in the Document
   4802           for public access to a Transparent copy of the Document, and
   4803           likewise the network locations given in the Document for
   4804           previous versions it was based on.  These may be placed in
   4805           the "History" section.  You may omit a network location for a
   4806           work that was published at least four years before the
   4807           Document itself, or if the original publisher of the version
   4808           it refers to gives permission.
   4809 
   4810        K. For any section Entitled "Acknowledgements" or "Dedications",
   4811           Preserve the Title of the section, and preserve in the
   4812           section all the substance and tone of each of the contributor
   4813           acknowledgements and/or dedications given therein.
   4814 
   4815        L. Preserve all the Invariant Sections of the Document,
   4816           unaltered in their text and in their titles.  Section numbers
   4817           or the equivalent are not considered part of the section
   4818           titles.
   4819 
   4820        M. Delete any section Entitled "Endorsements".  Such a section
   4821           may not be included in the Modified Version.
   4822 
   4823        N. Do not retitle any existing section to be Entitled
   4824           "Endorsements" or to conflict in title with any Invariant
   4825           Section.
   4826 
   4827        O. Preserve any Warranty Disclaimers.
   4828 
   4829      If the Modified Version includes new front-matter sections or
   4830      appendices that qualify as Secondary Sections and contain no
   4831      material copied from the Document, you may at your option
   4832      designate some or all of these sections as invariant.  To do this,
   4833      add their titles to the list of Invariant Sections in the Modified
   4834      Version's license notice.  These titles must be distinct from any
   4835      other section titles.
   4836 
   4837      You may add a section Entitled "Endorsements", provided it contains
   4838      nothing but endorsements of your Modified Version by various
   4839      parties--for example, statements of peer review or that the text
   4840      has been approved by an organization as the authoritative
   4841      definition of a standard.
   4842 
   4843      You may add a passage of up to five words as a Front-Cover Text,
   4844      and a passage of up to 25 words as a Back-Cover Text, to the end
   4845      of the list of Cover Texts in the Modified Version.  Only one
   4846      passage of Front-Cover Text and one of Back-Cover Text may be
   4847      added by (or through arrangements made by) any one entity.  If the
   4848      Document already includes a cover text for the same cover,
   4849      previously added by you or by arrangement made by the same entity
   4850      you are acting on behalf of, you may not add another; but you may
   4851      replace the old one, on explicit permission from the previous
   4852      publisher that added the old one.
   4853 
   4854      The author(s) and publisher(s) of the Document do not by this
   4855      License give permission to use their names for publicity for or to
   4856      assert or imply endorsement of any Modified Version.
   4857 
   4858   5. COMBINING DOCUMENTS
   4859 
   4860      You may combine the Document with other documents released under
   4861      this License, under the terms defined in section 4 above for
   4862      modified versions, provided that you include in the combination
   4863      all of the Invariant Sections of all of the original documents,
   4864      unmodified, and list them all as Invariant Sections of your
   4865      combined work in its license notice, and that you preserve all
   4866      their Warranty Disclaimers.
   4867 
   4868      The combined work need only contain one copy of this License, and
   4869      multiple identical Invariant Sections may be replaced with a single
   4870      copy.  If there are multiple Invariant Sections with the same name
   4871      but different contents, make the title of each such section unique
   4872      by adding at the end of it, in parentheses, the name of the
   4873      original author or publisher of that section if known, or else a
   4874      unique number.  Make the same adjustment to the section titles in
   4875      the list of Invariant Sections in the license notice of the
   4876      combined work.
   4877 
   4878      In the combination, you must combine any sections Entitled
   4879      "History" in the various original documents, forming one section
   4880      Entitled "History"; likewise combine any sections Entitled
   4881      "Acknowledgements", and any sections Entitled "Dedications".  You
   4882      must delete all sections Entitled "Endorsements."
   4883 
   4884   6. COLLECTIONS OF DOCUMENTS
   4885 
   4886      You may make a collection consisting of the Document and other
   4887      documents released under this License, and replace the individual
   4888      copies of this License in the various documents with a single copy
   4889      that is included in the collection, provided that you follow the
   4890      rules of this License for verbatim copying of each of the
   4891      documents in all other respects.
   4892 
   4893      You may extract a single document from such a collection, and
   4894      distribute it individually under this License, provided you insert
   4895      a copy of this License into the extracted document, and follow
   4896      this License in all other respects regarding verbatim copying of
   4897      that document.
   4898 
   4899   7. AGGREGATION WITH INDEPENDENT WORKS
   4900 
   4901      A compilation of the Document or its derivatives with other
   4902      separate and independent documents or works, in or on a volume of
   4903      a storage or distribution medium, is called an "aggregate" if the
   4904      copyright resulting from the compilation is not used to limit the
   4905      legal rights of the compilation's users beyond what the individual
   4906      works permit.  When the Document is included in an aggregate, this
   4907      License does not apply to the other works in the aggregate which
   4908      are not themselves derivative works of the Document.
   4909 
   4910      If the Cover Text requirement of section 3 is applicable to these
   4911      copies of the Document, then if the Document is less than one half
   4912      of the entire aggregate, the Document's Cover Texts may be placed
   4913      on covers that bracket the Document within the aggregate, or the
   4914      electronic equivalent of covers if the Document is in electronic
   4915      form.  Otherwise they must appear on printed covers that bracket
   4916      the whole aggregate.
   4917 
   4918   8. TRANSLATION
   4919 
   4920      Translation is considered a kind of modification, so you may
   4921      distribute translations of the Document under the terms of section
   4922      4.  Replacing Invariant Sections with translations requires special
   4923      permission from their copyright holders, but you may include
   4924      translations of some or all Invariant Sections in addition to the
   4925      original versions of these Invariant Sections.  You may include a
   4926      translation of this License, and all the license notices in the
   4927      Document, and any Warranty Disclaimers, provided that you also
   4928      include the original English version of this License and the
   4929      original versions of those notices and disclaimers.  In case of a
   4930      disagreement between the translation and the original version of
   4931      this License or a notice or disclaimer, the original version will
   4932      prevail.
   4933 
   4934      If a section in the Document is Entitled "Acknowledgements",
   4935      "Dedications", or "History", the requirement (section 4) to
   4936      Preserve its Title (section 1) will typically require changing the
   4937      actual title.
   4938 
   4939   9. TERMINATION
   4940 
   4941      You may not copy, modify, sublicense, or distribute the Document
   4942      except as expressly provided for under this License.  Any other
   4943      attempt to copy, modify, sublicense or distribute the Document is
   4944      void, and will automatically terminate your rights under this
   4945      License.  However, parties who have received copies, or rights,
   4946      from you under this License will not have their licenses
   4947      terminated so long as such parties remain in full compliance.
   4948 
   4949  10. FUTURE REVISIONS OF THIS LICENSE
   4950 
   4951      The Free Software Foundation may publish new, revised versions of
   4952      the GNU Free Documentation License from time to time.  Such new
   4953      versions will be similar in spirit to the present version, but may
   4954      differ in detail to address new problems or concerns.  See
   4955      `http://www.gnu.org/copyleft/'.
   4956 
   4957      Each version of the License is given a distinguishing version
   4958      number.  If the Document specifies that a particular numbered
   4959      version of this License "or any later version" applies to it, you
   4960      have the option of following the terms and conditions either of
   4961      that specified version or of any later version that has been
   4962      published (not as a draft) by the Free Software Foundation.  If
   4963      the Document does not specify a version number of this License,
   4964      you may choose any version ever published (not as a draft) by the
   4965      Free Software Foundation.
   4966 
   4967 ADDENDUM: How to use this License for your documents
   4968 ====================================================
   4969 
   4970 To use this License in a document you have written, include a copy of
   4971 the License in the document and put the following copyright and license
   4972 notices just after the title page:
   4973 
   4974        Copyright (C)  YEAR  YOUR NAME.
   4975        Permission is granted to copy, distribute and/or modify this document
   4976        under the terms of the GNU Free Documentation License, Version 1.2
   4977        or any later version published by the Free Software Foundation;
   4978        with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
   4979        Texts.  A copy of the license is included in the section entitled ``GNU
   4980        Free Documentation License''.
   4981 
   4982    If you have Invariant Sections, Front-Cover Texts and Back-Cover
   4983 Texts, replace the "with...Texts." line with this:
   4984 
   4985          with the Invariant Sections being LIST THEIR TITLES, with
   4986          the Front-Cover Texts being LIST, and with the Back-Cover Texts
   4987          being LIST.
   4988 
   4989    If you have Invariant Sections without Cover Texts, or some other
   4990 combination of the three, merge those two alternatives to suit the
   4991 situation.
   4992 
   4993    If your document contains nontrivial examples of program code, we
   4994 recommend releasing these examples in parallel under your choice of
   4995 free software license, such as the GNU General Public License, to
   4996 permit their use in free software.
   4997 
   4998 
   4999 File: cpp.info,  Node: Index of Directives,  Next: Option Index,  Prev: GNU Free Documentation License,  Up: Top
   5000 
   5001 Index of Directives
   5002 *******************
   5003 
   5004 [index]
   5005 * Menu:
   5006 
   5007 * #assert:                               Assertions.           (line 41)
   5008 * #define:                               Object-like Macros.   (line 11)
   5009 * #elif:                                 Elif.                 (line  6)
   5010 * #else:                                 Else.                 (line  6)
   5011 * #endif:                                Ifdef.                (line  6)
   5012 * #error:                                Diagnostics.          (line  6)
   5013 * #ident:                                Other Directives.     (line  6)
   5014 * #if:                                   Conditional Syntax.   (line  6)
   5015 * #ifdef:                                Ifdef.                (line  6)
   5016 * #ifndef:                               Ifdef.                (line 40)
   5017 * #import:                               Obsolete once-only headers.
   5018                                                                (line 10)
   5019 * #include:                              Include Syntax.       (line  6)
   5020 * #include_next:                         Wrapper Headers.      (line  6)
   5021 * #line:                                 Line Control.         (line 20)
   5022 * #pragma GCC dependency:                Pragmas.              (line 53)
   5023 * #pragma GCC poison:                    Pragmas.              (line 65)
   5024 * #pragma GCC system_header <1>:         Pragmas.              (line 92)
   5025 * #pragma GCC system_header:             System Headers.       (line 31)
   5026 * #sccs:                                 Other Directives.     (line  6)
   5027 * #unassert:                             Assertions.           (line 52)
   5028 * #undef:                                Undefining and Redefining Macros.
   5029                                                                (line  6)
   5030 * #warning:                              Diagnostics.          (line 27)
   5031 
   5032 
   5033 File: cpp.info,  Node: Option Index,  Next: Concept Index,  Prev: Index of Directives,  Up: Top
   5034 
   5035 Option Index
   5036 ************
   5037 
   5038 CPP's command line options and environment variables are indexed here
   5039 without any initial `-' or `--'.
   5040 
   5041 [index]
   5042 * Menu:
   5043 
   5044 * A:                                     Invocation.          (line 525)
   5045 * ansi:                                  Invocation.          (line 311)
   5046 * C:                                     Invocation.          (line 577)
   5047 * C_INCLUDE_PATH:                        Environment Variables.
   5048                                                               (line  16)
   5049 * CPATH:                                 Environment Variables.
   5050                                                               (line  15)
   5051 * CPLUS_INCLUDE_PATH:                    Environment Variables.
   5052                                                               (line  17)
   5053 * D:                                     Invocation.          (line  39)
   5054 * dD:                                    Invocation.          (line 557)
   5055 * DEPENDENCIES_OUTPUT:                   Environment Variables.
   5056                                                               (line  44)
   5057 * dI:                                    Invocation.          (line 566)
   5058 * dM:                                    Invocation.          (line 541)
   5059 * dN:                                    Invocation.          (line 563)
   5060 * fdirectives-only:                      Invocation.          (line 433)
   5061 * fdollars-in-identifiers:               Invocation.          (line 455)
   5062 * fexec-charset:                         Invocation.          (line 482)
   5063 * fextended-identifiers:                 Invocation.          (line 458)
   5064 * finput-charset:                        Invocation.          (line 495)
   5065 * fno-show-column:                       Invocation.          (line 520)
   5066 * fno-working-directory:                 Invocation.          (line 505)
   5067 * fpreprocessed:                         Invocation.          (line 463)
   5068 * ftabstop:                              Invocation.          (line 476)
   5069 * fwide-exec-charset:                    Invocation.          (line 487)
   5070 * fworking-directory:                    Invocation.          (line 505)
   5071 * H:                                     Invocation.          (line 622)
   5072 * help:                                  Invocation.          (line 614)
   5073 * I:                                     Invocation.          (line  71)
   5074 * I-:                                    Invocation.          (line 348)
   5075 * idirafter:                             Invocation.          (line 390)
   5076 * imacros:                               Invocation.          (line 381)
   5077 * imultilib:                             Invocation.          (line 413)
   5078 * include:                               Invocation.          (line 370)
   5079 * iprefix:                               Invocation.          (line 397)
   5080 * iquote:                                Invocation.          (line 425)
   5081 * isysroot:                              Invocation.          (line 409)
   5082 * isystem:                               Invocation.          (line 417)
   5083 * iwithprefix:                           Invocation.          (line 403)
   5084 * iwithprefixbefore:                     Invocation.          (line 403)
   5085 * M:                                     Invocation.          (line 183)
   5086 * MD:                                    Invocation.          (line 272)
   5087 * MF:                                    Invocation.          (line 218)
   5088 * MG:                                    Invocation.          (line 227)
   5089 * MM:                                    Invocation.          (line 208)
   5090 * MMD:                                   Invocation.          (line 288)
   5091 * MP:                                    Invocation.          (line 237)
   5092 * MQ:                                    Invocation.          (line 263)
   5093 * MT:                                    Invocation.          (line 249)
   5094 * nostdinc:                              Invocation.          (line 360)
   5095 * nostdinc++:                            Invocation.          (line 365)
   5096 * o:                                     Invocation.          (line  82)
   5097 * OBJC_INCLUDE_PATH:                     Environment Variables.
   5098                                                               (line  18)
   5099 * P:                                     Invocation.          (line 570)
   5100 * pedantic:                              Invocation.          (line 173)
   5101 * pedantic-errors:                       Invocation.          (line 178)
   5102 * remap:                                 Invocation.          (line 609)
   5103 * std=:                                  Invocation.          (line 311)
   5104 * SUNPRO_DEPENDENCIES:                   Environment Variables.
   5105                                                               (line  60)
   5106 * target-help:                           Invocation.          (line 614)
   5107 * traditional-cpp:                       Invocation.          (line 602)
   5108 * trigraphs:                             Invocation.          (line 606)
   5109 * U:                                     Invocation.          (line  62)
   5110 * undef:                                 Invocation.          (line  66)
   5111 * v:                                     Invocation.          (line 618)
   5112 * version:                               Invocation.          (line 631)
   5113 * w:                                     Invocation.          (line 169)
   5114 * Wall:                                  Invocation.          (line  88)
   5115 * Wcomment:                              Invocation.          (line  96)
   5116 * Wcomments:                             Invocation.          (line  96)
   5117 * Wendif-labels:                         Invocation.          (line 146)
   5118 * Werror:                                Invocation.          (line 159)
   5119 * Wimport:                               Invocation.          (line 119)
   5120 * Wsystem-headers:                       Invocation.          (line 163)
   5121 * Wtraditional:                          Invocation.          (line 113)
   5122 * Wtrigraphs:                            Invocation.          (line 101)
   5123 * Wundef:                                Invocation.          (line 122)
   5124 * Wunused-macros:                        Invocation.          (line 127)
   5125 * x:                                     Invocation.          (line 295)
   5126 
   5127 
   5128 File: cpp.info,  Node: Concept Index,  Prev: Option Index,  Up: Top
   5129 
   5130 Concept Index
   5131 *************
   5132 
   5133 [index]
   5134 * Menu:
   5135 
   5136 * # operator:                            Stringification.     (line   6)
   5137 * ## operator:                           Concatenation.       (line   6)
   5138 * _Pragma:                               Pragmas.             (line  25)
   5139 * alternative tokens:                    Tokenization.        (line 106)
   5140 * arguments:                             Macro Arguments.     (line   6)
   5141 * arguments in macro definitions:        Macro Arguments.     (line   6)
   5142 * assertions:                            Assertions.          (line   6)
   5143 * assertions, canceling:                 Assertions.          (line  52)
   5144 * backslash-newline:                     Initial processing.  (line  61)
   5145 * block comments:                        Initial processing.  (line  77)
   5146 * C++ named operators:                   C++ Named Operators. (line   6)
   5147 * character constants:                   Tokenization.        (line  85)
   5148 * character set, execution:              Invocation.          (line 482)
   5149 * character set, input:                  Invocation.          (line 495)
   5150 * character set, wide execution:         Invocation.          (line 487)
   5151 * command line:                          Invocation.          (line   6)
   5152 * commenting out code:                   Deleted Code.        (line   6)
   5153 * comments:                              Initial processing.  (line  77)
   5154 * common predefined macros:              Common Predefined Macros.
   5155                                                               (line   6)
   5156 * computed includes:                     Computed Includes.   (line   6)
   5157 * concatenation:                         Concatenation.       (line   6)
   5158 * conditional group:                     Ifdef.               (line  14)
   5159 * conditionals:                          Conditionals.        (line   6)
   5160 * continued lines:                       Initial processing.  (line  61)
   5161 * controlling macro:                     Once-Only Headers.   (line  35)
   5162 * defined:                               Defined.             (line   6)
   5163 * dependencies for make as output:       Environment Variables.
   5164                                                               (line  45)
   5165 * dependencies, make:                    Invocation.          (line 183)
   5166 * diagnostic:                            Diagnostics.         (line   6)
   5167 * differences from previous versions:    Differences from previous versions.
   5168                                                               (line   6)
   5169 * digraphs:                              Tokenization.        (line 106)
   5170 * directive line:                        The preprocessing language.
   5171                                                               (line   6)
   5172 * directive name:                        The preprocessing language.
   5173                                                               (line   6)
   5174 * directives:                            The preprocessing language.
   5175                                                               (line   6)
   5176 * empty macro arguments:                 Macro Arguments.     (line  66)
   5177 * environment variables:                 Environment Variables.
   5178                                                               (line   6)
   5179 * expansion of arguments:                Argument Prescan.    (line   6)
   5180 * FDL, GNU Free Documentation License:   GNU Free Documentation License.
   5181                                                               (line   6)
   5182 * function-like macros:                  Function-like Macros.
   5183                                                               (line   6)
   5184 * grouping options:                      Invocation.          (line  34)
   5185 * guard macro:                           Once-Only Headers.   (line  35)
   5186 * header file:                           Header Files.        (line   6)
   5187 * header file names:                     Tokenization.        (line  85)
   5188 * identifiers:                           Tokenization.        (line  34)
   5189 * implementation limits:                 Implementation limits.
   5190                                                               (line   6)
   5191 * implementation-defined behavior:       Implementation-defined behavior.
   5192                                                               (line   6)
   5193 * including just once:                   Once-Only Headers.   (line   6)
   5194 * invocation:                            Invocation.          (line   6)
   5195 * iso646.h:                              C++ Named Operators. (line   6)
   5196 * line comments:                         Initial processing.  (line  77)
   5197 * line control:                          Line Control.        (line   6)
   5198 * line endings:                          Initial processing.  (line  14)
   5199 * linemarkers:                           Preprocessor Output. (line  28)
   5200 * macro argument expansion:              Argument Prescan.    (line   6)
   5201 * macro arguments and directives:        Directives Within Macro Arguments.
   5202                                                               (line   6)
   5203 * macros in include:                     Computed Includes.   (line   6)
   5204 * macros with arguments:                 Macro Arguments.     (line   6)
   5205 * macros with variable arguments:        Variadic Macros.     (line   6)
   5206 * make:                                  Invocation.          (line 183)
   5207 * manifest constants:                    Object-like Macros.  (line   6)
   5208 * named operators:                       C++ Named Operators. (line   6)
   5209 * newlines in macro arguments:           Newlines in Arguments.
   5210                                                               (line   6)
   5211 * null directive:                        Other Directives.    (line  15)
   5212 * numbers:                               Tokenization.        (line  61)
   5213 * object-like macro:                     Object-like Macros.  (line   6)
   5214 * options:                               Invocation.          (line  38)
   5215 * options, grouping:                     Invocation.          (line  34)
   5216 * other tokens:                          Tokenization.        (line 120)
   5217 * output format:                         Preprocessor Output. (line  12)
   5218 * overriding a header file:              Wrapper Headers.     (line   6)
   5219 * parentheses in macro bodies:           Operator Precedence Problems.
   5220                                                               (line   6)
   5221 * pitfalls of macros:                    Macro Pitfalls.      (line   6)
   5222 * predefined macros:                     Predefined Macros.   (line   6)
   5223 * predefined macros, system-specific:    System-specific Predefined Macros.
   5224                                                               (line   6)
   5225 * predicates:                            Assertions.          (line  19)
   5226 * preprocessing directives:              The preprocessing language.
   5227                                                               (line   6)
   5228 * preprocessing numbers:                 Tokenization.        (line  61)
   5229 * preprocessing tokens:                  Tokenization.        (line   6)
   5230 * prescan of macro arguments:            Argument Prescan.    (line   6)
   5231 * problems with macros:                  Macro Pitfalls.      (line   6)
   5232 * punctuators:                           Tokenization.        (line 106)
   5233 * redefining macros:                     Undefining and Redefining Macros.
   5234                                                               (line   6)
   5235 * repeated inclusion:                    Once-Only Headers.   (line   6)
   5236 * reporting errors:                      Diagnostics.         (line   6)
   5237 * reporting warnings:                    Diagnostics.         (line   6)
   5238 * reserved namespace:                    System-specific Predefined Macros.
   5239                                                               (line   6)
   5240 * self-reference:                        Self-Referential Macros.
   5241                                                               (line   6)
   5242 * semicolons (after macro calls):        Swallowing the Semicolon.
   5243                                                               (line   6)
   5244 * side effects (in macro arguments):     Duplication of Side Effects.
   5245                                                               (line   6)
   5246 * standard predefined macros.:           Standard Predefined Macros.
   5247                                                               (line   6)
   5248 * string constants:                      Tokenization.        (line  85)
   5249 * string literals:                       Tokenization.        (line  85)
   5250 * stringification:                       Stringification.     (line   6)
   5251 * symbolic constants:                    Object-like Macros.  (line   6)
   5252 * system header files <1>:               System Headers.      (line   6)
   5253 * system header files:                   Header Files.        (line  13)
   5254 * system-specific predefined macros:     System-specific Predefined Macros.
   5255                                                               (line   6)
   5256 * testing predicates:                    Assertions.          (line  30)
   5257 * token concatenation:                   Concatenation.       (line   6)
   5258 * token pasting:                         Concatenation.       (line   6)
   5259 * tokens:                                Tokenization.        (line   6)
   5260 * trigraphs:                             Initial processing.  (line  32)
   5261 * undefining macros:                     Undefining and Redefining Macros.
   5262                                                               (line   6)
   5263 * unsafe macros:                         Duplication of Side Effects.
   5264                                                               (line   6)
   5265 * variable number of arguments:          Variadic Macros.     (line   6)
   5266 * variadic macros:                       Variadic Macros.     (line   6)
   5267 * wrapper #ifndef:                       Once-Only Headers.   (line   6)
   5268 * wrapper headers:                       Wrapper Headers.     (line   6)
   5269 
   5270 
   5271 
   5272 Tag Table:
   5273 Node: Top1102
   5274 Node: Overview3798
   5275 Node: Character sets6619
   5276 Ref: Character sets-Footnote-18802
   5277 Node: Initial processing8983
   5278 Ref: trigraphs10542
   5279 Node: Tokenization14744
   5280 Ref: Tokenization-Footnote-121880
   5281 Node: The preprocessing language21991
   5282 Node: Header Files24869
   5283 Node: Include Syntax26749
   5284 Node: Include Operation28386
   5285 Node: Search Path30234
   5286 Node: Once-Only Headers33424
   5287 Node: Computed Includes35069
   5288 Node: Wrapper Headers38213
   5289 Node: System Headers40639
   5290 Node: Macros42689
   5291 Node: Object-like Macros43830
   5292 Node: Function-like Macros47420
   5293 Node: Macro Arguments49036
   5294 Node: Stringification53181
   5295 Node: Concatenation56387
   5296 Node: Variadic Macros59495
   5297 Node: Predefined Macros64282
   5298 Node: Standard Predefined Macros64870
   5299 Node: Common Predefined Macros70806
   5300 Node: System-specific Predefined Macros83493
   5301 Node: C++ Named Operators85514
   5302 Node: Undefining and Redefining Macros86478
   5303 Node: Directives Within Macro Arguments88582
   5304 Node: Macro Pitfalls90130
   5305 Node: Misnesting90663
   5306 Node: Operator Precedence Problems91775
   5307 Node: Swallowing the Semicolon93641
   5308 Node: Duplication of Side Effects95664
   5309 Node: Self-Referential Macros97847
   5310 Node: Argument Prescan100256
   5311 Node: Newlines in Arguments104010
   5312 Node: Conditionals104961
   5313 Node: Conditional Uses106791
   5314 Node: Conditional Syntax108149
   5315 Node: Ifdef108469
   5316 Node: If111630
   5317 Node: Defined113934
   5318 Node: Else115217
   5319 Node: Elif115787
   5320 Node: Deleted Code117076
   5321 Node: Diagnostics118323
   5322 Node: Line Control119940
   5323 Node: Pragmas123744
   5324 Node: Other Directives128014
   5325 Node: Preprocessor Output129064
   5326 Node: Traditional Mode132265
   5327 Node: Traditional lexical analysis133323
   5328 Node: Traditional macros135826
   5329 Node: Traditional miscellany139628
   5330 Node: Traditional warnings140625
   5331 Node: Implementation Details142822
   5332 Node: Implementation-defined behavior143443
   5333 Ref: Identifier characters144176
   5334 Node: Implementation limits147261
   5335 Node: Obsolete Features149935
   5336 Node: Assertions150390
   5337 Node: Obsolete once-only headers152931
   5338 Node: Differences from previous versions154666
   5339 Node: Invocation158874
   5340 Ref: Wtrigraphs163326
   5341 Ref: dashMF168157
   5342 Ref: fdollars-in-identifiers177540
   5343 Node: Environment Variables185373
   5344 Node: GNU Free Documentation License188339
   5345 Node: Index of Directives210772
   5346 Node: Option Index212701
   5347 Node: Concept Index218885
   5348 
   5349 End Tag Table
   5350