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INFO-DIR-SECTION Programming
START-INFO-DIR-ENTRY
* Cpp: (cpp).		       The GNU C preprocessor.
END-INFO-DIR-ENTRY


File: cpp.info,  Node: Top,  Next: Overview,  Up: (dir)



   The C preprocessor implements the macro language used to transform C,
C++, and Objective-C programs before they are compiled.  It can also be
useful on its own.

* Menu:

* Overview::
* Header Files::
* Macros::
* Conditionals::
* Diagnostics::
* Line Control::
* Pragmas::
* Other Directives::
* Preprocessor Output::
* Traditional Mode::
* Implementation Details::
* Invocation::
* Environment Variables::
* GNU Free Documentation License::
* Option Index::
* Index of Directives::
* Concept Index::

 --- The Detailed Node Listing ---

Overview

* Initial processing::
* Tokenization::
* The preprocessing language::

Header Files

* Include Syntax::
* Include Operation::
* Search Path::
* Once-Only Headers::
* Computed Includes::
* Wrapper Headers::
* System Headers::

Macros

* Object-like Macros::
* Function-like Macros::
* Macro Arguments::
* Stringification::
* Concatenation::
* Variadic Macros::
* Predefined Macros::
* Undefining and Redefining Macros::
* Macro Pitfalls::

Predefined Macros

* Standard Predefined Macros::
* Common Predefined Macros::
* System-specific Predefined Macros::
* C++ Named Operators::

Macro Pitfalls

* Misnesting::
* Operator Precedence Problems::
* Swallowing the Semicolon::
* Duplication of Side Effects::
* Self-Referential Macros::
* Argument Prescan::
* Newlines in Arguments::

Conditionals

* Conditional Uses::
* Conditional Syntax::
* Deleted Code::

Conditional Syntax

* Ifdef::
* If::
* Defined::
* Else::
* Elif::

Implementation Details

* Implementation-defined behavior::
* Implementation limits::
* Obsolete Features::
* Differences from previous versions::

Obsolete Features

* Assertions::
* Obsolete once-only headers::
* Miscellaneous obsolete features::

   Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997,
1998, 1999, 2000, 2001 Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation.  A copy of
the license is included in the section entitled "GNU Free Documentation
License".

   This manual contains no Invariant Sections.  The Front-Cover Texts
are (a) (see below), and the Back-Cover Texts are (b) (see below).

   (a) The FSF's Front-Cover Text is:

   A GNU Manual

   (b) The FSF's Back-Cover Text is:

   You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise funds
for GNU development.


File: cpp.info,  Node: Overview,  Next: Header Files,  Prev: Top,  Up: Top

Overview
********

   The C preprocessor, often known as "cpp", is a "macro processor"
that is used automatically by the C compiler to transform your program
before compilation.  It is called a macro processor because it allows
you to define "macros", which are brief abbreviations for longer
constructs.

   The C preprocessor is intended to be used only with C, C++, and
Objective-C source code.  In the past, it has been abused as a general
text processor.  It will choke on input which does not obey C's lexical
rules.  For example, apostrophes will be interpreted as the beginning of
character constants, and cause errors.  Also, you cannot rely on it
preserving characteristics of the input which are not significant to
C-family languages.  If a Makefile is preprocessed, all the hard tabs
will be removed, and the Makefile will not work.

   Having said that, you can often get away with using cpp on things
which are not C.  Other Algol-ish programming languages are often safe
(Pascal, Ada, etc.) So is assembly, with caution.  `-traditional' mode
preserves more white space, and is otherwise more permissive.  Many of
the problems can be avoided by writing C or C++ style comments instead
of native language comments, and keeping macros simple.

   Wherever possible, you should use a preprocessor geared to the
language you are writing in.  Modern versions of the GNU assembler have
macro facilities.  Most high level programming languages have their own
conditional compilation and inclusion mechanism.  If all else fails,
try a true general text processor, such as GNU M4.

   C preprocessors vary in some details.  This manual discusses the GNU
C preprocessor, which provides a small superset of the features of ISO
Standard C.  In its default mode, the GNU C preprocessor does not do a
few things required by the standard.  These are features which are
rarely, if ever, used, and may cause surprising changes to the meaning
of a program which does not expect them.  To get strict ISO Standard C,
you should use the `-std=c89' or `-std=c99' options, depending on which
version of the standard you want.  To get all the mandatory
diagnostics, you must also use `-pedantic'.  *Note Invocation::.

* Menu:

* Initial processing::
* Tokenization::
* The preprocessing language::


File: cpp.info,  Node: Initial processing,  Next: Tokenization,  Up: Overview

Initial processing
==================

   The preprocessor performs a series of textual transformations on its
input.  These happen before all other processing.  Conceptually, they
happen in a rigid order, and the entire file is run through each
transformation before the next one begins.  GNU CPP actually does them
all at once, for performance reasons.  These transformations correspond
roughly to the first three "phases of translation" described in the C
standard.

  1. The input file is read into memory and broken into lines.

     GNU CPP expects its input to be a text file, that is, an
     unstructured stream of ASCII characters, with some characters
     indicating the end of a line of text.  Extended ASCII character
     sets, such as ISO Latin-1 or Unicode encoded in UTF-8, are also
     acceptable.  Character sets that are not strict supersets of
     seven-bit ASCII will not work.  We plan to add complete support
     for international character sets in a future release.

     Different systems use different conventions to indicate the end of
     a line.  GCC accepts the ASCII control sequences `LF', `CR LF',
     `CR', and `LF CR' as end-of-line markers.  The first three are the
     canonical sequences used by Unix, DOS and VMS, and the classic Mac
     OS (before OSX) respectively.  You may therefore safely copy
     source code written on any of those systems to a different one and
     use it without conversion.  (GCC may lose track of the current
     line number if a file doesn't consistently use one convention, as
     sometimes happens when it is edited on computers with different
     conventions that share a network file system.)  `LF CR' is
     included because it has been reported as an end-of-line marker
     under exotic conditions.

     If the last line of any input file lacks an end-of-line marker,
     the end of the file is considered to implicitly supply one.  The C
     standard says that this condition provokes undefined behavior, so
     GCC will emit a warning message.

  2. If trigraphs are enabled, they are replaced by their corresponding
     single characters.

     These are nine three-character sequences, all starting with `??',
     that are defined by ISO C to stand for single characters.  They
     permit obsolete systems that lack some of C's punctuation to use
     C.  For example, `??/' stands for `\', so '??/n' is a character
     constant for a newline.  By default, GCC ignores trigraphs, but if
     you request a strictly conforming mode with the `-std' option, then
     it converts them.

     Trigraphs are not popular and many compilers implement them
     incorrectly.  Portable code should not rely on trigraphs being
     either converted or ignored.  If you use the `-Wall' or
     `-Wtrigraphs' options, GCC will warn you when a trigraph would
     change the meaning of your program if it were converted.

     In a string constant, you can prevent a sequence of question marks
     from being confused with a trigraph by inserting a backslash
     between the question marks.  "(??\?)" is the string `(???)', not
     `(?]'.  Traditional C compilers do not recognize this idiom.

     The nine trigraphs and their replacements are

          Trigraph:       ??(  ??)  ??<  ??>  ??=  ??/  ??'  ??!  ??-
          Replacement:      [    ]    {    }    #    \    ^    |    ~

  3. Continued lines are merged into one long line.

     A continued line is a line which ends with a backslash, `\'.  The
     backslash is removed and the following line is joined with the
     current one.  No space is inserted, so you may split a line
     anywhere, even in the middle of a word.  (It is generally more
     readable to split lines only at white space.)

     The trailing backslash on a continued line is commonly referred to
     as a "backslash-newline".

     If there is white space between a backslash and the end of a line,
     that is still a continued line.  However, as this is usually the
     result of an editing mistake, and many compilers will not accept
     it as a continued line, GCC will warn you about it.

  4. All comments are replaced with single spaces.

     There are two kinds of comments.  "Block comments" begin with `/*'
     and continue until the next `*/'.  Block comments do not nest:

          /* this is /* one comment */ text outside comment

     "Line comments" begin with `//' and continue to the end of the
     current line.  Line comments do not nest either, but it does not
     matter, because they would end in the same place anyway.

          // this is // one comment
          text outside comment

   It is safe to put line comments inside block comments, or vice versa.

     /* block comment
        // contains line comment
        yet more comment
      */ outside comment
     
     // line comment /* contains block comment */

   But beware of commenting out one end of a block comment with a line
comment.

      // l.c.  /* block comment begins
         oops! this isn't a comment anymore */

   Comments are not recognized within string literals.  "/* blah */" is
the string constant `/* blah */', not an empty string.

   Line comments are not in the 1989 edition of the C standard, but they
are recognized by GCC as an extension.  In C++ and in the 1999 edition
of the C standard, they are an official part of the language.

   Since these transformations happen before all other processing, you
can split a line mechanically with backslash-newline anywhere.  You can
comment out the end of a line.  You can continue a line comment onto the
next line with backslash-newline.  You can even split `/*', `*/', and
`//' onto multiple lines with backslash-newline.  For example:

     /\
     *
     */ # /*
     */ defi\
     ne FO\
     O 10\
     20

is equivalent to `#define FOO 1020'.  All these tricks are extremely
confusing and should not be used in code intended to be readable.

   There is no way to prevent a backslash at the end of a line from
being interpreted as a backslash-newline.

     "foo\\
     bar"

is equivalent to `"foo\bar"', not to `"foo\\bar"'.  To avoid having to
worry about this, do not use the deprecated GNU extension which permits
multi-line strings.  Instead, use string literal concatenation:

        "foo\\"
        "bar"

Your program will be more portable this way, too.


File: cpp.info,  Node: Tokenization,  Next: The preprocessing language,  Prev: Initial processing,  Up: Overview

Tokenization
============

   After the textual transformations are finished, the input file is
converted into a sequence of "preprocessing tokens".  These mostly
correspond to the syntactic tokens used by the C compiler, but there are
a few differences.  White space separates tokens; it is not itself a
token of any kind.  Tokens do not have to be separated by white space,
but it is often necessary to avoid ambiguities.

   When faced with a sequence of characters that has more than one
possible tokenization, the preprocessor is greedy.  It always makes
each token, starting from the left, as big as possible before moving on
to the next token.  For instance, `a+++++b' is interpreted as
`a ++ ++ + b', not as `a ++ + ++ b', even though the latter
tokenization could be part of a valid C program and the former could
not.

   Once the input file is broken into tokens, the token boundaries never
change, except when the `##' preprocessing operator is used to paste
tokens together.  *Note Concatenation::.  For example,

     #define foo() bar
     foo()baz
          ==> bar baz
     _not_
          ==> barbaz

   The compiler does not re-tokenize the preprocessor's output.  Each
preprocessing token becomes one compiler token.

   Preprocessing tokens fall into five broad classes: identifiers,
preprocessing numbers, string literals, punctuators, and other.  An
"identifier" is the same as an identifier in C: any sequence of
letters, digits, or underscores, which begins with a letter or
underscore.  Keywords of C have no significance to the preprocessor;
they are ordinary identifiers.  You can define a macro whose name is a
keyword, for instance.  The only identifier which can be considered a
preprocessing keyword is `defined'.  *Note Defined::.

   This is mostly true of other languages which use the C preprocessor.
However, a few of the keywords of C++ are significant even in the
preprocessor.  *Note C++ Named Operators::.

   In the 1999 C standard, identifiers may contain letters which are not
part of the "basic source character set," at the implementation's
discretion (such as accented Latin letters, Greek letters, or Chinese
ideograms).  This may be done with an extended character set, or the
`\u' and `\U' escape sequences.  GCC does not presently implement
either feature in the preprocessor or the compiler.

   As an extension, GCC treats `$' as a letter.  This is for
compatibility with some systems, such as VMS, where `$' is commonly
used in system-defined function and object names.  `$' is not a letter
in strictly conforming mode, or if you specify the `-$' option.  *Note
Invocation::.

   A "preprocessing number" has a rather bizarre definition.  The
category includes all the normal integer and floating point constants
one expects of C, but also a number of other things one might not
initially recognize as a number.  Formally, preprocessing numbers begin
with an optional period, a required decimal digit, and then continue
with any sequence of letters, digits, underscores, periods, and
exponents.  Exponents are the two-character sequences `e+', `e-', `E+',
`E-', `p+', `p-', `P+', and `P-'.  (The exponents that begin with `p'
or `P' are new to C99.  They are used for hexadecimal floating-point
constants.)

   The purpose of this unusual definition is to isolate the preprocessor
from the full complexity of numeric constants.  It does not have to
distinguish between lexically valid and invalid floating-point numbers,
which is complicated.  The definition also permits you to split an
identifier at any position and get exactly two tokens, which can then be
pasted back together with the `##' operator.

   It's possible for preprocessing numbers to cause programs to be
misinterpreted.  For example, `0xE+12' is a preprocessing number which
does not translate to any valid numeric constant, therefore a syntax
error.  It does not mean `0xE + 12', which is what you might have
intended.

   "String literals" are string constants, character constants, and
header file names (the argument of `#include').(1)  String constants
and character constants are straightforward: "..." or '...'.  In either
case embedded quotes should be escaped with a backslash: '\'' is the
character constant for `''.  There is no limit on the length of a
character constant, but the value of a character constant that contains
more than one character is implementation-defined.  *Note
Implementation Details::.

   Header file names either look like string constants, "...", or are
written with angle brackets instead, <...>.  In either case, backslash
is an ordinary character.  There is no way to escape the closing quote
or angle bracket.  The preprocessor looks for the header file in
different places depending on which form you use.  *Note Include
Operation::.

   In standard C, no string literal may extend past the end of a line.
GNU CPP accepts multi-line string constants, but not multi-line
character constants or header file names.  This extension is deprecated
and will be removed in GCC 3.1.  You may use continued lines instead,
or string constant concatenation.  *Note Differences from previous
versions::.

   "Punctuators" are all the usual bits of punctuation which are
meaningful to C and C++.  All but three of the punctuation characters in
ASCII are C punctuators.  The exceptions are `@', `$', and ``'.  In
addition, all the two- and three-character operators are punctuators.
There are also six "digraphs", which the C++ standard calls
"alternative tokens", which are merely alternate ways to spell other
punctuators.  This is a second attempt to work around missing
punctuation in obsolete systems.  It has no negative side effects,
unlike trigraphs, but does not cover as much ground.  The digraphs and
their corresponding normal punctuators are:

     Digraph:        <%  %>  <:  :>  %:  %:%:
     Punctuator:      {   }   [   ]   #    ##

   Any other single character is considered "other." It is passed on to
the preprocessor's output unmolested.  The C compiler will almost
certainly reject source code containing "other" tokens.  In ASCII, the
only other characters are `@', `$', ``', and control characters other
than NUL (all bits zero).  (Note that `$' is normally considered a
letter.)  All characters with the high bit set (numeric range
0x7F-0xFF) are also "other" in the present implementation.  This will
change when proper support for international character sets is added to
GCC.

   NUL is a special case because of the high probability that its
appearance is accidental, and because it may be invisible to the user
(many terminals do not display NUL at all).  Within comments, NULs are
silently ignored, just as any other character would be.  In running
text, NUL is considered white space.  For example, these two directives
have the same meaning.

     #define X^@1
     #define X 1

(where `^@' is ASCII NUL).  Within string or character constants, NULs
are preserved.  In the latter two cases the preprocessor emits a
warning message.

   ---------- Footnotes ----------

   (1) The C standard uses the term "string literal" to refer only to
what we are calling "string constants".


File: cpp.info,  Node: The preprocessing language,  Prev: Tokenization,  Up: Overview

The preprocessing language
==========================

   After tokenization, the stream of tokens may simply be passed
straight to the compiler's parser.  However, if it contains any
operations in the "preprocessing language", it will be transformed
first.  This stage corresponds roughly to the standard's "translation
phase 4" and is what most people think of as the preprocessor's job.

   The preprocessing language consists of "directives" to be executed
and "macros" to be expanded.  Its primary capabilities are:

   * Inclusion of header files.  These are files of declarations that
     can be substituted into your program.

   * Macro expansion.  You can define "macros", which are abbreviations
     for arbitrary fragments of C code.  The preprocessor will replace
     the macros with their definitions throughout the program.  Some
     macros are automatically defined for you.

   * Conditional compilation.  You can include or exclude parts of the
     program according to various conditions.

   * Line control.  If you use a program to combine or rearrange source
     files into an intermediate file which is then compiled, you can
     use line control to inform the compiler where each source line
     originally came from.

   * Diagnostics.  You can detect problems at compile time and issue
     errors or warnings.

   There are a few more, less useful, features.

   Except for expansion of predefined macros, all these operations are
triggered with "preprocessing directives".  Preprocessing directives
are lines in your program that start with `#'.  Whitespace is allowed
before and after the `#'.  The `#' is followed by an identifier, the
"directive name".  It specifies the operation to perform.  Directives
are commonly referred to as `#NAME' where NAME is the directive name.
For example, `#define' is the directive that defines a macro.

   The `#' which begins a directive cannot come from a macro expansion.
Also, the directive name is not macro expanded.  Thus, if `foo' is
defined as a macro expanding to `define', that does not make `#foo' a
valid preprocessing directive.

   The set of valid directive names is fixed.  Programs cannot define
new preprocessing directives.

   Some directives require arguments; these make up the rest of the
directive line and must be separated from the directive name by
whitespace.  For example, `#define' must be followed by a macro name
and the intended expansion of the macro.

   A preprocessing directive cannot cover more than one line.  The line
may, however, be continued with backslash-newline, or by a block comment
which extends past the end of the line.  In either case, when the
directive is processed, the continuations have already been merged with
the first line to make one long line.


File: cpp.info,  Node: Header Files,  Next: Macros,  Prev: Overview,  Up: Top

Header Files
************

   A header file is a file containing C declarations and macro
definitions (*note Macros::) to be shared between several source files.
You request the use of a header file in your program by "including"
it, with the C preprocessing directive `#include'.

   Header files serve two purposes.

   * System header files declare the interfaces to parts of the
     operating system.  You include them in your program to supply the
     definitions and declarations you need to invoke system calls and
     libraries.

   * Your own header files contain declarations for interfaces between
     the source files of your program.  Each time you have a group of
     related declarations and macro definitions all or most of which
     are needed in several different source files, it is a good idea to
     create a header file for them.

   Including a header file produces the same results as copying the
header file into each source file that needs it.  Such copying would be
time-consuming and error-prone.  With a header file, the related
declarations appear in only one place.  If they need to be changed, they
can be changed in one place, and programs that include the header file
will automatically use the new version when next recompiled.  The header
file eliminates the labor of finding and changing all the copies as well
as the risk that a failure to find one copy will result in
inconsistencies within a program.

   In C, the usual convention is to give header files names that end
with `.h'.  It is most portable to use only letters, digits, dashes, and
underscores in header file names, and at most one dot.

* Menu:

* Include Syntax::
* Include Operation::
* Search Path::
* Once-Only Headers::
* Computed Includes::
* Wrapper Headers::
* System Headers::


File: cpp.info,  Node: Include Syntax,  Next: Include Operation,  Up: Header Files

Include Syntax
==============

   Both user and system header files are included using the
preprocessing directive `#include'.  It has two variants:

`#include <FILE>'
     This variant is used for system header files.  It searches for a
     file named FILE in a standard list of system directories.  You can
     prepend directories to this list with the `-I' option (*note
     Invocation::).

`#include "FILE"'
     This variant is used for header files of your own program.  It
     searches for a file named FILE first in the directory containing
     the current file, then in the same directories used for `<FILE>'.

   The argument of `#include', whether delimited with quote marks or
angle brackets, behaves like a string constant in that comments are not
recognized, and macro names are not expanded.  Thus, `#include <x/*y>'
specifies inclusion of a system header file named `x/*y'.

   However, if backslashes occur within FILE, they are considered
ordinary text characters, not escape characters.  None of the character
escape sequences appropriate to string constants in C are processed.
Thus, `#include "x\n\\y"' specifies a filename containing three
backslashes.  (Some systems interpret `\' as a pathname separator.  All
of these also interpret `/' the same way.  It is most portable to use
only `/'.)

   It is an error if there is anything (other than comments) on the line
after the file name.


File: cpp.info,  Node: Include Operation,  Next: Search Path,  Prev: Include Syntax,  Up: Header Files

Include Operation
=================

   The `#include' directive works by directing the C preprocessor to
scan the specified file as input before continuing with the rest of the
current file.  The output from the preprocessor contains the output
already generated, followed by the output resulting from the included
file, followed by the output that comes from the text after the
`#include' directive.  For example, if you have a header file
`header.h' as follows,

     char *test (void);

and a main program called `program.c' that uses the header file, like
this,

     int x;
     #include "header.h"
     
     int
     main (void)
     {
       puts (test ());
     }

the compiler will see the same token stream as it would if `program.c'
read

     int x;
     char *test (void);
     
     int
     main (void)
     {
       puts (test ());
     }

   Included files are not limited to declarations and macro definitions;
those are merely the typical uses.  Any fragment of a C program can be
included from another file.  The include file could even contain the
beginning of a statement that is concluded in the containing file, or
the end of a statement that was started in the including file.  However,
a comment or a string or character constant may not start in the
included file and finish in the including file.  An unterminated
comment, string constant or character constant in an included file is
considered to end (with an error message) at the end of the file.

   To avoid confusion, it is best if header files contain only complete
syntactic units--function declarations or definitions, type
declarations, etc.

   The line following the `#include' directive is always treated as a
separate line by the C preprocessor, even if the included file lacks a
final newline.


File: cpp.info,  Node: Search Path,  Next: Once-Only Headers,  Prev: Include Operation,  Up: Header Files

Search Path
===========

   GCC looks in several different places for headers.  On a normal Unix
system, if you do not instruct it otherwise, it will look for headers
requested with `#include <FILE>' in:

     /usr/local/include
     /usr/lib/gcc-lib/TARGET/VERSION/include
     /usr/TARGET/include
     /usr/include

   For C++ programs, it will also look in `/usr/include/g++-v3', first.
In the above, TARGET is the canonical name of the system GCC was
configured to compile code for; often but not always the same as the
canonical name of the system it runs on.  VERSION is the version of GCC
in use.

   You can add to this list with the `-IDIR' command line option.  All
the directories named by `-I' are searched, in left-to-right order,
_before_ the default directories.  The only exception is when `dir' is
already searched by default.  In this case, the option is ignored and
the search order for system directories remains unchanged.

   Duplicate directories are removed from the quote and bracket search
chains before the two chains are merged to make the final search chain.
Thus, it is possible for a directory to occur twice in the final search
chain if it was specified in both the quote and bracket chains.

   You can prevent GCC from searching any of the default directories
with the `-nostdinc' option.  This is useful when you are compiling an
operating system kernel or some other program that does not use the
standard C library facilities, or the standard C library itself.  `-I'
options are not ignored as described above when `-nostdinc' is in
effect.

   GCC looks for headers requested with `#include "FILE"' first in the
directory containing the current file, then in the same places it would
have looked for a header requested with angle brackets.  For example,
if `/usr/include/sys/stat.h' contains `#include "types.h"', GCC looks
for `types.h' first in `/usr/include/sys', then in its usual search
path.

   `#line' (*note Line Control::) does not change GCC's idea of the
directory containing the current file.

   You may put `-I-' at any point in your list of `-I' options.  This
has two effects.  First, directories appearing before the `-I-' in the
list are searched only for headers requested with quote marks.
Directories after `-I-' are searched for all headers.  Second, the
directory containing the current file is not searched for anything,
unless it happens to be one of the directories named by an `-I' switch.

   `-I. -I-' is not the same as no `-I' options at all, and does not
cause the same behavior for `<>' includes that `""' includes get with
no special options.  `-I.' searches the compiler's current working
directory for header files.  That may or may not be the same as the
directory containing the current file.

   If you need to look for headers in a directory named `-', write
`-I./-'.

   There are several more ways to adjust the header search path.  They
are generally less useful.  *Note Invocation::.


File: cpp.info,  Node: Once-Only Headers,  Next: Computed Includes,  Prev: Search Path,  Up: Header Files

Once-Only Headers
=================

   If a header file happens to be included twice, the compiler will
process its contents twice.  This is very likely to cause an error,
e.g. when the compiler sees the same structure definition twice.  Even
if it does not, it will certainly waste time.

   The standard way to prevent this is to enclose the entire real
contents of the file in a conditional, like this:

     /* File foo.  */
     #ifndef FILE_FOO_SEEN
     #define FILE_FOO_SEEN
     
     THE ENTIRE FILE
     
     #endif /* !FILE_FOO_SEEN */

   This construct is commonly known as a "wrapper #ifndef".  When the
header is included again, the conditional will be false, because
`FILE_FOO_SEEN' is defined.  The preprocessor will skip over the entire
contents of the file, and the compiler will not see it twice.

   GNU CPP optimizes even further.  It remembers when a header file has
a wrapper `#ifndef'.  If a subsequent `#include' specifies that header,
and the macro in the `#ifndef' is still defined, it does not bother to
rescan the file at all.

   You can put comments outside the wrapper.  They will not interfere
with this optimization.

   The macro `FILE_FOO_SEEN' is called the "controlling macro" or
"guard macro".  In a user header file, the macro name should not begin
with `_'.  In a system header file, it should begin with `__' to avoid
conflicts with user programs.  In any kind of header file, the macro
name should contain the name of the file and some additional text, to
avoid conflicts with other header files.


File: cpp.info,  Node: Computed Includes,  Next: Wrapper Headers,  Prev: Once-Only Headers,  Up: Header Files

Computed Includes
=================

   Sometimes it is necessary to select one of several different header
files to be included into your program.  They might specify
configuration parameters to be used on different sorts of operating
systems, for instance.  You could do this with a series of conditionals,

     #if SYSTEM_1
     # include "system_1.h"
     #elif SYSTEM_2
     # include "system_2.h"
     #elif SYSTEM_3
     ...
     #endif

   That rapidly becomes tedious.  Instead, the preprocessor offers the
ability to use a macro for the header name.  This is called a "computed
include".  Instead of writing a header name as the direct argument of
`#include', you simply put a macro name there instead:

     #define SYSTEM_H "system_1.h"
     ...
     #include SYSTEM_H

`SYSTEM_H' will be expanded, and the preprocessor will look for
`system_1.h' as if the `#include' had been written that way originally.
`SYSTEM_H' could be defined by your Makefile with a `-D' option.

   You must be careful when you define the macro.  `#define' saves
tokens, not text.  The preprocessor has no way of knowing that the macro
will be used as the argument of `#include', so it generates ordinary
tokens, not a header name.  This is unlikely to cause problems if you
use double-quote includes, which are close enough to string constants.
If you use angle brackets, however, you may have trouble.

   The syntax of a computed include is actually a bit more general than
the above.  If the first non-whitespace character after `#include' is
not `"' or `<', then the entire line is macro-expanded like running
text would be.

   If the line expands to a single string constant, the contents of that
string constant are the file to be included.  CPP does not re-examine
the string for embedded quotes, but neither does it process backslash
escapes in the string.  Therefore

     #define HEADER "a\"b"
     #include HEADER

looks for a file named `a\"b'.  CPP searches for the file according to
the rules for double-quoted includes.

   If the line expands to a token stream beginning with a `<' token and
including a `>' token, then the tokens between the `<' and the first
`>' are combined to form the filename to be included.  Any whitespace
between tokens is reduced to a single space; then any space after the
initial `<' is retained, but a trailing space before the closing `>' is
ignored.  CPP searches for the file according to the rules for
angle-bracket includes.

   In either case, if there are any tokens on the line after the file
name, an error occurs and the directive is not processed.  It is also
an error if the result of expansion does not match either of the two
expected forms.

   These rules are implementation-defined behavior according to the C
standard.  To minimize the risk of different compilers interpreting your
computed includes differently, we recommend you use only a single
object-like macro which expands to a string constant.  This will also
minimize confusion for people reading your program.


File: cpp.info,  Node: Wrapper Headers,  Next: System Headers,  Prev: Computed Includes,  Up: Header Files

Wrapper Headers
===============

   Sometimes it is necessary to adjust the contents of a system-provided
header file without editing it directly.  GCC's `fixincludes' operation
does this, for example.  One way to do that would be to create a new
header file with the same name and insert it in the search path before
the original header.  That works fine as long as you're willing to
replace the old header entirely.  But what if you want to refer to the
old header from the new one?

   You cannot simply include the old header with `#include'.  That will
start from the beginning, and find your new header again.  If your
header is not protected from multiple inclusion (*note Once-Only
Headers::), it will recurse infinitely and cause a fatal error.

   You could include the old header with an absolute pathname:
     #include "/usr/include/old-header.h"

This works, but is not clean; should the system headers ever move, you
would have to edit the new headers to match.

   There is no way to solve this problem within the C standard, but you
can use the GNU extension `#include_next'.  It means, "Include the
_next_ file with this name."  This directive works like `#include'
except in searching for the specified file: it starts searching the
list of header file directories _after_ the directory in which the
current file was found.

   Suppose you specify `-I /usr/local/include', and the list of
directories to search also includes `/usr/include'; and suppose both
directories contain `signal.h'.  Ordinary `#include <signal.h>' finds
the file under `/usr/local/include'.  If that file contains
`#include_next <signal.h>', it starts searching after that directory,
and finds the file in `/usr/include'.

   `#include_next' does not distinguish between `<FILE>' and `"FILE"'
inclusion, nor does it check that the file you specify has the same
name as the current file.  It simply looks for the file named, starting
with the directory in the search path after the one where the current
file was found.

   The use of `#include_next' can lead to great confusion.  We
recommend it be used only when there is no other alternative.  In
particular, it should not be used in the headers belonging to a specific
program; it should be used only to make global corrections along the
lines of `fixincludes'.


File: cpp.info,  Node: System Headers,  Prev: Wrapper Headers,  Up: Header Files

System Headers
==============

   The header files declaring interfaces to the operating system and
runtime libraries often cannot be written in strictly conforming C.
Therefore, GCC gives code found in "system headers" special treatment.
All warnings, other than those generated by `#warning' (*note
Diagnostics::), are suppressed while GCC is processing a system header.
Macros defined in a system header are immune to a few warnings
wherever they are expanded.  This immunity is granted on an ad-hoc
basis, when we find that a warning generates lots of false positives
because of code in macros defined in system headers.

   Normally, only the headers found in specific directories are
considered system headers.  These directories are determined when GCC
is compiled.  There are, however, two ways to make normal headers into
system headers.

   The `-isystem' command line option adds its argument to the list of
directories to search for headers, just like `-I'.  Any headers found
in that directory will be considered system headers.

   All directories named by `-isystem' are searched _after_ all
directories named by `-I', no matter what their order was on the
command line.  If the same directory is named by both `-I' and
`-isystem', the `-I' option is ignored.  GCC provides an informative
message when this occurs if `-v' is used.

   There is also a directive, `#pragma GCC system_header', which tells
GCC to consider the rest of the current include file a system header,
no matter where it was found.  Code that comes before the `#pragma' in
the file will not be affected.  `#pragma GCC system_header' has no
effect in the primary source file.

   On very old systems, some of the pre-defined system header
directories get even more special treatment.  GNU C++ considers code in
headers found in those directories to be surrounded by an `extern "C"'
block.  There is no way to request this behavior with a `#pragma', or
from the command line.


File: cpp.info,  Node: Macros,  Next: Conditionals,  Prev: Header Files,  Up: Top

Macros
******

   A "macro" is a fragment of code which has been given a name.
Whenever the name is used, it is replaced by the contents of the macro.
There are two kinds of macros.  They differ mostly in what they look
like when they are used.  "Object-like" macros resemble data objects
when used, "function-like" macros resemble function calls.

   You may define any valid identifier as a macro, even if it is a C
keyword.  The preprocessor does not know anything about keywords.  This
can be useful if you wish to hide a keyword such as `const' from an
older compiler that does not understand it.  However, the preprocessor
operator `defined' (*note Defined::) can never be defined as a macro,
and C++'s named operators (*note C++ Named Operators::) cannot be
macros when you are compiling C++.

* Menu:

* Object-like Macros::
* Function-like Macros::
* Macro Arguments::
* Stringification::
* Concatenation::
* Variadic Macros::
* Predefined Macros::
* Undefining and Redefining Macros::
* Macro Pitfalls::


File: cpp.info,  Node: Object-like Macros,  Next: Function-like Macros,  Up: Macros

Object-like Macros
==================

   An "object-like macro" is a simple identifier which will be replaced
by a code fragment.  It is called object-like because it looks like a
data object in code that uses it.  They are most commonly used to give
symbolic names to numeric constants.

   You create macros with the `#define' directive.  `#define' is
followed by the name of the macro and then the token sequence it should
be an abbreviation for, which is variously referred to as the macro's
"body", "expansion" or "replacement list".  For example,

     #define BUFFER_SIZE 1024

defines a macro named `BUFFER_SIZE' as an abbreviation for the token
`1024'.  If somewhere after this `#define' directive there comes a C
statement of the form

     foo = (char *) malloc (BUFFER_SIZE);

then the C preprocessor will recognize and "expand" the macro
`BUFFER_SIZE'.  The C compiler will see the same tokens as it would if
you had written

     foo = (char *) malloc (1024);

   By convention, macro names are written in upper case.  Programs are
easier to read when it is possible to tell at a glance which names are
macros.

   The macro's body ends at the end of the `#define' line.  You may
continue the definition onto multiple lines, if necessary, using
backslash-newline.  When the macro is expanded, however, it will all
come out on one line.  For example,

     #define NUMBERS 1, \
                     2, \
                     3
     int x[] = { NUMBERS };
          ==> int x[] = { 1, 2, 3 };

The most common visible consequence of this is surprising line numbers
in error messages.

   There is no restriction on what can go in a macro body provided it
decomposes into valid preprocessing tokens.  Parentheses need not
balance, and the body need not resemble valid C code.  (If it does not,
you may get error messages from the C compiler when you use the macro.)

   The C preprocessor scans your program sequentially.  Macro
definitions take effect at the place you write them.  Therefore, the
following input to the C preprocessor

     foo = X;
     #define X 4
     bar = X;

produces

     foo = X;
     bar = 4;

   When the preprocessor expands a macro name, the macro's expansion
replaces the macro invocation, then the expansion is examined for more
macros to expand.  For example,

     #define TABLESIZE BUFSIZE
     #define BUFSIZE 1024
     TABLESIZE
          ==> BUFSIZE
          ==> 1024

`TABLESIZE' is expanded first to produce `BUFSIZE', then that macro is
expanded to produce the final result, `1024'.

   Notice that `BUFSIZE' was not defined when `TABLESIZE' was defined.
The `#define' for `TABLESIZE' uses exactly the expansion you
specify--in this case, `BUFSIZE'--and does not check to see whether it
too contains macro names.  Only when you _use_ `TABLESIZE' is the
result of its expansion scanned for more macro names.

   This makes a difference if you change the definition of `BUFSIZE' at
some point in the source file.  `TABLESIZE', defined as shown, will
always expand using the definition of `BUFSIZE' that is currently in
effect:

     #define BUFSIZE 1020
     #define TABLESIZE BUFSIZE
     #undef BUFSIZE
     #define BUFSIZE 37

Now `TABLESIZE' expands (in two stages) to `37'.

   If the expansion of a macro contains its own name, either directly or
via intermediate macros, it is not expanded again when the expansion is
examined for more macros.  This prevents infinite recursion.  *Note
Self-Referential Macros::, for the precise details.


File: cpp.info,  Node: Function-like Macros,  Next: Macro Arguments,  Prev: Object-like Macros,  Up: Macros

Function-like Macros
====================

   You can also define macros whose use looks like a function call.
These are called "function-like macros".  To define a function-like
macro, you use the same `#define' directive, but you put a pair of
parentheses immediately after the macro name.  For example,

     #define lang_init()  c_init()
     lang_init()
          ==> c_init()

   A function-like macro is only expanded if its name appears with a
pair of parentheses after it.  If you write just the name, it is left
alone.  This can be useful when you have a function and a macro of the
same name, and you wish to use the function sometimes.

     extern void foo(void);
     #define foo() /* optimized inline version */
     ...
       foo();
       funcptr = foo;

   Here the call to `foo()' will use the macro, but the function
pointer will get the address of the real function.  If the macro were to
be expanded, it would cause a syntax error.

   If you put spaces between the macro name and the parentheses in the
macro definition, that does not define a function-like macro, it defines
an object-like macro whose expansion happens to begin with a pair of
parentheses.

     #define lang_init ()    c_init()
     lang_init()
          ==> () c_init()()

   The first two pairs of parentheses in this expansion come from the
macro.  The third is the pair that was originally after the macro
invocation.  Since `lang_init' is an object-like macro, it does not
consume those parentheses.


File: cpp.info,  Node: Macro Arguments,  Next: Stringification,  Prev: Function-like Macros,  Up: Macros

Macro Arguments
===============

   Function-like macros can take "arguments", just like true functions.
To define a macro that uses arguments, you insert "parameters" between
the pair of parentheses in the macro definition that make the macro
function-like.  The parameters must be valid C identifiers, separated
by commas and optionally whitespace.

   To invoke a macro that takes arguments, you write the name of the
macro followed by a list of "actual arguments" in parentheses, separated
by commas.  The invocation of the macro need not be restricted to a
single logical line--it can cross as many lines in the source file as
you wish.  The number of arguments you give must match the number of
parameters in the macro definition.  When the macro is expanded, each
use of a parameter in its body is replaced by the tokens of the
corresponding argument.  (You need not use all of the parameters in the
macro body.)

   As an example, here is a macro that computes the minimum of two
numeric values, as it is defined in many C programs, and some uses.

     #define min(X, Y)  ((X) < (Y) ? (X) : (Y))
       x = min(a, b);          ==>  x = ((a) < (b) ? (a) : (b));
       y = min(1, 2);          ==>  y = ((1) < (2) ? (1) : (2));
       z = min(a + 28, *p);    ==>  z = ((a + 28) < (*p) ? (a + 28) : (*p));

(In this small example you can already see several of the dangers of
macro arguments.  *Note Macro Pitfalls::, for detailed explanations.)

   Leading and trailing whitespace in each argument is dropped, and all
whitespace between the tokens of an argument is reduced to a single
space.  Parentheses within each argument must balance; a comma within
such parentheses does not end the argument.  However, there is no
requirement for square brackets or braces to balance, and they do not
prevent a comma from separating arguments.  Thus,

     macro (array[x = y, x + 1])

passes two arguments to `macro': `array[x = y' and `x + 1]'.  If you
want to supply `array[x = y, x + 1]' as an argument, you can write it
as `array[(x = y, x + 1)]', which is equivalent C code.

   All arguments to a macro are completely macro-expanded before they
are substituted into the macro body.  After substitution, the complete
text is scanned again for macros to expand, including the arguments.
This rule may seem strange, but it is carefully designed so you need
not worry about whether any function call is actually a macro
invocation.  You can run into trouble if you try to be too clever,
though.  *Note Argument Prescan::, for detailed discussion.

   For example, `min (min (a, b), c)' is first expanded to

       min (((a) < (b) ? (a) : (b)), (c))

and then to

     ((((a) < (b) ? (a) : (b))) < (c)
      ? (((a) < (b) ? (a) : (b)))
      : (c))

(Line breaks shown here for clarity would not actually be generated.)

   You can leave macro arguments empty; this is not an error to the
preprocessor (but many macros will then expand to invalid code).  You
cannot leave out arguments entirely; if a macro takes two arguments,
there must be exactly one comma at the top level of its argument list.
Here are some silly examples using `min':

     min(, b)        ==> ((   ) < (b) ? (   ) : (b))
     min(a, )        ==> ((a  ) < ( ) ? (a  ) : ( ))
     min(,)          ==> ((   ) < ( ) ? (   ) : ( ))
     min((,),)       ==> (((,)) < ( ) ? ((,)) : ( ))
     
     min()      error--> macro "min" requires 2 arguments, but only 1 given
     min(,,)    error--> macro "min" passed 3 arguments, but takes just 2

   Whitespace is not a preprocessing token, so if a macro `foo' takes
one argument, `foo ()' and `foo ( )' both supply it an empty argument.
Previous GNU preprocessor implementations and documentation were
incorrect on this point, insisting that a function-like macro that
takes a single argument be passed a space if an empty argument was
required.

   Macro parameters appearing inside string literals are not replaced by
their corresponding actual arguments.

     #define foo(x) x, "x"
     foo(bar)        ==> bar, "x"

