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1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software
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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

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INFO-DIR-SECTION Software development
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
******************

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::
* Index of Directives::
* Option Index::
* Concept Index::

--- The Detailed Node Listing ---

Overview

* Character sets::
* 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::
* Directives Within Macro Arguments::
* 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::

Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997,
1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software
Foundation, Inc.

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

1 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-cpp'
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::.

This manual describes the behavior of the ISO preprocessor. To
minimize gratuitous differences, where the ISO preprocessor's behavior
does not conflict with traditional semantics, the traditional
preprocessor should behave the same way. The various differences that
do exist are detailed in the section *Note Traditional Mode::.

For clarity, unless noted otherwise, references to `CPP' in this
manual refer to GNU CPP.

* Menu:

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

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

1.1 Character sets
==================

Source code character set processing in C and related languages is
rather complicated. The C standard discusses two character sets, but
there are really at least four.

The files input to CPP might be in any character set at all. CPP's
very first action, before it even looks for line boundaries, is to
convert the file into the character set it uses for internal
processing. That set is what the C standard calls the "source"
character set. It must be isomorphic with ISO 10646, also known as
Unicode. CPP uses the UTF-8 encoding of Unicode.

The character sets of the input files are specified using the
`-finput-charset=' option.

All preprocessing work (the subject of the rest of this manual) is
carried out in the source character set. If you request textual output
from the preprocessor with the `-E' option, it will be in UTF-8.

After preprocessing is complete, string and character constants are
converted again, into the "execution" character set. This character
set is under control of the user; the default is UTF-8, matching the
source character set. Wide string and character constants have their
own character set, which is not called out specifically in the
standard. Again, it is under control of the user. The default is
UTF-16 or UTF-32, whichever fits in the target's `wchar_t' type, in the
target machine's byte order.(1) Octal and hexadecimal escape sequences
do not undergo conversion; '\x12' has the value 0x12 regardless of the
currently selected execution character set. All other escapes are
replaced by the character in the source character set that they
represent, then converted to the execution character set, just like
unescaped characters.

Unless the experimental `-fextended-identifiers' option is used, GCC
does not permit the use of characters outside the ASCII range, nor `\u'
and `\U' escapes, in identifiers. Even with that option, characters
outside the ASCII range can only be specified with the `\u' and `\U'
escapes, not used directly in identifiers.

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

(1) UTF-16 does not meet the requirements of the C standard for a
wide character set, but the choice of 16-bit `wchar_t' is enshrined in
some system ABIs so we cannot fix this.

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

1.2 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. 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.

Different systems use different conventions to indicate the end of
a line. GCC accepts the ASCII control sequences `LF', `CR LF' and
`CR' as end-of-line markers. These 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.)

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. By default GCC ignores trigraphs, but if you
request a strictly conforming mode with the `-std' option, or you
specify the `-trigraphs' option, then it converts them.

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.

Trigraphs are not popular and many compilers implement them
incorrectly. Portable code should not rely on trigraphs being
either converted or ignored. With `-Wtrigraphs' GCC will warn you
when a trigraph may change the meaning of your program if it were
converted. *Note Wtrigraphs::.

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, or by separating the string literal at
the trigraph and making use of string literal concatenation.
"(??\?)" is the string `(???)', not `(?]'. Traditional C
compilers do not recognize these idioms.

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. This cannot affect any
correct program, however.

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

1.3 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. The implementation of this feature in
GCC is experimental; such characters are only accepted in the `\u' and
`\U' forms and only if `-fextended-identifiers' is used.

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::.

No string literal may extend past the end of a line. Older versions
of GCC accepted multi-line string constants. 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

1.4 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

2 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

2.1 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 quote directories and then the same
directories used for `<FILE>'. You can prepend directories to the
list of quote directories with the `-iquote' option.

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

2.2 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,
an included file must consist of complete tokens. Comments and string
literals which have not been closed by the end of an included file are
invalid. For error recovery, they are considered to end 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

2.3 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
LIBDIR/gcc/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 directories as
specified by `-iquote' options, 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-' is deprecated, `-iquote' should be used instead.

`-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

2.4 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.

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

2.5 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

2.6 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

2.7 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

3 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:

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

3.1 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 uppercase. 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

3.2 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

3.3 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"

File: cpp.info, Node: Stringification, Next: Concatenation, Prev: Macro Arguments, Up: Macros

3.4 Stringification
===================

Sometimes you may want to convert a macro argument into a string
constant. Parameters are not replaced inside string constants, but you
can use the `#' preprocessing operator instead. When a macro parameter
is used with a leading `#', the preprocessor replaces it with the
literal text of the actual argument, converted to a string constant.
Unlike normal parameter replacement, the argument is not macro-expanded
first. This is called "stringification".

There is no way to combine an argument with surrounding text and
stringify it all together. Instead, you can write a series of adjacent
string constants and stringified arguments. The preprocessor will
replace the stringified arguments with string constants. The C
compiler will then combine all the adjacent string constants into one
long string.

Here is an example of a macro definition that uses stringification:

#define WARN_IF(EXP) \
do { if (EXP) \
fprintf (stderr, "Warning: " #EXP "\n"); } \
while (0)
WARN_IF (x == 0);
==> do { if (x == 0)
fprintf (stderr, "Warning: " "x == 0" "\n"); } while (0);

The argument for `EXP' is substituted once, as-is, into the `if'
statement, and once, stringified, into the argument to `fprintf'. If
`x' were a macro, it would be expanded in the `if' statement, but not
in the string.

The `do' and `while (0)' are a kludge to make it possible to write
`WARN_IF (ARG);', which the resemblance of `WARN_IF' to a function
would make C programmers want to do; see *Note Swallowing the
Semicolon::.

Stringification in C involves more than putting double-quote
characters around the fragment. The preprocessor backslash-escapes the
quotes surrounding embedded string constants, and all backslashes
within string and character constants, in order to get a valid C string
constant with the proper contents. Thus, stringifying `p = "foo\n";'
results in "p = \"foo\\n\";". However, backslashes that are not inside
string or character constants are not duplicated: `\n' by itself
stringifies to "\n".

All leading and trailing whitespace in text being stringified is
ignored. Any sequence of whitespace in the middle of the text is
converted to a single space in the stringified result. Comments are
replaced by whitespace long before stringification happens, so they
never appear in stringified text.

There is no way to convert a macro argument into a character
constant.

If you want to stringify the result of expansion of a macro argument,
you have to use two levels of macros.

#define xstr(s) str(s)
#define str(s) #s
#define foo 4
str (foo)
==> "foo"
xstr (foo)
==> xstr (4)
==> str (4)
==> "4"

`s' is stringified when it is used in `str', so it is not
macro-expanded first. But `s' is an ordinary argument to `xstr', so it
is completely macro-expanded before `xstr' itself is expanded (*note
Argument Prescan::). Therefore, by the time `str' gets to its
argument, it has already been macro-expanded.

File: cpp.info, Node: Concatenation, Next: Variadic Macros, Prev: Stringification, Up: Macros

3.5 Concatenation
=================

It is often useful to merge two tokens into one while expanding macros.
This is called "token pasting" or "token concatenation". The `##'
preprocessing operator performs token pasting. When a macro is
expanded, the two tokens on either side of each `##' operator are
combined into a single token, which then replaces the `##' and the two
original tokens in the macro expansion. Usually both will be
identifiers, or one will be an identifier and the other a preprocessing
number. When pasted, they make a longer identifier. This isn't the
only valid case. It is also possible to concatenate two numbers (or a
number and a name, such as `1.5' and `e3') into a number. Also,
multi-character operators such as `+=' can be formed by token pasting.

However, two tokens that don't together form a valid token cannot be
pasted together. For example, you cannot concatenate `x' with `+' in
either order. If you try, the preprocessor issues a warning and emits
the two tokens. Whether it puts white space between the tokens is
undefined. It is common to find unnecessary uses of `##' in complex
macros. If you get this warning, it is likely that you can simply
remove the `##'.

Both the tokens combined by `##' could come from the macro body, but
you could just as well write them as one token in the first place.
Token pasting is most useful when one or both of the tokens comes from a
macro argument. If either of the tokens next to an `##' is a parameter
name, it is replaced by its actual argument before `##' executes. As
with stringification, the actual argument is not macro-expanded first.
If the argument is empty, that `##' has no effect.

Keep in mind that the C preprocessor converts comments to whitespace
before macros are even considered. Therefore, you cannot create a
comment by concatenating `/' and `*'. You can put as much whitespace
between `##' and its operands as you like, including comments, and you
can put comments in arguments that will be concatenated. However, it
is an error if `##' appears at either end of a macro body.

Consider a C program that interprets named commands. There probably
needs to be a table of commands, perhaps an array of structures declared
as follows:

struct command
{
char *name;
void (*function) (void);
};

struct command commands[] =
{
{ "quit", quit_command },
{ "help", help_command },
...
};

It would be cleaner not to have to give each command name twice,
once in the string constant and once in the function name. A macro
which takes the name of a command as an argument can make this
unnecessary. The string constant can be created with stringification,
and the function name by concatenating the argument with `_command'.
Here is how it is done:

#define COMMAND(NAME) { #NAME, NAME ## _command }

struct command commands[] =
{
COMMAND (quit),
COMMAND (help),
...
};

File: cpp.info, Node: Variadic Macros, Next: Predefined Macros, Prev: Concatenation, Up: Macros

3.6 Variadic Macros
===================

A macro can be declared to accept a variable number of arguments much as
a function can. The syntax for defining the macro is similar to that of
a function. Here is an example:

#define eprintf(...) fprintf (stderr, __VA_ARGS__)

This kind of macro is called "variadic". When the macro is invoked,
all the tokens in its argument list after the last named argument (this
macro has none), including any commas, become the "variable argument".
This sequence of tokens replaces the identifier `__VA_ARGS__' in the
macro body wherever it appears. Thus, we have this expansion:

eprintf ("%s:%d: ", input_file, lineno)
==> fprintf (stderr, "%s:%d: ", input_file, lineno)

The variable argument is completely macro-expanded before it is
inserted into the macro expansion, just like an ordinary argument. You
may use the `#' and `##' operators to stringify the variable argument
or to paste its leading or trailing token with another token. (But see
below for an important special case for `##'.)

If your macro is complicated, you may want a more descriptive name
for the variable argument than `__VA_ARGS__'. CPP permits this, as an
extension. You may write an argument name immediately before the
`...'; that name is used for the variable argument. The `eprintf'
macro above could be written

#define eprintf(args...) fprintf (stderr, args)

using this extension. You cannot use `__VA_ARGS__' and this extension
in the same macro.

You can have named arguments as well as variable arguments in a
variadic macro. We could define `eprintf' like this, instead:

#define eprintf(format, ...) fprintf (stderr, format, __VA_ARGS__)

This formulation looks more descriptive, but unfortunately it is less
flexible: you must now supply at least one argument after the format
string. In standard C, you cannot omit the comma separating the named
argument from the variable arguments. Furthermore, if you leave the
variable argument empty, you will get a syntax error, because there
will be an extra comma after the format string.

eprintf("success!\n", );
==> fprintf(stderr, "success!\n", );

GNU CPP has a pair of extensions which deal with this problem.
First, you are allowed to leave the variable argument out entirely:

eprintf ("success!\n")
==> fprintf(stderr, "success!\n", );

Second, the `##' token paste operator has a special meaning when placed
between a comma and a variable argument. If you write

#define eprintf(format, ...) fprintf (stderr, format, ##__VA_ARGS__)

and the variable argument is left out when the `eprintf' macro is used,
then the comma before the `##' will be deleted. This does _not_ happen
if you pass an empty argument, nor does it happen if the token
preceding `##' is anything other than a comma.

eprintf ("success!\n")
==> fprintf(stderr, "success!\n");

The above explanation is ambiguous about the case where the only macro
parameter is a variable arguments parameter, as it is meaningless to
try to distinguish whether no argument at all is an empty argument or a
missing argument. In this case the C99 standard is clear that the
comma must remain, however the existing GCC extension used to swallow
the comma. So CPP retains the comma when conforming to a specific C
standard, and drops it otherwise.

C99 mandates that the only place the identifier `__VA_ARGS__' can
appear is in the replacement list of a variadic macro. It may not be
used as a macro name, macro argument name, or within a different type
of macro. It may also be forbidden in open text; the standard is
ambiguous. We recommend you avoid using it except for its defined
purpose.

Variadic macros are a new feature in C99. GNU CPP has supported them
for a long time, but only with a named variable argument (`args...',
not `...' and `__VA_ARGS__'). If you are concerned with portability to
previous versions of GCC, you should use only named variable arguments.
On the other hand, if you are concerned with portability to other
conforming implementations of C99, you should use only `__VA_ARGS__'.

Previous versions of CPP implemented the comma-deletion extension
much more generally. We have restricted it in this release to minimize
the differences from C99. To get the same effect with both this and
previous versions of GCC, the token preceding the special `##' must be
a comma, and there must be white space between that comma and whatever
comes immediately before it:

#define eprintf(format, args...) fprintf (stderr, format , ##args)

*Note Differences from previous versions::, for the gory details.

File: cpp.info, Node: Predefined Macros, Next: Undefining and Redefining Macros, Prev: Variadic Macros, Up: Macros

3.7 Predefined Macros
=====================

Several object-like macros are predefined; you use them without
supplying their definitions. They fall into three classes: standard,
common, and system-specific.

In C++, there is a fourth category, the named operators. They act
like predefined macros, but you cannot undefine them.

* Menu:

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

File: cpp.info, Node: Standard Predefined Macros, Next: Common Predefined Macros, Up: Predefined Macros

3.7.1 Standard Predefined Macros
--------------------------------

The standard predefined macros are specified by the relevant language
standards, so they are available with all compilers that implement
those standards. Older compilers may not provide all of them. Their
names all start with double underscores.

`__FILE__'
This macro expands to the name of the current input file, in the
form of a C string constant. This is the path by which the
preprocessor opened the file, not the short name specified in
`#include' or as the input file name argument. For example,
`"/usr/local/include/myheader.h"' is a possible expansion of this
macro.

`__LINE__'
This macro expands to the current input line number, in the form
of a decimal integer constant. While we call it a predefined
macro, it's a pretty strange macro, since its "definition" changes
with each new line of source code.

`__FILE__' and `__LINE__' are useful in generating an error message
to report an inconsistency detected by the program; the message can
state the source line at which the inconsistency was detected. For
example,

fprintf (stderr, "Internal error: "
"negative string length "
"%d at %s, line %d.",
length, __FILE__, __LINE__);

An `#include' directive changes the expansions of `__FILE__' and
`__LINE__' to correspond to the included file. At the end of that
file, when processing resumes on the input file that contained the
`#include' directive, the expansions of `__FILE__' and `__LINE__'
revert to the values they had before the `#include' (but `__LINE__' is
then incremented by one as processing moves to the line after the
`#include').

A `#line' directive changes `__LINE__', and may change `__FILE__' as
well. *Note Line Control::.

C99 introduces `__func__', and GCC has provided `__FUNCTION__' for a
long time. Both of these are strings containing the name of the
current function (there are slight semantic differences; see the GCC
manual). Neither of them is a macro; the preprocessor does not know the
name of the current function. They tend to be useful in conjunction
with `__FILE__' and `__LINE__', though.

`__DATE__'
This macro expands to a string constant that describes the date on
which the preprocessor is being run. The string constant contains
eleven characters and looks like `"Feb 12 1996"'. If the day of
the month is less than 10, it is padded with a space on the left.

If GCC cannot determine the current date, it will emit a warning
message (once per compilation) and `__DATE__' will expand to
`"??? ?? ????"'.

`__TIME__'
This macro expands to a string constant that describes the time at
which the preprocessor is being run. The string constant contains
eight characters and looks like `"23:59:01"'.

If GCC cannot determine the current time, it will emit a warning
message (once per compilation) and `__TIME__' will expand to
`"??:??:??"'.

`__STDC__'
In normal operation, this macro expands to the constant 1, to
signify that this compiler conforms to ISO Standard C. If GNU CPP
is used with a compiler other than GCC, this is not necessarily
true; however, the preprocessor always conforms to the standard
unless the `-traditional-cpp' option is used.

This macro is not defined if the `-traditional-cpp' option is used.

On some hosts, the system compiler uses a different convention,
where `__STDC__' is normally 0, but is 1 if the user specifies
strict conformance to the C Standard. CPP follows the host
convention when processing system header files, but when
processing user files `__STDC__' is always 1. This has been
reported to cause problems; for instance, some versions of Solaris
provide X Windows headers that expect `__STDC__' to be either
undefined or 1. *Note Invocation::.

`__STDC_VERSION__'
This macro expands to the C Standard's version number, a long
integer constant of the form `YYYYMML' where YYYY and MM are the
year and month of the Standard version. This signifies which
version of the C Standard the compiler conforms to. Like
`__STDC__', this is not necessarily accurate for the entire
implementation, unless GNU CPP is being used with GCC.

The value `199409L' signifies the 1989 C standard as amended in
1994, which is the current default; the value `199901L' signifies
the 1999 revision of the C standard. Support for the 1999
revision is not yet complete.

This macro is not defined if the `-traditional-cpp' option is
used, nor when compiling C++ or Objective-C.

`__STDC_HOSTED__'
This macro is defined, with value 1, if the compiler's target is a
"hosted environment". A hosted environment has the complete
facilities of the standard C library available.

`__cplusplus'
This macro is defined when the C++ compiler is in use. You can use
`__cplusplus' to test whether a header is compiled by a C compiler
or a C++ compiler. This macro is similar to `__STDC_VERSION__', in
that it expands to a version number. A fully conforming
implementation of the 1998 C++ standard will define this macro to
`199711L'. The GNU C++ compiler is not yet fully conforming, so
it uses `1' instead. It is hoped to complete the implementation
of standard C++ in the near future.

`__OBJC__'
This macro is defined, with value 1, when the Objective-C compiler
is in use. You can use `__OBJC__' to test whether a header is
compiled by a C compiler or a Objective-C compiler.

`__ASSEMBLER__'
This macro is defined with value 1 when preprocessing assembly
language.

File: cpp.info, Node: Common Predefined Macros, Next: System-specific Predefined Macros, Prev: Standard Predefined Macros, Up: Predefined Macros

3.7.2 Common Predefined Macros
------------------------------

The common predefined macros are GNU C extensions. They are available
with the same meanings regardless of the machine or operating system on
which you are using GNU C. Their names all start with double
underscores.

`__COUNTER__'
This macro expands to sequential integral values starting from 0.
In conjuction with the `##' operator, this provides a convenient
means to generate unique identifiers. Care must be taken to
ensure that `__COUNTER__' is not expanded prior to inclusion of
precompiled headers which use it. Otherwise, the precompiled
headers will not be used.

`__GNUC__'
`__GNUC_MINOR__'
`__GNUC_PATCHLEVEL__'
These macros are defined by all GNU compilers that use the C
preprocessor: C, C++, and Objective-C. Their values are the major
version, minor version, and patch level of the compiler, as integer
constants. For example, GCC 3.2.1 will define `__GNUC__' to 3,
`__GNUC_MINOR__' to 2, and `__GNUC_PATCHLEVEL__' to 1. These
macros are also defined if you invoke the preprocessor directly.

`__GNUC_PATCHLEVEL__' is new to GCC 3.0; it is also present in the
widely-used development snapshots leading up to 3.0 (which identify
themselves as GCC 2.96 or 2.97, depending on which snapshot you
have).

If all you need to know is whether or not your program is being
compiled by GCC, or a non-GCC compiler that claims to accept the
GNU C dialects, you can simply test `__GNUC__'. If you need to
write code which depends on a specific version, you must be more
careful. Each time the minor version is increased, the patch
level is reset to zero; each time the major version is increased
(which happens rarely), the minor version and patch level are
reset. If you wish to use the predefined macros directly in the
conditional, you will need to write it like this:

/* Test for GCC > 3.2.0 */
#if __GNUC__ > 3 || \
(__GNUC__ == 3 && (__GNUC_MINOR__ > 2 || \
(__GNUC_MINOR__ == 2 && \
__GNUC_PATCHLEVEL__ > 0))

Another approach is to use the predefined macros to calculate a
single number, then compare that against a threshold:

#define GCC_VERSION (__GNUC__ * 10000 \
+ __GNUC_MINOR__ * 100 \
+ __GNUC_PATCHLEVEL__)
...
/* Test for GCC > 3.2.0 */
#if GCC_VERSION > 30200

Many people find this form easier to understand.

`__GNUG__'
The GNU C++ compiler defines this. Testing it is equivalent to
testing `(__GNUC__ && __cplusplus)'.

`__STRICT_ANSI__'
GCC defines this macro if and only if the `-ansi' switch, or a
`-std' switch specifying strict conformance to some version of ISO
C, was specified when GCC was invoked. It is defined to `1'.
This macro exists primarily to direct GNU libc's header files to
restrict their definitions to the minimal set found in the 1989 C
standard.

`__BASE_FILE__'
This macro expands to the name of the main input file, in the form
of a C string constant. This is the source file that was specified
on the command line of the preprocessor or C compiler.

`__INCLUDE_LEVEL__'
This macro expands to a decimal integer constant that represents
the depth of nesting in include files. The value of this macro is
incremented on every `#include' directive and decremented at the
end of every included file. It starts out at 0, it's value within
the base file specified on the command line.

`__ELF__'
This macro is defined if the target uses the ELF object format.

`__VERSION__'
This macro expands to a string constant which describes the
version of the compiler in use. You should not rely on its
contents having any particular form, but it can be counted on to
contain at least the release number.

`__OPTIMIZE__'
`__OPTIMIZE_SIZE__'
`__NO_INLINE__'
These macros describe the compilation mode. `__OPTIMIZE__' is
defined in all optimizing compilations. `__OPTIMIZE_SIZE__' is
defined if the compiler is optimizing for size, not speed.
`__NO_INLINE__' is defined if no functions will be inlined into
their callers (when not optimizing, or when inlining has been
specifically disabled by `-fno-inline').

These macros cause certain GNU header files to provide optimized
definitions, using macros or inline functions, of system library
functions. You should not use these macros in any way unless you
make sure that programs will execute with the same effect whether
or not they are defined. If they are defined, their value is 1.

`__GNUC_GNU_INLINE__'
GCC defines this macro if functions declared `inline' will be
handled in GCC's traditional gnu89 mode. In this mode an `extern
inline' function will never be compiled as a standalone function,
and an `inline' function which is neither `extern' nor `static'
will always be compiled as a standalone function.

`__GNUC_STDC_INLINE__'
GCC defines this macro if functions declared `inline' will be
handled according to the ISO C99 standard. In this mode an
`extern inline' function will always be compiled as a standalone
externally visible function, and an `inline' function which is
neither `extern' nor `static' will never be compiled as a
standalone function.

If this macro is defined, GCC supports the `gnu_inline' function
attribute as a way to always get the gnu89 behaviour. Support for
this and `__GNUC_GNU_INLINE__' was added in GCC 4.1.3. If neither
macro is defined, an older version of GCC is being used: `inline'
functions will be compiled in gnu89 mode, and the `gnu_inline'
function attribute will not be recognized.

`__CHAR_UNSIGNED__'
GCC defines this macro if and only if the data type `char' is
unsigned on the target machine. It exists to cause the standard
header file `limits.h' to work correctly. You should not use this
macro yourself; instead, refer to the standard macros defined in
`limits.h'.

`__WCHAR_UNSIGNED__'
Like `__CHAR_UNSIGNED__', this macro is defined if and only if the
data type `wchar_t' is unsigned and the front-end is in C++ mode.

`__REGISTER_PREFIX__'
This macro expands to a single token (not a string constant) which
is the prefix applied to CPU register names in assembly language
for this target. You can use it to write assembly that is usable
in multiple environments. For example, in the `m68k-aout'
environment it expands to nothing, but in the `m68k-coff'
environment it expands to a single `%'.

`__USER_LABEL_PREFIX__'
This macro expands to a single token which is the prefix applied to
user labels (symbols visible to C code) in assembly. For example,
in the `m68k-aout' environment it expands to an `_', but in the
`m68k-coff' environment it expands to nothing.

This macro will have the correct definition even if
`-f(no-)underscores' is in use, but it will not be correct if
target-specific options that adjust this prefix are used (e.g. the
OSF/rose `-mno-underscores' option).

`__SIZE_TYPE__'
`__PTRDIFF_TYPE__'
`__WCHAR_TYPE__'
`__WINT_TYPE__'
`__INTMAX_TYPE__'
`__UINTMAX_TYPE__'
These macros are defined to the correct underlying types for the
`size_t', `ptrdiff_t', `wchar_t', `wint_t', `intmax_t', and
`uintmax_t' typedefs, respectively. They exist to make the
standard header files `stddef.h' and `wchar.h' work correctly.
You should not use these macros directly; instead, include the
appropriate headers and use the typedefs.

`__CHAR_BIT__'
Defined to the number of bits used in the representation of the
`char' data type. It exists to make the standard header given
numerical limits work correctly. You should not use this macro
directly; instead, include the appropriate headers.

`__SCHAR_MAX__'
`__WCHAR_MAX__'
`__SHRT_MAX__'
`__INT_MAX__'
`__LONG_MAX__'
`__LONG_LONG_MAX__'
`__INTMAX_MAX__'
Defined to the maximum value of the `signed char', `wchar_t',
`signed short', `signed int', `signed long', `signed long long',
and `intmax_t' types respectively. They exist to make the
standard header given numerical limits work correctly. You should
not use these macros directly; instead, include the appropriate
headers.

`__DEPRECATED'
This macro is defined, with value 1, when compiling a C++ source
file with warnings about deprecated constructs enabled. These
warnings are enabled by default, but can be disabled with
`-Wno-deprecated'.

`__EXCEPTIONS'
This macro is defined, with value 1, when compiling a C++ source
file with exceptions enabled. If `-fno-exceptions' was used when
compiling the file, then this macro will not be defined.

`__USING_SJLJ_EXCEPTIONS__'
This macro is defined, with value 1, if the compiler uses the old
mechanism based on `setjmp' and `longjmp' for exception handling.

`__GXX_WEAK__'
This macro is defined when compiling a C++ source file. It has the
value 1 if the compiler will use weak symbols, COMDAT sections, or
other similar techniques to collapse symbols with "vague linkage"
that are defined in multiple translation units. If the compiler
will not collapse such symbols, this macro is defined with value
0. In general, user code should not need to make use of this
macro; the purpose of this macro is to ease implementation of the
C++ runtime library provided with G++.

`__NEXT_RUNTIME__'
This macro is defined, with value 1, if (and only if) the NeXT
runtime (as in `-fnext-runtime') is in use for Objective-C. If
the GNU runtime is used, this macro is not defined, so that you
can use this macro to determine which runtime (NeXT or GNU) is
being used.

`__LP64__'
`_LP64'
These macros are defined, with value 1, if (and only if) the
compilation is for a target where `long int' and pointer both use
64-bits and `int' uses 32-bit.

`__SSP__'
This macro is defined, with value 1, when `-fstack-protector' is in
use.

`__SSP_ALL__'
This macro is defined, with value 2, when `-fstack-protector-all'
is in use.

`__TIMESTAMP__'
This macro expands to a string constant that describes the date
and time of the last modification of the current source file. The
string constant contains abbreviated day of the week, month, day
of the month, time in hh:mm:ss form, year and looks like
`"Sun Sep 16 01:03:52 1973"'. If the day of the month is less
than 10, it is padded with a space on the left.

If GCC cannot determine the current date, it will emit a warning
message (once per compilation) and `__TIMESTAMP__' will expand to
`"??? ??? ?? ??:??:?? ????"'.

File: cpp.info, Node: System-specific Predefined Macros, Next: C++ Named Operators, Prev: Common Predefined Macros, Up: Predefined Macros

3.7.3 System-specific Predefined Macros
---------------------------------------

The C preprocessor normally predefines several macros that indicate what
type of system and machine is in use. They are obviously different on
each target supported by GCC. This manual, being for all systems and
machines, cannot tell you what their names are, but you can use `cpp
-dM' to see them all. *Note Invocation::. All system-specific
predefined macros expand to the constant 1, so you can test them with
either `#ifdef' or `#if'.

The C standard requires that all system-specific macros be part of
the "reserved namespace". All names which begin with two underscores,
or an underscore and a capital letter, are reserved for the compiler and
library to use as they wish. However, historically system-specific
macros have had names with no special prefix; for instance, it is common
to find `unix' defined on Unix systems. For all such macros, GCC
provides a parallel macro with two underscores added at the beginning
and the end. If `unix' is defined, `__unix__' will be defined too.
There will never be more than two underscores; the parallel of `_mips'
is `__mips__'.

When the `-ansi' option, or any `-std' option that requests strict
conformance, is given to the compiler, all the system-specific
predefined macros outside the reserved namespace are suppressed. The
parallel macros, inside the reserved namespace, remain defined.

We are slowly phasing out all predefined macros which are outside the
reserved namespace. You should never use them in new programs, and we
encourage you to correct older code to use the parallel macros whenever
you find it. We don't recommend you use the system-specific macros that
are in the reserved namespace, either. It is better in the long run to
check specifically for features you need, using a tool such as
`autoconf'.

File: cpp.info, Node: C++ Named Operators, Prev: System-specific Predefined Macros, Up: Predefined Macros

3.7.4 C++ Named Operators
-------------------------

In C++, there are eleven keywords which are simply alternate spellings
of operators normally written with punctuation. These keywords are
treated as such even in the preprocessor. They function as operators in
`#if', and they cannot be defined as macros or poisoned. In C, you can
request that those keywords take their C++ meaning by including
`iso646.h'. That header defines each one as a normal object-like macro
expanding to the appropriate punctuator.

These are the named operators and their corresponding punctuators:

Named Operator Punctuator
`and' `&&'
`and_eq' `&='
`bitand' `&'
`bitor' `|'
`compl' `~'
`not' `!'
`not_eq' `!='
`or' `||'
`or_eq' `|='
`xor' `^'
`xor_eq' `^='

File: cpp.info, Node: Undefining and Redefining Macros, Next: Directives Within Macro Arguments, Prev: Predefined Macros, Up: Macros

3.8 Undefining and Redefining Macros
====================================

If a macro ceases to be useful, it may be "undefined" with the `#undef'
directive. `#undef' takes a single argument, the name of the macro to
undefine. You use the bare macro name, even if the macro is
function-like. It is an error if anything appears on the line after
the macro name. `#undef' has no effect if the name is not a macro.

#define FOO 4
x = FOO; ==> x = 4;
#undef FOO
x = FOO; ==> x = FOO;

Once a macro has been undefined, that identifier may be "redefined"
as a macro by a subsequent `#define' directive. The new definition
need not have any resemblance to the old definition.

However, if an identifier which is currently a macro is redefined,
then the new definition must be "effectively the same" as the old one.
Two macro definitions are effectively the same if:
* Both are the same type of macro (object- or function-like).

* All the tokens of the replacement list are the same.

* If there are any parameters, they are the same.

* Whitespace appears in the same places in both. It need not be
exactly the same amount of whitespace, though. Remember that
comments count as whitespace.

These definitions are effectively the same:
#define FOUR (2 + 2)
#define FOUR (2 + 2)
#define FOUR (2 /* two */ + 2)
but these are not:
#define FOUR (2 + 2)
#define FOUR ( 2+2 )
#define FOUR (2 * 2)
#define FOUR(score,and,seven,years,ago) (2 + 2)

If a macro is redefined with a definition that is not effectively the
same as the old one, the preprocessor issues a warning and changes the
macro to use the new definition. If the new definition is effectively
the same, the redefinition is silently ignored. This allows, for
instance, two different headers to define a common macro. The
preprocessor will only complain if the definitions do not match.

File: cpp.info, Node: Directives Within Macro Arguments, Next: Macro Pitfalls, Prev: Undefining and Redefining Macros, Up: Macros

3.9 Directives Within Macro Arguments
=====================================

Occasionally it is convenient to use preprocessor directives within the
arguments of a macro. The C and C++ standards declare that behavior in
these cases is undefined.

Versions of CPP prior to 3.2 would reject such constructs with an
error message. This was the only syntactic difference between normal
functions and function-like macros, so it seemed attractive to remove
this limitation, and people would often be surprised that they could
not use macros in this way. Moreover, sometimes people would use
conditional compilation in the argument list to a normal library
function like `printf', only to find that after a library upgrade
`printf' had changed to be a function-like macro, and their code would
no longer compile. So from version 3.2 we changed CPP to successfully
process arbitrary directives within macro arguments in exactly the same
way as it would have processed the directive were the function-like
macro invocation not present.

If, within a macro invocation, that macro is redefined, then the new
definition takes effect in time for argument pre-expansion, but the
original definition is still used for argument replacement. Here is a
pathological example:

#define f(x) x x
f (1
#undef f
#define f 2
f)

which expands to

1 2 1 2

with the semantics described above.

File: cpp.info, Node: Macro Pitfalls, Prev: Directives Within Macro Arguments, Up: Macros

3.10 Macro Pitfalls
===================

In this section we describe some special rules that apply to macros and
macro expansion, and point out certain cases in which the rules have
counter-intuitive consequences that you must watch out for.

* Menu:

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

File: cpp.info, Node: Misnesting, Next: Operator Precedence Problems, Up: Macro Pitfalls

3.10.1 Misnesting
-----------------

When a macro is called with arguments, the arguments are substituted
into the macro body and the result is checked, together with the rest of
the input file, for more macro calls. It is possible to piece together
a macro call coming partially from the macro body and partially from the
arguments. For example,

#define twice(x) (2*(x))
#define call_with_1(x) x(1)
call_with_1 (twice)
==> twice(1)
==> (2*(1))

Macro definitions do not have to have balanced parentheses. By
writing an unbalanced open parenthesis in a macro body, it is possible
to create a macro call that begins inside the macro body but ends
outside of it. For example,

#define strange(file) fprintf (file, "%s %d",
...
strange(stderr) p, 35)
==> fprintf (stderr, "%s %d", p, 35)

The ability to piece together a macro call can be useful, but the
use of unbalanced open parentheses in a macro body is just confusing,
and should be avoided.

File: cpp.info, Node: Operator Precedence Problems, Next: Swallowing the Semicolon, Prev: Misnesting, Up: Macro Pitfalls

3.10.2 Operator Precedence Problems
-----------------------------------

You may have noticed that in most of the macro definition examples shown
above, each occurrence of a macro argument name had parentheses around
it. In addition, another pair of parentheses usually surround the
entire macro definition. Here is why it is best to write macros that
way.

Suppose you define a macro as follows,

#define ceil_div(x, y) (x + y - 1) / y

whose purpose is to divide, rounding up. (One use for this operation is
to compute how many `int' objects are needed to hold a certain number
of `char' objects.) Then suppose it is used as follows:

a = ceil_div (b & c, sizeof (int));
==> a = (b & c + sizeof (int) - 1) / sizeof (int);

This does not do what is intended. The operator-precedence rules of C
make it equivalent to this:

a = (b & (c + sizeof (int) - 1)) / sizeof (int);

What we want is this:

a = ((b & c) + sizeof (int) - 1)) / sizeof (int);

Defining the macro as

#define ceil_div(x, y) ((x) + (y) - 1) / (y)

provides the desired result.

Unintended grouping can result in another way. Consider `sizeof
ceil_div(1, 2)'. That has the appearance of a C expression that would
compute the size of the type of `ceil_div (1, 2)', but in fact it means
something very different. Here is what it expands to:

sizeof ((1) + (2) - 1) / (2)

This would take the size of an integer and divide it by two. The
precedence rules have put the division outside the `sizeof' when it was
intended to be inside.

Parentheses around the entire macro definition prevent such problems.
Here, then, is the recommended way to define `ceil_div':

#define ceil_div(x, y) (((x) + (y) - 1) / (y))

File: cpp.info, Node: Swallowing the Semicolon, Next: Duplication of Side Effects, Prev: Operator Precedence Problems, Up: Macro Pitfalls

3.10.3 Swallowing the Semicolon
-------------------------------

Often it is desirable to define a macro that expands into a compound
statement. Consider, for example, the following macro, that advances a
pointer (the argument `p' says where to find it) across whitespace
characters:

#define SKIP_SPACES(p, limit) \
{ char *lim = (limit); \
while (p < lim) { \
if (*p++ != ' ') { \
p--; break; }}}

Here backslash-newline is used to split the macro definition, which must
be a single logical line, so that it resembles the way such code would
be laid out if not part of a macro definition.

A call to this macro might be `SKIP_SPACES (p, lim)'. Strictly
speaking, the call expands to a compound statement, which is a complete
statement with no need for a semicolon to end it. However, since it
looks like a function call, it minimizes confusion if you can use it
like a function call, writing a semicolon afterward, as in `SKIP_SPACES
(p, lim);'

This can cause trouble before `else' statements, because the
semicolon is actually a null statement. Suppose you write

if (*p != 0)
SKIP_SPACES (p, lim);
else ...

The presence of two statements--the compound statement and a null
statement--in between the `if' condition and the `else' makes invalid C
code.

The definition of the macro `SKIP_SPACES' can be altered to solve
this problem, using a `do ... while' statement. Here is how:

#define SKIP_SPACES(p, limit) \
do { char *lim = (limit); \
while (p < lim) { \
if (*p++ != ' ') { \
p--; break; }}} \
while (0)

Now `SKIP_SPACES (p, lim);' expands into

do {...} while (0);

which is one statement. The loop executes exactly once; most compilers
generate no extra code for it.

File: cpp.info, Node: Duplication of Side Effects, Next: Self-Referential Macros, Prev: Swallowing the Semicolon, Up: Macro Pitfalls

3.10.4 Duplication of Side Effects
----------------------------------

Many C programs define a macro `min', for "minimum", like this:

#define min(X, Y) ((X) < (Y) ? (X) : (Y))

When you use this macro with an argument containing a side effect,
as shown here,

next = min (x + y, foo (z));

it expands as follows:

next = ((x + y) < (foo (z)) ? (x + y) : (foo (z)));

where `x + y' has been substituted for `X' and `foo (z)' for `Y'.

The function `foo' is used only once in the statement as it appears
in the program, but the expression `foo (z)' has been substituted twice
into the macro expansion. As a result, `foo' might be called two times
when the statement is executed. If it has side effects or if it takes
a long time to compute, the results might not be what you intended. We
say that `min' is an "unsafe" macro.

The best solution to this problem is to define `min' in a way that
computes the value of `foo (z)' only once. The C language offers no
standard way to do this, but it can be done with GNU extensions as
follows:

#define min(X, Y) \
({ typeof (X) x_ = (X); \
typeof (Y) y_ = (Y); \
(x_ < y_) ? x_ : y_; })

The `({ ... })' notation produces a compound statement that acts as
an expression. Its value is the value of its last statement. This
permits us to define local variables and assign each argument to one.
The local variables have underscores after their names to reduce the
risk of conflict with an identifier of wider scope (it is impossible to
avoid this entirely). Now each argument is evaluated exactly once.

If you do not wish to use GNU C extensions, the only solution is to
be careful when _using_ the macro `min'. For example, you can
calculate the value of `foo (z)', save it in a variable, and use that
variable in `min':

#define min(X, Y) ((X) < (Y) ? (X) : (Y))
...
{
int tem = foo (z);
next = min (x + y, tem);
}

(where we assume that `foo' returns type `int').

File: cpp.info, Node: Self-Referential Macros, Next: Argument Prescan, Prev: Duplication of Side Effects, Up: Macro Pitfalls

3.10.5 Self-Referential Macros
------------------------------

A "self-referential" macro is one whose name appears in its definition.
Recall that all macro definitions are rescanned for more macros to
replace. If the self-reference were considered a use of the macro, it
would produce an infinitely large expansion. To prevent this, the
self-reference is not considered a macro call. It is passed into the
preprocessor output unchanged. Consider an example:

#define foo (4 + foo)

where `foo' is also a variable in your program.

Following the ordinary rules, each reference to `foo' will expand
into `(4 + foo)'; then this will be rescanned and will expand into `(4
+ (4 + foo))'; and so on until the computer runs out of memory.

The self-reference rule cuts this process short after one step, at
`(4 + foo)'. Therefore, this macro definition has the possibly useful
effect of causing the program to add 4 to the value of `foo' wherever
`foo' is referred to.

In most cases, it is a bad idea to take advantage of this feature. A
person reading the program who sees that `foo' is a variable will not
expect that it is a macro as well. The reader will come across the
identifier `foo' in the program and think its value should be that of
the variable `foo', whereas in fact the value is four greater.

One common, useful use of self-reference is to create a macro which
expands to itself. If you write

#define EPERM EPERM

then the macro `EPERM' expands to `EPERM'. Effectively, it is left
alone by the preprocessor whenever it's used in running text. You can
tell that it's a macro with `#ifdef'. You might do this if you want to
define numeric constants with an `enum', but have `#ifdef' be true for
each constant.

If a macro `x' expands to use a macro `y', and the expansion of `y'
refers to the macro `x', that is an "indirect self-reference" of `x'.
`x' is not expanded in this case either. Thus, if we have

#define x (4 + y)
#define y (2 * x)

then `x' and `y' expand as follows:

x ==> (4 + y)
==> (4 + (2 * x))

y ==> (2 * x)
==> (2 * (4 + y))

Each macro is expanded when it appears in the definition of the other
macro, but not when it indirectly appears in its own definition.

File: cpp.info, Node: Argument Prescan, Next: Newlines in Arguments, Prev: Self-Referential Macros, Up: Macro Pitfalls

3.10.6 Argument Prescan
-----------------------

Macro arguments are completely macro-expanded before they are
substituted into a macro body, unless they are stringified or pasted
with other tokens. After substitution, the entire macro body, including
the substituted arguments, is scanned again for macros to be expanded.
The result is that the arguments are scanned _twice_ to expand macro
calls in them.

Most of the time, this has no effect. If the argument contained any
macro calls, they are expanded during the first scan. The result
therefore contains no macro calls, so the second scan does not change
it. If the argument were substituted as given, with no prescan, the
single remaining scan would find the same macro calls and produce the
same results.

You might expect the double scan to change the results when a
self-referential macro is used in an argument of another macro (*note
Self-Referential Macros::): the self-referential macro would be
expanded once in the first scan, and a second time in the second scan.
However, this is not what happens. The self-references that do not
expand in the first scan are marked so that they will not expand in the
second scan either.

You might wonder, "Why mention the prescan, if it makes no
difference? And why not skip it and make the preprocessor faster?"
The answer is that the prescan does make a difference in three special
cases:

* Nested calls to a macro.

We say that "nested" calls to a macro occur when a macro's argument
contains a call to that very macro. For example, if `f' is a macro
that expects one argument, `f (f (1))' is a nested pair of calls to
`f'. The desired expansion is made by expanding `f (1)' and
substituting that into the definition of `f'. The prescan causes
the expected result to happen. Without the prescan, `f (1)' itself
would be substituted as an argument, and the inner use of `f' would
appear during the main scan as an indirect self-reference and
would not be expanded.

* Macros that call other macros that stringify or concatenate.

If an argument is stringified or concatenated, the prescan does not
occur. If you _want_ to expand a macro, then stringify or
concatenate its expansion, you can do that by causing one macro to
call another macro that does the stringification or concatenation.
For instance, if you have

#define AFTERX(x) X_ ## x
#define XAFTERX(x) AFTERX(x)
#define TABLESIZE 1024
#define BUFSIZE TABLESIZE

then `AFTERX(BUFSIZE)' expands to `X_BUFSIZE', and
`XAFTERX(BUFSIZE)' expands to `X_1024'. (Not to `X_TABLESIZE'.
Prescan always does a complete expansion.)

* Macros used in arguments, whose expansions contain unshielded
commas.

This can cause a macro expanded on the second scan to be called
with the wrong number of arguments. Here is an example:

#define foo a,b
#define bar(x) lose(x)
#define lose(x) (1 + (x))

We would like `bar(foo)' to turn into `(1 + (foo))', which would
then turn into `(1 + (a,b))'. Instead, `bar(foo)' expands into
`lose(a,b)', and you get an error because `lose' requires a single
argument. In this case, the problem is easily solved by the same
parentheses that ought to be used to prevent misnesting of
arithmetic operations:

#define foo (a,b)
or
#define bar(x) lose((x))

The extra pair of parentheses prevents the comma in `foo''s
definition from being interpreted as an argument separator.

File: cpp.info, Node: Newlines in Arguments, Prev: Argument Prescan, Up: Macro Pitfalls

3.10.7 Newlines in Arguments
----------------------------

The invocation of a function-like macro can extend over many logical
lines. However, in the present implementation, the entire expansion
comes out on one line. Thus line numbers emitted by the compiler or
debugger refer to the line the invocation started on, which might be
different to the line containing the argument causing the problem.

Here is an example illustrating this:

#define ignore_second_arg(a,b,c) a; c

ignore_second_arg (foo (),
ignored (),
syntax error);

The syntax error triggered by the tokens `syntax error' results in an
error message citing line three--the line of ignore_second_arg-- even
though the problematic code comes from line five.

We consider this a bug, and intend to fix it in the near future.

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

4 Conditionals
**************

A "conditional" is a directive that instructs the preprocessor to
select whether or not to include a chunk of code in the final token
stream passed to the compiler. Preprocessor conditionals can test
arithmetic expressions, or whether a name is defined as a macro, or both
simultaneously using the special `defined' operator.

A conditional in the C preprocessor resembles in some ways an `if'
statement in C, but it is important to understand the difference between
them. The condition in an `if' statement is tested during the
execution of your program. Its purpose is to allow your program to
behave differently from run to run, depending on the data it is
operating on. The condition in a preprocessing conditional directive is
tested when your program is compiled. Its purpose is to allow different
code to be included in the program depending on the situation at the
time of compilation.

However, the distinction is becoming less clear. Modern compilers
often do test `if' statements when a program is compiled, if their
conditions are known not to vary at run time, and eliminate code which
can never be executed. If you can count on your compiler to do this,
you may find that your program is more readable if you use `if'
statements with constant conditions (perhaps determined by macros). Of
course, you can only use this to exclude code, not type definitions or
other preprocessing directives, and you can only do it if the code
remains syntactically valid when it is not to be used.

GCC version 3 eliminates this kind of never-executed code even when
not optimizing. Older versions did it only when optimizing.

* Menu:

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

File: cpp.info, Node: Conditional Uses, Next: Conditional Syntax, Up: Conditionals

4.1 Conditional Uses
====================

There are three general reasons to use a conditional.

* A program may need to use different code depending on the machine
or operating system it is to run on. In some cases the code for
one operating system may be erroneous on another operating system;
for example, it might refer to data types or constants that do not
exist on the other system. When this happens, it is not enough to
avoid executing the invalid code. Its mere presence will cause
the compiler to reject the program. With a preprocessing
conditional, the offending code can be effectively excised from
the program when it is not valid.

* You may want to be able to compile the same source file into two
different programs. One version might make frequent time-consuming
consistency checks on its intermediate data, or print the values of
those data for debugging, and the other not.

* A conditional whose condition is always false is one way to
exclude code from the program but keep it as a sort of comment for
future reference.

Simple programs that do not need system-specific logic or complex
debugging hooks generally will not need to use preprocessing
conditionals.

File: cpp.info, Node: Conditional Syntax, Next: Deleted Code, Prev: Conditional Uses, Up: Conditionals

4.2 Conditional Syntax
======================

A conditional in the C preprocessor begins with a "conditional
directive": `#if', `#ifdef' or `#ifndef'.

* Menu:

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

File: cpp.info, Node: Ifdef, Next: If, Up: Conditional Syntax

4.2.1 Ifdef
-----------

The simplest sort of conditional is

#ifdef MACRO

CONTROLLED TEXT

#endif /* MACRO */

This block is called a "conditional group". CONTROLLED TEXT will be
included in the output of the preprocessor if and only if MACRO is
defined. We say that the conditional "succeeds" if MACRO is defined,
"fails" if it is not.

The CONTROLLED TEXT inside of a conditional can include
preprocessing directives. They are executed only if the conditional
succeeds. You can nest conditional groups inside other conditional
groups, but they must be completely nested. In other words, `#endif'
always matches the nearest `#ifdef' (or `#ifndef', or `#if'). Also,
you cannot start a conditional group in one file and end it in another.

Even if a conditional fails, the CONTROLLED TEXT inside it is still
run through initial transformations and tokenization. Therefore, it
must all be lexically valid C. Normally the only way this matters is
that all comments and string literals inside a failing conditional group
must still be properly ended.

The comment following the `#endif' is not required, but it is a good
practice if there is a lot of CONTROLLED TEXT, because it helps people
match the `#endif' to the corresponding `#ifdef'. Older programs
sometimes put MACRO directly after the `#endif' without enclosing it in
a comment. This is invalid code according to the C standard. CPP
accepts it with a warning. It never affects which `#ifndef' the
`#endif' matches.

Sometimes you wish to use some code if a macro is _not_ defined.
You can do this by writing `#ifndef' instead of `#ifdef'. One common
use of `#ifndef' is to include code only the first time a header file
is included. *Note Once-Only Headers::.

Macro definitions can vary between compilations for several reasons.
Here are some samples.

* Some macros are predefined on each kind of machine (*note
System-specific Predefined Macros::). This allows you to provide
code specially tuned for a particular machine.

* System header files define more macros, associated with the
features they implement. You can test these macros with
conditionals to avoid using a system feature on a machine where it
is not implemented.

* Macros can be defined or undefined with the `-D' and `-U' command
line options when you compile the program. You can arrange to
compile the same source file into two different programs by
choosing a macro name to specify which program you want, writing
conditionals to test whether or how this macro is defined, and
then controlling the state of the macro with command line options,
perhaps set in the Makefile. *Note Invocation::.

* Your program might have a special header file (often called
`config.h') that is adjusted when the program is compiled. It can
define or not define macros depending on the features of the
system and the desired capabilities of the program. The
adjustment can be automated by a tool such as `autoconf', or done
by hand.

File: cpp.info, Node: If, Next: Defined, Prev: Ifdef, Up: Conditional Syntax

4.2.2 If
--------

The `#if' directive allows you to test the value of an arithmetic
expression, rather than the mere existence of one macro. Its syntax is

#if EXPRESSION

CONTROLLED TEXT

#endif /* EXPRESSION */

EXPRESSION is a C expression of integer type, subject to stringent
restrictions. It may contain

* Integer constants.

* Character constants, which are interpreted as they would be in
normal code.

* Arithmetic operators for addition, subtraction, multiplication,
division, bitwise operations, shifts, comparisons, and logical
operations (`&&' and `||'). The latter two obey the usual
short-circuiting rules of standard C.

* Macros. All macros in the expression are expanded before actual
computation of the expression's value begins.

* Uses of the `defined' operator, which lets you check whether macros
are defined in the middle of an `#if'.

* Identifiers that are not macros, which are all considered to be the
number zero. This allows you to write `#if MACRO' instead of
`#ifdef MACRO', if you know that MACRO, when defined, will always
have a nonzero value. Function-like macros used without their
function call parentheses are also treated as zero.

In some contexts this shortcut is undesirable. The `-Wundef'
option causes GCC to warn whenever it encounters an identifier
which is not a macro in an `#if'.

The preprocessor does not know anything about types in the language.
Therefore, `sizeof' operators are not recognized in `#if', and neither
are `enum' constants. They will be taken as identifiers which are not
macros, and replaced by zero. In the case of `sizeof', this is likely
to cause the expression to be invalid.

The preprocessor calculates the value of EXPRESSION. It carries out
all calculations in the widest integer type known to the compiler; on
most machines supported by GCC this is 64 bits. This is not the same
rule as the compiler uses to calculate the value of a constant
expression, and may give different results in some cases. If the value
comes out to be nonzero, the `#if' succeeds and the CONTROLLED TEXT is
included; otherwise it is skipped.

File: cpp.info, Node: Defined, Next: Else, Prev: If, Up: Conditional Syntax

4.2.3 Defined
-------------

The special operator `defined' is used in `#if' and `#elif' expressions
to test whether a certain name is defined as a macro. `defined NAME'
and `defined (NAME)' are both expressions whose value is 1 if NAME is
defined as a macro at the current point in the program, and 0
otherwise. Thus, `#if defined MACRO' is precisely equivalent to
`#ifdef MACRO'.

`defined' is useful when you wish to test more than one macro for
existence at once. For example,

#if defined (__vax__) || defined (__ns16000__)

would succeed if either of the names `__vax__' or `__ns16000__' is
defined as a macro.

Conditionals written like this:

#if defined BUFSIZE && BUFSIZE >= 1024

can generally be simplified to just `#if BUFSIZE >= 1024', since if
`BUFSIZE' is not defined, it will be interpreted as having the value
zero.

If the `defined' operator appears as a result of a macro expansion,
the C standard says the behavior is undefined. GNU cpp treats it as a
genuine `defined' operator and evaluates it normally. It will warn
wherever your code uses this feature if you use the command-line option
`-pedantic', since other compilers may handle it differently.

File: cpp.info, Node: Else, Next: Elif, Prev: Defined, Up: Conditional Syntax

4.2.4 Else
----------

The `#else' directive can be added to a conditional to provide
alternative text to be used if the condition fails. This is what it
looks like:

#if EXPRESSION
TEXT-IF-TRUE
#else /* Not EXPRESSION */
TEXT-IF-FALSE
#endif /* Not EXPRESSION */

If EXPRESSION is nonzero, the TEXT-IF-TRUE is included and the
TEXT-IF-FALSE is skipped. If EXPRESSION is zero, the opposite happens.

You can use `#else' with `#ifdef' and `#ifndef', too.

File: cpp.info, Node: Elif, Prev: Else, Up: Conditional Syntax

4.2.5 Elif
----------

One common case of nested conditionals is used to check for more than
two possible alternatives. For example, you might have

#if X == 1
...
#else /* X != 1 */
#if X == 2
...
#else /* X != 2 */
...
#endif /* X != 2 */
#endif /* X != 1 */

Another conditional directive, `#elif', allows this to be
abbreviated as follows:

#if X == 1
...
#elif X == 2
...
#else /* X != 2 and X != 1*/
...
#endif /* X != 2 and X != 1*/

`#elif' stands for "else if". Like `#else', it goes in the middle
of a conditional group and subdivides it; it does not require a
matching `#endif' of its own. Like `#if', the `#elif' directive
includes an expression to be tested. The text following the `#elif' is
processed only if the original `#if'-condition failed and the `#elif'
condition succeeds.

More than one `#elif' can go in the same conditional group. Then
the text after each `#elif' is processed only if the `#elif' condition
succeeds after the original `#if' and all previous `#elif' directives
within it have failed.

`#else' is allowed after any number of `#elif' directives, but
`#elif' may not follow `#else'.

File: cpp.info, Node: Deleted Code, Prev: Conditional Syntax, Up: Conditionals

4.3 Deleted Code
================

If you replace or delete a part of the program but want to keep the old
code around for future reference, you often cannot simply comment it
out. Block comments do not nest, so the first comment inside the old
code will end the commenting-out. The probable result is a flood of
syntax errors.

One way to avoid this problem is to use an always-false conditional
instead. For instance, put `#if 0' before the deleted code and
`#endif' after it. This works even if the code being turned off
contains conditionals, but they must be entire conditionals (balanced
`#if' and `#endif').

Some people use `#ifdef notdef' instead. This is risky, because
`notdef' might be accidentally defined as a macro, and then the
conditional would succeed. `#if 0' can be counted on to fail.

Do not use `#if 0' for comments which are not C code. Use a real
comment, instead. The interior of `#if 0' must consist of complete
tokens; in particular, single-quote characters must balance. Comments
often contain unbalanced single-quote characters (known in English as
apostrophes). These confuse `#if 0'. They don't confuse `/*'.

File: cpp.info, Node: Diagnostics, Next: Line Control, Prev: Conditionals, Up: Top

5 Diagnostics
*************

The directive `#error' causes the preprocessor to report a fatal error.
The tokens forming the rest of the line following `#error' are used as
the error message.

You would use `#error' inside of a conditional that detects a
combination of parameters which you know the program does not properly
support. For example, if you know that the program will not run
properly on a VAX, you might write

#ifdef __vax__
#error "Won't work on VAXen. See comments at get_last_object."
#endif

If you have several configuration parameters that must be set up by
the installation in a consistent way, you can use conditionals to detect
an inconsistency and report it with `#error'. For example,

#if !defined(UNALIGNED_INT_ASM_OP) && defined(DWARF2_DEBUGGING_INFO)
#error "DWARF2_DEBUGGING_INFO requires UNALIGNED_INT_ASM_OP."
#endif

The directive `#warning' is like `#error', but causes the
preprocessor to issue a warning and continue preprocessing. The tokens
following `#warning' are used as the warning message.

You might use `#warning' in obsolete header files, with a message
directing the user to the header file which should be used instead.

Neither `#error' nor `#warning' macro-expands its argument.
Internal whitespace sequences are each replaced with a single space.
The line must consist of complete tokens. It is wisest to make the
argument of these directives be a single string constant; this avoids
problems with apostrophes and the like.

File: cpp.info, Node: Line Control, Next: Pragmas, Prev: Diagnostics, Up: Top

6 Line Control
**************

The C preprocessor informs the C compiler of the location in your source
code where each token came from. Presently, this is just the file name
and line number. All the tokens resulting from macro expansion are
reported as having appeared on the line of the source file where the
outermost macro was used. We intend to be more accurate in the future.

If you write a program which generates source code, such as the
`bison' parser generator, you may want to adjust the preprocessor's
notion of the current file name and line number by hand. Parts of the
output from `bison' are generated from scratch, other parts come from a
standard parser file. The rest are copied verbatim from `bison''s
input. You would like compiler error messages and symbolic debuggers
to be able to refer to `bison''s input file.

`bison' or any such program can arrange this by writing `#line'
directives into the output file. `#line' is a directive that specifies
the original line number and source file name for subsequent input in
the current preprocessor input file. `#line' has three variants:

`#line LINENUM'
LINENUM is a non-negative decimal integer constant. It specifies
the line number which should be reported for the following line of
input. Subsequent lines are counted from LINENUM.

`#line LINENUM FILENAME'
LINENUM is the same as for the first form, and has the same
effect. In addition, FILENAME is a string constant. The
following line and all subsequent lines are reported to come from
the file it specifies, until something else happens to change that.
FILENAME is interpreted according to the normal rules for a string
constant: backslash escapes are interpreted. This is different
from `#include'.

Previous versions of CPP did not interpret escapes in `#line'; we
have changed it because the standard requires they be interpreted,
and most other compilers do.

`#line ANYTHING ELSE'
ANYTHING ELSE is checked for macro calls, which are expanded. The
result should match one of the above two forms.

`#line' directives alter the results of the `__FILE__' and
`__LINE__' predefined macros from that point on. *Note Standard
Predefined Macros::. They do not have any effect on `#include''s idea
of the directory containing the current file. This is a change from
GCC 2.95. Previously, a file reading

#line 1 "../src/gram.y"
#include "gram.h"

would search for `gram.h' in `../src', then the `-I' chain; the
directory containing the physical source file would not be searched.
In GCC 3.0 and later, the `#include' is not affected by the presence of
a `#line' referring to a different directory.

We made this change because the old behavior caused problems when
generated source files were transported between machines. For instance,
it is common practice to ship generated parsers with a source release,
so that people building the distribution do not need to have yacc or
Bison installed. These files frequently have `#line' directives
referring to the directory tree of the system where the distribution was
created. If GCC tries to search for headers in those directories, the
build is likely to fail.

The new behavior can cause failures too, if the generated file is not
in the same directory as its source and it attempts to include a header
which would be visible searching from the directory containing the
source file. However, this problem is easily solved with an additional
`-I' switch on the command line. The failures caused by the old
semantics could sometimes be corrected only by editing the generated
files, which is difficult and error-prone.

File: cpp.info, Node: Pragmas, Next: Other Directives, Prev: Line Control, Up: Top

7 Pragmas
*********

The `#pragma' directive is the method specified by the C standard for
providing additional information to the compiler, beyond what is
conveyed in the language itself. Three forms of this directive
(commonly known as "pragmas") are specified by the 1999 C standard. A
C compiler is free to attach any meaning it likes to other pragmas.

GCC has historically preferred to use extensions to the syntax of the
language, such as `__attribute__', for this purpose. However, GCC does
define a few pragmas of its own. These mostly have effects on the
entire translation unit or source file.

In GCC version 3, all GNU-defined, supported pragmas have been given
a `GCC' prefix. This is in line with the `STDC' prefix on all pragmas
defined by C99. For backward compatibility, pragmas which were
recognized by previous versions are still recognized without the `GCC'
prefix, but that usage is deprecated. Some older pragmas are
deprecated in their entirety. They are not recognized with the `GCC'
prefix. *Note Obsolete Features::.

C99 introduces the `_Pragma' operator. This feature addresses a
major problem with `#pragma': being a directive, it cannot be produced
as the result of macro expansion. `_Pragma' is an operator, much like
`sizeof' or `defined', and can be embedded in a macro.

Its syntax is `_Pragma (STRING-LITERAL)', where STRING-LITERAL can
be either a normal or wide-character string literal. It is
destringized, by replacing all `\\' with a single `\' and all `\"' with
a `"'. The result is then processed as if it had appeared as the right
hand side of a `#pragma' directive. For example,

_Pragma ("GCC dependency \"parse.y\"")

has the same effect as `#pragma GCC dependency "parse.y"'. The same
effect could be achieved using macros, for example

#define DO_PRAGMA(x) _Pragma (#x)
DO_PRAGMA (GCC dependency "parse.y")

The standard is unclear on where a `_Pragma' operator can appear.
The preprocessor does not accept it within a preprocessing conditional
directive like `#if'. To be safe, you are probably best keeping it out
of directives other than `#define', and putting it on a line of its own.

This manual documents the pragmas which are meaningful to the
preprocessor itself. Other pragmas are meaningful to the C or C++
compilers. They are documented in the GCC manual.

`#pragma GCC dependency'
`#pragma GCC dependency' allows you to check the relative dates of
the current file and another file. If the other file is more
recent than the current file, a warning is issued. This is useful
if the current file is derived from the other file, and should be
regenerated. The other file is searched for using the normal
include search path. Optional trailing text can be used to give
more information in the warning message.

#pragma GCC dependency "parse.y"
#pragma GCC dependency "/usr/include/time.h" rerun fixincludes

`#pragma GCC poison'
Sometimes, there is an identifier that you want to remove
completely from your program, and make sure that it never creeps
back in. To enforce this, you can "poison" the identifier with
this pragma. `#pragma GCC poison' is followed by a list of
identifiers to poison. If any of those identifiers appears
anywhere in the source after the directive, it is a hard error.
For example,

#pragma GCC poison printf sprintf fprintf
sprintf(some_string, "hello");

will produce an error.

If a poisoned identifier appears as part of the expansion of a
macro which was defined before the identifier was poisoned, it
will _not_ cause an error. This lets you poison an identifier
without worrying about system headers defining macros that use it.

For example,

#define strrchr rindex
#pragma GCC poison rindex
strrchr(some_string, 'h');

will not produce an error.

`#pragma GCC system_header'
This pragma takes no arguments. It causes the rest of the code in
the current file to be treated as if it came from a system header.
*Note System Headers::.

File: cpp.info, Node: Other Directives, Next: Preprocessor Output, Prev: Pragmas, Up: Top

8 Other Directives
******************

The `#ident' directive takes one argument, a string constant. On some
systems, that string constant is copied into a special segment of the
object file. On other systems, the directive is ignored. The `#sccs'
directive is a synonym for `#ident'.

These directives are not part of the C standard, but they are not
official GNU extensions either. What historical information we have
been able to find, suggests they originated with System V.

The "null directive" consists of a `#' followed by a newline, with
only whitespace (including comments) in between. A null directive is
understood as a preprocessing directive but has no effect on the
preprocessor output. The primary significance of the existence of the
null directive is that an input line consisting of just a `#' will
produce no output, rather than a line of output containing just a `#'.
Supposedly some old C programs contain such lines.

File: cpp.info, Node: Preprocessor Output, Next: Traditional Mode, Prev: Other Directives, Up: Top

9 Preprocessor Output
*********************

When the C preprocessor is used with the C, C++, or Objective-C
compilers, it is integrated into the compiler and communicates a stream
of binary tokens directly to the compiler's parser. However, it can
also be used in the more conventional standalone mode, where it produces
textual output.

The output from the C preprocessor looks much like the input, except
that all preprocessing directive lines have been replaced with blank
lines and all comments with spaces. Long runs of blank lines are
discarded.

The ISO standard specifies that it is implementation defined whether
a preprocessor preserves whitespace between tokens, or replaces it with
e.g. a single space. In GNU CPP, whitespace between tokens is collapsed
to become a single space, with the exception that the first token on a
non-directive line is preceded with sufficient spaces that it appears in
the same column in the preprocessed output that it appeared in the
original source file. This is so the output is easy to read. *Note
Differences from previous versions::. CPP does not insert any
whitespace where there was none in the original source, except where
necessary to prevent an accidental token paste.

Source file name and line number information is conveyed by lines of
the form

# LINENUM FILENAME FLAGS

These are called "linemarkers". They are inserted as needed into the
output (but never within a string or character constant). They mean
that the following line originated in file FILENAME at line LINENUM.
FILENAME will never contain any non-printing characters; they are
replaced with octal escape sequences.

After the file name comes zero or more flags, which are `1', `2',
`3', or `4'. If there are multiple flags, spaces separate them. Here
is what the flags mean:

`1'
This indicates the start of a new file.

`2'
This indicates returning to a file (after having included another
file).

`3'
This indicates that the following text comes from a system header
file, so certain warnings should be suppressed.

`4'
This indicates that the following text should be treated as being
wrapped in an implicit `extern "C"' block.

As an extension, the preprocessor accepts linemarkers in
non-assembler input files. They are treated like the corresponding
`#line' directive, (*note Line Control::), except that trailing flags
are permitted, and are interpreted with the meanings described above.
If multiple flags are given, they must be in ascending order.

Some directives may be duplicated in the output of the preprocessor.
These are `#ident' (always), `#pragma' (only if the preprocessor does
not handle the pragma itself), and `#define' and `#undef' (with certain
debugging options). If this happens, the `#' of the directive will
always be in the first column, and there will be no space between the
`#' and the directive name. If macro expansion happens to generate
tokens which might be mistaken for a duplicated directive, a space will
be inserted between the `#' and the directive name.

File: cpp.info, Node: Traditional Mode, Next: Implementation Details, Prev: Preprocessor Output, Up: Top

10 Traditional Mode
*******************

Traditional (pre-standard) C preprocessing is rather different from the
preprocessing specified by the standard. When GCC is given the
`-traditional-cpp' option, it attempts to emulate a traditional
preprocessor.

GCC versions 3.2 and later only support traditional mode semantics in
the preprocessor, and not in the compiler front ends. This chapter
outlines the traditional preprocessor semantics we implemented.

The implementation does not correspond precisely to the behavior of
earlier versions of GCC, nor to any true traditional preprocessor.
After all, inconsistencies among traditional implementations were a
major motivation for C standardization. However, we intend that it
should be compatible with true traditional preprocessors in all ways
that actually matter.

* Menu:

* Traditional lexical analysis::
* Traditional macros::
* Traditional miscellany::
* Traditional warnings::

File: cpp.info, Node: Traditional lexical analysis, Next: Traditional macros, Up: Traditional Mode

10.1 Traditional lexical analysis
=================================

The traditional preprocessor does not decompose its input into tokens
the same way a standards-conforming preprocessor does. The input is
simply treated as a stream of text with minimal internal form.

This implementation does not treat trigraphs (*note trigraphs::)
specially since they were an invention of the standards committee. It
handles arbitrarily-positioned escaped newlines properly and splices
the lines as you would expect; many traditional preprocessors did not
do this.

The form of horizontal whitespace in the input file is preserved in
the output. In particular, hard tabs remain hard tabs. This can be
useful if, for example, you are preprocessing a Makefile.

Traditional CPP only recognizes C-style block comments, and treats
the `/*' sequence as introducing a comment only if it lies outside
quoted text. Quoted text is introduced by the usual single and double
quotes, and also by an initial `<' in a `#include' directive.

Traditionally, comments are completely removed and are not replaced
with a space. Since a traditional compiler does its own tokenization
of the output of the preprocessor, this means that comments can
effectively be used as token paste operators. However, comments behave
like separators for text handled by the preprocessor itself, since it
doesn't re-lex its input. For example, in

#if foo/**/bar

`foo' and `bar' are distinct identifiers and expanded separately if
they happen to be macros. In other words, this directive is equivalent
to

#if foo bar

rather than

#if foobar

Generally speaking, in traditional mode an opening quote need not
have a matching closing quote. In particular, a macro may be defined
with replacement text that contains an unmatched quote. Of course, if
you attempt to compile preprocessed output containing an unmatched quote
you will get a syntax error.

However, all preprocessing directives other than `#define' require
matching quotes. For example:

#define m This macro's fine and has an unmatched quote
"/* This is not a comment. */
/* This is a comment. The following #include directive
is ill-formed. */
#include <stdio.h

Just as for the ISO preprocessor, what would be a closing quote can
be escaped with a backslash to prevent the quoted text from closing.

File: cpp.info, Node: Traditional macros, Next: Traditional miscellany, Prev: Traditional lexical analysis, Up: Traditional Mode

10.2 Traditional macros
=======================

The major difference between traditional and ISO macros is that the
former expand to text rather than to a token sequence. CPP removes all
leading and trailing horizontal whitespace from a macro's replacement
text before storing it, but preserves the form of internal whitespace.

One consequence is that it is legitimate for the replacement text to
contain an unmatched quote (*note Traditional lexical analysis::). An
unclosed string or character constant continues into the text following
the macro call. Similarly, the text at the end of a macro's expansion
can run together with the text after the macro invocation to produce a
single token.

Normally comments are removed from the replacement text after the
macro is expanded, but if the `-CC' option is passed on the command
line comments are preserved. (In fact, the current implementation
removes comments even before saving the macro replacement text, but it
careful to do it in such a way that the observed effect is identical
even in the function-like macro case.)

The ISO stringification operator `#' and token paste operator `##'
have no special meaning. As explained later, an effect similar to
these operators can be obtained in a different way. Macro names that
are embedded in quotes, either from the main file or after macro
replacement, do not expand.

CPP replaces an unquoted object-like macro name with its replacement
text, and then rescans it for further macros to replace. Unlike
standard macro expansion, traditional macro expansion has no provision
to prevent recursion. If an object-like macro appears unquoted in its
replacement text, it will be replaced again during the rescan pass, and
so on _ad infinitum_. GCC detects when it is expanding recursive
macros, emits an error message, and continues after the offending macro
invocation.

#define PLUS +
#define INC(x) PLUS+x
INC(foo);
==> ++foo;

Function-like macros are similar in form but quite different in
behavior to their ISO counterparts. Their arguments are contained
within parentheses, are comma-separated, and can cross physical lines.
Commas within nested parentheses are not treated as argument
separators. Similarly, a quote in an argument cannot be left unclosed;
a following comma or parenthesis that comes before the closing quote is
treated like any other character. There is no facility for handling
variadic macros.

This implementation removes all comments from macro arguments, unless
the `-C' option is given. The form of all other horizontal whitespace
in arguments is preserved, including leading and trailing whitespace.
In particular

f( )

is treated as an invocation of the macro `f' with a single argument
consisting of a single space. If you want to invoke a function-like
macro that takes no arguments, you must not leave any whitespace
between the parentheses.

If a macro argument crosses a new line, the new line is replaced with
a space when forming the argument. If the previous line contained an
unterminated quote, the following line inherits the quoted state.

Traditional preprocessors replace parameters in the replacement text
with their arguments regardless of whether the parameters are within
quotes or not. This provides a way to stringize arguments. For example

#define str(x) "x"
str(/* A comment */some text )
==> "some text "

Note that the comment is removed, but that the trailing space is
preserved. Here is an example of using a comment to effect token
pasting.

#define suffix(x) foo_/**/x
suffix(bar)
==> foo_bar

File: cpp.info, Node: Traditional miscellany, Next: Traditional warnings, Prev: Traditional macros, Up: Traditional Mode

10.3 Traditional miscellany
===========================

Here are some things to be aware of when using the traditional
preprocessor.

* Preprocessing directives are recognized only when their leading
`#' appears in the first column. There can be no whitespace
between the beginning of the line and the `#', but whitespace can
follow the `#'.

* A true traditional C preprocessor does not recognize `#error' or
`#pragma', and may not recognize `#elif'. CPP supports all the
directives in traditional mode that it supports in ISO mode,
including extensions, with the exception that the effects of
`#pragma GCC poison' are undefined.

* __STDC__ is not defined.

* If you use digraphs the behavior is undefined.

* If a line that looks like a directive appears within macro
arguments, the behavior is undefined.

File: cpp.info, Node: Traditional warnings, Prev: Traditional miscellany, Up: Traditional Mode

10.4 Traditional warnings
=========================

You can request warnings about features that did not exist, or worked
differently, in traditional C with the `-Wtraditional' option. GCC
does not warn about features of ISO C which you must use when you are
using a conforming compiler, such as the `#' and `##' operators.

Presently `-Wtraditional' warns about:

* Macro parameters that appear within string literals in the macro
body. In traditional C macro replacement takes place within
string literals, but does not in ISO C.

* In traditional C, some preprocessor directives did not exist.
Traditional preprocessors would only consider a line to be a
directive if the `#' appeared in column 1 on the line. Therefore
`-Wtraditional' warns about directives that traditional C
understands but would ignore because the `#' does not appear as the
first character on the line. It also suggests you hide directives
like `#pragma' not understood by traditional C by indenting them.
Some traditional implementations would not recognize `#elif', so it
suggests avoiding it altogether.

* A function-like macro that appears without an argument list. In
some traditional preprocessors this was an error. In ISO C it
merely means that the macro is not expanded.

* The unary plus operator. This did not exist in traditional C.

* The `U' and `LL' integer constant suffixes, which were not
available in traditional C. (Traditional C does support the `L'
suffix for simple long integer constants.) You are not warned
about uses of these suffixes in macros defined in system headers.
For instance, `UINT_MAX' may well be defined as `4294967295U', but
you will not be warned if you use `UINT_MAX'.

You can usually avoid the warning, and the related warning about
constants which are so large that they are unsigned, by writing the
integer constant in question in hexadecimal, with no U suffix.
Take care, though, because this gives the wrong result in exotic
cases.

File: cpp.info, Node: Implementation Details, Next: Invocation, Prev: Traditional Mode, Up: Top

11 Implementation Details
*************************

Here we document details of how the preprocessor's implementation
affects its user-visible behavior. You should try to avoid undue
reliance on behavior described here, as it is possible that it will
change subtly in future implementations.

Also documented here are obsolete features and changes from previous
versions of CPP.

* Menu:

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

File: cpp.info, Node: Implementation-defined behavior, Next: Implementation limits, Up: Implementation Details

11.1 Implementation-defined behavior
====================================

This is how CPP behaves in all the cases which the C standard describes
as "implementation-defined". This term means that the implementation
is free to do what it likes, but must document its choice and stick to
it.

* The mapping of physical source file multi-byte characters to the
execution character set.

Currently, CPP requires its input to be ASCII or UTF-8. The
execution character set may be controlled by the user, with the
`-fexec-charset' and `-fwide-exec-charset' options.

* Identifier characters.

The C and C++ standards allow identifiers to be composed of `_'
and the alphanumeric characters. C++ and C99 also allow universal
character names, and C99 further permits implementation-defined
characters. GCC currently only permits universal character names
if `-fextended-identifiers' is used, because the implementation of
universal character names in identifiers is experimental.

GCC allows the `$' character in identifiers as an extension for
most targets. This is true regardless of the `std=' switch, since
this extension cannot conflict with standards-conforming programs.
When preprocessing assembler, however, dollars are not identifier
characters by default.

Currently the targets that by default do not permit `$' are AVR,
IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC targets for the AIX
and BeOS operating systems.

You can override the default with `-fdollars-in-identifiers' or
`fno-dollars-in-identifiers'. *Note fdollars-in-identifiers::.

* Non-empty sequences of whitespace characters.

In textual output, each whitespace sequence is collapsed to a
single space. For aesthetic reasons, the first token on each
non-directive line of output is preceded with sufficient spaces
that it appears in the same column as it did in the original
source file.

* The numeric value of character constants in preprocessor
expressions.

The preprocessor and compiler interpret character constants in the
same way; i.e. escape sequences such as `\a' are given the values
they would have on the target machine.

The compiler values a multi-character character constant a
character at a time, shifting the previous value left by the
number of bits per target character, and then or-ing in the
bit-pattern of the new character truncated to the width of a
target character. The final bit-pattern is given type `int', and
is therefore signed, regardless of whether single characters are
signed or not (a slight change from versions 3.1 and earlier of
GCC). If there are more characters in the constant than would fit
in the target `int' the compiler issues a warning, and the excess
leading characters are ignored.

For example, `'ab'' for a target with an 8-bit `char' would be
interpreted as
`(int) ((unsigned char) 'a' * 256 + (unsigned char) 'b')', and
`'\234a'' as
`(int) ((unsigned char) '\234' * 256 + (unsigned char) 'a')'.

* Source file inclusion.

For a discussion on how the preprocessor locates header files,
*Note Include Operation::.

* Interpretation of the filename resulting from a macro-expanded
`#include' directive.

*Note Computed Includes::.

* Treatment of a `#pragma' directive that after macro-expansion
results in a standard pragma.

No macro expansion occurs on any `#pragma' directive line, so the
question does not arise.

Note that GCC does not yet implement any of the standard pragmas.

File: cpp.info, Node: Implementation limits, Next: Obsolete Features, Prev: Implementation-defined behavior, Up: Implementation Details

11.2 Implementation limits
==========================

CPP has a small number of internal limits. This section lists the
limits which the C standard requires to be no lower than some minimum,
and all the others known. It is intended that there should be as few
limits as possible. If you encounter an undocumented or inconvenient
limit, please report that as a bug. *Note Reporting Bugs: (gcc)Bugs.

Where we say something is limited "only by available memory", that
means that internal data structures impose no intrinsic limit, and space
is allocated with `malloc' or equivalent. The actual limit will
therefore depend on many things, such as the size of other things
allocated by the compiler at the same time, the amount of memory
consumed by other processes on the same computer, etc.

* Nesting levels of `#include' files.

We impose an arbitrary limit of 200 levels, to avoid runaway
recursion. The standard requires at least 15 levels.

* Nesting levels of conditional inclusion.

The C standard mandates this be at least 63. CPP is limited only
by available memory.

* Levels of parenthesized expressions within a full expression.

The C standard requires this to be at least 63. In preprocessor
conditional expressions, it is limited only by available memory.

* Significant initial characters in an identifier or macro name.

The preprocessor treats all characters as significant. The C
standard requires only that the first 63 be significant.

* Number of macros simultaneously defined in a single translation
unit.

The standard requires at least 4095 be possible. CPP is limited
only by available memory.

* Number of parameters in a macro definition and arguments in a
macro call.

We allow `USHRT_MAX', which is no smaller than 65,535. The minimum
required by the standard is 127.

* Number of characters on a logical source line.

The C standard requires a minimum of 4096 be permitted. CPP places
no limits on this, but you may get incorrect column numbers
reported in diagnostics for lines longer than 65,535 characters.

* Maximum size of a source file.

The standard does not specify any lower limit on the maximum size
of a source file. GNU cpp maps files into memory, so it is
limited by the available address space. This is generally at
least two gigabytes. Depending on the operating system, the size
of physical memory may or may not be a limitation.

File: cpp.info, Node: Obsolete Features, Next: Differences from previous versions, Prev: Implementation limits, Up: Implementation Details

11.3 Obsolete Features
======================

CPP has a number of features which are present mainly for compatibility
with older programs. We discourage their use in new code. In some
cases, we plan to remove the feature in a future version of GCC.

* Menu:

* Assertions::
* Obsolete once-only headers::

File: cpp.info, Node: Assertions, Next: Obsolete once-only headers, Up: Obsolete Features

11.3.1 Assertions
-----------------

"Assertions" are a deprecated alternative to macros in writing
conditionals to test what sort of computer or system the compiled
program will run on. Assertions are usually predefined, but you can
define them with preprocessing directives or command-line options.

Assertions were intended to provide a more systematic way to describe
the compiler's target system. However, in practice they are just as
unpredictable as the system-specific predefined macros. In addition,
they are not part of any standard, and only a few compilers support
them. Therefore, the use of assertions is *less* portable than the use
of system-specific predefined macros. We recommend you do not use them
at all.

An assertion looks like this:

#PREDICATE (ANSWER)

PREDICATE must be a single identifier. ANSWER can be any sequence of
tokens; all characters are significant except for leading and trailing
whitespace, and differences in internal whitespace sequences are
ignored. (This is similar to the rules governing macro redefinition.)
Thus, `(x + y)' is different from `(x+y)' but equivalent to
`( x + y )'. Parentheses do not nest inside an answer.

To test an assertion, you write it in an `#if'. For example, this
conditional succeeds if either `vax' or `ns16000' has been asserted as
an answer for `machine'.

#if #machine (vax) || #machine (ns16000)

You can test whether _any_ answer is asserted for a predicate by
omitting the answer in the conditional:

#if #machine

Assertions are made with the `#assert' directive. Its sole argument
is the assertion to make, without the leading `#' that identifies
assertions in conditionals.

#assert PREDICATE (ANSWER)

You may make several assertions with the same predicate and different
answers. Subsequent assertions do not override previous ones for the
same predicate. All the answers for any given predicate are
simultaneously true.

Assertions can be canceled with the `#unassert' directive. It has
the same syntax as `#assert'. In that form it cancels only the answer
which was specified on the `#unassert' line; other answers for that
predicate remain true. You can cancel an entire predicate by leaving
out the answer:

#unassert PREDICATE

In either form, if no such assertion has been made, `#unassert' has no
effect.

You can also make or cancel assertions using command line options.
*Note Invocation::.

File: cpp.info, Node: Obsolete once-only headers, Prev: Assertions, Up: Obsolete Features

11.3.2 Obsolete once-only headers
---------------------------------

CPP supports two more ways of indicating that a header file should be
read only once. Neither one is as portable as a wrapper `#ifndef', and
we recommend you do not use them in new programs.

In the Objective-C language, there is a variant of `#include' called
`#import' which includes a file, but does so at most once. If you use
`#import' instead of `#include', then you don't need the conditionals
inside the header file to prevent multiple inclusion of the contents.
GCC permits the use of `#import' in C and C++ as well as Objective-C.
However, it is not in standard C or C++ and should therefore not be
used by portable programs.

`#import' is not a well designed feature. It requires the users of
a header file to know that it should only be included once. It is much
better for the header file's implementor to write the file so that users
don't need to know this. Using a wrapper `#ifndef' accomplishes this
goal.

In the present implementation, a single use of `#import' will
prevent the file from ever being read again, by either `#import' or
`#include'. You should not rely on this; do not use both `#import' and
`#include' to refer to the same header file.

Another way to prevent a header file from being included more than
once is with the `#pragma once' directive. If `#pragma once' is seen
when scanning a header file, that file will never be read again, no
matter what.

`#pragma once' does not have the problems that `#import' does, but
it is not recognized by all preprocessors, so you cannot rely on it in
a portable program.

File: cpp.info, Node: Differences from previous versions, Prev: Obsolete Features, Up: Implementation Details

11.4 Differences from previous versions
=======================================

This section details behavior which has changed from previous versions
of CPP. We do not plan to change it again in the near future, but we
do not promise not to, either.

The "previous versions" discussed here are 2.95 and before. The
behavior of GCC 3.0 is mostly the same as the behavior of the widely
used 2.96 and 2.97 development snapshots. Where there are differences,
they generally represent bugs in the snapshots.

* -I- deprecated

This option has been deprecated in 4.0. `-iquote' is meant to
replace the need for this option.

* Order of evaluation of `#' and `##' operators

The standard does not specify the order of evaluation of a chain of
`##' operators, nor whether `#' is evaluated before, after, or at
the same time as `##'. You should therefore not write any code
which depends on any specific ordering. It is possible to
guarantee an ordering, if you need one, by suitable use of nested
macros.

An example of where this might matter is pasting the arguments `1',
`e' and `-2'. This would be fine for left-to-right pasting, but
right-to-left pasting would produce an invalid token `e-2'.

GCC 3.0 evaluates `#' and `##' at the same time and strictly left
to right. Older versions evaluated all `#' operators first, then
all `##' operators, in an unreliable order.

* The form of whitespace between tokens in preprocessor output

*Note Preprocessor Output::, for the current textual format. This
is also the format used by stringification. Normally, the
preprocessor communicates tokens directly to the compiler's
parser, and whitespace does not come up at all.

Older versions of GCC preserved all whitespace provided by the
user and inserted lots more whitespace of their own, because they
could not accurately predict when extra spaces were needed to
prevent accidental token pasting.

* Optional argument when invoking rest argument macros

As an extension, GCC permits you to omit the variable arguments
entirely when you use a variable argument macro. This is
forbidden by the 1999 C standard, and will provoke a pedantic
warning with GCC 3.0. Previous versions accepted it silently.

* `##' swallowing preceding text in rest argument macros

Formerly, in a macro expansion, if `##' appeared before a variable
arguments parameter, and the set of tokens specified for that
argument in the macro invocation was empty, previous versions of
CPP would back up and remove the preceding sequence of
non-whitespace characters (*not* the preceding token). This
extension is in direct conflict with the 1999 C standard and has
been drastically pared back.

In the current version of the preprocessor, if `##' appears between
a comma and a variable arguments parameter, and the variable
argument is omitted entirely, the comma will be removed from the
expansion. If the variable argument is empty, or the token before
`##' is not a comma, then `##' behaves as a normal token paste.

* `#line' and `#include'

The `#line' directive used to change GCC's notion of the
"directory containing the current file", used by `#include' with a
double-quoted header file name. In 3.0 and later, it does not.
*Note Line Control::, for further explanation.

* Syntax of `#line'

In GCC 2.95 and previous, the string constant argument to `#line'
was treated the same way as the argument to `#include': backslash
escapes were not honored, and the string ended at the second `"'.
This is not compliant with the C standard. In GCC 3.0, an attempt
was made to correct the behavior, so that the string was treated
as a real string constant, but it turned out to be buggy. In 3.1,
the bugs have been fixed. (We are not fixing the bugs in 3.0
because they affect relatively few people and the fix is quite
invasive.)

File: cpp.info, Node: Invocation, Next: Environment Variables, Prev: Implementation Details, Up: Top

12 Invocation
*************

Most often when you use the C preprocessor you will not have to invoke
it explicitly: the C compiler will do so automatically. However, the
preprocessor is sometimes useful on its own. All the options listed
here are also acceptable to the C compiler and have the same meaning,
except that the C compiler has different rules for specifying the output
file.

_Note:_ Whether you use the preprocessor by way of `gcc' or `cpp',
the "compiler driver" is run first. This program's purpose is to
translate your command into invocations of the programs that do the
actual work. Their command line interfaces are similar but not
identical to the documented interface, and may change without notice.

The C preprocessor expects two file names as arguments, INFILE and
OUTFILE. The preprocessor reads INFILE together with any other files
it specifies with `#include'. All the output generated by the combined
input files is written in OUTFILE.

Either INFILE or OUTFILE may be `-', which as INFILE means to read
from standard input and as OUTFILE means to write to standard output.
Also, if either file is omitted, it means the same as if `-' had been
specified for that file.

Unless otherwise noted, or the option ends in `=', all options which
take an argument may have that argument appear either immediately after
the option, or with a space between option and argument: `-Ifoo' and
`-I foo' have the same effect.

Many options have multi-letter names; therefore multiple
single-letter options may _not_ be grouped: `-dM' is very different from
`-d -M'.

`-D NAME'
Predefine NAME as a macro, with definition `1'.

`-D NAME=DEFINITION'
The contents of DEFINITION are tokenized and processed as if they
appeared during translation phase three in a `#define' directive.
In particular, the definition will be truncated by embedded
newline characters.

If you are invoking the preprocessor from a shell or shell-like
program you may need to use the shell's quoting syntax to protect
characters such as spaces that have a meaning in the shell syntax.

If you wish to define a function-like macro on the command line,
write its argument list with surrounding parentheses before the
equals sign (if any). Parentheses are meaningful to most shells,
so you will need to quote the option. With `sh' and `csh',
`-D'NAME(ARGS...)=DEFINITION'' works.

`-D' and `-U' options are processed in the order they are given on
the command line. All `-imacros FILE' and `-include FILE' options
are processed after all `-D' and `-U' options.

`-U NAME'
Cancel any previous definition of NAME, either built in or
provided with a `-D' option.

`-undef'
Do not predefine any system-specific or GCC-specific macros. The
standard predefined macros remain defined. *Note Standard
Predefined Macros::.

`-I DIR'
Add the directory DIR to the list of directories to be searched
for header files. *Note Search Path::. Directories named by `-I'
are searched before the standard system include directories. If
the directory DIR is a standard system include directory, the
option is ignored to ensure that the default search order for
system directories and the special treatment of system headers are
not defeated (*note System Headers::) .

`-o FILE'
Write output to FILE. This is the same as specifying FILE as the
second non-option argument to `cpp'. `gcc' has a different
interpretation of a second non-option argument, so you must use
`-o' to specify the output file.

`-Wall'
Turns on all optional warnings which are desirable for normal code.
At present this is `-Wcomment', `-Wtrigraphs', `-Wmultichar' and a
warning about integer promotion causing a change of sign in `#if'
expressions. Note that many of the preprocessor's warnings are on
by default and have no options to control them.

`-Wcomment'
`-Wcomments'
Warn whenever a comment-start sequence `/*' appears in a `/*'
comment, or whenever a backslash-newline appears in a `//' comment.
(Both forms have the same effect.)

`-Wtrigraphs'
Most trigraphs in comments cannot affect the meaning of the
program. However, a trigraph that would form an escaped newline
(`??/' at the end of a line) can, by changing where the comment
begins or ends. Therefore, only trigraphs that would form escaped
newlines produce warnings inside a comment.

This option is implied by `-Wall'. If `-Wall' is not given, this
option is still enabled unless trigraphs are enabled. To get
trigraph conversion without warnings, but get the other `-Wall'
warnings, use `-trigraphs -Wall -Wno-trigraphs'.

`-Wtraditional'
Warn about certain constructs that behave differently in
traditional and ISO C. Also warn about ISO C constructs that have
no traditional C equivalent, and problematic constructs which
should be avoided. *Note Traditional Mode::.

`-Wimport'
Warn the first time `#import' is used.

`-Wundef'
Warn whenever an identifier which is not a macro is encountered in
an `#if' directive, outside of `defined'. Such identifiers are
replaced with zero.

`-Wunused-macros'
Warn about macros defined in the main file that are unused. A
macro is "used" if it is expanded or tested for existence at least
once. The preprocessor will also warn if the macro has not been
used at the time it is redefined or undefined.

Built-in macros, macros defined on the command line, and macros
defined in include files are not warned about.

_Note:_ If a macro is actually used, but only used in skipped
conditional blocks, then CPP will report it as unused. To avoid
the warning in such a case, you might improve the scope of the
macro's definition by, for example, moving it into the first
skipped block. Alternatively, you could provide a dummy use with
something like:

#if defined the_macro_causing_the_warning
#endif

`-Wendif-labels'
Warn whenever an `#else' or an `#endif' are followed by text.
This usually happens in code of the form

#if FOO
...
#else FOO
...
#endif FOO

The second and third `FOO' should be in comments, but often are not
in older programs. This warning is on by default.

`-Werror'
Make all warnings into hard errors. Source code which triggers
warnings will be rejected.

`-Wsystem-headers'
Issue warnings for code in system headers. These are normally
unhelpful in finding bugs in your own code, therefore suppressed.
If you are responsible for the system library, you may want to see
them.

`-w'
Suppress all warnings, including those which GNU CPP issues by
default.

`-pedantic'
Issue all the mandatory diagnostics listed in the C standard.
Some of them are left out by default, since they trigger
frequently on harmless code.

`-pedantic-errors'
Issue all the mandatory diagnostics, and make all mandatory
diagnostics into errors. This includes mandatory diagnostics that
GCC issues without `-pedantic' but treats as warnings.

`-M'
Instead of outputting the result of preprocessing, output a rule
suitable for `make' describing the dependencies of the main source
file. The preprocessor outputs one `make' rule containing the
object file name for that source file, a colon, and the names of
all the included files, including those coming from `-include' or
`-imacros' command line options.

Unless specified explicitly (with `-MT' or `-MQ'), the object file
name consists of the basename of the source file with any suffix
replaced with object file suffix. If there are many included
files then the rule is split into several lines using `\'-newline.
The rule has no commands.

This option does not suppress the preprocessor's debug output,
such as `-dM'. To avoid mixing such debug output with the
dependency rules you should explicitly specify the dependency
output file with `-MF', or use an environment variable like
`DEPENDENCIES_OUTPUT' (*note Environment Variables::). Debug
output will still be sent to the regular output stream as normal.

Passing `-M' to the driver implies `-E', and suppresses warnings
with an implicit `-w'.

`-MM'
Like `-M' but do not mention header files that are found in system
header directories, nor header files that are included, directly
or indirectly, from such a header.

This implies that the choice of angle brackets or double quotes in
an `#include' directive does not in itself determine whether that
header will appear in `-MM' dependency output. This is a slight
change in semantics from GCC versions 3.0 and earlier.

`-MF FILE'
When used with `-M' or `-MM', specifies a file to write the
dependencies to. If no `-MF' switch is given the preprocessor
sends the rules to the same place it would have sent preprocessed
output.

When used with the driver options `-MD' or `-MMD', `-MF' overrides
the default dependency output file.

`-MG'
In conjunction with an option such as `-M' requesting dependency
generation, `-MG' assumes missing header files are generated files
and adds them to the dependency list without raising an error.
The dependency filename is taken directly from the `#include'
directive without prepending any path. `-MG' also suppresses
preprocessed output, as a missing header file renders this useless.

This feature is used in automatic updating of makefiles.

`-MP'
This option instructs CPP to add a phony target for each dependency
other than the main file, causing each to depend on nothing. These
dummy rules work around errors `make' gives if you remove header
files without updating the `Makefile' to match.

This is typical output:

test.o: test.c test.h

test.h:

`-MT TARGET'
Change the target of the rule emitted by dependency generation. By
default CPP takes the name of the main input file, including any
path, deletes any file suffix such as `.c', and appends the
platform's usual object suffix. The result is the target.

An `-MT' option will set the target to be exactly the string you
specify. If you want multiple targets, you can specify them as a
single argument to `-MT', or use multiple `-MT' options.

For example, `-MT '$(objpfx)foo.o'' might give

$(objpfx)foo.o: foo.c

`-MQ TARGET'
Same as `-MT', but it quotes any characters which are special to
Make. `-MQ '$(objpfx)foo.o'' gives

$$(objpfx)foo.o: foo.c

The default target is automatically quoted, as if it were given
with `-MQ'.

`-MD'
`-MD' is equivalent to `-M -MF FILE', except that `-E' is not
implied. The driver determines FILE based on whether an `-o'
option is given. If it is, the driver uses its argument but with
a suffix of `.d', otherwise it take the basename of the input file
and applies a `.d' suffix.

If `-MD' is used in conjunction with `-E', any `-o' switch is
understood to specify the dependency output file (*note -MF:
dashMF.), but if used without `-E', each `-o' is understood to
specify a target object file.

Since `-E' is not implied, `-MD' can be used to generate a
dependency output file as a side-effect of the compilation process.

`-MMD'
Like `-MD' except mention only user header files, not system
header files.

`-x c'
`-x c++'
`-x objective-c'
`-x assembler-with-cpp'
Specify the source language: C, C++, Objective-C, or assembly.
This has nothing to do with standards conformance or extensions;
it merely selects which base syntax to expect. If you give none
of these options, cpp will deduce the language from the extension
of the source file: `.c', `.cc', `.m', or `.S'. Some other common
extensions for C++ and assembly are also recognized. If cpp does
not recognize the extension, it will treat the file as C; this is
the most generic mode.

_Note:_ Previous versions of cpp accepted a `-lang' option which
selected both the language and the standards conformance level.
This option has been removed, because it conflicts with the `-l'
option.

`-std=STANDARD'
`-ansi'
Specify the standard to which the code should conform. Currently
CPP knows about C and C++ standards; others may be added in the
future.

STANDARD may be one of:
`iso9899:1990'
`c89'
The ISO C standard from 1990. `c89' is the customary
shorthand for this version of the standard.

The `-ansi' option is equivalent to `-std=c89'.

`iso9899:199409'
The 1990 C standard, as amended in 1994.

`iso9899:1999'
`c99'
`iso9899:199x'
`c9x'
The revised ISO C standard, published in December 1999.
Before publication, this was known as C9X.

`gnu89'
The 1990 C standard plus GNU extensions. This is the default.

`gnu99'
`gnu9x'
The 1999 C standard plus GNU extensions.

`c++98'
The 1998 ISO C++ standard plus amendments.

`gnu++98'
The same as `-std=c++98' plus GNU extensions. This is the
default for C++ code.

`-I-'
Split the include path. Any directories specified with `-I'
options before `-I-' are searched only for headers requested with
`#include "FILE"'; they are not searched for `#include <FILE>'.
If additional directories are specified with `-I' options after
the `-I-', those directories are searched for all `#include'
directives.

In addition, `-I-' inhibits the use of the directory of the current
file directory as the first search directory for `#include "FILE"'.
*Note Search Path::. This option has been deprecated.

`-nostdinc'
Do not search the standard system directories for header files.
Only the directories you have specified with `-I' options (and the
directory of the current file, if appropriate) are searched.

`-nostdinc++'
Do not search for header files in the C++-specific standard
directories, but do still search the other standard directories.
(This option is used when building the C++ library.)

`-include FILE'
Process FILE as if `#include "file"' appeared as the first line of
the primary source file. However, the first directory searched
for FILE is the preprocessor's working directory _instead of_ the
directory containing the main source file. If not found there, it
is searched for in the remainder of the `#include "..."' search
chain as normal.

If multiple `-include' options are given, the files are included
in the order they appear on the command line.

`-imacros FILE'
Exactly like `-include', except that any output produced by
scanning FILE is thrown away. Macros it defines remain defined.
This allows you to acquire all the macros from a header without
also processing its declarations.

All files specified by `-imacros' are processed before all files
specified by `-include'.

`-idirafter DIR'
Search DIR for header files, but do it _after_ all directories
specified with `-I' and the standard system directories have been
exhausted. DIR is treated as a system include directory.

`-iprefix PREFIX'
Specify PREFIX as the prefix for subsequent `-iwithprefix'
options. If the prefix represents a directory, you should include
the final `/'.

`-iwithprefix DIR'
`-iwithprefixbefore DIR'
Append DIR to the prefix specified previously with `-iprefix', and
add the resulting directory to the include search path.
`-iwithprefixbefore' puts it in the same place `-I' would;
`-iwithprefix' puts it where `-idirafter' would.

`-isysroot DIR'
This option is like the `--sysroot' option, but applies only to
header files. See the `--sysroot' option for more information.

`-imultilib DIR'
Use DIR as a subdirectory of the directory containing
target-specific C++ headers.

`-isystem DIR'
Search DIR for header files, after all directories specified by
`-I' but before the standard system directories. Mark it as a
system directory, so that it gets the same special treatment as is
applied to the standard system directories. *Note System
Headers::.

`-iquote DIR'
Search DIR only for header files requested with `#include "FILE"';
they are not searched for `#include <FILE>', before all
directories specified by `-I' and before the standard system
directories. *Note Search Path::.

`-fdirectives-only'
This option provides a simplified preprocessor to improve the
performance of distributed build systems such as distcc. It's
behavior depends on a number of other flags.

If the `-E' option is enabled, it suppresses things like macro
expansion, trigraph conversion, and escaped newline splicing
outside of directives. All directives are processed normally,
except that macro definitions are output similar to the `-dD'
option.

If the `-fpreprocessed' option is enabled, it suppresses
predefinition of most builtin and command line macros. This
prevents duplicate definition of macros output with the `-E'
option.

`-fdollars-in-identifiers'
Accept `$' in identifiers. *Note Identifier characters::.

`-fextended-identifiers'
Accept universal character names in identifiers. This option is
experimental; in a future version of GCC, it will be enabled by
default for C99 and C++.

`-fpreprocessed'
Indicate to the preprocessor that the input file has already been
preprocessed. This suppresses things like macro expansion,
trigraph conversion, escaped newline splicing, and processing of
most directives. The preprocessor still recognizes and removes
comments, so that you can pass a file preprocessed with `-C' to
the compiler without problems. In this mode the integrated
preprocessor is little more than a tokenizer for the front ends.

`-fpreprocessed' is implicit if the input file has one of the
extensions `.i', `.ii' or `.mi'. These are the extensions that
GCC uses for preprocessed files created by `-save-temps'.

`-ftabstop=WIDTH'
Set the distance between tab stops. This helps the preprocessor
report correct column numbers in warnings or errors, even if tabs
appear on the line. If the value is less than 1 or greater than
100, the option is ignored. The default is 8.

`-fexec-charset=CHARSET'
Set the execution character set, used for string and character
constants. The default is UTF-8. CHARSET can be any encoding
supported by the system's `iconv' library routine.

`-fwide-exec-charset=CHARSET'
Set the wide execution character set, used for wide string and
character constants. The default is UTF-32 or UTF-16, whichever
corresponds to the width of `wchar_t'. As with `-fexec-charset',
CHARSET can be any encoding supported by the system's `iconv'
library routine; however, you will have problems with encodings
that do not fit exactly in `wchar_t'.

`-finput-charset=CHARSET'
Set the input character set, used for translation from the
character set of the input file to the source character set used
by GCC. If the locale does not specify, or GCC cannot get this
information from the locale, the default is UTF-8. This can be
overridden by either the locale or this command line option.
Currently the command line option takes precedence if there's a
conflict. CHARSET can be any encoding supported by the system's
`iconv' library routine.

`-fworking-directory'
Enable generation of linemarkers in the preprocessor output that
will let the compiler know the current working directory at the
time of preprocessing. When this option is enabled, the
preprocessor will emit, after the initial linemarker, a second
linemarker with the current working directory followed by two
slashes. GCC will use this directory, when it's present in the
preprocessed input, as the directory emitted as the current
working directory in some debugging information formats. This
option is implicitly enabled if debugging information is enabled,
but this can be inhibited with the negated form
`-fno-working-directory'. If the `-P' flag is present in the
command line, this option has no effect, since no `#line'
directives are emitted whatsoever.

`-fno-show-column'
Do not print column numbers in diagnostics. This may be necessary
if diagnostics are being scanned by a program that does not
understand the column numbers, such as `dejagnu'.

`-A PREDICATE=ANSWER'
Make an assertion with the predicate PREDICATE and answer ANSWER.
This form is preferred to the older form `-A PREDICATE(ANSWER)',
which is still supported, because it does not use shell special
characters. *Note Assertions::.

`-A -PREDICATE=ANSWER'
Cancel an assertion with the predicate PREDICATE and answer ANSWER.

`-dCHARS'
CHARS is a sequence of one or more of the following characters,
and must not be preceded by a space. Other characters are
interpreted by the compiler proper, or reserved for future
versions of GCC, and so are silently ignored. If you specify
characters whose behavior conflicts, the result is undefined.

`M'
Instead of the normal output, generate a list of `#define'
directives for all the macros defined during the execution of
the preprocessor, including predefined macros. This gives
you a way of finding out what is predefined in your version
of the preprocessor. Assuming you have no file `foo.h', the
command

touch foo.h; cpp -dM foo.h

will show all the predefined macros.

`D'
Like `M' except in two respects: it does _not_ include the
predefined macros, and it outputs _both_ the `#define'
directives and the result of preprocessing. Both kinds of
output go to the standard output file.

`N'
Like `D', but emit only the macro names, not their expansions.

`I'
Output `#include' directives in addition to the result of
preprocessing.

`-P'
Inhibit generation of linemarkers in the output from the
preprocessor. This might be useful when running the preprocessor
on something that is not C code, and will be sent to a program
which might be confused by the linemarkers. *Note Preprocessor
Output::.

`-C'
Do not discard comments. All comments are passed through to the
output file, except for comments in processed directives, which
are deleted along with the directive.

You should be prepared for side effects when using `-C'; it causes
the preprocessor to treat comments as tokens in their own right.
For example, comments appearing at the start of what would be a
directive line have the effect of turning that line into an
ordinary source line, since the first token on the line is no
longer a `#'.

`-CC'
Do not discard comments, including during macro expansion. This is
like `-C', except that comments contained within macros are also
passed through to the output file where the macro is expanded.

In addition to the side-effects of the `-C' option, the `-CC'
option causes all C++-style comments inside a macro to be
converted to C-style comments. This is to prevent later use of
that macro from inadvertently commenting out the remainder of the
source line.

The `-CC' option is generally used to support lint comments.

`-traditional-cpp'
Try to imitate the behavior of old-fashioned C preprocessors, as
opposed to ISO C preprocessors. *Note Traditional Mode::.

`-trigraphs'
Process trigraph sequences. *Note Initial processing::.

`-remap'
Enable special code to work around file systems which only permit
very short file names, such as MS-DOS.

`--help'
`--target-help'
Print text describing all the command line options instead of
preprocessing anything.

`-v'
Verbose mode. Print out GNU CPP's version number at the beginning
of execution, and report the final form of the include path.

`-H'
Print the name of each header file used, in addition to other
normal activities. Each name is indented to show how deep in the
`#include' stack it is. Precompiled header files are also
printed, even if they are found to be invalid; an invalid
precompiled header file is printed with `...x' and a valid one
with `...!' .

`-version'
`--version'
Print out GNU CPP's version number. With one dash, proceed to
preprocess as normal. With two dashes, exit immediately.

File: cpp.info, Node: Environment Variables, Next: GNU Free Documentation License, Prev: Invocation, Up: Top

13 Environment Variables
************************

This section describes the environment variables that affect how CPP
operates. You can use them to specify directories or prefixes to use
when searching for include files, or to control dependency output.

Note that you can also specify places to search using options such as
`-I', and control dependency output with options like `-M' (*note
Invocation::). These take precedence over environment variables, which
in turn take precedence over the configuration of GCC.

`CPATH'
`C_INCLUDE_PATH'
`CPLUS_INCLUDE_PATH'
`OBJC_INCLUDE_PATH'
Each variable's value is a list of directories separated by a
special character, much like `PATH', in which to look for header
files. The special character, `PATH_SEPARATOR', is
target-dependent and determined at GCC build time. For Microsoft
Windows-based targets it is a semicolon, and for almost all other
targets it is a colon.

`CPATH' specifies a list of directories to be searched as if
specified with `-I', but after any paths given with `-I' options
on the command line. This environment variable is used regardless
of which language is being preprocessed.

The remaining environment variables apply only when preprocessing
the particular language indicated. Each specifies a list of
directories to be searched as if specified with `-isystem', but
after any paths given with `-isystem' options on the command line.

In all these variables, an empty element instructs the compiler to
search its current working directory. Empty elements can appear
at the beginning or end of a path. For instance, if the value of
`CPATH' is `:/special/include', that has the same effect as
`-I. -I/special/include'.