@c @cropmarks
@c @finalout
-@macro copyrightnotice
+@include gcc-common.texi
+
+@copying
@c man begin COPYRIGHT
Copyright @copyright{} 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996,
-1997, 1998, 1999, 2000, 2001
+1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005
Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation. A copy of
-the license is included in the accompanying manual for GNU CC, in the
-section ``GNU Free Documentation License''.
+the license is included in the
@c man end
-@end macro
-
-@c The manpage doesn't have Front-Cover and Back-Cover Texts, but the
-@c complete manual does. -zw
-
+section entitled ``GNU Free Documentation License''.
@ignore
@c man begin COPYRIGHT
-This manual contains no Invariant Sections, and has no Front-Cover Texts
-or Back-Cover Texts.
+man page gfdl(7).
@c man end
@end ignore
-@macro covertexts
+@c man begin COPYRIGHT
This manual contains no Invariant Sections. The Front-Cover Texts are
-(a) (see below), and the the Back-Cover Texts are (b) (see below).
+(a) (see below), and the Back-Cover Texts are (b) (see below).
(a) The FSF's Front-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.
-@end macro
+@c man end
+@end copying
-@c Better form for references to other manuals.
-@ifinfo
-@macro docref{docname}
-GNU \docname\. @ref{Top, \docname\, , \docname\}@c
-@end macro
-@end ifinfo
-@ifnotinfo
-@macro docref{docname}
-GNU \docname\@c
-@end macro
-@end ifnotinfo
+@c Create a separate index for command line options.
+@defcodeindex op
+@syncodeindex vr op
+
+@c Used in cppopts.texi and cppenv.texi.
+@set cppmanual
@ifinfo
@dircategory Programming
@titlepage
@title The C Preprocessor
-@subtitle Last revised April 2001
-@subtitle for GCC version 3
+@subtitle for GCC version @value{version-GCC}
@author Richard M. Stallman
@author Zachary Weinberg
@page
@c There is a fill at the bottom of the page, so we need a filll to
@c override it.
@vskip 0pt plus 1filll
-@copyrightnotice{}
-@covertexts{}
+@insertcopying
@end titlepage
@contents
@page
-@ifinfo
+@ifnottex
@node Top
@top
The C preprocessor implements the macro language used to transform C,
-C++, and Objective C programs before they are compiled. It can also be
+C++, and Objective-C programs before they are compiled. It can also be
useful on its own.
@menu
* Traditional Mode::
* Implementation Details::
* Invocation::
+* Environment Variables::
+* GNU Free Documentation License::
* Index of Directives::
+* Option Index::
* Concept Index::
@detailmenu
Overview
+* Character sets::
* Initial processing::
* Tokenization::
* The preprocessing language::
* Variadic Macros::
* Predefined Macros::
* Undefining and Redefining Macros::
+* Directives Within Macro Arguments::
* Macro Pitfalls::
Predefined Macros
* Assertions::
* Obsolete once-only headers::
-* Miscellaneous obsolete features::
@end detailmenu
@end menu
-@copyrightnotice{}
-@covertexts{}
-@end ifinfo
+@insertcopying
+@end ifnottex
@node Overview
@chapter Overview
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
+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
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. @option{-traditional}
+are not C@. Other Algol-ish programming languages are often safe
+(Pascal, Ada, etc.) So is assembly, with caution. @option{-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.
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 @docref{M4}.
+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
you should use the @option{-std=c89} or @option{-std=c99} options, depending
on which version of the standard you want. To get all the mandatory
diagnostics, you must also use @option{-pedantic}. @xref{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 @ref{Traditional
+Mode}.
+
+For clarity, unless noted otherwise, references to @samp{CPP} in this
+manual refer to GNU CPP@.
@c man end
@menu
+* Character sets::
* Initial processing::
* Tokenization::
* The preprocessing language::
@end menu
+@node Character sets
+@section 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 @dfn{source}
+character set. It must be isomorphic with ISO 10646, also known as
+Unicode. CPP uses the UTF-8 encoding of Unicode.
+
+At present, GNU CPP does not implement conversion from arbitrary file
+encodings to the source character set. Use of any encoding other than
+plain ASCII or UTF-8, except in comments, will cause errors. Use of
+encodings that are not strict supersets of ASCII, such as Shift JIS,
+may cause errors even if non-ASCII characters appear only in comments.
+We plan to fix this in the near future.
+
+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 @option{-E} option, it will be
+in UTF-8.
+
+After preprocessing is complete, string and character constants are
+converted again, into the @dfn{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
+@code{wchar_t} type, in the target machine's byte
+order.@footnote{UTF-16 does not meet the requirements of the C
+standard for a wide character set, but the choice of 16-bit
+@code{wchar_t} is enshrined in some system ABIs so we cannot fix
+this.} Octal and hexadecimal escape sequences do not undergo
+conversion; @t{'\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 @option{-fextended-identifiers} option is used,
+GCC does not permit the use of characters outside the ASCII range, nor
+@samp{\u} and @samp{\U} escapes, in identifiers. Even with that
+option, characters outside the ASCII range can only be specified with
+the @samp{\u} and @samp{\U} escapes, not used directly in identifiers.
+
@node Initial processing
@section Initial processing
The preprocessor performs a series of textual transformations on its
input. These happen before all other processing. Conceptually, they
happen in a rigid order, and the entire file is run through each
-transformation before the next one begins. GNU CPP actually does them
+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.
@enumerate
@item
-@cindex character sets
@cindex line endings
The input file is read into memory and broken into lines.
-GNU CPP expects its input to be a text file, that is, an unstructured
-stream of ASCII characters, with some characters indicating the end of a
-line of text. Extended ASCII character sets, such as ISO Latin-1 or
-Unicode encoded in UTF-8, are also acceptable. Character sets that are
-not strict supersets of seven-bit ASCII will not work. We plan to add
-complete support for international character sets in a future release.
-
Different systems use different conventions to indicate the end of a
line. GCC accepts the ASCII control sequences @kbd{LF}, @kbd{@w{CR
-LF}}, @kbd{CR}, and @kbd{@w{LF CR}} as end-of-line markers. The first
-three are the canonical sequences used by Unix, DOS and VMS, and the
-classic Mac OS (before OSX) respectively. You may therefore safely copy
-source code written on any of those systems to a different one and use
-it without conversion. (GCC may lose track of the current line number
-if a file doesn't consistently use one convention, as sometimes happens
-when it is edited on computers with different conventions that share a
-network file system.) @kbd{@w{LF CR}} is included because it has been
-reported as an end-of-line marker under exotic conditions.
+LF}} and @kbd{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
@item
@cindex trigraphs
-If trigraphs are enabled, they are replaced by their corresponding
-single characters.
+@anchor{trigraphs}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 @option{-std}
+option, or you specify the @option{-trigraphs} option, then it
+converts them.
These are nine three-character sequences, all starting with @samp{??},
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
+obsolete systems that lack some of C's punctuation to use C@. For
example, @samp{??/} stands for @samp{\}, so @t{'??/n'} is a character
-constant for a newline. By default, GCC ignores trigraphs, but if you
-request a strictly conforming mode with the @option{-std} option, then
-it converts them.
+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. If you use the @option{-Wall} or @option{-Wtrigraphs} options,
-GCC will warn you when a trigraph would change the meaning of your
-program if it were converted.
+Trigraphs are not popular and many compilers implement them
+incorrectly. Portable code should not rely on trigraphs being either
+converted or ignored. With @option{-Wtrigraphs} GCC will warn you
+when a trigraph may change the meaning of your program if it were
+converted. @xref{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. @t{"(??\?)"} is the string @samp{(???)}, not
-@samp{(?]}. Traditional C compilers do not recognize this idiom.
+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. @t{"(??\?)"}
+is the string @samp{(???)}, not @samp{(?]}. Traditional C compilers
+do not recognize these idioms.
The nine trigraphs and their replacements are
-@example
+@smallexample
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] @{ @} # \ ^ | ~
-@end example
+@end smallexample
@item
@cindex continued lines
@samp{/*} and continue until the next @samp{*/}. Block comments do not
nest:
-@example
+@smallexample
/* @r{this is} /* @r{one comment} */ @r{text outside comment}
-@end example
+@end smallexample
@dfn{Line comments} begin with @samp{//} 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.
-@example
+@smallexample
// @r{this is} // @r{one comment}
@r{text outside comment}
-@end example
+@end smallexample
+@end enumerate
It is safe to put line comments inside block comments, or vice versa.
-@example
+@smallexample
@group
/* @r{block comment}
// @r{contains line comment}
// @r{line comment} /* @r{contains block comment} */
@end group
-@end example
+@end smallexample
But beware of commenting out one end of a block comment with a line
comment.
-@example
+@smallexample
@group
// @r{l.c.} /* @r{block comment begins}
@r{oops! this isn't a comment anymore} */
@end group
-@end example
+@end smallexample
-Comments are not recognized within string literals. @t{@w{"/* blah
-*/"}} is the string constant @samp{@w{/* blah */}}, not an empty string.
+Comments are not recognized within string literals.
+@t{@w{"/* blah */"}} is the string constant @samp{@w{/* 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.
-@end enumerate
Since these transformations happen before all other processing, you can
split a line mechanically with backslash-newline anywhere. You can
@samp{*/}, and @samp{//} onto multiple lines with backslash-newline.
For example:
-@example
+@smallexample
@group
/\
*
O 10\
20
@end group
-@end example
+@end smallexample
@noindent
-is equivalent to @samp{@w{#define FOO 1020}}. All these tricks are
+is equivalent to @code{@w{#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.
-
-@example
-"foo\\
-bar"
-@end example
-
-@noindent
-is equivalent to @code{"foo\bar"}, not to @code{"foo\\bar"}. To avoid
-having to worry about this, do not use the deprecated GNU extension
-which permits multi-line strings. Instead, use string literal
-concatenation:
-
-@example
- "foo\\"
- "bar"
-@end example
-
-@noindent
-Your program will be more portable this way, too.
+interpreted as a backslash-newline. This cannot affect any correct
+program, however.
@node Tokenization
@section Tokenization
After the textual transformations are finished, the input file is
converted into a sequence of @dfn{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
+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, @samp{a+++++b} is interpreted as
-@samp{@w{a ++ ++ + b}}, not as @samp{@w{a ++ + ++ b}}, even though the
+token. For instance, @code{a+++++b} is interpreted as
+@code{@w{a ++ ++ + b}}, not as @code{@w{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 @samp{##} preprocessing operator is used to glue
+change, except when the @samp{##} preprocessing operator is used to paste
tokens together. @xref{Concatenation}. For example,
-@example
+@smallexample
@group
#define foo() bar
foo()baz
@emph{not}
@expansion{} barbaz
@end group
-@end example
+@end smallexample
The compiler does not re-tokenize the preprocessor's output. Each
preprocessing token becomes one compiler token.
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 @samp{defined}. @xref{Defined}.
+preprocessing keyword is @code{defined}. @xref{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. @xref{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
+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
-@samp{\u} and @samp{\U} escape sequences. GNU C does not presently
-implement either feature in the preprocessor or the compiler.
+@samp{\u} and @samp{\U} escape sequences. The implementation of this
+feature in GCC is experimental; such characters are only accepted in
+the @samp{\u} and @samp{\U} forms and only if
+@option{-fextended-identifiers} is used.
+
+As an extension, GCC treats @samp{$} as a letter. This is for
+compatibility with some systems, such as VMS, where @samp{$} is commonly
+used in system-defined function and object names. @samp{$} is not a
+letter in strictly conforming mode, or if you specify the @option{-$}
+option. @xref{Invocation}.
@cindex numbers
@cindex preprocessing numbers
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
-glued back together with the @samp{##} operator.
+pasted back together with the @samp{##} operator.
It's possible for preprocessing numbers to cause programs to be
-misinterpreted. For example, @samp{0xE+12} is a preprocessing number
+misinterpreted. For example, @code{0xE+12} is a preprocessing number
which does not translate to any valid numeric constant, therefore a
-syntax error. It does not mean @samp{@w{0xE + 12}}, which is what you
+syntax error. It does not mean @code{@w{0xE + 12}}, which is what you
might have intended.
@cindex string literals
header file names (the argument of @samp{#include}).@footnote{The C
standard uses the term @dfn{string literal} to refer only to what we are
calling @dfn{string constants}.} String constants and character
-constants are straightforward: @t{"..."} or @t{'...'}. In either case
-the closing quote may be escaped with a backslash: @t{'\'@:'} is the
-character constant for @samp{'}. 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. @xref{Implementation
-Details}.
-
-Header file names either look like string constants, @t{"..."}, or are
-written with angle brackets instead, @t{<...>}. In either case,
+constants are straightforward: @t{"@dots{}"} or @t{'@dots{}'}. In
+either case embedded quotes should be escaped with a backslash:
+@t{'\'@:'} is the character constant for @samp{'}. 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. @xref{Implementation Details}.
+
+Header file names either look like string constants, @t{"@dots{}"}, or are
+written with angle brackets instead, @t{<@dots{}>}. 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. @xref{Include
Operation}.
-In standard C, no string literal may extend past the end of a line. GNU
-CPP accepts multi-line string constants, but not character constants or
-header file names. This extension is deprecated and will be removed in
-GCC 3.1. You may use continued lines instead, or string constant
-concatenation. @xref{Differences from previous versions}.
+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. @xref{Differences
+from previous versions}.
@cindex punctuators
+@cindex digraphs
+@cindex alternative tokens
@dfn{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 @samp{@@}, @samp{$}, and
@samp{`}. In addition, all the two- and three-character operators are
-punctuators. There are also six @dfn{digraphs}, which are merely
-alternate ways to spell other punctuators. This is a second attempt to
-work around missing punctuation in obsolete systems. It has no negative
-side effects, unlike trigraphs, but does not cover as much ground. The
-digraphs and their corresponding normal punctuators are:
-
-@example
+punctuators. There are also six @dfn{digraphs}, which the C++ standard
+calls @dfn{alternative tokens}, which are merely alternate ways to spell
+other punctuators. This is a second attempt to work around missing
+punctuation in obsolete systems. It has no negative side effects,
+unlike trigraphs, but does not cover as much ground. The digraphs and
+their corresponding normal punctuators are:
+
+@smallexample
Digraph: <% %> <: :> %: %:%:
Punctuator: @{ @} [ ] # ##
-@end example
+@end smallexample
@cindex other tokens
-Any other single character is considered ``other.'' It is passed on to
+Any other single character is considered ``other''. It is passed on to
the preprocessor's output unmolested. The C compiler will almost
certainly reject source code containing ``other'' tokens. In ASCII, the
only other characters are @samp{@@}, @samp{$}, @samp{`}, and control
-characters other than NUL (all bits zero). All characters with the high
-bit set (numeric range 0x7F--0xFF) are also ``other'' in the present
+characters other than NUL (all bits zero). (Note that @samp{$} 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.
+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
text, NUL is considered white space. For example, these two directives
have the same meaning.
-@example
+@smallexample
#define X^@@1
#define X 1
-@end example
+@end smallexample
@noindent
-(where @samp{^@@} is ASCII NUL). Within string or character constants,
+(where @samp{^@@} is ASCII NUL)@. Within string or character constants,
NULs are preserved. In the latter two cases the preprocessor emits a
warning message.
or warnings.
@end itemize
-There are a few more, less useful features.
+There are a few more, less useful, features.
Except for expansion of predefined macros, all these operations are
triggered with @dfn{preprocessing directives}. Preprocessing directives
The @samp{#} which begins a directive cannot come from a macro
expansion. Also, the directive name is not macro expanded. Thus, if
-@samp{foo} is defined as a macro expanding to @samp{define}, that does
+@code{foo} is defined as a macro expanding to @code{define}, that does
not make @samp{#foo} a valid preprocessing directive.
The set of valid directive names is fixed. Programs cannot define new
directories to this list with the @option{-I} option (@pxref{Invocation}).
@item #include "@var{file}"
-This variant is used for header files of your own program. It searches
-for a file named @var{file} first in the directory containing the
-current file, then in the same directories used for @code{<@var{file}>}.
+This variant is used for header files of your own program. It
+searches for a file named @var{file} first in the directory containing
+the current file, then in the quote directories and then the same
+directories used for @code{<@var{file}>}. You can prepend directories
+to the list of quote directories with the @option{-iquote} option.
@end table
The argument of @samp{#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, @samp{@w{#include
+recognized, and macro names are not expanded. Thus, @code{@w{#include
<x/*y>}} specifies inclusion of a system header file named @file{x/*y}.
However, if backslashes occur within @var{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, @samp{@w{#include "x\n\\y"}} specifies a filename containing three
+Thus, @code{@w{#include "x\n\\y"}} specifies a filename containing three
backslashes. (Some systems interpret @samp{\} as a pathname separator.
All of these also interpret @samp{/} the same way. It is most portable
to use only @samp{/}.)
@samp{#include} directive. For example, if you have a header file
@file{header.h} as follows,
-@example
+@smallexample
char *test (void);
-@end example
+@end smallexample
@noindent
and a main program called @file{program.c} that uses the header file,
like this,
-@example
+@smallexample
int x;
#include "header.h"
@{
puts (test ());
@}
-@end example
+@end smallexample
@noindent
the compiler will see the same token stream as it would if
@file{program.c} read
-@example
+@smallexample
int x;
char *test (void);
@{
puts (test ());
@}
-@end example
+@end smallexample
Included files are not limited to declarations and macro definitions;
those are merely the typical uses. Any fragment of a C program can be
included from another file. The include file could even contain the
beginning of a statement that is concluded in the containing file, or
the end of a statement that was started in the including file. However,
-a comment or a string or character constant may not start in the
-included file and finish in the including file. An unterminated
-comment, string constant or character constant in an included file is
-considered to end (with an error message) at the end of the file.
+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
+syntactic units---function declarations or definitions, type
declarations, etc.
The line following the @samp{#include} directive is always treated as a
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 @samp{@w{#include <@var{file}>}} in:
+requested with @code{@w{#include <@var{file}>}} in:
-@example
+@smallexample
/usr/local/include
-/usr/lib/gcc-lib/@var{target}/@var{version}/include
+@var{libdir}/gcc/@var{target}/@var{version}/include
/usr/@var{target}/include
/usr/include
-@end example
+@end smallexample
-For C++ programs, it will also look in @samp{/usr/include/g++-v3},
+For C++ programs, it will also look in @file{/usr/include/g++-v3},
first. In the above, @var{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. @var{version} is the
You can add to this list with the @option{-I@var{dir}} command line
option. All the directories named by @option{-I} are searched, in
-left-to-right order, @emph{before} the default directories. You can
-also prevent GCC from searching any of the default directories with the
-@option{-nostdinc} option. This is useful when you are compiling an
+left-to-right order, @emph{before} the default directories. The only
+exception is when @file{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 @option{-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.
-
-GCC looks for headers requested with @samp{@w{#include "@var{file}"}}
-first in the directory containing the current file, then in the same
-places it would have looked for a header requested with angle brackets.
-For example, if @file{/usr/include/sys/stat.h} contains
-@samp{@w{#include "types.h"}}, GCC looks for @file{types.h} first in
+@option{-I} options are not ignored as described above when
+@option{-nostdinc} is in effect.
+
+GCC looks for headers requested with @code{@w{#include "@var{file}"}}
+first in the directory containing the current file, then in the
+directories as specified by @option{-iquote} options, then in the same
+places it would have looked for a header requested with angle
+brackets. For example, if @file{/usr/include/sys/stat.h} contains
+@code{@w{#include "types.h"}}, GCC looks for @file{types.h} first in
@file{/usr/include/sys}, then in its usual search path.
@samp{#line} (@pxref{Line Control}) does not change GCC's idea of the
quote marks. Directories after @option{-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 @option{-I} switch.
+named by an @option{-I} switch. @option{-I-} is deprecated, @option{-iquote}
+should be used instead.
@option{-I. -I-} is not the same as no @option{-I} options at all, and does
not cause the same behavior for @samp{<>} includes that @samp{""}
@option{-I./-}.
There are several more ways to adjust the header search path. They are
-less generally useful. @xref{Invocation}.
+generally less useful. @xref{Invocation}.
@node Once-Only Headers
@section Once-Only Headers
@cindex wrapper @code{#ifndef}
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
+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:
-@example
+@smallexample
@group
/* File foo. */
#ifndef FILE_FOO_SEEN
@var{the entire file}
-#endif /* FILE_FOO_SEEN */
+#endif /* !FILE_FOO_SEEN */
@end group
-@end example
+@end smallexample
This construct is commonly known as a @dfn{wrapper #ifndef}.
When the header is included again, the conditional will be false,
over the entire contents of the file, and the compiler will not see it
twice.
-GNU CPP optimizes even further. It remembers when a header file has a
+CPP optimizes even further. It remembers when a header file has a
wrapper @samp{#ifndef}. If a subsequent @samp{#include} specifies that
header, and the macro in the @samp{#ifndef} is still defined, it does
not bother to rescan the file at all.
configuration parameters to be used on different sorts of operating
systems, for instance. You could do this with a series of conditionals,
-@example
+@smallexample
#if SYSTEM_1
# include "system_1.h"
#elif SYSTEM_2
#elif SYSTEM_3
@dots{}
#endif
-@end example
+@end smallexample
That rapidly becomes tedious. Instead, the preprocessor offers the
ability to use a macro for the header name. This is called a
@dfn{computed include}. Instead of writing a header name as the direct
argument of @samp{#include}, you simply put a macro name there instead:
-@example
+@smallexample
#define SYSTEM_H "system_1.h"
@dots{}
#include SYSTEM_H
-@end example
+@end smallexample
-@samp{SYSTEM_H} will be expanded, and the preprocessor will look for
+@noindent
+@code{SYSTEM_H} will be expanded, and the preprocessor will look for
@file{system_1.h} as if the @samp{#include} had been written that way
-originally. @samp{SYSTEM_H} could be defined by your Makefile with a
+originally. @code{SYSTEM_H} could be defined by your Makefile with a
@option{-D} option.
You must be careful when you define the macro. @samp{#define} saves
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 reexamine the
+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
-@example
+@smallexample
#define HEADER "a\"b"
#include HEADER
-@end example
+@end smallexample
+@noindent
looks for a file named @file{a\"b}. CPP searches for the file according
to the rules for double-quoted includes.
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 of people reading your program.
+minimize confusion for people reading your program.
@node Wrapper Headers
@section Wrapper Headers
@findex #include_next
Sometimes it is necessary to adjust the contents of a system-provided
-header file without editing it directly. GCC's @samp{fixincludes}
+header file without editing it directly. GCC's @command{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
Headers}), it will recurse infinitely and cause a fatal error.
You could include the old header with an absolute pathname:
-@example
+@smallexample
#include "/usr/include/old-header.h"
-@end example
+@end smallexample
@noindent
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 @samp{#include_next}. It means, ``Include the
-@emph{next} file with this name.'' This directive works like
+@emph{next} file with this name''. This directive works like
@samp{#include} except in searching for the specified file: it starts
searching the list of header file directories @emph{after} the directory
in which the current file was found.
Suppose you specify @option{-I /usr/local/include}, and the list of
directories to search also includes @file{/usr/include}; and suppose
-both directories contain @file{signal.h}. Ordinary @samp{#include
-<signal.h>} finds the file under @file{/usr/local/include}. If that
-file contains @samp{#include_next <signal.h>}, it starts searching
+both directories contain @file{signal.h}. Ordinary @code{@w{#include
+<signal.h>}} finds the file under @file{/usr/local/include}. If that
+file contains @code{@w{#include_next <signal.h>}}, it starts searching
after that directory, and finds the file in @file{/usr/include}.
-@samp{#include_next} does not distinguish between @samp{<@var{file}>}
-and @samp{"@var{file}"} inclusion, nor does it check that the file you
+@samp{#include_next} does not distinguish between @code{<@var{file}>}
+and @code{"@var{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 @samp{#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, only to make global corrections along the lines
-of @samp{fixincludes}.
+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 @command{fixincludes}.
@node System Headers
@section System Headers
The header files declaring interfaces to the operating system and
runtime libraries often cannot be written in strictly conforming C@.
-Therefore, GNU C gives code found in @dfn{system headers} special
+Therefore, GCC gives code found in @dfn{system headers} special
treatment. All warnings, other than those generated by @samp{#warning}
(@pxref{Diagnostics}), are suppressed while GCC is processing a system
header. Macros defined in a system header are immune to a few warnings
The @option{-isystem} command line option adds its argument to the list of
directories to search for headers, just like @option{-I}. Any headers
-found in that directory will be considered system headers. Note that
-unlike @option{-I}, you must put a space between @option{-isystem} and its
-argument.
+found in that directory will be considered system headers.
All directories named by @option{-isystem} are searched @emph{after} all
directories named by @option{-I}, no matter what their order was on the
command line. If the same directory is named by both @option{-I} and
-@option{-isystem}, @option{-I} wins; it is as if the @option{-isystem} option
-had never been specified at all.
+@option{-isystem}, the @option{-I} option is ignored. GCC provides an
+informative message when this occurs if @option{-v} is used.
@findex #pragma GCC system_header
-There is also a directive, @samp{@w{#pragma GCC system_header}}, which
+There is also a directive, @code{@w{#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
-@samp{#pragma} in the file will not be affected. @samp{@w{#pragma GCC
+@samp{#pragma} in the file will not be affected. @code{@w{#pragma GCC
system_header}} has no effect in the primary source file.
On very old systems, some of the pre-defined system header directories
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 @samp{const} from an
+can be useful if you wish to hide a keyword such as @code{const} from an
older compiler that does not understand it. However, the preprocessor
-operator @samp{defined} (@pxref{Defined}) can never be defined as a
+operator @code{defined} (@pxref{Defined}) can never be defined as a
macro, and C++'s named operators (@pxref{C++ Named Operators}) cannot be
macros when you are compiling C++.
* Variadic Macros::
* Predefined Macros::
* Undefining and Redefining Macros::
+* Directives Within Macro Arguments::
* Macro Pitfalls::
@end menu
be an abbreviation for, which is variously referred to as the macro's
@dfn{body}, @dfn{expansion} or @dfn{replacement list}. For example,
-@example
+@smallexample
#define BUFFER_SIZE 1024
-@end example
+@end smallexample
@noindent
-defines a macro named @samp{BUFFER_SIZE} as an abbreviation for the
-token @samp{1024}. If somewhere after this @samp{#define} directive
+defines a macro named @code{BUFFER_SIZE} as an abbreviation for the
+token @code{1024}. If somewhere after this @samp{#define} directive
there comes a C statement of the form
-@example
+@smallexample
foo = (char *) malloc (BUFFER_SIZE);
-@end example
+@end smallexample
@noindent
then the C preprocessor will recognize and @dfn{expand} the macro
-@samp{BUFFER_SIZE}. The C compiler will see the same tokens as it would
+@code{BUFFER_SIZE}. The C compiler will see the same tokens as it would
if you had written
-@example
+@smallexample
foo = (char *) malloc (1024);
-@end example
+@end smallexample
-By convention, macro names are written in upper case. Programs are
+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.
backslash-newline. When the macro is expanded, however, it will all
come out on one line. For example,
-@example
+@smallexample
#define NUMBERS 1, \
2, \
3
int x[] = @{ NUMBERS @};
@expansion{} int x[] = @{ 1, 2, 3 @};
-@end example
+@end smallexample
@noindent
The most common visible consequence of this is surprising line numbers
take effect at the place you write them. Therefore, the following input
to the C preprocessor
-@example
+@smallexample
foo = X;
#define X 4
bar = X;
-@end example
+@end smallexample
@noindent
produces
-@example
+@smallexample
foo = X;
bar = 4;
-@end example
+@end smallexample
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,
-@example
+@smallexample
@group
#define TABLESIZE BUFSIZE
#define BUFSIZE 1024
@expansion{} BUFSIZE
@expansion{} 1024
@end group
-@end example
+@end smallexample
@noindent
-@samp{TABLESIZE} is expanded first to produce @samp{BUFSIZE}, then that
-macro is expanded to produce the final result, @samp{1024}.
+@code{TABLESIZE} is expanded first to produce @code{BUFSIZE}, then that
+macro is expanded to produce the final result, @code{1024}.
-Notice that @samp{BUFSIZE} was not defined when @samp{TABLESIZE} was
-defined. The @samp{#define} for @samp{TABLESIZE} uses exactly the
-expansion you specify --- in this case, @samp{BUFSIZE} --- and does not
+Notice that @code{BUFSIZE} was not defined when @code{TABLESIZE} was
+defined. The @samp{#define} for @code{TABLESIZE} uses exactly the
+expansion you specify---in this case, @code{BUFSIZE}---and does not
check to see whether it too contains macro names. Only when you
-@emph{use} @samp{TABLESIZE} is the result of its expansion scanned for
+@emph{use} @code{TABLESIZE} is the result of its expansion scanned for
more macro names.
-This makes a difference if you change the definition of @samp{BUFSIZE}
-at some point in the source file. @samp{TABLESIZE}, defined as shown,
-will always expand using the definition of @samp{BUFSIZE} that is
+This makes a difference if you change the definition of @code{BUFSIZE}
+at some point in the source file. @code{TABLESIZE}, defined as shown,
+will always expand using the definition of @code{BUFSIZE} that is
currently in effect:
-@example
+@smallexample
#define BUFSIZE 1020
#define TABLESIZE BUFSIZE
#undef BUFSIZE
#define BUFSIZE 37
-@end example
+@end smallexample
@noindent
-Now @samp{TABLESIZE} expands (in two stages) to @samp{37}.
+Now @code{TABLESIZE} expands (in two stages) to @code{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
you use the same @samp{#define} directive, but you put a pair of
parentheses immediately after the macro name. For example,
-@example
+@smallexample
#define lang_init() c_init()
lang_init()
@expansion{} c_init()
-@end example
+@end smallexample
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.
-@example
+@smallexample
extern void foo(void);
-#define foo() /* optimized inline version */
+#define foo() /* @r{optimized inline version} */
@dots{}
foo();
funcptr = foo;
-@end example
+@end smallexample
-Here the call to @samp{foo()} will use the macro, but the function
+Here the call to @code{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.
an object-like macro whose expansion happens to begin with a pair of
parentheses.
-@example
+@smallexample
#define lang_init () c_init()
lang_init()
@expansion{} () c_init()()
-@end example
+@end smallexample
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 @samp{lang_init} is an object-like macro, it does not
+invocation. Since @code{lang_init} is an object-like macro, it does not
consume those parentheses.
@node Macro Arguments
To invoke a macro that takes arguments, you write the name of the macro
followed by a list of @dfn{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
+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
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.
-@example
+@smallexample
#define min(X, Y) ((X) < (Y) ? (X) : (Y))
x = min(a, b); @expansion{} x = ((a) < (b) ? (a) : (b));
y = min(1, 2); @expansion{} y = ((1) < (2) ? (1) : (2));
z = min(a + 28, *p); @expansion{} z = ((a + 28) < (*p) ? (a + 28) : (*p));
-@end example
+@end smallexample
@noindent
(In this small example you can already see several of the dangers of
requirement for square brackets or braces to balance, and they do not
prevent a comma from separating arguments. Thus,
-@example
+@smallexample
macro (array[x = y, x + 1])
-@end example
+@end smallexample
@noindent
-passes two arguments to @code{macro}: @samp{array[x = y} and @samp{x +
-1]}. If you want to supply @samp{array[x = y, x + 1]} as an argument,
-you can write it as @samp{array[(x = y, x + 1)]}, which is equivalent C
+passes two arguments to @code{macro}: @code{array[x = y} and @code{x +
+1]}. If you want to supply @code{array[x = y, x + 1]} as an argument,
+you can write it as @code{array[(x = y, x + 1)]}, which is equivalent C
code.
All arguments to a macro are completely macro-expanded before they are
run into trouble if you try to be too clever, though. @xref{Argument
Prescan}, for detailed discussion.
-For example, @samp{min (min (a, b), c)} is first expanded to
+For example, @code{min (min (a, b), c)} is first expanded to
-@example
+@smallexample
min (((a) < (b) ? (a) : (b)), (c))
-@end example
+@end smallexample
@noindent
and then to
-@example
+@smallexample
@group
((((a) < (b) ? (a) : (b))) < (c)
? (((a) < (b) ? (a) : (b)))
: (c))
@end group
-@end example
+@end smallexample
@noindent
(Line breaks shown here for clarity would not actually be generated.)
@cindex empty macro arguments
-You can leave macro arguments empty; this is not an error for the
+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 @samp{min}:
+Here are some silly examples using @code{min}:
-@example
+@smallexample
min(, b) @expansion{} (( ) < (b) ? ( ) : (b))
min(a, ) @expansion{} ((a ) < ( ) ? (a ) : ( ))
min(,) @expansion{} (( ) < ( ) ? ( ) : ( ))
min() @error{} macro "min" requires 2 arguments, but only 1 given
min(,,) @error{} macro "min" passed 3 arguments, but takes just 2
-@end example
+@end smallexample
Whitespace is not a preprocessing token, so if a macro @code{foo} takes
-one argument, @samp{@w{foo ()}} and @samp{@w{foo ( )}} both supply it an
+one argument, @code{@w{foo ()}} and @code{@w{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 are not replaced by their corresponding actual
-arguments if they appear inside string literals.
+Macro parameters appearing inside string literals are not replaced by
+their corresponding actual arguments.
-@example
+@smallexample
#define foo(x) x, "x"
foo(bar) @expansion{} bar, "x"
-@end example
+@end smallexample
@node Stringification
@section 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 more string constants. The C
+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:
-@example
+@smallexample
@group
#define WARN_IF(EXP) \
do @{ if (EXP) \
@expansion{} do @{ if (x == 0)
fprintf (stderr, "Warning: " "x == 0" "\n"); @} while (0);
@end group
-@end example
+@end smallexample
@noindent
-The argument for @samp{EXP} is substituted once, as-is, into the
-@samp{if} statement, and once, stringified, into the argument to
-@samp{fprintf}. If @samp{x} were a macro, it would be expanded in the
-@samp{if} statement, but not in the string.
-
-The @samp{do} and @samp{while (0)} are a kludge to make it possible to
-write @samp{WARN_IF (@var{arg});}, which the resemblance of
-@samp{WARN_IF} to a function would make C programmers want to do; see
+The argument for @code{EXP} is substituted once, as-is, into the
+@code{if} statement, and once, stringified, into the argument to
+@code{fprintf}. If @code{x} were a macro, it would be expanded in the
+@code{if} statement, but not in the string.
+
+The @code{do} and @code{while (0)} are a kludge to make it possible to
+write @code{WARN_IF (@var{arg});}, which the resemblance of
+@code{WARN_IF} to a function would make C programmers want to do; see
@ref{Swallowing the Semicolon}.
Stringification in C involves more than putting double-quote characters
-around the fragment. The preprocessor backslash-escapes the surrounding
-quotes of string literals, and all backslashes within string and
+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 @samp{@w{p = "foo\n";}} results in
+proper contents. Thus, stringifying @code{@w{p = "foo\n";}} results in
@t{@w{"p = \"foo\\n\";"}}. However, backslashes that are not inside string
or character constants are not duplicated: @samp{\n} by itself
stringifies to @t{"\n"}.
If you want to stringify the result of expansion of a macro argument,
you have to use two levels of macros.
-@example
+@smallexample
#define xstr(s) str(s)
#define str(s) #s
#define foo 4
@expansion{} xstr (4)
@expansion{} str (4)
@expansion{} "4"
-@end example
+@end smallexample
@code{s} is stringified when it is used in @code{str}, so it is not
macro-expanded first. But @code{s} is an ordinary argument to
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 @samp{1.5} and @samp{e3}) into a number.
-Also, multi-character operators such as @samp{+=} can be formed by
+number and a name, such as @code{1.5} and @code{e3}) into a number.
+Also, multi-character operators such as @code{+=} 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 @samp{x} with
-@samp{+} in either order. If you try, the preprocessor issues a warning
-and emits the two tokens as if they had been written next to each other.
-It is common to find unnecessary uses of @samp{##} in complex macros.
-If you get this warning, it is likely that you can simply remove the
-@samp{##}.
+pasted together. For example, you cannot concatenate @code{x} with
+@code{+} 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 @samp{##}
+in complex macros. If you get this warning, it is likely that you can
+simply remove the @samp{##}.
Both the tokens combined by @samp{##} could come from the macro body,
but you could just as well write them as one token in the first place.
needs to be a table of commands, perhaps an array of structures declared
as follows:
-@example
+@smallexample
@group
struct command
@{
@dots{}
@};
@end group
-@end example
+@end smallexample
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
constant can be created with stringification, and the function name by
concatenating the argument with @samp{_command}. Here is how it is done:
-@example
+@smallexample
#define COMMAND(NAME) @{ #NAME, NAME ## _command @}
struct command commands[] =
COMMAND (help),
@dots{}
@};
-@end example
+@end smallexample
@node Variadic Macros
@section Variadic Macros
a function can. The syntax for defining the macro is similar to that of
a function. Here is an example:
-@example
-#define eprintf(...) fprintf (stderr, __VA_ARGS__)
-@end example
+@smallexample
+#define eprintf(@dots{}) fprintf (stderr, __VA_ARGS__)
+@end smallexample
This kind of macro is called @dfn{variadic}. When the macro is invoked,
all the tokens in its argument list after the last named argument (this
@code{@w{__VA_ARGS__}} in the macro body wherever it appears. Thus, we
have this expansion:
-@example
+@smallexample
eprintf ("%s:%d: ", input_file, lineno)
@expansion{} fprintf (stderr, "%s:%d: ", input_file, lineno)
-@end example
+@end smallexample
The variable argument is completely macro-expanded before it is inserted
into the macro expansion, just like an ordinary argument. You may use
below for an important special case for @samp{##}.)
If your macro is complicated, you may want a more descriptive name for
-the variable argument than @code{@w{__VA_ARGS__}}. GNU CPP permits
+the variable argument than @code{@w{__VA_ARGS__}}. CPP permits
this, as an extension. You may write an argument name immediately
before the @samp{@dots{}}; that name is used for the variable argument.
The @code{eprintf} macro above could be written
-@example
-#define eprintf(args...) fprintf (stderr, args)
-@end example
+@smallexample
+#define eprintf(args@dots{}) fprintf (stderr, args)
+@end smallexample
@noindent
-using this extension. You cannot use @code{__VA_ARGS__} and this
+using this extension. You cannot use @code{@w{__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 @code{eprintf} like this, instead:
-@example
-#define eprintf(format, ...) fprintf (stderr, format, __VA_ARGS__)
-@end example
+@smallexample
+#define eprintf(format, @dots{}) fprintf (stderr, format, __VA_ARGS__)
+@end smallexample
+@noindent
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
variable argument empty, you will get a syntax error, because
there will be an extra comma after the format string.
-@example
+@smallexample
eprintf("success!\n", );
@expansion{} fprintf(stderr, "success!\n", );
-@end example
+@end smallexample
-GNU CPP has a pair of extensions which deal with this problem. First,
+GNU CPP has a pair of extensions which deal with this problem. First,
you are allowed to leave the variable argument out entirely:
-@example
+@smallexample
eprintf ("success!\n")
@expansion{} fprintf(stderr, "success!\n", );
-@end example
+@end smallexample
+@noindent
Second, the @samp{##} token paste operator has a special meaning when
placed between a comma and a variable argument. If you write
-@example
-#define eprintf(format, ...) fprintf (stderr, format, ##__VA_ARGS__)
-@end example
+@smallexample
+#define eprintf(format, @dots{}) fprintf (stderr, format, ##__VA_ARGS__)
+@end smallexample
-and the variable argument is left out when the @samp{eprintf} macro is
+@noindent
+and the variable argument is left out when the @code{eprintf} macro is
used, then the comma before the @samp{##} will be deleted. This does
@emph{not} happen if you pass an empty argument, nor does it happen if
the token preceding @samp{##} is anything other than a comma.
-@example
+@smallexample
eprintf ("success!\n")
@expansion{} fprintf(stderr, "success!\n");
-@end example
+@end smallexample
+
+@noindent
+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 @code{@w{__VA_ARGS__}}
can appear is in the replacement list of a variadic macro. It may not
Variadic macros are a new feature in C99. GNU CPP has supported them
for a long time, but only with a named variable argument
-(@samp{args...}, not @samp{...} and @code{@w{__VA_ARGS__}}). If you are
+(@samp{args@dots{}}, not @samp{@dots{}} and @code{@w{__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 @code{@w{__VA_ARGS__}}.
-Previous versions of GNU CPP implemented the comma-deletion extension
+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 @samp{##} must
be a comma, and there must be white space between that comma and
whatever comes immediately before it:
-@example
-#define eprintf(format, args...) fprintf (stderr, format , ##args)
-@end example
+@smallexample
+#define eprintf(format, args@dots{}) fprintf (stderr, format , ##args)
+@end smallexample
@noindent
@xref{Differences from previous versions}, for the gory details.
@subsection Standard Predefined Macros
@cindex standard predefined macros.
-The standard predefined macros are specified by the C and/or C++
+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.
a C string constant. This is the path by which the preprocessor opened
the file, not the short name specified in @samp{#include} or as the
input file name argument. For example,
-@samp{"/usr/local/include/myheader.h"} is a possible expansion of this
+@code{"/usr/local/include/myheader.h"} is a possible expansion of this
macro.
@item __LINE__
new line of source code.
@end table
-@samp{__FILE__} and @samp{__LINE__} are useful in generating an error
+@code{__FILE__} and @code{__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,
-@example
+@smallexample
fprintf (stderr, "Internal error: "
"negative string length "
"%d at %s, line %d.",
length, __FILE__, __LINE__);
-@end example
+@end smallexample
-An @samp{#include} directive changes the expansions of @samp{__FILE__}
-and @samp{__LINE__} to correspond to the included file. At the end of
+An @samp{#include} directive changes the expansions of @code{__FILE__}
+and @code{__LINE__} to correspond to the included file. At the end of
that file, when processing resumes on the input file that contained
-the @samp{#include} directive, the expansions of @samp{__FILE__} and
-@samp{__LINE__} revert to the values they had before the
-@samp{#include} (but @samp{__LINE__} is then incremented by one as
+the @samp{#include} directive, the expansions of @code{__FILE__} and
+@code{__LINE__} revert to the values they had before the
+@samp{#include} (but @code{__LINE__} is then incremented by one as
processing moves to the line after the @samp{#include}).
-A @samp{#line} directive changes @samp{__LINE__}, and may change
-@samp{__FILE__} as well. @xref{Line Control}.
+A @samp{#line} directive changes @code{__LINE__}, and may change
+@code{__FILE__} as well. @xref{Line Control}.
-C99 introduces @samp{__func__}, and GCC has provided @samp{__FUNCTION__}
+C99 introduces @code{__func__}, and GCC has provided @code{__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 @samp{__FILE__} and @samp{__LINE__}, though.
+with @code{__FILE__} and @code{__LINE__}, though.
@table @code
@item __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 @w{@samp{"Feb 12 1996"}}. If the day of the
+characters and looks like @code{@w{"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 @code{__DATE__} will expand to
+@code{@w{"??? ?? ????"}}.
+
@item __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 @samp{"23:59:01"}.
+eight characters and looks like @code{"23:59:01"}.
+
+If GCC cannot determine the current time, it will emit a warning message
+(once per compilation) and @code{__TIME__} will expand to
+@code{"??:??:??"}.
@item __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 GNU CC, this is not necessarily true; however, the
-preprocessor always conforms to the standard, unless the
-@option{-traditional} option is used.
+a compiler other than GCC, this is not necessarily true; however, the
+preprocessor always conforms to the standard unless the
+@option{-traditional-cpp} option is used.
-This macro is not defined if the @option{-traditional} option is used.
+This macro is not defined if the @option{-traditional-cpp} option is used.
On some hosts, the system compiler uses a different convention, where
@code{__STDC__} is normally 0, but is 1 if the user specifies strict
-conformance to the C Standard. GNU CPP follows the host convention when
+conformance to the C Standard. CPP follows the host convention when
processing system header files, but when processing user files
@code{__STDC__} is always 1. This has been reported to cause problems;
for instance, some versions of Solaris provide X Windows headers that
-expect @code{__STDC__} to be either undefined or 1. You may be able to
-work around this sort of problem by using an @option{-I} option to
-cancel treatment of those headers as system headers. @xref{Invocation}.
+expect @code{__STDC__} to be either undefined or 1. @xref{Invocation}.
@item __STDC_VERSION__
This macro expands to the C Standard's version number, a long integer
-constant of the form @samp{@var{yyyy}@var{mm}L} where @var{yyyy} and
+constant of the form @code{@var{yyyy}@var{mm}L} where @var{yyyy} and
@var{mm} are the year and month of the Standard version. This signifies
which version of the C Standard the compiler conforms to. Like
-@samp{__STDC__}, this is not necessarily accurate for the entire
-implementation, unless GNU CPP is being used with GNU CC.
+@code{__STDC__}, this is not necessarily accurate for the entire
+implementation, unless GNU CPP is being used with GCC@.
-The value @samp{199409L} signifies the 1989 C standard as amended in
-1994, which is the current default; the value @samp{199901L} signifies
+The value @code{199409L} signifies the 1989 C standard as amended in
+1994, which is the current default; the value @code{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 @option{-traditional} option is used, nor
-when compiling C++ or Objective C@.
+This macro is not defined if the @option{-traditional-cpp} option is
+used, nor when compiling C++ or Objective-C@.
@item __STDC_HOSTED__
This macro is defined, with value 1, if the compiler's target is a
@item __cplusplus
This macro is defined when the C++ compiler is in use. You can use
-@samp{__cplusplus} to test whether a header is compiled by a C compiler
-or a C++ compiler. This macro is similar to @samp{__STDC_VERSION__}, in
+@code{__cplusplus} to test whether a header is compiled by a C compiler
+or a C++ compiler. This macro is similar to @code{__STDC_VERSION__}, in
that it expands to a version number. A fully conforming implementation
-of the 1998 C++ standard will define this macro to @samp{199711L}. The
-GNU C++ compiler is not yet fully conforming, so it uses @samp{1}
-instead. We hope to complete our implementation in the near future.
+of the 1998 C++ standard will define this macro to @code{199711L}. The
+GNU C++ compiler is not yet fully conforming, so it uses @code{1}
+instead. It is hoped to complete the implementation of standard C++
+in the near future.
+
+@item __OBJC__
+This macro is defined, with value 1, when the Objective-C compiler is in
+use. You can use @code{__OBJC__} to test whether a header is compiled
+by a C compiler or a Objective-C compiler.
+
+@item __ASSEMBLER__
+This macro is defined with value 1 when preprocessing assembly
+language.
@end table
@itemx __GNUC_MINOR__
@itemx __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
+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 @code{__GNUC__} to 3,
-@code{__GNUC_MINOR__} to 2, and @code{__GNUC_PATCHLEVEL__} to 1. They
-are defined only when the entire compiler is in use; if you invoke the
-preprocessor directly, they are not defined.
-
-
+@code{__GNUC_MINOR__} to 2, and @code{__GNUC_PATCHLEVEL__} to 1. These
+macros are also defined if you invoke the preprocessor directly.
@code{__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, you can simply test @code{__GNUC__}. If you need to write code
+by GCC, or a non-GCC compiler that claims to accept the GNU C dialects,
+you can simply test @code{__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. The simplest way to write the
-conditional is like this:
+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:
-@example
+@smallexample
/* @r{Test for GCC > 3.2.0} */
-#if __GNUC__ > 3 || __GNUC_MINOR__ > 2 \
- || __GNUC_PATCHLEVEL__ > 0
-@end example
+#if __GNUC__ > 3 || \
+ (__GNUC__ == 3 && (__GNUC_MINOR__ > 2 || \
+ (__GNUC_MINOR__ == 2 && \
+ __GNUC_PATCHLEVEL__ > 0))
+@end smallexample
@noindent
-This takes advantage of the rule that an undefined macro evaluates to
-zero in an @samp{#if}. The conditional will be false if
-@code{__GNUC__} is not defined (that is, if you are not using @w{GNU CC}).
+Another approach is to use the predefined macros to
+calculate a single number, then compare that against a threshold:
+
+@smallexample
+#define GCC_VERSION (__GNUC__ * 10000 \
+ + __GNUC_MINOR__ * 100 \
+ + __GNUC_PATCHLEVEL__)
+@dots{}
+/* @r{Test for GCC > 3.2.0} */
+#if GCC_VERSION > 30200
+@end smallexample
+
+@noindent
+Many people find this form easier to understand.
@item __GNUG__
The GNU C++ compiler defines this. Testing it is equivalent to
-testing @samp{(__GNUC__ && __cplusplus)}.
+testing @code{@w{(__GNUC__ && __cplusplus)}}.
@item __STRICT_ANSI__
-GNU C defines this macro if and only if the @option{-ansi} switch, or a
+GCC defines this macro if and only if the @option{-ansi} switch, or a
@option{-std} switch specifying strict conformance to some version of ISO C,
-was specified when GNU C was invoked. Its definition is the null string.
+was specified when GCC was invoked. It is defined to @samp{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.
end of every included file. It starts out at 0, it's value within the
base file specified on the command line.
+@item __ELF__
+This macro is defined if the target uses the ELF object format.
+
@item __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
are defined. If they are defined, their value is 1.
@item __CHAR_UNSIGNED__
-GNU C defines this macro if and only if the data type @code{char} is
+GCC defines this macro if and only if the data type @code{char} is
unsigned on the target machine. It exists to cause the standard header
file @file{limits.h} to work correctly. You should not use this macro
yourself; instead, refer to the standard macros defined in @file{limits.h}.
+@item __WCHAR_UNSIGNED__
+Like @code{__CHAR_UNSIGNED__}, this macro is defined if and only if the
+data type @code{wchar_t} is unsigned and the front-end is in C++ mode.
+
@item __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 @samp{m68k-aout} environment it
-expands to nothing, but in the @samp{m68k-coff} environment it expands
+environments. For example, in the @code{m68k-aout} environment it
+expands to nothing, but in the @code{m68k-coff} environment it expands
to a single @samp{%}.
@item __USER_LABEL_PREFIX__
-This macro expands to a single token which is the the prefix applied to
+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 @samp{m68k-aout} environment it expands to an @samp{_}, but in the
-@samp{m68k-coff} environment it expands to nothing.
+the @code{m68k-aout} environment it expands to an @samp{_}, but in the
+@code{m68k-coff} environment it expands to nothing.
This macro will have the correct definition even if
@option{-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
+target-specific options that adjust this prefix are used (e.g.@: the
OSF/rose @option{-mno-underscores} option).
@item __SIZE_TYPE__
@itemx __PTRDIFF_TYPE__
@itemx __WCHAR_TYPE__
@itemx __WINT_TYPE__
+@itemx __INTMAX_TYPE__
+@itemx __UINTMAX_TYPE__
These macros are defined to the correct underlying types for the
-@code{size_t}, @code{ptrdiff_t}, @code{wchar_t}, and @code{wint_t}
+@code{size_t}, @code{ptrdiff_t}, @code{wchar_t}, @code{wint_t},
+@code{intmax_t}, and @code{uintmax_t}
typedefs, respectively. They exist to make the standard header files
@file{stddef.h} and @file{wchar.h} work correctly. You should not use
these macros directly; instead, include the appropriate headers and use
the typedefs.
+@item __CHAR_BIT__
+Defined to the number of bits used in the representation of the
+@code{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.
+
+@item __SCHAR_MAX__
+@itemx __WCHAR_MAX__
+@itemx __SHRT_MAX__
+@itemx __INT_MAX__
+@itemx __LONG_MAX__
+@itemx __LONG_LONG_MAX__
+@itemx __INTMAX_MAX__
+Defined to the maximum value of the @code{signed char}, @code{wchar_t},
+@code{signed short},
+@code{signed int}, @code{signed long}, @code{signed long long}, and
+@code{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.
+
+@item __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 @option{-Wno-deprecated}.
+
+@item __EXCEPTIONS
+This macro is defined, with value 1, when compiling a C++ source file
+with exceptions enabled. If @option{-fno-exceptions} was used when
+compiling the file, then this macro will not be defined.
+
+@item __USING_SJLJ_EXCEPTIONS__
+This macro is defined, with value 1, if the compiler uses the old
+mechanism based on @code{setjmp} and @code{longjmp} for exception
+handling.
+
+@item __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++.
+
+@item __NEXT_RUNTIME__
+This macro is defined, with value 1, if (and only if) the NeXT runtime
+(as in @option{-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.
+
+@item __LP64__
+@itemx _LP64
+These macros are defined, with value 1, if (and only if) the compilation
+is for a target where @code{long int} and pointer both use 64-bits and
+@code{int} uses 32-bit.
+
+@item __SSP__
+This macro is defined, with value 1, when @option{-fstack-protector} is in
+use.
+
+@item __SSP_ALL__
+This macro is defined, with value 2, when @option{-fstack-protector-all} is
+in use.
+
@end table
@node 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 GNU C@. This manual, being for all systems and
+each target supported by GCC@. This manual, being for all systems and
machines, cannot tell you what their names are, but you can use
-@samp{cpp -dM} to see them all. @xref{Invocation}. All system-specific
+@command{cpp -dM} to see them all. @xref{Invocation}. All system-specific
predefined macros expand to the constant 1, so you can test them with
either @samp{#ifdef} or @samp{#if}.
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 @samp{unix} defined on Unix systems. For all such macros, GCC
+to find @code{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 @samp{unix} is defined, @samp{__unix__} will be defined
+and the end. If @code{unix} is defined, @code{__unix__} will be defined
too. There will never be more than two underscores; the parallel of
-@samp{_mips} is @samp{__mips__}.
+@code{_mips} is @code{__mips__}.
When the @option{-ansi} option, or any @option{-std} option that
requests strict conformance, is given to the compiler, all the
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
-@docref{autoconf}.
+@command{autoconf}.
@node C++ Named Operators
@subsection C++ Named Operators
after the macro name. @samp{#undef} has no effect if the name is not a
macro.
-@example
+@smallexample
#define FOO 4
x = FOO; @expansion{} x = 4;
#undef FOO
x = FOO; @expansion{} x = FOO;
-@end example
+@end smallexample
Once a macro has been undefined, that identifier may be @dfn{redefined}
as a macro by a subsequent @samp{#define} directive. The new definition
@noindent
These definitions are effectively the same:
-@example
+@smallexample
#define FOUR (2 + 2)
#define FOUR (2 + 2)
-#define FOUR (2 /* two */ + 2)
-@end example
+#define FOUR (2 /* @r{two} */ + 2)
+@end smallexample
@noindent
but these are not:
-@example
+@smallexample
#define FOUR (2 + 2)
#define FOUR ( 2+2 )
#define FOUR (2 * 2)
#define FOUR(score,and,seven,years,ago) (2 + 2)
-@end example
+@end smallexample
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 compiler
-will only complain if the definitions do not match.
+instance, two different headers to define a common macro. The
+preprocessor will only complain if the definitions do not match.
+
+@node Directives Within Macro Arguments
+@section Directives Within Macro Arguments
+@cindex macro arguments and directives
+
+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 @samp{printf}, only to find that after a library upgrade
+@samp{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:
+
+@smallexample
+#define f(x) x x
+f (1
+#undef f
+#define f 2
+f)
+@end smallexample
+
+@noindent
+which expands to
+
+@smallexample
+1 2 1 2
+@end smallexample
+
+@noindent
+with the semantics described above.
@node Macro Pitfalls
@section 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
-counterintuitive consequences that you must watch out for.
+counter-intuitive consequences that you must watch out for.
@menu
* Misnesting::
a macro call coming partially from the macro body and partially from the
arguments. For example,
-@example
+@smallexample
#define twice(x) (2*(x))
#define call_with_1(x) x(1)
call_with_1 (twice)
@expansion{} twice(1)
@expansion{} (2*(1))
-@end example
+@end smallexample
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,
-@example
+@smallexample
#define strange(file) fprintf (file, "%s %d",
@dots{}
strange(stderr) p, 35)
@expansion{} fprintf (stderr, "%s %d", p, 35)
-@end example
+@end smallexample
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
Suppose you define a macro as follows,
-@example
+@smallexample
#define ceil_div(x, y) (x + y - 1) / y
-@end example
+@end smallexample
@noindent
whose purpose is to divide, rounding up. (One use for this operation is
-to compute how many @samp{int} objects are needed to hold a certain
-number of @samp{char} objects.) Then suppose it is used as follows:
+to compute how many @code{int} objects are needed to hold a certain
+number of @code{char} objects.) Then suppose it is used as follows:
-@example
+@smallexample
a = ceil_div (b & c, sizeof (int));
@expansion{} a = (b & c + sizeof (int) - 1) / sizeof (int);
-@end example
+@end smallexample
@noindent
This does not do what is intended. The operator-precedence rules of
C make it equivalent to this:
-@example
+@smallexample
a = (b & (c + sizeof (int) - 1)) / sizeof (int);
-@end example
+@end smallexample
@noindent
What we want is this:
-@example
+@smallexample
a = ((b & c) + sizeof (int) - 1)) / sizeof (int);
-@end example
+@end smallexample
@noindent
Defining the macro as
-@example
+@smallexample
#define ceil_div(x, y) ((x) + (y) - 1) / (y)
-@end example
+@end smallexample
@noindent
provides the desired result.
-Unintended grouping can result in another way. Consider @samp{sizeof
+Unintended grouping can result in another way. Consider @code{sizeof
ceil_div(1, 2)}. That has the appearance of a C expression that would
-compute the size of the type of @samp{ceil_div (1, 2)}, but in fact it
+compute the size of the type of @code{ceil_div (1, 2)}, but in fact it
means something very different. Here is what it expands to:
-@example
+@smallexample
sizeof ((1) + (2) - 1) / (2)
-@end example
+@end smallexample
@noindent
This would take the size of an integer and divide it by two. The
-precedence rules have put the division outside the @samp{sizeof} when it
+precedence rules have put the division outside the @code{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 @samp{ceil_div}:
+Here, then, is the recommended way to define @code{ceil_div}:
-@example
+@smallexample
#define ceil_div(x, y) (((x) + (y) - 1) / (y))
-@end example
+@end smallexample
@node Swallowing the Semicolon
@subsection 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 @samp{p} says where to find it) across whitespace
+pointer (the argument @code{p} says where to find it) across whitespace
characters:
-@example
+@smallexample
#define SKIP_SPACES(p, limit) \
@{ char *lim = (limit); \
while (p < lim) @{ \
if (*p++ != ' ') @{ \
p--; break; @}@}@}
-@end example
+@end smallexample
@noindent
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 @samp{SKIP_SPACES (p, lim)}. Strictly
+A call to this macro might be @code{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
-@samp{SKIP_SPACES (p, lim);}
+@code{SKIP_SPACES (p, lim);}
-This can cause trouble before @samp{else} statements, because the
+This can cause trouble before @code{else} statements, because the
semicolon is actually a null statement. Suppose you write
-@example
+@smallexample
if (*p != 0)
SKIP_SPACES (p, lim);
else @dots{}
-@end example
+@end smallexample
@noindent
-The presence of two statements --- the compound statement and a null
-statement --- in between the @samp{if} condition and the @samp{else}
+The presence of two statements---the compound statement and a null
+statement---in between the @code{if} condition and the @code{else}
makes invalid C code.
-The definition of the macro @samp{SKIP_SPACES} can be altered to solve
-this problem, using a @samp{do @dots{} while} statement. Here is how:
+The definition of the macro @code{SKIP_SPACES} can be altered to solve
+this problem, using a @code{do @dots{} while} statement. Here is how:
-@example
+@smallexample
#define SKIP_SPACES(p, limit) \
do @{ char *lim = (limit); \
while (p < lim) @{ \
if (*p++ != ' ') @{ \
p--; break; @}@}@} \
while (0)
-@end example
+@end smallexample
-Now @samp{SKIP_SPACES (p, lim);} expands into
+Now @code{SKIP_SPACES (p, lim);} expands into
-@example
+@smallexample
do @{@dots{}@} while (0);
-@end example
+@end smallexample
@noindent
which is one statement. The loop executes exactly once; most compilers
@cindex side effects (in macro arguments)
@cindex unsafe macros
-Many C programs define a macro @samp{min}, for ``minimum'', like this:
+Many C programs define a macro @code{min}, for ``minimum'', like this:
-@example
+@smallexample
#define min(X, Y) ((X) < (Y) ? (X) : (Y))
-@end example
+@end smallexample
When you use this macro with an argument containing a side effect,
as shown here,
-@example
+@smallexample
next = min (x + y, foo (z));
-@end example
+@end smallexample
@noindent
it expands as follows:
-@example
+@smallexample
next = ((x + y) < (foo (z)) ? (x + y) : (foo (z)));
-@end example
+@end smallexample
@noindent
-where @samp{x + y} has been substituted for @samp{X} and @samp{foo (z)}
-for @samp{Y}.
+where @code{x + y} has been substituted for @code{X} and @code{foo (z)}
+for @code{Y}.
-The function @samp{foo} is used only once in the statement as it appears
-in the program, but the expression @samp{foo (z)} has been substituted
-twice into the macro expansion. As a result, @samp{foo} might be called
+The function @code{foo} is used only once in the statement as it appears
+in the program, but the expression @code{foo (z)} has been substituted
+twice into the macro expansion. As a result, @code{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 @samp{min} is an @dfn{unsafe} macro.
+intended. We say that @code{min} is an @dfn{unsafe} macro.
-The best solution to this problem is to define @samp{min} in a way that
-computes the value of @samp{foo (z)} only once. The C language offers
+The best solution to this problem is to define @code{min} in a way that
+computes the value of @code{foo (z)} only once. The C language offers
no standard way to do this, but it can be done with GNU extensions as
follows:
-@example
+@smallexample
#define min(X, Y) \
(@{ typeof (X) x_ = (X); \
typeof (Y) y_ = (Y); \
(x_ < y_) ? x_ : y_; @})
-@end example
+@end smallexample
The @samp{(@{ @dots{} @})} notation produces a compound statement that
acts as an expression. Its value is the value of its last statement.
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 @emph{using} the macro @samp{min}. For example, you can
-calculate the value of @samp{foo (z)}, save it in a variable, and use
-that variable in @samp{min}:
+careful when @emph{using} the macro @code{min}. For example, you can
+calculate the value of @code{foo (z)}, save it in a variable, and use
+that variable in @code{min}:
-@example
+@smallexample
+@group
#define min(X, Y) ((X) < (Y) ? (X) : (Y))
@dots{}
@{
int tem = foo (z);
next = min (x + y, tem);
@}
-@end example
+@end group
+@end smallexample
@noindent
-(where we assume that @samp{foo} returns type @samp{int}).
+(where we assume that @code{foo} returns type @code{int}).
@node Self-Referential Macros
@subsection Self-Referential Macros
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. Let's consider an example:
+the preprocessor output unchanged. Consider an example:
-@example
+@smallexample
#define foo (4 + foo)
-@end example
+@end smallexample
@noindent
-where @samp{foo} is also a variable in your program.
+where @code{foo} is also a variable in your program.
-Following the ordinary rules, each reference to @samp{foo} will expand
-into @samp{(4 + foo)}; then this will be rescanned and will expand into
-@samp{(4 + (4 + foo))}; and so on until the computer runs out of memory.
+Following the ordinary rules, each reference to @code{foo} will expand
+into @code{(4 + foo)}; then this will be rescanned and will expand into
+@code{(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
-@samp{(4 + foo)}. Therefore, this macro definition has the possibly
-useful effect of causing the program to add 4 to the value of @samp{foo}
-wherever @samp{foo} is referred to.
+@code{(4 + foo)}. Therefore, this macro definition has the possibly
+useful effect of causing the program to add 4 to the value of @code{foo}
+wherever @code{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 @samp{foo} is a variable will
+person reading the program who sees that @code{foo} is a variable will
not expect that it is a macro as well. The reader will come across the
-identifier @samp{foo} in the program and think its value should be that
-of the variable @samp{foo}, whereas in fact the value is four greater.
+identifier @code{foo} in the program and think its value should be that
+of the variable @code{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
-@example
+@smallexample
#define EPERM EPERM
-@end example
+@end smallexample
@noindent
-then the macro @code{EPERM} expands to @samp{EPERM}. Effectively, it is
+then the macro @code{EPERM} expands to @code{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 @samp{#ifdef}. You might do this if you
-want to define numeric constants with an @samp{enum}, but have
+want to define numeric constants with an @code{enum}, but have
@samp{#ifdef} be true for each constant.
If a macro @code{x} expands to use a macro @code{y}, and the expansion of
self-reference} of @code{x}. @code{x} is not expanded in this case
either. Thus, if we have
-@example
+@smallexample
#define x (4 + y)
#define y (2 * x)
-@end example
+@end smallexample
@noindent
-then @samp{x} and @samp{y} expand as follows:
+then @code{x} and @code{y} expand as follows:
-@example
+@smallexample
@group
x @expansion{} (4 + y)
@expansion{} (4 + (2 * x))
y @expansion{} (2 * x)
@expansion{} (2 * (4 + y))
@end group
-@end example
+@end smallexample
@noindent
Each macro is expanded when it appears in the definition of the other
Nested calls to a macro.
We say that @dfn{nested} calls to a macro occur when a macro's argument
-contains a call to that very macro. For example, if @samp{f} is a macro
-that expects one argument, @samp{f (f (1))} is a nested pair of calls to
-@samp{f}. The desired expansion is made by expanding @samp{f (1)} and
-substituting that into the definition of @samp{f}. The prescan causes
-the expected result to happen. Without the prescan, @samp{f (1)} itself
-would be substituted as an argument, and the inner use of @samp{f} would
+contains a call to that very macro. For example, if @code{f} is a macro
+that expects one argument, @code{f (f (1))} is a nested pair of calls to
+@code{f}. The desired expansion is made by expanding @code{f (1)} and
+substituting that into the definition of @code{f}. The prescan causes
+the expected result to happen. Without the prescan, @code{f (1)} itself
+would be substituted as an argument, and the inner use of @code{f} would
appear during the main scan as an indirect self-reference and would not
be expanded.
another macro that does the stringification or concatenation. For
instance, if you have
-@example
+@smallexample
#define AFTERX(x) X_ ## x
#define XAFTERX(x) AFTERX(x)
#define TABLESIZE 1024
#define BUFSIZE TABLESIZE
-@end example
+@end smallexample
-then @samp{AFTERX(BUFSIZE)} expands to @samp{X_BUFSIZE}, and
-@samp{XAFTERX(BUFSIZE)} expands to @samp{X_1024}. (Not to
-@samp{X_TABLESIZE}. Prescan always does a complete expansion.)
+then @code{AFTERX(BUFSIZE)} expands to @code{X_BUFSIZE}, and
+@code{XAFTERX(BUFSIZE)} expands to @code{X_1024}. (Not to
+@code{X_TABLESIZE}. Prescan always does a complete expansion.)
@item
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:
-@example
+@smallexample
#define foo a,b
#define bar(x) lose(x)
#define lose(x) (1 + (x))
-@end example
+@end smallexample
-We would like @samp{bar(foo)} to turn into @samp{(1 + (foo))}, which
-would then turn into @samp{(1 + (a,b))}. Instead, @samp{bar(foo)}
-expands into @samp{lose(a,b)}, and you get an error because @code{lose}
+We would like @code{bar(foo)} to turn into @code{(1 + (foo))}, which
+would then turn into @code{(1 + (a,b))}. Instead, @code{bar(foo)}
+expands into @code{lose(a,b)}, and you get an error because @code{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:
-@example
+@smallexample
#define foo (a,b)
@exdent or
#define bar(x) lose((x))
-@end example
+@end smallexample
The extra pair of parentheses prevents the comma in @code{foo}'s
definition from being interpreted as an argument separator.
Here is an example illustrating this:
-@example
+@smallexample
#define ignore_second_arg(a,b,c) a; c
ignore_second_arg (foo (),
ignored (),
syntax error);
-@end example
+@end smallexample
@noindent
-The syntax error triggered by the tokens @samp{syntax error} results in
-an error message citing line three --- the line of ignore_second_arg ---
+The syntax error triggered by the tokens @code{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.
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 @samp{defined} operator.
+simultaneously using the special @code{defined} operator.
-A conditional in the C preprocessor resembles in some ways an @samp{if}
+A conditional in the C preprocessor resembles in some ways an @code{if}
statement in C, but it is important to understand the difference between
-them. The condition in an @samp{if} statement is tested during the
+them. The condition in an @code{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
time of compilation.
However, the distinction is becoming less clear. Modern compilers often
-do test @samp{if} statements when a program is compiled, if their
+do test @code{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 @samp{if}
+you may find that your program is more readable if you use @code{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.
-GNU C version 3 eliminates this kind of never-executed code even when
+GCC version 3 eliminates this kind of never-executed code even when
not optimizing. Older versions did it only when optimizing.
@menu
The simplest sort of conditional is
-@example
+@smallexample
@group
#ifdef @var{MACRO}
#endif /* @var{MACRO} */
@end group
-@end example
+@end smallexample
@cindex conditional group
This block is called a @dfn{conditional group}. @var{controlled text}
Even if a conditional fails, the @var{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
+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.
helps people match the @samp{#endif} to the corresponding @samp{#ifdef}.
Older programs sometimes put @var{MACRO} directly after the
@samp{#endif} without enclosing it in a comment. This is invalid code
-according to the C standard. GNU CPP accepts it with a warning. It
+according to the C standard. CPP accepts it with a warning. It
never affects which @samp{#ifndef} the @samp{#endif} matches.
@findex #ifndef
@file{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 @docref{autoconf}, or done by hand.
+automated by a tool such as @command{autoconf}, or done by hand.
@end itemize
@node If
The @samp{#if} directive allows you to test the value of an arithmetic
expression, rather than the mere existence of one macro. Its syntax is
-@example
+@smallexample
@group
#if @var{expression}
#endif /* @var{expression} */
@end group
-@end example
+@end smallexample
@var{expression} is a C expression of integer type, subject to stringent
restrictions. It may contain
@item
Arithmetic operators for addition, subtraction, multiplication,
division, bitwise operations, shifts, comparisons, and logical
-operations (@samp{&&} and @samp{||}). The latter two obey the usual
-short-circuiting rules of standard C.
+operations (@code{&&} and @code{||}). The latter two obey the usual
+short-circuiting rules of standard C@.
@item
Macros. All macros in the expression are expanded before actual
computation of the expression's value begins.
@item
-Uses of the @samp{defined} operator, which lets you check whether macros
+Uses of the @code{defined} operator, which lets you check whether macros
are defined in the middle of an @samp{#if}.
@item
Identifiers that are not macros, which are all considered to be the
-number zero. This allows you to write @samp{@w{#if MACRO}} instead of
-@samp{@w{#ifdef MACRO}}, if you know that MACRO, when defined, will
+number zero. This allows you to write @code{@w{#if MACRO}} instead of
+@code{@w{#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.
comes out to be nonzero, the @samp{#if} succeeds and the @var{controlled
text} is included; otherwise it is skipped.
-If @var{expression} is not correctly formed, GCC issues an error and
-treats the conditional as having failed.
-
@node Defined
@subsection Defined
@cindex @code{defined}
-The special operator @samp{defined} is used in @samp{#if} and
+The special operator @code{defined} is used in @samp{#if} and
@samp{#elif} expressions to test whether a certain name is defined as a
-macro. @samp{defined @var{name}} and @samp{defined (@var{name})} are
+macro. @code{defined @var{name}} and @code{defined (@var{name})} are
both expressions whose value is 1 if @var{name} is defined as a macro at
-the current point in the program, and 0 otherwise. Thus, @samp{@w{#if
-defined MACRO}} is precisely equivalent to @samp{@w{#ifdef MACRO}}.
+the current point in the program, and 0 otherwise. Thus, @code{@w{#if
+defined MACRO}} is precisely equivalent to @code{@w{#ifdef MACRO}}.
-@samp{defined} is useful when you wish to test more than one macro for
+@code{defined} is useful when you wish to test more than one macro for
existence at once. For example,
-@example
+@smallexample
#if defined (__vax__) || defined (__ns16000__)
-@end example
+@end smallexample
@noindent
-would succeed if either of the names @samp{__vax__} or
-@samp{__ns16000__} is defined as a macro.
+would succeed if either of the names @code{__vax__} or
+@code{__ns16000__} is defined as a macro.
Conditionals written like this:
-@example
+@smallexample
#if defined BUFSIZE && BUFSIZE >= 1024
-@end example
+@end smallexample
@noindent
-can generally be simplified to just @samp{@w{#if BUFSIZE >= 1024}},
-since if @samp{BUFSIZE} is not defined, it will be interpreted as having
+can generally be simplified to just @code{@w{#if BUFSIZE >= 1024}},
+since if @code{BUFSIZE} is not defined, it will be interpreted as having
the value zero.
-If the @samp{defined} operator appears as a result of a macro expansion,
+If the @code{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 @samp{defined} operator and evaluates it normally. It will warn
+genuine @code{defined} operator and evaluates it normally. It will warn
wherever your code uses this feature if you use the command-line option
@option{-pedantic}, since other compilers may handle it differently.
alternative text to be used if the condition fails. This is what it
looks like:
-@example
+@smallexample
+@group
#if @var{expression}
@var{text-if-true}
#else /* Not @var{expression} */
@var{text-if-false}
#endif /* Not @var{expression} */
-@end example
+@end group
+@end smallexample
+@noindent
If @var{expression} is nonzero, the @var{text-if-true} is included and
the @var{text-if-false} is skipped. If @var{expression} is zero, the
opposite happens.
One common case of nested conditionals is used to check for more than two
possible alternatives. For example, you might have
-@example
+@smallexample
#if X == 1
@dots{}
#else /* X != 1 */
@dots{}
#endif /* X != 2 */
#endif /* X != 1 */
-@end example
+@end smallexample
Another conditional directive, @samp{#elif}, allows this to be
abbreviated as follows:
-@example
+@smallexample
#if X == 1
@dots{}
#elif X == 2
#else /* X != 2 and X != 1*/
@dots{}
#endif /* X != 2 and X != 1*/
-@end example
+@end smallexample
@samp{#elif} stands for ``else if''. Like @samp{#else}, it goes in the
middle of a conditional group and subdivides it; it does not require a
syntax errors.
One way to avoid this problem is to use an always-false conditional
-instead. For instance, put @samp{#if 0} before the deleted code and
-@samp{#endif} after it. This This works even if the code being turned
+instead. For instance, put @code{#if 0} before the deleted code and
+@code{#endif} after it. This works even if the code being turned
off contains conditionals, but they must be entire conditionals
(balanced @samp{#if} and @samp{#endif}).
-Some people use @samp{#ifdef notdef} instead. This is risky, because
-@samp{notdef} might be accidentally defined as a macro, and then the
-conditional would succeed. @samp{#if 0} can be counted on to fail.
+Some people use @code{#ifdef notdef} instead. This is risky, because
+@code{notdef} might be accidentally defined as a macro, and then the
+conditional would succeed. @code{#if 0} can be counted on to fail.
-Do not use @samp{#if 0} for comments which are not C code. Use a real
-comment, instead. The interior of @samp{#if 0} must consist of complete
+Do not use @code{#if 0} for comments which are not C code. Use a real
+comment, instead. The interior of @code{#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 @samp{#if 0}. They don't confuse
+apostrophes). These confuse @code{#if 0}. They don't confuse
@samp{/*}.
@node Diagnostics
You would use @samp{#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
+properly on a VAX, you might write
-@example
+@smallexample
@group
#ifdef __vax__
-#error "Won't work on Vaxen. See comments at get_last_object."
+#error "Won't work on VAXen. See comments at get_last_object."
#endif
@end group
-@end example
+@end smallexample
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 @samp{#error}. For example,
-@example
+@smallexample
#if !defined(UNALIGNED_INT_ASM_OP) && defined(DWARF2_DEBUGGING_INFO)
#error "DWARF2_DEBUGGING_INFO requires UNALIGNED_INT_ASM_OP."
#endif
-@end example
+@end smallexample
@findex #warning
The directive @samp{#warning} is like @samp{#error}, but causes 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
-@code{bison} parser generator, you may want to adjust the preprocessor's
+@command{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 @code{bison} are generated from scratch, other parts come
+output from @command{bison} are generated from scratch, other parts come
from a standard parser file. The rest are copied verbatim from
-@code{bison}'s input. You would like compiler error messages and
+@command{bison}'s input. You would like compiler error messages and
symbolic debuggers to be able to refer to @code{bison}'s input file.
@findex #line
-@code{bison} or any such program can arrange this by writing
+@command{bison} or any such program can arrange this by writing
@samp{#line} directives into the output file. @samp{#line} is a
directive that specifies the original line number and source file name
for subsequent input in the current preprocessor input file.
-@samp{#line} has four variants:
+@samp{#line} has three variants:
@table @code
@item #line @var{linenum}
-@var{linenum} is a decimal integer constant. It specifies the line
-number which should be reported for the following line of input.
-Subsequent lines are counted from @var{linenum}.
+@var{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 @var{linenum}.
@item #line @var{linenum} @var{filename}
@var{linenum} is the same as for the first form, and has the same
effect. In addition, @var{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.
+@var{filename} is interpreted according to the normal rules for a string
+constant: backslash escapes are interpreted. This is different from
+@samp{#include}.
-@item #line @var{linenum} @var{filename} @var{flags@dots{}}
-This form is a GCC extension. @var{linenum} and @var{filename} are the
-same as the second form. The @var{flags} are small positive integers,
-which have the same meaning as the flags which can appear at the end of a
-linemarker in the preprocessor's output. @xref{Preprocessor Output}.
+Previous versions of CPP did not interpret escapes in @samp{#line};
+we have changed it because the standard requires they be interpreted,
+and most other compilers do.
@item #line @var{anything else}
@var{anything else} is checked for macro calls, which are expanded.
-The result should match one of the above three forms.
+The result should match one of the above two forms.
@end table
@samp{#line} directives alter the results of the @code{__FILE__} and
@code{__LINE__} predefined macros from that point on. @xref{Standard
Predefined Macros}. They do not have any effect on @samp{#include}'s
-idea of the directory containing the current file.
+idea of the directory containing the current file. This is a change
+from GCC 2.95. Previously, a file reading
+
+@smallexample
+#line 1 "../src/gram.y"
+#include "gram.h"
+@end smallexample
+
+would search for @file{gram.h} in @file{../src}, then the @option{-I}
+chain; the directory containing the physical source file would not be
+searched. In GCC 3.0 and later, the @samp{#include} is not affected by
+the presence of a @samp{#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 @samp{#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
+@option{-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.
@node Pragmas
@chapter Pragmas
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 @samp{__attribute__}, for this purpose. However, GCC
+language, such as @code{__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
-@samp{GCC} prefix. This is in line with the @samp{STDC} prefix on all
+@code{GCC} prefix. This is in line with the @code{STDC} prefix on all
pragmas defined by C99. For backward compatibility, pragmas which were
recognized by previous versions are still recognized without the
-@samp{GCC} prefix, but that usage is deprecated. Some older pragmas are
+@code{GCC} prefix, but that usage is deprecated. Some older pragmas are
deprecated in their entirety. They are not recognized with the
-@samp{GCC} prefix. @xref{Obsolete Features}.
+@code{GCC} prefix. @xref{Obsolete Features}.
@cindex @code{_Pragma}
-C99 introduces the @samp{_Pragma} operator. This feature addresses a
+C99 introduces the @code{@w{_Pragma}} operator. This feature addresses a
major problem with @samp{#pragma}: being a directive, it cannot be
-produced as the result of macro expansion. @samp{_Pragma} is an
-operator, much like @samp{sizeof} or @samp{defined}, and can be embedded
+produced as the result of macro expansion. @code{@w{_Pragma}} is an
+operator, much like @code{sizeof} or @code{defined}, and can be embedded
in a macro.
-The syntax is @code{_Pragma (string-literal)}, where
-@samp{string-literal} can be either a normal or wide-character string
+Its syntax is @code{@w{_Pragma (@var{string-literal})}}, where
+@var{string-literal} can be either a normal or wide-character string
literal. It is destringized, by replacing all @samp{\\} with a single
@samp{\} and all @samp{\"} with a @samp{"}. The result is then
processed as if it had appeared as the right hand side of a
@samp{#pragma} directive. For example,
-@example
+@smallexample
_Pragma ("GCC dependency \"parse.y\"")
-@end example
+@end smallexample
@noindent
-has the same effect as @samp{#pragma GCC dependency "parse.y"}. The
+has the same effect as @code{#pragma GCC dependency "parse.y"}. The
same effect could be achieved using macros, for example
-@example
+@smallexample
#define DO_PRAGMA(x) _Pragma (#x)
DO_PRAGMA (GCC dependency "parse.y")
-@end example
+@end smallexample
-The standard is unclear on where a @samp{_Pragma} operator can appear.
-The preprocessor accepts it even within a preprocessing conditional
+The standard is unclear on where a @code{_Pragma} operator can appear.
+The preprocessor does not accept it within a preprocessing conditional
directive like @samp{#if}. To be safe, you are probably best keeping it
out of directives other than @samp{#define}, and putting it on a line of
its own.
preprocessor itself. Other pragmas are meaningful to the C or C++
compilers. They are documented in the GCC manual.
-@ftable @samp
+@ftable @code
@item #pragma GCC dependency
-@samp{#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
+@code{#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.
-@example
+@smallexample
#pragma GCC dependency "parse.y"
#pragma GCC dependency "/usr/include/time.h" rerun fixincludes
-@end example
+@end smallexample
@item #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 @dfn{poison} the identifier with this pragma.
-@samp{#pragma GCC poison} is followed by a list of identifiers to
+@code{#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,
-@example
+@smallexample
#pragma GCC poison printf sprintf fprintf
sprintf(some_string, "hello");
-@end example
+@end smallexample
@noindent
will produce an error.
For example,
-@example
+@smallexample
#define strrchr rindex
#pragma GCC poison rindex
strrchr(some_string, 'h');
-@end example
+@end smallexample
@noindent
will not produce an error.
@chapter Other Directives
@findex #ident
+@findex #sccs
The @samp{#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 object file. On other systems, the directive is ignored. The
+@samp{#sccs} directive is a synonym for @samp{#ident}.
-This directive is not part of the C standard, but it is not an official
-GNU extension either. We believe it came from System V.
-
-@findex #sccs
-The @samp{#sccs} directive is recognized on some systems, because it
-appears in their header files. It is a very old, obscure, extension
-which we did not invent, and we have been unable to find any
-documentation of what it should do, so GCC simply ignores it.
+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@.
@cindex null directive
The @dfn{null directive} consists of a @samp{#} followed by a newline,
@node Preprocessor Output
@chapter Preprocessor Output
-When the C preprocessor is used with the C, C++, or Objective C
+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
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 the GNU C preprocessor, 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 in the original source file. This is so the output
-is easy to read. @xref{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.
+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.
+@xref{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.
@cindex linemarkers
Source file name and line number information is conveyed by lines
of the form
-@example
+@smallexample
# @var{linenum} @var{filename} @var{flags}
-@end example
+@end smallexample
@noindent
These are called @dfn{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 @var{filename} at line
-@var{linenum}.
+@var{linenum}. @var{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 @samp{1},
@samp{2}, @samp{3}, or @samp{4}. If there are multiple flags, spaces
so certain warnings should be suppressed.
@item 4
This indicates that the following text should be treated as being
-wrapped in an implicit @samp{extern "C"} block.
+wrapped in an implicit @code{extern "C"} block.
@c maybe cross reference NO_IMPLICIT_EXTERN_C
@end table
+As an extension, the preprocessor accepts linemarkers in non-assembler
+input files. They are treated like the corresponding @samp{#line}
+directive, (@pxref{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 @samp{#ident} (always), @samp{#pragma} (only if the
preprocessor does not handle the pragma itself), and @samp{#define} and
Traditional (pre-standard) C preprocessing is rather different from
the preprocessing specified by the standard. When GCC is given the
-@option{-traditional} option, it attempts to emulate a traditional
-preprocessor. We do not guarantee that GCC's behavior under
-@option{-traditional} matches any pre-standard preprocessor exactly.
+@option{-traditional-cpp} option, it attempts to emulate a traditional
+preprocessor.
-Traditional mode exists only for backward compatibility. We have no
-plans to augment it in any way nor will we change it except to fix
-catastrophic bugs. You should be aware that modern C libraries often
-have header files which are incompatible with traditional mode.
+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.
-This is a list of the differences. It may not be complete, and may not
-correspond exactly to the behavior of either GCC or a true traditional
-preprocessor.
+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.
-@itemize @bullet
-@item
-Traditional macro expansion pays no attention to single-quote or
-double-quote characters; macro argument symbols are replaced by the
-argument values even when they appear within apparent string or
-character constants.
+@menu
+* Traditional lexical analysis::
+* Traditional macros::
+* Traditional miscellany::
+* Traditional warnings::
+@end menu
-@item
-Traditionally, it is permissible for a macro expansion to end in the
-middle of a string or character constant. The constant continues into
-the text surrounding the macro call.
+@node Traditional lexical analysis
+@section Traditional lexical analysis
-@item
-However, the end of the line terminates a string or character constant,
-with no error. (This is a kluge. Traditional mode is commonly used to
-preprocess things which are not C, and have a different comment syntax.
-Single apostrophes often appear in comments. This kluge prevents the
-traditional preprocessor from issuing errors on such comments.)
+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.
-@item
-Preprocessing directives are recognized in traditional C only when their
-leading @samp{#} appears in the first column. There can be no
-whitespace between the beginning of the line and the @samp{#}.
+This implementation does not treat trigraphs (@pxref{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.
-@item
-In traditional C, a comment is equivalent to no text at all. (In ISO
-C, a comment counts as whitespace.) It can be used sort of the same way
-that @samp{##} is used in ISO C, to paste macro arguments together.
+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.
-@item
-Traditional C does not have the concept of a preprocessing number.
+Traditional CPP only recognizes C-style block comments, and treats the
+@samp{/*} 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 @samp{<} in a @code{#include}
+directive.
-@item
-A macro is not suppressed within its own definition, in traditional C@.
-Thus, any macro that is used recursively inevitably causes an error.
+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
-@item
-The @samp{#} and @samp{##} operators are not available in traditional
-C@.
+@smallexample
+#if foo/**/bar
+@end smallexample
+
+@noindent
+@samp{foo} and @samp{bar} are distinct identifiers and expanded
+separately if they happen to be macros. In other words, this
+directive is equivalent to
+
+@smallexample
+#if foo bar
+@end smallexample
+
+@noindent
+rather than
+
+@smallexample
+#if foobar
+@end smallexample
+
+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 @code{#define}
+require matching quotes. For example:
+
+@smallexample
+#define m This macro's fine and has an unmatched quote
+"/* This is not a comment. */
+/* @r{This is a comment. The following #include directive
+ is ill-formed.} */
+#include <stdio.h
+@end smallexample
+
+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.
+
+@node Traditional macros
+@section 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 (@pxref{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 @option{-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 @samp{#} and token paste operator
+@samp{##} 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 @emph{ad infinitum}. GCC detects when it is expanding
+recursive macros, emits an error message, and continues after the
+offending macro invocation.
+
+@smallexample
+#define PLUS +
+#define INC(x) PLUS+x
+INC(foo);
+ @expansion{} ++foo;
+@end smallexample
+
+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 @option{-C} option is given. The form of all other horizontal
+whitespace in arguments is preserved, including leading and trailing
+whitespace. In particular
+
+@smallexample
+f( )
+@end smallexample
+
+@noindent
+is treated as an invocation of the macro @samp{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
+
+@smallexample
+#define str(x) "x"
+str(/* @r{A comment} */some text )
+ @expansion{} "some text "
+@end smallexample
+
+@noindent
+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.
+@smallexample
+#define suffix(x) foo_/**/x
+suffix(bar)
+ @expansion{} foo_bar
+@end smallexample
+
+@node Traditional miscellany
+@section Traditional miscellany
+
+Here are some things to be aware of when using the traditional
+preprocessor.
+
+@itemize @bullet
@item
-In traditional C, the text at the end of a macro expansion can run
-together with the text after the macro call, to produce a single token.
-This is impossible in ISO C@.
+Preprocessing directives are recognized only when their leading
+@samp{#} appears in the first column. There can be no whitespace
+between the beginning of the line and the @samp{#}, but whitespace can
+follow the @samp{#}.
@item
-None of the GNU extensions to the preprocessor are available in
-traditional mode, with the exception of assertions, and those may be
-removed in the future.
+A true traditional C preprocessor does not recognize @samp{#error} or
+@samp{#pragma}, and may not recognize @samp{#elif}. CPP supports all
+the directives in traditional mode that it supports in ISO mode,
+including extensions, with the exception that the effects of
+@samp{#pragma GCC poison} are undefined.
@item
-A true traditional C preprocessor does not recognize @samp{#elif},
-@samp{#error}, or @samp{#pragma}. GCC supports @samp{#elif} and
-@samp{#error} even in traditional mode, but not @samp{#pragma}.
+__STDC__ is not defined.
@item
-Traditional mode is text-based, not token-based, and comments are
-stripped after macro expansion. Therefore, @samp{/**/} can be used to
-paste tokens together provided that there is no whitespace between it
-and the tokens to be pasted.
+If you use digraphs the behavior is undefined.
@item
-Traditional mode preserves the amount and form of whitespace provided by
-the user. Hard tabs remain hard tabs. This can be useful, e.g. if you
-are preprocessing a Makefile (which we do not encourage).
+If a line that looks like a directive appears within macro arguments,
+the behavior is undefined.
+
@end itemize
+@node Traditional warnings
+@section Traditional warnings
You can request warnings about features that did not exist, or worked
differently, in traditional C with the @option{-Wtraditional} option.
-This works only if you do @emph{not} specify @option{-traditional}. GCC
-does not warn about features of ISO C which you must use when you are
-using a conforming compiler, such as the @samp{#} and @samp{##}
+GCC does not warn about features of ISO C which you must use when you
+are using a conforming compiler, such as the @samp{#} and @samp{##}
operators.
Presently @option{-Wtraditional} warns about:
understands but would ignore because the @samp{#} does not appear as the
first character on the line. It also suggests you hide directives like
@samp{#pragma} not understood by traditional C by indenting them. Some
-traditional implementations would not recognise @samp{#elif}, so it
+traditional implementations would not recognize @samp{#elif}, so it
suggests avoiding it altogether.
@item
-A function-like macro that appears without an argument list. In
-traditional C this was an error. In ISO C it merely means that the
-macro is not expanded.
+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.
@item
The unary plus operator. This did not exist in traditional C@.
available in traditional C@. (Traditional C does support the @samp{L}
suffix for simple long integer constants.) You are not warned about
uses of these suffixes in macros defined in system headers. For
-instance, @samp{UINT_MAX} may well be defined as @samp{4294967295U}, but
-you will not be warned if you use @samp{UINT_MAX}.
+instance, @code{UINT_MAX} may well be defined as @code{4294967295U}, but
+you will not be warned if you use @code{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
Here we document details of how the preprocessor's implementation
affects its user-visible behavior. You should try to avoid undue
-reliance on behaviour described here, as it is possible that it will
+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 GNU CPP.
+versions of CPP@.
@menu
* Implementation-defined behavior::
@section Implementation-defined behavior
@cindex implementation-defined behavior
-This is how GNU CPP behaves in all the cases which the C standard
+This is how CPP behaves in all the cases which the C standard
describes as @dfn{implementation-defined}. This term means that the
implementation is free to do what it likes, but must document its choice
and stick to it.
@item The mapping of physical source file multi-byte characters to the
execution character set.
-Currently, GNU cpp only supports character sets that are strict supersets
-of ASCII, and performs no translation of characters.
+Currently, CPP requires its input to be ASCII or UTF-8. The execution
+character set may be controlled by the user, with the
+@option{-ftarget-charset} and @option{-ftarget-wide-charset} options.
+
+@item Identifier characters.
+@anchor{Identifier characters}
+
+The C and C++ standards allow identifiers to be composed of @samp{_}
+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
+@option{-fextended-identifiers} is used, because the implementation of
+universal character names in identifiers is experimental.
+
+GCC allows the @samp{$} character in identifiers as an extension for
+most targets. This is true regardless of the @option{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 @samp{$} 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 @option{-fdollars-in-identifiers} or
+@option{fno-dollars-in-identifiers}. @xref{fdollars-in-identifiers}.
@item Non-empty sequences of whitespace characters.
@item The numeric value of character constants in preprocessor expressions.
-The preprocessor and compiler interpret character constants in the same
-way; escape sequences such as @code{\a} are given the values they would
-have on the target machine.
-
-Multi-character character constants are interpreted a character at a
-time, shifting the previous result left by the number of bits per
-character on the host, and adding the new character. For example, 'ab'
-on an 8-bit host would be interpreted as @w{'a' * 256 + 'b'}. If there
-are more characters in the constant than can fit in the widest native
-integer type on the host, usually a @samp{long}, the excess characters
-are ignored and a diagnostic is given.
+The preprocessor and compiler interpret character constants in the
+same way; i.e.@: escape sequences such as @samp{\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 @code{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 @code{int} the
+compiler issues a warning, and the excess leading characters are
+ignored.
+
+For example, @code{'ab'} for a target with an 8-bit @code{char} would be
+interpreted as @w{@samp{(int) ((unsigned char) 'a' * 256 + (unsigned char)
+'b')}}, and @code{'\234a'} as @w{@samp{(int) ((unsigned char) '\234' *
+256 + (unsigned char) 'a')}}.
@item Source file inclusion.
@section Implementation limits
@cindex implementation limits
-GNU CPP has a small number of internal limits. This section lists the
+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 we are aware of. We intend there to be as few limits
+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 to us as a bug. (See the section on reporting bugs in
-the GCC manual.)
+please report that as a bug. @xref{Bugs, , Reporting Bugs, gcc, Using
+the GNU Compiler Collection (GCC)}.
Where we say something is limited @dfn{only by available memory}, that
means that internal data structures impose no intrinsic limit, and space
@item Nesting levels of conditional inclusion.
-The C standard mandates this be at least 63. The GNU C preprocessor
-is limited only by available memory.
+The C standard mandates this be at least 63. CPP is limited only by
+available memory.
-@item Levels of parenthesised expressions within a full expression.
+@item 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.
@item Number of macros simultaneously defined in a single translation unit.
-The standard requires at least 4095 be possible; GNU cpp is limited only
+The standard requires at least 4095 be possible. CPP is limited only
by available memory.
@item Number of parameters in a macro definition and arguments in a macro call.
-We allow @code{USHRT_MAX}, which is no smaller than 65,535. The minimum
+We allow @code{USHRT_MAX}, which is no smaller than 65,535. The minimum
required by the standard is 127.
@item Number of characters on a logical source line.
-The C standard requires a minimum of 4096 be permitted. GNU CPP places
+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.
@node Obsolete Features
@section Obsolete Features
-GNU CPP has a number of features which are present mainly for
+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.
+In some cases, we plan to remove the feature in a future version of GCC@.
@menu
* Assertions::
* Obsolete once-only headers::
-* Miscellaneous obsolete features::
@end menu
@node Assertions
@cindex predicates
An assertion looks like this:
-@example
+@smallexample
#@var{predicate} (@var{answer})
-@end example
+@end smallexample
@noindent
-@var{predicate} must be a single identifier. @var{answer} can be any
+@var{predicate} must be a single identifier. @var{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, @samp{(x + y)} is different from @samp{(x+y)} but
-equivalent to @samp{@w{( x + y )}}. Parentheses do not nest inside an
+redefinition.) Thus, @code{(x + y)} is different from @code{(x+y)} but
+equivalent to @code{@w{( x + y )}}. Parentheses do not nest inside an
answer.
@cindex testing predicates
conditional succeeds if either @code{vax} or @code{ns16000} has been
asserted as an answer for @code{machine}.
-@example
+@smallexample
#if #machine (vax) || #machine (ns16000)
-@end example
+@end smallexample
@noindent
You can test whether @emph{any} answer is asserted for a predicate by
omitting the answer in the conditional:
-@example
+@smallexample
#if #machine
-@end example
+@end smallexample
@findex #assert
Assertions are made with the @samp{#assert} directive. Its sole
argument is the assertion to make, without the leading @samp{#} that
identifies assertions in conditionals.
-@example
+@smallexample
#assert @var{predicate} (@var{answer})
-@end example
+@end smallexample
@noindent
You may make several assertions with the same predicate and different
same predicate. All the answers for any given predicate are
simultaneously true.
-@cindex assertions, cancelling
+@cindex assertions, canceling
@findex #unassert
-Assertions can be cancelled with the the @samp{#unassert} directive. It
+Assertions can be canceled with the @samp{#unassert} directive. It
has the same syntax as @samp{#assert}. In that form it cancels only the
answer which was specified on the @samp{#unassert} line; other answers
for that predicate remain true. You can cancel an entire predicate by
leaving out the answer:
-@example
+@smallexample
#unassert @var{predicate}
-@end example
+@end smallexample
@noindent
In either form, if no such assertion has been made, @samp{#unassert} has
@node Obsolete once-only headers
@subsection Obsolete once-only headers
-GNU C supports two more ways of indicating that a header file should be
+CPP supports two more ways of indicating that a header file should be
read only once. Neither one is as portable as a wrapper @samp{#ifndef},
and we recommend you do not use them in new programs.
@findex #import
-In the Objective C language, there is a variant of @samp{#include}
+In the Objective-C language, there is a variant of @samp{#include}
called @samp{#import} which includes a file, but does so at most once.
If you use @samp{#import} instead of @samp{#include}, then you don't
need the conditionals inside the header file to prevent multiple
inclusion of the contents. GCC permits the use of @samp{#import} in C
-and C++ as well as Objective C. However, it is not in standard C or 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.
@samp{#import} is not a well designed feature. It requires the users of
but it is not recognized by all preprocessors, so you cannot rely on it
in a portable program.
-@node Miscellaneous obsolete features
-@subsection Miscellaneous obsolete features
-
-Here are a few more obsolete features.
-
-@itemize @bullet
-@cindex invalid token paste
-@item Attempting to paste two tokens which together do not form a valid
-preprocessing token.
-
-The preprocessor currently warns about this and outputs the two tokens
-adjacently, which is probably the behavior the programmer intends. It
-may not work in future, though.
-
-Most of the time, when you get this warning, you will find that @samp{##}
-is being used superstitiously, to guard against whitespace appearing
-between two tokens. It is almost always safe to delete the @samp{##}.
-
-@cindex pragma poison
-@item @code{#pragma poison}
-
-This is the same as @samp{#pragma GCC poison}. The version without the
-@code{GCC} prefix is deprecated. @xref{Pragmas}.
-
-@cindex multi-line string constants
-@item Multi-line string constants
-
-GCC currently allows a string constant to extend across multiple logical
-lines of the source file. This extension is deprecated and will be
-removed in a future version of GCC. Such string constants are already
-rejected in all directives apart from @samp{#define}.
-
-Instead, make use of ISO C concatenation of adjacent string literals, or
-use @samp{\n} followed by a backslash-newline.
-
-@end itemize
-
@node Differences from previous versions
@section Differences from previous versions
@cindex differences from previous versions
This section details behavior which has changed from previous versions
-of GNU CPP. We do not plan to change it again in the near future, but
+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
@itemize @bullet
+@item -I- deprecated
+
+This option has been deprecated in 4.0. @option{-iquote} is meant to
+replace the need for this option.
+
@item Order of evaluation of @samp{#} and @samp{##} operators
The standard does not specify the order of evaluation of a chain of
left to right. Older versions evaluated all @samp{#} operators first,
then all @samp{##} operators, in an unreliable order.
-@item The form of whitespace betwen tokens in preprocessor output
+@item The form of whitespace between tokens in preprocessor output
@xref{Preprocessor Output}, for the current textual format. This is
also the format used by stringification. Normally, the preprocessor
@item @samp{##} swallowing preceding text in rest argument macros
Formerly, in a macro expansion, if @samp{##} appeared before a variable
-arguments parameter, and the set of tokens specified for that argument in
-the macro invocation was empty, previous versions of the GNU C
-preprocessor would back up and remove the preceding sequence of
-non-whitespace characters (@strong{not} the preceding token). This
-extension is in direct conflict with the 1999 C standard and has been
-drastically pared back.
+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
+(@strong{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 @samp{##} appears between
a comma and a variable arguments parameter, and the variable argument is
variable argument is empty, or the token before @samp{##} is not a
comma, then @samp{##} behaves as a normal token paste.
-@item Traditional mode and GNU extensions
+@item @samp{#line} and @samp{#include}
+
+The @samp{#line} directive used to change GCC's notion of the
+``directory containing the current file'', used by @samp{#include} with
+a double-quoted header file name. In 3.0 and later, it does not.
+@xref{Line Control}, for further explanation.
+
+@item Syntax of @samp{#line}
+
+In GCC 2.95 and previous, the string constant argument to @samp{#line}
+was treated the same way as the argument to @samp{#include}: backslash
+escapes were not honored, and the string ended at the second @samp{"}.
+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.)
-Traditional mode used to be implemented in the same program as normal
-preprocessing. Therefore, all the GNU extensions to the preprocessor
-were still available in traditional mode. It is now a separate program,
-and does not implement any of the GNU extensions except assertions.
-Even those may be removed in a future release.
@end itemize
@node Invocation
except that the C compiler has different rules for specifying the output
file.
-@strong{Note:} Whether you use the preprocessor by way of @command{gcc}
+@emph{Note:} Whether you use the preprocessor by way of @command{gcc}
or @command{cpp}, the @dfn{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
@ignore
@c man begin SYNOPSIS
-cpp [@option{-D}@var{macro}[=@var{defn}]...] [@option{-U}@var{macro}]
- [@option{-I}@var{dir}...] [@option{-W}@var{warn}...]
+cpp [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}]
+ [@option{-I}@var{dir}@dots{}] [@option{-iquote}@var{dir}@dots{}]
+ [@option{-W}@var{warn}@dots{}]
[@option{-M}|@option{-MM}] [@option{-MG}] [@option{-MF} @var{filename}]
- [@option{-MP}] [@option{-MQ} @var{target}...] [@option{-MT} @var{target}...]
+ [@option{-MP}] [@option{-MQ} @var{target}@dots{}]
+ [@option{-MT} @var{target}@dots{}]
+ [@option{-P}] [@option{-fno-working-directory}]
[@option{-x} @var{language}] [@option{-std=}@var{standard}]
@var{infile} @var{outfile}
Only the most useful options are listed here; see below for the remainder.
@c man end
@c man begin SEEALSO
+gpl(7), gfdl(7), fsf-funding(7),
gcc(1), as(1), ld(1), and the Info entries for @file{cpp}, @file{gcc}, and
@file{binutils}.
@c man end
means to write to standard output. Also, if either file is omitted, it
means the same as if @option{-} had been specified for that file.
-All single-letter options which take an argument may have that argument
-appear immediately after the option letter, or with a space between
-option and argument: @option{-Ifoo} and @option{-I foo} have the same
-effect. Long options that take arguments require a space between option
-and argument.
+Unless otherwise noted, or the option ends in @samp{=}, all options
+which take an argument may have that argument appear either immediately
+after the option, or with a space between option and argument:
+@option{-Ifoo} and @option{-I foo} have the same effect.
-@cindex options
-@table @code
-@item -D @var{name}
-Predefine @var{name} as a macro, with definition @samp{1}.
-
-@item -D @var{name}=@var{definition}
-Predefine @var{name} as a macro, with definition @var{definition}.
-There are no restrictions on the contents of @var{definition}, but 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 use more than
-one @option{-D} for the same @var{name}, the rightmost definition takes
-effect.
+@cindex grouping options
+@cindex options, grouping
+Many options have multi-letter names; therefore multiple single-letter
+options may @emph{not} be grouped: @option{-dM} is very different from
+@w{@samp{-d -M}}.
-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 @command{sh} and @command{csh},
-@option{-D'@var{name}(@var{args...})=@var{definition}'} works.
-
-@item -U @var{name}
-Cancel any previous definition of @var{name}, either built in or
-provided with a @option{-D} option.
-
-All @option{-imacros @var{file}} and @option{-include @var{file}} options
-are processed after all @option{-D} and @option{-U} options.
-
-@item -undef
-Do not predefine any system-specific macros. The common predefined
-macros remain defined.
-
-@item -I @var{dir}
-Add the directory @var{dir} to the list of directories to be searched
-for header files. @xref{Search Path}. Directories named by @option{-I}
-are searched before the standard system include directories.
-
-It is dangerous to specify a standard system include directory in an
-@option{-I} option. This defeats the special treatment of system
-headers (@pxref{System Headers}). It can also defeat the repairs to
-buggy system headers which GCC makes when it is installed.
-
-@item -o @var{file}
-Write output to @var{file}. This is the same as specifying @var{file}
-as the second non-option argument to @command{cpp}. @command{gcc} has a
-different interpretation of a second non-option argument, so you must
-use @option{-o} to specify the output file.
-
-@item -Wall
-Turns on all optional warnings which are desirable for normal code. At
-present this is @option{-Wcomment} and @option{-Wtrigraphs}. Note that
-many of the preprocessor's warnings are on by default and have no
-options to control them.
-
-@item -Wcomment
-@itemx -Wcomments
-Warn whenever a comment-start sequence @samp{/*} appears in a @samp{/*}
-comment, or whenever a backslash-newline appears in a @samp{//} comment.
-(Both forms have the same effect.)
-
-@item -Wtrigraphs
-Warn if any trigraphs are encountered. This option used to take effect
-only if @option{-trigraphs} was also specified, but now works
-independently. Warnings are not given for trigraphs within comments, as
-they do not affect the meaning of the program.
-
-@item -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.
-@xref{Traditional Mode}.
-
-@item -Wimport
-Warn the first time @samp{#import} is used.
-
-@item -Wundef
-Warn whenever an identifier which is not a macro is encountered in an
-@samp{#if} directive, outside of @samp{defined}. Such identifiers are
-replaced with zero.
-
-@item -Werror
-Make all warnings into hard errors. Source code which triggers warnings
-will be rejected.
-
-@item -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.
-
-@item -w
-Suppress all warnings, including those which GNU CPP issues by default.
-
-@item -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.
-
-@item -pedantic-errors
-Issue all the mandatory diagnostics, and make all mandatory diagnostics
-into errors. This includes mandatory diagnostics that GCC issues
-without @samp{-pedantic} but treats as warnings.
-
-@item -M
-Instead of outputting the result of preprocessing, output a rule
-suitable for @code{make} describing the dependencies of the main source
-file. The preprocessor outputs one @code{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 @option{-include} or
-@option{-imacros} command line options.
-
-Unless specified explicitly (with @option{-MT} or @option{-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 @samp{\}-newline.
-The rule has no commands.
-
-@item -MM
-Like @option{-M}, but mention only the files included with @samp{#include
-"@var{file}"} or with @option{-include} or @option{-imacros} command line
-options. System header files included with @samp{#include <@var{file}>}
-are omitted.
-
-@item -MF @var{file}
-When used with @option{-M} or @option{-MM}, specifies a file to write the
-dependencies to. This allows the preprocessor to write the preprocessed
-file to stdout normally. If no @option{-MF} switch is given, CPP sends
-the rules to stdout and suppresses normal preprocessed output.
-
-@item -MG
-When used with @option{-M} or @option{-MM}, @option{-MG} says to treat missing
-header files as generated files and assume they live in the same
-directory as the source file. It suppresses preprocessed output, as a
-missing header file is ordinarily an error.
-
-This feature is used in automatic updating of makefiles.
-
-@item -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 @code{make} gives if you remove header
-files without updating the @code{Makefile} to match.
-
-This is typical output:
-
-@example
-test.o: test.c test.h
-
-test.h:
-@end example
-
-@item -MT @var{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 @samp{.c}, and appends the platform's
-usual object suffix. The result is the target.
-
-An @option{-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 @option{-MT}, or use multiple @option{-MT} options.
-
-For example, @samp{@w{-MT '$(objpfx)foo.o'}} might give
-
-@example
-$(objpfx)foo.o: foo.c
-@end example
-
-@item -MQ @var{target}
-
-Same as @option{-MT}, but it quotes any characters which are special to
-Make. @samp{@w{-MQ '$(objpfx)foo.o'}} gives
-
-@example
-$$(objpfx)foo.o: foo.c
-@end example
-
-The default target is automatically quoted, as if it were given with
-@option{-MQ}.
-
-@item -MD @var{file}
-@itemx -MMD @var{file}
-@option{-MD @var{file}} is equivalent to @option{-M -MF @var{file}}, and
-@option{-MMD @var{file}} is equivalent to @option{-MM -MF @var{file}}.
-
-Due to limitations in the compiler driver, you must use these switches
-when you want to generate a dependency file as a side-effect of normal
-compilation.
-
-@item -x c
-@itemx -x c++
-@itemx -x objective-c
-@itemx -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:
-@samp{.c}, @samp{.cc}, @samp{.m}, or @samp{.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.
-
-@strong{Note:} Previous versions of cpp accepted a @option{-lang} option
-which selected both the language and the standards conformance level.
-This option has been removed, because it conflicts with the @option{-l}
-option.
-
-@item -std=@var{standard}
-@itemx -ansi
-Specify the standard to which the code should conform. Currently cpp
-only knows about the standards for C; other language standards will be
-added in the future.
-
-@var{standard}
-may be one of:
-@table @code
-@item iso9899:1990
-@itemx c89
-The ISO C standard from 1990. @samp{c89} is the customary shorthand for
-this version of the standard.
-
-The @option{-ansi} option is equivalent to @option{-std=c89}.
-
-@item iso9899:199409
-The 1990 C standard, as amended in 1994.
-
-@item iso9899:1999
-@itemx c99
-@itemx iso9899:199x
-@itemx c9x
-The revised ISO C standard, published in December 1999. Before
-publication, this was known as C9X.
+@cindex options
+@include cppopts.texi
+@c man end
-@item gnu89
-The 1990 C standard plus GNU extensions. This is the default.
+@node Environment Variables
+@chapter Environment Variables
+@cindex environment variables
+@c man begin ENVIRONMENT
-@item gnu99
-@itemx gnu9x
-The 1999 C standard plus GNU extensions.
-@end table
+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.
-@item -I-
-Split the include path. Any directories specified with @option{-I}
-options before @option{-I-} are searched only for headers requested with
-@samp{@w{#include "@var{file}"}}; they are not searched for
-@samp{@w{#include <@var{file}>}}. If additional directories are
-specified with @option{-I} options after the @option{-I-}, those
-directories are searched for all @samp{#include} directives.
-
-In addition, @option{-I-} inhibits the use of the directory of the current
-file directory as the first search directory for @samp{@w{#include
-"@var{file}"}}. @xref{Search Path}.
-
-@item -nostdinc
-Do not search the standard system directories for header files.
-Only the directories you have specified with @option{-I} options
-(and the directory of the current file, if appropriate) are searched.
-
-@item -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.)
-
-@item -include @var{file}
-
-Process @var{file} as if @code{#include "file"} appeared as the first
-line of the primary source file. However, the first directory searched
-for @var{file} is the preprocessor's working directory @emph{instead of}
-the directory containing the main source file. If not found there, it
-is searched for in the remainder of the @code{#include "..."} search
-chain as normal.
-
-If multiple @option{-include} options are given, the files are included
-in the order they appear on the command line.
-
-@item -imacros @var{file}
-
-Exactly like @option{-include}, except that any output produced by
-scanning @var{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 @option{-imacros} are processed before all files
-specified by @option{-include}.
-
-@item -idirafter @var{dir}
-Search @var{dir} for header files, but do it @emph{after} all
-directories specified with @option{-I} and the standard system directories
-have been exhausted. @var{dir} is treated as a system include directory.
-
-@item -iprefix @var{prefix}
-Specify @var{prefix} as the prefix for subsequent @option{-iwithprefix}
-options. If the prefix represents a directory, you should include the
-final @samp{/}.
-
-@item -iwithprefix @var{dir}
-@itemx -iwithprefixbefore @var{dir}
-
-Append @var{dir} to the prefix specified previously with
-@option{-iprefix}, and add the resulting directory to the include search
-path. @option{-iwithprefixbefore} puts it in the same place @option{-I}
-would; @option{-iwithprefix} puts it where @option{-idirafter} would.
-
-Use of these options is discouraged.
-
-@item -isystem @var{dir}
-Search @var{dir} for header files, after all directories specified by
-@option{-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. @xref{System Headers}.
-
-@item -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.
-In this mode the integrated preprocessor is little more than a tokenizer
-for the front ends.
-
-@samp{-fpreprocessed} is implicit if the input file has one of the
-extensions @samp{.i}, @samp{.ii} or @samp{.mi}. These are the
-extensions that GCC uses for preprocessed files created by
-@option{-save-temps}.
-
-@item -ftabstop=@var{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.
-
-@item -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 @code{dejagnu}.
-
-@item -A @var{predicate}=@var{answer}
-Make an assertion with the predicate @var{predicate} and answer
-@var{answer}. This form is preferred to the older form @option{-A
-@var{predicate}(@var{answer})}, which is still supported, because
-it does not use shell special characters. @xref{Assertions}.
-
-@item -A -@var{predicate}=@var{answer}
-Cancel an assertion with the predicate @var{predicate} and answer
-@var{answer}.
-
-@item -A-
-Cancel all predefined assertions and all assertions preceding it on
-the command line. Also, undefine all predefined macros and all
-macros preceding it on the command line. (This is a historical wart and
-may change in the future.)
-
-@item -dM
-Instead of the normal output, generate a list of @samp{#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 @samp{foo.h}, the command
-
-@example
-touch foo.h; cpp -dM foo.h
-@end example
+Note that you can also specify places to search using options such as
+@option{-I}, and control dependency output with options like
+@option{-M} (@pxref{Invocation}). These take precedence over
+environment variables, which in turn take precedence over the
+configuration of GCC@.
-@noindent
-will show all the predefined macros.
-
-@item -dD
-Like @option{-dM} except in two respects: it does @emph{not} include the
-predefined macros, and it outputs @emph{both} the @samp{#define}
-directives and the result of preprocessing. Both kinds of output go to
-the standard output file.
-
-@item -dN
-Like @option{-dD}, but emit only the macro names, not their expansions.
-
-@item -dI
-Output @samp{#include} directives in addition to the result of
-preprocessing.
-
-@item -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. @xref{Preprocessor Output}.
-
-@item -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. Comments appearing in the expansion list of a
-macro will be preserved, and appear in place wherever the macro is
-invoked.
-
-You should be prepared for side effects when using @option{-C}; it causes
-the preprocessor to treat comments as tokens in their own right. For
-example, macro redefinitions that were trivial when comments were
-replaced by a single space might become significant when comments are
-retained. Also, 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 @samp{#}.
-
-@item -gcc
-Define the macros @sc{__gnuc__}, @sc{__gnuc_minor__} and
-@sc{__gnuc_patchlevel__}. These are defined automatically when you use
-@samp{gcc -E}; you can turn them off in that case with @option{-no-gcc}.
-
-@item -traditional
-Try to imitate the behavior of old-fashioned C, as opposed to ISO
-C@. @xref{Traditional Mode}.
-
-@item -trigraphs
-Process trigraph sequences. @xref{Initial processing}.
-
-@item -remap
-Enable special code to work around file systems which only permit very
-short file names, such as MS-DOS.
-
-@item -$
-Forbid the use of @samp{$} in identifiers. The C standard allows
-implementations to define extra characters that can appear in
-identifiers. By default the GNU C preprocessor permits @samp{$}, a
-common extension.
-
-@item -h
-@itemx --help
-@itemx --target-help
-Print text describing all the command line options instead of
-preprocessing anything.
-
-@item -v
-Verbose mode. Print out GNU CPP's version number at the beginning of
-execution, and report the final form of the include path.
-
-@item -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
-@samp{#include} stack it is.
-
-@item -version
-@itemx --version
-Print out GNU CPP's version number. With one dash, proceed to
-preprocess as normal. With two dashes, exit immediately.
-@end table
+@include cppenv.texi
@c man end
@page
+@include fdl.texi
+
+@page
@node Index of Directives
@unnumbered Index of Directives
@printindex fn
+@node Option Index
+@unnumbered Option Index
+@noindent
+CPP's command line options and environment variables are indexed here
+without any initial @samp{-} or @samp{--}.
+@printindex op
+
+@page
@node Concept Index
@unnumbered Concept Index
@printindex cp
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