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Paul Eggert 1997-03-21 21:05:54 +00:00
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@ -45,7 +45,7 @@ into another language, under the above conditions for modified versions.
@titlepage @titlepage
@c @finalout @c @finalout
@title The C Preprocessor @title The C Preprocessor
@subtitle Last revised July 1992 @subtitle Last revised March 1997
@subtitle for GCC version 2 @subtitle for GCC version 2
@author Richard M. Stallman @author Richard M. Stallman
@page @page
@ -54,7 +54,7 @@ This booklet is eventually intended to form the first chapter of a GNU
C Language manual. C Language manual.
@vskip 0pt plus 1filll @vskip 0pt plus 1filll
Copyright @copyright{} 1987, 1989, 1991, 1992, 1993, 1994, 1995 Free Copyright @copyright{} 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997 Free
Software Foundation, Inc. Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of Permission is granted to make and distribute verbatim copies of
@ -105,7 +105,7 @@ to inform the compiler of where each source line originally came from.
C preprocessors vary in some details. This manual discusses the GNU C C preprocessors vary in some details. This manual discusses the GNU C
preprocessor, the C Compatible Compiler Preprocessor. The GNU C preprocessor, the C Compatible Compiler Preprocessor. The GNU C
preprocessor provides a superset of the features of ANSI Standard C. preprocessor provides a superset of the features of ANSI Standard C@.
ANSI Standard C requires the rejection of many harmless constructs commonly ANSI Standard C requires the rejection of many harmless constructs commonly
used by today's C programs. Such incompatibility would be inconvenient for used by today's C programs. Such incompatibility would be inconvenient for
@ -115,6 +115,12 @@ options @samp{-trigraphs}, @samp{-undef} and @samp{-pedantic}, but in
practice the consequences of having strict ANSI Standard C make it practice the consequences of having strict ANSI Standard C make it
undesirable to do this. @xref{Invocation}. undesirable to do this. @xref{Invocation}.
The C preprocessor is designed for C-like languages; you may run into
problems if you apply it to other kinds of languages, because it assumes
that it is dealing with C@. For example, the C preprocessor sometimes
outputs extra white space to avoid inadvertent C token concatenation,
and this may cause problems with other languages.
@menu @menu
* Global Actions:: Actions made uniformly on all input files. * Global Actions:: Actions made uniformly on all input files.
* Directives:: General syntax of preprocessing directives. * Directives:: General syntax of preprocessing directives.
@ -490,14 +496,14 @@ names, then inheritance is straightforward: simply write @samp{#include
Sometimes it is necessary to give the inheriting file the same name as Sometimes it is necessary to give the inheriting file the same name as
the base file. This is less straightforward. the base file. This is less straightforward.
For example, suppose an application program uses the system header file For example, suppose an application program uses the system header
@file{sys/signal.h}, but the version of @file{/usr/include/sys/signal.h} @file{sys/signal.h}, but the version of @file{/usr/include/sys/signal.h}
on a particular system doesn't do what the application program expects. on a particular system doesn't do what the application program expects.
It might be convenient to define a ``local'' version, perhaps under the It might be convenient to define a ``local'' version, perhaps under the
name @file{/usr/local/include/sys/signal.h}, to override or add to the name @file{/usr/local/include/sys/signal.h}, to override or add to the
one supplied by the system. one supplied by the system.
You can do this by using the option @samp{-I.} for compilation, and You can do this by compiling with the option @samp{-I.}, and
writing a file @file{sys/signal.h} that does what the application writing a file @file{sys/signal.h} that does what the application
program expects. But making this file include the standard program expects. But making this file include the standard
@file{sys/signal.h} is not so easy---writing @samp{#include @file{sys/signal.h} is not so easy---writing @samp{#include
@ -521,8 +527,8 @@ file directories @emph{after} the directory in which the current file
was found. was found.
Suppose you specify @samp{-I /usr/local/include}, and the list of Suppose you specify @samp{-I /usr/local/include}, and the list of
directories to search also includes @file{/usr/include}; and suppose that directories to search also includes @file{/usr/include}; and suppose
both directories contain a file named @file{sys/signal.h}. Ordinary both directories contain @file{sys/signal.h}. Ordinary
@samp{#include <sys/signal.h>} finds the file under @samp{#include <sys/signal.h>} finds the file under
@file{/usr/local/include}. If that file contains @samp{#include_next @file{/usr/local/include}. If that file contains @samp{#include_next
<sys/signal.h>}, it starts searching after that directory, and finds the <sys/signal.h>}, it starts searching after that directory, and finds the
@ -671,7 +677,7 @@ values, as it is defined in many C programs:
@end example @end example
@noindent @noindent
(This is not the best way to define a ``minimum'' macro in GNU C. (This is not the best way to define a ``minimum'' macro in GNU C@.
@xref{Side Effects}, for more information.) @xref{Side Effects}, for more information.)
To use a macro that expects arguments, you write the name of the macro To use a macro that expects arguments, you write the name of the macro
@ -814,7 +820,7 @@ system-specific macros.
@cindex standard predefined macros @cindex standard predefined macros
The standard predefined macros are available with the same meanings The standard predefined macros are available with the same meanings
regardless of the machine or operating system on which you are using GNU C. regardless of the machine or operating system on which you are using GNU C@.
Their names all start and end with double underscores. Those preceding Their names all start and end with double underscores. Those preceding
@code{__GNUC__} in this table are standardized by ANSI C; the rest are @code{__GNUC__} in this table are standardized by ANSI C; the rest are
GNU C extensions. GNU C extensions.
@ -872,7 +878,7 @@ eight characters and looks like @samp{"23:59:01"}.
@item __STDC__ @item __STDC__
@findex __STDC__ @findex __STDC__
This macro expands to the constant 1, to signify that this is ANSI This macro expands to the constant 1, to signify that this is ANSI
Standard C. (Whether that is actually true depends on what C compiler Standard C@. (Whether that is actually true depends on what C compiler
will operate on the output from the preprocessor.) will operate on the output from the preprocessor.)
@item __STDC_VERSION__ @item __STDC_VERSION__
@ -887,7 +893,7 @@ will operate on the output from the preprocessor.
@item __GNUC__ @item __GNUC__
@findex __GNUC__ @findex __GNUC__
This macro is defined if and only if this is GNU C. This macro is This macro is defined if and only if this is GNU C@. This macro is
defined only when the entire GNU C compiler is in use; if you invoke the defined only when the entire GNU C compiler is in use; if you invoke the
preprocessor directly, @samp{__GNUC__} is undefined. The value preprocessor directly, @samp{__GNUC__} is undefined. The value
identifies the major version number of GNU CC (@samp{1} for GNU CC identifies the major version number of GNU CC (@samp{1} for GNU CC
@ -922,7 +928,7 @@ This macro is defined if and only if the @samp{-ansi} switch was
specified when GNU C was invoked. Its definition is the null string. specified when GNU C was invoked. Its definition is the null string.
This macro exists primarily to direct certain GNU header files not to This macro exists primarily to direct certain GNU header files not to
define certain traditional Unix constructs which are incompatible with define certain traditional Unix constructs which are incompatible with
ANSI C. ANSI C@.
@item __BASE_FILE__ @item __BASE_FILE__
@findex __BASE_FILE__ @findex __BASE_FILE__
@ -941,7 +947,7 @@ the nesting level is zero.
@item __VERSION__ @item __VERSION__
@findex __VERSION__ @findex __VERSION__
This macro expands to a string which describes the version number of This macro expands to a string which describes the version number of
GNU C. The string is normally a sequence of decimal numbers separated GNU C@. The string is normally a sequence of decimal numbers separated
by periods, such as @samp{"2.6.0"}. The only reasonable use of this by periods, such as @samp{"2.6.0"}. The only reasonable use of this
macro is to incorporate it into a string constant. macro is to incorporate it into a string constant.
@ -1240,7 +1246,7 @@ possible to piece together a string constant. However, two pieces of text
that don't together form a valid lexical unit cannot be concatenated. For that don't together form a valid lexical unit cannot be concatenated. For
example, concatenation with @samp{x} on one side and @samp{+} on the other example, concatenation with @samp{x} on one side and @samp{+} on the other
is not meaningful because those two characters can't fit together in any is not meaningful because those two characters can't fit together in any
lexical unit of C. The ANSI standard says that such attempts at lexical unit of C@. The ANSI standard says that such attempts at
concatenation are undefined, but in the GNU C preprocessor it is well concatenation are undefined, but in the GNU C preprocessor it is well
defined: it puts the @samp{x} and @samp{+} side by side with no particular defined: it puts the @samp{x} and @samp{+} side by side with no particular
special results. special results.
@ -1440,7 +1446,7 @@ Defining the macro as
@noindent @noindent
provides the desired result. provides the desired result.
However, unintended grouping can result in another way. Consider Unintended grouping can result in another way. Consider
@samp{sizeof ceil_div(1, 2)}. That has the appearance of a C expression @samp{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 that would compute the size of the type of @samp{ceil_div (1, 2)}, but in
fact it means something very different. Here is what it expands to: fact it means something very different. Here is what it expands to:
@ -1961,7 +1967,7 @@ practice because it helps people match the @samp{#endif} to the
corresponding @samp{#if}. Such comments should always be used, except in corresponding @samp{#if}. Such comments should always be used, except in
short conditionals that are not nested. In fact, you can put anything at short conditionals that are not nested. In fact, you can put anything at
all after the @samp{#endif} and it will be ignored by the GNU C preprocessor, all after the @samp{#endif} and it will be ignored by the GNU C preprocessor,
but only comments are acceptable in ANSI Standard C. but only comments are acceptable in ANSI Standard C@.
@var{expression} is a C expression of integer type, subject to stringent @var{expression} is a C expression of integer type, subject to stringent
restrictions. It may contain restrictions. It may contain
@ -2444,8 +2450,8 @@ rather than a line of output containing just a @samp{#}. Supposedly
some old C programs contain such lines. some old C programs contain such lines.
@findex #pragma @findex #pragma
The ANSI standard specifies that the @samp{#pragma} directive has an The ANSI standard specifies that the effect of the @samp{#pragma}
arbitrary, implementation-defined effect. In the GNU C preprocessor, directive is implementation-defined. In the GNU C preprocessor,
@samp{#pragma} directives are not used, except for @samp{#pragma once} @samp{#pragma} directives are not used, except for @samp{#pragma once}
(@pxref{Once-Only}). However, they are left in the preprocessor output, (@pxref{Once-Only}). However, they are left in the preprocessor output,
so they are available to the compilation pass. so they are available to the compilation pass.
@ -2493,7 +2499,7 @@ This indicates returning to a file (after having included another file).
This indicates that the following text comes from a system header file, This indicates that the following text comes from a system header file,
so certain warnings should be suppressed. so certain warnings should be suppressed.
@item 4 @item 4
This indicates that the following text should be treated as C. This indicates that the following text should be treated as C@.
@c maybe cross reference NO_IMPLICIT_EXTERN_C @c maybe cross reference NO_IMPLICIT_EXTERN_C
@end table @end table
@ -2538,7 +2544,7 @@ output before the expansion of the macro call.
@item -traditional @item -traditional
@findex -traditional @findex -traditional
Try to imitate the behavior of old-fashioned C, as opposed to ANSI C. Try to imitate the behavior of old-fashioned C, as opposed to ANSI C@.
@itemize @bullet @itemize @bullet
@item @item
@ -2566,17 +2572,17 @@ It considers @samp{1.0e+4} to be three tokens: @samp{1.0e}, @samp{+},
and @samp{4}. and @samp{4}.
@item @item
A macro is not suppressed within its own definition, in traditional C. A macro is not suppressed within its own definition, in traditional C@.
Thus, any macro that is used recursively inevitably causes an error. Thus, any macro that is used recursively inevitably causes an error.
@item @item
The character @samp{#} has no special meaning within a macro definition The character @samp{#} has no special meaning within a macro definition
in traditional C. in traditional C@.
@item @item
In traditional C, the text at the end of a macro expansion can run 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. together with the text after the macro call, to produce a single token.
(This is impossible in ANSI C.) (This is impossible in ANSI C@.)
@item @item
Traditionally, @samp{\} inside a macro argument suppresses the syntactic Traditionally, @samp{\} inside a macro argument suppresses the syntactic
@ -2629,7 +2635,7 @@ Requests both @samp{-Wtrigraphs} and @samp{-Wcomment} (but not
@item -Wtraditional @item -Wtraditional
@findex -Wtraditional @findex -Wtraditional
Warn about certain constructs that behave differently in traditional and Warn about certain constructs that behave differently in traditional and
ANSI C. ANSI C@.
@item -I @var{directory} @item -I @var{directory}
@findex -I @findex -I