The C preprocessor is a macro processor that is used
automatically by the C compiler to transform your program before
actual compilation. It is called a macro processor because it
allows you to define macros which are brief abbreviations
for longer constructs.
The C preprocessor provides four separate facilities that you
can use as you see fit:
Inclusion of header files. These are files of declarations that
can be substituted into your program.
Macro expansion. You can define macros which are
abbreviations for arbitrary fragments of C code, and then the C
preprocessor will replace the macros with their definitions
throughout the program.
Conditional compilation. Using special preprocessing
directives, you can include or exclude parts of the program
according to various conditions.
Line control. If you use a program to combine or rearrange
source files into an intermediate file which is then compiled, you
can use line control to inform the compiler of where each source
line originally came from.
C preprocessors vary in some details. For a full explanation of
the GNU C preprocessor, see the info(1) file
cpp.info, or the manual The C Preprocessor Both of
these are built from the same documentation source file,
cpp.texinfo. The GNU C preprocessor provides a superset of
the features of ANSI Standard C.
ANSI Standard C requires the rejection of many harmless
constructs commonly used by today's C programs. Such
incompatibility would be inconvenient for users, so the GNU C
preprocessor is configured to accept these constructs by default.
Strictly speaking, to get ANSI Standard C, you must use the options
-trigraphs, -undef and -pedantic, but in
practice the consequences of having strict ANSI Standard C make it
undesirable to do this.
Most often when you use the C preprocessor you will not have to
invoke it explicitly: the C compiler will do so automatically.
However, the preprocessor is sometimes useful individually.
When you call the preprocessor individually, either name
(cpp(1) or
cccp(1)) will do--they are completely synonymous.
The C preprocessor expects two file names as arguments,
infile and outfile. The preprocessor reads
infile together with any other files it specifies with
#include. All the output generated by the combined input
files is written in outfile.
Either infile or outfile may be -, which as
infile means to read from standard input and as
outfile means to write to standard output. Also, if
outfile or both file names are omitted, the standard output
and standard input are used for the omitted file names.
Here is a table of command options accepted by the C
preprocessor. These options can also be given when compiling a C
program; they are passed along automatically to the preprocessor
when it is invoked by the compiler.
-P
Inhibit generation of # with line-number information 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 #
-C
Do not discard comments: pass them through to the output file.
Comments appearing in arguments of a macro call will be copied to
the output before the expansion of the macro call.
-traditional
Try to imitate the behavior of old-fashioned C, as opposed to
ANSI C.
-trigraphs
Process standard ANSI trigraph sequences. These are
three-character sequences, all starting with ?? that are
defined by ANSI C to stand for single characters. For example,
??/ stands for \, so '??/n' is a character
constant for a newline. Strictly speaking, the GNU C preprocessor
does not support all programs in ANSI Standard C unless
-trigraphs is used, but if you ever notice the difference it
will be with relief. You don't want to know any more about
trigraphs.
-pedantic
Issue warnings required by the ANSI C standard in certain cases
such as when text other than a comment follows #else or
#endif.
-pedantic-errors
Like -pedantic, except that errors are produced rather
than warnings.
-Wtrigraphs
Warn if any trigraphs are encountered (assuming they are
enabled).
-Wcomment, -Wcomments
Warn whenever a comment-start sequence /* appears in a
comment. (Both forms have the same effect).
-Wall
Requests both -Wtrigraphs and -Wcomment (but not
--Wtraditional).
-Wtraditional
Warn about certain constructs that behave differently in
traditional and ANSI C.
-Idirectory
Add the directory directory to the end of the list of
directories to be searched for header files. This can be used to
override a system header file, substituting your own version, since
these directories are searched before the system header file
directories. If you use more than one -I option, the
directories are scanned in left-to-right order; the standard system
directories come after.
-I-
Any directories specified with -I options before the
-I- option are searched only for the case of #includefile they are not searched for #include<file>.
If additional directories are specified with -I options
after the -I-, these directories are searched for all
#include directives.
In addition, the -I- option inhibits the use of the
current directory as the first search directory for #include
(dqfile". Therefore, the current directory is
searched only if it is requested explicitly with -I.
Specifying both -I- and -I. allows you to control
precisely which directories are searched before the current one and
which are searched after.
-nostdinc
Do not search the standard system directories for header files.
Only the directories you have specified with -I options (and
the current directory, if appropriate) are searched.
-nostdinc++
Do not search for header files in the C++ specific standard
directories, but do still search the other standard directories.
(This option is used when building libg++.)
-Dname
Predefine name as a macro, with definition 1
-Dname=definition
Predefine name as a macro, with definition
definition There are no restrictions on the contents of
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 -D for the same
name the rightmost definition takes effect.
-Uname
Do not predefine name If both -U and -D
are specified for one name, the -U beats the -D and
the name is not predefined.
-undef
Do not predefine any nonstandard macros.
-Aname(value)
Assert (in the same way as the #assert directive) the
predicate name with tokenlist value Remember to
escape or quote the parentheses on shell command lines.
You can use -A- to disable all predefined assertions; it
also undefines all predefined macros.
-dM
Instead of outputting the result of preprocessing, output a
list of #define directives for all the macros defined during
the execution of the preprocessor, including predefined macros.
This gives you a way of finding out what is predefined in your
version of the preprocessor; assuming you have no file foo.h
the command
touch foo.h; cpp -dM foo.h
will show the values of any predefined macros.
-dD
Like -dM except in two respects: it does not
include the predefined macros, and it outputs both the
#define directives and the result of preprocessing. Both
kinds of output go to the standard output file.
-M
-M [-MG]
Instead of outputting the result of preprocessing, output a
rule suitable for make(1) describing the dependencies of the
main source file. The preprocessor outputs one make(1) rule
containing the object file name for that source file, a colon, and
the names of all the included files. If there are many included
files then the rule is split into several lines using \
-MG says to treat missing header files as generated files
and assume they live in the same directory as the source file. It
must be specified in addition to -M.
This feature is used in automatic updating of
makefiles.
-MM[-MG]
Like -M but mention only the files included with
#include "file". System header files included with
#include <file> are omitted.
-MDfile
Like -M, but the dependency information is written to
file This is in addition to compiling the file as
specified---MD does not inhibit ordinary compilation the way
-M does.
When invoking gcc, do not specify the file argument. Gcc
will create file names made by replacing .c with .d
at the end of the input file names.
In Mach, you can use the utility md to merge multiple
files into a single dependency file suitable for using with the
make(1) command.
-MMDfile
Like -MD except mention only user header files, not
system header files.
-H
Print the name of each header file used, in addition to other
normal activities.
-imacrosfile
Process file as input, discarding the resulting output,
before processing the regular input file. Because the output
generated from file is discarded, the only effect of
-imacrosfile is to make the macros defined in
file available for use in the main input. The preprocessor
evaluates any -D and -U options on the command line
before processing -imacrosfile
-includefile
Process file as input, and include all the resulting
output, before processing the regular input file.
-idirafterdir
Add the directory dir to the second include path. The
directories on the second include path are searched when a header
file is not found in any of the directories in the main include
path (the one that -I adds to).
-iprefixprefix
Specify prefix as the prefix for subsequent
-iwithprefix options.
-iwithprefixdir
Add a directory to the second include path. The directory's
name is made by concatenating prefix and dir where
prefix was specified previously with -iprefix.
-lang-c, -lang-c++, -lang-objc, -lang-objc++
Specify the source language. -lang-c++ makes the
preprocessor handle C++ comment syntax, and includes extra default
include directories for C++, and -lang-objc enables the
Objective C #import directive. -lang-c explicitly
turns off both of these extensions, and -lang-objc++ enables
both.
These options are generated by the compiler driver
gcc(1), but not passed from the gcc(1) command
line.
-lint
Look for commands to the program checker lint(1)
embedded in comments, and emit them preceded by #pragma lint
For example, the comment /* NOTREACHED */ becomes #pragma
lint NOTREACHED
This option is available only when you call cpp(1)
directly; gcc(1) will not pass it from its command
line.
-$
Forbid the use of $ in identifiers. This was formerly
required for strict conformance to the C Standard before the
standard was corrected.
This option is available only when you call cpp(1)
directly; gcc(1) will not pass it from its command
line.
Copyright (c) 1991, 1992, 1993 Free Software Foundation,
Inc.
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