cpp(1) GNU Tools cpp(1)
cpp - The GNU C-Compatible Compiler Preprocessor.
cpp [-$] [-Apredicate[(value)]] [-C] [-Dname[=definition]] [-dD]
[-dM] [-I directory] [-H] [-I-] [-imacros file] [-include file]
[-idirafter dir] [-iprefix prefix] [-iwithprefix dir] [-lang-c]
[-lang-c++] [-lang-objc] [-lang-objc++] [-lint] [-M [-MG]]
[-MM [-MG]] [-MD file ] [-MMD file ] [-nostdinc] [-nostdinc++]
[-P] [-pedantic] [-pedantic-errors] [-traditional] [-trigraphs]
[-Uname] [-undef] [-Wtrigraphs] [-Wcomment] [-Wall]
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:
o Inclusion of header files. These are files of declarations that
can be substituted into your program.
o 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
o Conditional compilation. Using special preprocessing direc-
tives, you can include or exclude parts of the program according
to various conditions.
o 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 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 com-
monly used by today's C programs. Such incompatibility would be incon-
venient 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.
The C preprocessor expects two file names as arguments, infile and out-
file. 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
-P Inhibit generation of `#'-lines 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.
Try to imitate the behavior of old-fashioned C, as opposed to
Process ANSI standard trigraph sequences. These are three-char-
acter sequences, all starting with `??', that are defined by AN-
SI 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.
Issue warnings required by the ANSI C standard in certain cases
such as when text other than a comment follows `#else' or `#en-
Like `-pedantic', except that errors are produced rather than
Warn if any trigraphs are encountered (assuming they are en-
Warn whenever a comment-start sequence `/*' appears in a com-
ment. (Both forms have the same effect).
-Wall Requests both `-Wtrigraphs' and `-Wcomment' (but not `-Wtradi-
Warn about certain constructs that behave differently in tradi-
tional and ANSI C.
Add the directory directory to the end of the list of directo-
ries to be searched for header files. This can be used to over-
ride a system header file, substituting your own version, since
these directories are searched before the system header file di-
rectories. If you use more than one `-I' option, the directo-
ries are scanned in left-to-right order; the standard system di-
rectories come after.
-I- Any directories specified with `-I' options before the `-I-' op-
tion are searched only for the case of `#include file"'; 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' di-
In addition, the `-I-' option inhibits the use of the current
directory as the first search directory for `#include file"'.
Therefore, the current directory is searched only if it is re-
quested 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.
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.
Do not search for header files in the C++ specific standard di-
rectories, but do still search the other standard directories.
(This option is used when building libg++.)
Predefine name as a macro, with definition `1'.
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 char-
acters 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.
Do not predefine name. If both `-U' and `-D' are specified for
one name, the `-U' beats the `-D' and the name is not prede-
-undef Do not predefine any nonstandard macros.
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',
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 pre-
defined macros, and it outputs both the `#define' directives and
the result of preprocessing. Both kinds of output go to the
standard output file.
Instead of outputting the result of preprocessing, output a rule
suitable for make describing the dependencies of the main source
file. The preprocessor outputs one make rule containing the ob-
ject 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 `\'-newline.
`-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.
Like `-M' but mention only the files included with `#include
"file"'. System header files included with `#include <file>'
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' com-
Like `-MD' except mention only user header files, not system
-H Print the name of each header file used, in addition to other
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 `-imacros file' 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 `-imacros file' .
Process file as input, and include all the resulting output, be-
fore processing the regular input file.
Add the directory dir to the second include path. The directo-
ries 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).
Specify prefix as the prefix for subsequent `-iwithprefix' op-
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'.
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, but not
passed from the `gcc' command line.
-lint Look for commands to the program checker lint embedded in com-
ments, and emit them preceded by `#pragma lint'. For example,
the comment `/* NOTREACHED */' becomes `#pragma lint
This option is available only when you call cpp directly; gcc
will not pass it from its command line.
-$ Forbid the use of `$' in identifiers. This was formerly re-
quired for strict conformance to the C Standard before the stan-
dard was corrected. This option is available only when you
call cpp directly; gcc will not pass it from its command line.
`Cpp' entry in info; The C Preprocessor, Richard M. Stallman.
gcc(1); `Gcc' entry in info; Using and Porting GNU CC (for version
2.0), Richard M. Stallman.
Copyright (c) 1991, 1992, 1993 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the en-
tire resulting derived work is distributed under the terms of a permis-
sion notice identical to this one.
Permission is granted to copy and distribute translations of this manu-
al into another language, under the above conditions for modified ver-
sions, except that this permission notice may be included in transla-
tions approved by the Free Software Foundation instead of in the origi-
FreeBSD April 30, 1993 cpp(1)