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Only the most useful options are listed here; see below for the remainder.
The C preprocessor is intended to be used only with C, [C+], and Objective-C source code. In the past, it has been abused as a general text processor. It will choke on input which does not obey C's lexical rules. For example, apostrophes will be interpreted as the beginning of character constants, and cause errors. Also, you cannot rely on it preserving characteristics of the input which are not significant to C-family languages. If a Makefile is preprocessed, all the hard tabs will be removed, and the Makefile will not work.
Having said that, you can often get away with using cpp on things which are not C. Other Algol-ish programming languages are often safe (Pascal, Ada, etc.) So is assembly, with caution. -traditional mode preserves more white space, and is otherwise more permissive. Many of the problems can be avoided by writing C or [C+] style comments instead of native language comments, and keeping macros simple.
Wherever possible, you should use a preprocessor geared to the language you are writing in. Modern versions of the GNU assembler have macro facilities. Most high level programming languages have their own conditional compilation and inclusion mechanism. If all else fails, try a true general text processor, such as GNU M4.
C preprocessors vary in some details. This manual discusses the GNU C preprocessor, which provides a small superset of the features of ISO Standard C. In its default mode, the GNU C preprocessor does not do a few things required by the standard. These are features which are rarely, if ever, used, and may cause surprising changes to the meaning of a program which does not expect them. To get strict ISO Standard C, you should use the -std=c89 or -std=c99 options, depending on which version of the standard you want. To get all the mandatory diagnostics, you must also use -pedantic.
Either infile or outfile may be -, which as infile means to read from standard input and as outfile means to write to standard output. Also, if either file is omitted, it means the same as if - had been specified for that file.
Unless otherwise noted, or the option ends in =, all options which take an argument may have that argument appear either immediately after the option, or with a space between option and argument: -Ifoo and -I foo have the same effect.
Many options have multi-letter names; therefore multiple single-letter
options may not be grouped: -dM is very different from
-d -M.
Predefine name as a macro, with definition [C`]1[C'].
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 wish to define a function-like macro on the command line, write
its argument list with surrounding parentheses before the equals sign
(if any). Parentheses are meaningful to most shells, so you will need
to quote the option. With sh and csh,
-D'name(args...)=definition' works.
-D and -U options are processed in the order they
are given on the command line. All -imacros file and
-include file options are processed after all
-D and -U options.
Cancel any previous definition of name, either built in or
provided with a -D option.
Do not predefine any system-specific macros. The common predefined
macros remain defined.
Add the directory dir to the list of directories to be searched
for header files.
Directories named by -I are searched before the standard
system include directories.
It is dangerous to specify a standard system include directory in an
-I option. This defeats the special treatment of system
headers
. It can also defeat the repairs to buggy system headers which GCC
makes when it is installed.
Write output to file. This is the same as specifying file
as the second non-option argument to cpp. gcc has a
different interpretation of a second non-option argument, so you must
use -o to specify the output file.
Turns on all optional warnings which are desirable for normal code. At
present this is -Wcomment and -Wtrigraphs. Note that
many of the preprocessor's warnings are on by default and have no
options to control them.
Warn whenever a comment-start sequence /* appears in a /*
comment, or whenever a backslash-newline appears in a // comment.
(Both forms have the same effect.)
Warn if any trigraphs are encountered. This option used to take effect
only if -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.
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.
Warn the first time #import is used.
Warn whenever an identifier which is not a macro is encountered in an
#if directive, outside of defined. Such identifiers are
replaced with zero.
Make all warnings into hard errors. Source code which triggers warnings
will be rejected.
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.
Suppress all warnings, including those which GNU CPP issues by default.
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.
Issue all the mandatory diagnostics, and make all mandatory diagnostics
into errors. This includes mandatory diagnostics that GCC issues
without -pedantic but treats as warnings.
Instead of outputting the result of preprocessing, output a rule
suitable for make describing the dependencies of the main
source file. The preprocessor outputs one make rule containing
the object file name for that source file, a colon, and the names of all
the included files, including those coming from -include or
-imacros command line options.
Unless specified explicitly (with -MT or -MQ), the
object file name consists of the basename of the source file with any
suffix replaced with object file suffix. If there are many included
files then the rule is split into several lines using \-newline.
The rule has no commands.
This option does not suppress the preprocessor's debug output, such as
-dM. To avoid mixing such debug output with the dependency
rules you should explicitly specify the dependency output file with
-MF, or use an environment variable like
DEPENDENCIES_OUTPUT. Debug output
will still be sent to the regular output stream as normal.
Passing -M to the driver implies -E.
Like -M but do not mention header files that are found in
system header directories, nor header files that are included,
directly or indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in an
#include directive does not in itself determine whether that
header will appear in -MM dependency output. This is a
slight change in semantics from GCC versions 3.0 and earlier.
@anchor{-MF}
When used with -M or -MM, specifies a
file to write the dependencies to. If no -MF switch is given
the preprocessor sends the rules to the same place it would have sent
preprocessed output.
When used with the driver options -MD or -MMD,
-MF overrides the default dependency output file.
When used with -M or -MM, -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.
This option instructs CPP to add a phony target for each dependency
other than the main file, causing each to depend on nothing. These
dummy rules work around errors make gives if you remove header
files without updating the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
Change the target of the rule emitted by dependency generation. By
default CPP takes the name of the main input file, including any path,
deletes any file suffix such as .c, and appends the platform's
usual object suffix. The result is the target.
An -MT option will set the target to be exactly the string you
specify. If you want multiple targets, you can specify them as a single
argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
Same as -MT, but it quotes any characters which are special to
Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with
-MQ.
-MD is equivalent to -M -MF file, except that
-E is not implied. The driver determines file based on
whether an -o option is given. If it is, the driver uses its
argument but with a suffix of .d, otherwise it take the
basename of the input file and applies a .d suffix.
If -MD is used in conjunction with -E, any
-o switch is understood to specify the dependency output file
(but @pxref{-MF}), but if used without -E, each -o
is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate
a dependency output file as a side-effect of the compilation process.
Like -MD except mention only user header files, not system
-header files.
Specify the source language: C, [C+], Objective-C, or assembly. This has
nothing to do with standards conformance or extensions; it merely
selects which base syntax to expect. If you give none of these options,
cpp will deduce the language from the extension of the source file:
.c, .cc, .m, or .S. Some other common
extensions for [C+] and assembly are also recognized. If cpp does not
recognize the extension, it will treat the file as C; this is the most
generic mode.
Note: Previous versions of cpp accepted a -lang option
which selected both the language and the standards conformance level.
This option has been removed, because it conflicts with the -l
option.
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.
standard
may be one of:
Note that you can also specify places to search using options such as
-I, and control dependency output with options like
-M. These take precedence over
environment variables, which in turn take precedence over the
configuration of GCC.
Each variable's value is a list of directories separated by a special
character, much like PATH, in which to look for header files.
The special character, [C`]PATH_SEPARATOR[C'], is target-dependent and
determined at GCC build time. For Windows-based targets it is a
semicolon, and for almost all other targets it is a colon.
CPATH specifies a list of directories to be searched as if
specified with -I, but after any paths given with -I
options on the command line. The environment variable is used
regardless of which language is being preprocessed.
The remaining environment variables apply only when preprocessing the
particular language indicated. Each specifies a list of directories
to be searched as if specified with -isystem, but after any
paths given with -isystem options on the command line.
See also @ref{Search Path}.
@anchor{DEPENDENCIES_OUTPUT}
If this variable is set, its value specifies how to output
dependencies for Make based on the non-system header files processed
by the compiler. System header files are ignored in the dependency
output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in
which case the Make rules are written to that file, guessing the target
name from the source file name. Or the value can have the form
file target, in which case the rules are written to
file file using target as the target name.
In other words, this environment variable is equivalent to combining
the options -MM and -MF,
with an optional -MT switch too.
This variable is the same as the environment variable
DEPENDENCIES_OUTPUT, except that
system header files are not ignored, so it implies -M rather
than -MM. However, the dependence on the main input file is
omitted.
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 man page gfdl(7). This manual contains no Invariant Sections. The Front-Cover Texts are (a) (see below), and the Back-Cover Texts are (b) (see below).
(a) The FSF's Front-Cover Text is:
A GNU Manual (b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.
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