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 on its own. All the options listed here are also acceptable to the C compiler and have the same meaning, except that the C compiler has different rules for specifying the output file.
Note: Whether you use the preprocessor by way of
cpp, the 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
similar but not identical to the documented interface, and may change
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 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:
-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
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
-U options are processed in the order they
are given on the command line. All
options are processed after all
-Iare 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
(see System Headers)
. It can also defeat the repairs to buggy system headers which GCC
makes when it is installed.
gcchas a different interpretation of a second non-option argument, so you must use
-oto specify the output file.
-Wtrigraphs. Note that many of the preprocessor's warnings are on by default and have no options to control them.
/*appears in a
/*comment, or whenever a backslash-newline appears in a
//comment. (Both forms have the same effect.)
-trigraphswas also specified, but now works independently. Warnings are not given for trigraphs within comments, as they do not affect the meaning of the program.
#ifdirective, outside of
defined. Such identifiers are replaced with zero.
-pedanticbut treats as warnings.
makedescribing the dependencies of the main source file. The preprocessor outputs one
makerule containing the object file name for that source file, a colon, and the names of all the included files, including those coming from
-imacroscommand line options.
Unless specified explicitly (with
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
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 (see DEPENDENCIES_OUTPUT). Debug output
will still be sent to the regular output stream as normal.
-M to the driver implies
-Mbut 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.
-MM, specifies a file to write the dependencies to. If no
-MFswitch is given the preprocessor sends the rules to the same place it would have sent preprocessed output.
When used with the driver options
-MF overrides the default dependency output file.
-MGsays 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.
makegives if you remove header files without updating the
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.
-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
-MT, or use multiple
-MT '$(objpfx)foo.o' might give
-MT, but it quotes any characters which are special to
-MQ '$(objpfx)foo.o' gives
The default target is automatically quoted, as if it were given with
-MDis equivalent to
, except that
-Eis not implied. The driver determines file based on whether an
-ooption 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
-MD is used in conjunction with
-o switch is understood to specify the dependency output file
(but see -MF), but if used without
is understood to specify a target object file.
-E is not implied,
-MD can be used to generate
a dependency output file as a side-effect of the compilation process.
-MDexcept mention only user header files, not system -header files.
.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
which selected both the language and the standards conformance level.
This option has been removed, because it conflicts with the
standard may be one of:
c89is the customary shorthand for this version of the standard.
-ansi option is equivalent to
-I-are searched only for headers requested with
"; they are not searched for
>. If additional directories are specified with
-Ioptions after the
-I-, those directories are searched for all
-I- inhibits the use of the directory of the current
file directory as the first search directory for
See Search Path.
-Ioptions (and the directory of the current file, if appropriate) are searched.
#include "file"appeared as the first line of the primary source file. However, the first directory searched for file is the preprocessor's working directory instead of the directory containing the main source file. If not found there, it is searched for in the remainder of the
#include "..."search chain as normal.
-include options are given, the files are included
in the order they appear on the command line.
-include, except that any output produced by scanning 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
-imacros are processed before all files
-Iand the standard system directories have been exhausted. dir is treated as a system include directory.
-iwithprefixoptions. If the prefix represents a directory, you should include the final
-iprefix, and add the resulting directory to the include search path.
-iwithprefixbeforeputs it in the same place
-iwithprefixputs it where
Use of these options is discouraged.
-Ibut 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. See System Headers.
-Cto the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the
.mi. These are the
extensions that GCC uses for preprocessed files created by
), which is still supported, because it does not use shell special characters. See Assertions.
#definedirectives 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 all the predefined macros.
Mexcept in two respects: it does not include the predefined macros, and it outputs both the
#definedirectives and the result of preprocessing. Both kinds of output go to the standard output file.
D, but emit only the macro names, not their expansions.
#includedirectives in addition to the result of preprocessing.
You should be prepared for side effects when using
causes the preprocessor to treat comments as tokens in their own right.
For example, 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
gcc -E; you can turn them off in that case with
$in identifiers. The C standard allows implementations to define extra characters that can appear in identifiers. By default GNU CPP permits
$, a common extension.
#includestack it is.