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 gcc
or 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
without notice.
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:
-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.
-D name1.
-D name=definitionIf 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.
-U name-D option.
-undef
-I dir-I are searched before the standard
system include directories. If the directory dir is a standard
system include directory, the option is ignored to ensure that the
default search order for system directories and the special treatment
of system headers are not defeated
(see System Headers)
.
-o filecpp. gcc has a
different interpretation of a second non-option argument, so you must
use -o to specify the output file.
-Wall
-Wcomment and -Wtrigraphs. Note that
many of the preprocessor's warnings are on by default and have no
options to control them.
-Wcomment
-Wcomments
/* appears in a /*
comment, or whenever a backslash-newline appears in a // comment.
(Both forms have the same effect.)
-Wtrigraphs
-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.
-Wtraditional
-Wimport
#import is used.
-Wundef
#if directive, outside of defined. Such identifiers are
replaced with zero.
-Wunused-macros
Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then CPP will report it as unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by, for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with something like:
#if defined the_macro_causing_the_warning
#endif
-Wendif-labels
#else or an #endif are followed by text.
This usually happens in code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third FOO should be in comments, but often are not
in older programs. This warning is on by default.
-Werror
-Wsystem-headers
-w
-pedantic
-pedantic-errors
-pedantic but treats as warnings.
-M
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 (see Environment Variables). Debug output
will still be sent to the regular output stream as normal.
Passing -M to the driver implies -E, and suppresses
warnings with an implicit -w.
-MM
-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.
-MF file-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.
-MG
-M requesting
dependency generation, -MG assumes missing header files are
generated files and adds them to the dependency list without raising
an error. The dependency filename is taken directly from the
#include directive without prepending any path. -MG
also suppresses preprocessed output, as a missing header file renders
this useless.
This feature is used in automatic updating of makefiles.
-MP
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:
-MT targetChange 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
$(objpfx)foo.o: foo.c
-MQ targetSame as -MT, but it quotes any characters which are special to
Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with
-MQ.
-MD
-MD is equivalent to -M -MF file-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 see -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.
-MMD
-MD except mention only user header files, not system
-header files.
-x c
-x c++
-x objective-c
-x assembler-with-cpp
.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.
-std=standard-ansi
standard may be one of:
iso9899:1990
c89
c89 is the customary shorthand for
this version of the standard.
The -ansi option is equivalent to -std=c89.
iso9899:199409
iso9899:1999
c99
iso9899:199x
c9x
gnu89
gnu99
gnu9x
c++98
gnu++98
-std=c++98 plus GNU extensions. This is the
default for C++ code.
-I-
-I
options before -I- are searched only for headers requested with
#include "file"; they are not searched for
#include <file>. If additional directories are
specified with -I options after the -I-, those
directories are searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current
file directory as the first search directory for #include "file".
See Search Path.
-nostdinc
-I options
(and the directory of the current file, if appropriate) are searched.
-nostdinc++
-include file#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.
If multiple -include options are given, the files are included
in the order they appear on the command line.
-imacros file-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
specified by -include.
-idirafter dir-I and the standard system directories
have been exhausted. dir is treated as a system include directory.
-iprefix prefix-iwithprefix
options. If the prefix represents a directory, you should include the
final /.
-iwithprefix dir-iwithprefixbefore dir-iprefix, and add the resulting directory to the include search
path. -iwithprefixbefore puts it in the same place -I
would; -iwithprefix puts it where -idirafter would.
Use of these options is discouraged.
-isystem dir-I but 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.
-fpreprocessed
-C to 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
extensions .i, .ii or .mi. These are the
extensions that GCC uses for preprocessed files created by
-save-temps.
-ftabstop=width-fno-show-column
dejagnu.
-A predicate=answer-A
predicate(answer), which is still supported, because
it does not use shell special characters.
See Assertions.
-A -predicate=answer-dCHARS
M
#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 all the predefined macros.
D
M 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.
N
D, but emit only the macro names, not their expansions.
I
#include directives in addition to the result of
preprocessing.
-P
-C
You should be prepared for side effects when using -C; it
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 #.
-CC
-C, except that comments contained within macros are
also passed through to the output file where the macro is expanded.
In addition to the side-effects of the -C option, the
-CC option causes all C++-style comments inside a macro
to be converted to C-style comments. This is to prevent later use
of that macro from inadvertently commenting out the remainder of
the source line.
The -CC option is generally used to support lint comments.
-traditional-cpp
-trigraphs
-remap
--help
--target-help
-v
-H
#include stack it is.
-version
--version