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Most often when you use the C preprocessor you do not have to invoke it explicitly: the C compiler does so automatically. However, the preprocessor is sometimes useful on its own. You can invoke the preprocessor either with the cpp command, or via gcc -E. In GCC, the preprocessor is actually integrated with the compiler rather than a separate program, and both of these commands invoke GCC and tell it to stop after the preprocessing phase.
The cpp options listed here are also accepted by gcc and have the same meaning. Likewise the cpp command accepts all the usual gcc driver options, although those pertaining to compilation phases after preprocessing are ignored.
Only options specific to preprocessing behavior are documented here. Refer to the GCC manual for full documentation of other driver options.
The cpp command 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. If either file is omitted, it means the same as if - had been specified for that file. You can also use the -o outfile option to specify the output 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 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 should 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-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
fileAll files specified by -imacros are processed before all files
specified by -include.
-undef
-pthread
-M
Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with object file suffix and with any leading directory parts removed. 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 is still sent to the regular output stream as normal.
Passing -M to the driver implies -E, and suppresses
warnings with an implicit -w.
-MM
This implies that the choice of angle brackets or double quotes in an `#include' directive does not in itself determine whether that header appears in -MM dependency output.
-MF
fileWhen used with the driver options -MD or -MMD, -MF overrides the default dependency output file.
If file is -, then the dependencies are written to stdout.
-MG
#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.
-Mno-modules
-MP
This is typical output:
test.o: test.c test.h test.h:
-MT
targetAn -MT option sets 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
target$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with
-MQ.
-MD
If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file (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
-fpreprocessed
-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.
-fdirectives-only
The option's behavior depends on the -E and -fpreprocessed options.
With -E, preprocessing is limited to the handling of directives
such as #define
, #ifdef
, and #error
. Other
preprocessor operations, such as macro expansion and trigraph
conversion are not performed. In addition, the -dD option is
implicitly enabled.
With -fpreprocessed, predefinition of command line and most
builtin macros is disabled. Macros such as __LINE__
, which are
contextually dependent, are handled normally. This enables compilation of
files previously preprocessed with -E -fdirectives-only
.
With both -E and -fpreprocessed, the rules for
-fpreprocessed take precedence. This enables full preprocessing of
files previously preprocessed with -E -fdirectives-only
.
-fdollars-in-identifiers
-fextended-identifiers
-fno-canonical-system-headers
-fmax-include-depth=
depth-ftabstop=
width-ftrack-macro-expansion
[=
level]Note that -ftrack-macro-expansion=2
is activated by default.
-fmacro-prefix-map=
old=
new__FILE__
and __BASE_FILE__
macros as if the
files resided in directory new instead. This can be used
to change an absolute path to a relative path by using . for
new which can result in more reproducible builds that are
location independent. This option also affects
__builtin_FILE()
during compilation. See also
-ffile-prefix-map.
-fexec-charset=
charseticonv
library routine.
-fwide-exec-charset=
charsetwchar_t
. As with
-fexec-charset, charset can be any encoding supported
by the system's iconv
library routine; however, you will have
problems with encodings that do not fit exactly in wchar_t
.
-finput-charset=
charseticonv
library routine.
-fworking-directory
#line
directives are emitted whatsoever.
-A
predicate=
answer-A -
predicate=
answer-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
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.
-P
-traditional
-traditional-cpp
Note that GCC does not otherwise attempt to emulate a pre-standard
C compiler, and these options are only supported with the -E
switch, or when invoking CPP explicitly.
-trigraphs
By default, GCC ignores trigraphs, but in
standard-conforming modes it converts them. See the -std and
-ansi options.
-remap
-H
-d
letters-dM
touch foo.h; cpp -dM foo.h
shows all the predefined macros.
-dD
-dN
-dI
-dU
-fdebug-cpp
When used from GCC without -E, this option has no effect.
-I
dir-iquote
dir-isystem
dir-idirafter
dir$SYSROOT
, then the `='
or $SYSROOT
is replaced by the sysroot prefix; see
--sysroot and -isysroot.
Directories specified with -iquote apply only to the quote
form of the directive, #include "
file"
.
Directories specified with -I, -isystem,
or -idirafter apply to lookup for both the
#include "
file"
and
#include <
file>
directives.
You can specify any number or combination of these options on the command line to search for header files in several directories. The lookup order is as follows:
You can use -I to override a system header file, substituting your own version, since these directories are searched before the standard system header file directories. However, you should not use this option to add directories that contain vendor-supplied system header files; use -isystem for that.
The -isystem and -idirafter options also mark the directory as a system directory, so that it gets the same special treatment that is applied to the standard system directories. See System Headers.
If a standard system include directory, or a directory specified with
-isystem, is also specified with -I, the -I
option is ignored. The directory is still searched but as a
system directory at its normal position in the system include chain.
This is to ensure that GCC's procedure to fix buggy system headers and
the ordering for the #include_next
directive are not inadvertently
changed.
If you really need to change the search order for system directories,
use the -nostdinc and/or -isystem options.
See System Headers.
-I-
Any directories specified with -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"
. There is no way to override this effect of -I-.
See Search Path.
-iprefix
prefix-iwithprefix
dir-iwithprefixbefore
dir-isysroot
dir-imultilib
dir-nostdinc
-nostdinc++
-Wcomment
-Wcomments
-Wtrigraphs
This option is implied by -Wall. If -Wall is not
given, this option is still enabled unless trigraphs are enabled. To
get trigraph conversion without warnings, but get the other
-Wall warnings, use `-trigraphs -Wall -Wno-trigraphs'.
-Wundef
#if
directive.
Such identifiers are replaced with zero.
-Wexpansion-to-defined
-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 the preprocessor reports 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
-Wno-endif-labels
#else
or an #endif
are followed by text.
This sometimes happens in older programs with code of the form
#if FOO ... #else FOO ... #endif FOO
The second and third FOO
should be in comments.
This warning is on by default.