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
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.
cpp options listed here are also accepted by
gcc and have the same meaning. Likewise the
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.
cpp command expects two file names as arguments,
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 may be
-, which as
infile means to read from standard input and as
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
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
name as a macro, with definition
The contents of
definition are tokenized and processed as if
they appeared during translation phase three in a ‘
directive. In particular, the definition is truncated by
embedded newline characters.
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 should
quote the option. With
-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
Cancel any previous definition of
name, either built in or
provided with a
file as if
#include "file" appeared as the first
line of the primary source file. However, the first directory searched
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
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
Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined. See Standard Predefined Macros.
Define additional macros required for using the POSIX threads library. You should use this option consistently for both compilation and linking. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets.
Instead of outputting the result of preprocessing, output a rule
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
-imacros command-line options.
Unless specified explicitly (with
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
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.
-M to the driver implies
-E, and suppresses
warnings with an implicit
-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 appears in
-MM dependency output.
When used with
-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 send
When used with the driver options
-MF overrides the default dependency output file.
-, then the dependencies are written to
In conjunction with an option such as
-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.
also suppresses preprocessed output, as a missing header file renders
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, deletes any
directory components and any file suffix such as ‘
appends the platform’s usual object suffix. The result is the target.
-MT option sets 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
-MD is equivalent to
-M -MF file, except that
-E is not implied. The driver determines
file based on
-o option is given. If it is, the driver uses its
argument but with a suffix of
.d, otherwise it takes the name
of the input file, removes any directory components and suffix, and
-MD is used in conjunction with
-o switch is understood to specify the dependency output file
(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.
-MD except mention only user header files, not system
Indicate to the preprocessor that the input file has already been
preprocessed. This suppresses things like macro expansion, trigraph
conversion, escaped newline splicing, and processing of most directives.
The preprocessor still recognizes and removes comments, so that you can
pass a file preprocessed with
-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
.ii’ or ‘
.mi’. These are the
extensions that GCC uses for preprocessed files created by
When preprocessing, handle directives, but do not expand macros.
The option’s behavior depends on the
-E, preprocessing is limited to the handling of directives
preprocessor operations, such as macro expansion and trigraph
conversion are not performed. In addition, the
-dD option is
-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
-fpreprocessed, the rules for
-fpreprocessed take precedence. This enables full preprocessing of
files previously preprocessed with
$’ in identifiers.
See Identifier characters.
Accept universal character names and extended characters in identifiers. This option is enabled by default for C99 (and later C standard versions) and C++.
When preprocessing, do not shorten system header paths with canonicalization.
Set the maximum depth of the nested #include. The default is 200.
Set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored. The default is 8.
Track locations of tokens across macro expansions. This allows the
compiler to emit diagnostic about the current macro expansion stack
when a compilation error occurs in a macro expansion. Using this
option makes the preprocessor and the compiler consume more
level parameter can be used to choose the level of
precision of token location tracking thus decreasing the memory
consumption if necessary. Value ‘
this option. Value ‘
1’ tracks tokens locations in a
degraded mode for the sake of minimal memory overhead. In this mode
all tokens resulting from the expansion of an argument of a
function-like macro have the same location. Value ‘
tokens locations completely. This value is the most memory hungry.
When this option is given no argument, the default parameter value is
-ftrack-macro-expansion=2 is activated by default.
When preprocessing files residing in directory
__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
new which can result in more reproducible builds that are
location independent. This option also affects
__builtin_FILE() during compilation. See also
Set the execution character set, used for string and character
constants. The default is UTF-8.
charset can be any encoding
supported by the system’s
iconv library routine.
Set the wide execution character set, used for wide string and
character constants. The default is UTF-32 or UTF-16, whichever
corresponds to the width of
wchar_t. As with
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
Set the input character set, used for translation from the character
set of the input file to the source character set used by GCC. If the
locale does not specify, or GCC cannot get this information from the
locale, the default is UTF-8. This can be overridden by either the locale
or this command-line option. Currently the command-line option takes
precedence if there’s a conflict.
charset can be any encoding
supported by the system’s
iconv library routine.
Enable generation of linemarkers in the preprocessor output that
let the compiler know the current working directory at the time of
preprocessing. When this option is enabled, the preprocessor
emits, after the initial linemarker, a second linemarker with the
current working directory followed by two slashes. GCC uses this
directory, when it’s present in the preprocessed input, as the
directory emitted as the current working directory in some debugging
information formats. This option is implicitly enabled if debugging
information is enabled, but this can be inhibited with the negated
-fno-working-directory. If the
-P flag is
present in the command line, this option has no effect, since no
#line directives are emitted whatsoever.
Make an assertion with the predicate
predicate and answer
answer. This form is preferred to the older form
-A predicate(answer), which is still supported, because
it does not use shell special characters.
See Obsolete Features.
Cancel an assertion with the predicate
predicate and answer
Do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive.
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 ‘
Do not discard comments, including during macro expansion. This is
-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.
-CC option is generally used to support lint comments.
Inhibit generation of linemarkers 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 linemarkers. See Preprocessor Output.
Try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors. See Traditional Mode.
Note that GCC does not otherwise attempt to emulate a pre-standard
C compiler, and these options are only supported with the
switch, or when invoking CPP explicitly.
Support ISO C trigraphs.
These are three-character sequences, all starting with ‘
are defined by ISO C to stand for single characters. For example,
??/’ stands for ‘
\’, so ‘
'??/n'’ is a character
constant for a newline.
See Initial processing.
By default, GCC ignores trigraphs, but in
standard-conforming modes it converts them. See the
Enable special code to work around file systems which only permit very short file names, such as MS-DOS.
Print the name of each header file used, in addition to other normal
activities. Each name is indented to show how deep in the
#include’ stack it is. Precompiled header files are also
printed, even if they are found to be invalid; an invalid precompiled
header file is printed with ‘
...x’ and a valid one with ‘
Says to make debugging dumps during compilation as specified by
letters. The flags documented here are those relevant to the
letters are interpreted
by the compiler proper, or reserved for future versions of GCC, and so
are silently ignored. If you specify
letters whose behavior
conflicts, the result is undefined.
Instead of the normal output, generate a list of ‘
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
shows all the predefined macros.
-dM except in two respects: it does not include the
predefined macros, and it outputs both the ‘
directives and the result of preprocessing. Both kinds of output go to
the standard output file.
-dD, but emit only the macro names, not their expansions.
#include’ directives in addition to the result of
-dD except that only macros that are expanded, or whose
definedness is tested in preprocessor directives, are output; the
output is delayed until the use or test of the macro; and
#undef’ directives are also output for macros tested but
undefined at the time.
This option is only useful for debugging GCC. When used from CPP or with
-E, it dumps debugging information about location maps. Every
token in the output is preceded by the dump of the map its location
When used from GCC without
-E, this option has no effect.
Add the directory
dir to the list of directories to be searched
for header files during preprocessing.
See Search Path.
dir begins with ‘
$SYSROOT, then the ‘
$SYSROOT is replaced by the sysroot prefix; see
Directories specified with
-iquote apply only to the quote
form of the directive,
Directories specified with
-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:
-iquoteoptions are searched in left-to-right order, as they appear on the command line.
-Ioptions are scanned in left-to-right order.
-isystemoptions are scanned in left-to-right order.
-idirafteroptions are scanned in left-to-right order.
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.
-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
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
If you really need to change the search order for system directories,
See System Headers.
Split the include path.
This option has been deprecated. Please use
-iquote instead for
-I directories before the
-I- and remove the
Any directories specified with
-I- are searched only for headers requested with
#include "file"; they are not searched for
#include <file>. If additional directories are
-I options after the
directories are searched for all ‘
-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
See Search Path.
prefix as the prefix for subsequent
options. If the prefix represents a directory, you should include the
dir to the prefix specified previously with
-iprefix, and add the resulting directory to the include search
-iwithprefixbefore puts it in the same place
-iwithprefix puts it where
This option is like the
--sysroot option, but applies only to
header files (except for Darwin targets, where it applies to both header
files and libraries). See the
--sysroot option for more
dir as a subdirectory of the directory containing
target-specific C++ headers.
Do not search the standard system directories for header files.
Only the directories explicitly specified with
options (and the directory of the current file, if appropriate)
Do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building the C++ library.)
Warn whenever a comment-start sequence ‘
/*’ appears in a ‘
comment, or whenever a backslash-newline appears in a ‘
This warning is enabled by
Warn if any trigraphs are encountered that might change the meaning of the program. Trigraphs within comments are not warned about, except those that would form escaped newlines.
This option is implied by
-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’.
Warn if an undefined identifier is evaluated in an
Such identifiers are replaced with zero.
Warn whenever ‘
defined’ is encountered in the expansion of a macro
(including the case where the macro is expanded by an ‘
Such usage is not portable.
This warning is also enabled by
Warn about macros defined in the main file that are unused. A macro is used if it is expanded or tested for existence at least once. The preprocessor also warns if the macro has not been used at the time it is redefined or undefined.
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
Do not warn whenever an
#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.