|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
NAME | SYNOPSIS | DESCRIPTION | OPTIONS | ENVIRONMENT | SEE ALSO | COPYRIGHT | COLOPHON |
|
|
|
CPP(1) GNU CPP(1)
cpp - The C Preprocessor
cpp [-Dmacro[=defn]...] [-Umacro]
[-Idir...] [-iquotedir...]
[-M|-MM] [-MG] [-MF filename]
[-MP] [-MQ target...]
[-MT target...]
infile [[-o] outfile]
Only the most useful options are given above; see below for a more
complete list of preprocessor-specific options. In addition, cpp
accepts most gcc driver options, which are not listed here. Refer
to the GCC documentation for details.
The C preprocessor, often known as cpp, is a macro processor that
is used automatically by the C compiler to transform your program
before compilation. It is called a macro processor because it
allows you to define macros, which are brief abbreviations for
longer constructs.
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 (Ada, etc.) So is assembly, with caution. -traditional-cpp
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=c90, -std=c99, -std=c11 or -std=c17 options, depending on
which version of the standard you want. To get all the mandatory
diagnostics, you must also use -pedantic.
This manual describes the behavior of the ISO preprocessor. To
minimize gratuitous differences, where the ISO preprocessor's
behavior does not conflict with traditional semantics, the
traditional preprocessor should behave the same way. The various
differences that do exist are detailed in the section Traditional
Mode.
For clarity, unless noted otherwise, references to CPP in this
manual refer to GNU CPP.
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 name
Predefine name as a macro, with definition 1.
-D name=definition
The contents of definition are tokenized and processed as if
they appeared during translation phase three in a #define
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 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
Cancel any previous definition of name, either built in or
provided with a -D option.
-include file
Process file as if "#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
Exactly like -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.
-undef
Do not predefine any system-specific or GCC-specific macros.
The standard predefined macros remain defined.
-pthread
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.
-M 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 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. 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 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 appears in -MM dependency output.
-MF file
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 send preprocessed
output.
When 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 In conjunction with an option such as -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 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:
-MT target
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 .c, and appends the platform's usual object suffix.
The result is the target.
An -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
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special to
Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were
given with -MQ.
-MD -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 takes the name of the input
file, removes any directory components and suffix, 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 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
Like -MD except mention only user header files, not system
header files.
-fpreprocessed
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
extensions .i, .ii or .mi. These are the extensions that GCC
uses for preprocessed files created by -save-temps.
-fdirectives-only
When preprocessing, handle directives, but do not expand
macros.
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
Accept $ in identifiers.
-fextended-identifiers
Accept universal character names in identifiers. This option
is enabled by default for C99 (and later C standard versions)
and C++.
-fno-canonical-system-headers
When preprocessing, do not shorten system header paths with
canonicalization.
-ftabstop=width
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.
-ftrack-macro-expansion[=level]
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 memory. The level parameter can be used
to choose the level of precision of token location tracking
thus decreasing the memory consumption if necessary. Value 0
of level de-activates 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 2 tracks tokens locations completely. This
value is the most memory hungry. When this option is given no
argument, the default parameter value is 2.
Note that "-ftrack-macro-expansion=2" is activated by default.
-fmacro-prefix-map=old=new
When preprocessing files residing in directory old, expand the
"__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=charset
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.
-fwide-exec-charset=charset
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
-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=charset
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.
-fworking-directory
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 form
-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.
-A predicate=answer
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.
-A -predicate=answer
Cancel an assertion with the predicate predicate and answer
answer.
-C 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 -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 Do not discard comments, including during macro expansion.
This is like -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.
-P 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.
-traditional
-traditional-cpp
Try to imitate the behavior of pre-standard C preprocessors,
as opposed to ISO C preprocessors.
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
Support ISO C trigraphs. These are three-character sequences,
all starting with ??, that are defined by ISO C to stand for
single characters. For example, ??/ stands for \, so '??/n'
is a character constant for a newline.
By default, GCC ignores trigraphs, but in standard-conforming
modes it converts them. See the -std and -ansi options.
-remap
Enable special code to work around file systems which only
permit very short file names, such as MS-DOS.
-H 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 ...! .
-dletters
Says to make debugging dumps during compilation as specified
by letters. The flags documented here are those relevant to
the preprocessor. Other 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.
-dM Instead of the normal output, generate a list of #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
shows all the predefined macros.
-dD Like -dM 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.
-dN Like -dD, but emit only the macro names, not their
expansions.
-dI Output #include directives in addition to the result of
preprocessing.
-dU Like -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.
-fdebug-cpp
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 belongs to.
When used from GCC without -E, this option has no effect.
-I dir
-iquote dir
-isystem dir
-idirafter dir
Add the directory dir to the list of directories to be
searched for header files during preprocessing.
If dir begins with = or $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:
1. For the quote form of the include directive, the directory
of the current file is searched first.
2. For the quote form of the include directive, the
directories specified by -iquote options are searched in
left-to-right order, as they appear on the command line.
3. Directories specified with -I options are scanned in left-
to-right order.
4. Directories specified with -isystem options are scanned in
left-to-right order.
5. Standard system directories are scanned.
6. Directories specified with -idirafter options 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.
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.
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.
-I- Split the include path. This option has been deprecated.
Please use -iquote instead for -I directories before the -I-
and remove the -I- option.
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-.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix
options. If the prefix represents a directory, you should
include the final /.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with -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.
-isysroot dir
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 information.
-imultilib dir
Use dir as a subdirectory of the directory containing target-
specific C++ headers.
-nostdinc
Do not search the standard system directories for header
files. Only the directories explicitly specified with -I,
-iquote, -isystem, and/or -idirafter options (and the
directory of the current file, if appropriate) are searched.
-nostdinc++
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.)
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /*
comment, or whenever a backslash-newline appears in a //
comment. This warning is enabled by -Wall.
-Wtrigraphs
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. 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
Warn if an undefined identifier is evaluated in an "#if"
directive. Such identifiers are replaced with zero.
-Wexpansion-to-defined
Warn whenever defined is encountered in the expansion of a
macro (including the case where the macro is expanded by an
#if directive). Such usage is not portable. This warning is
also enabled by -Wpedantic and -Wextra.
-Wunused-macros
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
-Wno-endif-labels
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.
This section describes the environment variables that affect how
CPP operates. You can use them to specify directories or prefixes
to use when searching for include files, or to control dependency
output.
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.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
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, "PATH_SEPARATOR", is target-
dependent and determined at GCC build time. For Microsoft
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. This 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.
In all these variables, an empty element instructs the
compiler to search its current working directory. Empty
elements can appear at the beginning or end of a path. For
instance, if the value of CPATH is ":/special/include", that
has the same effect as -I. -I/special/include.
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.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above),
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.
SOURCE_DATE_EPOCH
If this variable is set, its value specifies a UNIX timestamp
to be used in replacement of the current date and time in the
"__DATE__" and "__TIME__" macros, so that the embedded
timestamps become reproducible.
The value of SOURCE_DATE_EPOCH must be a UNIX timestamp,
defined as the number of seconds (excluding leap seconds)
since 01 Jan 1970 00:00:00 represented in ASCII; identical to
the output of @command{date +%s} on GNU/Linux and other
systems that support the %s extension in the "date" command.
The value should be a known timestamp such as the last
modification time of the source or package and it should be
set by the build process.
gpl(7), gfdl(7), fsf-funding(7), gcc(1), and the Info entries for
cpp and gcc.
Copyright (c) 1987-2019 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License,
Version 1.3 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.
This page is part of the gcc (GNU Compiler Collection) project.
Information about the project can be found at
⟨http://gcc.gnu.org/⟩. If you have a bug report for this manual
page, see ⟨http://gcc.gnu.org/bugs/⟩. This page was obtained from
the tarball gcc-9.5.0.tar.xz fetched from
⟨https://ftp.fu-berlin.de/unix/languages/gcc/releases/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
gcc-9.5.0 2022-05-27 CPP(1)
Pages that refer to this page: pmcpp(1), pmgenmap(1), suffixes(7)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|