CPP(1) GNU CPP(1)NAMEcpp - The C Preprocessor
SYNOPSIScpp [-Dmacro[=defn]...] [-Umacro]
[-M⎪-MM] [-MG] [-MF filename]
[-MP] [-MQ target...]
[-x language] [-std=standard]
Only the most useful options are listed here; see below for the remain‐
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 Objec‐
tive-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 (Pas‐
cal, 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 con‐
ditional 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=c89 or -std=c99 options, depending on which
version of the standard you want. To get all the mandatory diagnos‐
tics, you must also use -pedantic.
This manual describes the behavior of the ISO preprocessor. To mini‐
mize 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 C preprocessor expects two file names as arguments, infile and out‐
file. 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.
Predefine name as a macro, with definition 1.
The contents of definition are tokenized and processed as if they
appeared during translation phase three in a #define directive. In
particular, the definition will be truncated by embedded newline
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 will need to quote the option. With sh and csh,
-D and -U options are processed in the order they are given on the
command line. All -imacros file and -include file options are pro‐
cessed after all -D and -U options.
Cancel any previous definition of name, either built in or provided
with a -D option.
Do not predefine any system-specific or GCC-specific macros. The
standard predefined macros remain defined.
Add the directory dir to the list of directories to be searched for
Directories named by -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 .
Write output to file. This is the same as specifying file as the
second non-option argument to cpp. gcc has a different interpreta‐
tion of a second non-option argument, so you must use -o to specify
the output file.
Turns on all optional warnings which are desirable for normal code.
At present this is -Wcomment, -Wtrigraphs, -Wmultichar and a warn‐
ing about integer promotion causing a change of sign in "#if"
expressions. Note that many of the preprocessor's warnings are on
by default and have no options to control them.
Warn whenever a comment-start sequence /* appears in a /* comment,
or whenever a backslash-newline appears in a // comment. (Both
forms have the same effect.)
Most trigraphs in comments cannot affect the meaning of the pro‐
gram. However, a trigraph that would form an escaped newline (??/
at the end of a line) can, by changing where the comment begins or
ends. Therefore, only trigraphs that would form escaped newlines
produce warnings inside a comment.
This option is implied by -Wall. If -Wall is not given, this
option is still enabled unless trigraphs are enabled. To get tri‐
graph conversion without warnings, but get the other -Wall warn‐
ings, use -trigraphs -Wall -Wno-trigraphs.
Warn about certain constructs that behave differently in tradi‐
tional and ISO C. Also warn about ISO C constructs that have no
traditional C equivalent, and problematic constructs which should
Warn the first time #import is used.
Warn whenever an identifier which is not a macro is encountered in
an #if directive, outside of defined. Such identifiers are
replaced with zero.
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 will also warn 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 condi‐
tional 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
Warn whenever an #else or an #endif are followed by text. This
usually happens in code of the form
The second and third "FOO" should be in comments, but often are not
in older programs. This warning is on by default.
Make all warnings into hard errors. Source code which triggers
warnings will be rejected.
Issue warnings for code in system headers. These are normally
unhelpful in finding bugs in your own code, therefore suppressed.
If you are responsible for the system library, you may want to see
-w Suppress all warnings, including those which GNU CPP issues by
Issue all the mandatory diagnostics listed in the C standard. Some
of them are left out by default, since they trigger frequently on
Issue all the mandatory diagnostics, and make all mandatory diag‐
nostics into errors. This includes mandatory diagnostics that GCC
issues without -pedantic but treats as warnings.
-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 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.
Debug output will still be sent to the regular output stream as
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 will appear in -MM dependency output. This is a slight
change in semantics from GCC versions 3.0 and earlier.
When used with -M or -MM, specifies a file to write the dependen‐
cies 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 In conjunction with an option such as -M requesting dependency gen‐
eration, -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 out‐
put, 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
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 plat‐
form'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 '$(objpfx)foo.o' might give
Same as -MT, but it quotes any characters which are special to
Make. -MQ '$(objpfx)foo.o' gives
The default target is automatically quoted, as if it were given
-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 take the basename of the input file and applies a .d
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.
Like -MD except mention only user header files, not system header
Specify the source language: C, C++, Objective-C, or assembly.
This has nothing to do with standards conformance or extensions; it
merely selects which base syntax to expect. If you give none of
these options, cpp will deduce the language from the extension of
the source file: .c, .cc, .m, or .S. Some other common extensions
for C++ and assembly are also recognized. If cpp does not recog‐
nize the extension, it will treat the file as C; this is the most
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
Specify the standard to which the code should conform. Currently
CPP knows about C and C++ standards; others may be added in the
standard may be one of:
The ISO C standard from 1990. c89 is the customary shorthand
for this version of the standard.
The -ansi option is equivalent to -std=c89.
The 1990 C standard, as amended in 1994.
The revised ISO C standard, published in December 1999. Before
publication, this was known as C9X.
The 1990 C standard plus GNU extensions. This is the default.
The 1999 C standard plus GNU extensions.
The 1998 ISO C++ standard plus amendments.
The same as -std=c++98 plus GNU extensions. This is the
default for C++ code.
-I- Split the include path. 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"".
This option has been deprecated.
Do not search the standard system directories for header files.
Only the directories you have specified with -I options (and the
directory of the current file, if appropriate) are searched.
Do not search for header files in the C++-specific standard direc‐
tories, but do still search the other standard directories. (This
option is used when building the C++ library.)
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 direc‐
tory containing the main source file. If not found there, it is
searched for in the remainder of the "#include "..."" search chain
If multiple -include options are given, the files are included in
the order they appear on the command line.
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.
Search dir for header files, but do it after all directories speci‐
fied with -I and the standard system directories have been
exhausted. dir is treated as a system include directory.
Specify prefix as the prefix for subsequent -iwithprefix options.
If the prefix represents a directory, you should include the final
Append dir to the prefix specified previously with -iprefix, and
add the resulting directory to the include search path. -iwithpre‐
fixbefore puts it in the same place -I would; -iwithprefix puts it
where -idirafter would.
This option is like the --sysroot option, but applies only to
header files. See the --sysroot option for more information.
Use dir as a subdirectory of the directory containing target-spe‐
cific C++ headers.
Search dir for header files, after all directories specified by -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.
Search dir only for header files requested with "#include "file"";
they are not searched for "#include <file>", before all directories
specified by -I and before the standard system directories.
Accept $ in identifiers.
Accept universal character names in identifiers. This option is
experimental; in a future version of GCC, it will be enabled by
default for C99 and C++.
Indicate to the preprocessor that the input file has already been
preprocessed. This suppresses things like macro expansion, tri‐
graph conversion, escaped newline splicing, and processing of most
directives. The preprocessor still recognizes and removes com‐
ments, so that you can pass a file preprocessed with -C to the com‐
piler 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 exten‐
sions .i, .ii or .mi. These are the extensions that GCC uses for
preprocessed files created by -save-temps.
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.
Set the execution character set, used for string and character con‐
stants. The default is UTF-8. charset can be any encoding sup‐
ported 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 -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".
Set the input character set, used for translation from the charac‐
ter 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 com‐
mand line option takes precedence if there's a conflict. charset
can be any encoding supported by the system's "iconv" library rou‐
Enable generation of linemarkers in the preprocessor output that
will let the compiler know the current working directory at the
time of preprocessing. When this option is enabled, the preproces‐
sor will emit, after the initial linemarker, a second linemarker
with the current working directory followed by two slashes. GCC
will use 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 inhib‐
ited 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.
Do not print column numbers in diagnostics. This may be necessary
if diagnostics are being scanned by a program that does not under‐
stand the column numbers, such as dejagnu.
Make an assertion with the predicate predicate and answer answer.
This form is preferred to the older form -Apredicate(answer),
which is still supported, because it does not use shell special
Cancel an assertion with the predicate predicate and answer answer.
CHARS is a sequence of one or more of the following characters, and
must not be preceded by a space. Other characters are interpreted
by the compiler proper, or reserved for future versions of GCC, and
so are silently ignored. If you specify characters whose behavior
conflicts, the result is undefined.
M Instead of the normal output, generate a list of #define direc‐
tives 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 Like M except in two respects: it does not include the prede‐
fined macros, and it outputs both the #define directives and
the result of preprocessing. Both kinds of output go to the
standard output file.
N Like D, but emit only the macro names, not their expansions.
I Output #include directives in addition to the result of prepro‐
-P Inhibit generation of linemarkers in the output from the preproces‐
sor. This might be useful when running the preprocessor on some‐
thing that is not C code, and will be sent to a program which might
be confused by the linemarkers.
-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 ordi‐
nary 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.
Try to imitate the behavior of old-fashioned C preprocessors, as
opposed to ISO C preprocessors.
Process trigraph sequences.
Enable special code to work around file systems which only permit
very short file names, such as MS-DOS.
Print text describing all the command line options instead of pre‐
-v Verbose mode. Print out GNU CPP's version number at the beginning
of execution, and report the final form of the include path.
-H Print the name of each header file used, in addition to other nor‐
mal 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 ...! .
Print out GNU CPP's version number. With one dash, proceed to pre‐
process as normal. With two dashes, exit immediately.
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.
Each variable's value is a list of directories separated by a spe‐
cial 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 speci‐
fied with -I, but after any paths given with -I options on the com‐
mand 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 direc‐
tories 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
If this variable is set, its value specifies how to output depen‐
dencies for Make based on the non-system header files processed by
the compiler. System header files are ignored in the dependency
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 combin‐
ing the options -MM and -MF, with an optional -MT switch too.
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
SEE ALSOgpl(7), gfdl(7), fsf-funding(7), gcc(1), as(1), ld(1), and the Info
entries for cpp, gcc, and binutils.
Copyright (c) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997,
1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software Founda‐
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 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.
gcc-4.2.2 2007-10-07 CPP(1)