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CPP(1) GNU CPP(1)
NAME
cpp - The C Preprocessor
SYNOPSIS
cpp [-Dmacro[=defn]...] [-Umacro]
[-Idir...] [-iquotedir...]
[-Wwarn...]
[-M│-MM] [-MG] [-MF filename]
[-MP] [-MQ target...]
[-MT target...]
[-P] [-fno-working-directory]
[-x language] [-std=standard]
infile outfile
Only the most useful options are listed here; see below for the remainder.
DESCRIPTION
The C preprocessor, often known as cpp, is a macro processor that is used automati-
cally 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 abbrevia-
tions 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 (Pascal, 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 preproces-
sor, 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 stan-
dard. 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 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 tradi-
tional 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.
OPTIONS
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 name
Predefine name as a macro, with definition 1.
-D name=definition
The contents of definition are tokenized and processed as if they appeared dur-
ing translation phase three in a #define directive. In particular, the defini-
tion will be 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 will need to quote the
option. With sh and csh, -Dâ€â€™name(args...)=definitionâ€â€™ 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.
-undef
Do not predefine any system-specific or GCC-specific macros. The standard pre-
defined macros remain defined.
-I dir
Add the directory dir to the list of directories to be searched for header
files.
Directories named by -I are searched before the standard system include direc-
tories. If the directory dir is a standard system include directory, the
option is ignored to ensure that the default search order for system directo-
ries and the special treatment of system headers are not defeated .
-o file
Write output to file. This is the same as specifying file as the second non-
option argument to cpp. gcc has a different interpretation of a second non-
option argument, so you must use -o to specify the output file.
-Wall
Turns on all optional warnings which are desirable for normal code. At present
this is -Wcomment, -Wtrigraphs, -Wmultichar and a warning about integer promo-
tion causing a change of sign in "#if" expressions. Note that many of the pre-
processor’s warnings are on by default and have no options to control them.
-Wcomment
-Wcomments
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.)
-Wtrigraphs
@anchor{Wtrigraphs} Most trigraphs in comments cannot affect the meaning of the
program. 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 com-
ment.
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 warn-
ings, but get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs.
-Wtraditional
Warn about certain constructs that behave differently in traditional and ISO C.
Also warn about ISO C constructs that have no traditional C equivalent, and
problematic constructs which should be avoided.
-Wimport
Warn the first time #import is used.
-Wundef
Warn whenever an identifier which is not a macro is encountered in an #if
directive, outside of defined. Such identifiers are replaced with zero.
-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 will
also warn if the macro has not been used at the time it is redefined or unde-
fined.
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
Warn whenever an #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
Make all warnings into hard errors. Source code which triggers warnings will
be rejected.
-Wsystem-headers
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 them.
-w Suppress all warnings, including those which GNU CPP issues by default.
-pedantic
Issue all the mandatory diagnostics listed in the C standard. Some of them are
left out by default, since they trigger frequently on harmless code.
-pedantic-errors
Issue all the mandatory diagnostics, and make all mandatory diagnostics 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 suf-
fix. 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 out-
put 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 direc-
tories, 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.
@anchor{dashMF}
-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
have sent preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the default depen-
dency output file.
-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 depen-
dency 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 Make-
file 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, 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 â€â€™$(objpfx)foo.oâ€â€™ might give
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special to Make.
-MQ â€â€™$(objpfx)foo.oâ€â€™ 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 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 @pxref{dashMF,,-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
Like -MD except mention only user header files, not system header files.
-x c
-x c++
-x objective-c
-x assembler-with-cpp
Specify the source language: C, C++, Objective-C, or assembly. This has noth-
ing 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
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
Specify the standard to which the code should conform. Currently CPP knows
about C and C++ standards; others may be added in the future.
standard may be one of:
"iso9899:1990"
"c89"
The ISO C standard from 1990. c89 is the customary shorthand for this ver-
sion of the standard.
The -ansi option is equivalent to -std=c89.
"iso9899:199409"
The 1990 C standard, as amended in 1994.
"iso9899:1999"
"c99"
"iso9899:199x"
"c9x"
The revised ISO C standard, published in December 1999. Before publica-
tion, this was known as C9X.
"gnu89"
The 1990 C standard plus GNU extensions. This is the default.
"gnu99"
"gnu9x"
The 1999 C standard plus GNU extensions.
"c++98"
The 1998 ISO C++ standard plus amendments.
"gnu++98"
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 direc-
tory as the first search directory for "#include "file"".
This option has been deprecated.
-nostdinc
Do not search the standard system directories for header files. Only the
directories you have specified with -I options (and the directory of the cur-
rent 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 build-
ing the C++ library.)
-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 preproces-
sor’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.
-idirafter dir
Search dir for header files, but do it after all directories specified with -I
and the standard system directories have been exhausted. dir is treated as a
system include directory.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix options. If the pre-
fix 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.
-isystem dir
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 directo-
ries.
-iquote dir
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.
-fdollars-in-identifiers
@anchor{fdollars-in-identifiers} Accept $ in identifiers.
-fpreprocessed
Indicate to the preprocessor that the input file has already been preprocessed.
This suppresses things like macro expansion, trigraph conversion, escaped new-
line splicing, and processing of most directives. The preprocessor still rec-
ognizes 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.
-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.
-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 con-
stants. 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 con-
flict. charset can be any encoding supported by the system’s "iconv" library
routine.
-fworking-directory
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 preprocessor will emit, after the initial line-
marker, 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 debug-
ging 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.
-fno-show-column
Do not print column numbers in diagnostics. This may be necessary if diagnos-
tics are being scanned by a program that does not understand the column num-
bers, such as dejagnu.
-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.
-dCHARS
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 directives for all
the macros defined during the execution of the preprocessor, including pre-
defined 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 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 Like D, but emit only the macro names, not their expansions.
I Output #include directives in addition to the result of preprocessing.
-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.
-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 preproces-
sor 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.
-traditional-cpp
Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO
C preprocessors.
-trigraphs
Process trigraph sequences.
-remap
Enable special code to work around file systems which only permit very short
file names, such as MS-DOS.
--help
--target-help
Print text describing all the command line options instead of preprocessing
anything.
-v Verbose mode. Print out GNU CPP’s version number at the beginning of execu-
tion, and report the final form of the include path.
-H Print the name of each header file used, in addition to other normal activi-
ties. 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 ...! .
-version
--version
Print out GNU CPP’s version number. With one dash, proceed to preprocess as
normal. With two dashes, exit immediately.
ENVIRONMENT
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 con-
trol dependency output with options like -M. These take precedence over environ-
ment 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 charac-
ter, 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 environ-
ment variable is used regardless of which language is being preprocessed.
The remaining environment variables apply only when preprocessing the particu-
lar 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 sys-
tem header files are not ignored, so it implies -M rather than -MM. However,
the dependence on the main input file is omitted.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), gcc(1), as(1), ld(1), and the Info entries for
cpp, gcc, and binutils.
COPYRIGHT
Copyright (c) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
2000, 2001, 2002, 2003, 2004, 2005 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 1.1 or any later version pub-
lished 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.
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