CPP(1) GNU CPP(1)NAMEcpp - The C Preprocessor
SYNOPSIScpp [-Dmacro[=defn]...] [-Umacro]
[-Idir...] [-Wwarn...]
[-M|-MM] [-MG] [-MF filename]
[-MP] [-MQ target...] [-MT target...]
[-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 proces-
sor that is used automatically by the C compiler to trans-
form 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 char-
acter 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 lan-
guages 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 pro-
gramming 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 dis-
cusses 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 diag-
nostics, you must also use -pedantic.
This manual describes the behavior of the ISO preproces-
sor. 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 @ref{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 dif-
ferent from -d -M.
-D name
Predefine name as a macro, with definition 1.
-D name=definition
Predefine name as a macro, with definition definition.
There are no restrictions on the contents of defini-
tion, but 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). Paren-
theses are meaningful to most shells, so you will need
to 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.
-undef
Do not predefine any system-specific macros. The com-
mon predefined 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 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 .
-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 and -Wtri-
graphs. Note that many of the preprocessor'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
Warn if any trigraphs are encountered. This option
used to take effect only if -trigraphs was also speci-
fied, but now works independently. Warnings are not
given for trigraphs within comments, as they do not
affect the meaning of the program.
-Wtraditional
Warn about certain constructs that behave differently
in traditional and ISO C. Also warn about ISO C con-
structs 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 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 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 -pedan-
tic but treats as warnings.
-M Instead of outputting the result of preprocessing,
output a rule suitable for make describing the depen-
dencies 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 out-
put 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 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 dou-
ble 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 seman-
tics from GCC versions 3.0 and earlier.
-MF file
@anchor{-MF} 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 dependency 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" direc-
tive 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 make-
files.
-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, including any path, deletes any file suf-
fix 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 '$(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 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{-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 compi-
lation 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 nothing to do with standards con-
formance 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 custom-
ary shorthand for this version 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 publication, 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 head-
ers requested with "#include "file""; they are not
searched for "#include <file>". If additional direc-
tories are specified with -I options after the -I-,
those directories are searched for all #include direc-
tives.
In addition, -I- inhibits the use of the directory of
the current file directory as the first search direc-
tory for "#include "file"".
-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 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.)
-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 con-
taining 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 dec-
larations.
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 -iwithpre-
fix 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.
Use of these options is discouraged.
-isystem dir
Search dir for header files, after all directories
specified by -I but before the standard system direc-
tories. Mark it as a system directory, so that it
gets the same special treatment as is applied to the
standard system directories.
-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 com-
ments, 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 tok-
enizer 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 cre-
ated 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.
-fno-show-column
Do not print column numbers in diagnostics. This may
be necessary if diagnostics are being scanned by a
program that does not understand the column numbers,
such as dejagnu.
-A predicate=answer
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 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 dur-
ing the execution of the preprocessor, including
predefined macros. This gives you a way of find-
ing 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 preprocess-
ing. Both kinds of output go to the standard out-
put 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 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 expan-
sion. 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 com-
ments.
-gcc
Define the macros __GNUC__, __GNUC_MINOR__ and
__GNUC_PATCHLEVEL__. These are defined automatically
when you use gcc -E; you can turn them off in that
case with -no-gcc.
-traditional-cpp
Try to imitate the behavior of old-fashioned C prepro-
cessors, 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 execution, and report the final form
of the include path.
-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.
-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 control dependency output with
options like -M. These take precedence over environment
variables, which in turn take precedence over the configu-
ration of GCC.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable's value is a list of directories sepa-
rated 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 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 ":/spe-
cial/include", that has the same effect as -I. -I/spe-
cial/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 equiva-
lent 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.
SEE ALSOgpl(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 Free Soft-
ware Foundation, Inc.
Permission is granted to copy, distribute and/or modify
this document under the terms of the GNU Free Documenta-
tion 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-3.3 2003-05-14 CPP(1)
