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 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
(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
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
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 @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 different from
-d -M.
-Dname
Predefine name as a macro, with definition 1.
-Dname=definition
Predefine name as a macro, with definition definition.
There are no restrictions on the contents of definition, 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). Parentheses 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 -imacrosfile and
-includefile options are processed after all
-D and -U options.
-Uname
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 predefined macros remain defined.
-Idir
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
.
-ofile
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 -Wtrigraphs. 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 specified, 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 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 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 -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 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 will appear in -MM dependency output. This is a
slight change in semantics from GCC versions 3.0 and earlier.
-MFfile
@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 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:
-MTtarget
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 '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQtarget
Same as -MT, but it quotes any characters which are special to
Make. -MQ '$(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 -MFfile, 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 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
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
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 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 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"".
-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.)
-includefile
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.
-imacrosfile
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.
-idirafterdir
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.
-iprefixprefix
Specify prefix as the prefix for subsequent -iwithprefix
options. If the prefix represents a directory, you should include the
final /.
-iwithprefixdir
-iwithprefixbeforedir
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.
-isystemdir
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.
-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.
-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.
-Apredicate=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 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
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 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.
-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
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
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 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
filetarget, 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.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7),
gcc(1), as(1), ld(1), and the Info entries for cpp, gcc, and
binutils.
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.
