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NAME
cpp - The C Preprocessor
SYNOPSIS
cpp [-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 remain-
der.
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 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 mode pre-
serves 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.
OPTIONS
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.
-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 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 -imacros file and -include file options are pro-
cessed 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 common 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.
It is dangerous to specify a standard system include directory in
an -I option. This defeats the special treatment of system headers
. It can also defeat the repairs to buggy system headers which GCC
makes when it is installed.
-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 interpreta-
tion 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 inde-
pendently. 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 tradi-
tional 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.
-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 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
normal.
Passing -M to the driver implies -E.
-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.
-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 When used with -M or -MM, -MG says to treat missing header files as
generated files and assume they live in the same directory as the
source file. It suppresses preprocessed output, as a missing
header file is ordinarily an error.
This feature is used in automatic updating of makefiles.
-MP This option instructs CPP to add a phony target for each dependency
other than the main file, causing each to depend on nothing. These
dummy rules work around errors make gives if you remove header
files without updating the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
-MT target
Change the target of the rule emitted by dependency generation. By
default CPP takes the name of the main input file, 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
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special to
Make. -MQ '$(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 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 recog-
nize 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 only knows about the standards for C; other language standards
will 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.
-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 direc-
tories, 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 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
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 speci-
fied 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 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. -iwithpre-
fixbefore 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 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, 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.
-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 under-
stand 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 -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.
-A- Cancel all predefined assertions and all assertions preceding it on
the command line. Also, undefine all predefined macros and all
macros preceding it on the command line. (This is a historical
wart and may change in the future.)
-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 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-
cessing.
-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
#.
-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
Try to imitate the behavior of old-fashioned C, as opposed to ISO
C.
-trigraphs
Process trigraph sequences.
-remap
Enable special code to work around file systems which only permit
very short file names, such as MS-DOS.
-$ Forbid the use of $ in identifiers. The C standard allows imple-
mentations to define extra characters that can appear in identi-
fiers. By default GNU CPP permits $, a common extension.
-h
--help
--target-help
Print text describing all the command line options instead of pre-
processing 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 nor-
mal 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 pre-
process 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 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 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. The 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.
See also @ref{Search Path}.
DEPENDENCIES_OUTPUT
@anchor{DEPENDENCIES_OUTPUT} If this variable is set, its value
specifies how to output dependencies for Make based on the non-sys-
tem 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 combin-
ing the options -MM and -MF, with an optional -MT switch too.
SUNPRO_DEPENDENCIES
This variable is the same as the environment variable DEPENDEN-
CIES_OUTPUT, 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.
COPYRIGHT
Copyright (c) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997,
1998, 1999, 2000, 2001 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 published by the Free Software Foundation. A copy of
the license is included in the man pagegfdl(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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