cpp(1)

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 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 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.

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 stan
dard 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 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.
-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 usi-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.
-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 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 sup
presses preprocessed output, as a missing header file
is ordinarily an error.; 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 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 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.
-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 -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 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 #.
-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 implementations to define extra characters that can appear in identifiers. By default GNU CPP permits $, a common extension.
-h
--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. 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 directories 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-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 the environment variable
DEPENDENCIES_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.

COPYRIGHT

Copyright (c) 1987, 1989, 1991, 1992, 1993, 1994, 1995,
1996, 1997, 1998, 1999, 2000, 2001 Free Software Founda
tion, 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.

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See also