cpp − The C Preprocessor

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.

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 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.

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

     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 use more
    than one −D for the same name, the rightmost definition
    takes effect.

    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,

−U name
    Cancel any previous definition of name, either built in
    or provided with a −D option.

    All −imacros file and −include file options are
    processed after all −D and −U options.

    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

    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.

    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.


    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.)

    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.

    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.

    Warn the first time #import is used.

    Warn whenever an identifier which is not a macro is
    encountered in an #if directive, outside of defined.
    Such identifiers are replaced with zero.


    Make all warnings into hard errors.  Source code which
    triggers warnings will be rejected.

    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.

    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.

    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

−MM Like −M, but mention only the files included with
    "#include "file"" or with −include or −imacros command
    line options.  System header files included with
    "#include <file>" are omitted.

−MF file
    When used with −M or −MM, specifies a file to write the
    dependencies to.  This allows the preprocessor to write
    the preprocessed file to stdout normally.  If no −MF
    switch is given, CPP sends the rules to stdout and
    suppresses normal preprocessed output.

−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


−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 ’$(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 file

−MMD file
    −MD file is equivalent to −M −MF file, and −MMD file is
    equivalent to −MM −MF file.

    Due to limitations in the compiler driver, you must use
    these switches when you want to generate a dependency
    file as a side‐effect of normal compilation.

−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.


    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:


        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.

        The 1990 C standard, as amended in 1994.




        The revised ISO C standard, published in December
        1999.  Before publication, this was known as C9X.

        The 1990 C standard plus GNU extensions.  This is
        the default.



        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"".

    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.

    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 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.

−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

    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 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

    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.

    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

    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.

    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


−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.)

    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

                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

    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


−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

    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

    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.

    Try to imitate the behavior of old‐fashioned C, as
    opposed to ISO C.

    Process trigraph sequences.

    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.



    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.



    Print out GNU CPP’s version number.  With one dash,
    proceed to preprocess as normal.  With two dashes, exit

gpl(7), gfdl(7), fsf−funding(7), gcc(1), as(1), ld(1), and
the Info entries for cpp, gcc, and binutils.

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

     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.