cpp − The C Preprocessor

cpp [−Dmacro[=defn]...] [−Umacro]
    [−Idir...] [−iquotedir...]
    [−M|−MM] [−MG] [−MF filename]
    [−MP] [−MQ target...]
    [−MT target...]
    [−P] [−fno−working−directory]
    [−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−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

     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=c90, −std=c99
or −std=c11 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 Traditional Mode.

     For clarity, unless noted otherwise, references to CPP
in this manual refer to GNU CPP.

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
    The contents of definition are tokenized and processed
    as if they appeared during translation phase three in a
    #define directive.  In particular, the definition will
    be truncated by embedded newline characters.

    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 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

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

    Do not predefine any system‐specific or GCC‐specific
    macros.  The standard 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 .  If dir begins with "=", then the "="
    will be replaced by the sysroot prefix; see −−sysroot
    and −isysroot.

−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, −Wtrigraphs,
    −Wmultichar and a warning about integer promotion
    causing a change of sign in "#if" expressions.  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.)

    Most trigraphs in comments cannot affect the meaning of
    the program.  However, a trigraph that would form an
    escaped newline (??/ at the end of a line) can, by


    changing where the comment begins or ends.  Therefore,
    only trigraphs that would form escaped newlines produce
    warnings inside a comment.

    This option is implied by −Wall.  If −Wall is not given,
    this option is still enabled unless trigraphs are
    enabled.  To get trigraph conversion without warnings,
    but get the other −Wall warnings, use −trigraphs −Wall

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

    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

    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



    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 name of the source file
    with any suffix replaced with object file suffix and
    with any leading directory parts removed.  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.

−MF file
    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


−MT target
    Change the target of the rule emitted by dependency
    generation.  By default CPP takes the name of the main
    input file, deletes any directory components and 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 −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 takes the
    name of the input file, removes any directory components
    and suffix, 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 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.

    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.



    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:



        The ISO C standard from 1990.  c90 is the customary
        shorthand for this version of the standard.

        The −ansi option is equivalent to −std=c90.

        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 revised ISO C standard, published in December
        2011.  Before publication, this was known as C1X.


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


        The 1999 C standard plus GNU extensions.


        The 2011 C standard plus GNU extensions.


        The 1998 ISO C++ standard plus amendments.

        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"".

    This option has been deprecated.

    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.  If dir begins with "=", then
    the "=" will be replaced by the sysroot prefix; see
    −−sysroot and −isysroot.

−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

−isysroot dir
    This option is like the −−sysroot option, but applies
    only to header files (except for Darwin targets, where
    it applies to both header files and libraries).  See the
    −−sysroot option for more information.

−imultilib dir
    Use dir as a subdirectory of the directory containing
    target‐specific C++ headers.

−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.

    If dir begins with "=", then the "=" will be replaced by
    the sysroot prefix; see −−sysroot and −isysroot.

−iquote dir
    Search dir only for header files requested with
    "#include "file""; they are not searched for
    "#include <file>", before all directories specified by
    −I and before the standard system directories.

    If dir begins with "=", then the "=" will be replaced by
    the sysroot prefix; see −−sysroot and −isysroot.

    When preprocessing, handle directives, but do not expand


    The option’s behavior depends on the −E and
    −fpreprocessed options.

    With −E, preprocessing is limited to the handling of
    directives such as "#define", "#ifdef", and "#error".
    Other preprocessor operations, such as macro expansion
    and trigraph conversion are not performed.  In addition,
    the −dD option is implicitly enabled.

    With −fpreprocessed, predefinition of command line and
    most builtin macros is disabled.  Macros such as
    "__LINE__", which are contextually dependent, are
    handled normally.  This enables compilation of files
    previously preprocessed with "−E −fdirectives−only".

    With both −E and −fpreprocessed, the rules for
    −fpreprocessed take precedence.  This enables full
    preprocessing of files previously preprocessed with "−E

    Accept $ in identifiers.

    Accept universal character names in identifiers.  This
    option is experimental; in a future version of GCC, it
    will be enabled by default for C99 and C++.

    When preprocessing, do not shorten system header paths
    with canonicalization.

    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.


    This option is only useful for debugging GCC.  When used
    with −E, dumps debugging information about location
    maps.  Every token in the output is preceded by the dump
    of the map its location belongs to.  The dump of the map
    holding the location of a token would be:


    When used without −E, this option has no effect.

    Track locations of tokens across macro expansions. This
    allows the compiler to emit diagnostic about the current
    macro expansion stack when a compilation error occurs in
    a macro expansion. Using this option makes the
    preprocessor and the compiler consume more memory. The
    level parameter can be used to choose the level of
    precision of token location tracking thus decreasing the
    memory consumption if necessary. Value 0 of level de‐
    activates this option just as if no
    −ftrack−macro−expansion was present on the command line.
    Value 1 tracks tokens locations in a degraded mode for
    the sake of minimal memory overhead. In this mode all
    tokens resulting from the expansion of an argument of a
    function‐like macro have the same location. Value 2
    tracks tokens locations completely. This value is the
    most memory hungry.  When this option is given no
    argument, the default parameter value is 2.

    Note that −ftrack−macro−expansion=2 is activated by

    Set the execution character set, used for string and
    character constants.  The default is UTF−8.  charset can
    be any encoding supported by the system’s "iconv"
    library routine.

    Set the wide execution character set, used for wide
    string and character constants.  The default is UTF−32
    or UTF−16, whichever corresponds to the width of
    "wchar_t".  As with −fexec−charset, charset can be any
    encoding supported by the system’s "iconv" library
    routine; however, you will have problems with encodings
    that do not fit exactly in "wchar_t".

    Set the input character set, used for translation from
    the character set of the input file to the source
    character set used by GCC.  If the locale does not
    specify, or GCC cannot get this information from the
    locale, the default is UTF−8.  This can be overridden by


    either the locale or this command line option.
    Currently the command line option takes precedence if
    there’s a conflict.  charset can be any encoding
    supported by the system’s "iconv" library routine.

    Enable generation of linemarkers in the preprocessor
    output that will let the compiler know the current
    working directory at the time of preprocessing.  When
    this option is enabled, the preprocessor will emit,
    after the initial linemarker, a second linemarker with
    the current working directory followed by two slashes.
    GCC will use this directory, when it’s present in the
    preprocessed input, as the directory emitted as the
    current working directory in some debugging information
    formats.  This option is implicitly enabled if debugging
    information is enabled, but this can be inhibited with
    the negated form −fno−working−directory.  If the −P flag
    is present in the command line, this option has no
    effect, since no "#line" directives are emitted

    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.

    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.

        If you use −dM without the −E option, −dM is
        interpreted as a synonym for −fdump−rtl−mach.

    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.

    U   Like D except that only macros that are expanded, or
        whose definedness is tested in preprocessor
        directives, are output; the output is delayed until
        the use or test of the macro; and #undef directives
        are also output for macros tested but undefined at
        the time.

−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

−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

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

    Process trigraph sequences.

    Enable special code to work around file systems which
    only permit very short file names, such as MS‐DOS.


    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.  Precompiled
    header files are also printed, even if they are found to
    be invalid; an invalid precompiled header file is
    printed with ...x and a valid one with ...! .


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

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.





    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 Microsoft Windows‐based targets it
    is a semicolon, and for almost all other targets it is a

    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

    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.

    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 equivalent
    to combining the options −MM and −MF, with an optional
    −MT switch too.

    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.


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

Copyright (c) 1987−2014 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 1.3 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.