cpp − The C Preprocessor

cpp [−Dmacro[=defn]...] [−Umacro]
    [−Idir...] [−iquotedir...]
    [−M|−MM] [−MG] [−MF filename]
    [−MP] [−MQ target...]
    [−MT target...]
    infile [[−o] outfile]

     Only the most useful options are given above; see below
for a more complete list of preprocessor‐specific options.
In addition, cpp accepts most gcc driver options, which are
not listed here.  Refer to the GCC documentation for

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 (Ada, etc.) So is assembly, with
caution.  −traditional−cpp mode preserves more white space,
and is otherwise more permissive.  Many of the problems can
be avoided by writing C or C++ style comments instead of
native language comments, and keeping macros simple.

     Wherever possible, you should use a preprocessor geared
to the language you are writing in.  Modern versions of the
GNU assembler have macro facilities.  Most high level
programming languages have their own conditional compilation
and inclusion mechanism.  If all else fails, try a true
general text processor, such as GNU M4.

     C preprocessors vary in some details.  This manual
discusses the GNU C preprocessor, which provides a small
superset of the features of ISO Standard C.  In its default
mode, the GNU C preprocessor does not do a few things
required by the standard.  These are features which are
rarely, if ever, used, and may cause surprising changes to
the meaning of a program which does not expect them.  To get


strict ISO Standard C, you should use the −std=c90,
−std=c99, −std=c11 or −std=c17 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 cpp command 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.  If either file is omitted, it
means the same as if had been specified for that file.
You can also use the −o outfile option to specify the output

     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 is
    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 should


    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.

−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.

    Do not predefine any system‐specific or GCC‐specific
    macros.  The standard predefined macros remain defined.

    Define additional macros required for using the POSIX
    threads library.  You should use this option
    consistently for both compilation and linking.  This
    option is supported on GNU/Linux targets, most other
    Unix derivatives, and also on x86 Cygwin and MinGW

−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 is still sent to the regular output stream as

    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 appears in −MM dependency

−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
    send preprocessed output.

    When used with the driver options −MD or −MMD, −MF
    overrides the default dependency output file.

    If file is , then the dependencies are written to

−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 sets 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.

    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

    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 and extended characters
    in identifiers.  This option is enabled by default for
    C99 (and later C standard versions) and C++.

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

    Set the maximum depth of the nested #include. The
    default is 200.


    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.

    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. 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

    When preprocessing files residing in directory old,
    expand the "__FILE__" and "__BASE_FILE__" macros as if
    the files resided in directory new instead.  This can be
    used to change an absolute path to a relative path by
    using . for new which can result in more reproducible
    builds that are location independent.  This option also
    affects "__builtin_FILE()" during compilation.  See also

    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 let the compiler know the current working
    directory at the time of preprocessing.  When this
    option is enabled, the preprocessor emits, after the
    initial linemarker, a second linemarker with the current
    working directory followed by two slashes.  GCC uses
    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 whatsoever.

−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.

−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

−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


    Try to imitate the behavior of pre‐standard C
    preprocessors, as opposed to ISO C preprocessors.

    Note that GCC does not otherwise attempt to emulate a
    pre‐standard C compiler, and these options are only
    supported with the −E switch, or when invoking CPP

    Support ISO C trigraphs.  These are three‐character
    sequences, all starting with ??, that are defined by ISO
    C to stand for single characters.  For example, ??/
    stands for \, so ’??/n’ is a character constant for a

    By default, GCC ignores trigraphs, but in standard‐
    conforming modes it converts them.  See the −std and
    −ansi options.

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

−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 ...! .

    Says to make debugging dumps during compilation as
    specified by letters.  The flags documented here are
    those relevant to the preprocessor.  Other letters are
    interpreted by the compiler proper, or reserved for
    future versions of GCC, and so are silently ignored.  If


    you specify letters whose behavior conflicts, the result
    is undefined.

    −dM 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

        shows all the predefined macros.

    −dD Like −dM 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.

    −dN Like −dD, but emit only the macro names, not their

    −dI Output #include directives in addition to the result
        of preprocessing.

    −dU Like −dD 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.

    This option is only useful for debugging GCC.  When used
    from CPP or with −E, it dumps debugging information
    about location maps.  Every token in the output is
    preceded by the dump of the map its location belongs to.

    When used from GCC without −E, this option has no

−I dir

−iquote dir

−isystem dir

−idirafter dir
    Add the directory dir to the list of directories to be
    searched for header files during preprocessing.

    If dir begins with = or $SYSROOT, then the = or $SYSROOT
    is replaced by the sysroot prefix; see −−sysroot and



    Directories specified with −iquote apply only to the
    quote form of the directive, "#include "file"".
    Directories specified with −I, −isystem, or −idirafter
    apply to lookup for both the "#include "file"" and
    "#include <file>" directives.

    You can specify any number or combination of these
    options on the command line to search for header files
    in several directories.  The lookup order is as follows:

    1.  For the quote form of the include directive, the
        directory of the current file is searched first.

    2.  For the quote form of the include directive, the
        directories specified by −iquote options are
        searched in left‐to‐right order, as they appear on
        the command line.

    3.  Directories specified with −I options are scanned in
        left‐to‐right order.

    4.  Directories specified with −isystem options are
        scanned in left‐to‐right order.

    5.  Standard system directories are scanned.

    6.  Directories specified with −idirafter options are
        scanned in left‐to‐right order.

        You can use −I to override a system header file,
        substituting your own version, since these
        directories are searched before the standard system
        header file directories.  However, you should not
        use this option to add directories that contain
        vendor‐supplied system header files; use −isystem
        for that.

        The −isystem and −idirafter options also mark the
        directory as a system directory, so that it gets the
        same special treatment that is applied to the
        standard system directories.

        If a standard system include directory, or a
        directory specified with −isystem, is also specified
        with −I, the −I option is ignored.  The directory is
        still searched but as a system directory at its
        normal position in the system include chain.  This
        is to ensure that GCC’s procedure to fix buggy
        system headers and the ordering for the
        "#include_next" directive are not inadvertently
        changed.  If you really need to change the search
        order for system directories, use the −nostdinc


        and/or −isystem options.

−I− Split the include path.  This option has been
    deprecated.  Please use −iquote instead for −I
    directories before the −I− and remove the −I− option.

    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"".  There is no way to override this
    effect of −I−.

−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.

    Do not search the standard system directories for header
    files.  Only the directories explicitly specified with
    −I, −iquote, −isystem, and/or −idirafter options (and
    the directory of the current file, if appropriate) are

    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.)



    Warn whenever a comment‐start sequence /* appears in a
    /* comment, or whenever a backslash‐newline appears in a
    // comment.  This warning is enabled by −Wall.

    Warn if any trigraphs are encountered that might change
    the meaning of the program.  Trigraphs within comments
    are not warned about, except those that would form
    escaped newlines.

    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 if an undefined identifier is evaluated in an "#if"
    directive.  Such identifiers are replaced with zero.

    Warn whenever defined is encountered in the expansion of
    a macro (including the case where the macro is expanded
    by an #if directive).  Such usage is not portable.  This
    warning is also enabled by −Wpedantic and −Wextra.

    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 also warns 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 the preprocessor
    reports 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

    Do not warn whenever an "#else" or an "#endif" are
    followed by text.  This sometimes happens in older
    programs with code of the form


            #if FOO
            #else FOO
            #endif FOO

    The second and third "FOO" should be in comments.  This
    warning is on by default.

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.

    If this variable is set, its value specifies a UNIX
    timestamp to be used in replacement of the current date
    and time in the "__DATE__" and "__TIME__" macros, so
    that the embedded timestamps become reproducible.

    The value of SOURCE_DATE_EPOCH must be a UNIX timestamp,
    defined as the number of seconds (excluding leap
    seconds) since 01 Jan 1970 00:00:00 represented in
    ASCII; identical to the output of @command{date +%s} on
    GNU/Linux and other systems that support the %s
    extension in the "date" command.

    The value should be a known timestamp such as the last
    modification time of the source or package and it should
    be set by the build process.

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

Copyright (c) 1987−2020 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.