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NAME

       cpp - The C Preprocessor

SYNOPSIS

       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 details.

DESCRIPTION

       The C preprocessor, often known as cpp, is a macro processor that is used automatically by
       the C compiler to transform your program before compilation.  It is called a macro
       processor because it allows you to define macros, which are brief abbreviations for longer
       constructs.

       The C preprocessor is intended to be used only with C, C++, and 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.

OPTIONS

       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 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 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'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.

       -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 declarations.

           All files specified by -imacros are processed before all files specified by -include.

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

       -pthread
           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
           targets.

       -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
           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 appears in -MM dependency
           output.

       -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 stdout.

       -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

                   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.

       -MMD
           Like -MD except mention only user header files, not system header files.

       -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
           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 -save-temps.

       -fdirectives-only
           When preprocessing, handle directives, but do not expand macros.

           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
           -fdirectives-only".

       -fdollars-in-identifiers
           Accept $ in identifiers.

       -fextended-identifiers
           Accept universal character names in identifiers.  This option is enabled by default
           for C99 (and later C standard versions) and C++.

       -fno-canonical-system-headers
           When preprocessing, do not shorten system header paths with canonicalization.

       -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.

       -ftrack-macro-expansion[=level]
           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 default.

       -fmacro-prefix-map=old=new
           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 -ffile-prefix-map.

       -fexec-charset=charset
           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.

       -fwide-exec-charset=charset
           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".

       -finput-charset=charset
           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.

       -fworking-directory
           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 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 #.

       -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 comments.

       -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.

       -traditional
       -traditional-cpp
           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 explicitly.

       -trigraphs
           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 newline.

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

       -remap
           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 ...! .

       -dletters
           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 command

                       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 expansions.

           -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.

       -fdebug-cpp
           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 effect.

       -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 -isysroot.

           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 would.

       -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.

       -nostdinc
           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 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.)

       -Wcomment
       -Wcomments
           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.

       -Wtrigraphs
           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 -Wno-trigraphs.

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

       -Wexpansion-to-defined
           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.

       -Wunused-macros
           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
                   #endif

       -Wno-endif-labels
           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.

ENVIRONMENT

       This section describes the environment variables that affect how CPP operates.  You can
       use them to specify directories or prefixes to use when searching for include files, or to
       control dependency output.

       Note that you can also specify places to search using options such as -I, and control
       dependency output with options like -M.  These take precedence over environment variables,
       which in turn take precedence over the configuration of GCC.

       CPATH
       C_INCLUDE_PATH
       CPLUS_INCLUDE_PATH
       OBJC_INCLUDE_PATH
           Each variable's value is a list of directories separated by a 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
           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.  This 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.

           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.

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

       SUNPRO_DEPENDENCIES
           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.

       SOURCE_DATE_EPOCH
           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.

SEE ALSO

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

COPYRIGHT

       Copyright (c) 1987-2019 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.