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        cpp - The C Preprocessor


        cpp [-Dmacro[=defn]...] [-Umacro]
            [-Idir...] [-Wwarn...]
            [-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 remain‐


        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 Objec‐
        tive-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 (Pas‐
        cal, 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 con‐
        ditional compilation and inclusion mechanism.  If all else fails, try a
        true general text processor, such as GNU M4.
        C preprocessors vary in some details.  This manual discusses the GNU C
        preprocessor, which provides a small superset of the features of ISO
        Standard C.  In its default mode, the GNU C preprocessor does not do a
        few things required by the standard.  These are features which are
        rarely, if ever, used, and may cause surprising changes to the meaning
        of a program which does not expect them.  To get strict ISO Standard C,
        you should use the -std=c89 or -std=c99 options, depending on which
        version of the standard you want.  To get all the mandatory diagnos‐
        tics, you must also use -pedantic.
        This manual describes the behavior of the ISO preprocessor.  To mini‐
        mize 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 out‐
        file.  The preprocessor reads infile together with any other files it
        specifies with #include.  All the output generated by the combined
        input files is written in outfile.
        Either infile or outfile may be -, which as infile means to read from
        standard input and as outfile means to write to standard output.  Also,
        if either file is omitted, it means the same as if - had been specified
        for that file.
        Unless otherwise noted, or the option ends in =, all options which take
        an argument may have that argument appear either immediately after the
        option, or with a space between option and argument: -Ifoo and -I foo
        have the same effect.
        Many options have multi-letter names; therefore multiple single-letter
        options may not be grouped: -dM is very different from -d -M.
        -D name
            Predefine name as a macro, with definition 1.
        -D name=definition
            Predefine name as a macro, with definition definition.  The con‐
            tents of definition are tokenized and processed as if they appeared
            during translation phase three in a #define directive.  In particu‐
            lar, the definition will be truncated by embedded newline charac‐
            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     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 pro‐
            cessed after all -D and -U options.
        -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 .
        -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 interpreta‐
            tion 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 warn‐
            ing 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.)
            @anchor{Wtrigraphs} 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 tri‐
            graph conversion without warnings, but get the other -Wall warn‐
            ings, use -trigraphs -Wall -Wno-trigraphs.
            Warn about certain constructs that behave differently in tradi‐
            tional and ISO C.  Also warn about ISO C constructs that have no
            traditional C equivalent, and problematic constructs which should
            be avoided.
            Warn the first time #import is used.
            Warn whenever an identifier which is not a macro is encountered in
            an #if directive, outside of defined.  Such identifiers are
            replaced with zero.
            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 condi‐
            tional 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 default.
            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
        -w  Suppress all warnings, including those which GNU CPP issues by
            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 diag‐
            nostics into errors.  This includes mandatory diagnostics that GCC
            issues without -pedantic but treats as warnings.
        -M  Instead of outputting the result of preprocessing, output a rule
            suitable for make describing the dependencies of the main source
            file.  The preprocessor outputs one make rule containing the object
            file name for that source file, a colon, and the names of all the
            included files, including those coming from -include or -imacros
            command line options.
            Unless specified explicitly (with -MT or -MQ), the object file name
            consists of the basename of the source file with any suffix
            replaced with object file suffix.  If there are many included files
            then the rule is split into several lines using \-newline.  The
            rule has no 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
            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 dependen‐
            cies 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 gen‐
            eration, -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 out‐
            put, 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, including any
            path, deletes any file suffix such as .c, and appends the plat‐
            form’s usual object suffix.  The result is the target.
            An -MT option will set the target to be exactly the string you
            specify.  If you want multiple targets, you can specify them as a
            single argument to -MT, or use multiple -MT options.
            For example, -MT      $(objpfx)foo.o      might give
                    $(objpfx)foo.o: foo.c
        -MQ target
            Same as -MT, but it quotes any characters which are special to
            Make.  -MQ      $(objpfx)foo.o      gives
                    $$(objpfx)foo.o: foo.c
            The default target is automatically quoted, as if it were given
            with -MQ.
        -MD -MD is equivalent to -M -MF file, except that -E is not implied.
            The driver determines file based on whether an -o option is given.
            If it is, the driver uses its argument but with a suffix of .d,
            otherwise it take the basename of the input file and applies a .d
            If -MD is used in conjunction with -E, any -o switch is understood
            to specify the dependency output file (but @pxref{dashMF,,-MF}),
            but if used without -E, each -o is understood to specify a target
            object file.
            Since -E is not implied, -MD can be used to generate a dependency
            output file as a side-effect of the compilation process.
            Like -MD except mention only user header files, not system -header
        -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 recog‐
            nize 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
            Specify the standard to which the code should conform.  Currently
            CPP knows about C and C++ standards; others may be added in the
            standard may be one of:
                The ISO C standard from 1990.  c89 is the customary shorthand
                for this version of the standard.
                The -ansi option is equivalent to -std=c89.
                The 1990 C standard, as amended in 1994.
                The revised ISO C standard, published in December 1999.  Before
                publication, this was known as C9X.
                The 1990 C standard plus GNU extensions.  This is the default.
                The 1999 C standard plus GNU extensions.
                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"".
            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 direc‐
            tories, 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 direc‐
            tory 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.
        -idirafter dir
            Search dir for header files, but do it after all directories speci‐
            fied with -I and the standard system directories have been
            exhausted.  dir is treated as a system include directory.
        -iprefix prefix
            Specify prefix as the prefix for subsequent -iwithprefix options.
            If the prefix represents a directory, you should include the final
        -iwithprefix dir
        -iwithprefixbefore dir
            Append dir to the prefix specified previously with -iprefix, and
            add the resulting directory to the include search path.  -iwithpre     
            fixbefore puts it in the same place -I would; -iwithprefix puts it
            where -idirafter would.
        -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.
            @anchor{fdollars-in-identifiers} Accept $ in identifiers.
            Indicate to the preprocessor that the input file has already been
            preprocessed.  This suppresses things like macro expansion, tri‐
            graph conversion, escaped newline splicing, and processing of most
            directives.  The preprocessor still recognizes and removes com‐
            ments, so that you can pass a file preprocessed with -C to the com‐
            piler 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.
            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.
            Set the execution character set, used for string and character con‐
            stants.  The default is UTF-8.  charset can be any encoding sup‐
            ported 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 -ftarget-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 charac‐
            ter 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 com‐
            mand line option takes precedence if there’s a conflict. charset
            can be any encoding supported by the system’s "iconv" library rou‐
            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 preproces‐
            sor 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 inhib‐
            ited 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.
            Do not print column numbers in diagnostics.  This may be necessary
            if diagnostics are being scanned by a program that does not under‐
            stand the column numbers, such as dejagnu.
        -A predicate=answer
            Make an assertion with the predicate predicate and answer answer.
            This form is preferred to the older form -A predicate(answer),
            which is still supported, because it does not use shell special
        -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 direc‐
                tives 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
                will show all the predefined macros.
            D   Like M except in two respects: it does not include the prede‐
                fined 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 expansions.
            I   Output #include directives in addition to the result of prepro‐
        -P  Inhibit generation of linemarkers in the output from the preproces‐
            sor.  This might be useful when running the preprocessor on some‐
            thing that is not C code, and will be sent to a program which might
            be confused by the linemarkers.
        -C  Do not discard comments.  All comments are passed through to the
            output file, except for comments in processed directives, which are
            deleted along with the directive.
            You should be prepared for side effects when using -C; it causes
            the preprocessor to treat comments as tokens in their own right.
            For example, comments appearing at the start of what would be a
            directive line have the effect of turning that line into an ordi‐
            nary 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.
            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 pre‐
            processing 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 nor‐
            mal 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 pre‐
            process as normal.  With two dashes, exit immediately.


        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 spe‐
            cial 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 speci‐
            fied with -I, but after any paths given with -I options on the com‐
            mand 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 direc‐
            tories 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 depen‐
            dencies for Make based on the non-system header files processed by
            the compiler.  System header files are ignored in the dependency
            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 combin‐
            ing 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
ld(1), and the Info
        entries for cpp, gcc, and binutils.


        Copyright (c) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 1997,
        1998, 1999, 2000, 2001, 2002, 2003, 2004 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 1.1 or
        any later version published by the Free Software Foundation.  A copy of
gfdl(7).  This manual contains
        no Invariant Sections.  The Front-Cover Texts are (a) (see below), and
        the Back-Cover Texts are (b) (see below).
        (a) The FSF’s Front-Cover Text is:
             A GNU Manual
        (b) The FSF’s Back-Cover Text is:
             You have freedom to copy and modify this GNU Manual, like GNU
             software.  Copies published by the Free Software Foundation raise
             funds for GNU development.


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