CPP(1)

NAME
cpp - The C Preprocessor

SYNOPSIS
cpp [-P] [-C] [-gcc] [-traditional]
    [-undef] [-trigraphs] [-pedantic]
    [-Wwarn...] [-Idir...]
    [-Dmacro[=defn]...] [-Umacro]
    [-Apredicate(answer)]
    [-M|-MM][-MG][-MFfilename]
    [-MP][-MQtarget...][-MTtarget...]
    [-x language] [-std=standard]
    infile outfile

Only the most useful options are listed here; see below for the remainder.

DESCRIPTION
The C preprocessor is a macro processor that is used automatically by the C compiler to transform your program before actual 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 only for macro processing of C, C++ and Objective C source files. For macro processing of other files, you are strongly encouraged to use alternatives like M4, which will likely give you better results and avoid many problems. For example, normally the C preprocessor does not preserve arbitrary whitespace verbatim, but instead replaces each sequence with a single space.

For use on C-like source files, the C preprocessor provides four separate facilities that you can use as you see fit:

•   Inclusion of header files. These are files of declarations that can be substituted into your program.

•   Macro expansion. You can define macros, which are abbreviations for arbitrary fragments of C code, and then the C preprocessor will replace the macros with their definitions throughout the program.

•   Conditional compilation. Using special preprocessing directives, you can include or exclude parts of the program according to various conditions.

•   Line control. If you use a program to combine or rearrange source files into an intermediate file which is then compiled, you can use line control to inform the compiler of where each source line originally came from.

C preprocessors vary in some details. This manual discusses the GNU C preprocessor, which provides a small superset of the features of ISO Standard C.

In its default mode, the GNU C preprocessor does not do a few things required by the standard. These are features which are rarely, if ever, used, and may cause surprising changes to the meaning of a program which does not expect them. To get strict ISO Standard C, you should use the -std=c89 or -std=c99 options, depending on which version of the standard you want. To get all the mandatory diagnostics, you must also use -pedantic.

OPTIONS
The C preprocessor expects two file names as arguments, infile and outfile. The preprocessor reads infile together with any other files it specifies with #include. All the output generated by the combined input files is written in 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.

Here is a table of command options accepted by the C preprocessor. These options can also be given when compiling a C program; they are passed along automatically to the preprocessor when it is invoked by the compiler.

-P  Inhibit generation of #-lines with line-number information 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 #-lines.

-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. Comments appearing in the expansion list of a macro will be preserved, and appear in place wherever the macro is invoked.

    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, macro redefinitions that were trivial when comments were replaced by a single space might become significant when comments are retained. Also, 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 #.

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

    •   Traditional macro expansion pays no attention to single-quote or double-quote characters; macro argument symbols are replaced by the argument values even when they appear within apparent string or character constants.

    •   Traditionally, it is permissible for a macro expansion to end in the middle of a string or character constant. The constant continues into the text surrounding the macro call.

    •   However, traditionally the end of the line terminates a string or character constant, with no error.

    •   In traditional C, a comment is equivalent to no text at all. (In ISO C, a comment counts as whitespace.)

    •   Traditional C does not have the concept of a ``preprocessing number''. It considers 1.0e+4 to be three tokens: 1.0e, +, and 4.

    •   A macro is not suppressed within its own definition, in traditional C. Thus, any macro that is used recursively inevitably causes an error.

    •   The character # has no special meaning within a macro definition in traditional C.

    •   In traditional C, the text at the end of a macro expansion can run together with the text after the macro call, to produce a single token. (This is impossible in ISO C.)

    •   None of the GNU extensions to the preprocessor are available in -traditional mode.

    Use the -traditional option when preprocessing Fortran code, so that single-quotes and double-quotes within Fortran comment lines (which are generally not recognized as such by the preprocessor) do not cause diagnostics about unterminated character or string constants.

    However, this option does not prevent diagnostics about unterminated comments when a C-style comment appears to start, but not end, within Fortran-style commentary.

    So, the following Fortran comment lines are accepted with -traditional:

            C This isn't an unterminated character constant
            C Neither is "20000000000, an octal constant
            C in some dialects of Fortran
    However, this type of comment line will likely produce a diagnostic, or at least unexpected output from the preprocessor, due to the unterminated comment:

            C Some Fortran compilers accept /* as starting
            C an inline comment.
    Note that "g77" automatically supplies the -traditional option when it invokes the preprocessor. However, a future version of "g77" might use a different, more-Fortran-aware preprocessor in place of "cpp".

-trigraphs
    Process ISO standard trigraph sequences. 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 option.

    The nine trigraph sequences are

    ??( -> [

    ??) -> ]

    ??< -> {

    ??> -> }

    ??= -> #

    ??/ -> \

    ??' -> ^

    ??! -> |

    ??- -> ~

    Trigraph support is not popular, so many compilers do not implement it properly. Portable code should not rely on trigraphs being either converted or ignored.

-pedantic
    Issue warnings required by the ISO C standard in certain cases such as when text other than a comment follows #else or #endif.

-pedantic-errors
    Like -pedantic, except that errors are produced rather than warnings.

-Wcomment
-Wcomments
    (Both forms have the same effect). Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears in a // comment.

-Wtrigraphs
    Warn if any trigraphs are encountered. This option used to take effect only if -trigraphs was also specified, but now works independently. Warnings are not given for trigraphs within comments, as we feel this is obnoxious.

-Wwhite-space
    Warn about possible white space confusion, e.g. white space between a backslash and a newline.

-Wall
    Requests -Wcomment, -Wtrigraphs, and -Wwhite-space (but not -Wtraditional or -Wundef).

-Wtraditional
    Warn about certain constructs that behave differently in traditional and ISO C.

    •   Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but does not in ISO C.

    •   In traditional C, some preprocessor directives did not exist. Traditional preprocessors would only consider a line to be a directive if the # appeared in column 1 on the line. Therefore -Wtraditional warns about directives that traditional C understands but would ignore because the # does not appear as the first character on the line. It also suggests you hide directives like #pragma not understood by traditional C by indenting them. Some traditional implementations would not recognise #elif, so it suggests avoiding it altogether.

    •   A function-like macro that appears without arguments.

    •   The unary plus operator.

    •   The `U' integer constant suffix. (Traditonal C does support the `L' suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in limits.h. Use of these macros can lead to spurious warnings as they do not necessarily reflect whether the code in question is any less portable to traditional C given that suitable backup definitions are provided.

-Wundef
    Warn if an undefined identifier is evaluated in an #if directive.

-I directory
    Add the directory directory to the head of the list of directories to be searched for header files. This can be used to override a system header file, substituting your own version, since these directories are searched before the system header file directories. If you use more than one -I option, the directories are scanned in left-to-right order; the standard system directories come after.

-I- Any directories specified with -I options before the -I- option are searched only for the case of #include "file"; they are not searched for #include <file>.

    If additional directories are specified with -I options after the -I-, these directories are searched for all #include directives.

    In addition, the -I- option inhibits the use of the current directory as the first search directory for #include "file". Therefore, the current directory is searched only if it is requested explicitly with -I.. Specifying both -I- and -I. allows you to control precisely which directories are searched before the current one and which are searched after.

-nostdinc
    Do not search the standard system directories for header files. Only the directories you have specified with -I options (and the current directory, if appropriate) are searched.

    By using both -nostdinc and -I-, you can limit the include-file search path to only those directories you specify explicitly.

-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.)

-remap
    When searching for a header file in a directory, remap file names if a file named header.gcc exists in that directory. This can be used to work around limitations of file systems with file name restrictions. The header.gcc file should contain a series of lines with two tokens on each line: the first token is the name to map, and the second token is the actual name to use.

-D name
    Predefine name as a macro, with definition 1.

-D name=definition
    Predefine name as a macro, with definition definition. There are no restrictions on the contents of definition, but if you are invoking the preprocessor from a shell or shell-like program you may need to use the shell's quoting syntax to protect characters such as spaces that have a meaning in the shell syntax. If you use more than one -D for the same name, the rightmost definition takes effect.

    Any -D and -U options on the command line are processed in order, and always before -imacros file, regardless of the order in which they are written.

-U name
    Do not predefine name.

    Any -D and -U options on the command line are processed in order, and always before -imacros file, regardless of the order in which they are written.

-undef
    Do not predefine any nonstandard macros.

-gcc
    Define the macros __GNUC__, __GNUC_MINOR__ and __GNUC_PATCHLEVEL__. These are defined automatically when you use gcc -E; you can turn them off in that case with -no-gcc.

-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
    Disable an assertion with the predicate predicate and answer answer. Specifying no predicate, by -A- or -A -, disables all predefined assertions and all assertions preceding it on the command line; and also undefines all predefined macros and all macros preceding it on the command line.

-dM Instead of outputting the result of preprocessing, output 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
    will show the values of any 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.

-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.

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

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

-MG When used with -M or -MM, -MG says to treat missing header files as generated files and assume they live in the same directory as the source file. It suppresses preprocessed output, as a missing header file is ordinarily an error.

    This feature is used in automatic updating of makefiles.

-MP This option instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing. These dummy rules work around errors "make" gives if you remove header files without updating the "Makefile" to match.

    This is typical output:-

            /tmp/test.o: /tmp/test.c /tmp/test.h
            /tmp/test.h:

-MQ target
-MT target
    By default CPP uses the main file name, including any path, and appends the object suffix, normally ``.o'', to it to obtain the name of the target for dependency generation. With -MT you can specify a target yourself, overriding the default one.

    If you want multiple targets, you can specify them as a single argument to -MT, or use multiple -MT options.

    The targets you specify are output in the order they appear on the command line. -MQ is identical to -MT, except that the target name is quoted for Make, but with -MT it isn't. For example, -MT '$(objpfx)foo.o' gives

            $(objpfx)foo.o: /tmp/foo.c
    but -MQ '$(objpfx)foo.o' gives

            $$(objpfx)foo.o: /tmp/foo.c
    The default target is automatically quoted, as if it were given with -MQ.

-H  Print the name of each header file used, in addition to other normal activities.

-imacros file
    Process file as input, discarding the resulting output, before processing the regular input file. Because the output generated from file is discarded, the only effect of -imacros file is to make the macros defined in file available for use in the main input.

-include file
    Process file as input, and include all the resulting output, before processing the regular input file.

-idirafter dir
    Add the directory dir to the second include path. The directories on the second include path are searched when a header file is not found in any of the directories in the main include path (the one that -I adds to).

-iprefix prefix
    Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final /.

-iwithprefix dir
    Add a directory to the second include path. The directory's name is made by concatenating prefix and dir, where prefix was specified previously with -iprefix.

-isystem dir
    Add a directory to the beginning of the second include path, marking it as a system directory, so that it gets the same special treatment as is applied to the standard system directories.

-x c
-x c++
-x objective-c
-x assembler-with-cpp
    Specify the source language: C, C++, Objective-C, or assembly. This has nothing to do with standards conformance or extensions; it merely selects which base syntax to expect. If you give none of these options, cpp will deduce the language from the extension of the source file: .c, .cc, .m, or .S. Some other common extensions for C++ and assembly are also recognized. If cpp does not recognize the extension, it will treat the file as C; this is the most generic mode.

    Note: Previous versions of cpp accepted a -lang option which selected both the language and the standards conformance level. This option has been removed, because it conflicts with the -l option.

-std=standard
-ansi
    Specify the standard to which the code should conform. Currently cpp only knows about the standards for C; other language standards will be added in the future.

    standard may be one of:

    ""iso9899:1990""
    ""c89""
        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.

    ""iso9899:199409""
        The 1990 C standard, as amended in 1994.

    ""iso9899:1999""
    ""c99""
    ""iso9899:199x""
    ""c9x""
        The revised ISO C standard, published in December 1999. Before publication, this was known as C9X.

    ""gnu89""
        The 1990 C standard plus GNU extensions. This is the default.

    ""gnu99""
    ""gnu9x""
        The 1999 C standard plus GNU extensions.

-ftabstop=NUMBER
    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. Values less than 1 or greater than 100 are ignored. The default is 8.

-$  Forbid the use of $ in identifiers. The C standard allows implementations to define extra characters that can appear in identifiers. By default the GNU C preprocessor permits $, a common extension.

SEE ALSO
gcc(1), as(1), ld(1), and the Info entries for cpp, gcc, and binutils.

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

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions.