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|-MD|-MMD [-MG]]
    [-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 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, the C Compatible Compiler Preprocessor. The GNU C preprocessor provides a superset of the features of ANSI Standard C.

ANSI Standard C requires the rejection of many harmless constructs commonly used by today's C programs. Such incompatibility would be inconvenient for users, so the GNU C preprocessor is configured to accept these constructs by default. Strictly speaking, to get ANSI Standard C, you must use the options -trigraphs, -undef and -pedantic, but in practice the consequences of having strict ANSI Standard C make it undesirable to do this.

The C preprocessor is designed for C-like languages; you may run into problems if you apply it to other kinds of languages, because it assumes that it is dealing with C. For example, the C preprocessor sometimes outputs extra white space to avoid inadvertent C token concatenation, and this may cause problems with other languages.

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. 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. Do not discard comments: pass them through to the output file. Comments appearing in arguments of a macro call will be copied to the output before the expansion of the macro call. Try to imitate the behavior of old-fashioned C, as opposed to ANSI C. Traditional macro expansion pays no attention to singlequote or doublequote 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 ANSI 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 ANSI C.) Traditionally, \ inside a macro argument suppresses the syntactic significance of the following character.

Use the -traditional option when preprocessing Fortran code, so that singlequotes and doublequotes 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. Process ANSI standard trigraph sequences. These are three-character sequences, all starting with ??, that are defined by ANSI C to stand for single characters. For example, ??/ stands for \, so '??/n' is a character constant for a newline. Strictly speaking, the GNU C preprocessor does not support all programs in ANSI Standard C unless -trigraphs is used, but if you ever notice the difference it will be with relief.

You don't want to know any more about trigraphs. Issue warnings required by the ANSI C standard in certain cases such as when text other than a comment follows #else or #endif. Like -pedantic, except that errors are produced rather than warnings. Warn if any trigraphs are encountered. Currently this only works if you have turned trigraphs on with -trigraphs or -ansi; in the future this restriction will be removed. Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a Backslash-Newline appears in a // comment. Requests both -Wtrigraphs and -Wcomment (but not -Wtraditional or -Wundef). Warn about certain constructs that behave differently in traditional and ANSI C. Warn if an undefined identifier is evaluated in an #if directive. 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. 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. 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. 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.) 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. Predefine name as a macro, with definition 1. 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. Do not predefine name. If both -U and -D are specified for one name, whichever one appears later on the command line wins. Do not predefine any nonstandard macros. 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. Make an assertion with the predicate predicate and answer answer.

You can use -A- to disable all predefined assertions; it also undefines all predefined macros and all macros that preceded it on the command line. 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. 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. Output #include directives in addition to the result of preprocessing. 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. If there are many included files then the rule is split into several lines using \-newline.

-MG says to treat missing header files as generated files and assume they live in the same directory as the source file. It must be specified in addition to -M.

This feature is used in automatic updating of makefiles. Like -M but mention only the files included with #include "file". System header files included with #include <file> are omitted. Like -M but the dependency information is written to file. This is in addition to compiling the file as specified----MD does not inhibit ordinary compilation the way -M does.

When invoking gcc, do not specify the file argument. gcc will create file names made by replacing ".c" with ".d" at the end of the input file names.

In Mach, you can use the utility md to merge multiple dependency files into a single dependency file suitable for using with the make command. Like -MD except mention only user header files, not system header files. Print the name of each header file used, in addition to other normal activities. 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. Process file as input, and include all the resulting output, before processing the regular input file. 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). Specify prefix as the prefix for subsequent -iwithprefix options. 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. 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. Specify the source language: C, C++, Objective-C, or assembly. This has nothing to do with standards conformance or extensions; it merely selects which base syntax to expect. If you give none of these options, cpp will deduce the language from the extension of the source file: .c, .cc, .m, or .S. Some other common extensions for C++ and assembly are also recognized. If cpp does not recognize the extension, it will treat the file as C; this is the most generic mode.

Note: Previous versions of cpp accepted a -lang option which selected both the language and the standards conformance level. This option has been removed, because it conflicts with the -l option. Specify the standard to which the code should conform. Currently cpp only knows about the standards for C; other language standards will be added in the future.

standard may be one of: The ISO C standard from 1990. The 1990 C standard, as amended in 1994. c89 is the customary shorthand for this version of the standard.

The -ansi option is equivalent to -std=c89. The revised ISO C standard, which is expected to be promulgated some time in 1999. It has not been approved yet, hence the x. The 1990 C standard plus GNU extensions. This is the default. The 199x C standard plus GNU extensions. Look for commands to the program checker lint embedded in comments, and emit them preceded by #pragma lint. For example, the comment /* NOTREACHED */ becomes #pragma lint NOTREACHED.

Because of the clash with -l, you must use the awkward syntax above. In a future release, this option will be replaced by -flint or -Wlint; we are not sure which yet. Forbid the use of $ in identifiers. The C standard does not permit this, but it is a common extension.

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

COPYRIGHT
Copyright 1987, 1989, 1991-1999 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.