(cpp.info)Invocation


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Invoking the C Preprocessor
===========================

   Most often when you use the C preprocessor you will not have to
invoke it explicitly: the C compiler will do so automatically.
However, the preprocessor is sometimes useful individually.

   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 OUTFILE or both file names are omitted, the standard output
and standard input are used for the omitted file names.

   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 (Note: Output.).  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: 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.

`-traditional'
     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.

`-trigraphs'
     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.

`-pedantic'
     Issue warnings required by the ANSI 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.

`-Wtrigraphs'
     Warn if any trigraphs are encountered (assuming they are enabled).

`-Wcomment'
     Warn whenever a comment-start sequence `/*' appears in a comment.

`-Wall'
     Requests both `-Wtrigraphs' and `-Wcomment' (but not
     `-Wtraditional').

`-Wtraditional'
     Warn about certain constructs that behave differently in
     traditional and ANSI C.

`-I DIRECTORY'
     Add the directory DIRECTORY to the end of the list of directories
     to be searched for header files (Note: Include Syntax.).  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'
     commands.

     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.

`-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 libg++.)

`-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.

`-U NAME'
     Do not predefine NAME.  If both `-U' and `-D' are specified for
     one name, the `-U' beats the `-D' and the name is not predefined.

`-undef'
     Do not predefine any nonstandard macros.

`-A PREDICATE(ANSWER)'
     Make an assertion with the predicate PREDICATE and answer ANSWER.
     Note: Assertions.

     You can use `-A-' to disable all predefined assertions; it also
     undefines all predefined macros that identify the type of target
     system.

`-dM'
     Instead of outputting the result of preprocessing, output a list of
     `#define' commands 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' commands
     and the result of preprocessing.  Both kinds of output go to the
     standard output file.

`-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.  If there are many included files then the
     rule is split into several lines using `\'-newline.

     This feature is used in automatic updating of makefiles.

`-MM'
     Like `-M' but mention only the files included with `#include
     "FILE"'.  System header files included with `#include <FILE>' are
     omitted.

`-MD'
     Like `-M' but the dependency information is written to files with
     names made by replacing `.c' with `.d' at the end of the input
     file names.  This is in addition to compiling the file as
     specified--`-MD' does not inhibit ordinary compilation the way
     `-M' does.

     In Mach, you can use the utility `md' to merge the `.d' files into
     a single dependency file suitable for using with the `make'
     command.

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

`-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.

`-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'.

`-lang-c'
`-lang-c++'
`-lang-objc'
`-lang-objc++'
     Specify the source language.  `-lang-c++' makes the preprocessor
     handle C++ comment syntax (comments may begin with `//', in which
     case they end at end of line), and includes extra default include
     directories for C++; and `-lang-objc' enables the Objective C
     `#import' command.  `-lang-c' explicitly turns off both of these
     extensions, and `-lang-objc++' enables both.

     These options are generated by the compiler driver `gcc', but not
     passed from the `gcc' command line.

`-lint'
     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'.

     This option is available only when you call `cpp' directly; `gcc'
     will not pass it from its command line.

`-$'
     Forbid the use of `$' in identifiers.  This is required for ANSI
     conformance.  `gcc' automatically supplies this option to the
     preprocessor if you specify `-ansi', but `gcc' doesn't recognize
     the `-$' option itself--to use it without the other effects of
     `-ansi', you must call the preprocessor directly.


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