NAME
cpp - The GNU C-Compatible Compiler Preprocessor.

SYNOPSIS
cpp [-$] [-C] [-Dname[=definition]] [-dD] [-dM] [-I directory]
[-H] [-I-] [-imacros file] [-include file] [-lang-c]
[-lang-c++] [-lang-objc] [-lang-objc++] [-lint] [-M] [-MD]
[-MM] [-MMD] [-nostdinc] [-P] [-pedantic] [-pedantic-errors]
[-trigraphs] [-Uname] [-undef] [-Wtrigraphs] [-Wcomment]
[-Wall] [-Wtraditional]
[infile|-] [outfile|-]

DESCRIPTION
The C preprocessor is a macro processor that is used automatically
by the C compiler to transform your program before actual compila-
tion. It is called a macro processor because it allows you to de-
fine 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 abbrevia-
tions 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 preprocessor commands,
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. For a full explanation of
the GNU C preprocessor, see the manual TheCPreprocessorThe 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.

Most often when you use the C preprocessor you will not have to in-
voke it explicitly: the C compiler will do so automatically. How-
ever, 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 out-
put. Also, if outfile or both file names are omitted, the standard
output and standard input are used for the omitted file names.

OPTIONS
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 in-
voked 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: 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.

-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 new-
line. Strictly speaking, the GNU C preprocessor does not sup-
port 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 en-
abled).

-Wcomment

-Wcomments
Warn whenever a comment-start sequence ‘/*’ appears in a com-
ment. (Both forms have the same effect).

-Wall
Requests both ‘-Wtrigraphs’ and ‘-Wcomment’ (but not ‘-Wtradi-
tional’).

-Wtraditional
Warn about certain constructs that behave differently in trad-
itional and ANSI C.

-I directory
Add the directory directory to the end of the list of direc-
tories 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 ‘#in-
clude’ 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 re-
quested 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’ op-
tions (and the current directory, if appropriate) are
searched.

-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 pro-
gram 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.

-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 mac-
ros. 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 contain-
ing the object file name for that source file, a colon, and
the names of all the included files. If there are many in-
cluded 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, be-
fore 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. The preprocessor evaluates any ‘-D’ and
‘-U’ options on the command line before processing ‘-imacros
file’ .

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

-lang-c

-lang-c++
Specify the source language. ‘-lang-c++’ makes the preproces-
sor handle C++ comment syntax, and includes extra default in-
clude directories for C++.

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 com-
ments, and emit them preceded by ‘#pragma lint’. For example,
the comment ‘/* NOTREACHED */’ becomes ‘#pragma lint NO-
TREACHED’.

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.

SEE ALSO
The C Preprocessor (for version 2.0), Richard M. Stallman, July
1992; The C Preprocessor (for version 1.38), Richard M. Stallman,
July 1990. The C Preprocessor, Richard M. Stallman.
cc(1); Using and Porting GNU CC (for version 2.3), Richard M.
Stallman, December 1992; Using and Porting GNU CC (for version
1.40), Richard M. Stallman, June 1991.

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