cpp - The C Preprocessor
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.
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:
o Inclusion of header files. These are files of declarations that
can be substituted into your program.
o 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.
o Conditional compilation. Using special preprocessing directives,
you can include or exclude parts of the program according to various
conditions.
o 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.
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.
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.
o 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.
o 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.
o However, traditionally the end of the line terminates a string
or character constant, with no error.
o In traditional C, a comment is equivalent to no text at all.
(In ISO C, a comment counts as whitespace.)
o Traditional C does not have the concept of a ``preprocessing
number''. It considers 1.0e+4 to be three tokens: 1.0e, +, and
4.
o A macro is not suppressed within its own definition, in traditional
C. Thus, any macro that is used recursively inevitably
causes an error.
o The character # has no special meaning within a macro definition
in traditional C.
o 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.)
o 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 -tradi-
tional:
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 -Wtra-
ditional or -Wundef).
-Wtraditional
Warn about certain constructs that behave differently in traditional
and ISO C.
-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. 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.
gcc(1), as(1), 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 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.
3rd Berkeley Distribution gcc-2.97 CPP(1)
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