CPP(1) GNU CPP(1)
NAME
cpp - The C Preprocessor
SYNOPSIS
cpp [-Dmacro[=defn]...] [-Umacro]
[-Idir...] [-iquotedir...]
[-Wwarn...]
[-M|-MM] [-MG] [-MF filename]
[-MP] [-MQ target...]
[-MT target...]
[-P] [-fno-working-directory]
[-x language] [-std=standard]
infile outfile
Only the most useful options are listed here; see below for the remainder.
DESCRIPTION
The C preprocessor, often known as cpp, is a macro processor that is used automatically by the C compiler to
transform your program before 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 to be used only with C, C++, and Objective-C source code. In the past, it has
been abused as a general text processor. It will choke on input which does not obey C's lexical rules. For
example, apostrophes will be interpreted as the beginning of character constants, and cause errors. Also, you
cannot rely on it preserving characteristics of the input which are not significant to C-family languages. If
a Makefile is preprocessed, all the hard tabs will be removed, and the Makefile will not work.
Having said that, you can often get away with using cpp on things which are not C. Other Algol-ish
programming languages are often safe (Pascal, Ada, etc.) So is assembly, with caution. -traditional-cpp mode
preserves more white space, and is otherwise more permissive. Many of the problems can be avoided by writing
C or C++ style comments instead of native language comments, and keeping macros simple.
Wherever possible, you should use a preprocessor geared to the language you are writing in. Modern versions
of the GNU assembler have macro facilities. Most high level programming languages have their own conditional
compilation and inclusion mechanism. If all else fails, try a true general text processor, such as GNU M4.
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=c90, -std=c99 or -std=c11 options, depending on which version of the standard you want. To get all
the mandatory diagnostics, you must also use -pedantic.
This manual describes the behavior of the ISO preprocessor. To minimize gratuitous differences, where the ISO
preprocessor's behavior does not conflict with traditional semantics, the traditional preprocessor should
behave the same way. The various differences that do exist are detailed in the section Traditional Mode.
For clarity, unless noted otherwise, references to CPP in this manual refer to GNU CPP.
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
-D name=definition
The contents of definition are tokenized and processed as if they appeared during translation phase three
in a #define directive. In particular, the definition will be truncated by embedded newline characters.
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 wish to define a function-like macro on the command line, write its argument list with surrounding
parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you will need
to quote the option. With sh and csh, -D'name(args...)=definition' works.
-D and -U options are processed in the order they are given on the command line. All -imacros file and
-include file options are processed after all -D and -U options.
-U name
Cancel any previous definition of name, either built in or provided with a -D option.
-undef
Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain
defined.
-I dir
Add the directory dir to the list of directories to be searched for header files.
Directories named by -I are searched before the standard system include directories. If the directory dir
is a standard system include directory, the option is ignored to ensure that the default search order for
system directories and the special treatment of system headers are not defeated . If dir begins with "=",
then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.
-o file
Write output to file. This is the same as specifying file as the second non-option argument to cpp. gcc
has a different interpretation of a second non-option argument, so you must use -o to specify the output
file.
-Wall
Turns on all optional warnings which are desirable for normal code. At present this is -Wcomment,
-Wtrigraphs, -Wmultichar and a warning about integer promotion causing a change of sign in "#if"
expressions. Note that many of the preprocessor's warnings are on by default and have no options to
control them.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears
in a // comment. (Both forms have the same effect.)
-Wtrigraphs
Most trigraphs in comments cannot affect the meaning of the program. However, a trigraph that would form
an escaped newline (??/ at the end of a line) can, by changing where the comment begins or ends.
Therefore, only trigraphs that would form escaped newlines produce warnings inside a comment.
This option is implied by -Wall. If -Wall is not given, this option is still enabled unless trigraphs are
enabled. To get trigraph conversion without warnings, but get the other -Wall warnings, use -trigraphs
-Wall -Wno-trigraphs.
Built-in macros, macros defined on the command line, and macros defined in include files are not warned
about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then CPP will report it as
unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by,
for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with
something like:
#if defined the_macro_causing_the_warning
#endif
-Wendif-labels
Warn whenever an #else or an #endif are followed by text. This usually happens in code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments, but often are not in older programs. This warning is on
by default.
-Werror
Make all warnings into hard errors. Source code which triggers warnings will be rejected.
-Wsystem-headers
Issue warnings for code in system headers. These are normally unhelpful in finding bugs in your own code,
therefore suppressed. If you are responsible for the system library, you may want to see them.
-w Suppress all warnings, including those which GNU CPP issues by default.
-pedantic
Issue all the mandatory diagnostics listed in the C standard. Some of them are left out by default, since
they trigger frequently on harmless code.
-pedantic-errors
Issue all the mandatory diagnostics, and make all mandatory diagnostics into errors. This includes
mandatory diagnostics that GCC issues without -pedantic but treats as warnings.
-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 name of the source
file with any suffix replaced with object file suffix and with any leading directory parts removed. If
there are many included files then the rule is split into several lines using \-newline. The rule has no
commands.
This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug
output with the dependency rules you should explicitly specify the dependency output file with -MF, or use
an environment variable like DEPENDENCIES_OUTPUT. Debug output will still be sent to the regular output
When used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given the
preprocessor sends the rules to the same place it would have sent preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.
-MG In conjunction with an option such as -M requesting dependency generation, -MG assumes missing header
files are generated files and adds them to the dependency list without raising an error. The dependency
filename is taken directly from the "#include" directive without prepending any path. -MG also suppresses
preprocessed output, as a missing header file renders this useless.
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:
test.o: test.c test.h
test.h:
-MT target
Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main
input file, deletes any directory components and any file suffix such as .c, and appends the platform's
usual object suffix. The result is the target.
An -MT option will set the target to be exactly the string you specify. If you want multiple targets, you
can specify them as a single argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special to Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with -MQ.
-MD -MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file based on
whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d, otherwise
it takes the name of the input file, removes any directory components and suffix, and applies a .d suffix.
If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file,
but if used without -E, each -o is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate a dependency output file as a side-effect of the
compilation process.
-MMD
Like -MD except mention only user header files, not system header files.
-std=standard
-ansi
Specify the standard to which the code should conform. Currently CPP knows about C and C++ standards;
others may be added in the future.
standard may be one of:
"c90"
"c89"
"iso9899:1990"
The ISO C standard from 1990. c90 is the customary shorthand for this version of the standard.
The -ansi option is equivalent to -std=c90.
"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.
"iso9899:2011"
"c11"
"c1x"
The revised ISO C standard, published in December 2011. Before publication, this was known as C1X.
"gnu90"
"gnu89"
The 1990 C standard plus GNU extensions. This is the default.
"gnu99"
"gnu9x"
The 1999 C standard plus GNU extensions.
"gnu11"
"gnu1x"
The 2011 C standard plus GNU extensions.
"c++98"
The 1998 ISO C++ standard plus amendments.
"gnu++98"
The same as -std=c++98 plus GNU extensions. This is the default for C++ code.
-I- Split the include path. Any directories specified with -I options before -I- are searched only for
headers requested with "#include "file""; they are not searched for "#include <file>". If additional
directories are specified with -I options after the -I-, those directories are searched for all #include
directives.
In addition, -I- inhibits the use of the directory of the current file directory as the first search
directory for "#include "file"".
Process file as if "#include "file"" appeared as the first line of the primary source file. However, the
first directory searched for file is the preprocessor's working directory instead of the directory
containing the main source file. If not found there, it is searched for in the remainder of the "#include
"..."" search chain as normal.
If multiple -include options are given, the files are included in the order they appear on the command
line.
-imacros file
Exactly like -include, except that any output produced by scanning file is thrown away. Macros it defines
remain defined. This allows you to acquire all the macros from a header without also processing its
declarations.
All files specified by -imacros are processed before all files specified by -include.
-idirafter dir
Search dir for header files, but do it after all directories specified with -I and the standard system
directories have been exhausted. dir is treated as a system include directory. If dir begins with "=",
then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory,
you should include the final /.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the
include search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where
-idirafter would.
-isysroot dir
This option is like the --sysroot option, but applies only to header files (except for Darwin targets,
where it applies to both header files and libraries). See the --sysroot option for more information.
-imultilib dir
Use dir as a subdirectory of the directory containing target-specific C++ headers.
-isystem dir
Search dir for header files, after all directories specified by -I but before the standard system
directories. Mark it as a system directory, so that it gets the same special treatment as is applied to
the standard system directories.
If dir begins with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.
-iquote dir
Search dir only for header files requested with "#include "file""; they are not searched for
"#include <file>", before all directories specified by -I and before the standard system directories.
If dir begins with "=", then the "=" will be replaced by the sysroot prefix; see --sysroot and -isysroot.
-fdirectives-only
When preprocessing, handle directives, but do not expand macros.
The option's behavior depends on the -E and -fpreprocessed options.
-fdollars-in-identifiers
Accept $ in identifiers.
-fextended-identifiers
Accept universal character names in identifiers. This option is experimental; in a future version of GCC,
it will be enabled by default for C99 and C++.
-fno-canonical-system-headers
When preprocessing, do not shorten system header paths with canonicalization.
-fpreprocessed
Indicate to the preprocessor that the input file has already been preprocessed. This suppresses things
like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives.
The preprocessor still recognizes and removes comments, so that you can pass a file preprocessed with -C
to the compiler without problems. In this mode the integrated preprocessor is little more than a
tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These are the
extensions that GCC uses for preprocessed files created by -save-temps.
-ftabstop=width
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. If the value is less than 1 or greater than 100, the option
is ignored. The default is 8.
-fdebug-cpp
This option is only useful for debugging GCC. When used with -E, dumps debugging information about
location maps. Every token in the output is preceded by the dump of the map its location belongs to. The
dump of the map holding the location of a token would be:
{"P":F</file/path>;"F":F</includer/path>;"L":<line_num>;"C":<col_num>;"S":<system_header_p>;"M":<map_address>;"E":<macro_expansion_p>,"loc":<location>}
When used without -E, this option has no effect.
-ftrack-macro-expansion[=level]
Track locations of tokens across macro expansions. This allows the compiler to emit diagnostic about the
current macro expansion stack when a compilation error occurs in a macro expansion. Using this option
makes the preprocessor and the compiler consume more memory. The level parameter can be used to choose the
level of precision of token location tracking thus decreasing the memory consumption if necessary. Value 0
of level de-activates this option just as if no -ftrack-macro-expansion was present on the command line.
Value 1 tracks tokens locations in a degraded mode for the sake of minimal memory overhead. In this mode
all tokens resulting from the expansion of an argument of a function-like macro have the same location.
Value 2 tracks tokens locations completely. This value is the most memory hungry. When this option is
given no argument, the default parameter value is 2.
Note that -ftrack-macro-expansion=2 is activated by default.
-fexec-charset=charset
Set the execution character set, used for string and character constants. The default is UTF-8. charset
can be any encoding supported by the system's "iconv" library routine.
-fwide-exec-charset=charset
Set the wide execution character set, used for wide string and character constants. The default is UTF-32
or UTF-16, whichever corresponds to the width of "wchar_t". As with -fexec-charset, charset can be any
working directory at the time of preprocessing. When this option is enabled, the preprocessor will emit,
after the initial linemarker, a second linemarker with the current working directory followed by two
slashes. GCC will use this directory, when it's present in the preprocessed input, as the directory
emitted as the current working directory in some debugging information formats. This option is implicitly
enabled if debugging information is enabled, but this can be inhibited with the negated form
-fno-working-directory. If the -P flag is present in the command line, this option has no effect, since
no "#line" directives are emitted whatsoever.
-fno-show-column
Do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a
program that does not understand the column numbers, such as dejagnu.
-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
Cancel an assertion with the predicate predicate and answer answer.
-dCHARS
CHARS is a sequence of one or more of the following characters, and must not be preceded by a space.
Other characters are interpreted by the compiler proper, or reserved for future versions of GCC, and so
are silently ignored. If you specify characters whose behavior conflicts, the result is undefined.
M Instead of the normal output, generate 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 all the predefined macros.
If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach.
D Like M 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.
N Like D, but emit only the macro names, not their expansions.
I Output #include directives in addition to the result of preprocessing.
U Like D except that only macros that are expanded, or whose definedness is tested in preprocessor
directives, are output; the output is delayed until the use or test of the macro; and #undef
directives are also output for macros tested but undefined at the time.
-P Inhibit generation of linemarkers 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 linemarkers.
-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.
You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as
-traditional-cpp
Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C preprocessors.
-trigraphs
Process trigraph sequences.
-remap
Enable special code to work around file systems which only permit very short file names, such as MS-DOS.
--help
--target-help
Print text describing all the command line options instead of preprocessing anything.
-v Verbose mode. Print out GNU CPP's version number at the beginning of execution, and report the final form
of the include path.
-H Print the name of each header file used, in addition to other normal activities. Each name is indented to
show how deep in the #include stack it is. Precompiled header files are also printed, even if they are
found to be invalid; an invalid precompiled header file is printed with ...x and a valid one with ...! .
-version
--version
Print out GNU CPP's version number. With one dash, proceed to preprocess as normal. With two dashes,
exit immediately.
ENVIRONMENT
This section describes the environment variables that affect how CPP operates. You can use them to specify
directories or prefixes to use when searching for include files, or to control dependency output.
Note that you can also specify places to search using options such as -I, and control dependency output with
options like -M. These take precedence over environment variables, which in turn take precedence over the
configuration of GCC.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable's value is a list of directories separated by a special character, much like PATH, in which
to look for header files. The special character, "PATH_SEPARATOR", is target-dependent and determined at
GCC build time. For Microsoft Windows-based targets it is a semicolon, and for almost all other targets
it is a colon.
CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given
with -I options on the command line. This environment variable is used regardless of which language is
being preprocessed.
The remaining environment variables apply only when preprocessing the particular language indicated. Each
specifies a list of directories to be searched as if specified with -isystem, but after any paths given
with -isystem options on the command line.
In all these variables, an empty element instructs the compiler to search its current working directory.
Empty elements can appear at the beginning or end of a path. For instance, if the value of CPATH is
":/special/include", that has the same effect as -I. -I/special/include.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not
ignored, so it implies -M rather than -MM. However, the dependence on the main input file is omitted.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), gcc(1), as(1), ld(1), and the Info entries for cpp, gcc, and binutils.
COPYRIGHT
Copyright (c) 1987-2015 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free
Documentation License, Version 1.3 or any later version published by the Free Software Foundation. A copy of
the license is included in the man page gfdl(7). This manual contains no Invariant Sections. The Front-Cover
Texts are (a) (see below), and the Back-Cover Texts are (b) (see below).
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
gcc-4.8.5 2015-06-23 CPP(1)
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