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cpp - The C Preprocessor


cpp [-Dmacro[=defn]...] [-Umacro]
[-Idir...] [-iquotedir...]
[-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.


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

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.


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.

Unless otherwise noted, or the option ends in =, all options which take an argument may
have that argument appear either immediately after the option, or with a space between
option and argument: -Ifoo and -I foo have the same effect.

Many options have multi-letter names; therefore multiple single-letter options may not be
grouped: -dM is very different from -d -M.

-D name
Predefine name as a macro, with definition 1.

-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.

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.

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.

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.)

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.

Warn about certain constructs that behave differently in traditional and ISO C. Also
warn about ISO C constructs that have no traditional C equivalent, and problematic
constructs which should be avoided.

Warn whenever an identifier which is not a macro is encountered in an #if directive,
outside of defined. Such identifiers are replaced with zero.

Warn about macros defined in the main file that are unused. A macro is used if it is
expanded or tested for existence at least once. The preprocessor will also warn if
the macro has not been used at the time it is redefined or undefined.

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

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.

Make all warnings into hard errors. Source code which triggers warnings will be

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.

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.

Issue all the mandatory diagnostics, and make all mandatory diagnostics into errors.
This includes mandatory diagnostics that GCC issues without -pedantic but treats as

-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 stream as

Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.

-MM Like -M but do not mention header files that are found in system header directories,
nor header files that are included, directly or indirectly, from such a header.

This implies that the choice of angle brackets or double quotes in an #include
directive does not in itself determine whether that header will appear in -MM
dependency output. This is a slight change in semantics from GCC versions 3.0 and

-MF file
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


-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.

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

-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.

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:

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.

The 1990 C standard, as amended in 1994.

The revised ISO C standard, published in December 1999. Before publication, this
was known as C9X.

The revised ISO C standard, published in December 2011. Before publication, this
was known as C1X.

The 1990 C standard plus GNU extensions. This is the default.

The 1999 C standard plus GNU extensions.

The 2011 C standard plus GNU extensions.

The 1998 ISO C++ standard plus amendments.

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"".

This option has been deprecated.

Do not search the standard system directories for header files. Only the directories
you have specified with -I options (and the directory of the current file, 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++

-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

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.

When preprocessing, handle directives, but do not expand macros.

The option's behavior depends on the -E and -fpreprocessed options.

With -E, preprocessing is limited to the handling of directives such as "#define",
"#ifdef", and "#error". Other preprocessor operations, such as macro expansion and
trigraph conversion are not performed. In addition, the -dD option is implicitly

With -fpreprocessed, predefinition of command line and most builtin macros is
disabled. Macros such as "__LINE__", which are contextually dependent, are handled
normally. This enables compilation of files previously preprocessed with "-E

With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. This
enables full preprocessing of files previously preprocessed with "-E

Accept $ in identifiers.

Accept universal character names in identifiers. This option is enabled by default
for C99 (and later C standard versions) and C++.

When preprocessing, do not shorten system header paths with canonicalization.

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.

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.

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:


When used without -E, this option has no effect.

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.

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

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 encoding supported by the system's "iconv"
library routine; however, you will have problems with encodings that do not fit
exactly in "wchar_t".

Set the input character set, used for translation from the character set of the input
file to the source character set used by GCC. If the locale does not specify, or GCC
cannot get this information from the locale, the default is UTF-8. This can be
overridden by either the locale or this command-line option. Currently the command-
line option takes precedence if there's a conflict. charset can be any encoding
supported by the system's "iconv" library routine.

Enable generation of linemarkers in the preprocessor output that will let the compiler
know the current 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.

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

-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.

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

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 tokens in their own right. For example, 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 #.

-CC Do not discard comments, including during macro expansion. This is like -C, except
that comments contained within macros are also passed through to the output file where
the macro is expanded.

In addition to the side-effects of the -C option, the -CC option causes all C++-style
comments inside a macro to be converted to C-style comments. This is to prevent later
use of that macro from inadvertently commenting out the remainder of the source line.

The -CC option is generally used to support lint comments.

Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C

Process trigraph sequences.

Enable special code to work around file systems which only permit very short file
names, such as MS-DOS.

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 ...! .

Print out GNU CPP's version number. With one dash, proceed to preprocess as normal.
With two dashes, exit immediately.


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.

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

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.

If this variable is set, its value specifies how to output dependencies for Make based
on the non-system header files processed by the compiler. System header files are
ignored in the dependency output.

The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules
are written to that file, guessing the target name from the source file name. Or the
value can have the form file target, in which case the rules are written to file file
using target as the target name.

In other words, this environment variable is equivalent to combining the options -MM
and -MF, with an optional -MT switch too.

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.

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