x86_64-linux-gnu-cpp Man page



cpp – The C Preprocessor


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


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.


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

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

-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 rejected.

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

-w Suppress all warnings, including those which GNU CPP issues by

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 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 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 earlier.

-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

-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

Specify the standard to which the code should conform. Currently
CPP knows about C and C++ standards; others may be added in the

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 “. 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++ library.)

-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

-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 “, 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 enabled.

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 -fdirectives-only”.

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

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.

-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.

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.

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 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

-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 -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

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 preprocessors.

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 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.

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

If this variable is set, its value specifies a UNIX timestamp to be
used in replacement of the current date and time in the “__DATE__”
and “__TIME__” macros, so that the embedded timestamps become

The value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined as
the number of seconds (excluding leap seconds) since 01 Jan 1970
00:00:00 represented in ASCII; identical to the output of
@command{date +%s} on GNU/Linux and other systems that support the
%s extension in the “date” command.

The value should be a known timestamp such as the last modification
time of the source or package and it should be set by the build


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