Ubuntu Manpage: cpp - The C Preprocessor

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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 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.

-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.

-Wtraditional
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.

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

-Wunused-macros
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
#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 stream as normal.

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

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.

-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 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"".

This option has been deprecated.

-nostdinc
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.

-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.)

-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.

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 -fdirectives-only".

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

-fdollars-in-identifiers
Accept $ in identifiers.

-fextended-identifiers
Accept universal character names in identifiers. This option is enabled by default for C99 (and
later C standard versions) 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 encoding supported by the system's "iconv" library routine; however, you will have
problems with encodings that do not fit exactly in "wchar_t".

-finput-charset=charset
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.

-fworking-directory
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.

-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 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.

-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.

DEPENDENCIES_OUTPUT
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.

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.

SOURCE_DATE_EPOCH
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
reproducible.

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 process.

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