cpp - The C Preprocessor
[-M⎪-MM] [-MG] [-MF filename]
[-MP] [-MQ target...]
[-x language] [-std=standard]
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 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=c89 or -std=c99 options, depending on which version of the standard you want. To get all the mandatory diagnostics, you must also use -pedantic.
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
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 .
- -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 the first time #import is used.
- 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 #endif
- 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 them.
- 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 warnings.
- 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
Unless specified explicitly (with -MT or -MQ), the object file name consists of the basename of the source file with any suffix replaced with object file suffix. 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.
- 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.
- 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.
- 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, including any path, deletes 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 '$(objpfx)foo.o' might give
- -MQ target
- Same as -MT, but it quotes any characters which are special to
Make. -MQ '$(objpfx)foo.o' gives
The default target is automatically quoted, as if it were given with -MQ.
- -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 take the basename of
the input file 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. c89 is the customary shorthand for
this version of the standard.
The -ansi option is equivalent to -std=c89.
- 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 1990 C standard plus GNU extensions. This is the default.
- The 1999 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.
- Split the include path. Any directories specified with -I options
before -I- are searched only for headers requested with
they are not searched for
If additional directories are specified with -I options after the
-I-, those directories are searched for all #include
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
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.
- -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. 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.
- -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.
- Accept $ in 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++.
- 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.
- 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 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.
- 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.
- 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.
- Like D, but emit only the macro names, not their expansions.
- Output #include directives in addition to the result of preprocessing.
- 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.
- 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 #.
- 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.
- Verbose mode. Print out GNU CPP's version number at the beginning of execution, and report the final form of the include path.
- 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 omitted.
gpl(7), gfdl(7), fsf-funding(7), gcc(1), as(1), ld(1), and the Info entries for cpp, gcc, and binutils.
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