Most often when you use the C preprocessor you will not have to invoke it explicitly: the C compiler will do so automatically. However, the preprocessor is sometimes useful on its own. All the options listed here are also acceptable to the C compiler and have the same meaning, except that the C compiler has different rules for specifying the output file.
Note: Whether you use the preprocessor by way of gcc or cpp, the compiler driver is run first. This program's purpose is to translate your command into invocations of the programs that do the actual work. Their command line interfaces are similar but not identical to the documented interface, and may change without notice.
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
1. -D name=definition
#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
-D option. -undef-I dir
-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 (see System Headers) . If dir begins with =, then the = will be replaced by the sysroot prefix; see --sysroot and -isysroot. -o file
cpp. gcc has a different interpretation of a second non-option argument, so you must use -o to specify the output file. -Wall-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/*’ appears in a ‘/*’ comment, or whenever a backslash-newline appears in a ‘//’ comment. (Both forms have the same effect.) -Wtrigraphs??/’ 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-Wundef#if’ directive, outside of ‘defined’. Such identifiers are replaced with zero. -Wunused-macrosBuilt-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#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-Wsystem-headers-w-pedantic-pedantic-errors-pedantic’ but treats as warnings. -Mmake 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 (see Environment Variables). 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-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
-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-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.
-MPmake 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
.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
$(objpfx)foo.o: foo.c
-MQ target
-MT, but it quotes any characters which are special to Make. -MQ '$(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 (see -MF), 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-MD except mention only user header files, not system header files. -x c-x c++-x objective-c-x assembler-with-cpp.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-ansistandard may be one of:
c90c89iso9899:1990c90’ is the customary shorthand for this version of the standard. The -ansi option is equivalent to -std=c90.
iso9899:199409iso9899:1999c99iso9899:199xc9xiso9899:2011c11c1xgnu90gnu89gnu99gnu9xgnu11gnu1xc++98gnu++98-std=c++98 plus GNU extensions. This is the default for C++ code. -I--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". See Search Path. This option has been deprecated.
-nostdinc-I options (and the directory of the current file, if appropriate) are searched. -nostdinc++-include file
#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
-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
-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
-iwithprefix options. If the prefix represents a directory, you should include the final ‘/’. -iwithprefix dir-iwithprefixbefore dir
-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
--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
-isystem dir
-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. See System Headers. If dir begins with =, then the = will be replaced by the sysroot prefix; see --sysroot and -isysroot. -iquote dir
#include "file"; they are not searched for #include <file>, before all directories specified by -I and before the standard system directories. See Search Path. If dir begins with =, then the = will be replaced by the sysroot prefix; see --sysroot and -isysroot. -fdirectives-onlyThe 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$’ in identifiers. See Identifier characters. -fextended-identifiers-fno-canonical-system-headers-fpreprocessed-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
-fdebug-cpp-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’:/file/path;‘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]
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
iconv library routine. -fwide-exec-charset=charset
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
iconv library routine. -fworking-directory-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-columndejagnu. -A predicate=answer
-A -predicate=answer
-dCHARSM’#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. See Debugging Options.
D’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’D’, but emit only the macro names, not their expansions. I’#include’ directives in addition to the result of preprocessing. U’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-CYou 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-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-trigraphs-remap--help--target-help-v-H#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
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Licensed under the GNU Free Documentation License, Version 1.3.
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