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Most often when you use the C preprocessor you do not have to invoke itexplicitly: the C compiler does so automatically. However, thepreprocessor is sometimes useful on its own. You can invoke the preprocessor either with thecpp command, or viagcc -E.In GCC, the preprocessor is actually integrated with the compilerrather than a separate program, and both of these commands invokeGCC and tell it to stop after the preprocessing phase.
Thecpp options listed here are also accepted bygcc and have the same meaning. Likewise thecppcommand accepts all the usualgcc driver options, although thosepertaining to compilation phases after preprocessing are ignored.
Only options specific to preprocessing behavior are documented here.Refer to the GCC manual for full documentation of other driver options.
Thecpp command expects two file names as arguments,infile andoutfile. The preprocessor readsinfile together with anyother files it specifies with ‘#include’. All the output generatedby the combined input files is written inoutfile.
Eitherinfile oroutfile may be-, which asinfile means to read from standard input and asoutfilemeans to write to standard output. If either file is omitted, itmeans the same as if- had been specified for that file.You can also use the-ooutfile option to specify the output file.
Unless otherwise noted, or the option ends in ‘=’, all optionswhich take an argument may have that argument appear either immediatelyafter 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-letteroptions maynot be grouped:-dM is very different from‘-d -M’.
-Dname ¶--define-macro=name--define-macronamePredefinename as a macro, with definition1.
-Dname=definition--define-macro=name=definition--define-macroname=definitionThe contents ofdefinition are tokenized and processed as ifthey appeared during translation phase three in a ‘#define’directive. In particular, the definition is truncated byembedded newline characters.
If you are invoking the preprocessor from a shell or shell-likeprogram you may need to use the shell’s quoting syntax to protectcharacters such as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line, writeits argument list with surrounding parentheses before the equals sign(if any). Parentheses are meaningful to most shells, so you shouldquote the option. Withsh andcsh,-D'name(args…)=definition' works.
-D and-U options are processed in the order theyare given on the command line. All-imacrosfile and-includefile options are processed after all-D and-U options.
-Uname ¶--undefine-macro=name--undefine-macronameCancel any previous definition ofname, either built in orprovided with a-D option.
-includefile ¶--include=file--includefileProcessfile as if#include "file" appeared as the firstline of the primary source file. However, the first directory searchedforfile is the preprocessor’s working directoryinstead ofthe directory containing the main source file. If not found there, itis searched for in the remainder of the#include "…" searchchain as normal.
If multiple-include options are given, the files are includedin the order they appear on the command line.
-imacrosfile ¶--imacros=file--imacrosfileExactly like-include, except that any output produced byscanningfile is thrown away. Macros it defines remain defined.This allows you to acquire all the macros from a header without alsoprocessing its declarations.
All files specified by-imacros are processed before all filesspecified by-include.
-undef ¶Do not predefine any system-specific or GCC-specific macros. Thestandard predefined macros remain defined.SeeStandard Predefined Macros.
-pthread ¶Define additional macros required for using the POSIX threads library.You should use this option consistently for both compilation and linking.This option is supported on GNU/Linux targets, most other Unix derivatives,and also on x86 Cygwin and MinGW targets.
-M ¶--dependenciesInstead of outputting the result of preprocessing, output a rulesuitable formake describing the dependencies of the mainsource file. The preprocessor outputs onemake rule containingthe object file name for that source file, a colon, and the names of allthe included files, including those coming from-include or-imacros command-line options.
Unless specified explicitly (with-MT or-MQ), theobject file name consists of the name of the source file with anysuffix replaced with object file suffix and with any leading directoryparts removed. If there are many included files then the rule issplit into several lines using ‘\’-newline. The rule has nocommands.
This option does not suppress the preprocessor’s debug output, such as-dM. To avoid mixing such debug output with the dependencyrules you should explicitly specify the dependency output file with-MF, or use an environment variable likeDEPENDENCIES_OUTPUT (seeEnvironment Variables). Debug outputis still sent to the regular output stream as normal.
Passing-M to the driver implies-E, and suppresseswarnings with an implicit-w.
-MM ¶--user-dependenciesLike-M but do not mention header files that are found insystem 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 thatheader appears in-MM dependency output.
-MFfile ¶When used with-M or-MM, specifies afile to write the dependencies to. If no-MF switch is giventhe preprocessor sends the rules to the same place it would sendpreprocessed output.
When used with the driver options-MD or-MMD,-MF overrides the default dependency output file.
Iffile is-, then the dependencies are written tostdout.
-MG ¶--print-missing-file-dependenciesIn conjunction with an option such as-M requestingdependency generation,-MG assumes missing header files aregenerated files and adds them to the dependency list without raisingan error. The dependency filename is taken directly from the#include directive without prepending any path.-MGalso suppresses preprocessed output, as a missing header file rendersthis useless.
This feature is used in automatic updating of makefiles.
-Mno-modules ¶Disable dependency generation for compiled module interfaces.
-MP ¶This option instructs CPP to add a phony target for each dependencyother than the main file, causing each to depend on nothing. Thesedummy rules work around errorsmake gives if you remove headerfiles without updating theMakefile to match.
This is typical output:
test.o: test.c test.htest.h:
-MTtarget ¶Change the target of the rule emitted by dependency generation. Bydefault CPP takes the name of the main input file, deletes anydirectory components and any file suffix such as ‘.c’, andappends the platform’s usual object suffix. The result is the target.
An-MT option sets the target to be exactly the string youspecify. If you want multiple targets, you can specify them as a singleargument to-MT, or use multiple-MT options.
For example,-MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQtarget ¶Same as-MT, but it quotes any characters which are special toMake.-MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with-MQ.
-MD ¶--write-dependencies-MD is equivalent to-M -MFfile, except that-E is not implied. The driver determinesfile based onwhether an-o option is given. If it is, the driver uses itsargument but with a suffix of.d, otherwise it takes the nameof the input file, removes any directory components and suffix, andapplies 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-ois understood to specify a target object file.
Since-E is not implied,-MD can be used to generatea dependency output file as a side effect of the compilation process.
-MMD ¶--write-user-dependenciesLike-MD except mention only user header files, not systemheader files.
-fpreprocessed ¶Indicate to the preprocessor that the input file has already beenpreprocessed. This suppresses things like macro expansion, trigraphconversion, escaped newline splicing, and processing of most directives.The preprocessor still recognizes and removes comments, so that you canpass a file preprocessed with-C to the compiler withoutproblems. In this mode the integrated preprocessor is little more thana tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of theextensions ‘.i’, ‘.ii’ or ‘.mi’. These are theextensions that GCC uses for preprocessed files created by-save-temps.
-fdirectives-only ¶When preprocessing, handle directives, but do not expand macros.
The option’s behavior depends on the-E and-fpreprocessedoptions.
With-E, preprocessing is limited to the handling of directivessuch as#define,#ifdef, and#error. Otherpreprocessor operations, such as macro expansion and trigraphconversion are not performed. In addition, the-dD option isimplicitly enabled.
With-fpreprocessed, predefinition of command line and mostbuiltin macros is disabled. Macros such as__LINE__, which arecontextually dependent, are handled normally. This enables compilation offiles previously preprocessed with-E -fdirectives-only.
With both-E and-fpreprocessed, the rules for-fpreprocessed take precedence. This enables full preprocessing offiles previously preprocessed with-E -fdirectives-only.
-fdollars-in-identifiers ¶Accept ‘$’ in identifiers.SeeIdentifier characters.
-fextended-identifiers ¶Accept universal character names and extended characters inidentifiers. This option is enabled by default for C99 (and later Cstandard versions) and C++.
-fno-canonical-system-headers ¶When preprocessing, do not shorten system header paths with canonicalization.
-fmax-include-depth=depth ¶Set the maximum depth of the nested #include. The default is 200.
-fsearch-include-path[=kind] ¶Look for input files on the #include path, not just the currentdirectory. This is particularly useful with C++20 modules, for whichboth header units and module interface units need to be compileddirectly:
g++ -c -std=c++20 -fmodules -fsearch-include-path bits/stdc++.h bits/std.cc
kind defaults to ‘user’, which looks on the#include"…" search path; you can also explicitly specify ‘system’for the#include <…> search path.
-ftabstop=width ¶Set the distance between tab stops. This helps the preprocessor reportcorrect column numbers in warnings or errors, even if tabs appear on theline. If the value is less than 1 or greater than 100, the option isignored. The default is 8.
-ftrack-macro-expansion[=level] ¶Track locations of tokens across macro expansions. This allows thecompiler to emit diagnostic about the current macro expansion stackwhen a compilation error occurs in a macro expansion. Using thisoption makes the preprocessor and the compiler consume morememory. Thelevel parameter can be used to choose the level ofprecision of token location tracking thus decreasing the memoryconsumption if necessary. Value ‘0’ oflevel de-activatesthis option. Value ‘1’ tracks tokens locations in adegraded mode for the sake of minimal memory overhead. In this modeall tokens resulting from the expansion of an argument of afunction-like macro have the same location. Value ‘2’ trackstokens 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.
-fmacro-prefix-map=old=new ¶When preprocessing files residing in directoryold,expand the__FILE__ and__BASE_FILE__ macros as if thefiles resided in directorynew instead. This can be usedto change an absolute path to a relative path by using. fornew which can result in more reproducible builds that arelocation independent. This option also affects__builtin_FILE() during compilation. See also-ffile-prefix-map and-fcanon-prefix-map.
-fexec-charset=charset ¶Set the execution character set, used for string and characterconstants. The default is UTF-8.charset can be any encodingsupported by the system’siconv library routine.
-fwide-exec-charset=charset ¶Set the wide execution character set, used for wide string andcharacter constants. The default is one of UTF-32BE, UTF-32LE, UTF-16BE,or UTF-16LE, whichever corresponds to the width ofwchar_t and thebig-endian or little-endian byte order being used for code generation. Aswith-fexec-charset,charset can be any encoding supportedby the system’siconv library routine; however, you will haveproblems with encodings that do not fit exactly inwchar_t.
-finput-charset=charset ¶Set the input character set, used for translation from the characterset of the input file to the source character set used by GCC. If thelocale does not specify, or GCC cannot get this information from thelocale, the default is UTF-8. This can be overridden by either the localeor this command-line option. Currently the command-line option takesprecedence if there’s a conflict.charset can be any encodingsupported by the system’siconv library routine.
-fworking-directory ¶Enable generation of linemarkers in the preprocessor output thatlet the compiler know the current working directory at the time ofpreprocessing. When this option is enabled, the preprocessoremits, after the initial linemarker, a second linemarker with thecurrent working directory followed by two slashes. GCC uses thisdirectory, when it’s present in the preprocessed input, as thedirectory emitted as the current working directory in some debugginginformation formats. This option is implicitly enabled if debugginginformation is enabled, but this can be inhibited with the negatedform-fno-working-directory. If the-P flag ispresent in the command line, this option has no effect, since no#line directives are emitted whatsoever.
-Apredicate=answer ¶--assertpredicate=answer--assert=predicate=answerMake an assertion with the predicatepredicate and answeranswer. This form is preferred to the older form-Apredicate(answer), which is still supported, becauseit does not use shell special characters.
Assertions are an obsolete feature of the preprocessor, supported onlyfor compatibility with legacy system headers on some targets.SeeObsolete Features, for more details on use.
-A -predicate=answer--assert -predicate=answer--assert=-predicate=answerCancel an assertion with the predicatepredicate and answeranswer.
-C ¶--commentsDo not discard comments. All comments are passed through to the outputfile, except for comments in processed directives, which are deletedalong with the directive.
You should be prepared for side effects when using-C; itcauses the preprocessor to treat comments as tokens in their own right.For example, comments appearing at the start of what would be adirective line have the effect of turning that line into an ordinarysource line, since the first token on the line is no longer a ‘#’.
-CC ¶--comments-in-macrosDo not discard comments, including during macro expansion. This islike-C, except that comments contained within macros arealso 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 macroto be converted to C-style comments. This is to prevent later useof that macro from inadvertently commenting out the remainder ofthe source line.
The-CC option is generally used to support lint comments.
-P ¶--no-line-commandsInhibit generation of linemarkers in the output from the preprocessor.This might be useful when running the preprocessor on something that isnot C code, and will be sent to a program which might be confused by thelinemarkers.SeePreprocessor Output.
-traditional ¶--traditional-traditional-cpp--traditional-cppTry to imitate the behavior of pre-standard C preprocessors, asopposed to ISO C preprocessors.SeeTraditional Mode.
Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the-E switch, or when invoking CPP explicitly.
-trigraphs ¶--trigraphsSupport ISO C trigraphs.These are three-character sequences, all starting with ‘??’, thatare defined by ISO C to stand for single characters. For example,‘??/’ stands for ‘\’, so ‘'??/n'’ is a characterconstant for a newline.SeeInitial processing.
By default, GCC ignores trigraphs, but instandard-conforming modes it converts them. See the-std and-ansi options.
-remap ¶Enable special code to work around file systems which only permit veryshort file names, such as MS-DOS.
-H ¶--trace-includesPrint the name of each header file used, in addition to other normalactivities. Each name is indented to show how deep in the‘#include’ stack it is. Precompiled header files are alsoprinted, even if they are found to be invalid; an invalid precompiledheader file is printed with ‘...x’ and a valid one with ‘...!’ .
-dletters ¶--dump=letters--dumplettersSays to make debugging dumps during compilation as specified byletters. The flags documented here are those relevant to thepreprocessor. Otherletters are interpretedby the compiler proper, or reserved for future versions of GCC, and soare silently ignored. If you specifyletters whose behaviorconflicts, the result is undefined.
-dM ¶--dump=MInstead of the normal output, generate a list of ‘#define’directives for all the macros defined during the execution of thepreprocessor, including predefined macros. This gives you a way offinding out what is predefined in your version of the preprocessor.Assuming you have no filefoo.h, the command
touch foo.h; cpp -dM foo.h
shows all the predefined macros.
-dD ¶--dump=DLike-dM except that it outputsboth the ‘#define’directives and the result of preprocessing. Both kinds of output go tothe standard output file.
-dN ¶--dump=NLike-dD, but emit only the macro names, not their expansions.
-dI ¶--dump=IOutput ‘#include’ directives in addition to the result ofpreprocessing.
-dU ¶--dump=ULike-dD except that only macros that are expanded, or whosedefinedness is tested in preprocessor directives, are output; theoutput is delayed until the use or test of the macro; and‘#undef’ directives are also output for macros tested butundefined at the time.
-fdebug-cpp ¶This option is only useful for debugging GCC. When used from CPP or with-E, it dumps debugging information about location maps. Everytoken in the output is preceded by the dump of the map its locationbelongs to.
When used from GCC without-E, this option has no effect.
-Idir ¶-iquotedir-isystemdir-idirafterdir--include-directory-after=dir--include-directory-afterdir--include-directory=dir--include-directorydirAdd the directorydir to the list of directories to be searchedfor header files during preprocessing.--include-directoryis an alias for-I, while--include-directory-afteris an alias for-idirafter.SeeSearch Path.Ifdir begins with ‘=’ or$SYSROOT, then the ‘=’or$SYSROOT is replaced by the sysroot prefix; see--sysroot and-isysroot.
Directories specified with-iquote apply only to the quote form of the directive,#include "file".Directories specified with-I,-isystem, or-idirafter apply to lookup for both the#include "file" and#include <file> directives.
You can specify any number or combination of these options on the command line to search for header files in several directories. The lookup order is as follows:
You can use-I to override a system headerfile, substituting your own version, since these directories aresearched before the standard system header file directories. However, you shouldnot use this option to add directories that contain vendor-suppliedsystem header files; use-isystem for that.
The-isystem and-idirafter options also mark the directoryas a system directory, so that it gets the same special treatment thatis applied to the standard system directories.SeeSystem Headers.
If a standard system include directory, or a directory specified with-isystem, is also specified with-I, the-Ioption is ignored. The directory is still searched but as asystem directory at its normal position in the system include chain.This is to ensure that GCC’s procedure to fix buggy system headers andthe ordering for the#include_next directive are not inadvertentlychanged.If you really need to change the search order for system directories,use the-nostdinc and/or-isystem options.SeeSystem Headers.
-I- ¶--include-barrierSplit the include path.This option has been deprecated. Please use-iquote instead for-I directories before the-I- and remove the-I-option.
Any directories specified with-Ioptions before-I- are searched only for headers requested with#include "file"; they are not searched for#include <file>. If additional directories arespecified with-I options after the-I-, thosedirectories are searched for all ‘#include’ directives.
In addition,-I- inhibits the use of the directory of the currentfile directory as the first search directory for#include "file". There is no way to override this effect of-I-.SeeSearch Path.
-iprefixprefix ¶--include-prefix=prefix--include-prefixprefixSpecifyprefix as the prefix for subsequent-iwithprefixoptions. If the prefix represents a directory, you should include thefinal ‘/’.
-iwithprefixdir ¶-iwithprefixbeforedir--include-with-prefix=prefix--include-with-prefixprefix--include-with-prefix-after=prefix--include-with-prefix-afterprefix--include-with-prefix-before=prefix--include-with-prefix-beforeprefixAppenddir to the prefix specified previously with-iprefix, and add the resulting directory to the include searchpath.-iwithprefixbefore puts it in the same place-Iwould;-iwithprefix puts it where-idirafter would.
--include-with-prefix and--include-with-prefix-after areboth aliases for-iwithprefix, while--include-with-prefix-before is an alias for-iwithprefixbefore.
-isysrootdir ¶This option is like the--sysroot option, but applies only toheader files (except for Darwin targets, where it applies to both headerfiles and libraries). See the--sysroot option for moreinformation.
-imultilibdir ¶Usedir as a subdirectory of the directory containingtarget-specific C++ headers.
-nostdinc ¶--no-standard-includesDo not search the standard system directories for header files.Only the directories explicitly specified with-I,-iquote,-isystem, and/or-idirafteroptions (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 isused when building the C++ library.)
--embed-dir=dir ¶--embed-directory=dir--embed-directorydirAppenddir directory to the list of searched directories for#embed preprocessing directive or__has_embed macro.There are no default directories for#embed.
Ifdir begins with ‘=’ or$SYSROOT, then the ‘=’or$SYSROOT is replaced by the sysroot prefix; see--sysroot and-isysroot.
-Wcomment ¶-WcommentsWarn whenever a comment-start sequence ‘/*’ appears in a ‘/*’comment, or whenever a backslash-newline appears in a ‘//’ comment.This warning is enabled by-Wall.
-Wtrigraphs ¶Warn if any trigraphs are encountered that might change the meaning ofthe program. Trigraphs within comments are not warned about,except those that would form escaped newlines.
This option is implied by-Wall. If-Wall is notgiven, this option is still enabled unless trigraphs are enabled. Toget trigraph conversion without warnings, but get the other-Wall warnings, use ‘-trigraphs -Wall -Wno-trigraphs’.
-Wundef ¶Warn if an undefined identifier is evaluated in an#if directive.Such identifiers are replaced with zero.
-Wexpansion-to-defined ¶Warn whenever ‘defined’ is encountered in the expansion of a macro(including the case where the macro is expanded by an ‘#if’ directive).Such usage is not portable.This warning is also enabled by-Wpedantic and-Wextra.
-Wunused-macros ¶Warn about macros defined in the main file that are unused. A macroisused if it is expanded or tested for existence at least once.The preprocessor also warns if the macro has not been used at thetime it is redefined or undefined.
Built-in macros, macros defined on the command line, and macrosdefined in include files are not warned about.
Note: If a macro is actually used, but only used in skippedconditional blocks, then the preprocessor reports it as unused. To avoid thewarning in such a case, you might improve the scope of the macro’sdefinition 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
-Wno-endif-labels ¶Do not warn whenever an#else or an#endif are followed by text.This sometimes happens in older programs with code of the form
#if FOO…#else FOO…#endif FOO
The second and thirdFOO should be in comments.This warning is on by default.
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