MAN page from openSUSE Tumbleweed cpp9-9.0.1+r268509-2.9.x86_64.rpm


Section: GNU (1)
Updated: 2019-02-04


cpp - The C Preprocessor 


cpp [-Dmacro[=defn]...] [-Umacro]
    [-Idir...] [-iquotedir...]
    [-M|-MM] [-MG] [-MF filename]
    [-MP] [-MQ target...]
    [-MT target...]
    infile [[-ooutfile]

Only the most useful options are given above; see below for a morecomplete list of preprocessor-specific options. In addition, cpp accepts most gcc driver options, whichare not listed here. Refer to the GCC documentation for details. 


The C preprocessor, often known as cpp, is a macro processorthat is used automatically by the C compiler to transform your programbefore compilation. It is called a macro processor because it allowsyou to define macros, which are brief abbreviations for longerconstructs.

The C preprocessor is intended to be used only with C, C++, andObjective-C source code. In the past, it has been abused as a generaltext processor. It will choke on input which does not obey C's lexicalrules. For example, apostrophes will be interpreted as the beginning ofcharacter constants, and cause errors. Also, you cannot rely on itpreserving characteristics of the input which are not significant toC-family languages. If a Makefile is preprocessed, all the hard tabswill be removed, and the Makefile will not work.

Having said that, you can often get away with using cpp on things whichare not C. Other Algol-ish programming languages are often safe(Ada, etc.) So is assembly, with caution. -traditional-cppmode preserves more white space, and is otherwise more permissive. Manyof the problems can be avoided by writing C or C++ style commentsinstead of native language comments, and keeping macros simple.

Wherever possible, you should use a preprocessor geared to the languageyou are writing in. Modern versions of the GNU assembler have macrofacilities. Most high level programming languages have their ownconditional 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 Cpreprocessor, which provides a small superset of the features of ISOStandard C. In its default mode, the GNU C preprocessor does not do afew things required by the standard. These are features which arerarely, if ever, used, and may cause surprising changes to the meaningof a program which does not expect them. To get strict ISO Standard C,you should use the -std=c90, -std=c99,-std=c11 or -std=c17 options, dependingon which version of the standard you want. To get all the mandatorydiagnostics, you must also use -pedantic.

This manual describes the behavior of the ISO preprocessor. Tominimize gratuitous differences, where the ISO preprocessor'sbehavior does not conflict with traditional semantics, thetraditional preprocessor should behave the same way. The variousdifferences that do exist are detailed in the section TraditionalMode.

For clarity, unless noted otherwise, references to CPP in thismanual refer to GNU CPP. 


The cpp command expects two file names as arguments, infile andoutfile. The preprocessor reads infile together with anyother files it specifies with #include. All the output generatedby the combined input files is written in outfile.

Either infile or outfile may be -, which asinfile means to read from standard input and as outfilemeans 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 -o outfile 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 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 ifthey appeared during translation phase three in a #definedirective. 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. With sh and csh,-D'name(args...)=definition' works.

-D and -U options are processed in the order theyare 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 orprovided with a -D option.
-include file
Process file as if "#include "file"" appeared as the firstline of the primary source file. However, the first directory searchedfor file is the preprocessor's working directory instead 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.

-imacros file
Exactly like -include, except that any output produced byscanning file 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.

Do not predefine any system-specific or GCC-specific macros. Thestandard predefined macros remain defined.
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.
Instead of outputting the result of preprocessing, output a rulesuitable for make describing the dependencies of the mainsource file. The preprocessor outputs one make 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. Debug outputis still sent to the regular output stream as normal.

Passing -M to the driver implies -E, and suppresseswarnings with an implicit -w.

Like -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.

-MF file
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.

If file is -, then the dependencies are written to stdout.

In 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.

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 errors make gives if you remove headerfiles 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. 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

        $(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special toMake. -MQ '$(objpfx)foo.o' gives

        $$(objpfx)foo.o: foo.c

The default target is automatically quoted, as if it were given with-MQ.

-MD is equivalent to -M -MF file, except that-E is not implied. The driver determines file 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, 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.

Like -MD except mention only user header files, not systemheader files.
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.

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".

Accept $ in identifiers.
Accept universal character names in identifiers. This option isenabled by default for C99 (and later C standard versions) and C++.
When preprocessing, do not shorten system header paths with canonicalization.
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.
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. The level parameter can be used to choose the level ofprecision of token location tracking thus decreasing the memoryconsumption if necessary. Value 0 of level 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 is2.

Note that "-ftrack-macro-expansion=2" is activated by default.

When preprocessing files residing in directory old,expand the "__FILE__" and "__BASE_FILE__" macros as if thefiles resided in directory new 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.
Set the execution character set, used for string and characterconstants. The default is UTF-8. charset can be any encodingsupported by the system's "iconv" library routine.
Set the wide execution character set, used for wide string andcharacter constants. The default is UTF-32 or UTF-16, whichevercorresponds to the width of "wchar_t". As with-fexec-charset, charset can be any encoding supportedby the system's "iconv" library routine; however, you will haveproblems with encodings that do not fit exactly in "wchar_t".
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's "iconv" library routine.
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.
-A predicate=answer
Make an assertion with the predicate predicate and answeranswer. This form is preferred to the older form -Apredicate(answer), which is still supported, becauseit does not use shell special characters.
-A -predicate=answer
Cancel an assertion with the predicate predicate and answeranswer.
Do 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 #.

Do 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.

Inhibit 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.
Try to imitate the behavior of pre-standard C preprocessors, asopposed to ISO C preprocessors.

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.

Support 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.

By default, GCC ignores trigraphs, but instandard-conforming modes it converts them. See the -std and-ansi options.

Enable special code to work around file systems which only permit veryshort file names, such as MS-DOS.
Print 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 ...! .
Says to make debugging dumps during compilation as specified byletters. The flags documented here are those relevant to thepreprocessor. Other letters are interpretedby the compiler proper, or reserved for future versions of GCC, and soare silently ignored. If you specify letters whose behaviorconflicts, the result is undefined.
Instead of the normal output, generate a list of #definedirectives 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 file foo.h, the command

        touch foo.h; cpp -dM foo.h

shows all the predefined macros.

Like -dM except in two respects: it does not include thepredefined macros, and it outputs both the #definedirectives and the result of preprocessing. Both kinds of output go tothe standard output file.
Like -dD, but emit only the macro names, not their expansions.
Output #include directives in addition to the result ofpreprocessing.
Like -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.
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.

-I dir
-iquote dir
-isystem dir
-idirafter dir
Add the directory dir to the list of directories to be searchedfor header files during preprocessing.

If dir 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:

For the quote form of the include directive, the directory of the currentfile is searched first.
For the quote form of the include directive, the directories specifiedby -iquote options are searched in left-to-right order,as they appear on the command line.
Directories specified with -I options are scanned inleft-to-right order.
Directories specified with -isystem options are scanned inleft-to-right order.
Standard system directories are scanned.
Directories specified with -idirafter options are scanned inleft-to-right order.

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.

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.

Split 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-.

-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefixoptions. If the prefix represents a directory, you should include thefinal /.
-iwithprefix dir
-iwithprefixbefore dir
Append dir 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.
-isysroot dir
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.
-imultilib dir
Use dir as a subdirectory of the directory containingtarget-specific C++ headers.
Do 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.
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.)
Warn whenever a comment-start sequence /* appears in a /*comment, or whenever a backslash-newline appears in a // comment.This warning is enabled by -Wall.
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.

Warn if an undefined identifier is evaluated in an "#if" directive.Such identifiers are replaced with zero.
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.
Warn about macros defined in the main file that are unused. A macrois used 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
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 third "FOO" should be in comments.This warning is on by default.



This section describes the environment variables that affect how CPPoperates. You can use them to specify directories or prefixes to usewhen 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 overenvironment variables, which in turn take precedence over theconfiguration of GCC.

Each variable's value is a list of directories separated by a specialcharacter, much like PATH, in which to look for header files.The special character, "PATH_SEPARATOR", is target-dependent anddetermined at GCC build time. For Microsoft Windows-based targets it is asemicolon, and for almost all other targets it is a colon.

CPATH specifies a list of directories to be searched as ifspecified with -I, but after any paths given with -Ioptions on the command line. This environment variable is usedregardless of which language is being preprocessed.

The remaining environment variables apply only when preprocessing theparticular language indicated. Each specifies a list of directoriesto be searched as if specified with -isystem, but after anypaths given with -isystem options on the command line.

In all these variables, an empty element instructs the compiler tosearch its current working directory. Empty elements can appear at thebeginning or end of a path. For instance, if the value ofCPATH is ":/special/include", that has the sameeffect as -I. -I/special/include.

If this variable is set, its value specifies how to outputdependencies for Make based on the non-system header files processedby the compiler. System header files are ignored in the dependencyoutput.

The value of DEPENDENCIES_OUTPUT can be just a file name, inwhich case the Make rules are written to that file, guessing the targetname from the source file name. Or the value can have the formfile target, in which case the rules are written tofile file using target as the target name.

In other words, this environment variable is equivalent to combiningthe 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 themain input file is omitted.
If this variable is set, its value specifies a UNIX timestamp to beused in replacement of the current date and time in the "__DATE__"and "__TIME__" macros, so that the embedded timestamps becomereproducible.

The value of SOURCE_DATE_EPOCH must be a UNIX timestamp,defined as the number of seconds (excluding leap seconds) since01 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 modificationtime of the source or package and it should be set by the buildprocess.



gpl(7), gfdl(7), fsf-funding(7),gcc(1), and the Info entries for cpp and gcc. 


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