MAN page from Trustix cpp-3.3-5tr.i586.rpm


Section: GNU (1)
Updated: 2003-05-14


cpp - The C Preprocessor 


cpp [-Dmacro[=defn]...] [-Umacro]
    [-Idir...] [-Wwarn...]
    [-M|-MM] [-MG] [-MF filename]
    [-MP] [-MQ target...] [-MT target...]
    [-x language] [-std=standard]
    infile outfile

Only the most useful options are listed here; see below for the remainder. 


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(Pascal, 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=c89 or -std=c99 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 @ref{TraditionalMode}.

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


The C preprocessor 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. Also, if either file is omitted, itmeans the same as if - had been specified for that 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
Predefine name as a macro, with definition definition.There are no restrictions on the contents of definition, but ifyou are invoking the preprocessor from a shell or shell-like program youmay need to use the shell's quoting syntax to protect characters such asspaces 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 will needto quote 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.
Do not predefine any system-specific macros. The common predefinedmacros remain defined.
-I dir
Add the directory dir to the list of directories to be searchedfor header files.

Directories named by -I are searched before the standardsystem include directories. If the directory dir is a standardsystem include directory, the option is ignored to ensure that thedefault search order for system directories and the special treatmentof system headers are not defeated.

-o file
Write output to file. This is the same as specifying fileas the second non-option argument to cpp. gcc has adifferent interpretation of a second non-option argument, so you mustuse -o to specify the output file.
Turns on all optional warnings which are desirable for normal code. Atpresent this is -Wcomment and -Wtrigraphs. Note thatmany of the preprocessor's warnings are on by default and have nooptions 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.)
Warn if any trigraphs are encountered. This option used to take effectonly if -trigraphs was also specified, but now worksindependently. Warnings are not given for trigraphs within comments, asthey do not affect the meaning of the program.
Warn about certain constructs that behave differently in traditional andISO C. Also warn about ISO C constructs that have no traditional Cequivalent, 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 arereplaced with zero.
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 will also warn 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 CPP will report 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
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 notin older programs. This warning is on by default.

Make all warnings into hard errors. Source code which triggers warningswill be rejected.
Issue warnings for code in system headers. These are normally unhelpfulin finding bugs in your own code, therefore suppressed. If you areresponsible 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 ofthem are left out by default, since they trigger frequently on harmlesscode.
Issue all the mandatory diagnostics, and make all mandatory diagnosticsinto errors. This includes mandatory diagnostics that GCC issueswithout -pedantic but treats as warnings.
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 basename of the source file with anysuffix replaced with object file suffix. If there are many includedfiles 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 dependencyrules you should explicitly specify the dependency output file with-MF, or use an environment variable likeDEPENDENCIES_OUTPUT. Debug outputwill still be 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 will appear in -MM dependency output. This is aslight change in semantics from GCC versions 3.0 and earlier.

-MF file
@anchor{-MF}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 have sentpreprocessed 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 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

-MT target
Change the target of the rule emitted by dependency generation. Bydefault CPP takes the name of the main input file, including any path,deletes any file suffix such as .c, and appends the platform'susual object suffix. The result is the target.

An -MT option will set 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 take thebasename 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 @pxref{-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.

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 hasnothing to do with standards conformance or extensions; it merelyselects 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 commonextensions for C++ and assembly are also recognized. If cpp does notrecognize the extension, it will treat the file as C; this is the mostgeneric mode.

Note: Previous versions of cpp accepted a -lang optionwhich selected both the language and the standards conformance level.This option has been removed, because it conflicts with the -loption.

Specify the standard to which the code should conform. Currently CPPknows about C and C++ standards; others may be added in the future.

standardmay be one of:

The ISO C standard from 1990. c89 is the customary shorthand forthis 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. Beforepublication, 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 thedefault for C++ code.
Split the include path. 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"".

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 isused when building the C++ library.)
-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.

-idirafter dir
Search dir for header files, but do it after alldirectories specified with -I and the standard system directorieshave been exhausted. dir is treated as a system include directory.
-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.

Use of these options is discouraged.

-isystem dir
Search dir for header files, after all directories specified by-I but before the standard system directories. Mark itas a system directory, so that it gets the same special treatment asis applied to the standard system directories.
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.

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.
Do not print column numbers in diagnostics. This may be necessary ifdiagnostics are being scanned by a program that does not understand thecolumn numbers, such as dejagnu.
-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.
CHARS is a sequence of one or more of the following characters,and must not be preceded by a space. Other characters are interpretedby the compiler proper, or reserved for future versions of GCC, and soare silently ignored. If you specify characters 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

will show all the predefined macros.

Like M 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 D, but emit only the macro names, not their expansions.
Output #include directives in addition to the result ofpreprocessing.
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.
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.

Define the macros __GNUC__, _0B5




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