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.\" ========================================================================

.\"

.IX Title "CPP 1"

.TH CPP 1 "2004-11-05" "gcc-3.4.3" "GNU"

.SH "NAME"

cpp \- The C Preprocessor

.SH "SYNOPSIS"

.IX Header "SYNOPSIS"

cpp [\fB\-D\fR\fImacro\fR[=\fIdefn\fR]...] [\fB\-U\fR\fImacro\fR]

    [\fB\-I\fR\fIdir\fR...] [\fB\-W\fR\fIwarn\fR...]

    [\fB\-M\fR|\fB\-MM\fR] [\fB\-MG\fR] [\fB\-MF\fR \fIfilename\fR]

    [\fB\-MP\fR] [\fB\-MQ\fR \fItarget\fR...]

    [\fB\-MT\fR \fItarget\fR...]

    [\fB\-P\fR] [\fB\-fno\-working\-directory\fR]

    [\fB\-x\fR \fIlanguage\fR] [\fB\-std=\fR\fIstandard\fR]

    \fIinfile\fR \fIoutfile\fR

.PP

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

.SH "DESCRIPTION"

.IX Header "DESCRIPTION"

The C preprocessor, often known as \fIcpp\fR, is a \fImacro processor\fR

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 \fImacros\fR, which are brief abbreviations for longer

constructs.

.PP

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.

.PP

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.  \fB\-traditional\-cpp\fR

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.

.PP

Wherever possible, you should use a preprocessor geared to the language

you are writing in.  Modern versions of the \s-1GNU\s0 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 \s-1GNU\s0 M4.

.PP

C preprocessors vary in some details.  This manual discusses the \s-1GNU\s0 C

preprocessor, which provides a small superset of the features of \s-1ISO\s0

Standard C.  In its default mode, the \s-1GNU\s0 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 \s-1ISO\s0 Standard C,

you should use the \fB\-std=c89\fR or \fB\-std=c99\fR options, depending

on which version of the standard you want.  To get all the mandatory

diagnostics, you must also use \fB\-pedantic\fR.  

.PP

This manual describes the behavior of the \s-1ISO\s0 preprocessor.  To

minimize gratuitous differences, where the \s-1ISO\s0 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 \fBTraditional

Mode\fR.

.PP

For clarity, unless noted otherwise, references to \fB\s-1CPP\s0\fR in this

manual refer to \s-1GNU\s0 \s-1CPP\s0.

.SH "OPTIONS"

.IX Header "OPTIONS"

The C preprocessor expects two file names as arguments, \fIinfile\fR and

\&\fIoutfile\fR.  The preprocessor reads \fIinfile\fR together with any

other files it specifies with \fB#include\fR.  All the output generated

by the combined input files is written in \fIoutfile\fR.

.PP

Either \fIinfile\fR or \fIoutfile\fR may be \fB\-\fR, which as

\&\fIinfile\fR means to read from standard input and as \fIoutfile\fR

means to write to standard output.  Also, if either file is omitted, it

means the same as if \fB\-\fR had been specified for that file.

.PP

Unless otherwise noted, or the option ends in \fB=\fR, all options

which take an argument may have that argument appear either immediately

after the option, or with a space between option and argument:

\&\fB\-Ifoo\fR and \fB\-I foo\fR have the same effect.

.PP

Many options have multi-letter names; therefore multiple single-letter

options may \fInot\fR be grouped: \fB\-dM\fR is very different from

\&\fB\-d\ \-M\fR.

.IP "\fB\-D\fR \fIname\fR" 4

.IX Item "-D name"

Predefine \fIname\fR as a macro, with definition \f(CW1\fR.

.IP "\fB\-D\fR \fIname\fR\fB=\fR\fIdefinition\fR" 4

.IX Item "-D name=definition"

Predefine \fIname\fR as a macro, with definition \fIdefinition\fR.

The contents of \fIdefinition\fR are tokenized and processed as if

they appeared during translation phase three in a \fB#define\fR

directive.  In particular, the definition will be truncated by

embedded newline characters.

.Sp

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.

.Sp

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 \fBsh\fR and \fBcsh\fR,

\&\fB\-D'\fR\fIname\fR\fB(\fR\fIargs...\fR\fB)=\fR\fIdefinition\fR\fB'\fR works.

.Sp

\&\fB\-D\fR and \fB\-U\fR options are processed in the order they

are given on the command line.  All \fB\-imacros\fR \fIfile\fR and

\&\fB\-include\fR \fIfile\fR options are processed after all

\&\fB\-D\fR and \fB\-U\fR options.

.IP "\fB\-U\fR \fIname\fR" 4

.IX Item "-U name"

Cancel any previous definition of \fIname\fR, either built in or

provided with a \fB\-D\fR option.

.IP "\fB\-undef\fR" 4

.IX Item "-undef"

Do not predefine any system-specific or GCC-specific macros.  The

standard predefined macros remain defined.

.IP "\fB\-I\fR \fIdir\fR" 4

.IX Item "-I dir"

Add the directory \fIdir\fR to the list of directories to be searched

for header files.

.Sp

Directories named by \fB\-I\fR are searched before the standard

system include directories.  If the directory \fIdir\fR 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

\&.

.IP "\fB\-o\fR \fIfile\fR" 4

.IX Item "-o file"

Write output to \fIfile\fR.  This is the same as specifying \fIfile\fR

as the second non-option argument to \fBcpp\fR.  \fBgcc\fR has a

different interpretation of a second non-option argument, so you must

use \fB\-o\fR to specify the output file.

.IP "\fB\-Wall\fR" 4

.IX Item "-Wall"

Turns on all optional warnings which are desirable for normal code.

At present this is \fB\-Wcomment\fR, \fB\-Wtrigraphs\fR,

\&\fB\-Wmultichar\fR and a warning about integer promotion causing a

change of sign in \f(CW\*(C`#if\*(C'\fR expressions.  Note that many of the

preprocessor's warnings are on by default and have no options to

control them.

.IP "\fB\-Wcomment\fR" 4

.IX Item "-Wcomment"

.PD 0

.IP "\fB\-Wcomments\fR" 4

.IX Item "-Wcomments"

.PD

Warn whenever a comment-start sequence \fB/*\fR appears in a \fB/*\fR

comment, or whenever a backslash-newline appears in a \fB//\fR comment.

(Both forms have the same effect.)

.IP "\fB\-Wtrigraphs\fR" 4

.IX Item "-Wtrigraphs"

@anchor{Wtrigraphs}

Most trigraphs in comments cannot affect the meaning of the program.

However, a trigraph that would form an escaped newline (\fB??/\fR 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.

.Sp

This option is implied by \fB\-Wall\fR.  If \fB\-Wall\fR is not

given, this option is still enabled unless trigraphs are enabled.  To

get trigraph conversion without warnings, but get the other

\&\fB\-Wall\fR warnings, use \fB\-trigraphs \-Wall \-Wno\-trigraphs\fR.

.IP "\fB\-Wtraditional\fR" 4

.IX Item "-Wtraditional"

Warn about certain constructs that behave differently in traditional and

\&\s-1ISO\s0 C.  Also warn about \s-1ISO\s0 C constructs that have no traditional C

equivalent, and problematic constructs which should be avoided.

.IP "\fB\-Wimport\fR" 4

.IX Item "-Wimport"

Warn the first time \fB#import\fR is used.

.IP "\fB\-Wundef\fR" 4

.IX Item "-Wundef"

Warn whenever an identifier which is not a macro is encountered in an

\&\fB#if\fR directive, outside of \fBdefined\fR.  Such identifiers are

replaced with zero.

.IP "\fB\-Wunused\-macros\fR" 4

.IX Item "-Wunused-macros"

Warn about macros defined in the main file that are unused.  A macro

is \fIused\fR 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.

.Sp

Built-in macros, macros defined on the command line, and macros

defined in include files are not warned about.

.Sp

\&\fBNote:\fR If a macro is actually used, but only used in skipped

conditional blocks, then \s-1CPP\s0 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:

.Sp

.Vb 2

\&        #if defined the_macro_causing_the_warning

\&        #endif

.Ve

.IP "\fB\-Wendif\-labels\fR" 4

.IX Item "-Wendif-labels"

Warn whenever an \fB#else\fR or an \fB#endif\fR are followed by text.

This usually happens in code of the form

.Sp

.Vb 5

\&        #if FOO

\&        ...

\&        #else FOO

\&        ...

\&        #endif FOO

.Ve

.Sp

The second and third \f(CW\*(C`FOO\*(C'\fR should be in comments, but often are not

in older programs.  This warning is on by default.

.IP "\fB\-Werror\fR" 4

.IX Item "-Werror"

Make all warnings into hard errors.  Source code which triggers warnings

will be rejected.

.IP "\fB\-Wsystem\-headers\fR" 4

.IX Item "-Wsystem-headers"

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.

.IP "\fB\-w\fR" 4

.IX Item "-w"

Suppress all warnings, including those which \s-1GNU\s0 \s-1CPP\s0 issues by default.

.IP "\fB\-pedantic\fR" 4

.IX Item "-pedantic"

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.

.IP "\fB\-pedantic\-errors\fR" 4

.IX Item "-pedantic-errors"

Issue all the mandatory diagnostics, and make all mandatory diagnostics

into errors.  This includes mandatory diagnostics that \s-1GCC\s0 issues

without \fB\-pedantic\fR but treats as warnings.

.IP "\fB\-M\fR" 4

.IX Item "-M"

Instead of outputting the result of preprocessing, output a rule

suitable for \fBmake\fR describing the dependencies of the main

source file.  The preprocessor outputs one \fBmake\fR rule containing

the object file name for that source file, a colon, and the names of all

the included files, including those coming from \fB\-include\fR or

\&\fB\-imacros\fR command line options.

.Sp

Unless specified explicitly (with \fB\-MT\fR or \fB\-MQ\fR), 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 \fB\e\fR\-newline.

The rule has no commands.

.Sp

This option does not suppress the preprocessor's debug output, such as

\&\fB\-dM\fR.  To avoid mixing such debug output with the dependency

rules you should explicitly specify the dependency output file with

\&\fB\-MF\fR, or use an environment variable like

\&\fB\s-1DEPENDENCIES_OUTPUT\s0\fR.  Debug output

will still be sent to the regular output stream as normal.

.Sp

Passing \fB\-M\fR to the driver implies \fB\-E\fR, and suppresses

warnings with an implicit \fB\-w\fR.

.IP "\fB\-MM\fR" 4

.IX Item "-MM"

Like \fB\-M\fR 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.

.Sp

This implies that the choice of angle brackets or double quotes in an

\&\fB#include\fR directive does not in itself determine whether that

header will appear in \fB\-MM\fR dependency output.  This is a

slight change in semantics from \s-1GCC\s0 versions 3.0 and earlier.

.Sp

@anchor{dashMF}

.IP "\fB\-MF\fR \fIfile\fR" 4

.IX Item "-MF file"

When used with \fB\-M\fR or \fB\-MM\fR, specifies a

file to write the dependencies to.  If no \fB\-MF\fR switch is given

the preprocessor sends the rules to the same place it would have sent

preprocessed output.

.Sp

When used with the driver options \fB\-MD\fR or \fB\-MMD\fR,

\&\fB\-MF\fR overrides the default dependency output file.

.IP "\fB\-MG\fR" 4

.IX Item "-MG"

In conjunction with an option such as \fB\-M\fR requesting

dependency generation, \fB\-MG\fR 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

\&\f(CW\*(C`#include\*(C'\fR directive without prepending any path.  \fB\-MG\fR

also suppresses preprocessed output, as a missing header file renders

this useless.

.Sp

This feature is used in automatic updating of makefiles.

.IP "\fB\-MP\fR" 4

.IX Item "-MP"

This option instructs \s-1CPP\s0 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 \fBmake\fR gives if you remove header

files without updating the \fIMakefile\fR to match.

.Sp

This is typical output:

.Sp

.Vb 1

\&        test.o: test.c test.h

.Ve

.Sp

.Vb 1

\&        test.h:

.Ve

.IP "\fB\-MT\fR \fItarget\fR" 4

.IX Item "-MT target"

Change the target of the rule emitted by dependency generation.  By

default \s-1CPP\s0 takes the name of the main input file, including any path,

deletes any file suffix such as \fB.c\fR, and appends the platform's

usual object suffix.  The result is the target.

.Sp

An \fB\-MT\fR 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 \fB\-MT\fR, or use multiple \fB\-MT\fR options.

.Sp

For example, \fB\-MT\ '$(objpfx)foo.o'\fR might give

.Sp

.Vb 1

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

.Ve

.IP "\fB\-MQ\fR \fItarget\fR" 4

.IX Item "-MQ target"

Same as \fB\-MT\fR, but it quotes any characters which are special to

Make.  \fB\-MQ\ '$(objpfx)foo.o'\fR gives

.Sp

.Vb 1

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

.Ve

.Sp

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

\&\fB\-MQ\fR.

.IP "\fB\-MD\fR" 4

.IX Item "-MD"

\&\fB\-MD\fR is equivalent to \fB\-M \-MF\fR \fIfile\fR, except that

\&\fB\-E\fR is not implied.  The driver determines \fIfile\fR based on

whether an \fB\-o\fR option is given.  If it is, the driver uses its

argument but with a suffix of \fI.d\fR, otherwise it take the

basename of the input file and applies a \fI.d\fR suffix.

.Sp

If \fB\-MD\fR is used in conjunction with \fB\-E\fR, any

\&\fB\-o\fR switch is understood to specify the dependency output file

(but \f(CW@pxref\fR{dashMF,,\-MF}), but if used without \fB\-E\fR, each \fB\-o\fR

is understood to specify a target object file.

.Sp

Since \fB\-E\fR is not implied, \fB\-MD\fR can be used to generate

a dependency output file as a side-effect of the compilation process.

.IP "\fB\-MMD\fR" 4

.IX Item "-MMD"

Like \fB\-MD\fR except mention only user header files, not system

\&\-header files.

.IP "\fB\-x c\fR" 4

.IX Item "-x c"

.PD 0

.IP "\fB\-x c++\fR" 4

.IX Item "-x c++"

.IP "\fB\-x objective-c\fR" 4

.IX Item "-x objective-c"

.IP "\fB\-x assembler-with-cpp\fR" 4

.IX Item "-x assembler-with-cpp"

.PD

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:

\&\fB.c\fR, \fB.cc\fR, \fB.m\fR, or \fB.S\fR.  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.

.Sp

\&\fBNote:\fR Previous versions of cpp accepted a \fB\-lang\fR option

which selected both the language and the standards conformance level.

This option has been removed, because it conflicts with the \fB\-l\fR

option.

.IP "\fB\-std=\fR\fIstandard\fR" 4

.IX Item "-std=standard"

.PD 0

.IP "\fB\-ansi\fR" 4

.IX Item "-ansi"

.PD

Specify the standard to which the code should conform.  Currently \s-1CPP\s0

knows about C and \*(C+ standards; others may be added in the future.

.Sp

\&\fIstandard\fR

may be one of:

.RS 4

.ie n .IP """iso9899:1990""" 4

.el .IP "\f(CWiso9899:1990\fR" 4

.IX Item "iso9899:1990"

.PD 0

.ie n .IP """c89""" 4

.el .IP "\f(CWc89\fR" 4

.IX Item "c89"

.PD

The \s-1ISO\s0 C standard from 1990.  \fBc89\fR is the customary shorthand for

this version of the standard.

.Sp