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.\" ========================================================================
.\"
.IX Title "CPP 1"
.TH CPP 1 "2022-06-28" "gcc-10.4.0" "GNU"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.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\-iquote\fR\fIdir\fR...]
    [\fB\-iremap\fR\fIsrc\fR:\fIdst\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...]
    \fIinfile\fR [[\fB\-o\fR] \fIoutfile\fR]
.PP
Only the most useful options are given above; see below for a more
complete list of preprocessor-specific options.  
In addition, \fBcpp\fR accepts most \fBgcc\fR driver options, which
are not listed here.  Refer to the \s-1GCC\s0 documentation for details.
.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
(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 M4.\s0
.PP
C preprocessors vary in some details.  This manual discusses the \s-1GNU C\s0
preprocessor, which provides a small superset of the features of \s-1ISO\s0
Standard C.  In its default mode, the \s-1GNU C\s0 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=c90\fR, \fB\-std=c99\fR,
\&\fB\-std=c11\fR or \fB\-std=c17\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 CPP.\s0
.SH "OPTIONS"
.IX Header "OPTIONS"
The \fBcpp\fR command 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.  If either file is omitted, it
means the same as if \fB\-\fR had been specified for that file.
You can also use the \fB\-o\fR \fIoutfile\fR option to specify the 
output 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"
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 is 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 should
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\-include\fR \fIfile\fR" 4
.IX Item "-include file"
Process \fIfile\fR as if \f(CW\*(C`#include "file"\*(C'\fR appeared as the first
line of the primary source file.  However, the first directory searched
for \fIfile\fR is the preprocessor's working directory \fIinstead of\fR
the directory containing the main source file.  If not found there, it
is searched for in the remainder of the \f(CW\*(C`#include "..."\*(C'\fR search
chain as normal.
.Sp
If multiple \fB\-include\fR options are given, the files are included
in the order they appear on the command line.
.IP "\fB\-imacros\fR \fIfile\fR" 4
.IX Item "-imacros file"
Exactly like \fB\-include\fR, except that any output produced by
scanning \fIfile\fR is thrown away.  Macros it defines remain defined.
This allows you to acquire all the macros from a header without also
processing its declarations.
.Sp
All files specified by \fB\-imacros\fR are processed before all files
specified by \fB\-include\fR.
.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\-pthread\fR" 4
.IX Item "-pthread"
Define additional macros required for using the \s-1POSIX\s0 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.
.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 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 \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
is still 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 appears in \fB\-MM\fR dependency output.
.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 send
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.
.Sp
If \fIfile\fR is \fI\-\fR, then the dependencies are written to \fIstdout\fR.
.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
\&        
\&        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, deletes any
directory components and 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 sets 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 takes the name
of the input file, removes any directory components and suffix, 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 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\-fpreprocessed\fR" 4
.IX Item "-fpreprocessed"
Indicate to the preprocessor that the input file has already been
preprocessed.  This suppresses things like macro expansion, trigraph
conversion, escaped newline splicing, and processing of most directives.
The preprocessor still recognizes and removes comments, so that you can
pass a file preprocessed with \fB\-C\fR to the compiler without
problems.  In this mode the integrated preprocessor is little more than
a tokenizer for the front ends.
.Sp
\&\fB\-fpreprocessed\fR is implicit if the input file has one of the
extensions \fB.i\fR, \fB.ii\fR or \fB.mi\fR.  These are the
extensions that \s-1GCC\s0 uses for preprocessed files created by
\&\fB\-save\-temps\fR.
.IP "\fB\-cxx\-isystem\fR \fIdir\fR" 4
.IX Item "-cxx-isystem dir"
Search \fIdir\fR for \*(C+ header files, after all directories specified by
\&\fB\-I\fR 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.
.IP "\fB\-fdirectives\-only\fR" 4
.IX Item "-fdirectives-only"
When preprocessing, handle directives, but do not expand macros.
.Sp
The option's behavior depends on the \fB\-E\fR and \fB\-fpreprocessed\fR
options.
.Sp
With \fB\-E\fR, preprocessing is limited to the handling of directives
such as \f(CW\*(C`#define\*(C'\fR, \f(CW\*(C`#ifdef\*(C'\fR, and \f(CW\*(C`#error\*(C'\fR.  Other
preprocessor operations, such as macro expansion and trigraph
conversion are not performed.  In addition, the \fB\-dD\fR option is
implicitly enabled.
.Sp
With \fB\-fpreprocessed\fR, predefinition of command line and most
builtin macros is disabled.  Macros such as \f(CW\*(C`_\|_LINE_\|_\*(C'\fR, which are
contextually dependent, are handled normally.  This enables compilation of
files previously preprocessed with \f(CW\*(C`\-E \-fdirectives\-only\*(C'\fR.
.Sp
With both \fB\-E\fR and \fB\-fpreprocessed\fR, the rules for
\&\fB\-fpreprocessed\fR take precedence.  This enables full preprocessing of
files previously preprocessed with \f(CW\*(C`\-E \-fdirectives\-only\*(C'\fR.
.IP "\fB\-iremap\fR \fIsrc\fR\fB:\fR\fIdst\fR" 4
.IX Item "-iremap src:dst"
Replace the prefix \fIsrc\fR in _\|_FILE_\|_ with \fIdst\fR at expansion time.
This option can be specified more than once.  Processing stops at the first
match.
.IP "\fB\-fdollars\-in\-identifiers\fR" 4
.IX Item "-fdollars-in-identifiers"
Accept \fB$\fR in identifiers.
.IP "\fB\-fextended\-identifiers\fR" 4
.IX Item "-fextended-identifiers"
Accept universal character names and extended characters in
identifiers.  This option is enabled by default for C99 (and later C
standard versions) and \*(C+.
.IP "\fB\-fno\-canonical\-system\-headers\fR" 4
.IX Item "-fno-canonical-system-headers"
When preprocessing, do not shorten system header paths with canonicalization.
.IP "\fB\-fmax\-include\-depth=\fR\fIdepth\fR" 4
.IX Item "-fmax-include-depth=depth"
Set the maximum depth of the nested #include. The default is 200.
.IP "\fB\-ftabstop=\fR\fIwidth\fR" 4
.IX Item "-ftabstop=width"
Set the distance between tab stops.  This helps the preprocessor report
correct column numbers in warnings or errors, even if tabs appear on the
line.  If the value is less than 1 or greater than 100, the option is
ignored.  The default is 8.
.IP "\fB\-ftrack\-macro\-expansion\fR[\fB=\fR\fIlevel\fR]" 4
.IX Item "-ftrack-macro-expansion[=level]"
Track locations of tokens across macro expansions. This allows the
compiler to emit diagnostic about the current macro expansion stack
when a compilation error occurs in a macro expansion. Using this
option makes the preprocessor and the compiler consume more
memory. The \fIlevel\fR parameter can be used to choose the level of
precision of token location tracking thus decreasing the memory
consumption if necessary. Value \fB0\fR of \fIlevel\fR de-activates
this option. Value \fB1\fR 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 \fB2\fR tracks
tokens locations completely. This value is the most memory hungry.
When this option is given no argument, the default parameter value is
\&\fB2\fR.
.Sp
Note that \f(CW\*(C`\-ftrack\-macro\-expansion=2\*(C'\fR is activated by default.
.IP "\fB\-fmacro\-prefix\-map=\fR\fIold\fR\fB=\fR\fInew\fR" 4
.IX Item "-fmacro-prefix-map=old=new"
When preprocessing files residing in directory \fI\fIold\fI\fR,
expand the \f(CW\*(C`_\|_FILE_\|_\*(C'\fR and \f(CW\*(C`_\|_BASE_FILE_\|_\*(C'\fR macros as if the
files resided in directory \fI\fInew\fI\fR instead.  This can be used
to change an absolute path to a relative path by using \fI.\fR for
\&\fInew\fR which can result in more reproducible builds that are
location independent.  This option also affects
\&\f(CW\*(C`_\|_builtin_FILE()\*(C'\fR during compilation.  See also
\&\fB\-ffile\-prefix\-map\fR.
.IP "\fB\-fexec\-charset=\fR\fIcharset\fR" 4
.IX Item "-fexec-charset=charset"
Set the execution character set, used for string and character
constants.  The default is \s-1UTF\-8.\s0  \fIcharset\fR can be any encoding
supported by the system's \f(CW\*(C`iconv\*(C'\fR library routine.
.IP "\fB\-fwide\-exec\-charset=\fR\fIcharset\fR" 4
.IX Item "-fwide-exec-charset=charset"
Set the wide execution character set, used for wide string and
character constants.  The default is \s-1UTF\-32\s0 or \s-1UTF\-16,\s0 whichever
corresponds to the width of \f(CW\*(C`wchar_t\*(C'\fR.  As with
\&\fB\-fexec\-charset\fR, \fIcharset\fR can be any encoding supported
by the system's \f(CW\*(C`iconv\*(C'\fR library routine; however, you will have
problems with encodings that do not fit exactly in \f(CW\*(C`wchar_t\*(C'\fR.
.IP "\fB\-finput\-charset=\fR\fIcharset\fR" 4
.IX Item "-finput-charset=charset"
Set the input character set, used for translation from the character
set of the input file to the source character set used by \s-1GCC.\s0  If the
locale does not specify, or \s-1GCC\s0 cannot get this information from the
locale, the default is \s-1UTF\-8.\s0  This can be overridden by either the locale
or this command-line option.  Currently the command-line option takes
precedence if there's a conflict.  \fIcharset\fR can be any encoding
supported by the system's \f(CW\*(C`iconv\*(C'\fR library routine.
.IP "\fB\-fworking\-directory\fR" 4
.IX Item "-fworking-directory"
Enable generation of linemarkers in the preprocessor output that
let the compiler know the current working directory at the time of
preprocessing.  When this option is enabled, the preprocessor
emits, after the initial linemarker, a second linemarker with the
current working directory followed by two slashes.  \s-1GCC\s0 uses this
directory, when it's present in the preprocessed input, as the
directory emitted as the current working directory in some debugging
information formats.  This option is implicitly enabled if debugging
information is enabled, but this can be inhibited with the negated
form \fB\-fno\-working\-directory\fR.  If the \fB\-P\fR flag is
present in the command line, this option has no effect, since no
\&\f(CW\*(C`#line\*(C'\fR directives are emitted whatsoever.
.IP "\fB\-A\fR \fIpredicate\fR\fB=\fR\fIanswer\fR" 4
.IX Item "-A predicate=answer"
Make an assertion with the predicate \fIpredicate\fR and answer
\&\fIanswer\fR.  This form is preferred to the older form \fB\-A\fR
\&\fIpredicate\fR\fB(\fR\fIanswer\fR\fB)\fR, which is still supported, because
it does not use shell special characters.
.IP "\fB\-A \-\fR\fIpredicate\fR\fB=\fR\fIanswer\fR" 4
.IX Item "-A -predicate=answer"
Cancel an assertion with the predicate \fIpredicate\fR and answer
\&\fIanswer\fR.
.IP "\fB\-C\fR" 4
.IX Item "-C"
Do not discard comments.  All comments are passed through to the output
file, except for comments in processed directives, which are deleted
along with the directive.
.Sp
You should be prepared for side effects when using \fB\-C\fR; 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 \fB#\fR.
.IP "\fB\-CC\fR" 4
.IX Item "-CC"
Do not discard comments, including during macro expansion.  This is
like \fB\-C\fR, except that comments contained within macros are
also passed through to the output file where the macro is expanded.
.Sp
In addition to the side effects of the \fB\-C\fR option, the
\&\fB\-CC\fR 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.
.Sp
The \fB\-CC\fR option is generally used to support lint comments.
.IP "\fB\-P\fR" 4
.IX Item "-P"
Inhibit generation of linemarkers in the output from the preprocessor.
This might be useful when running the preprocessor on something that is
not C code, and will be sent to a program which might be confused by the
linemarkers.
.IP "\fB\-traditional\fR" 4
.IX Item "-traditional"
.PD 0
.IP "\fB\-traditional\-cpp\fR" 4
.IX Item "-traditional-cpp"
.PD
Try to imitate the behavior of pre-standard C preprocessors, as
opposed to \s-1ISO C\s0 preprocessors.
.Sp
Note that \s-1GCC\s0 does not otherwise attempt to emulate a pre-standard 
C compiler, and these options are only supported with the \fB\-E\fR 
switch, or when invoking \s-1CPP\s0 explicitly.
.IP "\fB\-trigraphs\fR" 4
.IX Item "-trigraphs"
Support \s-1ISO C\s0 trigraphs.
These are three-character sequences, all starting with \fB??\fR, that
are defined by \s-1ISO C\s0 to stand for single characters.  For example,
\&\fB??/\fR stands for \fB\e\fR, so \fB'??/n'\fR is a character
constant for a newline.
.Sp
By default, \s-1GCC\s0 ignores trigraphs, but in
standard-conforming modes it converts them.  See the \fB\-std\fR and
\&\fB\-ansi\fR options.
.IP "\fB\-remap\fR" 4
.IX Item "-remap"
Enable special code to work around file systems which only permit very
short file names, such as MS-DOS.
.IP "\fB\-H\fR" 4
.IX Item "-H"
Print the name of each header file used, in addition to other normal
activities.  Each name is indented to show how deep in the
\&\fB#include\fR 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 \fB...x\fR and a valid one with \fB...!\fR .
.IP "\fB\-d\fR\fIletters\fR" 4
.IX Item "-dletters"
Says to make debugging dumps during compilation as specified by
\&\fIletters\fR.  The flags documented here are those relevant to the
preprocessor.  Other \fIletters\fR are interpreted
by the compiler proper, or reserved for future versions of \s-1GCC,\s0 and so
are silently ignored.  If you specify \fIletters\fR whose behavior
conflicts, the result is undefined.
.RS 4
.IP "\fB\-dM\fR" 4
.IX Item "-dM"
Instead of the normal output, generate a list of \fB#define\fR
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 \fIfoo.h\fR, the command
.Sp
.Vb 1
\&        touch foo.h; cpp \-dM foo.h
.Ve
.Sp
shows all the predefined macros.
.IP "\fB\-dD\fR" 4
.IX Item "-dD"
Like \fB\-dM\fR except in two respects: it does \fInot\fR include the
predefined macros, and it outputs \fIboth\fR the \fB#define\fR
directives and the result of preprocessing.  Both kinds of output go to
the standard output file.
.IP "\fB\-dN\fR" 4
.IX Item "-dN"
Like \fB\-dD\fR, but emit only the macro names, not their expansions.
.IP "\fB\-dI\fR" 4
.IX Item "-dI"
Output \fB#include\fR directives in addition to the result of
preprocessing.
.IP "\fB\-dU\fR" 4
.IX Item "-dU"
Like \fB\-dD\fR 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
\&\fB#undef\fR directives are also output for macros tested but
undefined at the time.
.RE
.RS 4
.RE
.IP "\fB\-fdebug\-cpp\fR" 4
.IX Item "-fdebug-cpp"
This option is only useful for debugging \s-1GCC.\s0  When used from \s-1CPP\s0 or with
\&\fB\-E\fR, it dumps debugging information about location maps.  Every
token in the output is preceded by the dump of the map its location
belongs to.
.Sp
When used from \s-1GCC\s0 without \fB\-E\fR, this option has no effect.
.IP "\fB\-I\fR \fIdir\fR" 4
.IX Item "-I dir"
.PD 0
.IP "\fB\-iquote\fR \fIdir\fR" 4
.IX Item "-iquote dir"
.IP "\fB\-isystem\fR \fIdir\fR" 4
.IX Item "-isystem dir"
.IP "\fB\-idirafter\fR \fIdir\fR" 4
.IX Item "-idirafter dir"
.PD
Add the directory \fIdir\fR to the list of directories to be searched
for header files during preprocessing.
.Sp
If \fIdir\fR begins with \fB=\fR or \f(CW$SYSROOT\fR, then the \fB=\fR
or \f(CW$SYSROOT\fR is replaced by the sysroot prefix; see
\&\fB\-\-sysroot\fR and \fB\-isysroot\fR.
.Sp
Directories specified with \fB\-iquote\fR apply only to the quote 
form of the directive, \f(CW\*(C`#include\ "\f(CIfile\f(CW"\*(C'\fR.
Directories specified with \fB\-I\fR, \fB\-isystem\fR, 
or \fB\-idirafter\fR apply to lookup for both the
\&\f(CW\*(C`#include\ "\f(CIfile\f(CW"\*(C'\fR and
\&\f(CW\*(C`#include\ <\f(CIfile\f(CW>\*(C'\fR directives.
.Sp
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:
.RS 4
.IP "1." 4
.IX Item "1."
For the quote form of the include directive, the directory of the current
file is searched first.
.IP "2." 4
.IX Item "2."
For the quote form of the include directive, the directories specified
by \fB\-iquote\fR options are searched in left-to-right order,
as they appear on the command line.
.IP "3." 4
.IX Item "3."
Directories specified with \fB\-I\fR options are scanned in
left-to-right order.
.IP "4." 4
.IX Item "4."
Directories specified with \fB\-isystem\fR options are scanned in
left-to-right order.
.IP "5." 4
.IX Item "5."
Standard system directories are scanned.
.IP "6." 4
.IX Item "6."
Directories specified with \fB\-idirafter\fR options are scanned in
left-to-right order.
.RE
.RS 4
.Sp
You can use \fB\-I\fR to override a system header
file, substituting your own version, since these directories are
searched before the standard system header file directories.  
However, you should
not use this option to add directories that contain vendor-supplied
system header files; use \fB\-isystem\fR for that.
.Sp
The \fB\-isystem\fR and \fB\-idirafter\fR options also mark the directory
as a system directory, so that it gets the same special treatment that
is applied to the standard system directories.
.Sp
If a standard system include directory, or a directory specified with
\&\fB\-isystem\fR, is also specified with \fB\-I\fR, the \fB\-I\fR
option is ignored.  The directory is still searched but as a
system directory at its normal position in the system include chain.
This is to ensure that \s-1GCC\s0's procedure to fix buggy system headers and
the ordering for the \f(CW\*(C`#include_next\*(C'\fR directive are not inadvertently
changed.
If you really need to change the search order for system directories,
use the \fB\-nostdinc\fR and/or \fB\-isystem\fR options.
.RE
.IP "\fB\-I\-\fR" 4
.IX Item "-I-"
Split the include path.
This option has been deprecated.  Please use \fB\-iquote\fR instead for
\&\fB\-I\fR directories before the \fB\-I\-\fR and remove the \fB\-I\-\fR
option.
.Sp
Any directories specified with \fB\-I\fR
options before \fB\-I\-\fR are searched only for headers requested with
\&\f(CW\*(C`#include\ "\f(CIfile\f(CW"\*(C'\fR; they are not searched for
\&\f(CW\*(C`#include\ <\f(CIfile\f(CW>\*(C'\fR.  If additional directories are
specified with \fB\-I\fR options after the \fB\-I\-\fR, those
directories are searched for all \fB#include\fR directives.
.Sp
In addition, \fB\-I\-\fR inhibits the use of the directory of the current
file directory as the first search directory for \f(CW\*(C`#include\ "\f(CIfile\f(CW"\*(C'\fR.  There is no way to override this effect of \fB\-I\-\fR.
.IP "\fB\-iprefix\fR \fIprefix\fR" 4
.IX Item "-iprefix prefix"
Specify \fIprefix\fR as the prefix for subsequent \fB\-iwithprefix\fR
options.  If the prefix represents a directory, you should include the
final \fB/\fR.
.IP "\fB\-iwithprefix\fR \fIdir\fR" 4
.IX Item "-iwithprefix dir"
.PD 0
.IP "\fB\-iwithprefixbefore\fR \fIdir\fR" 4
.IX Item "-iwithprefixbefore dir"
.PD
Append \fIdir\fR to the prefix specified previously with
\&\fB\-iprefix\fR, and add the resulting directory to the include search
path.  \fB\-iwithprefixbefore\fR puts it in the same place \fB\-I\fR
would; \fB\-iwithprefix\fR puts it where \fB\-idirafter\fR would.
.IP "\fB\-isysroot\fR \fIdir\fR" 4
.IX Item "-isysroot dir"
This option is like the \fB\-\-sysroot\fR option, but applies only to
header files (except for Darwin targets, where it applies to both header
files and libraries).  See the \fB\-\-sysroot\fR option for more
information.
.IP "\fB\-imultilib\fR \fIdir\fR" 4
.IX Item "-imultilib dir"
Use \fIdir\fR as a subdirectory of the directory containing
target-specific \*(C+ headers.
.IP "\fB\-nostdinc\fR" 4
.IX Item "-nostdinc"
Do not search the standard system directories for header files.
Only the directories explicitly specified with \fB\-I\fR,
\&\fB\-iquote\fR, \fB\-isystem\fR, and/or \fB\-idirafter\fR
options (and the directory of the current file, if appropriate) 
are searched.
.IP "\fB\-nostdinc++\fR" 4
.IX Item "-nostdinc++"
Do not search for header files in the \*(C+\-specific standard directories,
but do still search the other standard directories.  (This option is
used when building the \*(C+ library.)
.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.
This warning is enabled by \fB\-Wall\fR.
.IP "\fB\-Wtrigraphs\fR" 4
.IX Item "-Wtrigraphs"
Warn if any trigraphs are encountered that might change the meaning of
the program.  Trigraphs within comments are not warned about,
except those that would form escaped newlines.
.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\-Wundef\fR" 4
.IX Item "-Wundef"
Warn if an undefined identifier is evaluated in an \f(CW\*(C`#if\*(C'\fR directive.
Such identifiers are replaced with zero.
.IP "\fB\-Wexpansion\-to\-defined\fR" 4
.IX Item "-Wexpansion-to-defined"
Warn whenever \fBdefined\fR is encountered in the expansion of a macro
(including the case where the macro is expanded by an \fB#if\fR directive).
Such usage is not portable.
This warning is also enabled by \fB\-Wpedantic\fR and \fB\-Wextra\fR.
.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 also warns 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
\&\fINote:\fR If a macro is actually used, but only used in skipped
conditional blocks, then the preprocessor reports 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\-Wno\-endif\-labels\fR" 4
.IX Item "-Wno-endif-labels"
Do not warn whenever an \f(CW\*(C`#else\*(C'\fR or an \f(CW\*(C`#endif\*(C'\fR are followed by text.
This sometimes happens in older programs with 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.
This warning is on by default.
.SH "ENVIRONMENT"
.IX Header "ENVIRONMENT"
This section describes the environment variables that affect how \s-1CPP\s0
operates.  You can use them to specify directories or prefixes to use
when searching for include files, or to control dependency output.
.PP
Note that you can also specify places to search using options such as
\&\fB\-I\fR, and control dependency output with options like
\&\fB\-M\fR.  These take precedence over
environment variables, which in turn take precedence over the
configuration of \s-1GCC.\s0
.IP "\fB\s-1CPATH\s0\fR" 4
.IX Item "CPATH"
.PD 0
.IP "\fBC_INCLUDE_PATH\fR" 4
.IX Item "C_INCLUDE_PATH"
.IP "\fB\s-1CPLUS_INCLUDE_PATH\s0\fR" 4
.IX Item "CPLUS_INCLUDE_PATH"
.IP "\fB\s-1OBJC_INCLUDE_PATH\s0\fR" 4
.IX Item "OBJC_INCLUDE_PATH"
.PD
Each variable's value is a list of directories separated by a special
character, much like \fB\s-1PATH\s0\fR, in which to look for header files.
The special character, \f(CW\*(C`PATH_SEPARATOR\*(C'\fR, is target-dependent and
determined at \s-1GCC\s0 build time.  For Microsoft Windows-based targets it is a
semicolon, and for almost all other targets it is a colon.
.Sp
\&\fB\s-1CPATH\s0\fR specifies a list of directories to be searched as if
specified with \fB\-I\fR, but after any paths given with \fB\-I\fR
options on the command line.  This environment variable is used
regardless of which language is being preprocessed.
.Sp
The remaining environment variables apply only when preprocessing the
particular language indicated.  Each specifies a list of directories
to be searched as if specified with \fB\-isystem\fR, but after any
paths given with \fB\-isystem\fR options on the command line.
.Sp
In all these variables, an empty element instructs the compiler to
search its current working directory.  Empty elements can appear at the
beginning or end of a path.  For instance, if the value of
\&\fB\s-1CPATH\s0\fR is \f(CW\*(C`:/special/include\*(C'\fR, that has the same
effect as \fB\-I.\ \-I/special/include\fR.
.IP "\fB\s-1DEPENDENCIES_OUTPUT\s0\fR" 4
.IX Item "DEPENDENCIES_OUTPUT"
If this variable is set, its value specifies how to output
dependencies for Make based on the non-system header files processed
by the compiler.  System header files are ignored in the dependency
output.
.Sp
The value of \fB\s-1DEPENDENCIES_OUTPUT\s0\fR can be just a file name, in
which case the Make rules are written to that file, guessing the target
name from the source file name.  Or the value can have the form
\&\fIfile\fR\fB \fR\fItarget\fR, in which case the rules are written to
file \fIfile\fR using \fItarget\fR as the target name.
.Sp
In other words, this environment variable is equivalent to combining
the options \fB\-MM\fR and \fB\-MF\fR,
with an optional \fB\-MT\fR switch too.
.IP "\fB\s-1SUNPRO_DEPENDENCIES\s0\fR" 4
.IX Item "SUNPRO_DEPENDENCIES"
This variable is the same as \fB\s-1DEPENDENCIES_OUTPUT\s0\fR (see above),
except that system header files are not ignored, so it implies
\&\fB\-M\fR rather than \fB\-MM\fR.  However, the dependence on the
main input file is omitted.
.IP "\fB\s-1CPP_RESTRICTED\s0\fR" 4
.IX Item "CPP_RESTRICTED"
If this variable is defined, cpp will skip any include file which is not a
regular file, and will continue searching for the requested name (this is
always done if the found file is a directory).
.IP "\fB\s-1SOURCE_DATE_EPOCH\s0\fR" 4
.IX Item "SOURCE_DATE_EPOCH"
If this variable is set, its value specifies a \s-1UNIX\s0 timestamp to be
used in replacement of the current date and time in the \f(CW\*(C`_\|_DATE_\|_\*(C'\fR
and \f(CW\*(C`_\|_TIME_\|_\*(C'\fR macros, so that the embedded timestamps become
reproducible.
.Sp
The value of \fB\s-1SOURCE_DATE_EPOCH\s0\fR must be a \s-1UNIX\s0 timestamp,
defined as the number of seconds (excluding leap seconds) since
01 Jan 1970 00:00:00 represented in \s-1ASCII\s0; identical to the output of
\&\fB\f(CB@command\fB{date +%s\fR} on GNU/Linux and other systems that support the
\&\f(CW%s\fR extension in the \f(CW\*(C`date\*(C'\fR command.
.Sp
The value should be a known timestamp such as the last modification
time of the source or package and it should be set by the build
process.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBgpl\fR\|(7), \fBgfdl\fR\|(7), \fBfsf\-funding\fR\|(7),
\&\fBgcc\fR\|(1), and the Info entries for \fIcpp\fR and \fIgcc\fR.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright (c) 1987\-2020 Free Software Foundation, Inc.
.PP
Permission is granted to copy, distribute and/or modify this document
under the terms of the \s-1GNU\s0 Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation.  A copy of
the license is included in the
man page \fBgfdl\fR\|(7).
This manual contains no Invariant Sections.  The Front-Cover Texts are
(a) (see below), and the Back-Cover Texts are (b) (see below).
.PP
(a) The \s-1FSF\s0's Front-Cover Text is:
.PP
.Vb 1
\&     A GNU Manual
.Ve
.PP
(b) The \s-1FSF\s0's Back-Cover Text is:
.PP
.Vb 3
\&     You have freedom to copy and modify this GNU Manual, like GNU
\&     software.  Copies published by the Free Software Foundation raise
\&     funds for GNU development.
.Ve