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12 Invocation
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Most often when you use the C preprocessor you will not have to invoke
it explicitly: the C compiler will do so automatically. However, the
preprocessor is sometimes useful on its own. All the options listed
here are also acceptable to the C compiler and have the same meaning,
except that the C compiler has different rules for specifying the output
file.
_Note:_ Whether you use the preprocessor by way of `gcc' or `cpp',
the "compiler driver" is run first. This program's purpose is to
translate your command into invocations of the programs that do the
actual work. Their command line interfaces are similar but not
identical to the documented interface, and may change without notice.
The C preprocessor expects two file names as arguments, INFILE and
OUTFILE. The preprocessor reads INFILE together with any other files
it specifies with `#include'. All the output generated by the combined
input files is written in OUTFILE.
Either INFILE or OUTFILE may be `-', which as INFILE means to read
from standard input and as OUTFILE means to write to standard output.
Also, if either file is omitted, it means the same as if `-' had been
specified for that file.
Unless otherwise noted, or the option ends in `=', all options which
take an argument may have that argument appear either immediately after
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-letter options 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 if they
appeared during translation phase three in a `#define' directive.
In particular, the definition will be truncated by embedded
newline characters.
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.
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 `sh' and `csh',
`-D'NAME(ARGS...)=DEFINITION'' works.
`-D' and `-U' options are processed in the order they are 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 or
provided with a `-D' option.
`-undef'
Do not predefine any system-specific or GCC-specific macros. The
standard predefined macros remain defined. Standard
Predefined Macros.
`-I DIR'
Add the directory DIR to the list of directories to be searched
for header files. Search Path. Directories named by `-I'
are searched before the standard system include directories. If
the directory DIR 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 ( System Headers) . If DIR begins with `=',
then the `=' will be replaced by the sysroot prefix; see
`--sysroot' and `-isysroot'.
`-o FILE'
Write output to FILE. This is the same as specifying FILE as the
second non-option argument to `cpp'. `gcc' has a different
interpretation of a second non-option argument, so you must use
`-o' to specify the output file.
`-Wall'
Turns on all optional warnings which are desirable for normal code.
At present this is `-Wcomment', `-Wtrigraphs', `-Wmultichar' and a
warning about integer promotion causing a change of sign in `#if'
expressions. Note that many of the preprocessor's warnings are on
by default and have no options to control them.
`-Wcomment'
`-Wcomments'
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.)
`-Wtrigraphs'
Most trigraphs in comments cannot affect the meaning of the
program. However, a trigraph that would form an escaped newline
(`??/' 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.
This option is implied by `-Wall'. If `-Wall' is not given, this
option is still enabled unless trigraphs are enabled. To get
trigraph conversion without warnings, but get the other `-Wall'
warnings, use `-trigraphs -Wall -Wno-trigraphs'.
`-Wtraditional'
Warn about certain constructs that behave differently in
traditional and ISO C. Also warn about ISO C constructs that have
no traditional C equivalent, and problematic constructs which
should be avoided. Traditional Mode.
`-Wundef'
Warn whenever an identifier which is not a macro is encountered in
an `#if' directive, outside of `defined'. Such identifiers are
replaced with zero.
`-Wunused-macros'
Warn about macros defined in the main file that are unused. A
macro is "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 the time it is redefined or undefined.
Built-in macros, macros defined on the command line, and macros
defined in include files are not warned about.
_Note:_ If a macro is actually used, but only used in skipped
conditional blocks, then CPP 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:
#if defined the_macro_causing_the_warning
#endif
`-Wendif-labels'
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 not
in older programs. This warning is on by default.
`-Werror'
Make all warnings into hard errors. Source code which triggers
warnings will be rejected.
`-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.
`-w'
Suppress all warnings, including those which GNU CPP issues by
default.
`-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.
`-pedantic-errors'
Issue all the mandatory diagnostics, and make all mandatory
diagnostics into errors. This includes mandatory diagnostics that
GCC issues without `-pedantic' but treats as warnings.
`-M'
Instead of outputting the result of preprocessing, output a rule
suitable for `make' describing the dependencies of the main source
file. The preprocessor outputs one `make' rule containing the
object file name for that source file, a colon, and the names of
all the included files, including those coming from `-include' or
`-imacros' command line options.
Unless specified explicitly (with `-MT' or `-MQ'), 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 `\'-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
dependency rules you should explicitly specify the dependency
output file with `-MF', or use an environment variable like
`DEPENDENCIES_OUTPUT' ( Environment Variables). Debug
output will still be sent to the regular output stream as normal.
Passing `-M' to the driver implies `-E', and suppresses warnings
with an implicit `-w'.
`-MM'
Like `-M' 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.
This implies that the choice of angle brackets or double quotes in
an `#include' directive does not in itself determine whether that
header will appear in `-MM' dependency output. This is a slight
change in semantics from GCC versions 3.0 and earlier.
`-MF FILE'
When used with `-M' or `-MM', specifies a file to write the
dependencies to. If no `-MF' switch is given the preprocessor
sends the rules to the same place it would have sent preprocessed
output.
When used with the driver options `-MD' or `-MMD', `-MF' overrides
the default dependency output file.
`-MG'
In conjunction with an option such as `-M' requesting dependency
generation, `-MG' 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 `#include'
directive without prepending any path. `-MG' also suppresses
preprocessed output, as a missing header file renders this useless.
This feature is used in automatic updating of makefiles.
`-MP'
This option instructs CPP 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 `make' gives if you remove header
files 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. By
default CPP takes the name of the main input file, deletes any
directory components and any file suffix such as `.c', and appends
the platform's usual object suffix. The result is the target.
An `-MT' 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 `-MT', or use multiple `-MT' options.
For example, `-MT '$(objpfx)foo.o'' might give
$(objpfx)foo.o: foo.c
`-MQ TARGET'
Same as `-MT', but it quotes any characters which are special to
Make. `-MQ '$(objpfx)foo.o'' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given
with `-MQ'.
`-MD'
`-MD' is equivalent to `-M -MF FILE', except that `-E' is not
implied. The driver determines FILE based on whether an `-o'
option is given. If it is, the driver uses its argument but with
a suffix of `.d', otherwise it takes the name of the input file,
removes any directory components and suffix, and applies a `.d'
suffix.
If `-MD' is used in conjunction with `-E', any `-o' switch is
understood to specify the dependency output file ( -MF
dashMF.), but if used without `-E', each `-o' is understood to
specify a target object file.
Since `-E' is not implied, `-MD' can be used to generate a
dependency output file as a side-effect of the compilation process.
`-MMD'
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 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: `.c', `.cc', `.m', or `.S'. 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.
_Note:_ Previous versions of cpp accepted a `-lang' option which
selected both the language and the standards conformance level.
This option has been removed, because it conflicts with the `-l'
option.
`-std=STANDARD'
`-ansi'
Specify the standard to which the code should conform. Currently
CPP knows about C and C++ standards; others may be added in the
future.
STANDARD may be one of:
`iso9899:1990'
`c89'
The ISO C standard from 1990. `c89' is the customary
shorthand for this version of the standard.
The `-ansi' option is equivalent to `-std=c89'.
`iso9899:199409'
The 1990 C standard, as amended in 1994.
`iso9899:1999'
`c99'
`iso9899:199x'
`c9x'
The revised ISO C standard, published in December 1999.
Before publication, this was known as C9X.
`gnu89'
The 1990 C standard plus GNU extensions. This is the default.
`gnu99'
`gnu9x'
The 1999 C standard plus GNU extensions.
`c++98'
The 1998 ISO C++ standard plus amendments.
`gnu++98'
The same as `-std=c++98' plus GNU extensions. This is the
default for C++ code.
`-I-'
Split the include path. Any directories specified with `-I'
options before `-I-' are searched only for headers requested with
`#include "FILE"'; they are not searched for `#include <FILE>'.
If additional directories are specified with `-I' options after
the `-I-', those directories are searched for all `#include'
directives.
In addition, `-I-' inhibits the use of the directory of the current
file directory as the first search directory for `#include "FILE"'.
Search Path. This option has been deprecated.
`-nostdinc'
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.
`-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.)
`-include FILE'
Process FILE as if `#include "file"' appeared as the first line of
the primary source file. However, the first directory searched
for FILE is the preprocessor's working directory _instead of_ the
directory containing the main source file. If not found there, it
is searched for in the remainder of the `#include "..."' search
chain as normal.
If multiple `-include' options are given, the files are included
in the order they appear on the command line.
`-imacros FILE'
Exactly like `-include', except that any output produced by
scanning FILE is thrown away. Macros it defines remain defined.
This allows you to acquire all the macros from a header without
also processing its declarations.
All files specified by `-imacros' are processed before all files
specified by `-include'.
`-idirafter DIR'
Search DIR for header files, but do it _after_ all directories
specified with `-I' and the standard system directories have been
exhausted. DIR is treated as a system include directory. If DIR
begins with `=', then the `=' will be replaced by the sysroot
prefix; see `--sysroot' and `-isysroot'.
`-iprefix PREFIX'
Specify PREFIX as the prefix for subsequent `-iwithprefix'
options. If the prefix represents a directory, you should include
the final `/'.
`-iwithprefix DIR'
`-iwithprefixbefore DIR'
Append DIR to the prefix specified previously with `-iprefix', and
add the resulting directory to the include search path.
`-iwithprefixbefore' puts it in the same place `-I' would;
`-iwithprefix' puts it where `-idirafter' would.
`-isysroot DIR'
This option is like the `--sysroot' option, but applies only to
header files. See the `--sysroot' option for more information.
`-imultilib DIR'
Use DIR as a subdirectory of the directory containing
target-specific C++ headers.
`-isystem DIR'
Search DIR for header files, after all directories specified by
`-I' 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. System
Headers. If DIR begins with `=', then the `=' will be replaced
by the sysroot prefix; see `--sysroot' and `-isysroot'.
`-iquote DIR'
Search DIR only for header files requested with `#include "FILE"';
they are not searched for `#include <FILE>', before all
directories specified by `-I' and before the standard system
directories. Search Path. If DIR begins with `=', then
the `=' will be replaced by the sysroot prefix; see `--sysroot'
and `-isysroot'.
`-fdirectives-only'
When preprocessing, handle directives, but do not expand macros.
The option's behavior depends on the `-E' and `-fpreprocessed'
options.
With `-E', preprocessing is limited to the handling of directives
such as `#define', `#ifdef', and `#error'. Other preprocessor
operations, such as macro expansion and trigraph conversion are
not performed. In addition, the `-dD' option is implicitly
enabled.
With `-fpreprocessed', predefinition of command line and most
builtin macros is disabled. Macros such as `__LINE__', which are
contextually dependent, are handled normally. This enables
compilation of files previously preprocessed with `-E
-fdirectives-only'.
With both `-E' and `-fpreprocessed', the rules for
`-fpreprocessed' take precedence. This enables full preprocessing
of files previously preprocessed with `-E -fdirectives-only'.
`-fdollars-in-identifiers'
Accept `$' in identifiers. Identifier characters.
`-fextended-identifiers'
Accept universal character names in identifiers. This option is
experimental; in a future version of GCC, it will be enabled by
default for C99 and C++.
`-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 `-C' to
the compiler without problems. In this mode the integrated
preprocessor is little more than a tokenizer for the front ends.
`-fpreprocessed' is implicit if the input file has one of the
extensions `.i', `.ii' or `.mi'. These are the extensions that
GCC uses for preprocessed files created by `-save-temps'.
`-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.
`-fexec-charset=CHARSET'
Set the execution character set, used for string and character
constants. The default is UTF-8. CHARSET can be any encoding
supported by the system's `iconv' library routine.
`-fwide-exec-charset=CHARSET'
Set the wide execution character set, used for wide string and
character constants. The default is UTF-32 or UTF-16, whichever
corresponds to the width of `wchar_t'. As with `-fexec-charset',
CHARSET can be any encoding supported by the system's `iconv'
library routine; however, you will have problems with encodings
that do not fit exactly in `wchar_t'.
`-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 GCC. If the locale does not specify, or GCC cannot get this
information from the locale, the default is UTF-8. 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. CHARSET can be any encoding supported by the system's
`iconv' library routine.
`-fworking-directory'
Enable generation of linemarkers in the preprocessor output that
will let the compiler know the current working directory at the
time of preprocessing. When this option is enabled, the
preprocessor will emit, after the initial linemarker, a second
linemarker with the current working directory followed by two
slashes. GCC will use 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
`-fno-working-directory'. If the `-P' flag is present in the
command line, this option has no effect, since no `#line'
directives are emitted whatsoever.
`-fno-show-column'
Do not print column numbers in diagnostics. This may be necessary
if diagnostics are being scanned by a program that does not
understand the column numbers, such as `dejagnu'.
`-A PREDICATE=ANSWER'
Make an assertion with the predicate PREDICATE and answer ANSWER.
This form is preferred to the older form `-A PREDICATE(ANSWER)',
which is still supported, because it does not use shell special
characters. Obsolete Features.
`-A -PREDICATE=ANSWER'
Cancel an assertion with the predicate PREDICATE and answer ANSWER.
`-dCHARS'
CHARS is a sequence of one or more of the following characters,
and must not be preceded by a space. Other characters are
interpreted by the compiler proper, or reserved for future
versions of GCC, and so are silently ignored. If you specify
characters whose behavior conflicts, the result is undefined.
`M'
Instead of the normal output, generate a list of `#define'
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 `foo.h', the
command
touch foo.h; cpp -dM foo.h
will show all the predefined macros.
If you use `-dM' without the `-E' option, `-dM' is
interpreted as a synonym for `-fdump-rtl-mach'.
Debugging Options (gcc)Debugging Options.
`D'
Like `M' except in two respects: it does _not_ include the
predefined macros, and it outputs _both_ the `#define'
directives and the result of preprocessing. Both kinds of
output go to the standard output file.
`N'
Like `D', but emit only the macro names, not their expansions.
`I'
Output `#include' directives in addition to the result of
preprocessing.
`U'
Like `D' 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
`#undef' directives are also output for macros tested but
undefined at the time.
`-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. Preprocessor
Output.
`-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.
You should be prepared for side effects when using `-C'; 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 `#'.
`-CC'
Do not discard comments, including during macro expansion. This is
like `-C', except that comments contained within macros are also
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 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.
The `-CC' option is generally used to support lint comments.
`-traditional-cpp'
Try to imitate the behavior of old-fashioned C preprocessors, as
opposed to ISO C preprocessors. Traditional Mode.
`-trigraphs'
Process trigraph sequences. Initial processing.
`-remap'
Enable special code to work around file systems which only permit
very short file names, such as MS-DOS.
`--help'
`--target-help'
Print text describing all the command line options instead of
preprocessing anything.
`-v'
Verbose mode. Print out GNU CPP's version number at the beginning
of execution, and report the final form of the include path.
`-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
`#include' 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 `...x' and a valid one
with `...!' .
`-version'
`--version'
Print out GNU CPP's version number. With one dash, proceed to
preprocess as normal. With two dashes, exit immediately.
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