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Invoking the C Preprocessor
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Invoking
the C preprocessor
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.
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 outfile
or both file names are omitted, the standard output and standard input
are used for the omitted file names.
What
follows is a list of command options accepted by the C preprocessor. (These
options can also be given when compiling a C program; they are passed along
automatically to the preprocessor when it is invoked by the compiler.)
-
-P
Inhibit generation of #
-lines with line-number information in the output from the preprocessor
(see C preprocessor
output). 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 #-lines.
-
-C
Do not discard comments:
pass them through to the output file. Comments appearing in arguments of
a macro call will be copied to the output before the expansion of the macro
call.
-
-traditional
Try to imitate the behavior
of old-fashioned C, as opposed to ANSI C.
-
Traditional macro expansion
pays no attention to single-quote or doublequote characters; macro argument
symbols are replaced by the argument values even when they appear within
apparent string or character constants.
-
Traditionally, it is permissible
for a macro expansion to end in the middle of a string or character constant.
The constant continues
ffb
into the text surrounding the macro call.
-
However, traditionally the end
of the line terminates a string or character constant, with no error.
-
In traditional C, a comment
is equivalent to no text at all. (In ANSI C, a comment counts as whitespace.)
-
Traditional C does not have
the concept of a “preprocessing number”. It considers 1.0e+4
to be three tokens: 1.0e,
+,
and 4.
-
A macro is not suppressed within
its own definition, in traditional C. Thus, any macro that is used recursively
inevitably causes an error.
-
The #
character has
no special meaning within a macro definition in traditional C.
-
In traditional C, the text at
the end of a macro expansion can run together with the text after the macro
call, to produce a single token. (This is impossible in ANSI C.)
-
Traditionally, \
inside a macro argument suppresses the syntactic significance of the following
character.
-trigraphs
Process ANSI standard trigraph
sequences. These are three-character sequences, all starting with ??,
that are defined by ANSI C to stand for single characters. For example,
??/ stands
for \,
so ‘??/n’
is a character constant for a new-line. Strictly speaking, the GNU C preprocessor
does not support all programs in ANSI C Standard unless -trigraphs
is used, but if
you ever notice the difference it will be with relief.
-
You don’t want to know any more
about trigraphs.
-
-pedantic
Issue warnings required
by the ANSI C standard in certain cases such as when text other than a
comment follows #else
or #endif.
-
-pedantic-errors
Like -pedantic,
except that errors are produced rather than warnings.
-
-Wtrigraphs
Warn if any trigraphs are
encountered (assuming they are enabled).
-
-Wcomment
Warn whenever a comment-start
sequence /*
appears in a comment.
-
-Wall
Requests both -Wtrigraphs
and -Wcomment
(but not -Wtraditional).
-
-Wtraditional
Warn about certain constructs
that behave differently in traditional and ANSI C.
-
-I directory
Add the directory
to the head of the list of directories to be searched for header files
(see The #include
directive). This can be used to
override a system header file, substituting your own version, since these
directories are searched before the system header file directories. If
you use more than one -I
option, the directories are scanned in left-to-right
order; the standard system directories come after.
-
-I-
Any directories specified
with -I
options before the -I-
option are searched only for the case of #include
"file";
they are not searched for #include
<file>. If
additional directories are specified with -I
options after the -I-,
these directories are searched for all #include directives. In addition,
the -I-
option inhibits the use of the current directory as the first search directory
for #include " file".
Therefore, the current directory is searched only if -I.
is requested explicitly with it. Specifying both -I-
and -I.
allows you to control precisely which directories are searched before and
which after the current one is searched.
-
-nostdinc
Do not search the standard
system directories for header files. Only the directories you have specified
with -I
options (and the current directory, 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.)
-
-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 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 use more than
one -D
for the same name,
the rightmost definition takes effect.
-
-U name
Do not predefine name.
If both -U
and -D
are specified for one name, the -U
beats the -D
and the name is not predefined.
-
-undef
Do not predefine any nonstandard
macros.
-
-A predicate(answer)
Make an assertion with the
predicate
and answer.
See Assertions.
You can use -A-
to disable all predefined assertions; it also un-defines all predefined
macros that identify the type of target system.
-
-dM
Instead of outputting the
result of preprocessing, output 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 following command will show the values of any predefined macros.
touch foo.h; cpp -dM foo.h
-dD
Like -dM
except in two respects: it does not include the pre-defined macros,
and it outputs both the #define
directives and the result of preprocessing. Both kinds of output go to
the standard output file.
-M [-MG]
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. If there are many included files
then the rule is split into several lines using \-newline.
-MG
says to treat missing header files as generated files and assume they live
in the same directory as the source file. It must be specified in addition
to -M.
This feature is used in automatic updating of makefiles.
-MM [-MG]
Like -M,
mentions only the files included with #include"file".
System header files included with #include<file>
are omitted.
-MD file
Like -M
but the dependency information is written to file.
This is in addition to compiling the file as specified—-MD
does not inhibit ordinary compilation the way -M
does. When invoking gcc,
do not specify the file
argument. gcc
will create file names made by replacing ".c"
with ".d"
at the end of the input file names.
-
In Mach, you can use the utility
md
to merge multiple dependency files into a single dependency file suitable
for using with the make
command.
-MMD file
Like -MD
except mention only user header files, not system header files.
-H
Print the name of each header
file used, in addition to other normal activities.
-imacros file
Process
file
as input, discarding the resulting output, before processing the regular
input file. Because the output generated from file
is discarded, the only effect of -imacros
file
is to make the macros defined in file
available for use in the main input.
-include file
Process file
as input, and include all the resulting output, before processing the regular
input file.
-idirafter dir
Add the directory dir
to the second include path. The directories on the second include path
are searched when a header file is not found i
ffb
n any of the directories
in the main include path (the one that -I
adds to).
-iprefix prefix
Specify prefix
as a prefix for using subsequent -iwithprefix
options.
-iwithprefix dir
Add a directory to the second
include path. The directory’s name is made by concatenating prefix
and dir,
where prefix
was previously specified with -iprefix.
-isystem dir
Add a directory to the beginning
of the second include path, marking it as a system directory, so that it
gets the same special treatment as is applied to the standard system directories.
-lang-c
-lang-c89
-lang-c++
-lang-objc
-lang-objc++
Specify the source language.
-lang-c
is the default; it allows recognition of C++ comments (comments that begin
with //
and end at end of line), since this is a common feature and it will most
likely be in the next C standard. -lang-c89
disables recognition of C++ comments. -lang-c++
handles C++ comment syntax and includes extra default include directories
for C++. -lang-objc
enables the Objective C #import
directive. -lang-objc++
enables both C++ and Objective C extensions. These options are generated
by the compiler driver gcc,
but not passed from the gcc
command line unless you use the driver’s -Wp
option.
-lint
Look for commands to the
program checker lint
embedded in comments, and emit them preceded by #pragma
lint.
-
For example, /*
NOTREACHED */
becomes #pragma lint
NOTREACHED.
-
This option is available only
when you call cpp
directly; gcc
will not pass it from its command line.
-$
Forbid the use of $
in identifiers. This is required for ANSI conformance. gcc
automatically supplies this option to the preprocessor if you specify -ansi,
but gcc
doesn’t recognize the -$
option itself—to use it without the other effects of -ansi,
you must call the preprocessor directly.
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