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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 individually.
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.
Here is a table 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
Inhibit generation of `#'-lines with line-number information in
the output from the preprocessor (Note: 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
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.
Try to imitate the behavior of old-fashioned C, as opposed to ANSI
* Traditional macro expansion pays no attention to singlequote
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 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.
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
newline. Strictly speaking, the GNU C preprocessor does not
support all programs in ANSI Standard C 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.
Issue warnings required by the ANSI C standard in certain cases
such as when text other than a comment follows `#else' or `#endif'.
Like `-pedantic', except that errors are produced rather than
Warn if any trigraphs are encountered (assuming they are enabled).
Warn whenever a comment-start sequence `/*' appears in a comment.
Requests both `-Wtrigraphs' and `-Wcomment' (but not
Warn about certain constructs that behave differently in
traditional and ANSI C.
Add the directory DIRECTORY to the end of the list of directories
to be searched for header files (Note: Include Syntax.). 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.
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'
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 it is
requested explicitly with `-I.'. Specifying both `-I-' and `-I.'
allows you to control precisely which directories are searched
before the current one and which are searched after.
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.
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 libg++.)
Predefine NAME as a macro, with definition `1'.
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
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.
Do not predefine any nonstandard macros.
Make an assertion with the predicate PREDICATE and answer ANSWER.
You can use `-A-' to disable all predefined assertions; it also
undefines all predefined macros that identify the type of target
Instead of outputting the result of preprocessing, output a list of
`#define' commands 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 the values of any predefined macros.
Like `-dM' except in two respects: it does *not* include the
predefined macros, and it outputs *both* the `#define' commands
and the result of preprocessing. Both kinds of output go to the
standard output file.
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.
This feature is used in automatic updating of makefiles.
Like `-M' but mention only the files included with `#include
"FILE"'. System header files included with `#include <FILE>' are
Like `-M' but the dependency information is written to files with
names made by replacing `.c' with `.d' at the end of the input
file names. This is in addition to compiling the file as
specified--`-MD' does not inhibit ordinary compilation the way
In Mach, you can use the utility `md' to merge the `.d' files into
a single dependency file suitable for using with the `make'
Like `-MD' except mention only user header files, not system
Print the name of each header file used, in addition to other
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
Process FILE as input, and include all the resulting output,
before processing the regular input file.
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 in any of the directories in the main include path (the one
that `-I' adds to).
Specify PREFIX as the prefix for subsequent `-iwithprefix' options.
Add a directory to the second include path. The directory's name
is made by concatenating PREFIX and DIR, where PREFIX was
specified previously with `-iprefix'.
Specify the source language. `-lang-c++' makes the preprocessor
handle C++ comment syntax (comments may begin with `//', in which
case they end at end of line), and includes extra default include
directories for C++; and `-lang-objc' enables the Objective C
`#import' command. `-lang-c' explicitly turns off both of these
extensions, and `-lang-objc++' enables both.
These options are generated by the compiler driver `gcc', but not
passed from the `gcc' command line.
Look for commands to the program checker `lint' embedded in
comments, and emit them preceded by `#pragma lint'. For example,
the comment `/* 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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