(cpp.info) Concept Index
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.
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 ( 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.
Use the `-traditional' option when preprocessing Fortran code, so
that singlequotes and doublequotes within Fortran comment lines
(which are generally not recognized as such by the preprocessor)
do not cause diagnostics about unterminated character or string
However, this option does not prevent diagnostics about
unterminated comments when a C-style comment appears to start, but
not end, within Fortran-style commentary.
So, the following Fortran comment lines are accepted with
C This isn't an unterminated character constant
C Neither is "20000000000, an octal constant
C in some dialects of Fortran
However, this type of comment line will likely produce a
diagnostic, or at least unexpected output from the preprocessor,
due to the unterminated comment:
C Some Fortran compilers accept /* as starting
C an inline comment.
Note that `g77' automatically supplies the `-traditional' option
when it invokes the preprocessor. However, a future version of
`g77' might use a different, more-Fortran-aware preprocessor in
place of `cpp'.
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. Currently this only works
if you have turned trigraphs on with `-trigraphs' or `-ansi'; in
the future this restriction will be removed.
Warn whenever a comment-start sequence `/*' appears in a `/*'
comment, or whenever a Backslash-Newline appears in a `//' comment.
Requests both `-Wtrigraphs' and `-Wcomment' (but not
`-Wtraditional' or `-Wundef').
Warn about certain constructs that behave differently in
traditional and ANSI C.
Warn if an undefined identifier is evaluated in an `#if' directive.
Add the directory DIRECTORY to the head of the list of directories
to be searched for header files ( 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 the C++ library.)
When searching for a header file in a directory, remap file names
if a file named `header.gcc' exists in that directory. This can
be used to work around limitations of file systems with file name
restrictions. The `header.gcc' file should contain a series of
lines with two tokens on each line: the first token is the name to
map, and the second token is the actual name to use.
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.
Define the macros __GNUC__ and __GNUC_MINOR__. These are defined
automatically when you use `gcc -E'; you can turn them off in that
case with `-no-gcc'.
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 and all macros that preceded it on
the command line.
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 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' directives
and the result of preprocessing. Both kinds of output go to the
standard output file.
Output `#include' directives in addition to the result of
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.
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 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
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'.
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.
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.
This option has been removed, because it conflicts with the `-l'
Specify the standard to which the code should conform. Currently
cpp only knows about the standards for C; other language standards
will be added in the future.
STANDARD may be one of:
The ISO C standard from 1990.
The 1990 C standard, as amended in 1994. `c89' is the
customary shorthand for this version of the standard.
The `-ansi' option is equivalent to `-std=c89'.
The revised ISO C standard, which is expected to be
promulgated some time in 1999. It has not been approved yet,
hence the `x'.
The 1990 C standard plus GNU extensions. This is the default.
The 199x C standard plus GNU extensions.
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'.
Because of the clash with `-l', you must use the awkward syntax
above. In a future release, this option will be replaced by
`-flint' or `-Wlint'; we are not sure which yet.
Forbid the use of `$' in identifiers. The C standard does not
permit this, but it is a common extension.
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