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1.9 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 compiler.

`-P'

Inhibit generation of `#'-lines with line-number information in the output from the preprocessor (see section 1.8 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 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 constants.

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 `-traditional':

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.

`-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 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.

`-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. Currently this only works if you have turned trigraphs on with `-trigraphs' or `-ansi'; in the future this restriction will be removed.

`-Wcomment'

Warn whenever a comment-start sequence `/*' appears in a `/*' comment, or whenever a Backslash-Newline appears in a `//' comment.

`-Wall'

Requests both `-Wtrigraphs' and `-Wcomment' (but not `-Wtraditional' or `-Wundef').

`-Wtraditional'

Warn about certain constructs that behave differently in traditional and ANSI C.

`-Wundef'

Warn if an undefined identifier is evaluated in an `#if' directive.

`-I directory'

Add the directory directory to the head of the list of directories to be searched for header files (see section 1.3.2 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 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.

`-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.)

`-remap'

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.

`-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.

`-gcc'

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'.

`-A predicate(answer)'

Make an assertion with the predicate predicate and answer answer. See section 1.5.5 Assertions.

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.

`-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 command

touch foo.h; cpp -dM foo.h

will show the values of any predefined macros.

`-dD'

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.

`-dI'

Output `#include' directives in addition to the result of preprocessing.

`-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' but mention 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 in any of the directories in the main include path (the one that `-I' adds to).

`-iprefix prefix'

Specify prefix as the prefix for 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 specified previously 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.

`-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 only knows about the standards for C; other language standards will be added in the future.

standard may be one of:

iso9899:1990

The ISO C standard from 1990.

iso9899:199409

c89

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'.

iso9899:199x

c9x

The revised ISO C standard, which is expected to be promulgated some time in 1999. It has not been approved yet, hence the `x'.

gnu89

The 1990 C standard plus GNU extensions. This is the default.

gnu9x

The 199x C standard plus GNU extensions.

`-Wp,-lint'

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.