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Options Controlling the Preprocessor

The following options control the C preprocessor, which is run on each C source file before actual compilation.

If you use the -E option, nothing is done except preprocessing. Some of these options make sense only together with -E because they cause the preprocessor output to be unsuitable for actual compilation.

-include file
Process file as input before processing the regular input file. In effect, the contents of file are compiled first. Any -D and -U options on the command line are always processed before -include file, regardless of the order in which they are written. All the -include and -imacros options are processed in the order in which they are written.

-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.
Any -D and -U options on the command line are always processed before the -imacros file, regardless of the order in which they are written. All the -include and -imacros options are processed in the order in which they are written.

-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 directorys name is made by concatenating prefix and dir, where prefix was specified previously with -iprefix. If you have not specified a prefix yet, the directory containing the installed passes of the compiler is used as the default.

-iwithprefixbefore dir
Add a directory to the main include path. The directorys name is made by concatenating prefix and dir, as in the case of -iwithprefix.

-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.

-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. See Options for Directory Search for information on -I.
By using both -nostdinc and -I-, you can limit the include-file search path to only those directories you specify explicitly.

-undef
Do not predefine any nonstandard macros, including architecture flags.

-E
Run only the C preprocessor. Preprocess all the C source files specified and output the results to standard output or to the specified output file.

-C
Tell the preprocessor not to discard comments. Used with the -E option.

-P
Tell the preprocessor not to generate #line directives. Used with the -E option.

-M
Tell the preprocessor to output a rule suitable for make describing the dependencies of each object file. For each source file, the preprocessor outputs one make-rule whose target is the object filename for that source file and whose dependencies are all the #include header files it uses.
This rule may be a single line or may be continued with \-newline if it is long. The list of rules is printed on standard output instead of the preprocessed C program. -M implies -E.
Another way to specify output of a make rule is by setting the environment variable, DEPENDENCIES_OUTPUT (see Environment Variables Affecting GNU CC).

-MM
Like -M but the output mentions only the user header files included with #include "file".
System header files included with #include <file> are omitted.

-MD
Like -M but the dependency information is written to a file made by replacing .c with .d at the end of the input filenames. This is in addition to compiling the file as specified. -MD does not inhibit ordinary compilation the way -M does.
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
Like -MD except output mentions only user header files, not system header files.

-MG
Treat missing header files as generated files and assume they live in the same directory as the source file. If you specify -MG, you must also specify either -M or -MM. -MG is not supported with -MD or -MMD.

-H
Print the name of each header file used, in addition to other normal activities.

-Aquestion(answer)
Assert the answer answer for question, in case it is tested with a preprocessing conditional such as #if #question(answer). -A- disables the standard assertions that normally describe the target machine.

-Dmacro
Define macro macro with the string 1 as its definition.

-Dmacro=defn
Define macro macro as defn. All instances of -D on the command line are processed before any -U options.

-Umacro
Undefine macro macro. The -U options are evaluated after all -D options, but before any -include and -imacros options.

-dM
Tell the preprocessor to output only a list of the macro definitions that are in effect at the end of preprocessing. Used with the -E option.

-dD
Tell the preprocessing to pass all macro definitions into the output, in their proper sequence in the rest of the output.

-dN
Like -dD except that the macro arguments and contents are omitted. Only #define name is included in the output.

-trigraphs
Support ANSI C trigraphs. The -ansi option also has this effect.

-Wp, option
Pass option as an option to the preprocessor. If option contains commas, it is split into multiple options at the commas.