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 directory’s 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 directory’s 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.