ANSI Standard C requires the rejection of many harmless constructs commonly used by today’s C programs. Such incompatibility would be inconvenient for users, so the GNU C preprocessor is configured to accept these constructs by default. Strictly speaking, to get ANSI Standard C, you must use the options ‘ -trigraphs ’, ‘ -undef ’ and ‘ -pedantic ’, but in practice the consequences of having strict ANSI Standard C make it undesirable to do this.
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.
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| -P | Inhibit generation of ‘ # ’-lines with line-number information in the output from the preprocessor. 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. |
| -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 (assuming they are enabled). |
-Wcomment
-Wcomments |
| | Warn whenever a comment-start sequence ‘ /* ’ appears in a comment. (Both forms have the same effect). |
| -Wall | Requests both ‘ -Wtrigraphs ’ and ‘ -Wcomment ’ (but not ‘ -Wtraditional ’). |
| -Wtraditional |
| | Warn about certain constructs that behave differently in traditional and ANSI C. |
| "-I "directory |
| | Add the directory directory to the end of the list of directories to be searched for header files. 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 libg++.) |
| "-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. |
| "-A ""name("value) |
| | Assert (in the same way as the #assert directive) the predicate name with tokenlist value . Remember to escape or quote the parentheses on shell command lines. You can use ‘ -A- ’ to disable all predefined assertions; it also undefines all predefined macros. |
| -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. |
| -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 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. The preprocessor evaluates any ‘ -D ’ and ‘ -U ’ options on the command line before processing ‘ -imacros file ’ . |
| "-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 ’. |
-lang-c
-lang-c++
-lang-objc
-lang-objc++ | Specify the source language. ‘ -lang-c++ ’ makes the preprocessor handle C++ comment syntax, and includes extra default include directories for C++, and ‘ -lang-objc ’ enables the Objective C ‘ #import ’ directive. ‘ -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. |
| -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 ’. 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 was formerly required for strict conformance to the C Standard before the standard was corrected. This option is available only when you call cpp directly; gcc will not pass it from its command line. |
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