GCC dump preprocessor defines

Question

Is there a way for gcc/g++ to dump its preprocessor defines from the command line? I mean things like __GNUC__, __STDC__, and so on.

Answer 1

Yes, use -E -dM options instead of -c.

From the gcc manual:

Instead of the normal output, generate 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 all the predefined macros.

If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach.

Answer 2

I do it this way:

$ gcc -dM -E - < /dev/null