How do I run the GCC preprocessor to get the code after macros like #define are expanded?

Question

Is there an option that the GCC preprocessor could generate C source code and filter out irrelevant source code?

For example, a .c file has a #def

Answer 1

While the -E option will perform all pre-processing, it also produces some very 'raw' output that might not be what you want (depending on what you want).

If you need to debug a macro expansion that's not doing what you expect, E is a good way to go. If you simply want to filter out the 'inactive code', but leave the remaining code in more-or-less original form, you might want to look at the answers to the following Stack Overflow question:

• Is there a C pre-processor which eliminates #ifdef blocks based on values defined/undefined?

Answer 2

It sounds like you want unifdef, not the GCC preprocessor.

Answer 3

Yes. Use the -E option:

gcc -E foo.c