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 #define switch to define for many different platforms. I'm only interested in one platform, and I want the C preprocessor to filter out unrelated code.

Does GCC support this?

Note that after running the preprocessor, you'll also have all `#include` s at the top of the file (so prepare for a lot of scrolling). — , Oct 12 '10 at 16:45
There is a duplicate of this question at http://stackoverflow.com/questions/3917316/gcc-preprocessor with additional answers and discussion. — Kevin Vermeer, Mar 28 '11 at 21:18
Visual studio: http://stackoverflow.com/questions/277258/c-c-source-file-after-preprocessing — Ciro Santilli Путлер Капут 六四事, Jan 17 '16 at 11:42

score 37 · Answer 1 · answered Oct 12 '10 at 16:43

37

Yes. Use the -E option:

gcc -E foo.c

answered Oct 12 '10 at 16:43

JesperE

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score 18 · Answer 2 · edited May 23 '17 at 12:25

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?

score 5 · Answer 3 · edited Feb 04 '16 at 20:25

5

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

edited Feb 04 '16 at 20:25

Peter Mortensen

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answered Oct 13 '10 at 07:06

Christoffer

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

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