The "E" switch of gcc tells it to do only pre-processing and then stop. Output is to standard output, so you will want to capture that in a file.

However, the pre-processor goes all the way and resolves all macros, leaving no trace of them anymore. Its output is:

void main()
{
 int a, b, c;
 float d,e,f;

  c = a+b;

   f = e*d;
}
[download]

Actually it is much longer, related to the stdio.h and stdlib.h files.

And that is exactly where your problems will be: you will have to write a dedicated pre-processor which goes to all the included files to see which macros are and remain undefined (macros can be anywhere, not just in your header files) and then parse all files to see which code is to be deleted, leaving all other macro references in place.

Of course your example is trivial, but once you have many #if and #ifdef tests deeply nested, parsing the .c files will become more and more complicated. It will no longer be a straight search-and-delete thing, but you will actually have to parse what you are handling.

Anyhow, have a look at Text::EP3. It is an extensible pre-processor written in Perl, so you may be able to adapt it to your needs.