What is an easily hackable C preprocessor?

Question

I want to add a small feature to a C preprocessor, but for that, I need one that is easy to understand and can easily be modified. Specifically, I am looking for the following criteria:

• small codesize
• well-documented
• easy to modify
• free software (I want to be able to distribute the modified code without paying fees or so.)

I have already found tcc, which includes a preprocessor and is fairly small but, as far as I can see, lacks good documentation on how preprocessing is implemented. Should I just try to understand how tcc does it or does a better fit exist?

Answer 1

What about http://www.blitzbasic.com/Community/posts.php?topic=20396 or http://mcpp.sourceforge.net/

Seems good to me!