Build AST from C code

Question

How can I build an AST (Abstract Syntax Tree) from gcc C code in order to make some modifications, like converting some int variables to float, and reproduce(generate) the c

Answer 1

Eli Bendersky's pycparser is a C source-to-source tool written in Python: https://github.com/eliben/pycparser

It will parse C99 and can build a detailed Parse Tree with nodes matching the grammar in the K&R "The C Programming Language" Appendix A ch. 13 "Grammar". It's built on a Python pseudo-implementation of lex/yacc, flex/bison whatever called PLY.

It has examples and it's really easy to get going. Like the other posters said, it is a complex task to reduce the parse tree to a minimal AST with all the irrelevant details left out.

This project can do source-to-source transformations as well: https://github.com/axw/cmonster/ CMonster is written in Python and wraps the Clang API.

If you wanna use GCC for the task, you should look into MELT. There is another project where the scripting language is JavaScript, but I can't remember the name ATM..

EDIT: responding to comments

Yeah, the framework that handled the intermediate representation was called TreeHydra and it is abandoned, but still working as far as I can see. There is a video tutorial online somewhere with the young Ph.D. dude that designed TreeHydra - think I found it with google video - explaining his choice of JS as interface language because of the popularity etc. He appeared knowledgeable and charismatic and I guess that's the reason that particular project stuck with me :) Haven't tried it out myself though.

I myself am working on a hobby Control Flow Graph and Data Flow Analysis tool using Eli Bendersky's framework as a building block. Of the toolkits I've tried, Eli's kit really seems the most promising. Together with inspiration from this particular cool project: Atul's Mini-C Compiler that utilizes the same Lex/Yacc Python port (PLY). Haven't done much yet, but it was easier getting going than learning libclang, although I do consider that a very promising route as well.