For now, you need to add both -Wno-return-type and -m32 when compiling.

I think it doesn't work on x86-64, since the code assumes

    sizeof(int) == sizeof(void*)

Yep, you can compile it on 64 bit OS X with clang's -m32 option and it should work:

    ➜  c4 git:(master) ✗ clang -m32 c4.c
    ...
    ➜  c4 git:(master) ✗ ./a.out hello.c 
    hello, world
    exit(0) cycle = 9

Please try kbrock's fork at https://github.com/kbrock/c4 Check out his 'longs' branch.