Neat, but this is not a general purpose C interpreter. It seems to lack the preprocessor and is only able to execute the example program and itself because it has wrapper implementations for a static set of standard library functions including printf(), open(), read() and malloc().[1] Use a standard library function it does not readily support, and you're out of luck.

[1]: https://github.com/rswier/c4/blob/master/c4.c#L492-L498