The crash is because of an error in your C code:
in backend.c::
    void backend(char *output_filename)
    {
        C2py("backend", 1, output_filename);
    }

But your C2py function expects a variable number of PyObject*.

I found this by disabling the garbage collector: then the program
crashes in C2py.c at the instruction "Py_DECREF(pArgs);"