Michael, byref() is just a helper for passing an object’s address to a C function. Calling func(byref(b), ...) in Python is equivalent to the C code

unpack_bitfields(&b, ...)

I still think the root problem is in unpack_bitfields(). When compiled with XLC, your experiments confirm that it always returns unsigned values, and with GCC, it returns signed values.

It looks like you can detect XLC with the __IBMC__ and __xlC__ macros (not sure if there is a practical difference). So perhaps we can fix this with

#ifndef __xlC__