I'll get to this asap. Have to install a new "clean" environment aka a 
new virtual machine.

On 18-Aug-16 02:43, Martin Panter wrote:
> Martin Panter added the comment:
>
> 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__
>
> ----------
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <http://bugs.python.org/issue27643>
> _______________________________________