Antti, is there a place in the ctypes documentation that explicitly says ctypes is not meant to be used cross-platform? If not, shouldn't that be mentioned?

I think ultimately ctypes should default to standard OS/compiler behavior, but should allow the flexibility for one to override for cross-platform interchangeability.

In the C++ code here, the code is explicitly checking if Windows (or GCC) to choose behavior. In theory, that could be changed to allow runtime-choice for packing behavior. 

Of course, that is probably best for a feature issue. For this one, I'd just like to have the behavior on Linux match GCC, as that is the definitive bug here as our example has shown.