I didn't have time to look deeper into this. I'm sorry for not getting back to you. The rather-large example that you uploaded didn't and doesn't run for me, so I wasn't able to reproduce the problem.

The one bit of advice I have is to always declare required argument types in the `argtypes` attribute of ctypes functions. It can be very useful to take advantage of the type checking that ctypes provides in order to catch problems before they lead to stack or heap corruption, or bad values that lead to corruption later on. 

I'm closing this issue as not a bug. ctypes and libffi have been used in a broad range of cases in 32-bit Windows for many years, and I wasn't able to reproduce the problem with a function of the given call signature when I checked in 2016.