Interfacing a dynamic shared library gives me different results in 2.7 versus 3.5

[Siyi Deng]

I have a dynamic library doing some numerical computations. 

I used ctypes to interact it by passing numpy arrays back and forth. 

Python 3.5 gives me the correct results. 

Python 2.7 gives me different, erroneous results, but it never crashes. 

How is this possible? There is no string operations involved whatsoever.