It looks like resolving https://bugs.python.org/issue20160 didn't fix all the issues. When a large struct is used as a parameter, a reference to the object itself is passed to the function, not a reference to a temporary copy, as it is required in https://msdn.microsoft.com/en-us/library/zthk2dkh.aspx.

Here is the reproduction sample:

=== Sample.c: ===
typedef struct
{
  int v[3];
} Data;

__declspec(dllexport) void Cripple(Data data)
{
  data.v[0] = 0;
}

=== Sample.py ===
from ctypes import *

class Data(Structure):
    _fields_ = [("v", c_int * 3)]

lib = cdll.LoadLibrary('Sample.dll')
getattr(lib, 'Cripple').argtypes = [Data]

data = Data()
data.v[0] = 42
lib.Cripple(data)
assert data.v[0] == 42
print "OK"

=== repro.cmd ===
call "%VS140COMNTOOLS%/../../VC/vcvarsall.bat" x86_amd64
cl /LD Sample.c
python Sample.py