Python Ctype Initialising A Structure

My structure contains all unsigned char elements typedef struct { unsigned char bE; unsigned char cH; unsigned char cL; unsigned char EId1; unsigned char EId0;

Solution 1:

You passed pmsg by value, but the function expects a pointer. Since you've initialized to all zeros, the function ends up dereferencing a NULL pointer. Then ctypes uses Windows SEH to route the access violation to a Python exception.

You need to use byref(pmsg) to pass a reference. Also, define the function's argtypes to ensure proper handling of the pointer on 64-bit systems.

from ctypes import *
from ctypes.wintypes import *

classCMsg(Structure):
    _fields_ = [
        ('bE', c_ubyte),
        ('cH', c_ubyte),
        ('cL', c_ubyte),
        ('EId1', c_ubyte),
        ('EId0', c_ubyte),
        ('SId1', c_ubyte),
        ('SId0', c_ubyte),
        ('DLC', c_ubyte),
        ('D0', c_ubyte),
        ('D1', c_ubyte),
        ('D2', c_ubyte),
        ('D3', c_ubyte),
        ('D4', c_ubyte),
        ('D5', c_ubyte),
        ('D6', c_ubyte),
        ('D7', c_ubyte)]

hllDll = cdll...
hllDll.WriteCANMessage.argtypes = [HANDLE, POINTER(CMsg)]

handle = ...
pmsg = CMsg() #  initially memset to {0}
hllDll.WriteCANMessage(handle, byref(pmsg))