python ctype initialising a structure

Question

My structure contains all unsigned char elements

typedef struct
{
    unsigned char bE;
    unsigned char cH;
    unsigned char cL;
    unsigned char EId1;
    u         


        

Answer 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 *

class CMsg(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))