Python Ctypes Exception: access violation reading

Question

I try to communicate with a PLC through a DLL (C API interface distributed by the manufacturer of the PLC). I'm using Python 3.1 who is embedded as a scripting environment in a other software (x64 - Windows 7).

I managed to get a few DLL functions working, but now a get an "Access violation reading" that I can't solve.

Information about the DLL function:

LONG AdsSyncReadReq(
  PAmsAddr  pAddr,
  ULONG     nIndexGroup,
  ULONG     nIndexOffset,
  ULONG     nLength,
  PVOID     pData
);

Parameters:

• pAddr: [in] Structure with NetId and port number of the ADS server.
• nIndexGroup: [in] Index Group.
• nIndexOffset: [in] Index Offset.
• nLength:[in] Length of the data in bytes.
• pData: [out] Pointer to a data buffer that will receive the data.
• Return value: Returns the function's error status.

Structure AmsAddr:

typedef struct {
  AmsNetId        netId;
  USHORT          port;
} AmsAddr, *PAmsAddr;

Structure AmsNetId

typedef struct {
  UCHAR        b[6];
} AmsNetId, *PAmsNetId;

Python Implementation:

# -*- coding: utf-8 -*-
from ctypes import *

#I've tried OleDll and windll as wel..
ADS_DLL = CDLL("C:/Program Files/TwinCAT/Ads Api/TcAdsDll/x64/TcAdsDll.dll")

class AmsNetId(Structure):
    _fields_ = [('NetId',  c_ubyte*6)]

class AmsAddr(Structure):
    _fields_=[('AmsNetId',AmsNetId),('port',c_ushort)]

# DLL function working fine
version = ADS_DLL.AdsGetDllVersion()
print(version)

#DLL function working fine
errCode = ADS_DLL.AdsPortOpen()
print(errCode)

#DLL function using the AmsAddr() class, working fine
amsAddress = AmsAddr()
pointer_amsAddress = pointer(amsAddress)
errCode = ADS_DLL.AdsGetLocalAddress(pointer_amsAddress)
print(errCode)
contents_amsAddres = pointer_amsAddress.contents

#Function that doens't work:
errCode = ADS_DLL.AdsSyncReadReq()
print(errCode) # --> errCode = timeout error, normal because I didn't pass any arguments

# Now with arguments:
plcNetId = AmsNetId((c_ubyte*6)(5,18,18,27,1,1)) #correct adress to the PLC
plcAddress = AmsAddr(plcNetId,801) #correct port to the PLC
nIndexGroup = c_ulong(0xF020)
nIndexOffset = c_ulong(0x0) 
nLength = c_ulong(0x4)
data = c_void_p()
pointer_data = pointer(data)

#I tried with an without the following 2 lines, doesn't matters 
ADS_DLL.AdsSyncReadReq.argtypes=[AmsAddr,c_ulong,c_ulong,c_ulong,POINTER(c_void_p)]
ADS_DLL.AdsSyncReadReq.restype=None

#This line crashes
errCode = ADS_DLL.AdsSyncReadReq(plcAddress,nIndexGroup,nIndexOffset,nLength,pointer_data)
print(errCode)


>>>> Error in line 57: exception: access violation reading 0xFFFFFFFFFFFFFFFF

I hope anyone can't figure out what's wrong. I'm only a advanced novice in Python programming with no experience at all in C

Thanks in advance

Answer 1

You're passing invalid pointer, provide a valid memory buffer instead:

data = create_string_buffer(nLength)

The argument should be just c_void_p instead of POINTER(c_void_p) if PVOID means void *. Don't set restype to None (the function returns LONG).

Also pass pointer(plcAddress) (specify POINTER(AmsAddr) in argtypes).

Use correct calling convention (choose between cdll, windll, oledll).