Python Ctypes异常：读取访问冲突

Question

我尝试通过DLL（由PLC制造商分发的C API接口）与PLC通信。我正在使用Python 3.1，该Python 3.1作为脚本环境嵌入在其他软件（x64-Windows 7）中。

我设法使一些DLL函数正常工作，但现在却得到了我无法解决的“访问冲突阅读”。

有关DLL函数的信息：

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

参数：

• pAddr：[输入]具有NetId和ADS服务器的端口号的结构。
• nIndexGroup：[输入]索引组。
• nIndexOffset：[输入]索引偏移量。
• nLength：[in]数据的长度，以字节为单位。
• pData：[out]指向将接收数据的数据缓冲区的指针。
• 返回值：返回函数的错误状态。

结构AmsAddr：

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

结构AmsNetId

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

Python实现：

# -*- 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

我希望任何人都无法弄清楚出了什么问题。我只是Python编程的高级新手，完全没有C方面的经验

提前致谢

Answer 1

您传递了无效的指针，请提供有效的内存缓冲区：

data = create_string_buffer(nLength)

参数应该是 justc_void_p而不是POINTER(c_void_p)if PVOIDMeans void *。不要将 restype 设置为None（函数返回LONG）。

也通过pointer(plcAddress)（POINTER(AmsAddr)在 argtypes 中指定）。

使用正确的调用约定（在 cdll、windll、oledll 之间进行选择）。