Ctypes - Passing a Void Pointer from Python

Question

I am accessing a C++ DLL using Python Ctypes on Windows 7. I have the documentation for the DLL, but I can't actually open it. I'm trying to use a C++ function that takes in a function, which in turn takes in an unsigned int and a void pointer. Here is a short code sample that fails:

import ctypes
import os

root = os.path.dirname(__file__)
lib = ctypes.WinDLL(os.path.join(root, 'x86', 'toupcam.dll')) #works

cam = lib.Toupcam_Open(None) #works

def f(event, ctx): #Python version of function to pass in
    pass

#converting Python function to C function:
#CFUNTYPE params: return type, parameter types
func = ctypes.CFUNCTYPE(None, ctypes.c_uint, ctypes.c_void_p)(f)

res = lib.Toupcam_StartPullModeWithCallback(cam, func) #fails

Whenever I run this code I get this error on the last line:

OSError: exception: access violation writing 0x002CF330.

I don't really know how to approach this issue, since it's a C++ error not a Python error. I think it has to do with my void pointer, since similar errors I found online for C++ were pointer-related. Is there something wrong with the Ctypes void pointer, or am I doing something wrong?

A quick google shows that Toupcam_StartPullModeWithCallback takes three arguments: camera, callback, context. webastro.net/forum/archive/index.php/t-138355.html — kfsone

Mark Tolonen Mark Tolonen · Accepted Answer · 2016-07-06T21:11:59

You need to declare the argument types of the functions you call using argtypes. Since I don't know your exact API, here's an example:

Windows C DLL code with a callback:

typedef void (*CB)(int a);

__declspec(dllexport) void do_callback(CB func)
{
    int i;
    for(i=0;i<10;++i)
        func(i);
}

Python code:

from ctypes import *

# You can use as a Python decorator.
@CFUNCTYPE(None,c_int)
def callback(a):
  print(a)

# Use CDLL for __cdecl calling convention...WinDLL for __stdcall.
do_callback = CDLL('test').do_callback
do_callback.restype = None
do_callback.argtypes = [CFUNCTYPE(None,c_int)]

do_callback(callback)

Output:

Ctypes - Passing a Void Pointer from Python

1 Answers