Question

I have to define a callback function in Python, one that will be called from a DLL.

BOOL setCallback (LONG nPort, void ( _stdcall *pFileRefDone) (DWORD nPort, DWORD nUser), DWORD nUser);

I tried this code, that seems to work in Python 2.5 but with Python 2.7 it crashes and I assume I did something wrong.

import ctypes, ctypes.wintypes
from ctypes.wintypes import DWORD

def cbFunc(port, user_data):
        print "Hurrah!"

CB_Func = ctypes.WINFUNCTYPE(None, DWORD, DWORD)

mydll.setCallback(ctypes.c_long(0), CB_Func(cbFunc), DWORD(0))

Where is the mistake?

Note: The platform is running on 32bit only (both Windows and Python). The DLL loads successfully and also other functions from inside do work just fine while called from Python.

A full sample code that reproduces this is available at https://github.com/ssbarnea/pyHikvision – just run Video.py under py25 and py27.

Answer 1

In the context of Video class using a nested function:

class Video(object):

    def __init__(self, ...):
        self.__cbFileRefDone = []

    def open(self, filename):
        @WINFUNCTYPE(None, DWORD, DWORD)
        def cbFileRefDone(port, user_data):
            print "File indexed.", filename
        self.__cbFileRefDone.append(cbFileRefDone) # save reference

        if not self.hsdk.PlayM4_SetFileRefCallBack(
            c_long(self.port), cbFileRefDone, DWORD(0)):
            logging.error("Unable to set callback for indexing")
            return False

It is somewhat ugly but a more natural variant fails with TypeError during callback:

#XXX this won't work
@WINFUNCTYPE(None, DWORD, DWORD)
def cbFileRefDone(self, port, user_data):
    print "File indexed."

To fix that a special function descriptor could be created:

def method(prototype):
    class MethodDescriptor(object):
        def __init__(self, func):
            self.func = func
            self.bound_funcs = {} # hold on to references to prevent gc
        def __get__(self, obj, type=None):
            assert obj is not None # always require an instance
            try: return self.bound_funcs[obj,type]
            except KeyError:
                ret = self.bound_funcs[obj,type] = prototype(
                    self.func.__get__(obj, type))
                return ret
    return MethodDescriptor

Usage:

class Video(object):

    @method(WINFUNCTYPE(None, DWORD, DWORD))
    def cbFileRefDone(self, port, user_data):
        print "File indexed."

    def open(self, filename):
        # ...
        self.hsdk.PlayM4_SetFileRefCallBack(
            c_long(self.port), self.cbFileRefDone, DWORD(0))

Additionally MethodDescriptor saves references to C functions to prevent them being garbage collected.