Python Ctypes weird behavior

Question

I am trying to design a bot for an application called Virtual Paradise and the SDK that is given for building the bot is compiled into a shared library, therefore I have to use ctypes.

when I use

import threading
...
from ctypes import CDLL, CFUNCTYPE, c_char_p, c_int, c_void_p
vp = CDLL("libvpsdk.so")
vp.vp_string.restype = c_char_p
vp.vp_int.restype = c_int
...
class bot(threading.Thread):
    def initBot(self):
        ...
        instance = vp.vp_create()
        ...
        EventFunc = CFUNCTYPE(None)
        event_chat_func = EventFunc(self.event_chat)
        vp.vp_event_set(instance, 0, event_chat_func)
        ...
    def event_chat(self):
        print "Hello"
        ...

event_chat gets called correctly and prints "Hello"

but when I use this

import threading
import chat
...
from ctypes import CDLL, CFUNCTYPE, c_char_p, c_int, c_void_p
vp = CDLL("libvpsdk.so")
vp.vp_string.restype = c_char_p
vp.vp_int.restype = c_int
...
class bot(threading.Thread):
    def initBot(self):
        ...
        instance = vp.vp_create()
        ...
        chat.VPSDK(vp, instance)
        ...

Chat.py:

from ctypes import CFUNCTYPE
...
class VPSDK:
    def __init__(self, vp, instance):
        EventFunc = CFUNCTYPE(None)
        event_chat_func = EventFunc(self.event_chat)
        vp.vp_event_set(instance, 0, event_chat_func)

    def event_chat(self):
        print "Hello"
        ...

I get the error "Illegal instruction"

What am I doing wrong!? I need to use this separate class, otherwise other parts of my bot will loose functionality.

Answer 1

You must maintain a reference to the wrapped function for the lifetime it may be called. See the Important note... at the end of 15.16.1.17. Callback functions in the Python ctypes documentation.

One way is to use self.event_chat_func instead, storing it for the lifetime of the containing object.

Also, creating chat.VPSDK(vp, instance) creates an instance of chat.VPSDK that goes out of scope in the next line. You don't demonstrate how bot is instantiated in the first example, but the VPSDK object doesn't live very long.