Unable to stop python callbacks from a python C extension

Question

I'm trying to wrap a C++ camera frame acquisition library into Python. I'm having problems with the callback function. I call a python function from a C function with the following code:

PyGILState_STATE gstate;
gstate = PyGILState_Ensure();
result = PyObject_CallObject(my_callback, arglist);
if(result == NULL){
    caller.stopLive();
}
Py_DECREF(result);
Py_DECREF(arglist);
PyGILState_Release(gstate);

The callback works fine. The problem arises when I try to stop the callback with the camera library functions. If the python callback function is short the camera library function is able to stop the callbacks, but if the python callback function takes longer the camera library function is unable to stop the callbacks and the program freezes.

Has anybody had similar problems? Do you have any suggestions about what to try?

Edit: The code for stopping the callbacks is:

static PyObject * stopcallback(PyObject *self, PyObject *args)
{
    if(grabber->isListenerRegistered(pListener)){
        grabber->removeListener(pListener);
        while(grabber->isListenerRegistered(pListener)){
            Sleep(0);
        }
        delete pListener;
        printf("Callback stopped\n");
        Py_RETURN_NONE;
    }
}

I call this function from Python.

It isn't clear what you mean by "stop the callbacks"? Can you share the code you used to attempt this? Note that you have the GIL locked here (as you should). Does "stopping the callback" involve running Python code? If so, it will be blocked by the GIL. — Ned Batchelder
– Ned Batchelder, Commented Jul 17, 2012 at 13:54

MRAB · Accepted Answer · 2012-07-17 19:50:54Z

2

You say "If the python callback function is short the camera library function is able to stop the callbacks".

I wonder whether the library is unable to remove the callback while it is running (or pending?).

If the callback is short, there are periods of time when it's not being run (or is not pending), so the library is able to remove it, but if the callback is long, it's being run (or is pending) virtually all the time, so the library has no opportunity to remove it.

answered Jul 17, 2012 at 19:50

MRAB

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3 Comments

katrasnikj Over a year ago

I think you are correct. The question would then be, how to allow the library to remove the callback. I guess the GIL lock would have to be broken somehow from the C code? Is there a way to do this?

MRAB Over a year ago

Could you modify the callback function so that it can be short-circuited? For example, starting it with if stopping: return and doing stopping = True when you're about to remove the callback.

katrasnikj Over a year ago

I guess I could do it this way, although it seems a bit of a hack. I'll let you know how it goes.

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