problem with callback in python ( ctypes)

Question

Hi I have problem with CTYPES in python. I have ready dll library with some callback. On swift everything works, but I have some problem in python.

Python:

def set_up_callback(self):
    self.lib.set_callback(self.callback1)

@CFUNCTYPE(None, c_float, c_float, c_float, c_uint64)
def callback1( a, b, c, time):
    print(a, b, c, time)

c++ declaration of callback

typedef void(*callbackType)(float, float, float, uint64_t, void*);
        callbackType callback;

void* context;

c++ init

void setCallback(callbackType callback, void* context) {

        this->context = context;
        this->callback = callback;
}

c++ induction

callback(1.5f, 2.4f, 1.3f, timestamp, context);

shared.h

extern "C" void SHARED_EXPORT set_callback(callbackType callback, void* context);

and this works good but I would like have self In callback function so I try this

def set_up_callback(self):

    callback_type = CFUNCTYPE(None, c_float, c_float, c_float, c_uint64)

    callback = callback_type(self.callback1)

    self.lib.set_callback(callback)

def callback1(self, a, b, c, time):
    print(a, b, c, time)

and with this try I have error Segmentation fault: 11

Thank you in advance for help

Mark Tolonen · Accepted Answer · 2020-04-11 18:41:05Z

In set_up_callback, callback is a local variable that goes out-of-scope after calling self.lib.set_callback(callback). You must keep a reference to callback for the lifetime that it could be called, so store it as a member variable of the class instance.

Working demo:

demo.cpp

#include <time.h>
#include <stdint.h>

#if defined(_WIN32)
#   define API __declspec(dllexport)
#else
#   define API
#endif

typedef void(*CALLBACK)(float, float, float, uint64_t, void*);

CALLBACK g_callback;

extern "C" {

API void set_callback(CALLBACK callback) {
    g_callback = callback;
}

API void demo(void* context) {
    if(g_callback)
        g_callback(1.5f, 2.4f, 1.3f, time(nullptr), context);
}

}

demo.py

from ctypes import *
from datetime import datetime

CALLBACK = CFUNCTYPE(None,c_float,c_float,c_float,c_uint64,c_void_p)

class Demo:
    def __init__(self):
        self.lib = CDLL('./demo')
        self.lib.set_callback.argtypes = CALLBACK,
        self.lib.set_callback.restype = None
        self.lib.demo.argtypes = c_void_p,
        self.lib.demo.restype = None
        self.set_up_callback()

    def callback(self,a,b,c,timestamp,context):
        print(a,b,c,datetime.fromtimestamp(timestamp),self.context)

    def set_up_callback(self):
        self.callback = CALLBACK(self.callback)
        self.lib.set_callback(self.callback)

    def demo(self,context):
        self.context = context
        self.lib.demo(None)

demo = Demo()
demo.demo([1,2,3])
demo.demo(123.456)
demo.demo('a context')

Output:

1.5 2.4000000953674316 1.2999999523162842 2020-04-11 11:38:44 [1, 2, 3]
1.5 2.4000000953674316 1.2999999523162842 2020-04-11 11:38:44 123.456
1.5 2.4000000953674316 1.2999999523162842 2020-04-11 11:38:44 a context

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