Callbacks with ctypes (How to call a python function from C)

Question

Is it possible to call a Python function from a C dll function?

We consider this C function:

 void foo( void (*functionPtr)(int,int) , int a, int b);

On Python, I would like to call foo and set the callback to a Python function:

def callback(a, b):
    print("foo has finished its job (%d, %d)" % (a.value,b.value))

dll.foo( callback, c_int(a), c_int(b) )

Unfortunately, the ctypes documentation is pretty light on this topic and the above code does not work.

Answer 1

import ctypes as c

@c.CFUNCTYPE(None, c.c_int, c.c_int)
def callback(a, b):
    print("foo has finished its job (%d, %d)" % (a.value, b.value))

dll.foo(callback, a, b) # assuming a,b are ints

If you need stdcall calling conventions, use WINFUNCTYPE instead.

Note: if foo may store the callback to be called at a later time then make sure that Python callback is alive (it is enough if it is defined at the global level using the decorator as shown in the example -- modules are essentially immortal in Python unless you try to remove them explicitly).

Answer 2

Use CFUNCTYPE to create a callback type:

c_callback = CFUNCTYPE(None, c_int, c_int)(callback)
dll.foo(c_callback, a, b)