Passing a callback function from Python to C

To communicate between python and C, we have a library `ctypes`

Step — 1 : create a c program, in which one function receives another function pointer as parameter.

In the below example, the function divide receives another function pointer as first parameter, which receives 2 parameters, quotient and remainder.

save the program as `division.c`

#include <stdio.h>
void divide(void (*ptr)(int *, int *), int a, int b){
  int s = a / b;
  int r = a % b;
  (*ptr) (&s, &r);
}
void print_sum(int * s, int * r){
 printf("Quotient is %d, remainder is %d\n", *s, *r);
}
int main(){
  void (*ptr)() = &print_sum;
  divide(ptr, 7, 4);
}

To run the above program

$gcc division.c

$./a.out

Now create a shared library

$gcc -fPIC -shared -o libdiv.so division.c

Step — 2 : Create a python program

from ctypes import CFUNCTYPE
from ctypes import c_void_p
from ctypes import POINTER
from ctypes import c_int
from ctypes import cdll
lib = cdll.LoadLibrary('./libdiv.so')

CMPFUNC = CFUNCTYPE(c_void_p, POINTER(c_int), POINTER(c_int))
def py_cmp_func(s, r):
  print (f'Quotient is {s[0]} , remainder is {r[0]}')
cmp_func = CMPFUNC(py_cmp_func)
lib.divide(cmp_func, 3, 5)

save the program as test.py in the same directory

Now run it

$python3 test.py

You will get the following output

Quotient is 0 , remainder is 3