#include <stdio.h>

void worker(int arg, void (*function)(int fnarg)) {
    if (arg < 5) return;
    
    (*function)(arg);
}

void caller(int first_val) {
    int last_val = first_val;
    
    void difference(int eearg) {
        printf("difference between now and last call: %d\n", 
         eearg - last_val);
        last_val = eearg;
    }
    
    int i;
    for (i = first_val + 1; i < 10; i++) {
        worker(i, difference);
    }
}

int main(void) {
    caller(1);
    caller(3);
}