Как создайтся функция обратного вызова? - C (СИ)

#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
 
typedef int (*callback_t)(int, va_list);
 
void map_int_array(int *array, size_t n, callback_t f, ...)
{
    va_list extra;
    size_t i;
 
    va_start(extra, f);
 
    for(i = 0; i < n; ++i)
        array[i] = f(array[i], extra);
 
    va_end(extra);
}
 
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-parameter"
int identity(int arg, va_list extra)
{
    return arg;
}
 
int add(int arg, va_list extra)
{
    int addend = va_arg(extra, int);
    return arg + addend;
}
 
int add_two_numbers(int arg, va_list extra)
{
    int add1 = va_arg(extra, int);
    int add2 = va_arg(extra, int);
    return arg + add1 + add2;
}
 
int constantly(int arg, va_list extra)
{
    int value = va_arg(extra, int);
    return value;
}
 
int print_array(int arg, va_list extra)
{
    printf("%d\n", arg);
    return arg;
}
 
int find_max(int arg, va_list extra)
{
    int *max = va_arg(extra, int *);
    if(arg > *max)
        *max = arg;
    return arg;
}
 
#pragma GCC diagnostic pop
 
int main(void)
{
    int numbers[] = {5, 8, -2, 3, 6}, max;
    const size_t n = sizeof numbers / sizeof *numbers;
 
    puts("\nNumbers are");
    map_int_array(numbers, n, print_array);
 
    max = numbers[0];
    map_int_array(numbers + 1, n - 1, find_max, &max);
    printf("\nFind the max number: %d\n", max);
 
    puts("\n...then run the identity function:");
    map_int_array(numbers, n, identity);
    map_int_array(numbers, n, print_array);
 
    puts("\n...then add 3 to each number:");
    map_int_array(numbers, n, add, 3);
    map_int_array(numbers, n, print_array);
 
    puts("\n...then subtract 4 and add 2:");
    map_int_array(numbers, n, add_two_numbers, -4, 2);
    map_int_array(numbers, n, print_array);
 
    puts("\n...then replace each number with 42:");
    map_int_array(numbers, n, constantly, 42);
    map_int_array(numbers, n, print_array);
 
    exit(EXIT_SUCCESS);
}