code_example.cc

// callback declaration
void (*callback)(int);

void print_func(int a)
{
    printf("%d\n", a);
}

int register_print_callback()
{
    callback = print_func;
 
    // ...
    return 0;
}
    
// .. down the line in the code..

int d = 3;

register_callback();

if (callback)
    callback(d);