66. Pointer-iv

A timer driver allows users to register a callback function that fires on every tick:

typedef void (*TimerCallback)(void);

TimerCallback on_tick = NULL;

void timer_register_callback(___) {
   on_tick = cb;
}

void timer_isr() {
   if (on_tick != NULL)
       on_tick();
}

What is the correct parameter type for timer_register_callback?