Function pointers and callbacks in C

Question

I have started to review callbacks. I found this link on SO: What is a "callback" in C and how are they implemented? It has a good example of callback which is very similar to what we use at work. However, I have tried to get it to work, but I have many errors.

#include <stdio.h>

/* Is the actual function pointer? */
typedef void (*event_cb_t)(const struct event *evt, void *user_data);

struct event_cb
{
    event_cb_t cb;
    void *data;
};

int event_cb_register(event_ct_t cb, void *user_data);

static void my_event_cb(const struct event *evt, void *data)
{
    /* do some stuff */
}

int main(void)
{
    event_cb_register(my_event_cb, &my_custom_data);

    struct event_cb *callback;

    callback->cb(event, callback->data);

    return 0;
}

I know that callbacks use function pointers to store an address of a function. But there are a few things that I find I don't understand:

• What is meant by "registering the callback" and "event dispatcher"?

Answer 1

What is meant by "registering the callback" and "event dispatcher"?

"registering the callback" is the act of telling the underlying system which precise function to call, and (optionally) with which parameters, and possibly also for which particular class of events that callback should be invoked.

The "event dispatcher" receives events from the O/S (or GUI, etc), and actually invokes the callbacks, by looking in the list of registered callbacks to see which are interested in that event.