code_snippet.cc

static void (*event_callback)(void *user_priv, uint8_t *datap, size_t datap_len);
static void *user_priv;

int register_event_callback(void *user_ptr)
{
    event_callback = my_event_callback;
    user_priv = user_ptr;

    return 0;
}

// running for ever loop
void run()
{
    for ( ; ; ) {
        if (an_event) {
            uint8_t datap[100];
        
            datap_len = event_data_get(datap);
        
            // call the callback along with user's private data pointer
            if (event_callback)
                event_callback(user_priv, datap, datap_len);
        }
    }
}


// .. caller code..


// caller's private data
struct my_struct {
    int some_data;
};

void my_event_callback(void *user_priv, uint8_t *datap, size_t datap_len)
{
    struct my_struct *my = user_priv;
}

struct my_struct *my = calloc(1, sizeof(struct my_struct));
if (!my) {
    return -1;
}

// C takes care of type conversion automatically unlike the C++

register_event_callback(my);

// let the main loop run and this calls our event callback
run();