RTC on an mcu - function pointers and callbacks - c

Question

The code below is an example of how to use the real time clock on an mcu.
My question is in relation to callbacks and function pointers.
I have included the struct declaration for rtc_config_t below.
My question is, on the line cfg.callback = rtc_example_callback
Why is the & sign not used before rtc_example_callback.
Why is not necessary to pass the arguments to rtc_example_callback?
The last struct memeber void *callback_data; is set to NULL, I don't understand what this does?
When or what would you want to return?
Many tanks for your inputs
#include "rtc.h"
#include "interrupt.h"
#include "isr.h"
#define ALARM (QM_RTC_ALARM_MINUTE / 6)
#define MAX_RTC_FIRINGS (5)
void rtc_example_callback(void *);
static volatile uint32_t rtc_fired = 0;
/* RTC app example */
int main(void)
{
/* Variables */
rtc_config_t cfg; //create a struct variable to configure the RTC
PRINTF("Starting: RTC\n");
/* Initialise RTC configuration */
cfg.init_val = 0;
cfg.alarm_en = true;
cfg.alarm_val = ALARM;
cfg.callback = rtc_example_callback;
cfg.callback_data = NULL;
irq_request(IRQ_RTC_0, rtc_isr_0); //submit the RTC to the interrupt service routine
clk_periph_enable(CLK_PERIPH_RTC_REGISTER | CLK_PERIPH_CLK); //switch on RTC and Periphal clock
rtc_set_config(RTC_0, &cfg); //Set the RTC configuration
/* Wait for RTC to fire 5 times and then finish. */
while (rtc_fired < MAX_RTC_FIRINGS) {
}
PRINTF("Finished: RTC\n");
clk_periph_disable(CLK_PERIPH_RTC_REGISTER | CLK_PERIPH_CLK); //turn off the clocks
return 0;
}
void rtc_example_callback(void *data)
{
PUTS("Alarm!!\n");
qm_rtc_set_alarm(RTC_0, (RTC[RTC_0].rtc_ccvr + ALARM));
rtc_fired++;
}
-----------------------------------------------------------------------
/**
* RTC configuration type.
*/
typedef struct {
uint32_t init_val; /**< Initial value in RTC clocks. */
bool alarm_en; /**< Alarm enable. */
uint32_t alarm_val; /**< Alarm value in RTC clocks. */
/**
* User callback.
*
* #param[in] data User defined data.
*/
void (*callback)(void *data);
void *callback_data; /**< Callback user data. */
} rtc_config_t;

Answer 1

name of the function is a pointer to the function
function will be called with arguments from rtc library, you are not invoking it (you cannot pass arguments here).
I guess that NULL assigned to custom_callback will not call custom method from library (default function or no function will be called), just assign NULL if you dont want to use custom callback.
usually library code looks like:
if(custom_callback)
{
custom_callback(some_parameters);
}
else
{
default_callback(some_parameters);
}