RTC initialisation in an MCU - why use a global callback

Question

The code below is related to the initialization of an RTC in an MCU.

Would anybody know the rational for passing NULL to rtc_init() and then setting a global callback global_rtc_cb equal to it.

Why would you use a global callback at all when there is an other function called rtc_callback defined and used as the callback in the struct.

int main() {
rtc_init(NULL);   
} 

//-----------------------------------------------------------------
void ( * global_rtc_cb)(void *);  

int rtc_init(void (*cb)(void *)) {
rtc_config_t cfg;

  cfg.init_val = 0;
  cfg.alarm_en = true;
  cfg.alarm_val = ALARM;
  cfg.callback = rtc_callback;
  cfg.callback_data = NULL;

  global_rtc_cb = cb;

  irq_request(IRQ_RTC_0, rtc_isr_0);
  clk_periph_enable(CLK_PERIPH_RTC_REGISTER | CLK_PERIPH_CLK);
  rtc_set_config(QM_RTC_0, &cfg);

  return 0;
}

//---------------------------------------------------------------------
/**
 * 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

The rtc_ functions are part of the RTC driver. The RTC driver has something driver-specific to do when the event that prompts the callback occurs. This driver-specific stuff happens in rtc_callback. But there may also be other application-specific stuff that the application must do when the event occurs. The application-specific stuff should be done at the application layer, not within the driver. So if the application has something to do in response to the event it can provide a callback to rtc_init. Surely rtc_callback calls global_rtc_cb so that both the driver-specific stuff and the application-specific stuff is performed when the event occurs. Apparently your particular application doesn't need to do anything for this event so it passes NULL to rtc_init. But a different application that uses the same driver may provide a callback function.