typedef struct {
  volatile unsigned int lock;
} spinlock_t;

void spin_lock(spinlock_t *lock) {
  /* assembly code */
}

void spin_unlock(spinlock_t *lock) {
  /* assembly code */
}

spinlock_t rtc_lock;

int main(void)
{
  rtc_lock;
  spin_lock(&rtc_lock);
  rtc_lock;
  spin_unlock(&rtc_lock);
  rtc_lock;
}