int time_sleep

void handler(int sig)
{
  alarm(time_sleep);
}

int main()
{
  int i;
  signal(SIGUSR1, handler);
  while(1)
  {
    i = sleep(time_sleep);
    if(i!=0)
      printf("Awakened %d seconds before\n", i);
    //do things;
  }
}