docs: def _sig_handler(self, signum, stack):
          '''
          Handle process INT signal.
          '''
          log_debug("Got SIGINT.")
          if signum == signal.SIGINT:
              LLNetReal.running = False
              if self._pktqueue.qsize() == 0:
                  # put dummy pkt in queue to unblock a 
                  # possibly stuck user thread
                  self._pktqueue.put( (None,None,None) )