This function is called during sendrecv() routine to select
    the available sockets.

def _select_nonblock(sockets, remain=None):
    """This function is called during sendrecv() routine to select
    the available sockets.
    """
    # pcap sockets aren't selectable, so we return all of them
    # and ask the selecting functions to use nonblock_recv instead of recv
    def _sleep_nonblock_recv(self):
        try:
            res = self.nonblock_recv()
            if res is None:
                time.sleep(conf.recv_poll_rate)
            return res
        except TimeoutElapsed:
            return None
    return sockets, _sleep_nonblock_recv