'message receive routine for UDP link'

def recv_msg(self):

    self.pre_message()
    s = self.recv()
    if (len(s) > 0):
        if self.first_byte:
            self.auto_mavlink_version(s)
    m = self.mav.parse_char(s)
    if (m is not None):
        self.post_message(m)
    return m
