udp_multicast_bidi.py

# socket_multicast_sender.py
# https://pymotw.com/3/socket/multicast.html
#
#  This was working
#
# in one terminal
#
# sudo tcpdump -A -i lo port 20005
#  
# in a second terminal
#
# udp_broadcast_initialize(ip(192.168.254.255), ip(255,255,255,0)).
#
# listen(architrave(X), (writeln(X),X=42)).
#
# findall(X, broadcast_request(udp_subnet(architrave(X))), Xs).
#
# notice we get a single 42
#
# sudo tcpdump -A -i lo port 20005
#
# in a third terminal on same machine
#
#  python udp_multicast_bidi.py  
#
#  note that it prints an unbound variable, this is from python
#
#  It will hang for 30 seconds, use editline and repeat the findall above
#  before it times out. You wil see it report it 


import socket
import struct
import sys

message = b'\'$udp_request\'(architrave(X))'
# the IP addy must match exactly what is used in prolog udp_broadcast_initialize
# eg one can't be 192.168.254.47 and the other 127.0.0.1
multicast_group = ('192.168.254.47', 20005)

# Create the datagram socket
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

# Set a timeout so the socket does not block
# indefinitely when trying to receive data.
sock.settimeout(30.4)  # was 0.2

# Set the time-to-live for messages to 1 so they do not
# go past the local network segment.
ttl = struct.pack('b', 1)
sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, ttl)

try:

    # Send data to the multicast group
    # this gets picked up by the listen above
    print('sending {!r}'.format(message))
    sent = sock.sendto(message, multicast_group)

    # Look for responses from all recipients
    while True:
        print('waiting to receive')
        try:
            (data, server) = sock.recvfrom(4096)  # 16 is buffer size
        except socket.timeout:
            print('timed out, no more responses')
            break
        else:
            print('received {!r} from {}'.format(
                data, server))

finally:
    print('closing socket')
    sock.close()