2 program get same udp packets from a port

You need to use multicast if you want to do this with a single send / sendto on the server process. Here are quick examples done in Python 2.7.x for the sake of brevity / reuse of code I had laying around.

It's import for the transmit side to set IP_MULTICAST_LOOP if you are going to use this method with transmitter & receivers running on the same host.

sender.py:

#!/usr/bin/env python
import socket
import sys

MCAST_GROUP=sys.argv[1]
MCAST_PORT=int(sys.argv[2])

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.setsockopt( socket.SOL_SOCKET, socket.IP_MULTICAST_LOOP, 1 )

for ii in xrange(10):
    msg = 'message %d' %ii
    print 'sending: "%s"'  %msg
    s.sendto( msg, (MCAST_GROUP, MCAST_PORT) 

receiver.py:

#!/usr/bin/env python 
import socket
import sys
import struct

MCAST_GROUP=sys.argv[1]
MCAST_PORT=int(sys.argv[2])

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.setsockopt( socket.SOL_SOCKET, socket.SO_REUSEADDR, 1 )
s.bind( (MCAST_GROUP, MCAST_PORT) )

# In C, you'll want to use struct ip_mreq here.  See 'man 7 ip' for details.
# Python's socket module doesn't define a convenient way to do this, hence the
# 'manual' struct.pack
mreq = struct.pack( '4sI', socket.inet_aton(MCAST_GROUP), socket.INADDR_ANY )
s.setsockopt( socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, mreq )

while True:
    rx_data = s.recv(1000)
    print 'received: "%s"' %rx_data

Both programs expect two command line arguments, an IPv4 multicast IP (224.0.0.0 - 239.255.255.255), and a port. For example (./sender.py 239.10.10.10 5000).

You should be able to run as many instances of receiver.py as you like in different terminals, and see that a single instance of sender.py will transmit to all receivers.

To translate this to C, it's basically:

1. Convert s = socket.socket(...) -> s = socket(...)
2. Convert s.X(...) to X(s, ...) for X={setsockopt, bind, send, recv}
3. See notes about ip_mreq.