Using setsockopt to join a multicast group for a udp
socket fails on windows (works on linux).

Test reproduction:
run the attached file

expected behavior:
The snd thread sends three udp packets to the multicast
address.  The rcv thread reads three udp packets.

observed behavior on linux:
the expected behavior

observed behavior on windows:
Exception in thread rcv:
Traceback (most recent call last):
  File "C:\Python24\lib\threading.py", line 442, in
__bootstrap
    self.run()
  File "C:\Python24\lib\threading.py", line 422, in run
    self.__target(*self.__args, **self.__kwargs)
  File
"E:\svn\TTC\trunk\agrover\src\SmcTestCase\multicastbug.py",
line 12, in rcv
    s.setsockopt(SOL_IP, IP_ADD_MEMBERSHIP, sopt)
  File "<string>", line 1, in setsockopt
error: (10022, 'Invalid argument')

Logged In: YES 
user_id=1098341

More details of config:

Windows used is XP Pro SP2
Linuxen used include Fedora Core and Mandrake

I can't reproduce this on Windows Vista can someone please confirm my findings.

I tested it on a Windows XP box, and encountered the same problem.
The error is raised because Windows XP requires the socket to be bound before calling setsockopt(IPPROTO_IP, IP_ADD_MEMBERSHIP, mreq).
So calling bind() before setsockopt() solves this error.
But then you also need to specify for the sender the interface to use using setsockopt(IPPROTO_IP, IP_MULTICAST_IF, address)

Here's a working example:

---- sender ----
from socket import *

s = socket(AF_INET, SOCK_DGRAM)
s.setsockopt(IPPROTO_IP, IP_MULTICAST_IF, inet_aton('127.0.0.1'))
s.sendto(b'foo', ('224.0.0.1', 4242))
----------------

--- receiver ---
from socket import *

s = socket(AF_INET, SOCK_DGRAM)
s.bind(('127.0.0.1', 4242))
mreq = inet_aton('224.0.0.1') + inet_aton('127.0.0.1')
s.setsockopt(IPPROTO_IP, IP_ADD_MEMBERSHIP, mreq)
s.recv(100)
----------------

So it's not a Python bug.
Since multicast is tricky, it might be a good idea to add a short example somewhere in the documentation though.