setting a socket to multicast

[Adam Langley]

On Mon, 25 Nov 2002 21:46:51 +1100, Brad Hards wrote:

> s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
> s.setsockopt(socket.SOL_IP, socket.IP_ADD_MEMBERSHIP, socket.inet_aton(addr))
> s.bind(("", port))

> However when I run it, it bombs out:
> Traceback (most recent call last):
>   File "udpserver.py", line 8, in ?
>     s.setsockopt(socket.SOL_IP, socket.IP_ADD_MEMBERSHIP, 
>                  socket.inet_aton(addr))
> socket.error: (22, 'Invalid argument')

>From the ip(7) manpage:
	IP_ADD_MEMBERSHIP: Join  a  multicast  group.  Argument  is  a  struct
              ip_mreqn structure.

I don't think the Python setsockopt function is smart enough to see the
IP_ADD_MEMBERSHIP option and build the struct. I suggest you look into
   http://www.python.org/doc/current/lib/module-struct.html
and build a struct ip_mreqn

> I'm completely mystified by this. The man page for setsockopt(2) doesn't
> even mention EINVAL (at least on my system)

It is documented in the ip(7) manpage.

> except that the system call has five arguments and the python call only
> has three

because the first argument (the socket) comes from the object and the last
argument's value (data length) is implicit from the data being passed.