How to receive UDP Multicast in VxWorks 5.5

Question

I have been unable to receive UDP multicast under VxWorks 5.5. I've joined the multicast group:

setsockopt(soc, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char *) &ipMreq, sizeof (ipMreq));

Similar code on an adjacent Windows machine does receive multicast. I am able to send multicast from VxWorks; ifShow() indicates the interface is multicast capable; MCAST_ROUTING is enabled in the kernel config, but still unable to receive multicast.

Edit: I needed to set a bit in the RealTek Ethernet drive RX configuration register to enable multicast to be passed on to the application layer.

#define RTL_RXCG_AM           0x04      /* Accept Multicast */

Answer 1

Are you checking the return value on the Join setsockopt() call to be sure it's actually succeeding? I had a specific problem with VxWorks 5.5 in the past where my multicast joins were failing when they shouldn't be. I believe we had to get new libraries from WindRiver to fix the issue.

Edit: There is no specific trick that I'm aware of to getting multicast to work with VxWorks. It should use the standard BSD sockets operations. If the interface can receive unicast traffic properly, and a network analyzer (Wireshark, for instance) shows that the multicast JOINs are being sent and the inbound multicast packets are correctly formed, I would suspect a driver issue. WindRiver support has been very helpful for us in the past with these sorts of problems; I don't know if you have a support contract with them to get that level of assistance.