how do I fix failing multicast receive?

Question

I am using Visual Studio 6.0 VC++ on Windows XP (YES I know its old technology) I have a device which communicates via UDP all documentation says multicast and I wrote an app years ago that talks to it and uses multicast. my send and receive code will be below. My send works fine, using Wireshark I can see the message go out properly formatted and see the device respond with a properly formatted message. I CANNOT receive it into my code. It just sets there and listens infinitely, never receives anything. Here is what Wireshark displays coming back from the device:

Source 192.168.200.41 source port is 6311 (the device) Destination 192.168.200.72 destination port is 6303 (my development PC)

    ////////////////////////////Sender //////////////////////////////////
    //
    //  create a send udp socket descriptor
    // 
        SOCKET sUDPsocket = socket(AF_INET, SOCK_DGRAM, 0);

    //
    //initialize address struct 
    //

    memset(&address_send, 0, sizeof(address_send));
    address_send.sin_family = AF_INET;
    address_send.sin_port = htons(localPort);
    address_send.sin_addr.s_addr = inet_addr("239.255.255.250")

//
//  send the contents of cBuffer
//

    nBytesSent = sendto(sUDPsocket, cBuffer, nBufSize, 0,(SOCKADDR *)                       &address_send,sizeof(SOCKADDR_IN));

/////////////////////////// receiver /////////////////////////////////////////
//
//  create a receive udp socket descriptor 
//
    SOCKET rUDPsocket = socket(AF_INET, SOCK_DGRAM, 0);

/
//  initialize bind address struct 
//
    memset(&address_recv, 0, sizeof(address_recv));
    address_recv.sin_family = AF_INET;
    address_recv.sin_port = htons(6311);
    address_recv.sin_addr.s_addr = htonl(INADDR_ANY);
//
//  bind to this address/port
//
        result = bind(rUDPsocket, (struct sockaddr*)&address_recv,
                 sizeof(address_recv));

//
//  initialize recvfrom address struct
//
    sockaddr sender_address;
    rAddrsize = sizeof(sockaddr);

//
//  receive data
//
    rBytesRecv = recvfrom(rUDPsocket, rBuffer, rBufSize, 0,
                   (SOCKADDR *) &sender_address,
                   &rAddrsize);

/////////////////////////////////////////////////////////////////////////////

Answer 1

Your receiver isn't set up to receive multicast traffic. You need to use the IP_ADD_MEMBERSHIP socket option to join a multicast group:

struct ip_mreq mreq;
mreq.imr_multiaddr = inet_addr("239.255.255.250");
mreq.imr_interface = inet_addr("192.168.200.72");
if (setsockopt(rUDPsocket, IPPROTO_IP, IP_ADD_MEMBERSHIP,
                           (char *)&mreq, sizeof(mreq)) == -1) {
    char errbuf[300];
    FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM, NULL, WSAGetLastError(),
                  0, errbuf, sizeof(errbuf), NULL);
    fprintf(stderr, "(%d) %s", WSAGetLastError(), errbuf);
}

Also, make sure you're listening on the correct port. If the multicast packets have a destination port of 6311 you're fine. If not, change it to whatever that port is.