Subscribing to multiple multicast groups on one socket (Linux, C)

Question

Is it possible to receive data from more than one multicast group on a single socket?

For example:

void AddGroup(int sock,
              const char* mc_addr_str,
              int mc_port,
              const char* interface) {
  struct sockaddr_in mc_addr;
  memset(&mc_addr, 0, sizeof(mc_addr));
  mc_addr.sin_family      = AF_INET;
  mc_addr.sin_addr.s_addr = inet_addr(mc_addr_str);
  mc_addr.sin_port        = htons(mc_port);

  if ((bind(sock, (struct sockaddr *) &mc_addr,
            sizeof(mc_addr))) < 0) {
    perror("bind() failed");
    exit(1);
  }

  // construct an IGMP join request structure
  struct ip_mreq mc_req;
  mc_req.imr_multiaddr.s_addr = inet_addr(mc_addr_str);
  mc_req.imr_interface.s_addr = inet_addr(interface);

  if ((setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP,
                  (void*) &mc_req, sizeof(mc_req))) < 0) {
    perror("setsockopt() failed");
    exit(1);
  }
}

This code works when I add one multicast group. But when I try to add another, the "bind" fails. I don't quite understand why the bind needs to be there in the first place? (but the code doesn't work without it).

Ideally I would like to call AddGroup multiple times on the same socket. Is this possible? Or do I need one socket per group and then just use polling?

Answer 1

You can join as many multicast groups as you like, using the appropriate setsockopt() call with the IP_ADD_MEMBERSHIP option, rather than bind().

Answer 2

You only bind a socket once. Skip the bind the second time and see what happens.

Answer 3

You can join as many multicast groups you want to on a single socket. See setsockopt(), IP_PKTINFO for a way to recognize which multicast group you are reading data from.