Example: Receiving a multicast datagram

The following example enables a socket to perform the steps listed below and to receive multicast datagrams:

1. Create an AF_INET, SOCK_DGRAM type socket.
2. Set the SO_REUSEADDR option to allow multiple applications to receive datagrams that are destined to the same local port number.
3. Use the bind() verb to specify the local port number. Specify the IP address as INADDR_ANY in order to receive datagrams that are addressed to a multicast group.
4. Use the IP_ADD_MEMBERSHIP socket option to join the multicast group that receive the datagrams. When joining a group, specify the class D group address along with the IP address of a local interface. The system must call the IP_ADD_MEMBERSHIP socket option for each local interface receiving the multicast datagrams.
5. Receive the datagram.

#include 
#include 
#include 
#include 
#include 
#include 
 
 
 
struct sockaddr_in    localSock;
struct ip_mreq        group;
int                   sd;
int                   datalen;
char                  databuf[1024];
 
int main (int argc, char *argv[])
{
 
  /* ------------------------------------------------------------*/
  /*                                                             */
  /* Receive Multicast Datagram code example.                    */
  /*                                                             */
  /* ------------------------------------------------------------*/
 
  /*
   * Create a datagram socket on which to receive.
   */
  sd = socket(AF_INET, SOCK_DGRAM, 0);
  if (sd < 0) {
    perror("opening datagram socket");
    exit(1);
  }
 
  /*
   * Enable SO_REUSEADDR to allow multiple instances of this
   * application to receive copies of the multicast datagrams.
   */
  {
    int reuse=1;
 
    if (setsockopt(sd, SOL_SOCKET, SO_REUSEADDR,
                   (char *)&reuse, sizeof(reuse)) < 0) {
      perror("setting SO_REUSEADDR");
      close(sd);
      exit(1);
    }
  }
 
  /*
   * Bind to the proper port number with the IP address
   * specified as INADDR_ANY.
   */
  memset((char *) &localSock, 0, sizeof(localSock));
  localSock.sin_family = AF_INET;
  localSock.sin_port = htons(5555);;
  localSock.sin_addr.s_addr  = INADDR_ANY;
 
  if (bind(sd, (struct sockaddr*)&localSock, sizeof(localSock))) {
    perror("binding datagram socket");
    close(sd);
    exit(1);
  }
 
 
  /*
   * Join the multicast group 225.1.1.1 on the local 9.5.1.1
   * interface.  Note that this IP_ADD_MEMBERSHIP option must be
   * called for each local interface over which the multicast
   * datagrams are to be received.
   */
  group.imr_multiaddr.s_addr = inet_addr("225.1.1.1");
  group.imr_interface.s_addr = inet_addr("9.5.1.1");
  if (setsockopt(sd, IPPROTO_IP, IP_ADD_MEMBERSHIP,
                 (char *)&group, sizeof(group)) < 0) {
    perror("adding multicast group");
    close(sd);
    exit(1);
  }
 
  /*
   * Read from the socket.
   */
  datalen = sizeof(databuf);
  if (read(sd, databuf, datalen) < 0) {
    perror("reading datagram message");
    close(sd);
    exit(1);
  }
 
}