Example: Receive multicast datagrams

#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>



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);
  }

}