some description

sender.c

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

struct in_addr localInterface;
struct sockaddr_in groupSock;
int sd;
char buf[2048];

int main (int argc, char *argv[])
{
  fgets(buf, sizeof(buf), stdin);

  sd = socket(AF_INET, SOCK_DGRAM, 0);
  if(sd < 0) {
    exit(1);
  }

  memset((char *) &groupSock, 0, sizeof(groupSock));
  groupSock.sin_family = AF_INET;
  groupSock.sin_addr.s_addr = inet_addr("224.0.0.1");
  groupSock.sin_port = htons(atoi(argv[1]));
  localInterface.s_addr = inet_addr("127.0.0.1");

  if(setsockopt(sd, IPPROTO_IP, IP_MULTICAST_IF, (char *)&localInterface, sizeof(localInterface)) < 0) {
    exit(1);
  }

  if(sendto(sd, buf, sizeof(buf), 0, (struct sockaddr*)&groupSock, sizeof(groupSock)) < 0) {
    exit(1);
  }

  return 0;
}