我有一个发送多播数据报的代码。一个关键的代码：

uint32_t port;
int sockfd, err_ip;
const uint32_t sizebuff = 65535 - (20 + 8);
unsigned char *buff = (unsigned char *) malloc(sizebuff);
struct sockaddr_in servaddr, cliaddr;
struct in_addr serv_in_addr;
struct ip_mreq req;

port = str2uint16(cmdsrv->ipport);
bzero(buff, (size_t)sizebuff);
bzero(&servaddr, sizeof(servaddr));
bzero(&serv_in_addr, sizeof(serv_in_addr));
err_ip = inet_aton(cmdsrv->ipaddr, &serv_in_addr);

if(( err_ip != 0 ) && ( port != 0 )) {
   servaddr.sin_family = AF_INET;
   servaddr.sin_addr = serv_in_addr;
   servaddr.sin_port = htons(port);
   memcpy(&req.imr_multiaddr,&serv_in_addr,sizeof(req.imr_multiaddr));
   req.imr_interface.s_addr = INADDR_ANY;
   sockfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);

   if( sockfd == -1 ) {
      int outerror = errno;
      char *retstr = "Couldn't open socket\n";
      pthread_exit(retstr);
   }
   else {
      struct in_addr ifaddr;
      ifaddr.s_addr = INADDR_ANY;
      int optres3 =
         setsockopt( sockfd, IPPROTO_IP, IP_MULTICAST_IF, &ifaddr,
                     sizeof( ifaddr ));
      if( optres3 == -1 ) {
         int perrno = errno;
         char *retstr = "Can't set IP_MULTICAST_IF for socket\n";
         printf( "Error setsockopt: ERRNO = %s\n", strerror( perrno ));
         printf( "%s",retstr );
         pthread_exit(retstr);
      }

      unsigned char ttl = 32;
      int optres2 =
         setsockopt( sockfd, IPPROTO_IP, IP_MULTICAST_TTL, &ttl,
                     sizeof( ttl ));
      if( optres2 == -1 ) {
         int perrno = errno;
         char *retstr = "Can't set IP_MULTICAST_TTL for socket\n";
         printf("Error setsockopt: ERRNO = %s\n",strerror(perrno));
         printf("%s",retstr);
         pthread_exit(retstr);
      }

      int optres =
         setsockopt( sockfd, IPPROTO_IP, IP_ADD_MEMBERSHIP, &req,
                     sizeof( req ));
      if( optres == -1 ) {
         int perrno = errno;
         char *retstr = "Can't join to multicast-group\n";
         printf("Error setsockopt: ERRNO = %s\n",strerror(perrno));
         printf("%s",retstr);
         pthread_exit(retstr);
      }

      // Bind port with socket
      uint16_t cliport;
      cliaddr.sin_family = AF_INET;
      cliaddr.sin_addr.s_addr = INADDR_ANY;

      if( strcmp( cmdsrv->ipport, "16011" ) == 0 ) {
         cliport = str2uint16("16003");
         cliaddr.sin_port = htons(cliport);
      }
      else if( strcmp( cmdsrv->ipport, "16012" ) == 0 ) {
         cliport = str2uint16("16004");
         cliaddr.sin_port = htons(cliport);                     
      }
      else {
         printf("Device hasn't such port");
         pthread_exit(NULL);
      }

      int bindres =
         bind( sockfd, (struct sockaddr*)&cliaddr, sizeof( cliaddr ));
      if( bindres == -1 ) {
         int perrno = errno;
         perror("Error in bind\n");
      }
      // ADD 1 BYTE
      data rawdata;
      rawdata.desc = 23;
      printf( "SIZEOF = %d\n", sizeof( *( cmdsrv->cmd )));
      memcpy( &rawdata.cmd, cmdsrv->cmd, sizeof( *( cmdsrv->cmd )));
      printf( "RAWDATA: desc = %d, cmd = %d\n", rawdata.desc, rawdata.cmd );

      int outerror = 0;
      printf( "Send command to IP:\n addr = %s, port = %d\n",
         inet_ntoa( servaddr.sin_addr ), ntohs( servaddr.sin_port ));
      int size = sendto( sockfd, &rawdata, sizeof( rawdata ), 0,
         (struct sockaddr*)&servaddr, sizeof( servaddr ));
      if( size == -1 ) {
         perror("Can't send command to socket");
      }
      ...

有时程序执行成功（此时我有IP -192.168.80.122）。我可以通过wireshark捕获我的多播数据报。
但如果我将IP更改为192.168.1.2，则调用时会出现错误

int optres =
   setsockopt( sockfd, IPPROTO_IP, IP_ADD_MEMBERSHIP, &req,
               sizeof( req ));

我甚至不能捕获我的多播数据包。什么也没发送。bug在哪里？