int mcast_block_source(int sockfd, const SA *src, socklen_t srclen, const SA *grp, socklen_t grplen)
{
#ifdef MCAST_BLOCK_SOURCE
    struct group_source_req req;
    req.gsr_interface = 0;
    if (grplen > sizeof(req.gsr_group) || srclen > sizeof(req.gsr_source)) {
        errno = EINVAL;
        return -1;
    }
    memcpy(&req.gsr_group, grp, grplen);
    memcpy(&req.gsr_source, src, srclen);
    return (setsockopt(sockfd, family_to_level(grp->sa_family),
            MCAST_BLOCK_SOURCE, &req, sizeof(req)));
#else
    switch (grp->sa_family) {
#ifdef IP_BLOCK_SOURCE
    case AF_INET: {
        struct ip_mreq_source    mreq;

        memcpy(&mreq.imr_multiaddr,
               &((struct sockaddr_in *) grp)->sin_addr,
               sizeof(struct in_addr));
        memcpy(&mreq.imr_sourceaddr,
               &((struct sockaddr_in *) src)->sin_addr,
               sizeof(struct in_addr));
        mreq.imr_interface.s_addr = htonl(INADDR_ANY);

        return(setsockopt(sockfd, IPPROTO_IP, IP_BLOCK_SOURCE,
                          &mreq, sizeof(mreq)));
    }
#endif

#ifdef    IPV6
    case AF_INET6: /* IPv6 source-specific API is MCAST_BLOCK_SOURCE */
#endif
    default:
        errno = EAFNOSUPPORT;
        return(-1);
    }
#endif
}