mcast_join函数

int mcast_join(int sockfd, const SA *grp, socklen_t grplen, const char *ifname, u_int ifindex)
{
#ifdef MCAST_JOIN_GROUP
    struct group_req req;
    if (ifindex > 0) {
        req.gr_interface = ifindex;
    } else if (ifname != NULL) {
        if ( (req.gr_interface = if_nametoindex(ifname)) == 0) {
            errno = ENXIO;    /* i/f name not found */
            return(-1);
        }
    } else
        req.gr_interface = 0;
    if (grplen > sizeof(req.gr_group)) {
        errno = EINVAL;
        return -1;
    }
    memcpy(&req.gr_group, grp, grplen);
    return (setsockopt(sockfd, family_to_level(grp->sa_family),
            MCAST_JOIN_GROUP, &req, sizeof(req)));
#else
    switch (grp->sa_family) {
    case AF_INET: {
        struct ip_mreq        mreq;
        struct ifreq        ifreq;

        memcpy(&mreq.imr_multiaddr,
               &((const struct sockaddr_in *) grp)->sin_addr,
               sizeof(struct in_addr));

        if (ifindex > 0) {
            if (if_indextoname(ifindex, ifreq.ifr_name) == NULL) {
                errno = ENXIO;    /* i/f index not found */
                return(-1);
            }
            goto doioctl;
        } else if (ifname != NULL) {
            strncpy(ifreq.ifr_name, ifname, IFNAMSIZ);
doioctl:
            if (ioctl(sockfd, SIOCGIFADDR, &ifreq) < 0)
                return(-1);
            memcpy(&mreq.imr_interface,
                   &((struct sockaddr_in *) &ifreq.ifr_addr)->sin_addr,
                   sizeof(struct in_addr));
        } else
            mreq.imr_interface.s_addr = htonl(INADDR_ANY);

        return(setsockopt(sockfd, IPPROTO_IP, IP_ADD_MEMBERSHIP,
                          &mreq, sizeof(mreq)));
    }
#ifdef    IPV6
#ifndef    IPV6_JOIN_GROUP        /* APIv0 compatibility */
#define    IPV6_JOIN_GROUP        IPV6_ADD_MEMBERSHIP
#endif
    case AF_INET6: {
        struct ipv6_mreq    mreq6;

        memcpy(&mreq6.ipv6mr_multiaddr,
               &((const struct sockaddr_in6 *) grp)->sin6_addr,
               sizeof(struct in6_addr));

        if (ifindex > 0) {
            mreq6.ipv6mr_interface = ifindex;
        } else if (ifname != NULL) {
            if ( (mreq6.ipv6mr_interface = if_nametoindex(ifname)) == 0) {
                errno = ENXIO;    /* i/f name not found */
                return(-1);
            }
        } else
            mreq6.ipv6mr_interface = 0;

        return(setsockopt(sockfd, IPPROTO_IPV6, IPV6_JOIN_GROUP,
                          &mreq6, sizeof(mreq6)));
    }
#endif

    default:
        errno = EAFNOSUPPORT;
        return(-1);
    }
#endif
}