IPV6_V6ONLY socket option example

    for (res = res0; res; res = res->ai_next) {
        if (res->ai_family != PF_INET &&
            res->ai_family != PF_INET6)
            continue;
        s = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
        if (s < 0)
            continue;       /* Application dependent */
 #ifdef IPV6_V6ONLY
        if (res->ai_family == PF_INET6) {
            int on = 1;
            if (setsockopt(s, IPPROTO_IPV6, IPV6_V6ONLY, &on, sizeof(on)) < 0) {
                close(s);
                continue;   /* Application dependent */
            }
        }
 #endif
        if (bind(s, res->ai_addr, res->ai_addrlen) < 0) {
            close(s);
            continue;       /* Application dependent */
        }
        push_socket(socklist, s);
    }