Welcome KAME hack

link local addresses have lot of zeros

abuse the zeros to carry the scope ID
s6_addr[2] and s6_addr[3] carry the scope

using such IPs on other socket calls will fail horribly
routed to a black hole
userland is responsible the fiddle the scope id in and out

    struct sockaddr_in6     sa_in6;
    if (IN6_IS_ADDR_LINKLOCAL(&sa_in6.sin6_addr) ||
        IN6_IS_ADDR_MC_LINKLOCAL(&sa_in6.sin6_addr)) {
            memcpy(&tmp16, &sa_in6.sin6_addr.s6_addr[2], sizeof(tmp16));
            sa_in6.sin6_scope_id = ntohs(tmp16);
            sa_in6.sin6_addr.s6_addr[2] = 0;
            sa_in6.sin6_addr.s6_addr[3] = 0;
    }