#include    <errno.h>

int sockfd_to_family(int);

int mcast_get_if(int sockfd)

{

    switch (sockfd_to_family(sockfd)) {

        case AF_INET: {

        /* TODO: similar to mcast_set_if() */

            return(-);

        }

#ifdef    IPV6

        case AF_INET6: {

            u_int        idx;

            socklen_t    len;

            len = sizeof(idx);

            if (getsockopt(sockfd, IPPROTO_IPV6,

                IPV6_MULTICAST_IF, &idx, &len) < ) {

                return(-);

            }

            return(idx);

        }

#endif

        default: {

            errno = EAFNOSUPPORT;

            return (-);

        }

    }

}