#include    <errno.h>
#include    <sys/socket.h>
#include    <net/if.h>
#include    <netinet/in.h>

int sockfd_to_family(int);

int mcast_set_loop(int sockfd, int onoff)
{
    switch (sockfd_to_family(sockfd)) {
        case AF_INET: {
            u_char    flag;

            flag = onoff;
            return(setsockopt(sockfd, IPPROTO_IP, 
                   IP_MULTICAST_LOOP, &flag, sizeof(flag)));
        }

#ifdef    IPV6
        case AF_INET6: {
            u_int    flag;

            flag = onoff;
            return(setsockopt(sockfd, IPPROTO_IPV6, 
                   IPV6_MULTICAST_LOOP, &flag, sizeof(flag)));
        }
#endif
        default: {
            errno = EAFNOSUPPORT;
            return(-1);
        }
    }
}