mcast_set_if函数

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

int sockfd_to_family(int);

int mcast_set_if(int sockfd, const char *ifname, u_int ifindex)
{
    switch (sockfd_to_family(sockfd)) {
        case AF_INET: {
            struct in_addr    inaddr;
            struct ifreq    ifreq;

            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(&inaddr,
                   &((struct sockaddr_in *) &ifreq.ifr_addr)->sin_addr,
                   sizeof(struct in_addr));
            } else {
                /* remove prev. set default */
                inaddr.s_addr = htonl(INADDR_ANY);    
            }

            return(setsockopt(sockfd, IPPROTO_IP, 
                   IP_MULTICAST_IF, &inaddr, sizeof(struct in_addr)));
        }

#ifdef    IPV6
        case AF_INET6: {
            u_int    idx;

            if ( (idx = ifindex) == 0) {
                if (ifname == NULL) {
                    errno = EINVAL;    /* must supply either index or name */
                    return(-1);
                }
                if ( (idx = if_nametoindex(ifname)) == 0) {
                    errno = ENXIO;    /* i/f name not found */
                    return(-1);
                }
            }
            return(setsockopt(sockfd, IPPROTO_IPV6, 
                   IPV6_MULTICAST_IF, &idx, sizeof(idx)));
        }
#endif
        default: {
            errno = EAFNOSUPPORT;
            return(-1);
        }
    }
}