mcast_get_ttl函数

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

int sockfd_to_family(int);

int mcast_get_ttl(int sockfd)
{
    switch (sockfd_to_family(sockfd)) {
        case AF_INET: {
            u_char        ttl;
            socklen_t    len;

            len = sizeof(ttl);
            if (getsockopt(sockfd, IPPROTO_IP, 
                IP_MULTICAST_TTL, &ttl, &len) < 0) {
                return(-1);
            }
            return (ttl);
        }

#ifdef    IPV6
        case AF_INET6: {
            int            hop;
            socklen_t    len;

            len = sizeof(hop);
            if (getsockopt(sockfd, IPPROTO_IPV6, 
                IPV6_MULTICAST_HOPS, &hop, &len) < 0) {
                return(-1);
            }
            return (hop);
        }
#endif
        default: {
            errno = EAFNOSUPPORT;
            return(-1);
        }
    }
}