查找域名、由名字查找某个熟知的端口、由名字查找协议

struct servent{
    char   *s_name;     /* official service name */
    char **s_aliases;   /* other aliases */
    char     s_port;       /* prot ofr this service */
    char   *s_proto;     /* protocol to use */
};

struct servent *sptr;
sptr = getservbyname("smtp","tcp"));

struct protoent{
    char    *pname;
    char  **p_aliases;
    char      p_proto;
};

struct protoent *pptr;
pptr=getprotobyname("udp");

struct hostent{
    char     *h_name;
    char   **h_aliases;
    char       h_addrtype;
    char       h_length;                   /* address length */
    char   **h_addr_list;               /* list of addresses */
};

struct hostent *htpr;
char *str="dlut.edu.cn";
htpr = gethostbyname(str);