gethostbyname

<함수 prototype>
struct hostent *gethostbyname(const char *name);

int gethostbyname_r(const char *name,
               struct hostent *ret, char *buf, size_t buflen,
               struct hostent **result, int *h_errnop);

두 함수 모두 return type은 hostent  struct 이다.

<struct hostent 구조 >

struct hostent {
               char  *h_name;            /* official name of host */
               char **h_aliases;         /* alias list */
               int    h_addrtype;        /* host address type */
               int    h_length;          /* length of address */
               char **h_addr_list;       /* list of addresses */ 
==> 아래쪽에 in_addr구조체의 s_addr 변수 참조. 같은 형태임. 
0xff01ffff의 네트웍바이트(순서대로) 형태(255.1.255.255)  
           }
           #define h_addr h_addr_list[0] /* for backward compatibility */

       The members of the hostent structure are:


       h_addrtype
              The type of address; always AF_INET or AF_INET6 at present.

       h_length
              The length of the address in bytes.

       h_addr_list
              An  array of pointers to network addresses for the host (in net-

       HOST_NOT_FOUND
              The specified host is unknown.

       NO_ADDRESS or NO_DATA
              The requested name is valid but does not have an IP address.

       NO_RECOVERY
              A non-recoverable name server error occurred.

       TRY_AGAIN
              A temporary error occurred on an authoritative name server.  Try
              again later.


<그 외 network관련 struct들 >
           struct sockaddr_in {
               sa_family_t    sin_family; /* address family: AF_INET */
               in_port_t      sin_port;   /* port in network byte order */
               struct in_addr sin_addr;   /* internet address */
           };

           /* Internet address. */
           struct in_addr {
               uint32_t       s_addr;     /* address in network byte order */  
           };
0xffffffff의 네트웍바이트 형태.=255.255.255.255  

<in_addr_t  inet_addr(hostname) 함수> 이름을 0xffffffff (255.255.255.255 효과) 형태로 변환리턴.

리턴type:  typedef uint32_t in_addr_t;   

INADDR_NONE= -1 (255.255.255.255)을 리턴하는 경우. 약간문제가 될때도 있으나 거의 괜찮음.