gethostname gethostbyname gethostbyaddr 获得主机相关信息

网络编程里经常需要获得主机的相关信息，下面围绕相关的函数以及用到的结构来说明。

获得主机名：
int gethostname(
  char FAR *name,  //[out] Pointer to a buffer that receives the local host name.
  int namelen       //[in] Length of the buffer.
);
返回0表示成功，失败返回SOCKET_ERROR,错误代码通过调用WSAGetLastError查看

根据主机名获得主机信息：
struct hostent FAR *gethostbyname(
  const char FAR *name  
);
该函数返回一个hostent指针，"hostent"是"host entry"的缩写,接下来看一下hostent的结构：
struct  hostent {
        char    FAR * h_name;           /* official name of host */ //主机名
        char    FAR * FAR * h_aliases;  /* alias list */            //别名，一般为NULL
        short   h_addrtype;             /* host address type */     //地址类型  一般为2，即AF_INET
        short   h_length;               /* length of address */     //地址长度，ipv4地址为4
        char    FAR * FAR * h_addr_list; /* list of addresses */    //地址列表，假如有虚拟机的话，可能包含多个地址
#define h_addr  h_addr_list[0]          /* address, for backward compat */
};

举例说明：

   WSADATA wsaData;
    WSAStartup(MAKEWORD(2, 2), &wsaData);

    char szHostName[MAX_PATH] = { 0 };
    gethostname(szHostName, MAX_PATH);
    int err = GetLastError();

    hostent *ptent = gethostbyname(szHostName);
    printf("%s \t %s \t %d \n", ptent->h_name, ptent->h_aliases, ptent->h_addrtype);
    int i = 0;
    while(ptent->h_addr_list[i])
    {

      //一般机器上没有虚拟机，或者只哟一块儿网卡的话，这里只输出一项，即 h_addr_list[0]

        printf("addr:%s \n",inet_ntoa(*(in_addr*)ptent->h_addr_list[i]));
        i ++;
    }

WSACleanup();

不过gethostbyname已经被遗弃使用了，推荐使用getaddrinfo。看一下原著：

The gethostbyname function retrieves host information corresponding to a host name from a host database.

Note  The gethostbyname function has been deprecated by the introduction of the getaddrinfo function. Developers creating Windows Sockets 2 applications are urged to use the getaddrinfo function instead of gethostbyname.

gethostbyname只能解决传入的主机名，当不知道主机名，只知道IP地址时，此函数就无能为力了，这就得用两外一个函数：
gethostbyaddr。该函数同样返回hostent *先看一下该函数定义：
struct HOSTENT FAR * gethostbyaddr( 
 const char FAR *addr,  //[in] Pointer to an address in network byte order. 
 int len,      //[in] Length of the address. 
 int type    //[in] Type of the address, such as the AF_INET address family type (defined
             //     as TCP, UDP, and other associated Internet protocols). Address family types and
             //     their corresponding values are defined in the winsock2.h header file. 

);

此函数的第一个参数是一个网络序的地址形式，所以要想传入第一个参数，还得调用inet_addr(),将点分十进制的地址变为网络地址
//The inet_addr function converts a string containing an (Ipv4) Internet
Protocol dotted address into a proper address for the IN_ADDRstructure.
inet_addr原型：
unsigned long inet_addr(  const char   FAR *cp  );

举例说明：

    WSADATA wsaData;
    WSAStartup(MAKEWORD(2, 2), &wsaData);

    unsigned long ulong = inet_addr("127.0.0.1");
    hostent *P = gethostbyaddr((char *)&ulong, 4, AF_INET);
    printf("----------------------------------------------------\n");
    int   i = 0;
    printf("%s \t %s \t %d \n", P->h_name, P->h_aliases, P->h_addrtype);
    while(P->h_addr_list[i])
    {
        printf("addr :%s \n", inet_ntoa(*(in_addr *)(P->h_addr_list[i])));
        i++;
    }

WSACleanup();

不过有个前提，在使用以上任何函数之前先应该包含 #include <winsock2.h> 以及相应的lib  #pragma comment (lib, "ws2_32.lib")