hostent h_addr_list

struct  hostent {        char    FAR * h_name;           /* official name of host */        char    FAR * FAR * h_aliases;  /* alias list */        short   h_addrtype;             /* host address type */        short   h_length;               /* length of address */        char    FAR * FAR * h_addr_list; /* list of addresses */#define h_addr  h_addr_list[0]          /* address, for backward compat */};

h_addr_list其实是一个指针数组，数组中每个元素都是in_addr型指针。

typedef struct in_addr {        union {                struct { UCHAR s_b1,s_b2,s_b3,s_b4; } S_un_b;                struct { USHORT s_w1,s_w2; } S_un_w;                ULONG S_addr;        } S_un;#define s_addr  S_un.S_addr /* can be used for most tcp & ip code */#define s_host  S_un.S_un_b.s_b2    // host on imp#define s_net   S_un.S_un_b.s_b1    // network#define s_imp   S_un.S_un_w.s_w2    // imp#define s_impno S_un.S_un_b.s_b4    // imp ##define s_lh    S_un.S_un_b.s_b3    // logical host} IN_ADDR, *PIN_ADDR, FAR *LPIN_ADDR;

因为hostent结构支持多种地址类型，所以其定义的h_addr_list是char **型。gethostbyname以后，实际存储情况是这样：

hostent->h_addr_list[0][0] = 127hostent->h_addr_list[0][1] = 0hostent->h_addr_list[0][2] = 0hostent->h_addr_list[0][3] = 1

得到的地址是网络字节顺序。转成正常的字符：

char *ip;ip = inet_ntoa(*(struct in_addr *)*phost->h_addr_list);

向sockaddr_in赋值：

struct sockaddr whereto;struct sockaddr_in *to;memset(&whereto, 0, sizeof(struct sockaddr));to = (struct sockaddr_in *)&whereto;memcpy(&to->sin_addr, hp->h_addr, hp->h_length);