主机名称处理函数

#include <netdb.h>

struct hostent	*gethostbyaddr (const void *, socklen_t, int);
struct hostent	*gethostbyname (const char *);

hostent结构体：

/* Different from the linux versions - note the shorts.. */
struct	hostent {
	const char	*h_name;	/* official name of host */
	char	**h_aliases;	/* alias list */
	short	h_addrtype;		/* host address type */
	short	h_length;		/* length of address */
	char	**h_addr_list;	/* list of addresses from name server */
#define	h_addr	h_addr_list[0]	/* address, for backward compatiblity */
};

1、根据主机名获取主机信息

struct hostent * gethostbyname (const char *name)

Description:
The gethostbyname function returns information about the host named name. If the lookup fails, it 
 returns a null pointer. 

示例：

	struct hostent* ht = NULL;
	char name[] = "www.sina.com";
	ht = gethostbyname(name);

2、根据IP地址获取主机信息

struct hostent * gethostbyaddr (const char *addr, size_t length, int format)

Description:
The gethostbyaddr function returns information about the host with Internet address addr. The 
 parameter addr is not really a pointer to char - it can be a pointer to an IPv4 or an IPv6 address. The 
 length argument is the size (in bytes) of the address at addr. format specifies the address format; for 
 an IPv4 Internet address, specify a value of AF_INET; for an IPv6 Internet address, use AF_INET6. 

If the lookup fails, gethostbyaddr returns a null pointer. 

示例：

	struct hostent* ht1 = NULL;
	char ipaddr[] = "192.168.1.1";
	ht1 = gethostbyaddr(ipaddr, sizeof(ipaddr), AF_INET);

线程安全性：gethostbyname和gethostbyaddr是不可冲入的，调用结束后要及时将结果取出；目前已有线程安全的替代版本，并且适用于IPv4和IPv6，即getaddrinfo和getnameinfo。