兼容ipv6的C/S架构详解

讲到ipv4如何兼容ipv6，我觉得首先得介绍一个函数， getaddrinfo()

头文件：

#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>

函数原型：

int getaddrinfo(const char *node,     //identify an Internet host

　　　　　　　　const char *service,  //identify  a  service

　　　　　　　　const struct addrinfo *hints,

　　　　　　　　struct addrinfo **res);

getaddrinfo() returns one or more addrinfo structures, each of which contains an Internet address that can be specified in a call to bind() or connect().

addrinfo structure 如下：

struct addrinfo {
　　int ai_flags;
　　int ai_family;
　　int ai_socktype;
　　int ai_protocol;
　　size_t ai_addrlen;
　　struct sockaddr *ai_addr;
　　char *ai_canonname;
　　struct addrinfo *ai_next;
};

hints指向一个addrinfo结构体，但是addrinfo结构体内有一个next指针，说明可以将addrinfo结构体拉成一条链表，具体作用不清楚，继续往下看。

继续往下看addrinfo这个结构体：

ai_family:specifies the desired address family，可以值为 AF_INET  and  AF_INET6，或者为AF_UNSPEC ，而AF_UNSPEC 表示getaddrinfo() should return socket addresses for any address family (either IPv4 or IPv6, for example) that can be used with node and service.

ai_socktype：specifies the preferred socket type，for example SOCK_STREAM or

SOCK_DGRAM.假如指定为0，表示任何类型都可以。

SOCK_STREAM表示有保障的（即能保证数据正确传送到对方）面向连接的SOCKET，基于TCP的，数据传输比较有保障

SOCK_DGRAM表示是无保障的面向消息的socket ， 主要用于在网络上发广播信息。基于UDP的

 ai_protocol： specifies the protocol for the returned socket addresses.假如指定为0，表示任何协议皆可以。 For example ip, tcp, or udp.

 ai_flags：This field specifies additional options, described below. Multiple flags

are specified by logically OR-ing them together.for example, AI_V4MAPPED | AI_ADDRCONFIG。

All the other fields in the structure pointed to by hints must contain either 0 or a
null pointer, as appropriate. Specifying hints as NULL is equivalent to setting
ai_socktype and ai_protocol to 0; ai_family to AF_UNSPEC; and ai_flags to
(AI_V4MAPPED | AI_ADDRCONFIG).