typedef struct addrinfo {
?? ?int ai_flags;?? ??? ?//AI_PASSIVE,AI_CANONNAME,AI_NUMERICHOST
?? ?int ai_family;?? ??? ?//AF_INET,AF_INET6
?? ?int ai_socktype;?? ?//SOCK_STREAM,SOCK_DGRAM
?? ?int ai_protocol;?? ?//IPPROTO_IP, IPPROTO_IPV4, IPPROTO_IPV6 etc.
?? ?size_t ai_addrlen;?? ??? ??? ?//must be zero or a null pointer
?? ?char* ai_canonname;?? ??? ??? ?//must be zero or a null pointer
?? ?struct sockaddr* ai_addr;?? ?//must be zero or a null pointer
?? ?struct addrinfo* ai_next;?? ?//must be zero or a null pointer
}

其中ai_flags、ai_family、ai_socktype說(shuō)明如下：
參數(shù)?? ??? ??? ?取值?? ??? ??? ?值?? ??? ??? ?說(shuō)明
ai_family?? ??? ?AF_INET?? ??? ??? ?2?? ??? ??? ?IPv4
?? ??? ??? ??? ?AF_INET6?? ??? ?23?? ??? ??? ?IPv6
?? ??? ??? ??? ?AF_UNSPEC?? ??? ?0?? ??? ??? ?協(xié)議無(wú)關(guān)
ai_protocol?? ??? ?IPPROTO_IP?? ??? ?0?? ??? ??? ?IP協(xié)議
?? ??? ??? ??? ?IPPROTO_IPV4?? ?4?? ??? ??? ?IPv4
?? ??? ??? ??? ?IPPROTO_IPV6?? ?41?? ??? ??? ?IPv6
?? ??? ??? ??? ?IPPROTO_UDP?? ??? ?17?? ??? ??? ?UDP
?? ??? ??? ??? ?IPPROTO_TCP?? ??? ?6?? ??? ??? ?TCP
ai_socktype?? ??? ?SOCK_STREAM?? ??? ?1?? ??? ??? ?流
?? ??? ??? ??? ?SOCK_DGRAM?? ??? ?2?? ??? ??? ?數(shù)據(jù)報(bào)
ai_flags?? ??? ?AI_PASSIVE?? ??? ?1?? ??? ??? ?被動(dòng)的，用于bind，通常用于server socket
?? ??? ??? ??? ?AI_CANONNAME?? ?2
?? ??? ??? ??? ?AI_NUMERICHOST?? ?4?? ??? ??? ?地址為數(shù)字串

對(duì)于ai_flags值的說(shuō)明：
AI_NUMERICHOST | AI_CANONNAME | AI_PASSIVE
如上表所示，ai_flags的值范圍為0~7，取決于程序如何設(shè)置3個(gè)標(biāo)志位，比如設(shè)置ai_flags為 “AI_PASSIVE|AI_CANONNAME”，ai_flags值就為3。三個(gè)參數(shù)的含義分別為：
(1)AI_PASSIVE當(dāng)此標(biāo)志置位時(shí)，表示調(diào)用者將在bind()函數(shù)調(diào)用中使用返回的地址結(jié)構(gòu)。當(dāng)此標(biāo)志不置位時(shí)，表示將在connect()函數(shù)調(diào)用中使用。當(dāng)節(jié)點(diǎn)名位NULL，且此標(biāo)志置位，則返回的地址將是通配地址。如果節(jié)點(diǎn)名NULL，且此標(biāo)志不置位，則返回的地址將是回環(huán)地址。
(2)AI_CANNONAME當(dāng)此標(biāo)志置位時(shí)，在函數(shù)所返回的第一個(gè)addrinfo結(jié)構(gòu)中的ai_cannoname成員中，應(yīng)該包含一個(gè)以空字符結(jié)尾的字符串，字符串的內(nèi)容是節(jié)點(diǎn)名的正規(guī)名。
(3)AI_NUMERICHOST當(dāng)此標(biāo)志置位時(shí)，此標(biāo)志表示調(diào)用中的節(jié)點(diǎn)名必須是一個(gè)數(shù)字地址字符串。

addrinfo結(jié)構(gòu)體原型