socket编程之addrinfo结构体与getaddrinfo函数

原文地址 http://blog.chinaunix.net/uid-26583794-id-3167485.html

addrinfo结构体的定义如下：

struct addrinfo {     int ai_flags; /* customize behavior */     int ai_family; /* address family */     int ai_socktype; /* socket type */     int ai_protocol; /* protocol */     socklen_t ai_addrlen; /* length in bytes of address */     struct sockaddr *ai_addr; /* address */     char *ai_canonname; /* canonical name of host */     struct addrinfo *ai_next; /* next in list */     .     .     .   };

ai_family指定了地址族，可取值如下：
AF_INET 2 IPv4
AF_INET6 10 IPv6
AF_UNSPEC 0 协议无关
ai_socktype指定我套接字的类型
SOCK_STREAM 1 流
SOCK_DGRAM 2 数据报
在AF_INET通信域中套接字类型SOCK_STREAM的默认协议是TCP（传输控制协议）
在AF_INET通信域中套接字类型SOCK_DGRAM的默认协议是UDP（用户数据报协议）
ai_protocol指定协议类型。可取的值取决于ai_address和ai_socktype的值
ai_flags指定了如何来处理地址和名字,可取值如下：

getaddrinfo函数 定义及需要的头文件如下

#include <sys/socket.h>#include <netdb.h>int getaddrinfo(const char *restrict host,                const char *restrict service,                const struct addrinfo *restrict hint,                struct addrinfo **restrict res);Returns: 0 if OK, nonzero error code on errorvoid freeaddrinfo(struct addrinfo *ai);

getaddrinfo函数允许将一个主机名字和服务名字映射到一个地址。

使用示例如下：

#include<stdio.h>#include<stdlib.h>#include <netdb.h>#include <arpa/inet.h>voidprint_family(struct addrinfo *aip){    printf(" family ");    switch (aip->ai_family) {    case AF_INET:        printf("inet");        break;    case AF_INET6:        printf("inet6");        break;    case AF_UNIX:        printf("unix");        break;    case AF_UNSPEC:        printf("unspecified");        break;    default:        printf("unknown");    }}voidprint_type(struct addrinfo *aip){    printf(" type ");    switch (aip->ai_socktype) {    case SOCK_STREAM:        printf("stream");        break;    case SOCK_DGRAM:        printf("datagram");        break;    case SOCK_SEQPACKET:        printf("seqpacket");        break;    case SOCK_RAW:        printf("raw");        break;    default:        printf("unknown (%d)", aip->ai_socktype);    }}voidprint_protocol(struct addrinfo *aip){    printf(" protocol ");    switch (aip->ai_protocol) {    case 0:        printf("default");        break;    case IPPROTO_TCP:        printf("TCP");        break;    case IPPROTO_UDP:        printf("UDP");        break;    case IPPROTO_RAW:        printf("raw");        break;    default:        printf("unknown (%d)", aip->ai_protocol);    }}voidprint_flags(struct addrinfo *aip){    printf("flags");    if (aip->ai_flags == 0) {        printf(" 0");    } else {        if (aip->ai_flags & AI_PASSIVE)            printf(" passive");        if (aip->ai_flags & AI_CANONNAME)            printf(" canon");        if (aip->ai_flags & AI_NUMERICHOST)            printf(" numhost");#if defined(AI_NUMERICSERV)        if (aip->ai_flags & AI_NUMERICSERV)            printf(" numserv");#endif#if defined(AI_V4MAPPED)        if (aip->ai_flags & AI_V4MAPPED)            printf(" v4mapped");#endif#if defined(AI_ALL)        if (aip->ai_flags & AI_ALL)            printf(" all");#endif    }}intmain(int argc, char *argv[]){    struct addrinfo        *ailist, *aip;    struct addrinfo        hint;    struct sockaddr_in    *sinp;    const char             *addr;    int                 err;    char                 abuf[INET_ADDRSTRLEN];    if (argc != 3)        printf("usage: %s nodename service", argv[0]);    hint.ai_flags = AI_CANONNAME;    hint.ai_family = 0;    hint.ai_socktype = 0;    hint.ai_protocol = 0;    hint.ai_addrlen = 0;    hint.ai_canonname = NULL;    hint.ai_addr = NULL;    hint.ai_next = NULL;    if ((err = getaddrinfo(argv[1], argv[2], &hint, &ailist)) != 0)        printf("getaddrinfo error: %s", gai_strerror(err));    for (aip = ailist; aip != NULL; aip = aip->ai_next) {        print_flags(aip);        print_family(aip);        print_type(aip);        print_protocol(aip);        printf("\n\thost %s", aip->ai_canonname?aip->ai_canonname:"-");        if (aip->ai_family == AF_INET) {            sinp = (struct sockaddr_in *)aip->ai_addr;            addr = inet_ntop(AF_INET, &sinp->sin_addr, abuf,INET_ADDRSTRLEN);            printf(" address %s", addr?addr:"unknown");            printf(" port %d", ntohs(sinp->sin_port));        }        printf("\n");    }    exit(0);}

代码说明：
sinp = (struct sockaddr_in *)aip->ai_addr;会将struct sockaddr 变量强制转化为struct sockaddr_in 类型
inet_ntop函数用于在二进制格式与点分十进制格式表示（a.b.c.d）之间进行转换

执行结果：
root@X86-PC:/home# ./a X86-PC nfs
flags canon family inet type stream protocol TCP
host X86-PC address 127.0.1.1 port 2049
flags canon family inet type datagram protocol UDP
host - address 127.0.1.1 port 2049
root@X86-PC:/home#