socket编程之addrinfo结构体与getaddrinfo函数

原文链接：
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        23            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 error


void 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>

void
print_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");
    }
}

void
print_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);
    }
}

void
print_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);
    }
}

void
print_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
    }
}

int
main(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：#./a X86-PC nfs

root@X86-PC:/home/cheney/work-link/linux_C_Test/linux-program/fig16# ./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/cheney/work-link/linux_C_Test/linux-program/fig16#