gethostbyname() 及 getaddrinfo() 用法探究

主要是如何使用两个函数的返回结构。在gethostbyname函数中，返回hostent，linux参考手册介绍如下：

struct hostent {
    char  *h_name;            /* official name of host */
    char **h_aliases;         /* alias list */
    int    h_addrtype;        /* host address type */
    int    h_length;          /* length of address */
    char **h_addr_list;       /* list of addresses */
}

具体用法可以通过以下代码看到：

#include <arpa/inet.h>
#include <netdb.h>
#include <stdlib.h>
#include <stdio.h>

int main(int argc, char* argv[])
{
    char *ptr, **pptr;
    char str[INET_ADDRSTRLEN];
    struct hostent * hptr;

    while (--argc > 0)
    {
        ptr = *++argv;
        if ((hptr = gethostbyname(ptr)) == NULL)
        {
            fprintf(stderr, "gethostbyname error for host%s : %s",
                    ptr, hstrerror(h_errno));
            continue;
        }
        printf("official hostname %s\n", hptr->h_name);

        for (pptr = hptr->h_aliases; *pptr != NULL; ++pptr)
            printf("\talials: %s\n", *pptr);

        switch (hptr->h_addrtype)
        {
            case AF_INET:
                pptr = hptr->h_addr_list;
                for (; *pptr != NULL; ++pptr)
                    printf("\taddress: %s\n",
                            inet_ntop(hptr->h_addrtype, *pptr,
                                str, sizeof(str)));
                break;
            default:
                fprintf(stderr, "unknown address type");
                exit(EXIT_FAILURE);
                break;
        }
    }
    return 0;
}

执行./hostent www.baidu.com后
效果如下：

getaddrinfo函数则更加复杂，但是提供了更好的接口。
这是只展示一个示例程序。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <netdb.h>
#include <sys/types.h>
#include <arpa/inet.h>

void printAddressInfo(const struct addrinfo*);

int main(int argc, char* argv[])
{
    int n;
    struct addrinfo* res, *ressave;
    struct addrinfo hint;
    char* serv;

    if (argc < 2)
    {
        fprintf(stderr, "Usage: "
                "./getaddrinfo <hostname> [<service name"
                " or port name>]\n");
        exit(EXIT_FAILURE);
    }
    if (argc == 2)
        serv = NULL;
    else
        serv = argv[2];
    bzero(&hint, sizeof(hint));
    hint.ai_family = AF_UNSPEC;

    if ((n = getaddrinfo(argv[1], serv, &hint, &res)) != 0)
    {
        if (argc > 2)
            fprintf(stderr, "Cannot get information for "
                    "%s : %s\n\terror:%s\n",
                    argv[1], serv, gai_strerror(n));
        else
            fprintf(stderr, "Cannot get information for "
                    "%s\n\terror:%s\n",
                    argv[1], gai_strerror(n));
        exit(EXIT_FAILURE);
    }
    ressave = res;

    while (res != NULL)
    {
        printAddressInfo(res);
        res = res->ai_next;
    }

    freeaddrinfo(ressave);
    exit(EXIT_SUCCESS);
}

void printAddressInfo(const struct addrinfo* addr)
{
    char str[INET6_ADDRSTRLEN];
    short port;

    printf("ai_family: ");
    switch (addr->ai_family)
    {
        case AF_INET: {
            struct sockaddr_in* in;

            printf("ipv4\n");
            in = (struct sockaddr_in *)(addr->ai_addr);
            port = ntohs(in->sin_port);
            inet_ntop(AF_INET, &in->sin_addr, str, sizeof(str));
            break;
        }
        case AF_INET6: {
            struct sockaddr_in6* in6;

            in6 = (struct sockaddr_in6 *)(addr->ai_addr);
            port = ntohs(in6->sin6_port);
            inet_ntop(AF_INET6, &in6->sin6_addr, str, sizeof(str));
            printf("ipv6\n");
            break;
        }
        default:
            printf("Unknown\n");
            break;
    }
    printf("ai_socktype: ");
    switch (addr->ai_socktype)
    {
        case SOCK_STREAM:
            printf("stream\n");
            break;
        case SOCK_DGRAM:
            printf("dgram\n");
            break;
        case SOCK_SEQPACKET:
            printf("seqpacket\n");
            break;
        case SOCK_RAW:
            printf("raw\n");
            break;
        default:
            printf("others\n");
            break;
    }
    printf("protocol: ");
    switch (addr->ai_protocol)
    {
        case IPPROTO_TCP:
            printf("tcp\n");
            break;
        case IPPROTO_UDP:
            printf("udp\n");
            break;
        case IPPROTO_SCTP:
            printf("sctp\n");
            break;
        default:
            printf("others\n");
            break;
    }
    printf("address: %s\n", str);
    printf("port: %d\n", port);
    printf("canonical name: %s\n\n", addr->ai_canonname);
}

上述程序在输入：./getaddrinfo www.baidu.com http后，效果如下：

重点在于地址的显示。结构的内容。