有地址转换函数gethostbyname、gethostbyaddr、getservbyname、getservbyport，请选择其中三个进行举例应用

1. gethostbyname：该函数通过主机名获取主机的IP地址。例如：

#include <stdio.h>
#include <netdb.h>

int main() {
    const char *hostname = "www.google.com";
    struct hostent *host = gethostbyname(hostname);
    if (host == NULL) {
        printf("gethostbyname failed\n");
        return 1;
    }
    printf("%s's IP address is %s\n", hostname, host->h_addr_list[0]);
    return 0;
}

2. gethostbyaddr：该函数通过IP地址获取主机名和其他相关信息。例如：

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

int main() {
    const char *ip = "8.8.8.8";
    struct in_addr addr;
    if (inet_aton(ip, &addr) == 0) {
        printf("inet_aton failed\n");
        return 1;
    }
    struct hostent *host = gethostbyaddr(&addr, sizeof(addr), AF_INET);
    if (host == NULL) {
        printf("gethostbyaddr failed\n");
        return 1;
    }
    printf("%s's hostname is %s\n", ip, host->h_name);
    return 0;
}

3. getservbyname：该函数通过服务名获取服务的端口号。例如：

#include <stdio.h>
#include <netdb.h>

int main() {
    const char *servname = "http";
    struct servent *serv = getservbyname(servname, "tcp");
    if (serv == NULL) {
        printf("getservbyname failed\n");
        return 1;
    }
    printf("%s's port number is %d\n", servname, ntohs(serv->s_port));
    return 0;
}