5_12.cpp

// 访问daytime服务
#include<sys/socket.h>
#include<netinet/in.h>
#include<netdb.h>
#include<stdio.h>
#include<unistd.h>
#include<assert.h>

int main(int argc, char* argv[]){
    assert(argc == 2);
    char *host = argv[1];

    // 获取目标主机地址信息
    struct hostent* hostinfo = gethostbyname(host);
    assert(hostinfo);

    // 获取daytime服务信息
    struct servent* servinfo = getservbyname("daytime", "tcp");
    assert(servinfo);
    printf("daytime port is %d\n", ntohs(servinfo->s_port));

    struct sockaddr_in address;
    address.sin_family = AF_INET;
    address.sin_port = servinfo->s_port;

    // h_addr_list本身使用网络字节序地址列表，所以无须对目标IP地址进行地址转换
    address.sin_addr = *(struct in_addr*)*hostinfo->h_addr_list;

    int sockfd = socket(AF_INET, SOCK_STREAM, 0);
    int result = connect(sockfd, (struct sockaddr*)&address, sizeof(address));
    assert(result != -1);

    char buffer[128];
    result = read(sockfd, buffer, sizeof(buffer));
    assert(result > 0);
    buffer[result] = '\0';
    printf("the day time is : %s", buffer);
    close(sockfd);
    return 0;
}