客户端通过gethostbyname获取服务端IP地址

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <errno.h>
#include <string.h>
#include <netdb.h>
 
const int maxline=4096;
 
int main(int argc, char **argv) {
    int sockfd, n;
    char recvline[maxline+1], errbuff[maxline];;
    struct sockaddr_in servaddr;
    struct in_addr **pptr;
    struct in_addr *inetaddrp[2];
    struct in_addr inetaddr;
    struct hostent *hp;
    struct servent *sp;
 
    if(argc!=3) {
        fprintf(stderr, "usage: daytimetcpcli <hostname> <service>\n");
        exit(-1);
    }
 
    if((hp=gethostbyname(argv[1])) == NULL) {
        if(inet_aton(argv[1], &inetaddr)==0) {
            fprintf(stderr, "hostname error for %s: %s\n", argv[1], hstrerror(h_errno));
            exit(-1);
        } else {
            inetaddrp[0]=&inetaddr;
            inetaddrp[1]=NULL;
            pptr=inetaddrp;
        }
    } else {
        pptr=(struct in_addr **)hp->h_addr_list;
    }
 
    if((sp=getservbyname(argv[2], "tcp")) == NULL) {
        fprintf(stderr, "getservbyname error for %s\n", argv[2]);
        exit(-1);
    }
 
    for(; *pptr!=NULL; pptr++) {
        if((sockfd=socket(AF_INET, SOCK_STREAM, 0))<0) {
            strerror_r(errno, errbuff, maxline);
            fprintf(stderr, "socket error: %s\n", errbuff);
            continue;
        }
        memset(&servaddr, 0, sizeof(servaddr));
        servaddr.sin_family=AF_INET;
        servaddr.sin_port=sp->s_port;
        memcpy(&servaddr.sin_addr, *pptr, sizeof(struct in_addr));
        printf("trying %s: %d\n", inet_ntop(AF_INET, &servaddr.sin_addr, errbuff, sizeof(errbuff)), ntohs(servaddr.sin_port));
 
        if(connect(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr))==0) {
            break;
        } else {
            strerror_r(errno, errbuff, maxline);
            fprintf(stderr, "connect error: %s\n", errbuff);
            close(sockfd);
            exit(-1);
        }
    }
    if(*pptr==NULL) {
        fprintf(stderr, "unable to connect!\n");
        exit(-1);
    }
 
    while((n=read(sockfd, recvline, maxline))>0) {
        recvline[n]=0;
        if(fputs(recvline, stdout)==EOF) {
            fprintf(stderr, "fputs error\n");
            exit(-1);
        }
    }
    if(n<0) {
        fprintf(stderr, "read error.\n");
        exit(-1);
    }
    exit(0);
}