unsigned long int htonl(unsigned long int hostlong);
unsigned short int htons(unsigned short int hostshort);
unsigned long int ntohl(unsigned long int netlong);
unsigned short int ntohs(unsigned short int netshort);

struct hostent *gethostbyname(const char *hostname);
struct hostent *gethostbyaddr(const char *addr,int len,int type);

struct hostent{char *h_name;           /* 主机的正式名称  */char *h_aliases;        /* 主机的别名 */int   h_addrtype;       /* 主机的地址类型  AF_INET*/int   h_length;         /* 主机的地址长度  对于IP4 是4字节32位*/char **h_addr_list;     /* 主机的IP地址列表 */
};

#define h_addr h_addr_list[0]  /* 主机的第一个IP地址*/

int inet_aton(const char *cp,struct in_addr *inp);
char *inet_ntoa(struct in_addr in);

int getsockname(int sockfd,struct sockaddr *localaddr,int *addrlen);
int getpeername(int sockfd,struct sockaddr *peeraddr, int *addrlen);
struct servent *getservbyname(const char *servname,const char *protoname);
struct servent *getservbyport(int port,const char *protoname);
struct servent{char *s_name;          /* 正式服务名 */char **s_aliases;      /* 别名列表 */  int s_port;            /* 端口号 */char *s_proto;         /* 使用的协议 */ };

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <netdb.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/types.h>
#include <arpa/inet.h>int main(int argc ,char **argv)
{struct sockaddr_in addr;struct hostent *host;char **alias;if(argc<2){fprintf(stderr,"Usage:%s hostname|ip..\n\a",argv[0]);exit(1);}argv++;for(;*argv!=NULL;argv++){/* 这里我们假设是IP*/   if(inet_aton(*argv,&addr.sin_addr)!=0){host=gethostbyaddr((char   *)&addr.sin_addr,4,AF_INET); printf("Address information of Ip %s\n",*argv); } else {/* 失败,难道是域名?*/host=gethostbyname(*argv); printf("Address informationof host %s\n",*argv);
                }if(host==NULL){/* 都不是 ,算了不找了*/fprintf(stderr,"No address information of %s\n",*argv);continue;}printf("Official host name %s\n",host->h_name);printf("Name aliases:");for(alias=host->h_aliases;*alias!=NULL;alias++)printf("%s ,",*alias);printf("\nIp address:");for(alias=host->h_addr_list;*alias!=NULL;alias++)printf("%s ,",inet_ntoa(*(struct in_addr *)(*alias)));}
}