"Learning log" Linux network programming

The DNS Host entry contains DNS information about a domin name or IP address in DNS database.

1 struct hostent{2   Char *h_name; 3   Char *h_aliases; 4   int     H_addrtype; 5   int     h_length; 6   Char * *h_addr_list; 7 };

The Hostinfo program, which is used to resolve DNS info from IP or domin name.

1 /*modified from CSAPP:HOSTINFO.C2  */3#include <stdio.h>4#include <stdlib.h>5#include <unistd.h>6#include <string.h>7#include <ctype.h>8#include <setjmp.h>9#include <signal.h>Ten#include <sys/time.h> One#include <sys/types.h> A#include <sys/wait.h> -#include <sys/stat.h> -#include <fcntl.h> the#include <sys/mman.h> -#include <errno.h> -#include <math.h> -#include <pthread.h> +#include <semaphore.h> -#include <sys/socket.h> +#include <netdb.h> A#include <netinet/inch.h> at#include <arpa/inet.h> -#include <netinet/inch.h> -#include <netdb.h> -  - voidDns_error (Char*msg) - { infprintf (stderr,"%s:dns Error%d\n", MSG, h_errno); - } to  + structHostent *gethostbyaddr (Const Char* Addr,intLenintflag) - { the   structHostent *p; *   if(p = gethostbyaddr (addr, len, flag) = =NULL) $Dns_error ("gethostbyaddr Error");Panax Notoginseng   returnp; - } the  + structHostent *gethostbyname (Const Char*name) A { the   structHostent *p; +   if(p = gethostbyname (name)) = =NULL) -Dns_error ("gethostbyname Error"); $   returnp; $ } -  -  intMainintargcChar**argv) the { -   Char**pp;Wuyi   structin_addr addr; the   structHostent *HOSTP; -  Wu   if(ARGC! =2) { -fprintf (stderr,"Usage:%s <domain name or dotted-decimal>\n", Aboutargv[0]); $Exit0); -     } -   intAf_inet =af_inet; -   if(Inet_aton (argv[1], &addr)! =0) AHOSTP = gethostbyaddr ((Const Char*) &addr,sizeof(addr), af_inet); +   Else theHOSTP = gethostbyname (argv[1]); -  $   if(HOSTP = =NULL) theExit0); the  theprintf"official hostname:%s\n", hostp->h_name); the  -    for(pp = hostp->h_aliases;*pp! = null;pp++) inprintf"alias:%s\n", *pp); the  the    for(pp = hostp->h_addr_list;*pp = NULL; pp++) { AboutAddr.s_addr = ((structIN_ADDR *) *pp)s_addr; theprintf"Address:%s\n", Inet_ntoa (addr));  the   } theExit0); +}

Hostinfo