sethostent example C code

sethostent(3) Function sample C code

A short demonstration program for the function sethosent(3) has been provided below. This program accepts multiple hostnames on the command line and then queries the name server for each. All available information is reported to standard output, or an error is reported if the name cannot be resolved.

 /* lookup2.c:

  * sethostent(3) sample c code

  * supply the hostname as argument you want to lookup

  * see the sample output below the code

  */

  #include <stdio.h>

  #include <unistd.h>

  #include <stdlib.h>

  #include <string.h>

  #include <errno.h>

  #include <sys/socket.h>

  #include <netinet/in.h>

  #include <arpa/inet.h>

  #include <netdb.h>

  extern int h_errno;

  #define TRUE 1
  #define FALSE 0

  int main(int argc,char **argv) {

     int x, x2;

     struct hostent *hp;

     for ( x=1; x<argc; ++x ) {

  /*

   * Look up the hostname:

   */

        hp = gethostbyname(argv[x]);

        if ( !hp ) { //Report lookup failure

           fprintf(stderr,

                   "%s: host '%s'\n",

                   hstrerror(h_errno),

                   argv[x]);

           continue;

        }

  /* 

   * Report the findings:

   */

        printf("Host %s : \n" ,argv[x]);

        printf(" Officially:\t%s\n", hp->h_name);

        fputs(" Aliases:\t",stdout);

/*

 * The sethostent(3) function permits you, as the application

 * designer, to control how name server queries are performed.

 * This function can improve the overall network performance of

 * your application.

 */

        sethostent(TRUE);

        for ( x2=0; hp->h_aliases[x2]; ++x2 ) {

           if ( x2 ) {

              fputs(", ",stdout);

             }

        fputs(hp->h_aliases[x2],stdout);

        }    

        fputc('\n',stdout);

        printf(" Type:\t\t%s\n",

               hp->h_addrtype == AF_INET

               ? "AF_INET" : "AF_INET6");

        if ( hp->h_addrtype == AF_INET ) {

           for ( x2=0; hp->h_addr_list[x2]; ++x2 ) {

              printf(" Address:\t%s\n",

                     inet_ntoa( *(struct in_addr *)

                      hp->h_addr_list[x2]));

           }

        }

     putchar('\n');

     }

     return 0;

  }

/*

 * OUTPUT

 *

[sgupta@rhel55x86 chap9] gcc -c -D_GNU_SOURCE -Wall -Wreturn-type lookup2.c

gcc lookup.o -o lookup2

[sgupta@rhel55x86 chap9]$ ./lookup2 www.lwn.net sunsite.unc.edu ftp.redhat.com

Host www.lwn.net :

Officially: lwn.net

Aliases: www.lwn.net

Type: AF_INET

Address: 206.168.112.90

Host sunsite.unc.edu :

Officially: sunsite.unc.edu

Aliases:

Type: AF_INET

Address: 152.2.254.81

Host ftp.redhat.com :

Officially: ftp.redhat.com

Aliases:

Type: AF_INET

Address: 206.132.41.212

Address: 208.178.165.228

[sgupta@rhel55x86 chap9] $

 */