gethostbyname(3) Function sample C code
A short demonstration program for the function gethostbyname(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.
/* lookup.c:
* Example of gethostbyname(3):
* 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;
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);
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 lookup.c
gcc lookup.o -o lookup
[sgupta@rhel55x86 chap9]$ ./lookup 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] $
*/
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