PERL Beginners - Uninitialized value in unpack

Author

Uninitialized value in unpack

perl@fongo.de

2004-12-23, 3:58 pm

Can anyone point out what is uninitialized in line 162?

my @resolved = ();
foreach (@dns){
my $ip = gethostbyname($_);
my ($a,$b,$c,$d);
my $x = join (".", (unpack('C4',$ip ))); # line 162
$x ||= "????";
push (@resolved, join(" => ", $_,$x));

}

Thanks

JupiterHost.Net

2004-12-23, 3:58 pm

perl@fongo.de wrote:
> Can anyone point out what is uninitialized in line 162?
>
> my @resolved = ();
> foreach (@dns){
> my $ip = gethostbyname($_);
> my ($a,$b,$c,$d);
> my $x = join (".", (unpack('C4',$ip ))); # line 162
> $x ||= "????";
> push (@resolved, join(" => ", $_,$x));
>
> }

lets narrow it down a bit:

#!/usr/bin/perl

use strict;
use warnings;

for(@ARGV) {
my $ip = gethostbyname($_);
print "Host -$_-\nResolved: ";
print join ('.', unpack('C4',$ip)) || '????';
print "\n";
}

perl dns.pl is ok (is ok)

perl dns.pl google.com search.cpan.org (is ok)

perl dns.pl google.com search.cpan.org fake.fake (uninitialized value)

I imagine the join is only given one thing and not a list when the
hostname lookup fails:

IE like me saying to you "please tell me the sum of 8 +"

then you'd say "er, 8 + what? the second value you have not initialized?"

The solution would be to check for that before using it:

#!/usr/bin/perl

use strict;
use warnings;

for(@ARGV) {
print "Host -$_-\nResolved: ";
my $ip = gethostbyname($_);
if($ip) {
print join ('.', unpack('C4',$ip)),"\n";
} else {
print "????\n";
}
}

Now try it, no more warning :)

HTH :)

Lee.M - JupiterHhost.Net

John W. Krahn

2004-12-23, 3:58 pm

perl@fongo.de wrote:
> Can anyone point out what is uninitialized in line 162?

You don't say which line is number 162.

> my @resolved = ();
> foreach (@dns){
> my $ip = gethostbyname($_);

gethostbyname() will return undef if $_ does not contain a valid host name.

defined( my $ip = gethostbyname($_) ) or do {
warn "Host '$_' not valid.\n";
next;
};

> my ($a,$b,$c,$d);
> my $x = join (".", (unpack('C4',$ip ))); # line 162
> $x ||= "????";
> push (@resolved, join(" => ", $_,$x));
>
> }

John
--
use Perl;
program
fulfillment

Jonathan Paton

2004-12-23, 3:58 pm

On Thu, 23 Dec 2004 14:34:02 +0100, perl@fongo.de <perl@fongo.de> wrote:
>
> Can anyone point out what is uninitialized in line 162?
>
> my @resolved = ();
> foreach (@dns){
> my $ip = gethostbyname($_);

If the gethostbyname() call fails (the name can not be resolved), then you
get undef. Line 162 is the first time you use $ip.

> my ($a,$b,$c,$d);

Are these for the future? Not used anywhere.

> my $x = join (".", (unpack('C4',$ip ))); # line 162
> $x ||= "????";

You would be better having $x undefined if the resolve
fails.

> push (@resolved, join(" => ", $_,$x));
>
> }

Since I have made several suggestions, here is what I
would write:

my %resolved;

for my $name (@dns) {
my $ip = gethostbyname $name;

if (defined $ip) {
$ip = join ".", unpack('C4', $ip);
}

$resolved{$name} = $ip;
}

Notice I have introduced a more meaningful loop variable,
removed "????" as the default and used a hash instead.

I suggest you have the code reviewed.

Jonathan Paton

--
#!perl
$J=' 'x25 ;for (qq< 1+10 9+14 5-10 50-9 7+13 2-18 6+13
17+6 02+1 2-10 00+4 00+8 3-13 3+12 01-5 2-10 01+1 03+4
00+4 00+8 1-21 01+1 00+5 01-7 >=~/ \S\S \S\S /gx) {m/(
\d+) (.+) /x,, vec$ J,$p +=$2 ,8,= $c+= +$1} warn $J,,

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