Yesterday I looked in one of my apache error logs, and noticed that one of my CGI apps was spitting out lots of 'uninitialized value' warnings. This happens from time to time, and is no big deal. But they bug me, so any time I notice them I take a look at the code and tidy up whatever it was that was causing them.

However, this time I'm completely stumped.

Firstly, here is the offending code (for clarity, I removed some bits which are not relevant): 

for my $test (@testlist) {
    if (defined $tref->{$host}{$test}) {
        my $status = $tref->{$host}{$test}{status};
        my $duration = $tref->{$host}{$test}{dur};
        # debugging line
        if (!$tref->{$host}{$test}{status}) { print "$host=>$test\n";}

        # Some more processing here which is not relevent
    }
    else {
        print "\t<th>-</th>\n";
   }
}

[download]
The above code is part of a bigger loop, that is iterating through a list of around 500 'hosts'. For each of these 'hosts', it will iterate through a list of 'tests', and for each one it checks to see if we have data for that test/host combination. If data is present, it gets used. Otherwise just a dash '-' is ouput (the else part of the loop).

What I found was that just one single host/test combination (out of several thousand) was throwing the warning. With the print debugging line added, I narrowed it down to the 'http' test on the 'vnsgnnwwor-gw-1' host.

However - and this is the part that has me completely stumped - when I did a print Dumper($tref);1 immediately before this loop, the output for this particular host was: 

 'vnsgnnwwor-gw-1' => {
                         'conn' => {
                                     'dur' => '2.09 hours',
                                     'status' => 'red'
                                   },
                         'mrtg' => {
                                     'dur' => '4.00 hours',
                                     'status' => 'green'
                                   }
                       },

[download]
So I've confirmed that the 'http' data for this host does not exist. So therefore my question is - how on earth is this getting past the conditional in the 'if' statement at the top of the loop? Am I being bitten by autovivification? If I am, then my understanding of how autovivification works is obviously way off. And if is is autovivification, then it doesn't make sense. Because there would be dozens and dozens of other test/host combinations that would also slip through - but only this single one is.

I can't help having the feeling that I'm missing something blatantly obvious here. Anyway, somebody please put me out of my misery.

Update: Okay, I have figured this one out, and I was being bitten by autovivification. Here is a self-contained example that demonstrates: 

#!/usr/bin/perl -l
use strict;
use warnings;

my $tref = {
    'vnsgnnwwor-gw-1' => {
        'conn' => {
            'dur' => '2.09 hours',
            'status' => 'red'
        },
        'mrtg' => {
            'dur' => '4.00 hours',
            'status' => 'green',
        },
    },
    };

my $host = 'vnsgnnwwor-gw-1';
my $test = 'http';

print "$host is ", defined $tref->{$host} ? "True" : "False";
print "$test is ", defined $tref->{$host}{$test} ? "True" : "False";
print "$test=>status is ", defined $tref->{$host}{$test}{status} ? "Tr
+ue" : "False";
print "$test is ", defined $tref->{$host}{$test} ? "True" : "False";

[download]
And the output of the above is: 
vnsgnnwwor-gw-1 is True
http is False
http=>status is False
http is True

[download]
Note that the first time I test for 'http', it returns false. But the second time that I test for it, it returns true. This is because (if my understanding of autovivification is correct), when I test for $tref->{$host}{$test}{status} - Perl obligingly creates the $test part of that datastructure for me - thanks to autovivification.

I feel much better now :)

Thanks,
Darren :)

1. I use Data::Dumper::Simple for debugging

In reply to Tracking down an uninitialized value warning by McDarren

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