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Perl Variables Being Retained

by oioigazza (Initiate)
on Aug 01, 2013 at 22:08 UTC ( #1047506=perlquestion: print w/replies, xml ) Need Help??
oioigazza has asked for the wisdom of the Perl Monks concerning the following question:


I've never seen this behaviour before so am questioning my understanding of variables in a Perl CGI environment has been wrong from the start. I was always under the impression that the lifespan of a variable in a CGI script is just during the execution of that script (unless of course they are passed between pages).

I've just noticed that a variable I am using to calculate a value in a Perl CGI script is retained each time the script is called. So the net result is the variable never gets reset back to 0.

As a test, I've created a very simple CGI script to just add 1 to a variable and display it ($test++; print $test;). No other declarations. Run this as a CGI script and the numbers go 1, 2, 3, 4, 5, 6, etc. However, run this in a command shell and it always returns 1.

I'm amazed as I've never seen this behaviour before. And I'm sure I would have done with the Perl CGI I have done over the years.

Hoping someone may put me out of my misery and confirm it's a bug of some sort.

My environment is Windows 7 Ultimate 64-bit SP1, ActivePerl perl5 (revision 5 version 10 subversion 0), Windows IIS.

Many thanks

Replies are listed 'Best First'.
Re: Perl Variables Being Retained
by tobyink (Abbot) on Aug 01, 2013 at 23:18 UTC

    I think it's likely that you do not have IIS configured to run via CGI, but instead have it configured to run via FastCGI or some other persistent mechanism.

    That's generally considered a good thing. It means that your Perl script can save doing a bunch of initialization stuff for every single request (loading modules, connecting to databases, etc) and instead do it just once. This could allow you to process many more requests per second.

    However, it does mean that you've got to be careful not to persist stuff you don't want to persist. Generally making sure that variables are lexically scoped (i.e. my $var) should be sufficient to take care of that. The other thing you'll need to be careful about is leaking memory. This can happen if you have cyclical references pointing to each other, e.g.:

    my $a = []; my $b = []; push @$a, $b; push @$b, $a;

    ... these two arrays now each contain a reference to each other, so Perl doesn't ever realise it can free that memory, and doesn't free it until the end of the process. If every web request creates these two arrays, then the process ends up eating a little more memory for each request.

    But anyway, although you need to be a little more careful than you do with CGI, generally speaking a persistent Perl interpreter is a good way of serving web content.

    package Cow { use Moo; has name => (is => 'lazy', default => sub { 'Mooington' }) } say Cow->new->name

      Specifically in terms of “Perl not cleaning up the memory,” also note the weaken in Scalar::Util.   If you have data structures referring to one-another such that a circular reference is formed, memory does not clean up but rather “leaks.”   You avoid this by weakening one of the links in the chain.   Test::Memory::Cycle is another tool that is designed specifically to look for these circular reference-chains.

Re: Perl Variables Being Retained
by Anonymous Monk on Aug 02, 2013 at 03:47 UTC
Re: Perl Variables Being Retained
by sundialsvc4 (Abbot) on Aug 02, 2013 at 12:37 UTC
Re: Perl Variables Being Retained
by oioigazza (Initiate) on Aug 05, 2013 at 20:30 UTC
    Thank you all very much for your replies, much appreciated. I will check my IIS setup. As I mentioned in my post, I've never seen this behaviour before and am quite certain I would have. But saying that, I could have quite easily ticked the wrong box in IIS or something. And thanks for the links to the other pages provided. I will be sure to take a good look at them. Best regards, Gazza

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