The code below prevents having itself run more than once. The code is working in the sense that if you try to run it more than once, you will get a message of 'Another $script is already running.'.

But if you uncomment the "Check_Race_Condition()", and comment out the lines following it -- "open and flock", and try to run the code more than once, it will not give any error message. This means that the script is able to run more than once.

I'm not sure why this is happening... Am I missing something obvious? 

#!/usr/bin/perl

use warnings;
use strict;
use Fcntl qw( :flock );

print "Script $0 is running now.\n";

# Check_Race_Condition();

open( my $self, '<', $0 ) 
  or die( "Another $0 is already running.\n");
flock( $self, LOCK_EX | LOCK_NB ) 
  or die( "Another $0 is already running.\n");

sleep( 100 );
print "End.\n";

sub Check_Race_Condition {
    open( my $self, '<', $0 ) 
      or die( "Another $0 is already running.\n");
    flock( $self, LOCK_EX | LOCK_NB ) 
      or die( "Another $0 is already running.\n");
}

[download]

In reply to Flock and Subroutine by bichonfrise74

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.