Maybe I'm overlooking something, but given the following example code, it is not clear to me where there could occur a race condition.
#! /usr/bin/perl
+
use strict;
use warnings;
my $pidfile="testfile.pid";
my $scriptname = $0;
my $pid="$$";
if(-e $pidfile)
{
open(PFH, '<', $pidfile) or die $!;
$pid =<PFH>;
close(PFH);
print"$scriptname already running: pid is $pid";
exit;
}
open(PFH, '>', $pidfile) or die $!;
print(PFH $$);
close(PFH);
# do the job
+
sleep 13;
unlink($pidfile);
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