OP's code: 

open(PID, "> $CONF{upd_lockfile}") ;
flock(PID, 2);
seek(PID, 0, 2);
for my $a (@pids) {
    print(PID "$a\n") if ($a != $$);
}
flock(PID, 8);

[download]

Problem #1: You don't prevent anyone from modifying the file between the time you read the PIDs and the time you write them.

Problem #2: You release the lock before writing anything to the file. Closing the file would flush its buffers, but you didn't do that. In fact, if you closed the file, you wouldn't even have to unlock it since it's already unlocked. 

use Fcntl qw( LOCK_EX );

sub remove_pid {
   open(my $PID, '+>>', $CONF{upd_lockfile}") or die $!;
   flock($PID, LOCK_EX) or die $!;
   seek($PID, 0, SEEK_SET) or die $!;
   chomp( my @pids = <$PID> );
   @pids = grep $_ != $$, @pids;
   seek($PID, 0, SEEK_SET) or die $!;
   print($PID "$_\n") for @pids;
   truncate($PID, tell($PID)) or die $!;
   kill(CONT => $parend_pid) or die $! if !@pids;
   close($PID) or die $!;
}

[download]

But why go to all this trouble for a solution that's inherently fragile? (e.g. Consider what happens if a child gets killed.) Why not have the parent use wait or waitpid instead of suspending itself.

In reply to Re: File lockingProblem by ikegami
in thread File lockingProblem by ric.techow

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