I have several parallel processes which access a critical region. I implement this region using a lockfile, which holds a counter (which I need for my app - it contains "the number of clients" using a certain part of the system). Inside the critical region, there occurs one of two operations: Checkin (which increments the counter) and checkout (which decrements the counter). My implementation so far is straightforward. Here a simplified version which shows the essential part: 

use Fcntl qw(:DEFAULT :flock);
...
    sysopen(LOCKFILE,$lockfile,O_RDWR|O_CREAT) # create if necessary
        or die "Can not open/create $lockfile ($!)";
    flock(LOCKFILE, LOCK_EX) or die "Locking error on $lockfile ($!)";
    eval {
        my $number_of_clients=<LOCKFILE>||0;
        chomp($number_of_clients); 
        seek(LOCKFILE,0,0) or die "Rewind error on $lockfile ($!)";
        truncate(LOCKFILE,0) or die "Truncate error on $lockfile ($!)"
+;
        if($operation='checkin') {
          ++$number_of_clients;
          ...
        } else {
          ...
          --$number_of_clients;
        }
        print LOCKFILE $number_of_clients,"\n"; 
    }
    if($@) { warn "Exception: $@"; }
    flock(LOCKFILE, LOCK_UN) or die "Unlocking error on $lockfile ($!)
+";
    close(LOCKFILE) or warn "Error closing $lockfile ($!)";

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This seems to work fine. But now comes a new twist: If (in the checkout case) the number of clients becomes zero, instead of writing 0 to the file, I would like to *delete* the lockfile. Now the problem is that it is not nice to delete a lockfile while it is still open, so I would have to do it *after* the close, don't I? But at that time, I don't own the lock anymore, so another process could have already acquired the file and incremented the number of clients.

How can I solve this?

One more note: I know that my solution is flawed in that it would allow the number of clients to become negative (if checkout is called without a matching checking). I am aware of this, but this is something I can handle easily.

-- 
Ronald Fischer <ynnor@mm.st>

In reply to File Locking plus delete Lockfile question by rovf

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