File::CounterFile - Persistent counter class


File::CounterFile - Persistent counter class


 use File::CounterFile;
 $c = new File::CounterFile "COUNTER", "aa00";
 $id = $c->inc;
 open(F, ">F$id");


This module implements a persistent counter class. Each counter is represented by a separate file in the file system. File locking is applied, so multiple processes might try to access the same counters at the same time without risk of counter destruction.

You give the file name as the first parameter to the object constructor (new). The file is created if it does not exist.

If the file name does not start with ``/'' or ``.'', then it is interpreted as a file relative to $File::CounterFile::DEFAULT_DIR. The default value for this variable is initialized from the environment variable TMPDIR, or /usr/tmp is no environment variable is defined. You may want to assign a different value to this variable before creating counters.

If you pass a second parameter to the constructor, that sets the initial value for a new counter. This parameter only takes effect when the file is created (i.e. it does not exist before the call).

When you call the inc() method, you increment the counter value by one. When you call dec() the counter value is decrementd. In both cases the new value is returned. The dec() method only works for numerical counters (digits only).

You can peek at the value of the counter (without incrementing it) by using the value() method.

The counter can be locked and unlocked with the lock() and unlock() methods. Incrementing and value retrieval is faster when the counter is locked, because we do not have to update the counter file all the time. You can query whether the counter is locked with the locked() method.

There is also an operator overloading interface to the File::CounterFile object. This means that you might use the ++ operator for incrementing the counter, -- operator for decrementing and you can interpolate counters diretly into strings.


It uses flock(2) to lock the counter file. This does not work on all systems. Perhaps we should use the File::Lock module?


Copyright (c) 1995-1998 Gisle Aas. All rights reserved.

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.


Gisle Aas <>

 File::CounterFile - Persistent counter class