From: Paul Lutus <nospam@...>
Date: 2006-08-30T07:15:36+09:00
Subject: Re: process based locks for files

Radu Spineanu wrote:

> Hi,
> 
> I have two programs accessing the same file. Both programs open and
> close the file multiple times while running (for example reading it
> first, and then writing it back using global functions).
> 
> In order to avoid race conditions i was wondering if it's possible to
> lock the file at the start of the running process and then unlock it at
> the end. During that time the running process should be able to open and
> close the file without problems.

If both programs are willing to cooperate with each other, why not use a
flag file, as is often done to lock and unlock various resources in Linux?
This approach has the advantage of being platform-neutral and doesn't
require any explicit file-locking facilities.

-----------------------------------------

#!/usr/bin/ruby -w

flag_file = "FLAGFILE"

puts "Waiting for access ..."
while FileTest.exists? flag_file
   sleep 1
end

puts "Creating flag file ..."
ff = File.open(flag_file,"w") {}

puts "Processing ..."
sleep 5

puts "Removing flag file ..."
File.delete flag_file

-----------------------------------------

There is an obvious collision possibility in this arrangement, that may or
may not be an issue.

-- 
Paul Lutus
http://www.arachnoid.com