I have a program, where several instances run concurrently. They platforms to be supported are: Windows XP, Windows 2003 Server, Windows 2008 Server, and Windows 7 (64 Bit).

These processes 'share' a piece of code which should be executed as a critical region, i.e. only one of the processes is supposed to execute this part of the code at any time. To implement this, I use flock on a lockfile to guard entry and exit to the critical region, i.e. I basically do a

sysopen(LOCKFILE,$lockfile,O_WRONLY|O_CREAT);
while(!flock(LOCKFILE, LOCK_EX|LOCK_NB)) {
  sleep($some_random_time);
}

#########################
# CRITICAL REGION IS HERE
#########################

flock(LOCKFILE, LOCK_UN);
close(LOCKFILE);
[download]

This works well. Now I wonder: Could there be cases, where a process gets killed while being in the critical region, BUT the lock is not being released (and therefore all other processes would be locked out forever)?

I wrote a small test program to research this case, but no matter how I killed the process holding the lock, the lock was always freed afterwards. Of course this doesn't mean that what I'm doing is safe. It just means that I was not able to break it.

Could someone with good experience in Windows programming give me some enlightment here?

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