I have two Perl processes, let's call them A and B. When A runs, it (re)creates a file F (i.e. if F already exists, it will be erased, and then it will be created from scratch). When B runs, it reads the file F (if present).

To coordinate these activities, I would normally use file locking in processes A and B, using a separate lockfile L, like this (error handling omitted):

# Acquire lock
sysopen(LOCKFILE,'L',O_WRONLY|O_CREAT);
while(!flock(LOCKFILE, LOCK_EX|LOCK_NB)) {
        sleep(...);
}
# Lock granted!
if(I am process A) {
  open(my $file,'<','F');
  ...
} elsif(I am process B) {
  unlink 'F';
  open(my $file,'>','F);
  ...
}
# Release lock
flock(LOCKFILE, LOCK_UN);
close(LOCKFILE);
[download]

I think (hope) the basic logic is correct. However, in my case, process A runs on Unix and process B runs on Windows, and the file F needs to be accessed via the network in both cases. From what I have read in, for example, perlfaq5, locking may or may not work well when done over the network.

Now I have two questions:

• Can this locking algorithm be improved, or is it already "safe enough" for my concrete case?
• Do I really need a separate lockfile L, oder can I lock on file F? Of course I can't then unlink('F'), but if I would truncate the file to length zero after opening it, an unlink wouldn't be necessary.

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