in reply to Re: Flock Feedback
in thread Flock Feedback

If you just want a simple incrementing counter, one way to do this is to use the size of the file as the counter, rather than its contents. You don't need locking to do this: 

my $filename = '/path/to/file';

# Increment the counter (atomically, so no locking required)
open(FILE, ">> $filename") or warn "Missed a count";
print FILE '.'    # Ouput one (arbitrary) byte
close FILE;

# What is the counter set to?
my $countval = -s $filename || 0;

[download]

The disadvantage of this technique is that it uses one byte of disk space for each count.

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Re: Re: Re: Flock Feedback
by chipmunk (Parson) on Feb 26, 2001 at 20:04 UTC
    Not only does this use one byte of disk space per count, which is ridiculously wasteful, but you still have a race condition if two processes try to write to the file and then check its size at the same time. Even with this approach you still would need locking.
[reply]

      Even with this approach you still would need locking.

      Yes, as written. But the ">>" file mode does allow you to use something very similar to this without (additional) locking. 

      use IO::Handle;
my $filename = '/path/to/file';

# Increment the counter (atomically, so no locking required)
open(FILE, ">> $filename") or warn "Missed a count";
FILE->autoflush(1);
print FILE '.';    # Ouput one (arbitrary) byte
my $counter= tell(FILE);
close FILE;

      [download]

      Now, if you reset your counters every so often, this can be a really nice little trick. To reset the counters, just do a unlink($filename) once a day/week/month, which also doesn't require any locking on your part! (Though the location and permissions on the counter file are a non-trivial problem so you may want to use sysopen so you can specify permission bits for the case when the file is recreated.)

              - tye (but my friends call me "Tye")
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