Re: fcntl problems

On Aug 31, 12:23 am, Miles <semantic...@xxxxxxxxx> wrote:
Sorry, that last quote-only reply was accidental. :)

On 8/30/07, mhearne808 wrote:
I've been doing some experiments, and here are some specific examples
to try.

[snipped examples]

From these last two experiments I can only conclude that file locking
isn't doing a durned thing.

What's going on?

File locking isn't doing a durned thing in those cases because you're
only obtaining the lock from a single process.

According to my Python Cookbook:
"Exclusive lock: This denies all _other_ processes both read and write
access to the file."

This is only for mandatory locking; POSIX flock is advisory locking,
which states: "Only one process may hold an exclusive lock for a given
file at a given time." Advisory locks don't have any effect on
processes that don't use locks. Mandatory locks are kernel enforced,
but non-POSIX and not available in Mac OS X.

-Miles

I think I'm still confused. Maybe I should explain the behavior that
I want, and then figure out if it is possible.

I have a script that will be run from a cron job once a minute. One
of the things this script will do is open a file to stash some
temporary results. I expect that this script will always finish its
work in less than 15 seconds, but I didn't want to depend on that.
Thus I started to look into file locking, which I had hoped I could
use in the following fashion:

Process A opens file foo
Process A locks file foo
Process A takes more than a minute to do its work
Process B wakes up
Process B determines that file foo is locked
Process B quits in disgust
Process A finishes its work

Since I couldn't figure out file locking, I decided to just have
Process A create a "pid" file in the directory - analogous to the
"Occupied" sign on an airplane bathroom. This works, but it seems a
little hacky.

--Mike

.

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