Re: fcntl problems

"mhearne808[insert-at-sign-here]gmail[insert-dot-here]com" <mhearne808@xxxxxxxxx> writes:

I think I'm still confused.

What Miles tried to tell you is that you should call fcnt.flock from
both PA and PB. In the example you posted, you failed to call it from
PB. No lock call, so no locking happened.

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

File locking supports that scenario, as you suspected. You need to
use flock with LOCK_EX|LOCK_NB. If the call succeeds, you got the
lock. If you get an exception whose errno is EWOULDBLOCK, you quit in
disgust.
.