Re: fcntl problems

On 8/31/07, mhearne808 wrote:
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

That would look like (untested):

import fcntl, sys
f = open('foo', 'w+')
try:
fcntl.flock(f.fileno(), fcntl.LOCK_EX | fcntl.LOCK_NB)
except IOError, e:
if e.args[0] == 35:
sys.exit(1)
else:
raise
f.seek(0, 2) # seek to end
# do your thing with the file
f.flush()
fcntl.flock(f.fileno(), fcntl.LOCK_UN)
f.close()

-Miles
.

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