File locking in Python--help! (part 2)

, Will Kahn-Greene | share this (mastodon)
Note: This is an old post in a blog with a lot of posts over a long span of time. The world has changed, technologies have changed, and I've changed. It's likely this is out of date, the code doesn't work, the ideas haven't aged well, or the ideas were terrible to begin with. Let me know if you think this is something that needs updating.

I got a bunch of really helpful responses from my previous entry and I'm pretty sure that my problems were two-fold: my code was missing some stuff (the code I posted and what I actually had were different--go figure) and my testing program had a bug (or two or three).

Anyhow, my knowledge of file locking on Linux (and possibly other Unixs) is pretty abundant now (or so I think). This helped to fix a problem I had at work as well (not the file locking part, but the thing I needed file locking for helped me at work).

Python has the fcntl module which has a flock and lockf. I'm not entirely sure what the difference is, but there are man pages on both functions as the Python versions call their C counter-parts.

There's a good article on file locking that might be located at /usr/src/linux/Documentation/mandatory.txt (though it wasn't on my system--a Google search helped me out) that's pretty interesting. Also, there's explanations of file locking between processes and the issues involved in Advanced Programming in the Unix Environment and Linux Application Development.

My specific issue was with Exim and PINE. Both have documentation as to how they lock mbox files before playing with them--things a Google search reveals without much effort.

Anyhow, I'm pretty sure my code works now. I'll do some more sophisticated testing this weekend to make sure everything is fine.

Thanks to Chris, Jason, John and Lance for their help.

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