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Locking

2001-04-21 21:24:11
I've been using procmail under Linux for a while, but I don't really
understand locking. The man page and most FAQs presume a prior knowledge
of why locking is important and the types of locking necessary. I don't
have that prior knowledge. A couple of other possibly related issues
puzzle me as well. In particular:

0. Why are lockfiles needed at all?

1. Apparently, when piping through, say, formail, you don't need to use
a lockfile. Why? (Of course, I don't know why you'd use a lockfile in
the first place.)

2. Under certain circumstances, you would use the "f" flag in the
beginning of a recipe to indicate that you want procmail to filter the
header or whatever. But if I use a piping recipe (e.g. | formail ...),
why would I need to _tell_ procmail I'm filtering the email, since the
pipe automatically indicates some sort of filter? Perhaps more
important, how does procmail handle things internally differently if you
tell it to filter, versus when you don't?

3. Apparently, when you send mail to a directory, you don't need a
lockfile. Why?

4. When sending to a folder/file, you need a lockfile. If I've run
fetchmail, and fetchmail is delivering via procmail, there's only one
process accessing my mail folders. If only one process is accessing
them, why would a lockfile be needed? I've always assumed that lockfiles
were needed only if you expected more than one process/user to access a
file at the same time.

I've got more, but maybe the answers to these will answer the others as
well.

Paul

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