Re: File locking within zsh?

[DervishD <zsh@dervishd.net>]

    Hola Lloyd :)))

 * Lloyd Zusman <ljz@asfast.com> dixit:
> >> I know that I can do this with a number of compiled executables,
> >> but I'm looking for a zsh-only solution.
> >
> >     Apart from the 'ln' trick, you can do this in Perl if you don't
> > want compiled executables.
> 
> Well, the perl interpreter is a compiled executable, and a rather
> heavy one, at that.  I'm trying to avoid spawning any extra
> processes.

    OK. I thought that your problem was that you didn't want to write
it in C, for example. If you're trying to avoid heavy binaries, avoid
Perl ;))

> >     If you could ellaborate on the problem you're trying to solve, I
> > can try to give a better solution.
> 
> I'm writing a zsh shell function that might be invoked within concurrent
> terminal sessions.  There's a critical section of code within this shell
> function that I want to protect with an exclusive lock.  I only need to
> protect this critical section from other invocations of this same shell
> function.

    OK, then probably the 'ln' solution will do.
 
> Perhaps I can write a loadable zsh module which can supply this
> capability.

    Probably, but that's a lot of work for something you can achieve
using 'ln'. You can even write a couple of shell functions using 'ln'
to do the locking.

    The only problem I see with the 'ln' solution is that it doesn't
block, you're forced to use a busy-wait loop :((

    If you don't want to use a busy-wait loop, you can try to use a
FIFO (in the filesystem, I mean) or a socket. Synchronizing more than
two tasks using those mechanism is tricky, but I think it can be
done. Think of your problem not as a "locking problem" but just as an
"IPC problem". A bit complex, anyway, because the shell doesn't
provide atomic operations that block (AFAIK).

    Of course, a zsh module for doing locking would be great ;) (or
adding the needed functions to zsh/files, for example).

> >     Good luck :)
> 
> Muchas gracias. :)

    De nada :))) I'm happy of reading you again :)

    BTW, this issue has been partially addressed in zsh-users 8632
and 8633.

    Raúl Núñez de Arenas Coronado

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