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Re: File Locking

by Anonymous Monk
on Dec 12, 2003 at 07:19 UTC ( #314242=note: print w/ replies, xml ) Need Help??


in reply to File Locking

I really like to thank the person or people who wrote this "File Locking" tutorial because the text has given me the reason for opening a file in read-write mode before calling flock function. Now I feel more confident to implement flock function in my scripts.

Thanks.

Azim.


Comment on Re: File Locking
Re: Re: File Locking
by Anonymous Monk on Mar 19, 2004 at 15:03 UTC
    This was well written and informative, but I think I still see a bug in this code. After closing the file and opening it for read/write access but before putting the exclusive lock on it, someone else could swoop in and read the file...which would be minus your current entry, as it hasn't been added yet. After adding your entry, the previous persons then add theirs, minus yours. So the file is corrupted. Two alternatives: Append to the file instead of clearing it out OR open it for read/write from the very beginning with exclusive control. Thoughts?
      Thank you for your comment!!! I saw this bug also! but wasn't sure if it's a bug. read/write from the very beginning must be a good solution if it works, I haven't tried that out
      If you're pretty sure the friend is not on the list, then locking the file exclusive for the entire operation is a good idea.

      However, locking the file in LOCK_EX (instead of LOCK_SH) will limit the concurrency of the operation to 1. So if you just want to check if they are (not) on the list, a shared lock will scale better. Check again after you get an exclusive lock, to avoid race conditions. (For even better scalability, use a database.)

        OPTIMIST (likely to find the item in the list): Start with a shared lock, and without unlocking, "promote" to an exclusive lock once not found; make sure there is a timer on the attempt to prevent a deadlock, and if the timer goes off then unlock and retry from the top.

        PESSIMIST (unlikely to find the item in the list): Use the exclusive lock from the beginning.

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