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RE (tilly) 1: Creating a file with Open

by tilly (Archbishop)
on Oct 03, 2000 at 16:04 UTC ( #35087=note: print w/replies, xml ) Need Help??


in reply to Creating a file with Open

Everyone has already said to include $! in the error message. This is important but it is only a start.

You should also do as perlstyle says and include $user_name.

And personally I would begin sweating at trusting user input for filenames. Have you verified that the name looks like what you think it should? See perlsec for more on that issue.

And btw that code for flocking will not give you the protection you want, open it "+<$user_name" or else it gets wiped out before you try to lock it.

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(Guildenstern) RE: RE (tilly) 1: Creating a file with Open
by Guildenstern (Deacon) on Oct 03, 2000 at 17:58 UTC
    And btw that code for flocking will not give you the protection you want, open it "+<$user_name" or else it gets wiped out before you try to lock it.
    I'm confused here. Everyone else has been saying "+>$user_name" which makes sense to me. Is your suggestion a typo, or am I missing something fundamental here?

    Guildenstern
    Negaterd character class uber alles!
      My point is not a typo.

      Both "+<$user_name" and "+>$user_name" open the file for both reading and writing, but the first opens it for reading, and you can also write, while the second opens it for writing, but you can also read. Therefore the second wipes out the possibly existing file for you, while the first does not.

      Wiping out the file before you lock it and ensure it is not in use is unlikely to be desired. I call that, ERACE. (Sorry, didn't even try to hold that one back. :-)

      If this confuses you, just use sentinel files as in Simple Locking since I already dealt with all of this there.

      EDIT
      Somehow I said "suggestion" where I meant "typo". Fixed.

      EDIT 2
      Some days I should not post. Thanks isotope for pointing out I meant "while the first does not". *sigh*

      I'm confused here. Everyone else has been saying "+>$user_name" which makes sense to me. Is your suggestion a typo, or am I missing something fundamental here?
      No, not a typo. If you open something with +>, you are saying "zero out this file" before anything else happens. At that point, if someone else has the flock already, you are now erasing their work. You must open in a way that preserves any existing data, but if you're also going to write to it, you must use two-way mode, like +< or +>>.

      -- Randal L. Schwartz, Perl hacker

      Opening the file "+>" will wipe it out, so flocking it is then pretty pointless - you destroy the file before you've gained the lock on it.

      You need to open the file "+<", if that failed then open it "+>" then flock it then truncate it if you want to be safe with your flocking in the presence of other readers.

      Note that if you want to read the file back in with flocking you need to use "+<" so that you open the file with write intent otherwise the flocking may not work properly (I don't understand the reasons for this but that is what it says in the man pages!)

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