You're going to have trouble with this scenario. There's no good portable way to check if a file is being modified by someone else, especially if you're dealing with network mounted filesystems of any flavor. (And no, lsof isn't a good portable way, but you might check its man page if it's available anyway.)

The most secure method would be to centralize all read-write responsibilities to one process, such as a daemon. That's what databases are: one process that can manipulate the files in a centralized way.

Barring that, you'll have to erect some systems involving baling wire and twine. Er, I mean chmod, rename and/or creating/unlinking auxiliary files. The only benefit to these operations is that they are, for the most part, atomic on all useful filesystems.

As for flock(), it's not even guaranteed to work. It's a "suggestion" or "advisory" and not a stable effective lock. If it works, great, but don't count on it working in all situations.

--
[ e d @ h a l l e y . c c ]

In reply to Re: file locking by halley
in thread file locking by Tomtom

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.