My experience is that while locking is conceptually simple, virtually nobody ever gets it right. Read the thread starting at RE: RE: Flock Subroutine for a description of some common mistakes. The general theme is that you should lock overall tasks, and not access operations. For instance in your example above each process should get an exclusive lock before starting to read the file, and should not lose it until they are done writing. That is the only way to avoid races. Also remember that a close loses the lock. And put in error checks, flock can fail for many hard to spot reasons. (eg On Linux trying to lock a file that is available through NFS.)

Some old code of mine which does an OK job of this is at Simple Locking. It uses the sentinal lockfile approach.

Oh right, and if you can you want to use databases, not flatfiles. But you probably knew that...

In reply to Re (tilly) 1: File locking, lock files, and how it all sucks by tilly
in thread File locking, lock files, and how it all sucks by tocie

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.