I once thought that was the correct way to lock an SDBM file, until someone pointed out that there's a timing window 
    my $db = tie(%h, 'SDBM_File', 'filename', O_RDWR|O_CREAT, 0666)
       or die "Couldn't tie SDBM file 'filename': $!; aborting";
--> # right here
    flock($db->fd, LOCK_EX);

[download]
At first, you'd think this wasn't a hole. But opening an SDBM file (the last time I looked) actually causes some file I/O. So what happens in this window is that two processes have read some bookkeeping data from the file into memory, one process gets the lock and proceeds to do things that side effect bookkeeping, while the other process blocks. When the first process frees the lock, the second process unblocks, but has data in memory that is now inconsistent with what's on disk. Oops.

The way around this is to use a separate file for locking, and acquire the lock before opening the SDBM file.

I may be working off of stale info, though. Can someone who has access to SDBM source confirm that significant I/O happens when the file gets opened?

In reply to Re: Re: Locking a SDBM_File by dws
in thread Locking a SDBM_File by Brett Wraight

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.