Use an external file to lock the DB File, as described in RE: File Locking (using the Search box wouldl have given you an answer right away BTW).

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Ilya Martynov, ilya@iponweb.net
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For what it's worth, in my experience/experiments, using DB_File with the lastest version of the Berkeley DB(4x), there really isn't any need to do any sort of locking when all you want is to read.

The Berkeley DB mechanisms take care of any potential data corruption.

In the example below, run #1 and then #2 while #1 is still running (preferably in 2 separate shells -- simultaneous)

# 1
perl -MFcntl -MDB_File -e"tie%a,'DB_File','rat.test',O_CREAT|O_RDWR or
+ die $!;for(1..1_000){select undef,undef,undef,.01;$a{a}++;}"

# 2
perl -MFcntl -MDB_File -e"tie%a,'DB_File','rat.test',O_RDONLY or die $
+!;warn $a{a}"
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When you first run #1, and then #2 ( while #1 is still running), #2 will yield
Warning: something's wrong at -e line 1.
every time you invoke it, whilst #1 is still running.
After #1 is finished running, then run #2 again, and it will yield 1000 at -e line 1.

In my programs, I freely tie a DB_File for O_RDONLY, but for O_RDWR, I use a flocking mechanism like DB_File::Lock.

MJD says you can't just make shit up and expect the computer to know what you mean, retardo! ** The Third rule of perl club is a statement of fact: pod is sexy.

#!/usr/bin/perl -w
use strict;
use Fcntl ':flock'; # import LOCK_* constants

sub lock {
    flock(FH,LOCK_EX);
    # and, in case someone appended
    # while we were waiting...
    seek(FH, 0, 2);
}

sub unlock {
    flock(FH,LOCK_UN);
}

my $filename = "plain-db.pdb";

open(FH, ">>$filename")
    or die "Can't open file ($filename): $!";

lock();

# do what ever you want with it 

unlock();
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lock();
# do what ever you want with it 
unlock();
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If "whatever you want" involves tieing a DB_File, you're in for trouble. Some versions of BerkeleyDB close the file and reopen it, which loses the lock for you. This took me a lot of pain and agony to discover, but fortunately not too much data was corrupted before I figured it out.

Using a separate lock file is good. The mod_perl guide is good. Heed the other monks.

Update: If you close the same file you lose the lock, even if you open it through a different filehandle and close that one. Locks exist at the inode level, not the filehandle level.

Sorry,
flock works on filehandles, if you close the filehandle
you're in trouble (the lock will be released). That's right.

But only if you want to risk data corruption

That is the classic perl flock boo-boo. You should never "unlock", just close the file, and the lock will go away.

Also, do you really need a lock subroutine?

I didn't respond to this the other day in hopes that somebody else would, and much to my suprise, nobody paid attention.

MJD says you can't just make shit up and expect the computer to know what you mean, retardo! ** The Third rule of perl club is a statement of fact: pod is sexy.

Hope this helps someone.

-anelson

There is a snippet in Programming the Perl DBI. I'm not sure how up-to-date that is, but you may try:

my $db = tie %hash, 'DB_File', ...;

my $fd = $db->fd();
open my $handle, "+<&=$fd" or die ...

flock $handle, LOCK_WHATEVER;
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