Fellow monks, I would greatly appreciate your help in this problem of mine. I need to produce a piece of code that excercises flock() since I suspect a bug in one of the platforms I support.

Please take a look at the code below...

#!/usr/bin/perl

use strict;
use warnings;
use IO::File;
use Fcntl qw(:flock);

our $lock_file = 'test_lck';

                # Get rid of our garbage
END {
    unlink ($lock_file, $lock_file . '.A', $lock_file . '.B');
};

our %fh;
my $id = 'A';
my $pid;
my $buf = 'nothing';

$|++;

                # Spawn one helpful kid...

while (($pid = fork()) == -1)
{
    sleep 2;
}

unless ($pid)
{                # This is our child
    ++ $id;

    END {};

    my $fh = new IO::File "$lock_file.$id", "+>"
    or die "Failed to open $lock_file.$id: $!\n";

    print "$id: about to lock...\n";

    unless (flock($fh, LOCK_EX | LOCK_NB))
    {
        die "$id: Failed to acquire lock: $!\n";
    }

    print "$id: done\n";

    $fh->syswrite("$id\n", 10);
    $fh->seek(0, 0);
    sleep 3;
    $fh->sysread($buf, 10);
    chomp $buf;

    print "$id: got <$buf>\n";

    flock($fh, LOCK_UN);
    $fh->close;

    exit 0;
}

my $fh = new IO::File "$lock_file.$id", "+>"
    or die "Failed to open $lock_file.$id: $!\n";

sleep 1;

print "$id: about to lock...\n";

unless (flock($fh, LOCK_EX | LOCK_NB))
{
    die "$id: Failed to acquire lock: $!\n";
}

print "$id: done\n";

$fh->syswrite("$id\n", 10);
$fh->seek(0, 0);
$fh->sysread($buf, 10);
sleep 2;
chomp $buf;

print "$id: got <$buf>\n";

flock($fh, LOCK_UN);
$fh->close;

wait();                # Collect our child status

exit 0;

[download]

In my machine, it produces the following output:

bash-2.05a$ ./flock-test.pl 
B: about to lock...
B: done
A: about to lock...
A: done
B: got <B>
A: got <A>

But I have been moving/changing/adding sleep()s to father and child in order to have them invert what they get from the locked file (ie, to say something like B: got <A>) to no avail.

What am I doing wrong? More to the point, why is this code working at all?

Best regards

-lem, but some call me fokat

In reply to Testing flock by fokat

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