Safe Counter Follow Up

Hello Wise Monks! Here's the code I use to count. Somehow it resets to zero when 10 or more instances of it run at the same time. What's wrong with my code?

$completeadd = "counter.txt";
open(MFILE, "$completeadd") || die "file open failed: $!\n";
flock(MFILE, LOCK_EX) || die "Lock failed: $!";
@filedata1=<MFILE>;
chomp @filedata1;
close(MFILE);
if ($filedata1[0])
{
  $filedata1[0]=$filedata1[0] + 1;
}
else
{
  $filedata1[0] = 1;
}
open(MFILE, ">$completeadd") || die "file open failed: $!\n";
flock(MFILE, LOCK_EX) || die "Lock failed: $!";
print MFILE "$filedata1[0]";
close(MFILE);
[download]

Comment on Safe Counter Follow Up Download Code

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•Re: Safe Counter Follow Up by merlyn (Sage) on May 06, 2004 at 15:32 UTC
`open(MFILE, ">$completeadd") \|\| die "file open failed: $!\n"; flock(MFILE, LOCK_EX) \|\| die "Lock failed: $!";` [download] Yup, that's the culprit. You are zeroing the file before obtaining the flock, thus possibly stomping on someone else's read or write. Either use File::CounterFile, or read my columns that use flock for working examples. -- Randal L. Schwartz, Perl hacker Be sure to read my standard disclaimer if this is a reply.	[reply] [d/l]
Re: Safe Counter Follow Up by tachyon (Chancellor) on May 06, 2004 at 15:37 UTC
What's wrong with my code? You have several race conditions. You open the file. You have yet to apply the lock. Then you lock it. You read it then CLOSE THE FILE RELEASING THE LOCK BUT YOU HAVE NOT FINISHED! You then do stuff ;-) While you are doing this stuff any other program can open and lock the file. There are many solutions but in essence you need a 'lock' that persists until you have completed the update. Ie LOCK, read, increment, write, UNLOCK. This is a rather well worn chestnut. Could I suggest Super Search Gotta go. I have a boarding call.... cheers tachyon	[reply]
Re: Safe Counter Follow Up by Ovid (Cardinal) on May 06, 2004 at 15:33 UTC
Because you still have a race condition. KM has written a nice article about this. Cheers, Ovid New address of my CGI Course.	[reply]
Re: Safe Counter Follow Up by Ven'Tatsu (Deacon) on May 06, 2004 at 16:59 UTC
Take a look at the perlopentut man page (perlopentut on CPAN if perldoc is still down). Look in the section 'File Locking', there is example code for what you are trying to do.	[reply]
Re: Safe Counter Follow Up by Anonymous Monk on May 06, 2004 at 21:07 UTC
You'll want something like this: `#!/usr/bin/perl -w $\| = 1; use strict; use Fcntl qw(:flock); my $count_file = 'counter.txt'; open( my $fh, '+<', $count_file ) or die( "open failed: $!" ); flock( $fh, LOCK_EX ) or die( "flock failed: $!" ); ++ ( my $count = <$fh> ); seek( $fh, 0, 0 ) or die( "seek failed: $!" ); truncate( $fh, 0 ) or die( "truncate failed: $!" ); print $fh $count; close( $fh ) or die( "close failed: $!" );` [download]	[reply] [d/l]