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Addict
Not Yet a God
Joined: 21 Jan 2004
Posts: 473
  
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 Posted: Mon Apr 25, 2005 9:58 pm Post subject: |
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True, but who cares, just change it, if you use it.  |
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Dazzy
Agent
Joined: 09 Jan 2004
Posts: 1731
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| Posted: Tue Apr 26, 2005 7:07 am Post subject: |
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| Code: | | print "<b>There have been ".$counter->{Hits}." hit\(s\) to this page!</b>\n\n"; |
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mattaustin
Sentinel
Joined: 19 Jul 2004
Posts: 556
Location: Los Angeles, CA
votes: 1
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| Posted: Tue Apr 26, 2005 7:12 am Post subject: |
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lol good one daz
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eric256
The Keymaker
Joined: 03 May 2006
Posts: 2292
Location: Colorado
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| Posted: Tue Apr 26, 2005 2:22 pm Post subject: |
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When building counter scripts you should learn about locking. The problem is that web servers may run two of your script at the same time..
It is easier to see with an example.
| Code: | | <br />use Data::Dumper;<br />my $counter = do("./hits.dat"); #<- line a<br />sub Save<br />{<br /> open SAVE, "> ./hits.dat"; #<-- line b<br /> print SAVE Dumper($counter);<br /> close(SAVE);<br /> my $counter = do("./hits.dat");<br />}<br /> $counter->{Hits}++; Save;<br /> print "Content-type: text/html\n\n";<br /> print "<b>There has been ".$counter->{Hits}." to this page!</b>\n\n";<br /> |
So lets say that the first instance of the script starts. It gets to A and all is good. Now it opens the file to save but before it gets a chance to save the new counter data and second instance is started. The file is now empty because the first script opened it with > which blanks out the file. The second isntance gets to line A and find the file empty. The first instance gets control agian and saves the correct value for it and finishs. Now the second instance gets back control and saves its value, but its value is 0 because it opened an empty file. Now it saves that 0 over the correct number saved by A.
I know it sounds lke a long shot but if you page gets called a few times in a rush by one or more people then the odds of it dieing like this become very high. Locking would allow you to only have one instance reading and writing to the file at the same time. Look up flock on perldoc.org for more info.
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Eric256
Proud previous owner and current admin of Bot-depot.com |
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Dazzy
Agent
Joined: 09 Jan 2004
Posts: 1731
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| Posted: Tue Apr 26, 2005 2:35 pm Post subject: |
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it would have to be Very quick though i guess?
I mean, the amount of hashs you record in a second is quite a few....less that half a second i guess.....But the risk is still there... |
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Cer
Upgraded Agent
Joined: 03 Feb 2004
Posts: 3776
Location: Michigan
votes: 4
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| Posted: Tue Apr 26, 2005 6:46 pm Post subject: |
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What I do for file locking is use Fcntl ':flock';
| Code: | | use Fcntl ':flock';<br /><br />use Data::Dumper;<br />my $counter = do("./hits.dat"); #<- line a<br />sub Save<br />{<br /> open SAVE, "> ./hits.dat"; #<-- line b<br /><br /> # Gain exclusive lock (only this script has access)<br /> flock (SAVE, LOCK_EX);<br /><br /> print SAVE Dumper($counter);<br /><br /> # Unlock the file.<br /> flock (SAVE, LOCK_UN);<br /><br /> close(SAVE);<br /> my $counter = do("./hits.dat");<br />}<br /> $counter->{Hits}++; Save;<br /> print "Content-type: text/html\n\n";<br /> print "<b>There has been ".$counter->{Hits}." to this page!</b>\n\n"; |
I found that from some online tutorial but haven't properly tested it (or haven't purposely tested it). So if somebody sees something wrong there they can just reply about it. Probably File::Lock is an easier way to do it too.
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eric256
The Keymaker
Joined: 03 May 2006
Posts: 2292
Location: Colorado
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| Posted: Wed Apr 27, 2005 5:14 am Post subject: |
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Dazzy it is more likely to occur than you think. Plus if it occurs once and you arn't prepared you loose everything. Wanna take that risk?
Cer - The problem with that is that you are only locking it for the writing portion, which doesn't fix anything. Instead the normal way is to use a semaphore (or a flag if you prefer). At the start of your script you open a file (you can open the script itself if you want) and lock that file handle. Then do whatever you need to do. When the script ends the lock is released. This way your script will never have two instances running at once. If you have alot happening in your script you can follow the same idea but only lock the file for the portion where you read, add, and write. You lock a seperate file because you need to open the counter file 2 and you want to make sure that the value hasn't changed since you last read the file (meaning a nother script squeezed in the middle).
It is a kinda confusing concept I know but trust that it can happen as i have had it happen to me before. Of course I was saving the whole structure of a portol and not just a counter so I was quite upset when i figured out what happened. ;(
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draget
Not Yet a God
Joined: 29 Dec 2004
Posts: 367
Location: Australia
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| Posted: Wed Apr 27, 2005 10:42 am Post subject: |
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CGI/CGI::Carp ->  |
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Cer
Upgraded Agent
Joined: 03 Feb 2004
Posts: 3776
Location: Michigan
votes: 4
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| Posted: Wed Apr 27, 2005 11:32 am Post subject: |
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QUOTE(draget @ Apr 27 2005, 06:42 AM) CGI/CGI::Carp -> [right][snapback]48050[/snapback][/right] |
I don't see how CGI::Carp has anything to do with it.
Eric:
So locking the entire script would be like this?
| Code: | | #!/usr/bin/perl -w<br />use Fcntl ':flock';<br />open (SELF, "file.cgi");<br />flock (SELF, LOCK_EX);<br /><br /># do other file operations<br />open (FILE, ">file.dat");<br />print FILE "test";<br />close (FILE);<br /><br />__END__ |
Or is that still off?
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