file_exists doesn't seem to work correctly

Question

I've got a php-script that i want to execute, only if an other instance of the script isn't already running. I've got this code:

$lockfile = __DIR__.'/lock.file';

if(file_exists($lockfile) == false)
{
    echo 'no lockfile. Job will execute.';
    $handle = fopen($lockfile, 'x') or die('Cannot open file:  '.$lockfile);
    $data = 'This is the lockfile';
    fwrite($handle, $data);
    fclose($lockfile);

    createLinks();

    unlink($lockfile);
}
else
{
    echo 'Lockfile present, job will not execute. Please try again later';
}

But it doesn't correctly check if the file exists. If i call the script with my browser, the lock.file is correctly created (i can see it with ftp), and afterwards deleted. However, i still can run the script multiple times simultaneously. If i create a lock.file myself with ftp, it doesn't execute the script. I've been searching for hours now, what can it be? Maybe i'm stupid, but i think this should work, right?

Edit: Jep, flock did it. Thanks very much! Final code:

$lockfile = __DIR__.'/lock.file';
$handle = fopen($lockfile, "x");

if(flock($handle, LOCK_EX))
{
    echo 'no lockfile. Job will execute.';

    $data = 'This is the lockfile';
    fwrite($handle, $data);

    createLinks();

    fclose($lockfile);  
    unlink($lockfile);
}
else
{
    echo 'Lockfile present, job will not execute. Please try again later';
}

The code does not handle race conditions. You should assume you can `fopen` correctly and just handle a return code if it didn't work. [This article](http://blog.rodneyrehm.de/archives/12-Improving-Disk-IO-in-PHP-Apps.html) notes it. — Waleed Khan, Mar 03 '13 at 18:25
You are looking for `flock()` http://php.net/manual/en/function.flock.php — Tchoupi, Mar 03 '13 at 18:26

score 4 · Answer 1 · edited Mar 03 '13 at 22:09

4

From the documentation of file_exists:

Note: The results of this function are cached. See clearstatcache() for more details.`

and:

You should also note that PHP doesn't cache information about non-existent files. So, if you call file_exists() on a file that doesn't exist, it will return FALSE until you create the file. If you create the file, it will return TRUE even if you then delete the file. However unlink() clears the cache automatically

Try changing your function to:

if(file_exists($lockfile) == false)
{
    clearstatcache();
    ...

Have a look at clearstatcache().

edited Mar 03 '13 at 22:09

Lightness Races in Orbit

378,754
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answered Mar 03 '13 at 18:27

Marko D

7,576
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Please stop using code blocks for things that are not _code_. – Lightness Races in Orbit Mar 03 '13 at 22:08
Oh, sorry, I just realised now from your edit what is the proper way to write it, thanks! – Marko D Mar 03 '13 at 22:19
I managed to find it via Google (not using FAQ link here), so now I know how it works, thanks! – Marko D Mar 03 '13 at 23:34

score 1 · Answer 2 · edited May 23 '17 at 12:14

I've waisted today half a day dealing with almost the same problem as yours one. My PHP code also used file locks and had file_exists() call in it. And I was having the same problem with "strange" file_exists() results. I was completely confused when I changed the usage of files (to gain a lock) to the usage of the DB - a lock corresponded to a row in a table. The results were fully the same - I tried to launch the code concurrently in different tabs of a browser, but I still failed to gain the lock.

And then I realized what was the real reason of such strange behavior. In my case (it looks like that your case was the same) the main problem was in the fact that the script was launched "multiple times simultaneously" but in one browser. But requests were processed not concurrently but sequentially. A good explanation of such effect was given here: https://stackoverflow.com/a/3623584/1758680

So as all requests became sequentially - each request was waiting a preceding one to end. Each lock was released before the next script would start and try to gain the lock.

Thanks for your comment. However, this was not my problem and it's not the solution. As you can see, my scripts weren't run consecutively, they would just ignore the presence of a lockfile and start running. This didn't have anything to do with the order the process where running in, it had to do with the fact that fopen isn't the right way to create and search for the lockfile. flock did the trick, like i explained above. — wjhulzebosch, Aug 07 '13 at 13:26

score -1 · Answer 3 · answered Mar 03 '13 at 18:28

-1

Try

$lockfile = 'lock.file';

or other..

answered Mar 03 '13 at 18:28

Greg

1

file_exists doesn't seem to work correctly

3 Answers3