PHP lock text file for editing? - php

Question

I've got a form that writes its input to a textfile.
Would it be possible to lock a text file for editing, and perhaps give a friendly message "the file is edited by another user, please try again later."
I'd like to avoid conflicts if the file has multiple editors at the same time.
Here's how the entry is currently added.
$content = file_get_contents("./file.csv");
$fh = fopen("./file.csv", "w");
fwrite($fh, $date_yy . '-' . $date_mm . '-' . $date_dd . '|' . $address . '|' . $person . '|' . $time_hh . ':' . $time_mm);
fwrite($fh, "\n" . $content);
fclose($fh);
Any thoughts?

Answer 1

You can use flock() function to lock the file. For more see this
Something like:
<?php
$content = file_get_contents("./file.csv");
$fp = fopen("./file.csv", "w"); // open it for WRITING ("w")
if (flock($fp, LOCK_EX))
{
// do your file writes here
fwrite($fh, $date_yy . '-' . $date_mm . '-' . $date_dd . '|' . $address . '|' . $person . '|' . $time_hh . ':' . $time_mm);
fwrite($fh, "\n" . $content);
fclose($fh);
flock($fh, LOCK_UN); // unlock the file
}
?>

Answer 2

In order of desirability:
Use a database.
Use more than one text file.
Use locks:
eg:
$lockwait = 2; // seconds to wait for lock
$waittime = 250000; // microseconds to wait between lock attempts
// 2s / 250000us = 8 attempts.
$myfile = '/path/to/file.txt';
if( $fh = fopen($myfile, 'a') ) {
$waitsum = 0;
// attempt to get exclusive, non-blocking lock
$locked = flock($fh, LOCK_EX | LOCK_NB);
while( !$locked && ($waitsum <= $lockwait) ) {
$waitsum += $waittime/1000000; // microseconds to seconds
usleep($waittime);
$locked = flock($fh, LOCK_EX | LOCK_NB);
}
if( !$locked ) {
echo "Could not lock $myfile for write within $lockwait seconds.";
} else {
// write out your data here
flock($fh, LOCK_UN); // ALWAYS unlock
}
fclose($fh); // ALWAYS close your file handle
} else {
echo "Could not open $myfile";
exit 1;
}

Answer 3

You can use PHP's flock function to lock a file for writing, but that lock won't persist across web requests and doesn't work on NFS mounts (at least in my experience).
Your best be may be to create a token file in the same directory, check for its existence and report an error if it exists.
As with any locking scheme, you're going to have race conditions and locks that remain after the operation has completed, so you'll need a way to mitigate those.
I would recommend creating a hash of the file before editing and storing that value in the lock file. Also send that hash to the client as part of the edit form (so it comes back as data on the commit request). Before writing, compare the passed hash value to the value in the file. If they are the same, commit the data and remove the lock.
If they are different, show an error.

Answer 4

You could try flock — Portable advisory file locking ?
http://php.net/manual/en/function.flock.php

Answer 5

I would just use a simple integer or something like that.
$content = file_get_contents("./file.csv");
$fh = fopen("./file.csv", "w");
$status = 1;
...
if($status == 1){
fwrite($fh, $date_yy . '-' . $date_mm . '-' . $date_dd . '|' . $address . '|' . $person . '|' . $time_hh . ':' . $time_mm);
fwrite($fh, "\n" . $content);
fclose($fh);
$status = 0;
}
else
echo "the file is edited by another user, please try again later.";
Is that what you mean?