Programming Tutorials

Locking files in PHP

By: Andi, Stig and Derick in PHP Tutorials on 2008-11-23  

While writing to files that are possibly being read by other scripts at the same time, you will run into problems at some point because a write might not totally be completed while another script is reading the same file. The reading script will only see a partial file at that moment. Preventing this problem is not hard to do, and the method for this is called locking.

PHP can set locks on files with the flock() function. Locking a file prevents a reading script from reading a file when it is being written to by another script; the only prerequisites for this is that both scripts (the reader and the writer) implement the locking. A simple set of scripts may look like this:

<?php /* writer */
while (true) {
$fp = fopen('testfile', 'w');
echo "Waiting for lock...";
flock($fp, LOCK_EX);
echo "OK\n";

flock($filepointer, LOCK_EX); tries to acquire an exclusive lock on the file and blocks until this lock can be acquired. An exclusive lock will only be granted if there are no other locks on the file.

$date = date("Y-m-d H:i:s\n");
echo $date;
fputs($fp, $date);
sleep(1);
echo "Releasing lock...";
flock($fp, LOCK_UN);
echo "OK\n";

After we write to the file, we can release the lock with flock($fp,LOCK_UN);:

fclose($fp);
usleep(1);
}
?>

<?php /* reader */
while (true) {
$fp = fopen('testfile', 'r');
echo "Waiting for lock...";
flock($fp, LOCK_SH);
echo "OK\n";

Here, we request a shared lock. This lock will not be granted if there is an exclusive lock set on this file, but it will be granted if there is another shared lock, or no lock at all on this file. This means that it is possible to have multiple readers reading from the file at the same time, unless a writer process locks the file with its exclusive lock.

echo fgets($fp, 2048);
echo "Releasing lock...";
flock($fp, LOCK_UN);
echo "OK\n";
fclose($fp);
sleep(1);
}
?>

At the end of the script, we sleep for 1 second so that we are not using 100 percent CPU time.