flock() keeps waiting for lock to be released - php

Question

so i want to lock a file so i can see that a php process is already running. The example code looks as follows:
<?php
$file = fopen("test.txt","w+");
if (flock($file,LOCK_EX))
{
fwrite($file,"Write something");
sleep(10);
flock($file,LOCK_UN);
}
else
{
echo "Error locking file!";
}
fclose($file);
?>
But the problem is that when i executed this file, and execute the file again the second waits for the first one to be done with the lock. So then both are successfully executed. But only the first one has to be executed successfully. Anyknow know how to do this?

Answer 1

It sounds like you don't want flock to be blocking? You just want the first process to obtain the lock and the second one to fail?
To do that, you can use the LOCK_NB flag to stop the flock call from blocking:
$file = fopen("test.txt","r+");
if (flock($file,LOCK_EX | LOCK_NB))
{
fwrite($file,"Write something");
sleep(10);
flock($file,LOCK_UN);
}
else
{
echo "Error locking file!";
}
fclose($file);
More info is available on the PHP flock doc page - http://php.net/manual/en/function.flock.php

Answer 2

You could use a second file called e.g. "test.txt.updated" and maintain the state there -- i.e., whether "test.txt" was already updated or not. Something like the following. NOTE, I've opened "text.txt" in append-mode to see whether two concurrent runs really wrote only once to the file.
<?php
if (! ($f = fopen("text.txt", "a")) )
print("Cannot write text.txt\n");
elseif (! flock($f, LOCK_EX))
print("Error locking text.txt\n");
else {
print("Locked text.txt\n");
if (file_exists("text.txt.updated"))
print("text.txt already updated\n");
else {
print("Updating text.txt and creating text.txt.updated\n");
fwrite($f, "Write something\n");
if ($stamp = fopen("text.txt.updated", "w"))
fwrite($stamp, "whatever\n");
else
print("Oooooops, can't create the updated state file\n");
sleep(10);
}
flock($f, LOCK_UN);
}
?>