Check if a file is already locked using flock()? [duplicate]

Question

This question already has answers here:

Test if file is locked (2 answers)

Closed 5 years ago.

I have a file I'm writing to, but I need to lock it first (using flock()), to prevent any other script from writing to it. So I have:

$file=fopen($file_p);

if (flock($file, LOCK_EX)) {//lock was successful
    fwrite($file,$write_contents);          
}

But I need to check if it's already locked, to prevent other scripts from writing to it.

How can I do this?

Answer 1

I would check to see if I couldn't obtain a lock on the file, like this:

if (!flock($file, LOCK_EX)) {
    throw new Exception(sprintf('Unable to obtain lock on file: %s', $file));
}

fwrite($file, $write_contents);

Answer 2

As described in the docs, use LOCK_NB to make a non-blocking attempt to obtain the lock, and on failure check the $wouldblock argument to see if something else holds the lock.

if (!flock($fp, LOCK_EX|LOCK_NB, $wouldblock)) {
    if ($wouldblock) {
        // something already has a lock
    }
    else {
        // couldn't lock for some other reason
    }
}
else {
    // lock obtained
}

Answer 3

Your flock call is the check to see if it's already locked. If it's locked, that if() statement would fail, so you could just throw an else on it with something like:

if (flock($file, LOCK_EX)) {//lock was successful
    fwrite($file,$write_contents);
} else {
    echo "$file is locked.";
}