Editing a common temp file in php an preventing conflicts

Question

I want to have a temp file that gets updated from time to time. What I was thinking of doing is:

<!-- language: lang-php -->
// get the contents
$s = file_get_contents( ... );

// does it need updating?
if( needs_update() )
{
    $s = 'some new content';
    file_put_contents( ... );
}

The issue that I could see happening is that whatever condition causes 'needs_update()' to return true could cause more than one process to update the same file at, (almost), the same time.

In an ideal situation, I would have one single process updating the file and prevent all other processes from reading the file until I am done with it.

So as soon as 'needs_update()' return true is called I would prevent others processes from reading the file.

<!-- language: lang-php -->
// wait here if anybody is busy writing to the file.
wait_if_another_process_is_busy_with_the_file();

// get the contents
$s = file_get_contents( ... );

// does it need updating?
if( needs_update() )
{
    // prevent read/write access to the file for a moment
    prevent_read_write_to_file_and_wait();

    // rebuild the new content
    $s = 'some new content';
    file_put_contents( ... );
}

That way, only one process could possibly update the file and the files would all be getting the latest values.

Any suggestions on how I could prevent such a conflict?

Thanks

FFMG

Answer 1

You are looking for the flock function. flock will work as long as everyone that acess the file is using it. Example from php manual:

$fp = fopen("/tmp/lock.txt", "r+");

if (flock($fp, LOCK_EX)) {  // acquire an exclusive lock
    ftruncate($fp, 0);      // truncate file
    fwrite($fp, "Write something here\n");
    fflush($fp);            // flush output before releasing the lock
    flock($fp, LOCK_UN);    // release the lock
} else {
    echo "Couldn't get the lock!";
}

fclose($fp);

Manual: http://php.net/manual/en/function.flock.php