usage of flock

Hello All, I am confused with the flock function and its usage. I have jobs which are stored in a database, these jobs are run by a series of job_runners scripts but sometimes the job_runners stop ( server or php crash-down). So i put a job_controller in crontab to check regularly if the runners run. But after a while I have a bunch of job_controller running, so to avoid that I tried to use flock. I try to put this in the job_controller: $wouldblock=1; $f=fopen("controller.lock", "r"); flock($f, LOCK_EX+LOCK_NB, $wouldblock) or die("Error! cant lock!"); hoping that as long as the first job_controller run or don't close the file handle, a second job_controller won't be able to lock the controller.lockfile and die, but it didn't work. I also try this: $wouldblock=1; $f=fopen("controller.php", "r"); flock($f, LOCK_EX+LOCK_NB, $wouldblock) or die("Error! cant lock!"); hoping the first job_controller will lock it-self, but it didn't work. I also thought of writing in the lock file the PID of the first job_controller and then compare it and if it doesn't match then die, but my main concern is , if the server crash down the "surviving" lock file will prevent any job_controller to start. So how could prevent multiple instance of the same script? Is flock the best way? Thanks in advance for your time and help cheers yvan