File Locking

Trick #2: Only one process running at a time

Suppose you have an hourly cron job
But you don't want two instances running at the same time
- If the 8 AM job is still running at 9 AM, the 9 AM job should be cancelled
Easy solution:

        open SELF, "< $0" or die ...;
        flock SELF, LOCK_EX | LOCK_NB  or exit;

Try to get a lock on the program source code file
- If it's already locked, another instance is still running, so exit
- When the process that holds the lock exits, it releases the lock automatically

Trick #2a

        flock DATA, LOCK_EX | LOCK_NB  or exit;
        ...
        __DATA__