File Locking

Trap #2: Failing to check for failure

        flock FH, LOCK_EX;
        # modify the data

Serious problem here
- If the lock is unavailable, this waits until it is available
- If the lock is available, this gets the lock and continues immediately
But what if the system call fails for some reason?
- For example, if FH is not open?
Then it returns immediately
- The process then modifies the data
- Even though it does not have the lock!
Always check for failure!

        flock FH, LOCK_EX or die ...;

        if (flock FH, LOCK_EX) {
          # modify the file ...
        }