Checking if the file exists after the flock is not good enough. Assume the check is done with a stat. 
child 1   child 2   child 3
open
          open
flock                         (works for obvious reasons)
stat                          (works for obvious reasons)
unlink
close
                    open      (creates a new file)
                    flock     (works, no contention on new file)
                    stat      (works, new file exists)
          flock               (works, no contention on old file)
          stat                (works, new file exists)

In reply to Re^2: flock LOCK_EX not locking exclusively by Anonymous Monk
in thread flock LOCK_EX not locking exclusively by Anonymous Monk

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