This is similar to the discussion of locking when writing in Camle 3, where it states that the simpler open, flock sequence used for reading isn't adequate. BUT I believe OP's use of a separate semaphore file got around that issue, as he never does anything with the counter file until the flock on the semaphore file returns.

--Bob Niederman, http://bob-n.com

All code given here is UNTESTED unless otherwise stated.