Question

This may be silly, but I’m looking for so-called ‘real life’ examples of software which uses a semaphore with a count or more than 1.

In other words, no bouncers, librarians or toilet examples please.

Answer 1

I think I’ve used something like this for a “read/write” lock. That is, I want to allow up to N simultaneous readers, and only one writer at a time when there are no active readers.

So any reading thread locks the semaphore and increases the read count. If the read count is smaller than N the lock succeeds and the reader may continue. If not, the lock causes the reader thread to sched_yield.

If I want to lock for writing, I set the count to N so that no more readers can lock – e.g. they all start sleeping when they try locking. The writer thread keeps trying to lock the read semaphore. Every time a reader releases, the writer increments N again, until there are no more readers.

Then the write lock succeeds, the write operation happens, and the read semaphore is set back to zero so that reader threads may continue.