[dragontamer]

> but couldn't you make an idempotent semaphore by changing the signature to be increment/decrement(source, from, to)

Think about a mutex lock: if the semaphore (aka: mutex) is already locked, your signature is: sem_wait(mutex, 0, 0).

Which is not idempotent. Two "sem_waits(s, 0, 0)" mean that you need two (other) threads to unlock you. One sem_wait(s, 0, 0) means that only one other thread needs to unlock you.

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Semaphores used in this manner are how you implement reader/writer locks, as well as thread barriers. (If 100 threads are in existence, you wait for 100 semaphore_posts from those 100 other threads).

None of the semaphores or mutexes are idempotent. And never can be.