The 0 initied semaphore indicates that any process that uses sem_wait will have to wait until someone uses sem_post
If the semaphore is 0, anyone using sem_wait will have to wait until it becomes greater than 0. The only restriction of a semaphore is that it's value can not be less than zero. If the value of a semaphore is currently zero, then a sem_wait(3) operation will block until the value becomes greater than zero. Two operations can be performed on semaphores: increment the semaphore value by one (sem_post(3)) and decrement the semaphore value by one (sem_wait(3)). That zero means "unlocked" if you're using semaphores sensibly, it implies that nobody is using that data right now so you can lock the semaphore and go ahead.įrom the linux manpage sem_overview(7) which i consider a reliable source of information.Ī semaphore is an integer whose value is never allowed to fall below zero. It is common to decide to interpret a zero (or other such nullesque value) valued semaphore to mean "nobody is using this data right now". If you're using a semaphore system that someone else wrote, they will probably have already decided what a zero semaphore means and you should check the documentation.
not a simple yes/no), but a counting semaphore, then zero might mean "right now, there are zero accesses to this data going on" or zero might mean "there are no resources available so you can't have any". If you have decided that this is not a binary semaphore (i.e. If you have decided that you want "zero" to mean "the data this semaphore is referring to is being used, so you can't have the data" then that's what it means. If you have decided that you want "zero" to mean "the data this semaphore is referring to is not being used, so you can set the value of the semaphore to one and then you can go ahead and use the data" then that's what it means. If you initialise a semaphore value to zero, it means that when you look at it, it will have the value zero.
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