Good Information on Linux Semaphores
What is a Semaphore? An article I found here is very useful. It says that “Semaphores can be thought of as simple counters that indicate the status of a resource. This counter is a protected variable and cannot be accessed by the user directly.”
This recently came up when the Dell OpenManage storage service would not start on a Linux system. It complained that there were no more Linux semaphores available. Check out the article for more information.