Editing Semaphore (programming) (section)

===Important observations===
When used to control access to a [[pool (computer science)|pool]] of resources, a semaphore tracks only ''how many'' resources are free. It does not keep track of ''which'' of the resources are free. Some other mechanism (possibly involving more semaphores) may be required to select a particular free resource.

The paradigm is especially powerful because the semaphore count may serve as a useful trigger for a number of different actions. The librarian above may turn the lights off in the study hall when there are no students remaining, or may place a sign that says the rooms are very busy when most of the rooms are occupied.

The success of the protocol requires applications to follow it correctly. Fairness and safety are likely to be compromised (which practically means a program may behave slowly, act erratically, [[hang (computing)|hang]], or [[crash (computing)|crash]]) if even a single process acts incorrectly. This includes:
* requesting a resource and forgetting to release it;
* releasing a resource that was never requested;
* holding a resource for a long time without needing it;
* using a resource without requesting it first (or after releasing it).

Even if all processes follow these rules, ''multi-resource [[deadlock (computer science)|deadlock]]'' may still occur when there are different resources managed by different semaphores and when processes need to use more than one resource at a time, as illustrated by the [[dining philosophers problem]].