Using Semaphores

This document describes how to use semaphores.

Semaphores are provided to synchronise co-operating threads. They are Kernel objects and, as such, are managed by the Kernel.

Access to a semaphore is through an RSemaphore handle.

The semaphores are counting semaphores, having a TInt count value that is incremented by calling the Signal() member function of the semaphore handle and decremented by calling the Wait() member function of the semaphore handle. A semaphore with a negative value implies that a thread must wait for the completion of some event.

The mechanism by which a thread waits on a semaphore is part of the overall management of thread scheduling.

For general applications, it is not common to explicitly create a semaphore; most applications are interested in making asynchronous requests to service providers. When a thread is created, a request semaphore is also created that support asynchronous requests.

After making one or more asynchronous requests, a thread calls User::WaitForAnyRequest() to wait for one of the requests to complete or calls User::WaitForRequest() to wait for a specific request to complete. A service provider calls User::RequestComplete() to signal request completion.