Usually, when a thread attempts to get access to a resource which is unavailable, such as a semaphore, mutex, critical section, or synchronization object, the thread stops running while waiting for the resource to become available. This is called blocking. Unless a thread happens to block at exactly the time its time-slice quantum has expired, blocking prevents the thread from completing a full time slice. When the resource waited on becomes available, the blocked thread is unblocked and runs the next time it is scheduled.