JAVA-BASED REAL-TIME PROGRAMMING

give
give
give
3.2. Resources and mutual exclusion – Semaphores
T1 signal
T2
take
take
take
take
Figure 3.2: A thread T1 signaling to thread T2 via a counting semaphore. The
counter is assumed to be initialized to zero, resulting in the first call of take
blocking its caller. For each give, one take can be called without blocking.
of give but T2 never runs in advance of T1. This type of semaphore is referred
to as a counting semaphore.
Sometimes we do not want the signaled thread, T2 in this case, to catch
up when give has been called multiple times. That can be accomplished by
using a boolean instead of a counter as the state variable of the semaphore.
That corresponds to the counter being one binary digit, and such a semaphore
is therefore usually called a binary semaphore.
A less usual type of semaphore is the multistep semaphore, which is useful
when we want successive calls of take to be atomic. For instance, a producing
thread signals whenever a byte of data is put in a buffer, but the consuming
thread is reading a unicode (two bytes per character) string. We then want
to call take(2) to only get unblocked when there are two bytes available. One
can always solve the problem without such a multistep feature, but using a
multistep semaphore can be more convenient and efficient.
Mutual exclusion
Perhaps the most well-known usage of semaphores is for mutual exclusion.
Any of the semaphore types used for signaling can be used for mutual exclusion,
but there are reasons for providing a special mutual exclusion semaphore.
Such a semaphore is sometimes referred to as a ’mutex’. We will call it a mutex
semaphore. As depicted in Figure 3.3, the very same thread that has called
mutex.take(), thereafter having exclusive access to the shared data, should call
mutex.give() when the critical section is left. Any one activity not following
this protocol, for instance by calling give one extra time before take, would
damage the whole application. Therefore, it is a good idea to have a dedicated
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