JAVA-BASED REAL-TIME PROGRAMMING

3.2. Resources and mutual exclusion – Semaphores
queue depends on the scheduling of that particular run-time system. Normally,
threads are ordered according to priority, and equal priority threads
are placed in the order they arrived (FIFO).
Supporting timeout
In addition to the core methods, for convenience, the following method (return
type boolean) is also defined:
tryTake(long timeout) The same as take, but the caller gives up taking
the semaphore after timeout milliseconds. If the semaphore was taken,
true is returned. Otherwise false is returned.
Using this method requires some care. First, there is no guarantee that the
caller is unblocked exactly after the timeout time, it can be later (but not
earlier). The release time may depend on what other threads that have issued
blocking calls with timeouts, the CPU load, etc. Thus, the same type of
problems as with the Thread.sleep method.
Secondly, the returned boolean value must be taken care of (which is not
checked by the compiler in Java, just like in C/C++/C#). If false is returned,
perhaps after a long time of operation, and not considered due to a
programming error, shared resources can by accident be accessed without mutual
exclusion. The name tryTake, instead of overloading on take for instance,
is selected to emphasis the characteristics of the operation.
Finally, consider if your design is right if you use the tryTake operation.
It should be used instead of implementing additional timer threads, but not
for cyclic polling of the semaphore value. Some systems provide a way of
polling the value without actually trying to take the semaphore. Since such a
method easily could be misused by unskilled programmers, such a method is
not provided here.
Semaphores in other systems
When proposed by Dijkstra, the original name of the take operation was P,
from the Dutch word Passeren, referring to the situation that the thread wants
to Pass. The original name of the give operation was V, from Vrijgeven,
referring to the release (’free-giving’) of the resource. The names P and V are
still used in many systems today.
The most common names for take and give are wait and signal, respectively.
These names are more expressive and relates well to the English terms
used for railroad semaphores. The semaphore concept actually originates from
railroad traffic, where the tracks are the resources. As we will see later, wait
is not a useful name for a semaphore method in Java since it already is a final
method (with another meaning) in class Object.
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