INtime® 3.1 Software - tenAsys

INtime 3.1 Software
Semaphores
A semaphore is a counter that takes positive integer values. Threads use semaphores
for synchronization by sending units to and receiving units from the semaphores.
When a thread sends n units to a semaphore, the value of the counter increases by n;
when a thread receives n units from a semaphore, the value of the counter decreases
by n.
The next figure shows a typical example of a binary (one-unit) semaphore used for
synchronization.
Figure 3-2. Threads using a semaphore for synchronization
A
B
To ensure that thread A can do its work before thread B starts running, thread A creates a
semaphore that contains one unit. To enable synchronization, threads A and B should request and
obtain the unit before running.
Thread A begins to run and obtains the semaphore unit, leaving the semaphore empty. While the
semaphore has no units, thread B cannot run.
When thread A completes, it returns the unit to the semaphore. Thread B can now obtain the unit
and start running.
Semaphores:
• Enable synchronization; they don't enforce it. If threads do not request and obtain
units from the semaphore before running, synchronization does not occur. Each
thread must return the unit to the semaphore when it is no longer needed.
Otherwise, threads can be permanently prevented from running.
• Provide mutual exclusion from data or a resource as follows:
1. Thread A requests one unit from a binary semaphore, and uses the resource
when it receives the unit.
2. Thread B requests one unit from the semaphore before using the resource.
Thread B must wait at the semaphore until thread A returns the unit.
• Enable mutual exclusion; they do not enforce it.
• Have a queue where threads wait for units. The queue may be FIFO- or prioritybased.
There are system calls to create and delete semaphores, and to send and
receive units.
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