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15.6 Mbed TASK SYNCHRONIZATION—MUTEX, SEMAPHORE, AND SIGNALS 375

FIG. 15.8 Example program without mutex.

In Fig. 15.9 (program: MutexEx) the same program is given where a mutex is used to lock
the shared printf resource. This modified program displays the following messages on the
screen which is what is expected normally:
Before 1
After 1
Before 2
After 2

15.6.2 Semaphores

A semaphore is simply a nonnegative integer which increments and decrements and con-
trolsaccesstoasharedresource.Asemaphoreisinitiallysettoacountequivalenttothenumber
of free resources. A semaphore’s value must be positive to allow access to the shared resource.
The semaphore count is decremented by one when a thread uses the semaphore. Similarly, the
count is incremented by one when the thread releases the semaphore. A zero count does not
allow access to the shared resource. Semaphores with only one count are similar to mutexes.
Such semaphores are also known as Binary Semaphores. An analogy of semaphores is the us-
ageofaprinter(sharedresource),forexample,threeuserssendingprintrequestsallatthesame