It should be possible to implement general semaphores using binary semaphores. We can use the operations semWaitB and semSignalB and two binary semaphores, delay and mutex. Consider the following:
void semWait(semaphore s)
{
semWaitB(mutex);
s--;
if (s < 0) {
semSignalB(mutex);
semWaitB(delay);
}
else SemsignalB(mutex);
}
void semSignal(semaphore s);
{
semWaitB(mutex);
s++;
if (s <= 0)
semSignalB(delay);
semSignalB(mutex);
}
Initially, s is set to the desired semaphore value. Each semWait operation decrements s, and each semSignal operation increments s. The binary semaphore mutex, which is initialized to 1, assures that there is mutual exclusion for the updating of s. The binary semaphore delay, which is initialized to 0, is used to block processes.
There is a flaw in the preceding program. Demonstrate the flaw and propose a change that will fix it.