A Bounded Resource with a Counting Semaphore

semaphore->Init(N);

int AllocateEntry() {

int i;

semaphore->Down();

ASSERT(FindFreeItem(&i));

slot[i] = 1;

return(i);

}

void ReleaseEntry(int i) {

slot[i] = 0;

semaphore->Up();

}

A semaphore for an N-way resource

is called a counting semaphore.

A caller that gets past a Down is guaranteed that a resource instance is reserved for it.

Problems?

Note: the current value of the semaphore is the number of resource instances free to allocate.

But semaphores do not allow a thread to read this value directly. Why not?