Q40

Consider the following solution to the producer-consumer synchronization problem. The shared buffer size is $N$. Three semaphores $empty$, $full$ and $mutex$ are defined with respective initial values of $0, N$ and $1$. Semaphore $empty$ denotes the number of available slots in the buffer, for the consumer to read from. Semaphore $full$ denotes the number of available slots in the buffer, for the producer to write to. The placeholder variables, denoted by $P$, $Q$, $R$ and $S$, in the code below can be assigned either $empty$ or $full$. The valid semaphore operations are: $wait()$ and $sigmal()$.

$$\begin{array}{|l|l|}\hline \text{Producer:} & \text{Consumer:} \\\hline \text{do \{} & \text{do \{} \\ \quad\text{ wait (P);} & \quad\text{ wait (R);} \\ \quad\text{ wait (mutex);} & \quad\text{ wait (mutex);} \\ \quad \text{ //Add item to buffer} & \quad \text{ //consume item from buffer} \\ \quad\text{ signal (mutex);} & \quad\text{ signal (mutex);} \\ \quad \text{ signal (Q);} & \quad\text{ signal (S);} \\ \text{\}while (1); }& \text{\}while (1);} \\ \hline \end{array}$$

Which one of the following assignments tp $P$, $Q$, $R$ and $S$ will yield the correct solution?