Implementing Monitor using Semaphore

Let’s implement a a monitor mechanism using semaphores.

Following is a step-by-step implementation:

Step 1: Initialize a semaphore mutex to 1.

Step 2: Provide a semaphore mutex for each monitor.

Step 3: A process must execute wait (mutex) before entering the monitor and must execute signal (mutex) after leaving the monitor.

Step 4: Since a signaling process must wait until the resumed process either leaves or waits, introduce an additional semaphore, S, and initialize it to 0.

Step 5: The signaling processes can use S to suspend themselves. An integer variable S_count is also provided to count the number of processes suspended next. Thus, each external function Fun is replaced by

wait (mutex);

body of Fun

if (S_count > 0)

signal (S);

else

signal (mutex);

Mutual exclusion within a monitor is ensured.

Let’s see how condition variables are implemented.

Step 1: x is condition.

Step 2: Introduce a semaphore x_num and an integer variable x_count.

Step 3: Initialize both semaphores to 0.

x.wait() is now implemented as: x.wait()

x_count++;

if (S_count > 0)

signal (S);

else

signal (mutex);

wait (x_num);

x_count–;

The operation x.signal () can be implemented as

if (x_count > 0) {

S_count++;

signal (x_num);

wait (S);

S_count–;

}