Introduction to Monitors

Monitors :
- Consist of private data and operations on that data.
- It can contain types, constants, variables and procedures.
- Only the procedures explicitly marked can be seen outside the monitor.
- The monitor body allows the private data to be initialized.
- The compiler enforces mutual exclusion on a particular monitor.
- Each monitor has a boundary queue, and processes wanting to call a monitor routine
join this queue if the monitor is already in use.
- Monitors are an improvement over conditional critical regions because all the code
that accesses the shared data is localized.

To allow a process to wait within the monitor, a condition variable must be declared, as: condition x,y.
Condition variable can only be used with the operations wait and signal.
The operation x.wait()means that the process invoking this operation is suspended until another process invokes. Also called delay function.
The operation x.signal()resumes exactly one suspended process. If no process is suspended, then the signal operation has no effect.Also called resume function.

Monitor Implementation using Semaphores
- For each condition variable x, we have:
semaphore x-sem; // (initially = 0)
int x-count = 0;
- The operation x.wait can be implemented as:
x-count++;
if (next-count > 0)
signal(next);
else
signal(mutex);
wait(x-sem);
x-count--;
- The operation x.signal can be implemented as:
if (x-count > 0)
{
next-count++;
signal(x-sem);
wait(next);
next-count--;
}