solution to critical section problem monitor

1. Monitors
Prepared By: Mr. Sangram A. Patil
Assistant Professor PVPIT,Budhgaon

2. Monitors
 Semaphore provide convenient and effective mechanism for process synchronization,
using them incorrectly can result in timing errors that difficult to detect.
 Suppose process interchange order of wait() and signal()
Signal(mutex)
//critical section
Wait(mutex)
 Suppose signal(mutex) eplaced with wait(mutex)
wait(mutex)
//critical section
Wait(mutex)
Produce deadlock

4. Monitors
 A monitor is an ADT which has set of programmer defined operations that provide
mutual exclusion within monitor.
 Monitors define declaration of variables along with bodies of procedures and
functions that operates on those variables.
 Representation of monitor type cannot directly used by the various processes.
 Procedure within monitor only access those variables that are declared within
monitor and formal to those functions.
 Local variables accede by local procedures only.

5. Schematic view of monitor

6. Monitor monitor_name
{
//shared variable declaration
procedure p1(………….)
{
}
procedure p1(………….)
{
}
.
.
.
.
procedure p1(………….)
{
}
initialization code(………….)
{
}
}

7.  Monitor ensures that only one process at a time is active within a monitor.
 We need to provide additional synchronization mechanisms. And this can be
provided by condition Construct
 To implement synchronization programmer needs to define one or more variables
of type
condition x,y;
 Wait() and signal() operations are invoked by these variables.
x.wait()
Process invoking this operation is suspended until another process invokes
x.signal();
x.Signal() operation resumes exactly one suspended process.

8.  Suppose x.signal() operation invoked by process P there exits a suspended process
Q associated with condition x.
 Two possibilities:
1. Signal and wait: P either waits until Q leaves the monitor or wait for another
condition
2. Signal and continue: Q either waits until P leaves the monitor or wait for another
condition