Page 132 - DCAP103_Principle of operating system

Unit 4: Process Management-III

Operations P and V are done as single, indivisible, atomic action. It is guaranteed that once a Notes
semaphore operations has stared, no other process can access the semaphore until operation has
completed. Mutual exclusion on the semaphore, S, is enforced within P(S) and V(S).
If several processes attempt a P(S) simultaneously, only process will be allowed to proceed.
The other processes will be kept waiting, but the implementation of P and V guarantees that
processes will not suffer indefinite postponement. Semaphores solve the lost-wakeup problem.
4.5.1 Producer-Consumer Problem Using Semaphores

The solution to producer-consumer problem uses three semaphores, namely, full, empty and
mutex.
Initialization
• Set full buffer slots to 0.
i.e., semaphore full = 0.
• Set empty buffer slots to N.
i.e., semaphore empty = N.
• For control access to critical section set mutex to 1.
i.e., semaphore mutex = 1.

Producer ( )
WHILE (true)

produce-Item ( );
P (empty);
P (mutex);
enter-Item ( )
V (mutex)
V (full);
Consumer ( )
WHILE (true)
P (full)
P (mutex);
remove-Item ( );
V (mutex);
V (empty);
consume-Item (Item)
The semaphore ‘full’ is used for counting the number of slots in the buffer
that are full. The ‘empty’ for counting the number of slots that are empty
and semaphore ‘mutex’ to make sure that the producer and consumer do not
access modifiable shared section of the buffer simultaneously.

4.6 Deadlock Concept and Handling

In a multiprogramming environment, several processes may compete for a finite number of
resources. A process requests resources; if the resources are not available at that time, the process
enters a wait state. Waiting processes may never again change state, because the resources
they have requested are held by other waiting processes. This situation is called a deadlock.
Perhaps the best illustration of a deadlock can be drawn from a law passed by the Kansas
legislature early in the 20th century. It said, in part—“When two trains approach each other at a
crossing, both shall come to a full stop and neither shall start up again until the other has gone.”

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