Semaphore
Definition:
A semaphore is a protected variable whose value can be accessed
and altered only by the operations P and V and initialization
operation called 'Semaphoiinitislize'.
Binary
Semaphores can assume only the value 0 or the value 1 counting
semaphores also called general semaphores can assume only
nonnegative values.
The P
(or wait or sleep or down) operation on semaphores S, written
as P(S) or wait (S), operates as follows:
P(S):
IF S > 0
THEN S := S - 1
ELSE (wait on S)
The V
(or signal or wakeup or up) operation on semaphore S, written
as V(S) or signal (S), operates as follows:
V(S):
IF (one or more process are waiting on S)
THEN (let one of these processes proceed)
ELSE S := S +1
Operations
P and V are done as single, indivisible, atomic action. It
is guaranteed that once a 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.
Producer-Consumer
Problem Using Semaphores
The Solution to producer-consumer problem uses three semaphores,
namely, full, empty and mutex.
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.
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) |
