Question

What is semaphore? List the properties of semaphore variables. Define the wait function wait(s).

Answer 1

Semaphore :

1. Semaphore is a variable which contains an integer value that is used to solve the critical section problem
2. It is a synchronization tool which is used to achieve process synchronization in the multiprocessing environment.
3. It contains two variables P and V.

Properties of semaphore variables

1. V called as signal operation always increments the semaphore value and it is always successful on semaphore
2. P called as wait operation always decrements the semaphore value and it is successful until it becomes 0.
3. The variable will always hold a nonnegative integer value.
4. The two most common kinds of semaphores are counting semaphores and binary semaphores.
5. Counting semaphore can take non-negative integer values and Binary semaphore can take the value 0 & 1. only
6. They can be implemented using test operations and interrupts. They can also be executed using file descriptors

Wait operation(P) :

1. wait operation is denoted by variable P.
2. wait operaiton decrements the semaphore value
3. It is defined as follows.

wait(S)

{

while(S<=0)

S -- ;

}

Signal operation(V) :

1. It is denoted using variable V
2. It increments the semaphore value.
3. It is defined as follows

signal(S)

{

  while (S>=0);

S++ ;

}