Question

Operating Systems

how do semaphores support mutual exclusion ?

Answer 1

Answer:-

First of all semaphore is variable used to solve critical section problem.

There are two operations in semaphores. one is wait() and another one is signal()

implementation of wait:-

wait(Semaphore s)

{

            while(s==0);   //wait untill s==0//

s = s-1                 //decrements semaphore value

}

implementation of signal:-

signal(Semaphore s)

{

     s = s+1;

}

support for mutual exclusion through semaphore:-

first of all semaphore initialized to 1 and p1 and p2 are two processes

step1:-   s=1 p1,p2 //not in critical section//

step2:- if p1 enters into critical section.

             it will call wait() operation

             then s =0

step3:- another process p2 can not enter critical section until p1 calls signal opeartion //supports mutual exclusion//

step4:- p1 completes its operation then calls signal() function

step:-now s=1 then p2 can enter critical section.

process will implement following procedure for mutual exclusion:-

while(True)

{

//non critical section//

wait(s);

//critical section//

signal(s);

}