semaphore

Question

var occupied
var blocked
Enter Region:
{
If (occupied) {
then blocked= blocked +1
sleep ( );
}
else occupied= 1;
}
Exit Region:
{
occupied= 0
If (blocked) {
then wakeup (process);
blocked= blocked – 1;
}
}
(1) Mutual Exclusion is guaranteed
(2) Deadlock free Algorithm
(3) Progress is guaranteed
Which of the above statements are False?
(a) Only 1 (b) 1 & 2
(c) 1 & 3(d) none of the above

Comments

option A) Only mutual exclusion is guaranteed.

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how can u explain???

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Question is asking about false statement..

how deadlock is possible?@Shubhgupta

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because  of If else combination any process can enter into critical section. So mutual exclusion is not guaranteed and because of any number process can enter that's why there should be no deadlock.

Progress is satisfied because no process will stop other to enter into critical section.

So only 1 is false.

 

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suppose initial value of occupied and blocked is zero let p1 and p2 be two process and p1 check if condition does not satisfy then it will execute else part and put the value of occupied=1 suppose p1 preempt at this point then p2 will come and checks if condition which hold true then it will goes to sleep mode then p1 again came and goes to exit section put the value occupied=0 then it shows that mutual exclusion is gauranteed??can you explain in detail with example how it does not satisfy mutual exclusion??can you check my approach where it is wrong??

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@BASANT KUMAR,

Suppose there are two processes p1 and p2. and initially occupied and blocked both variable assigned with 0 then first p1 will come and execute if condition and preempt then p2 will come and execute if condition and go to else part and set occupied to 1 and enter into critical section now p1 start execution form where it left remember p1 already executed if statement so it will not execute again and will go to else part and again set the occupied variable with 1 and enter into critical section. So by this sequence mutual exclusion is not satisfied.

and your approach is correct in which mutual exclusion is satisfied but if one case is not satisfied then we can say that mutual exclusion is not satisfied.

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what is your initial value of occupied is it 0 or 1??

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its 0. I already mentioned.

initially occupied and blocked both variable assigned with 0 

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when p1 preempt after executing "if " statement then the value of occupied will become 1 then p2 will come go to sleep mode.am i right??

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Nope after executing 'if' p1 got preempted and not executed 'else' part and p2 also executed 'if' after preemption of p1 and go to 'else' part.