The Gateway to Computer Science Excellence
First time here? Checkout the FAQ!
+28 votes

Consider Peterson's algorithm for mutual exclusion between two concurrent processes i and j. The program executed by process is shown below.

    flag[i] = true;
    turn = j;
    while (P) do no-op;
    Enter critical section, perform actions, then
    exit critical section
    Flag[i] = false;
    Perform other non-critical section actions.
Until false;

For the program to guarantee mutual exclusion, the predicate P in the while loop should be

  1. flag[j] = true and turn = i
  2. flag[j] = true and turn = j
  3. flag[i] = true and turn = j
  4. flag[i] = true and turn = i
asked in Operating System by Veteran (59.5k points)
edited by | 2.7k views

I think if we will change 

turn = j; --> turn = i; 

Then also it will work. Please correct me if i am wrong.

yes it will work but condition P statement will change to flag[j]==true and turn==i
option B

5 Answers

+28 votes
Best answer

Answer is (B). suppose two processes $p1$ and $p2$. To gurantee mutual exclusion only $1$ process must be in $CS$ at a time. now, shared variable is turn $P1 P2 f[1]=$ true $f[2]=$true turn$=2$ $p1$ will now check for condition while($f[2]=$ true and turn $=2$) then it will go for busy wait. Then, $p2$ will execute and it will update turn $=1$ $p2$ will now check condition while($f[1]=$ true and turn $=1$) then it will go for busy wait, but here as the value of turn is updated by $p2$; $p1$ will be able to enter into CS as the condition will become false therefore, $p1$ will enter CS and $p2$ will be in busy wai until the $p1$ will come out and make $f[1]=$ false hence, no two process can enter into CS at a time so predicate $p$ is option (B).

answered by Loyal (8.2k points)
edited by
C is also correct ans...........

@ jayendra what about option c can you explain....

@bikram sir Are B and C both the correct answer?
Here the question should mention that this is the code executed by the process Pi.Otherwise it creates  confusion.

It is a tricky question.We should eliminate options if possible:-

c:) creates deadlock

a& d:)will fail mutual inclusion as the process has set turn =j. Now it will check turn =i in for loop .so,both can enter.So remaining is b.which is surely the answer

Let us say both the processes tries to enter now both set flag to 1 but the key is turn will have i or j ,so one of the process will fail and other will pass and enter.
Hi Bikram Sir,


Please correct me if I am wrong here for not taking option C.


If Pi enters first and doesn't wish to enter second time, then Pj will never be able to enter in CS as for PJ the condition is

flag[j] = true and turn = i both are true, so it will loop forever and Pj will never be able to enter CS.
+6 votes

If process Pi wants to enter into CS then it will make Flag[i]=True and turn=j. Now for while loop, consider 2 points:

(i)  If we take turn=i, then it already makes condition False in while loop and thus both processes can enter simultaneously into  CS. So, take turn=j

(ii)  if we take flag[i]=TRUE, then the condition will become like process ' i ' wants to enter which made flag[i]=TRUE and        turn=j already, so it will always remain in while loop forever. So, take Flag[j]=TRUE

Thus (B) is the answer for sure :)

answered by Active (1.5k points)
+3 votes

Option C is correct(Best option)

First of all its not a complete implementation of Peterson Algorithm. Here we need to check Mutual Exclusion. Now for that Process i is in CS and Process j is trying to enter into CS. How I know it ? It can be seen in code " turn = j " that means Process j is trying to enter CS. Therefore to make sure Mutual Exclusion is there Process j should keep on iterating in the while( ) loop while Process i is in CS . Now since Process i is in CS we need to make sure that when Process i exits CS then only Process j enters. Therefore for that to happen Process i must execute " Flag[i] = false". From this we can take condition that  "Flag[i] = true " should be put in while loop. Secondly since "turn = j" therefore " turn == j " should also be added in while loop. Since turn = j is given therefore i would best choose option c rather d.

answered by Junior (603 points)
edited by
Here first flag[i] and turn vaiables are set and then compared with same value:

flag[i] = true; 
turn = j; 
while(flag[i] == true and turn ==j); // it will always give infinite loop
but when the second process executes line no.2 won't the condition in while of the first process become(true&&false) and it would enter the critical section?
–1 vote

Ans should be (c) as it should be FLAG[other] which is i and turn of current is checked i.e j

answered by (431 points)
–3 votes
ans is C is correct because in case of option B, the process will enter simultaneously enter into the critical section but incase of C mutual exclusion will be ensured.
answered by (195 points)

Quick search syntax
tags tag:apple
author user:martin
title title:apple
content content:apple
exclude -tag:apple
force match +apple
views views:100
score score:10
answers answers:2
is accepted isaccepted:true
is closed isclosed:true

37,117 questions
44,701 answers
43,765 users