MadeEasy Full Length Test 2019: Operating System - Semaphore

Question

Let S be a binary semaphore variable, S=0

What will be the value of S when following operations are performed:-

2P, 4V, 5P, 2P, 8V, 3P, 2V  ?

Comments

Answer is 0.

initially s=0.

Now, 2P makes 2 processes waiting in queue

4V signals will make 2 of them to complete and final value of s=1 now

5P processes one of them will make s=0 and other 4 will wait

2P process further wait making in all 6 processes to wait

8V will make them complete and finally 2V extra signals make s=1

3P, 1p will complete s=0 and 2P will wait

2V will make them to complete and since no extra signal i.e., V so s remains 0

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0??

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should be 1

After 2P,4V,5P,2P.8V,3P semaphore will reach it's initial value '0' and 2V will change it to 1 as it is a binary semaphore.

Answer 1

Answer is 0.

initially s=0.

Now, 2P makes 2 processes waiting in queue

4V signals will make 2 of them to complete and final value of s=1 now

5P processes one of them will make s=0 and other 4 will wait

2P process further wait making in all 6 processes to wait

8V will make them complete and finally 2V extra signals make s=1

3P, 1p will complete s=0 and 2P will wait

2V will make them to complete and since no extra signal i.e., V so s remains 0

Comments

2V will make them to complete and since no extra signal i.e., V so s remains 0

In this when all signals are completed and it is a signal operation then why s is not becoming 1 (since S is available for use as no process is waiting) ?

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Because we only had 2V operations and we used those operations for completing the remaining 2 processes. so IF we do one more operation on V then we would get S=1 but our question stops us at 2V operation.