Gate2020 cs Q44

0. Each of a set of Q.28 n processes executes the following code using two semaphores a and b initialized to 1 and 0, respectively. Assume that count is a shared variable initialized to 0 and not used in CODE SECTION P.
CODE SECTION P
wait(a); count=count+1;
if (count==n) signal(b);
signal(a); wait(b); signal(b);
CODE SECTION Q

What does the code achieve?

A
It ensures that all processes execute CODE SECTION P mutually exclusively.
B
It ensures that at most n–1 processes are in CODE SECTION P at any time.
C
It ensures that no process executes CODE SECTION Q before every process has finished CODE SECTION P.
D
It ensures that at most two processes are in CODE SECTION Q at any time.

Answer
Comment
Array

Option : C
Explanation :
Subject Operating Systems
a = 1 b = 0 count = 0
Code section P: For process 1
Wait (a) ⇒ a = 0
Count ++ ⇒ count = 1
If (count = = n) ⇒ false
signal (a) ⇒ a = 0
wait (b) process blocked
As shown above, all processes execute wait (b) before, signal (b) in if condition.
Because, ‘if’ condition becomes True only for process P_n(nth process) [∵ if (count = = n)]
So, before Pn process finishes the code section, P, no other process executes code section Q.
Hence, no process executes code section Q before every process has finished code section P.

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