Question

Problem Consider three processes A, B, and C which have statements S1, S2 and 53 respectively. These statements should execute in a particular fashion. S3 should be executed only after executing S1 and S2. Statements S1 and 52 can be executed in any order. Implement the above using binary semaphore variables X1 and X2 which are initially set to zero. O A: S1; signal (x1); B: S2; signal (X2); C: wait(x1); wait (X2); S3;

Answer 1

SEM X1 = 0

SEM X2 = 0

A	B	C
S1 SIGNAL(X1)	S2 SIGNAL(X2)	WAIT(X1) WAIT(X2) S3 Explanation: When X1 and X2 are 0, S3 cannot be executed. Initially, X1 and X2 are 0. Hence, it will never run until signal(X1) and signal(X2) are executed after S1 and S2. So, first S1/S2 will execute, when they both call signal(X1) and signal(X2) then semaphores X1 and X2 will become 1, and then only C will be able to proceed.