Question

Use semaphore(s) to solve the following problem. There are three processes: P1, P2, and P3. Each process Pi has a segment of codes Ci, i=1, 2, 3. These three processes are executed only once, i.e., no repeat or loop at all, and their executions can start at any time. Your goal is to ensure that the execution of C1, C2, and C3 must satisfy the following conditions:
a. If C1 is executed ahead of C2 and C3, C2 must be executed ahead of C3.
b. Otherwise, C1 must be executed after both C2 and C3 are executed. In this case, the order of C2 and C3's execution doesn't matter. One of the possible execution orders is demonstrated below. Obviously, two other possible sequences in time are C2, C3, C1 and C3, C2, C1.
Please write your algorithm level code for semaphore initialization and usage in each code segment. [Hint: no if statements should ever be used. All you need is some semaphore function calls surrounding C1, C2, and C3 and their initial values.]

Answer 1

Answer:

See explaination

Explanation:

Here we will use two semaphore variables to satisfy our goal

We will initialize s1=1 and s2=1 globally and they are accessed by all 3 processes and use up and down operations in following way

Code:-

s1,s2=1

P1 P2 P3

P(s1)

P(s2)

C1

V(s2) .

P(s2). .

. C2

V(s1) .

P(s1)

. . C3

V(s2)

Explanation:-

The P(s1) stands for down operation for semaphore s1 and V(s1) stands for Up operation for semaphore s1.

The Down operation on s1=1 will make it s1=0 and our process will execute ,and down on s1=0 will block the process

The Up operation on s1=0 will unblock the process and on s1=1 will be normal execution of process

Now in the above code:

1)If C1 is executed first then it means down on s1,s2 will make it zero and up on s2 will make it 1, so in that case C3 cannot execute because P3 has down operation on s1 before C3 ,so C2 will execute by performing down on s2 and after that Up on s1 will be done by P2 and then C3 can execute

So our first condition gets satisfied

2)If C1 is not executed earlier means:-

a)If C2 is executed by performing down on S2 then s2=0,so definitely C3 will be executed because down(s2) in case of C1 will block the process P1 and after C3 execute Up operation on s2 ,C1 can execute because P1 gets unblocked .

b)If C3 is executed by performing down on s1 then s1=0 ,so definitely C2 will be executed now ,because down on s1 will block the process P1 and after that P2 will perform up on s1 ,so P1 gets unblocked

So C1 will be executed after C2 and C3 ,hence our 2nd condition satisfied.