Semaphores

Achieving mutual exclusion

Let us look at how one might use a semaphore to achieve mutual exclusion when updating a shared variable:

// Global variables (shared between processes):
int count;    // The shared variable.
// Create a semaphore for mutual exclusion---initialised to 1.
Semaphore mutex = new Semaphore(1);

void addAUser(){
    register int temp;
   wait(mutex); // Start of critical section.
    temp = count;
    temp ++;
    count = temp;
   signal(mutex); // End of critical section.
}

Let's look at the case when two processes attempt to update the shared variable:
 
 

process #1	process #2	mutex counter	blocked processes	critical section processes
		1
	wait(mutex)	0		P2
wait(mutex)		0	P1	P2
blocked	critical code	0	P1	P2
blocked	signal(mutex)	1	P1
(unblocked)		0		P1
critical code		0		P1
signal(mutex)		1
	wait(mutex)	0		P2
	critical code	0		P2
	signal(mutex)	1

 

The example above makes use of a 'binary semaphore'. It only ever has the values 0 and 1 (can you see why). Binary semaphores are ideal for mutual exclusion because we want to ensure that only one process is ever in a critical section.