semaphore in os

what is semaphore

• semaphore is used to solve critical section problem
• for exmaple, we have a variable

int shared = 5
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and two processes:
process 1:

int x = shared;  // storing the value of shared variable in the variable x
x++;
sleep(1);
shared = x;

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process 2:

int y = shared;
y--;
sleep(1);
shared = y;

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Because of process synchronization, we get shared = 6 in process one and shared = 4 in process 2.
However, we want something like shared increse one at first and then decrease one. As a result this value should remain unchanged.
To solve this problem, we use semaphore.

types of semaphores

binary semaphores

We set a semaphore for critial section, 1 means this section is available, 0 means this sectoin is unavailable.

counting semaphores

Here the semaphore count is the number of processes that can access the resources / code, which means “semaphore” number of processes can access this critical section at the same time. When semaphore equels zero, this critical section can not be accessed.

process of semaphore

shared variable semaphore = 1;
process i
	begin
	.
	.
	P(mutex);
	  execute Critical Section
	V(mutex);
	.
	.
	end;

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• P(mutex)

P(semaphore){
	if semaphore is greater than 0
	  then decrement semaphore by 1
}

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• V(mutex)

V(semaphore){
   increment semaphore by 1
}

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