This document discusses semaphores, which are integer variables that coordinate access to shared resources. It describes counting semaphores, which allow multiple processes to access a critical section simultaneously up to a set limit, and binary semaphores, which only permit one process at a time. Key differences are that counting semaphores can have any integer value while binary semaphores are limited to 0 or 1, and counting semaphores allow multiple slots while binary semaphores provide strict mutual exclusion. Limitations of semaphores include potential priority inversion issues and deadlocks if not used properly.

1. Topic:
Semaphore

2. Contents
• What is Semaphore
• Types of Semaphore
• Counting Semaphore
• Binary Semaphore
• Counting Semaphore vs Binary Semaphore
• Limitations of Semaphore
2

3. What is Semaphore?
• Semaphores are integer variables, stores the entire wake up calls
• On which read, modify, update happens automatically in kernel mode
• Needs support from OS to be implemented
3

4. Continue
P(Semaphore s) { s = s - 1;
if (s <= 0) {
// add process to queue
block(); }
}
V(Semaphore s) {
s = s + 1;
if (s <= 0) {
// remove process p from queue
wakeup(p); }
}
4

5. Types Of Semaphore
According to the demand of the situation
• Counting Semaphore
• Binary Semaphore or Mutex
5

6. Counting Semaphore
• When we need to execute more than one process in critical section at the same
time.
• The value of counting semaphore at any point of time indicates that the maximum
number of processes that can enter the critical section at the same time.
6

7. Example:
• A counting semaphore was initialized to 12. then 10P(wait) and 4V
signal operation was computed on this semaphore.
1. S= 12 (initially)
2. 10P wait
3. SS= S-P=2
4. Then signal 4V
5. So Total SS = 4+2 = 6
7

8. Binary Semaphore
• Strictly provides mutual exclusion.
• Rather than having more than 1 slot, we can only have one process in the critical
section.
• Have only two value 0 and 1
• Extended version of counting semaphore.
8

9. Counting Semaphore vs Binary Semaphore
Counting Binary
1. No mutual exclusion. 1. Mutual exclusion
2. Any integer value 2. Value is 0 and 1
3. More than one slot 3. Only one slot
4. Provide set of Processes 4. Have mutual exclusion mechanism.
9

10. Limitations of Semaphore
• Priority Inversion is a big limitation of semaphores.
• Their use is not enforced, but is by convention only.
• With improper use, a process may block indefinitely. Such a situation is
called Deadlock.
10

11. Thank You
11