This document discusses semaphores, which are integer variables that are used to solve critical section problems in operating systems. It describes semaphores as having two main operations - wait and signal. Wait decrements the semaphore value while signal increments it. Semaphores can be binary, ranging from 0 to 1, or counting, with an unrestricted domain. Examples where semaphores are used include producer-consumer problems and controlling access to shared resources like databases or bridges.

1. Operating
System
(CSA-CC-224)
Topic: Semaphores
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2. Contents
Introduction to semaphores
Two semaphore operations
Types of semaphores
Properties of semaphores
Limitations of semaphores
Examples
References

3. Introduction to Semaphores
Dijkestra proposed a significant
technique for managing concurrent
processes for complex mutual
exclusion problems.
A semaphore is an integer variable
which can hold only a non-negative
Integer value, that are used to solve
the critical section problem by using
two atomic operations.

4. Two Semaphore Operations
The wait() operation was originally
termed P and signal() was originally called V.
The classical definitions
of wait and signal are:
■ Wait: Decrements the value of its
argument S.
■ Signal: Increments the value of its
argument S.

5. Types of Semaphores
Semaphores are mainly of two types:
Binary Semaphore: The value of a
binary semaphores can range only
between 0 and 1.
Counting Semaphore: The value of a
counting semaphores can range over
an unrestricted domain.

6. Properties of Semaphores
It's simple and
always have a
non-negative
Integer value.
Works with many
processes.
Can have many
different critical
sections with
different
semaphores.
Each critical
section has
unique access
semaphores.
Can permit
multiple
processes into
the critical
section at once,
if desirable.

7. Limitations of Semaphores
 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.

8. Real Life
Example of
Semaphores
Producer and consumer problem
Retrieving databases
Trains crossing a bridge

9. References
■ Operating System Concepts 9th
Edi by A. Silberschatz, Peter
Galvin and G. Gagne
■ https://www.tutorialspoint.com/sema
phores-in-operating-system
managed by David Meador
■ https://www.studytonight.com/operat
ing-system/introduction-to-
semaphores

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