Semaphores - Operating System

1. SEMAPHORESSEMAPHORES
Raj Sahu
BTECH/10363/2018
OPERATING_SYSTEM_CS-211
CSE_A

2. ContentsContents
Topic Page_No
1. Definition 3
2. Operations supported 4-5
3. Types Of Semaphores 6
4. How mutual_Exclusion is acheived 7-8
5. Binary Semaphore 9-10
6. Counting Semaphore 11-12
7. References 13
THE_END

3. DefinitionDefinition
➔
Proposed by Dijkstra in 1965
➔
Used to manage concurrent processes by using
a simple integer value, which is known as a
semaphore.
➔
Semaphore is simply a variable which is non-
negative and shared between threads.
➔
This variable is used to solve the critical section
problem and to achieve process synchronization
in the multiprocessing environment.

4. Operations SupportedOperations Supported
Only Two Atomic operations
➔
P(sem) : Waits for semaphore to become
positive, then decrement it by 1 .
➔
V(sem) : [Signal] Increments semaphore
by 1 and wakes up a waiting thread at P(),
if any.

5. ➔
Atomic means that variable on which read,
modify and update happens at the same
time/moment with no pre-emption i.e. in
between read, modify and update no other
operation is performed that may change
the variable.

6. Types Of SemaphoresTypes Of Semaphores
Binary SemaphoreBinary Semaphore (aka mutex semaphore)(aka mutex semaphore)
➔
Initialized to 1
➔
guarantees mutually exclusive access to resource (e.g., a critical
section of code)
➔
only one thread/process allowed entry at a time
Counting Semaphore
➔
Initialized to N
➔
N = number of units Available
➔
represents resources with many (identical) units available
➔
allows threads to enter as long as more units are available
➔

7. How Mutual_ExclusionHow Mutual_Exclusion
is Acheivedis Acheived
➔
A critical section is surrounded by both
operations to implement process
synchronization.

9. Binary SemaphoreBinary Semaphore

11. Counting SemaphoreCounting Semaphore

13. References :References :
➔
https://en.wikipedia.org/wiki/
Semaphore_(programming)
➔
http://greenteapress.com/semaphores/
➔
http://www.opengroup.org/onlinepubs/
009695399/basedefs/semaphore.h.html

14. THE_ENDTHE_END