Semaphores are a synchronization tool that can solve critical section problems. They are integer variables that can be accessed atomically by multiple processes using wait and signal operations. Semaphores allow mutual exclusion without busy waiting, which wastes CPU cycles. Each process has an integer value and queue. When a process waits on a semaphore, it is added to the queue. A signal removes a process from the queue and resumes its execution. Semaphores can be implemented using binary semaphores with values of 0 or 1 to represent an empty or occupied critical section respectively. Processes are blocked or added to the queue depending on the semaphore value.

1. Operating Systems
UNIT-III
Semaphores
Presented by
Tmt.P.Tharani
AP/CSE
GCE,Salem
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2. Semaphores
 Synchronization tool –solves critical section problem
 It is an integer variable shared by multiple processes and can
be accesses through two standard atomic operation wait and
signal.
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3. • P1:
• P2:
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4.  The main disadvantage of mutual exclusion solutions requires
busy waiting.
 Busy waiting wastes CPU cycle and this type of semaphore is
called as Spinlock.
 Each Process has an integer value and a list of processes.
 When a process must wait on a semaphore it is added to the
list of processes.
 A signal operation removes one process from the list of
waiting processes and awakens that process.
 The wakeup operation resumes the execution of a blocked
process.
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5. • Mutual Exclusion with no busy waiting
Consider three values: value =1 (CS empty, Queue empty)
Value=0 (CS occupied, queue empty)
Value=-3 (CS occupied,three process are waiting in the queue)
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6. • Deadlock and Starvation
 Consider a system consisting of two processes P0 & P1 each
accessing two semaphore S and Q set to the value 1.
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7. • Binary Semaphore
 A binary semaphore is a semaphore with an integer value that
can range only between 0 and 1. Counting Semaphore has a
range over unrestricted domain.
 Let S be a counting semaphore .To implement it in terms of
binary semaphore.
type binary semaphore=record
value(0,1)
queue:list of process
end
var S: binary_semaphore
 Consider value=1 CS empty and value=0 CS occupied.
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8. wait B(s)
if s.value=1
then s.value=0
else begin
Place this process to s.queue
Block this process
End
Signal B(s)
if s.queue is empty
then s.value=1
else begin
Remove a process from s.queue
Place process in ready queue
end
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9. Thank You
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