Semaphores r software, blocking, OS assistance solution to
the mutual exclusion problem .It is
basically a non-negative integer variable that saves the
number of wakeup signals sent so they are not lost if the
process is not sleeping
another interpretation is that the semaphore value
represents the number of resources available
Semaphore is a machanism to resolve resources conflicts by
tallying resource seekers what is the state of sought
resources, achieving a mutual exclusive access to resources.
Often semaphore operates as a type of mutual exclusive
counters (such as mutexes) where it holds a number of access
keys to the resources. Process that seeks the resources must
obtain one of those access keys, one of semaphores, before
it proceeds further to utilize the resource. If there is no
more such a key available to the process, it has to wait for
the current resource user to release the key.
A semaphore, a new variable type.
A semaphore could have the value 0,indicating that no
wakeups were saved, or some positive values if one or more
wakeups were pending.
a semaphore s is an integer variable that apart from
initialization, is accesssed only through two standard
atomic operations, wait and signal. these operations were
orignially termed p(for wait to test) and v(for signal to
The classical defination of wait in psedocode is
The classical defination of signal in psedocode is
Modification to the integer value of smaphore in wait and
signal operations must be executed individually.
that is, when one process modifies the semaphore value no
other process can simultaneously modifiy that same
In computer science, a semaphore is a protected variable or
abstract data type which constitutes the classic method for
restricting access to shared resources such as shared
memory in a multiprogramming environment. A counting
semaphore is a counter for a set of available resources,
rather than a locked/unlocked flag of a single resource. It
was invented by Edsger Dijkstra. Semaphores are the classic
solution to preventing race conditions in the dining
philosophers problem, although they do not prevent resource
Semaphores can only be accessed using the following
operations. Those marked atomic should not be interrupted
(that is, if the system decides that the "turn is up" for
the program doing this, it shouldn't stop it in the middle
of those instructions) for the reasons explained below.
P(Semaphore s) // Acquire Resource
wait until s > 0, then s := s-1;
/* must be atomic because of race conditions */
V(Semaphore s) // Release Resource
s := s+1; /* must be atomic */
Synchronization tool that does not require busy waiting
Semaphore S–integer variable
Two standard operations modify S: wait()and signal()
Originally called P()andV()
Can only be accessed via two indivisible (atomic) operations
while S <= 0;
The semaphore abstract data type is perhaps the simplest
universally useful implementation of the variables uses as
arguments to the wait and signal operations although there
are many implementations of semaphores, they can be
intuitively thought of as integers, where wait(x) or the
object-orinted x.wait waits until x is greater then zero
and then decrements it, and signal(x)or x.signal increments
"semaphore " is an integer value that provide signalling
among the process
and it is an synchronization tool
semaphores are proposed by "DIJKSTRA"
generally semaphores are divided into 2 types
they are 1.General semaphore(or)counting semaphore
in general semaphores 2 primitives are used they are
semwait() checks decrements the semaphore value if the value
becomes negative then the process executing the semwait()is
blocked other wise the process continues execution
semsignal() increments the semaphore value if the value is
less than or equal to zero then a process blocked by a
semwait()operation is unblocked
->synchronization tool that does not require busy writing.
->semaphore S-integer variable
->can only be accessesd via two indivisible(atomic) operations
wait(S):while S<=0 do no-op;
semaphores are of two types
COUNTING SEMAPHORE-integer value can range over an
BINARY SEMAPHORE-integer value can range only between 0 and
1;can be simpler to implement.
can implement a counting semaphore S as a binary semaphore.
Assume that passwords are selected from four-character
combinations of 26 alphabetic characters. Assume that an
adversary is able to attempt passwords at a rate of one per
a) Assuming no feedback to the adversary until each attempt
has been completed, what is the expected time to discover
the correct password?