This document discusses semaphores, which are a synchronization tool used to control access to shared resources among multiple processes. Semaphores use wait() and signal() operations, where wait() decrements the semaphore value when acquiring a resource and signal() increments it when releasing a resource. The semaphore value indicates how many resources are available. Processes block on wait() if no resources are available until another process signals a release.

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SEMAPHORES

2. It is a synchronization tool.
It deal with n-process critical section problem
It was proposed by Dijkstra.
It is an integer value used for signaling among processes.
SEMAPHORES

3. Wait ( ) – To use a resource.
Signal ( ) – To release a resource.
 To transmit a signal via semaphore S, a process executes the primitive
semsignal (s).
To receive a signal via semaphore S, a process executes the primitive semwait (s).
Semaphores can be used to control access to a given resource.
SEMAPHORES FUNCTIONS

4. Semaphore is initialized to the number of resources available.
Each process that wishes to use a resource performs a wait ( )
operation.
wait (s)
{
while (s<=0)
// no operation
s--;
WAIT ( )

5. When a process releases a resource, if performs a signal ( )
operation.
Signal (s)
{
s++;
}
When the count for the semaphores goes to 0, resources
are being used.
After that, processes that with to use a resource will block
until the count becomes > 0.
SIGNAL ( )