Answer / preeti.j

Semaphore is a mechanism is an RTOS to serialize access to
a resource/resources for all the threads in the system.
There are 2 types of semaphore
1. counting semaphore - this is used when a set of
resources have to be shared between many threads. The
semaphore is initialized with the number of resources. Each
time a thread tries to access the semaphore, its value gets
decremented until it becomes 0. After this any try to
access the semaphore will block the calling thread.
2. binary semaphore - this is used to serialize access to a
single resource i.e the semaphore has only two values 0 and
1.
A mutex is similar to a binary semaphore except that it has
an owner i.e the thread where it was created. A mutex can
be freed only by this thread. This is not the case for
binary semaphore which can be freed by any thread.

Answer / vijay patil

semaphore is used to avoid the effect of dead lock.
there are three types of semaphore .
1)binary semaphore->it is only to acquire and release the
semaphore.zero is used for unavailabel and
one is used for available.
2)counting semaphore->when semaphore is acquired counter is
incremented and when semaphore is released conter is
decremented.when counter is zero it is goes to blocked state.
3)mutual exclusion->it is same as binary semaphore.but it is
used for safty purpose.it is two state locked state and
unlocked state.when semaphore is acquired it is goes to
locked state and when semaphore is released it goes to
unlocked state.

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