Counting semaphore -Locks a shared resource against 
simultaneous access by multiple threads 
A counting semaphore is a generalization of a mutex 
semaphore that locks a shared resource against simultaneous 
access by multiple threads. All resource allocation 
functions, such as wait() and post() , take an optional 
parameter defining the number of resources to allocate or 
deallocate.

Counting semaphores are a synchronization primitive that
allow threads to wait until an event has occurred. The event
may be generated by a producer thread, or by a DSR in
response to a hardware interrupt.