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    <p>A <strong>Monitor</strong> is an object designed to be accessed from multiple threads. The member functions or methods of a monitor object will enforce mutual exclusion, so only one thread may be performing any action on the object at a given time. If one thread is currently executing a member function of the object then any other thread that tries to call a member function of that object will have to wait until the first has finished.</p> <p>A <strong>Semaphore</strong> is a lower-level object. You might well use a semaphore to implement a monitor. A semaphore essentially is just a counter. When the counter is positive, if a thread tries to acquire the semaphore then it is allowed, and the counter is decremented. When a thread is done then it releases the semaphore, and increments the counter.</p> <p>If the counter is already zero when a thread tries to acquire the semaphore then it has to wait until another thread releases the semaphore. If multiple threads are waiting when a thread releases a semaphore then one of them gets it. The thread that releases a semaphore need not be the same thread that acquired it.</p> <p>A monitor is like a public toilet. Only one person can enter at a time. They lock the door to prevent anyone else coming in, do their stuff, and then unlock it when they leave.</p> <p>A semaphore is like a bike hire place. They have a certain number of bikes. If you try and hire a bike and they have one free then you can take it, otherwise you must wait. When someone returns their bike then someone else can take it. If you have a bike then you can give it to someone else to return --- the bike hire place doesn't care who returns it, as long as they get their bike back.</p>
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    1. POSemaphore vs. Monitors - what's the difference?
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    1. USAnthony Williams
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    1. CO+1 Great analogy with the public bathrooms and bike rental place. I will never forget the difference between the two now.
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      1. USDrupad Panchal
    2. COYour answer seems to contradict http://stackoverflow.com/a/7336799/632951.. so who is right?
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      1. USPacerier
    3. CO@Pacerier: I am :-) The only contradiction is the high-level/low-level thing. You **can** build a monitor from semaphores, it's just not very tidy, precisely **because** a monitor is a higher-level structure than a semaphore. A semaphore is just a counter with waiting. I suggest reading "The Little Book of Semaphores" http://greenteapress.com/semaphores/
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