Semaphore vs. Monitors - what's the difference?

A Monitor 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.

A Semaphore 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.

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.

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.

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.

Following explanation actually explains how wait() and signal() of monitor differ from P and V of semaphore.

The wait() and signal() operations on condition variables in a monitor are similar to P and V operations on counting semaphores.

A wait statement can block a process's execution, while a signal statement can cause another process to be unblocked. However, there are some differences between them. When a process executes a P operation, it does not necessarily block that process because the counting semaphore may be greater than zero. In contrast, when a wait statement is executed, it always blocks the process. When a task executes a V operation on a semaphore, it either unblocks a task waiting on that semaphore or increments the semaphore counter if there is no task to unlock. On the other hand, if a process executes a signal statement when there is no other process to unblock, there is no effect on the condition variable. Another difference between semaphores and monitors is that users awaken by a V operation can resume execution without delay. Contrarily, users awaken by a signal operation are restarted only when the monitor is unlocked. In addition, a monitor solution is more structured than the one with semaphores because the data and procedures are encapsulated in a single module and that the mutual exclusion is provided automatically by the implementation.

Link: here for further reading. Hope it helps.

Semaphore allows multiple threads (up to a set number) to access a shared object. Monitors allow mutually exclusive access to a shared object.

Monitor

Semaphore

One Line Answer:

Monitor: controls only ONE thread at a time can execute in the monitor. (need to acquire lock to execute the single thread)

Semaphore: a lock that protects a shared resource. (need to acquire the lock to access resource)