Semaphore vs Monitors - what s the difference

Question

What are the major differences between a Monitor and a Semaphore

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When a semaphore is used to guard a critical region, there is no direct relationship between the semaphore and the data being protected. This is part of the reason why semaphores may be dispersed around the code, and why it is easy to forget to call wait or notify, in which case the result will be, respectively, to violate mutual exclusion or to lock the resource permanently.

In contrast, niehter of these bad things can happen with a monitor. A monitor is tired directly to the data (it encapsulates the data) and, because the monitor operations are atomic actions, it is impossible to write code that can access the data without calling the entry protocol. The exit protocol is called automatically when the monitor operation is completed.

A monitor has a built-in mechanism for condition synchronisation in the form of condition variable before proceeding. If the condition is not satisfied, the process has to wait until it is notified of a change in the condition. When a process is waiting for condition synchronisation, the monitor implementation takes care of the mutual exclusion issue, and allows another process to gain access to the monitor.

Taken from The Open University M362 Unit 3 "Interacting process" course material.

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A semaphore is a signaling mechanism used to coordinate between threads  Example  One thread is downloading files from the internet and another thread is analyzing the files  This is a classic producer consumer scenario  The producer calls signal   on the semaphore when a file is downloaded  The consumer calls wait   on the same semaphore in order to be blocked until the signal indicates a file is ready  If the semaphore is already signaled when the consumer calls wait  the call does not block  Multiple threads can wait on a semaphore  but each signal will only unblock a single thread  A counting semaphore keeps track of the number of signals  E g  if the producer signals three times in a row  wait   can be called three times without blocking  A binary semaphore does not count but just have the  quot waiting quot  and  quot signalled quot  states  A mutex  mutual exclusion lock  is a lock which is owned by a single thread  Only the thread which have acquired the lock can realease it again  Other threads which try to acquire the lock will be blocked until the current owner thread releases it  A mutex lock does not in itself lock anything - it is really just a flag  But code can check for ownership of a mutex lock to ensure that only one thread at a time can access some object or resource  A monitor is a higher-level construct which uses an underlying mutex lock to ensure thread-safe access to some object  Unfortunately the word  quot monitor quot  is used in a few different meanings depending on context and platform and context  but in Java for example  a monitor is a mutex lock which is implicitly associated with an object  and which can be invoked with the synchronized keyword  The synchronized keyword can be applied to a class  method or block and ensures only one thread can execute the code at a time

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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

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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

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Semaphore    Using a counter or flag to control access some shared resources in a concurrent system  implies use of Semaphore    Example    A counter to allow only 50 Passengers to acquire the 50 seats  Shared resource  of any Theatre Bus Train Fun ride Classroom  And to allow a new Passenger only if someone vacates a seat  A binary flag indicating the free occupied status of any Bathroom  Traffic lights are good example of flags  They control flow by regulating passage of vehicles on Roads  Shared resource    Flags only reveal the current state of Resource  no count or any other information on the waiting or running objects on the resource   Monitor    A Monitor synchronizes access to an Object by communicating with threads interested in the object  asking them to acquire access or wait for some condition to become true   Example    A Father may acts as a monitor for her daughter  allowing her to date only one guy at a time   A school teacher using baton to allow only one child to speak in the class  Lastly a technical one  transactions  via threads  on an Account object synchronized to maintain integrity

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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

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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

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