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PART II
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Part II:
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// This thread decrements SharedResCount class DecThread { int num; public Thread Thrd; public DecThread(string name, int n) { Thrd = new Thread(new ThreadStart(thisRun)); num = n; ThrdName = name; ThrdStart(); } // Entry point of thread void Run() { ConsoleWriteLine(ThrdName + " is waiting for the mutex"); // Acquire the Mutex SharedResMtxWaitOne(); ConsoleWriteLine(ThrdName + " acquires the mutex"); do { ThreadSleep(500); SharedResCount--; ConsoleWriteLine("In " + ThrdName + ", SharedResCount is " + SharedResCount); num--; } while(num > 0); ConsoleWriteLine(ThrdName + " releases the mutex"); // Release the Mutex SharedResMtxReleaseMutex(); } } class MutexDemo { static void Main() { // Construct three threads IncThread mt1 = new IncThread("Increment Thread", 5); ThreadSleep(1); // let the Increment thread start DecThread mt2 = new DecThread("Decrement Thread", 5); mt1ThrdJoin(); mt2ThrdJoin(); } }
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The output is shown here:
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Increment Thread is waiting for the mutex Increment Thread acquires the mutex
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Decrement Thread is waiting for the mutex In Increment Thread, SharedResCount is 1 In Increment Thread, SharedResCount is 2 In Increment Thread, SharedResCount is 3 In Increment Thread, SharedResCount is 4 In Increment Thread, SharedResCount is 5 Increment Thread releases the mutex Decrement Thread acquires the mutex In Decrement Thread, SharedResCount is 4 In Decrement Thread, SharedResCount is 3 In Decrement Thread, SharedResCount is 2 In Decrement Thread, SharedResCount is 1 In Decrement Thread, SharedResCount is 0 Decrement Thread releases the mutex
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PART II
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As the output shows, access to SharedResCount is synchronized, with only one thread at a time being able to change its value To prove that the Mtx mutex was needed to produce the preceding output, try commenting out the calls to WaitOne( ) and ReleaseMutex( ) in the preceding program When you run the program, you will see the following sequence (the actual output you see may vary):
In In In In In In In In In Increment Decrement Increment Decrement Increment Decrement Increment Decrement Increment Thread, Thread, Thread, Thread, Thread, Thread, Thread, Thread, Thread, SharedResCount SharedResCount SharedResCount SharedResCount SharedResCount SharedResCount SharedResCount SharedResCount SharedResCount is is is is is is is is is 1 0 1 0 1 0 1 0 1
As this output shows, without the mutex, increments and decrements to SharedResCount are interspersed rather than sequenced The mutex created by the previous example is known only to the process that creates it However, it is possible to create a mutex that is known systemwide To do so, you must create a named mutex, using one of these constructors: public Mutex(bool owned, string name) public Mutex(bool owned, string name, out bool whatHappened) In both forms, the name of the mutex is passed in name In the first form, if owned is true, then ownership of the mutex is requested However, because a systemwide mutex might already be owned by another process, it is better to specify false for this parameter In the second form, on return, whatHappened will be true if ownership was requested and acquired It will be false if ownership was denied (There is also a third form of the Mutex constructor that allows you to specify a MutexSecurity object, which controls access) Using a named mutex enables you to manage interprocess synchronization One other point: It is legal for a thread that has acquired a mutex to make one or more additional calls to WaitOne( ) prior to calling ReleaseMutex( ), and these additional calls will succeed That is, redundant calls to WaitOne( ) will not block a thread that already owns the mutex However, the number of calls to WaitOne( ) must be balanced by the same number of calls to ReleaseMutex( ) before the mutex is released
Part II:
Exploring the C# Library
The Semaphore
A semaphore is similar to a mutex except that it can grant more than one thread access to a shared resource at the same time Thus, the semaphore is useful when a collection of resources is being synchronized A semaphore controls access to a shared resource through the use of a counter If the counter is greater than zero, then access is allowed If it is zero, access is denied What the counter is counting are permits Thus, to access the resource, a thread must be granted a permit from the semaphore In general, to use a semaphore, the thread that wants access to the shared resource tries to acquire a permit If the semaphore s counter is greater than zero, the thread acquires a permit, which causes the semaphore s count to be decremented Otherwise, the thread will block until a permit can be acquired When the thread no longer needs access to the shared resource, it releases the permit, which causes the semaphore s count to be incremented If there is another thread waiting for a permit, then that thread will acquire a permit at that time The number of simultaneous accesses permitted is specified when the semaphore is created If you create a semaphore that allows only one access, then a semaphore acts just like a mutex Semaphores are especially useful in situations in which a shared resource consists of a group or pool For example, a collection of network connections, any of which can be used for communication, is a resource pool A thread needing a network connection doesn t care which one it gets In this case, a semaphore offers a convenient mechanism to manage access to the connections The semaphore is implemented by SystemThreadingSemaphore It has several constructors The simplest form is shown here: public Semaphore(int initial, int max) Here, initial specifies the initial value of the semaphore permit counter, which is the number of permits available The maximum value of the counter is passed in max Thus, max represents the maximum number of permits that can granted by the semaphore The value in initial specifies how many of these permits are initially available Using a semaphore is similar to using a mutex, described earlier To acquire access, your code will call WaitOne( ) on the semaphore This method is inherited by Semaphore from the WaitHandle class WaitOne( ) waits until the semaphore on which it is called can be acquired Thus, it blocks execution of the calling thread until the specified semaphore can grant permission When your code no longer needs ownership of the semaphore, it releases it by calling Release( ), which is shown here: public int Release( ) public int Release(int num) The first form releases one permit The second form releases the number of permits specified by num Both return the permit count that existed prior to the release It is possible for a thread to call WaitOne( ) more than once before calling Release( ) However, the number of calls to WaitOne( ) must be balanced by the same number of calls to Release( ) before the permit is released Alternatively, you can call the Release(int) form, passing a number equal to the number of times that WaitOne( ) was called
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