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PART II
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Part II:
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Exploring the C# Library
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// This creates a semaphore that allows up to two // permits to be granted and that initially has // two permits available static Semaphore sem = new Semaphore(2, 2); public MyThread(string name) { Thrd = new Thread(thisRun); ThrdName = name; ThrdStart(); } // Entry point of thread void Run() { ConsoleWriteLine(ThrdName + " is waiting for a permit"); semWaitOne(); ConsoleWriteLine(ThrdName + " acquires a permit"); for(char ch='A'; ch < 'D'; ch++) { ConsoleWriteLine(ThrdName + " : " + ch + " "); ThreadSleep(500); } ConsoleWriteLine(ThrdName + " releases a permit"); // Release the semaphore semRelease(); } }
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class SemaphoreDemo { static void Main() { // Construct MyThread mt1 MyThread mt2 MyThread mt3 three = new = new = new threads MyThread("Thread #1"); MyThread("Thread #2"); MyThread("Thread #3");
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mt1ThrdJoin(); mt2ThrdJoin(); mt3ThrdJoin(); } }
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MyThread declares the semaphore sem, as shown here:
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static Semaphore sem = new Semaphore(2, 2);
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This creates a semaphore that can grant up to two permits and that initially has both permits available
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Multithreaded Programming, Part One
In MyThreadRun( ), notice that execution cannot continue until a permit is granted by the semaphore, sem If no permits are available, then execution of that thread suspends When a permit does become available, execution resumes and the thread can run In Main( ), three MyThread threads are created However, only the first two get to execute The third must wait until one of the other threads terminates The output, shown here, verifies this (The actual output you see may vary slightly)
Thread Thread Thread Thread Thread Thread Thread Thread Thread Thread Thread Thread Thread Thread Thread Thread Thread Thread #1 #1 #1 #2 #2 #2 #3 #1 #2 #1 #2 #1 #3 #3 #2 #3 #3 #3 is waiting acquires a : A is waiting acquires a : A is waiting : B : B : C : C releases a acquires a : A releases a : B : C releases a for a permit permit for a permit permit for a permit
PART II
permit permit permit
permit
The semaphore created by the previous example is known only to the process that creates it However, it is possible to create a semaphore that is known systemwide To do so, you must create a named semaphore To do this, use one of these constructors: public Semaphore(int initialCount, int maximumCount, string name) public Semaphore(int initialCount, int maximumCount, string name, out bool createdNew) In both forms, the name of the semaphore is passed in name In the first form, if a semaphore by the specified name does not already exist, it is created using the values of initialCount and maximumCount If it does already exist, then the values of initialCount and maximumCount are ignored In the second form, on return, createdNew will be true if the semaphore was created In this case, the values of initialCount and maximumCount will be used to create the semaphore If createdNew is false, then the semaphore already exists and the values of initialCount and maximumCount are ignored (There is also a third form of the Semaphore constructor that allows you to specify a SemaphoreSecurity object, which controls access) Using a named semaphore enables you to manage interprocess synchronization
Using Events
C# supports another type of synchronization object: the event There are two types of events: manual reset and auto reset These are supported by the classes ManualResetEvent and AutoResetEvent These classes are derived from the top-level class EventWaitHandle These classes are used in situations in which one thread is waiting for some event to occur
Part II:
Exploring the C# Library
in another thread When the event takes place, the second thread signals the first, allowing it to resume execution The constructors for ManualResetEvent and AutoResetEvent are shown here: public ManualResetEvent(bool initialState) public AutoResetEvent(bool initialState) Here, if initialState is true, the event is initially signaled If initialState is false, the event is initially non-signaled Events are easy to use For a ManualResetEvent, the procedure works like this A thread that is waiting for some event simply calls WaitOne( ) on the event object representing that event WaitOne( ) returns immediately if the event object is in a signaled state Otherwise, it suspends execution of the calling thread until the event is signaled After another thread performs the event, that thread sets the event object to a signaled state by calling Set( ) Thus, a call Set( ) can be understood as signaling that an event has occurred After the event object is set to a signaled state, the call to WaitOne( ) will return and the first thread will resume execution The event is returned to a non-signaled state by calling Reset( ) The difference between AutoResetEvent and ManualResetEvent is how the event gets reset For ManualResetEvent, the event remains signaled until a call to Reset( ) is made For AutoResetEvent, the event automatically changes to a non-signaled state as soon as a thread waiting on that event receives the event notification and resumes execution Thus, a call to Reset( ) is not necessary when using AutoResetEvent Here is an example that illustrates ManualResetEvent:
// Use a manual event object using System; using SystemThreading; // This thread signals the event passed to its constructor class MyThread { public Thread Thrd; ManualResetEvent mre; public MyThread(string name, ManualResetEvent evt) { Thrd = new Thread(thisRun); ThrdName = name; mre = evt; ThrdStart(); } // Entry point of thread void Run() { ConsoleWriteLine("Inside thread " + ThrdName); for(int i=0; i<5; i++) { ConsoleWriteLine(ThrdName); ThreadSleep(500); } ConsoleWriteLine(ThrdName + " Done!");
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