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MANUALRESETEVENT
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Suppose that you want to know if an AutoResetEvent object is signaled. One way you could do this is to call WaitOne on it, passing in zero for the wait time. If the AutoResetEvent were not signaled, it would return false. If it were signaled, it would return true. The problem is that when an AutoResetEvent is signaled and a wait is performed on it, the object automatically switches to being not signaled. This means that if one thread were inspecting the state of things it would change them by observing them. To address this and similar issues, we can use ManualResetEvent. As the name indicates, the state of the event does not change when it is waited on. The behavior can be thought of as being similar to a water faucet. When turned on, it will stay on until it is turned off. This contrasts with the AutoResetEvent, which turns itself off as soon as someone notices that it is on.
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The ManualResetEvent is a synchronization mechanism that remains in a signaled state regardless of how many times a wait method is called on it. It must be changed from the signaled state using the Reset method.
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Listing 8.5 shows an example that demonstrates using a ManualResetEvent object.
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Listing 8.5 The ManualResetEvent offers greater control (VB.NET).
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Private ReadyForWork() As ManualResetEvent Declares an array of . . . ManualResetEvents ReDim ReadyForWork(HowManyWorkers - 1) Resizes the array to the . . . number of workers For i = 0 To HowManyWorkers - 1 Workers(i) = New ClassWorkUnit("Worker" + i.ToString()) ReadyForWork(i) = Workers(i).ReadyForWork Retrieves the Worker s ResultsReady(i) = Workers(i).ResultsReady instance of the Next ManualResetEvent . . . Public Function WorkerThreadAvailability() As Boolean() Dim Results() As Boolean ReDim Results(Workers.Length - 1)
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Dim i As Long For i = 0 To Workers.Length - 1 Results(i) = ReadyForWork(i).WaitOne(0, False) Next Return Results End Function
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Inspects the signaled state of the ManualResetEvent
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To change an instance of the ManualResetEvent class from being signaled to not, we use the Reset method. Reset returns a Boolean indicating the success of the operation. As with all operations that return a value, ensure that the operation succeeds.
If Not ReadyForWorkEvent.Reset() Then Throw New Exception("Unable to reset ReadyForWorkEvent") End If
Instances of the ManualResetEvent class provide a robust means of synchronizing activity. They provide a high degree of control and are easy to use. They can be used with the WaitOne, WaitAny, and WaitAll methods.
The WaitHandle.WaitTimeout constant is currently 258.
Table 8.1 compares the results of the wait methods of AutoResetEvent and ManualResetEvent.
Table 8.1 Comparison of AutoResetEvent and ManualResetEvent AutoResetEvent Return Code TheEvent.Set() TheEvent.WaitOne(0, False) TheEvent.WaitOne(0, False) TheEvent.Reset() TheEvent.WaitOne(0, False) TheEvent.Set() WaitHandle.WaitAny(H, 0, False) WaitHandle.WaitAny(H, 0, False) TheEvent.Reset() WaitHandle.WaitAny(H, 0, False) TheEvent.Set() WaitHandle.WaitAll(H, 0, False) WaitHandle.WaitAll(H, 0, False) TheEvent.Reset() WaitHandle.WaitAll(H, 0, False) True True False True False True 0 258 True 258 True True False True False Yes Yes Yes Yes Signaled Yes Yes ManualResetEvent Return Code True True True True False True 0 0 True 258 True True True True False Yes Yes Yes Yes Yes Yes Signaled Yes Yes Yes
Statements
MANUALRESETEVENT
The Return Code column under each type of reset event indicates the value returned by the statement. The Signaled column indicates if the object is signaled. Notice that ManualResetEvent s signaled state does not change except for when Set and Reset are invoked on it. The value 258 corresponds to the WaitHandle.WaitTimeout constant.
If multiple threads manipulate the same ManualResetEvent, a synchronization block may be needed to ensure proper execution.
Both the manual and autoreset events are useful constructs. Many things can only be accomplished by using a ManualResetEvent. One word of warning, the following instruction is atomic.
TestAutoEvent.WaitOne()
While similar statements with a ManualResetEvent are not.
TestManualEvent.WaitOne() TestManualEvent.Reset()
To ensure proper execution, enclose the preceding lines in a synchronization block.
MUTEX CLASS: WAITONE AND RELEASEMUTEX
Suppose that you wanted to use a single text file to store the output of multiple threads. We ve seen how race conditions can happen. Any time a shared resource is used, there is the chance of a race condition. Since a file might be accessed not only by multiple threads but also multiple processes, the operating system provides for various file-sharing restrictions.
Listing 8.6 Using a Mutex to guard a shared text file (C#)
. . . public class ClassSafeFile { private string Filename; private Mutex TheMutex; public ClassSafeFile(bool UseMutex,string Filename ) { if (UseMutex) { TheMutex=new Mutex(false,"Manning.Dennis.Threading.Ch8.S8"); } this.Filename = Filename;
} public void Write(string Contents) { int ByteCount = System.Text.Encoding.Unicode.GetByteCount(Contents); byte[] Bytes = System.Text.Encoding.Unicode.GetBytes(Contents);
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