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CEvent Class

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Represents an event, which is a synchronization object that enables one thread to notify another that an event has occurred.

Syntax

class CEvent : public CSyncObject

Members

Public Constructors

Name Description
CEvent::CEvent Constructs a CEvent object.

Public Methods

Name Description
CEvent::PulseEvent Sets the event to available (signaled), releases waiting threads, and sets the event to unavailable (nonsignaled).
CEvent::ResetEvent Sets the event to unavailable (nonsignaled).
CEvent::SetEvent Sets the event to available (signaled) and releases any waiting threads.
CEvent::Unlock Releases the event object.

Remarks

Events are useful when a thread must know when to perform its task. For example, a thread that copies data to a data archive must be notified when new data is available. By using a CEvent object to notify the copy thread when new data is available, the thread can perform its task as soon as possible.

CEvent objects have two types: manual and automatic.

An automatic CEvent object automatically returns to a non-signaled (unavailable) state after at least one thread is released. By default, a CEvent object is automatic unless you pass TRUE for the bManualReset parameter during construction.

A manual CEvent object stays in the state set by SetEvent or ResetEvent until the other function is called. To create a manual CEvent object, pass TRUE for the bManualReset parameter during construction.

To use a CEvent object, construct the CEvent object when it's required. Specify the name of the event you want to wait on, and also specify that your application should initially own it. You can then access the event when the constructor returns. Call SetEvent to signal (make available) the event object and then call Unlock when you're done accessing the controlled resource.

An alternative method for using CEvent objects is to add a variable of type CEvent as a data member to the class you want to control. During construction of the controlled object, call the constructor of the CEvent data member and specify whether the event is initially signaled, and also specify the type of event object you want, the name of the event (if it will be used across process boundaries), and any security attributes you want.

To access a resource controlled by a CEvent object in this manner, first create a variable of either type CSingleLock or type CMultiLock in the access method of your resource. Then call the Lock method of the lock object (for example, CMultiLock::Lock). At this point, your thread will either gain access to the resource, wait for the resource to be released and gain access, or wait for the resource to be released, time out, and fail to gain access to the resource. In any case, your resource has been accessed in a thread-safe manner. To release the resource, call SetEvent to signal the event object, and then use the Unlock method of the lock object (for example, CMultiLock::Unlock), or let the lock object fall out of scope.

For more information about how to use CEvent objects, see Multithreading: How to Use the Synchronization Classes.

Example

// The following demonstrates trivial usage of the CEvent class.
// A CEvent object is created and passed as a parameter to another
// thread.  The other thread will wait for the event to be signaled
// and then exit

UINT __cdecl MyThreadProc(LPVOID lpParameter)
{
   CEvent *pEvent = (CEvent *)(lpParameter);
   VERIFY(pEvent != NULL);

   // Wait for the event to be signaled
   ::WaitForSingleObject(pEvent->m_hObject, INFINITE);

   // Terminate the thread
   ::AfxEndThread(0, FALSE);
   return 0L;
}

void CEvent_Test()
{
   // Create the CEvent object that will be passed to the thread routine
   CEvent *pEvent = new CEvent(FALSE, FALSE);

   // Create a thread that will wait on the event
   CWinThread *pThread;
   pThread = ::AfxBeginThread(&MyThreadProc, pEvent, 0, 0, CREATE_SUSPENDED, NULL);
   pThread->m_bAutoDelete = FALSE;
   pThread->ResumeThread();

   // Signal the thread to do the next work item
   pEvent->SetEvent();

   // Wait for the thread to consume the event and return
   ::WaitForSingleObject(pThread->m_hThread, INFINITE);
   delete pThread;
   delete pEvent;
}

 

// This example builds upon the previous one.
// A second thread is created to calculate prime numbers.
// The main thread will signal the second thread to calculate the next
// prime number in the series.  The second thread signals the first
// after each number is calculated. Finally, after several iterations
// the worker thread is signaled to terminate.

class CPrimeTest
{
public:
   CPrimeTest()
       : m_pCalcNext(new CEvent(FALSE, FALSE)),
         m_pCalcFinished(new CEvent(FALSE, FALSE)),
         m_pTerminateThread(new CEvent(FALSE, FALSE)),
         m_iCurrentPrime(0)
   {
      // Create a thread that will calculate the prime numbers
      CWinThread *pThread;
      pThread = ::AfxBeginThread(&PrimeCalcProc,
                                 this, 0, 0, CREATE_SUSPENDED, NULL);
      pThread->m_bAutoDelete = FALSE;
      pThread->ResumeThread();

      // Calcuate the first 10 prime numbers in the series on the thread
      for (UINT i = 0; i < 10; i++)
      {
         // Signal the thread to do the next work item
         m_pCalcNext->SetEvent();
         // Wait for the thread to complete the current task
         ::WaitForSingleObject(m_pCalcFinished->m_hObject, INFINITE);
         // Print the result
         TRACE(_T("The value of m_iCurrentPrime is: %d\n"), m_iCurrentPrime);
      }

      // Notify the worker thread to exit and wait for it to complete
      m_pTerminateThread->SetEvent();
      ::WaitForSingleObject(pThread->m_hThread, INFINITE);
      delete pThread;
   }
   ~CPrimeTest()
   {
      delete m_pCalcNext;
      delete m_pCalcFinished;
      delete m_pTerminateThread;
   }

private:
   // Determines whether the given number is a prime number
   static BOOL IsPrime(INT ThisPrime)
   {
      if (ThisPrime < 2)
         return FALSE;

      for (INT n = 2; n < ThisPrime; n++)
      {
         if (ThisPrime % n == 0)
            return FALSE;
      }
      return TRUE;
   }

   // Calculates the next prime number in the series
   static INT NextPrime(INT ThisPrime)
   {
      while (TRUE)
      {
         if (IsPrime(++ThisPrime))
         {
            return ThisPrime;
         }
      }
   }

   // Worker thread responsible for calculating the next prime
   // number in the series
   static UINT __cdecl PrimeCalcProc(LPVOID lpParameter)
   {
      CPrimeTest *pThis = static_cast<CPrimeTest *>(lpParameter);
      VERIFY(pThis != NULL);

      VERIFY(pThis->m_pCalcNext != NULL);
      VERIFY(pThis->m_pCalcFinished != NULL);
      VERIFY(pThis->m_pTerminateThread != NULL);

      // Create a CMultiLock object to wait on the various events
      // WAIT_OBJECT_0 refers to the first event in the array,
      // WAIT_OBJECT_0+1 refers to the second
      CSyncObject *pWaitObjects[] = {pThis->m_pCalcNext,
                                     pThis->m_pTerminateThread};
      CMultiLock MultiLock(pWaitObjects, 2L);
      while (MultiLock.Lock(INFINITE, FALSE) == WAIT_OBJECT_0)
      {
         // Calculate next prime
         pThis->m_iCurrentPrime = NextPrime(pThis->m_iCurrentPrime);
         // Notify main thread calculation is complete
         pThis->m_pCalcFinished->SetEvent();
      }

      // Terminate the thread
      ::AfxEndThread(0, FALSE);
      return 0L;
   }

   CEvent *m_pCalcNext;        // notifies worker thread to calculate next prime
   CEvent *m_pCalcFinished;    // notifies main thread current calculation is complete
   CEvent *m_pTerminateThread; // notifies worker thread to terminate

   INT m_iCurrentPrime; // current calculated prime number
};

Inheritance Hierarchy

CObject

CSyncObject

CEvent

Requirements

Header: afxmt.h

CEvent::CEvent

Constructs a named or unnamed CEvent object.

CEvent(
    BOOL bInitiallyOwn = FALSE,
    BOOL bManualReset = FALSE,
    LPCTSTR lpszName = NULL,
    LPSECURITY_ATTRIBUTES lpsaAttribute = NULL);

Parameters

bInitiallyOwn
If TRUE, the thread for the CMultilock or CSingleLock object is enabled. Otherwise, all threads wanting to access the resource must wait.

bManualReset
If TRUE, specifies that the event object is a manual event, otherwise the event object is an automatic event.

lpszName
Name of the CEvent object. Must be supplied if the object will be used across process boundaries. If the name matches an existing event, the constructor builds a new CEvent object that references the event of that name. If the name matches an existing synchronization object that isn't an event, the construction will fail. If NULL, the name will be null.

lpsaAttribute
Security attributes for the event object. For a full description of this structure, see SECURITY_ATTRIBUTES in the Windows SDK.

Remarks

To access or release a CEvent object, create a CMultiLock or CSingleLock object and call its Lock and Unlock member functions.

To change the state of a CEvent object to signaled (threads don't have to wait), call SetEvent or PulseEvent. To set the state of a CEvent object to nonsignaled (threads must wait), call ResetEvent.

Important

After creating the CEvent object, use GetLastError to ensure that the mutex didn't already exist. If the mutex did exist unexpectedly, it may indicate a rogue process is squatting and may be intending to use the mutex maliciously. In this case, the recommended security-conscious procedure is to close the handle and continue as if there was a failure in creating the object.

CEvent::PulseEvent

Sets the state of the event to signaled (available), releases any waiting threads, and resets it to nonsignaled (unavailable) automatically.

BOOL PulseEvent();

Return Value

Nonzero if the function was successful; otherwise 0.

Remarks

If the event is manual, all waiting threads are released, the event is set to nonsignaled, and PulseEvent returns. If the event is automatic, a single thread is released, the event is set to nonsignaled, and PulseEvent returns.

If no threads are waiting, or no threads can be released immediately, PulseEvent sets the state of the event to nonsignaled and returns.

PulseEvent uses the underlying Win32 PulseEvent function, which can be momentarily removed from the wait state by a kernel-mode asynchronous procedure call. Therefore, PulseEvent is unreliable and shouldn't be used by new applications. For more information, see the PulseEvent function.

CEvent::ResetEvent

Sets the state of the event to nonsignaled until explicitly set to signaled by the SetEvent member function.

BOOL ResetEvent();

Return Value

Nonzero if the function was successful; otherwise 0.

Remarks

This causes all threads wishing to access this event to wait.

This member function isn't used by automatic events.

CEvent::SetEvent

Sets the state of the event to signaled, releasing any waiting threads.

BOOL SetEvent();

Return Value

Nonzero if the function was successful, otherwise 0.

Remarks

If the event is manual, the event will remain signaled until ResetEvent is called. More than one thread can be released in this case. If the event is automatic, the event will remain signaled until a single thread is released. The system will then set the state of the event to nonsignaled. If no threads are waiting, the state remains signaled until one thread is released.

CEvent::Unlock

Releases the event object.

BOOL Unlock();

Return Value

Nonzero if the thread owned the event object and the event is an automatic event; otherwise 0.

Remarks

This member function is called by threads that currently own an automatic event to release it after they're done, if their lock object is to be reused. If the lock object isn't to be reused, this function will be called by the lock object's destructor.

See also

CSyncObject Class
Hierarchy Chart