Subj : (MFC)Multithreading : Main thread gets blocked ? What could be the reason ?
To   : comp.programming.threads
From : ajay.sonawane
Date : Thu Jun 30 2005 05:00 am

Its a threading problem. Let me explain it first.

I am using MFC.
My GUI (main) thread creates a worker thread that does some lengthy
operations using while loop.  I want to keep only one worker thread
alive for doing that lengthy operations. Main thread creates worker
thread when it receives a user defined messages and when no thread
(created eariler) is running ( I checked it by using
WaitForSingleObject(hThread) ) , I am using some of member variables to
abort thread and restart lengthy operation from start if list is
refilled from main thread. It means that without creating new thread, I
reused existing one to do lengthy operation.
I have used critical section to synchronize access to some variables
and operations.
Here is code,

LRSULT CMainFrame::OnUserMessage(WPARAM wParam,LPARAm lParam)
{

  if(!IsThreadRunning(0))
  {
//thread is not running
// delete old pointer and start new thread
     if(m_pAdvtThread)
     {
	delete m_pAdvtThread;
	m_pAdvtThread = NULL;
     }
  }
  else
  {
	TRACE("\nThread is still running !" );
	m_bRestartProcessAdvts = TRUE;
	g_CS.Unlock();
	return 1;
  }
  g_CS.Unlock();


  g_CS.Lock();
  // Start a worker thread to searck keyword in browsed page
  m_bRestartProcessAdvts = FALSE;
  m_bAbortProcessAdvts = FALSE;
  m_pAdvtThread = AfxBeginThread
(AdvtThreadProc,this,THREAD_PRIORITY_LOWEST,0,CREATE_SUSPENDED);
  m_pAdvtThread->SetThreadPriority(THREAD_PRIORITY_LOWEST);
  m_pAdvtThread->m_bAutoDelete = FALSE;
  m_pAdvtThread->ResumeThread();
  g_CS.Unlock();
}



// My worker thread
UINT AdvtThreadProc (LPVOID lpParam)
{

  // Process keywords
  for (int i=0; i < m_pList->GetCount(); i++)
  {
 	::Sleep(10);

	// Abort Processing
	if(pMainFrame->m_bAbortProcessAdvts)
	{
    	   AfxEndThread(0,TRUE);
	}

	// Restart Processing
	if(pMainFrame->m_bRestartProcessAdvts)
	{
	    g_CS.Lock();
	    pMainFrame->m_bRestartProcessAdvts = FALSE;
	    g_CS.Unlock();
	    continue;
	 }

	DoSomeLengthOperation();

	AfxEndThread(1,TRUE);
}


bool CMainFrame::IsThreadRunning(dwTimeOut)
{
  if(m_pAdvtThread)
  {
    if(WAIT_TIMEOUT == WaitForSingleObject(
                      m_pAdvtThread->m_hThread,dwTimeOut))
     return true;	// Thread is still running
  }
   return false;
}



Now my problem is , Many times my main thread becomes blocked and
utilized 100% CPU, I think that it happens due to some deadlock
situation. I am not getting where I did wrong ? I am using Sleep(10) to
sleep thread for some time so that it won't make thread busy. I used
WaitForSingleObject with value 0 means it will check and return state
of thread. I agree that I am using WaitForSingleObject in UI thread
which is not a good idea, But That is not the problem. I think that
critical section might be causing deadlocks. Please let me know the
solution.

.