MFC - CDialog Not Getting SetEvent with WaitForSingleObject From Thread

[steveq]

Hey Gurus,

I have a Dialog class that launches a worker thread. Under some circumstances, I want to tell the thread to stop work and exit. To know when the thread has exited cleanly I use SetEvent and WaitForSingleObject. The problem is, I don't seem to be getting the event in my dialog class. My code is as follows:

// My thread parameters
struct ThreadParams
{
HANDLE	threadStopEvent;
int		threadJob;

// Other variables removed for the sake of brevity
};

// My thread variables in the header of my dialog class
DWORD	m_psdbThreadID;
HANDLE	m_psdbThread;
HANDLE	m_psdbThreadStopEvent;
struct ThreadParams m_psdbThreadParams;

// Launch the thread
m_psdbThreadParams.threadJob = THREAD_UPDATEIMAGES;
m_psdbThreadStopEvent= CreateEvent( NULL, FALSE, FALSE, NULL );
m_psdbThreadParams.threadStopEvent = m_psdbThreadStopEvent;

// Launch the job
m_psdbThread= CreateThread( NULL, 0, ( LPTHREAD_START_ROUTINE ) PSDBThread, &m_ m_psdbThreadParams, 0, &m_psdbThreadID);

// My thread routine
void PSDBThread( void *param )
{
	struct ThreadParams *tp = ( struct ThreadParams * )param;

	bool finished = false;

	switch ( tp->threadJob )
	{
		case THREAD_UPDATEIMAGES:
		{
			do
			{
				// Check for any messages from the main app.
				while ( PeekMessage( &msg, NULL, THREAD_STOP, THREAD_OK, PM_REMOVE ) )
				{
					switch ( msg.message )
					{
						case THREAD_STOP:
						{
							quit = true;
						}
						break;

						case THREAD_PAUSE:
						{
							pause = true;
						}
						break;

						case THREAD_RESTART:
						{
							pause = false;
						}
						break;

						default:
						{
							DispatchMessage( &msg );
						}
						break;
					}
				}

				if ( !quit )
				{
					// Do stuff until finished
					finished = true;
				}

				else break;

				if ( finished ) break;

			} while( true );
		}

		break;
	}

	SetEvent( tp->threadStopEvent );
}

// My code to stop the thread before it has finished its task
if ( m_psdbThread)
{
	PostThreadMessage( m_psdbThreadID, THREAD_STOP, NULL, NULL );

	// Wait for the thread to stop
	DWORD dwEvent = WaitForSingleObject( m_psdbThreadStopEvent, INFINITE );

	switch ( dwEvent )
	{
		case WAIT_OBJECT_0 + 0:
			// TODO: Perform tasks required by this event
			printf( "First event was signaled.\n" );
		break;

		case WAIT_TIMEOUT:
			printf( "Wait timed out.\n" );
		break;

		// Return value is invalid.
		default:
			printf( "Wait error: %d\n", GetLastError() );
		break;
	}
}

In the above, my thread runs nicely and exits cleanly on its own, but if I try and stop it WaitForSingleObject never returns. If I change INFINTE to say 3000 (3 secs), then I get WAIT_TIMEOUT.

Perhaps I am not launching the thread correctly?

Could someone please give me some advice on what may be going wrong.

Thanks so much,

Steve Q.

[Arjay]

Where are you calling WaitForSingleObject? If you are doing that in the UI thread, it's a problem. If you are trying to wait on an object in the UI thread, you need to use MessageWaitForMultipleObjects.

[steveq]

Hey Arjay,

Thanks so much for your reply. Yes, the wait is in the UI. It's in the dialog class that launched the thread.

I have tried your suggestion, but I still don't seem to get the event. I changed my wait code to this:

HANDLE ghEvents[ 1 ];

ghEvents[ 0 ] =m_psdbThreadStopEvent;

while ( MsgWaitForMultipleObjects( 1, ghEvents, false, INFINITE, QS_ALLEVENTS ) != WAIT_OBJECT_0 );

It never gets out of the while loop?

It's almost like my thread stops processing and doesn't set the event because my main dialog is in a wait loop (makes no sense I know)?

SQ :-)

[Igor Vartanov]

I would say, your approach is unnecessary tricky.

When you have access to thread event object from both sides, signaler and listener, there's no need in sending messages. The event itself is a message.

The sample demonstrating my statement:
HOWTO_stop_thread_safely.zip

#include <windows.h>
#include <stdio.h>
#include <conio.h>

DWORD CALLBACK ThreadProc(LPVOID pVoid)
{
    DWORD dwRes;
    HANDLE hStop = (HANDLE)pVoid;

    while (1)
    {
        dwRes = WaitForSingleObject(hStop, 0);   // #1: test event without impeding thread

        switch(dwRes)
        {
        case WAIT_OBJECT_0:
            printf("Leaving thread...\n");
            return 0;

        case WAIT_TIMEOUT:
            printf("Doing worker stuff...\n");
            Sleep(10000);
            break;
        }
    }
    return 0;
}

int main()
{
    HANDLE hStop = CreateEvent(NULL, TRUE, FALSE, NULL);
    DWORD dwThreadId;
    HANDLE hThread = CreateThread(NULL, 0, ThreadProc, (LPVOID)hStop, 0, &dwThreadId);

    printf("Press any key to quit.\n\n");
    while(!kbhit());
    getchar();

    printf("Signalling thread to quit...\n");
    SetEvent(hStop);

    printf("Waiting for thread shutdown...\n");
    WaitForSingleObject(hThread, INFINITE);   // #2: waiting on thread itself, not event
    printf("App is going down...\n");
    CloseHandle(hThread);
    CloseHandle(hStop);

    return 0;
}

J:\Temp\127>127
Press any key to quit.

Doing worker stuff...
Doing worker stuff...
Doing worker stuff...
Doing worker stuff...
Doing worker stuff...

Signalling thread to quit...
Waiting for thread shutdown...
Leaving thread...
App is going down...

[steveq]

Hey Igor,

Thanks heaps for your help.

I have tried your suggestion, but with no luck.

The thread receives the SetEvent from the dialog with no problems, but the dialog always gets stuck in an infinite wait loop. I'm not sure, but the thread seems to stop once I hit the wait in my dialog. Perhaps that is just the debugger though?

This is really frustrating, as I just can't see what I am doing wrong!

*EDIT 1* I'm becoming more convinced that the thread stops running when I hit the MsgWaitForMultipleObjects in my main dialog. I did an AfxMessageBox just before the threads ends, and it doesn't get displayed. Is there anything I could be doing that could cause this? (I could also be way of the mark here!)

*EDIT 2* The thread is not quitting until AFTER the MsgWaitForMultipleObjects, if I put 1000 in instead of INFINITE, the threads starts again after the time has expired. This is why I'm not getting the event. Why does my thread stop processing when I'm in MsgWaitForMultipleObjects?

Steve :-(

[Igor Vartanov]

The only advice I'm able to come up with is: read documentation, use debugger.

[2kaud]

To try to provide further guidance, it would be helpful to see your code for the main dialog that includes MsgWaitForMultipleObjects() and the current code you have for the thread and dialog.

I would also suggest that you do the simplest possible program that re-produces the problem and post it here.

[steveq]

Thanks everyone,

I'll do exactly what 2kaud suggested. I'll write a basic dialog based app that launches a thread and see if it exhibits the same behaviour.

Watch this space.

Thanks so much for all your help, it is greatly appreciated.

Steve Q. :-)

[steveq]

Hey All,

Well I have built a new dialog based MFC application, that just launches a thread, and prints some stuff.

It exhibits exactly the same behaviour as my other program, as soon as I MsgWaitForMultipleObjects, the thread stops and the dialog goes into an infinite wait loop.

Here is the souce code, I have also attached the entire solution as a zip file:

// ThreadTestDlg.cpp : implementation file
//

#include "stdafx.h"
#include "ThreadTest.h"
#include "ThreadTestDlg.h"
#include "DlgProxy.h"
#include "afxdialogex.h"

#ifdef _DEBUG
#define new DEBUG_NEW
#endif

DWORD CALLBACK ThreadProc( void *param );


const int THREAD_STOP = WM_APP + 80;
const int THREAD_PAUSE = WM_APP + 81;
const int THREAD_RESTART = WM_APP + 82;
const int THREAD_OK = WM_APP + 83;
const int THREAD_FINISHED = WM_APP + 84;
const int THREAD_MESSAGE = WM_APP + 85;


// CAboutDlg dialog used for App About

class CAboutDlg : public CDialogEx
{
public:
	CAboutDlg();

// Dialog Data
	enum { IDD = IDD_ABOUTBOX };

	protected:
	virtual void DoDataExchange(CDataExchange* pDX);    // DDX/DDV support

// Implementation
protected:
	DECLARE_MESSAGE_MAP()
};

CAboutDlg::CAboutDlg() : CDialogEx(CAboutDlg::IDD)
{
}

void CAboutDlg::DoDataExchange(CDataExchange* pDX)
{
	CDialogEx::DoDataExchange(pDX);
}

BEGIN_MESSAGE_MAP(CAboutDlg, CDialogEx)
END_MESSAGE_MAP()


// CThreadTestDlg dialog


IMPLEMENT_DYNAMIC(CThreadTestDlg, CDialogEx);

CThreadTestDlg::CThreadTestDlg(CWnd* pParent /*=NULL*/)
	: CDialogEx(CThreadTestDlg::IDD, pParent)
{
	m_hIcon = AfxGetApp()->LoadIcon(IDR_MAINFRAME);
	m_pAutoProxy = NULL;
}

CThreadTestDlg::~CThreadTestDlg()
{
	// If there is an automation proxy for this dialog, set
	//  its back pointer to this dialog to NULL, so it knows
	//  the dialog has been deleted.
	if (m_pAutoProxy != NULL)
		m_pAutoProxy->m_pDialog = NULL;
}

void CThreadTestDlg::DoDataExchange(CDataExchange* pDX)
{
	CDialogEx::DoDataExchange( pDX );
	DDX_Control( pDX, IDC_STARTTHREAD, m_startThread );
	DDX_Control( pDX, IDC_STOPTHREAD, m_stopThread );
	DDX_Control( pDX, IDC_LISTINFO, m_listInfo );
}

BEGIN_MESSAGE_MAP(CThreadTestDlg, CDialogEx)
	ON_WM_SYSCOMMAND()
	ON_WM_CLOSE()
	ON_WM_PAINT()
	ON_WM_QUERYDRAGICON()
	ON_BN_CLICKED( IDC_STARTTHREAD, &CThreadTestDlg::OnBnClickedStartthread )
	ON_BN_CLICKED( IDC_STOPTHREAD, &CThreadTestDlg::OnBnClickedStopthread )
	ON_MESSAGE( THREAD_MESSAGE, ThreadMessage )
END_MESSAGE_MAP()


// CThreadTestDlg message handlers

BOOL CThreadTestDlg::OnInitDialog()
{
	CDialogEx::OnInitDialog();

	// Add "About..." menu item to system menu.

	// IDM_ABOUTBOX must be in the system command range.
	ASSERT((IDM_ABOUTBOX & 0xFFF0) == IDM_ABOUTBOX);
	ASSERT(IDM_ABOUTBOX < 0xF000);

	CMenu* pSysMenu = GetSystemMenu(FALSE);
	if (pSysMenu != NULL)
	{
		BOOL bNameValid;
		CString strAboutMenu;
		bNameValid = strAboutMenu.LoadString(IDS_ABOUTBOX);
		ASSERT(bNameValid);
		if (!strAboutMenu.IsEmpty())
		{
			pSysMenu->AppendMenu(MF_SEPARATOR);
			pSysMenu->AppendMenu(MF_STRING, IDM_ABOUTBOX, strAboutMenu);
		}
	}

	// Set the icon for this dialog.  The framework does this automatically
	//  when the application's main window is not a dialog
	SetIcon(m_hIcon, TRUE);			// Set big icon
	SetIcon(m_hIcon, FALSE);		// Set small icon

	// TODO: Add extra initialization here
	m_thread = NULL;

	return TRUE;  // return TRUE  unless you set the focus to a control
}

void CThreadTestDlg::OnSysCommand(UINT nID, LPARAM lParam)
{
	if ((nID & 0xFFF0) == IDM_ABOUTBOX)
	{
		CAboutDlg dlgAbout;
		dlgAbout.DoModal();
	}
	else
	{
		CDialogEx::OnSysCommand(nID, lParam);
	}
}

// If you add a minimize button to your dialog, you will need the code below
//  to draw the icon.  For MFC applications using the document/view model,
//  this is automatically done for you by the framework.

void CThreadTestDlg::OnPaint()
{
	if (IsIconic())
	{
		CPaintDC dc(this); // device context for painting

		SendMessage(WM_ICONERASEBKGND, reinterpret_cast<WPARAM>(dc.GetSafeHdc()), 0);

		// Center icon in client rectangle
		int cxIcon = GetSystemMetrics(SM_CXICON);
		int cyIcon = GetSystemMetrics(SM_CYICON);
		CRect rect;
		GetClientRect(&rect);
		int x = (rect.Width() - cxIcon + 1) / 2;
		int y = (rect.Height() - cyIcon + 1) / 2;

		// Draw the icon
		dc.DrawIcon(x, y, m_hIcon);
	}
	else
	{
		CDialogEx::OnPaint();
	}
}

// The system calls this function to obtain the cursor to display while the user drags
//  the minimized window.
HCURSOR CThreadTestDlg::OnQueryDragIcon()
{
	return static_cast<HCURSOR>(m_hIcon);
}

// Automation servers should not exit when a user closes the UI
//  if a controller still holds on to one of its objects.  These
//  message handlers make sure that if the proxy is still in use,
//  then the UI is hidden but the dialog remains around if it
//  is dismissed.

void CThreadTestDlg::OnClose()
{
	if (CanExit())
		CDialogEx::OnClose();
}

void CThreadTestDlg::OnOK()
{
	if (CanExit())
		CDialogEx::OnOK();
}

void CThreadTestDlg::OnCancel()
{
	if (CanExit())
		CDialogEx::OnCancel();
}

BOOL CThreadTestDlg::CanExit()
{
	// If the proxy object is still around, then the automation
	//  controller is still holding on to this application.  Leave
	//  the dialog around, but hide its UI.
	if (m_pAutoProxy != NULL)
	{
		ShowWindow(SW_HIDE);
		return FALSE;
	}

	return TRUE;
}



void CThreadTestDlg::OnBnClickedStartthread()
{
	// TODO: Add your control notification handler code here
	m_listInfo.ResetContent();

	// Set up the thread parameters
	m_tp.hWnd = m_hWnd;

	// Launch the job
	m_thread = CreateThread( NULL, 0, ( LPTHREAD_START_ROUTINE ) ThreadProc, &m_tp, 0, &m_threadID );
}


void CThreadTestDlg::OnBnClickedStopthread()
{
	// TODO: Add your control notification handler code here
	if ( m_thread )
	{
		HANDLE ghEvents[ 1 ];

		PostThreadMessage( m_threadID, THREAD_STOP, NULL, NULL );

		ghEvents[ 0 ] = m_thread;

		while ( MsgWaitForMultipleObjects( 1, ghEvents, false, INFINITE, QS_ALLINPUT ) != WAIT_OBJECT_0 );

		// This NEVER gets called.
		AfxMessageBox( _T( "MsgWaitForMultipleObjects has returned." ) );
	}
}

afx_msg LRESULT CThreadTestDlg::ThreadMessage( WPARAM param1, LPARAM param2 )
{
	CString *message = ( CString * ) param2;

	m_listInfo.AddString( *message );

	delete message;

	return false;
}

DWORD CALLBACK ThreadProc( void *param )
{
	MSG msg;

	int count = 0;

	bool quit = false;

	struct ThreadParams *tp = ( struct ThreadParams * )param;

	CString *message;

	message = new CString;

	message->Format( _T( "Thread started..." ) );

	PostMessage( tp->hWnd, THREAD_MESSAGE, NULL, ( LPARAM ) message );

	do
	{
		while ( PeekMessage( &msg, NULL, THREAD_STOP, THREAD_MESSAGE, PM_REMOVE ) )
		{
			switch ( msg.message )
			{
				case THREAD_STOP:
				{
					quit = true;

					message = new CString;

					message->Format( _T( "Thread stopping..." ) );

					PostMessage( tp->hWnd, THREAD_MESSAGE, NULL, ( LPARAM ) message );
				}

				break;

				default:
				{
					DispatchMessage( &msg );
				}

				break;
			}
		}

		if ( !quit )
		{
			Sleep( 1000 );

			message = new CString;

			message->Format( _T( "Thread processing - loop %d." ), ++count );

			PostMessage( tp->hWnd, THREAD_MESSAGE, NULL, ( LPARAM ) message );
		}

	} while ( !quit );

	message = new CString;

	message->Format( _T( "Thread exiting." ), ++count );

	PostMessage( tp->hWnd, THREAD_MESSAGE, NULL, ( LPARAM ) message );

	return 1;
}

Any ideas?

Thanks again for all your help.

Steve Q.

ThreadTest_SQ.zip

[Igor Vartanov]

It looks like I posted my sample in vain. You keep trying with thread message, don't you?

[Igor Vartanov]

        while ( MsgWaitForMultipleObjects( 1, ghEvents, true, INFINITE, QS_ALLINPUT ) != WAIT_OBJECT_0 );

This solves your issue.

[Igor Vartanov]

        while ( MsgWaitForMultipleObjects( 1, ghEvents, false, INFINITE, QS_ALLINPUT ) != (WAIT_OBJECT_0 + 1) );

And this does that too.

[steveq]

Hey Igor,

Your first suggestion worked in my test program, but not in my main program. Very strange.

This still waits forever:

while ( MsgWaitForMultipleObjects( 1, ghEvents, true, INFINITE, QS_ALLINPUT ) != WAIT_OBJECT_0 );

and this returns immediately (I put a 5 second sleep in my thread before it exited to make sure my MagWaitForMultipleObjects also waited for 5 secs before returning, alas it returned straight away):

while ( MsgWaitForMultipleObjects( 1, ghEvents, false, INFINITE, QS_ALLINPUT ) != WAIT_OBJECT_0 + 1 );

I'll have to look a little deeper into my main program to see why it differs from my test program.

With respect to the thread message, I send multiple messages to my thread, such as STOP, PAUSE, RESTART, RELOAD and there may be more as I further develop my code. I should have made this clearer in my original post. The thread is a background processing task that works while the user continues to use my software. From what I can tell, this appeared to be the most efficient way of passing various messages to the thread. Using an event meant multiple thread parameter variables and multiple CreateEvent calls. Posting a message, means all I have to do is add another thread ID and then case for it in the message loop.

I'm quite happy to be proven wrong if you still believe an event is a better way of handling it, and I'll modify my code accordingly.

Thanks so much for you patience and help,

Steve Q.

[2kaud]

while ( MsgWaitForMultipleObjects( 1, ghEvents, true, INFINITE, QS_ALLINPUT ) != WAIT_OBJECT_0 );

The while loop may not be terminating as expected, but what values are being returned by MsgWaitForMultipleObjects()?

If the parameter bWaitAll is true (as here) then the return value indicates that the state of all specified objects is signalled - ie that the thread has terminated - so as the timeout is INFINITE you shouldn't need to check the return value. What does

MsgWaitForMultipleObjects( 1, ghEvents, true, INFINITE, QS_ALLINPUT );

do?

PS
MsgWaitForMultipleObjects() with bWaitAll true and dwWakeMask set to QS_ALLINPUT will wait until the specified handle is signalled and an input type has occurred. From MSDN

When bWaitAll is TRUE, the function's wait is completed only when the states of all objects have been set to signaled and an input event has been received.

so even if the thread has been terminated and the handle signalled, unless an input event also occurs MsgWaitForMultipleObjects(..true..) won't return. This may expalin why it works in the test program but not in the main program.

[steveq]

Hey 2kaud,

Without the while loop, MsgWaitForMultipleObjects still does not return. I changed the code to this:

do
{
	dwEvent = MsgWaitForMultipleObjects( 1, ghEvents, true, INFINITE, QS_ALLINPUT );

} while ( dwEvent != WAIT_OBJECT_0 );

and MsgWaitForMultipleObjects never returned. So I am unable to check the values of dwEvent. If I replace INFINITE with 1000, then dwEvent returns WAIT_TIMEOUT (as you would expect).

You say:

MsgWaitForMultipleObjects() with bWaitAll true and dwWakeMask set to QS_ALLINPUT will wait until the specified handle is signalled and an input type has occurred. From MSDN

Perhaps I should set QS_ALLINPUT to something else?

Thanks again,

Steve Q.