Change value to a thread

_Flaviu

I indent to use FindFirstChangeNotification in a class static method in order to use it in a thread, something like this:

DWORD WINAPI FilesWatchDogThread(LPVOID lpParam)
{
HANDLE hFileChange = ::FindFirstChangeNotification((LPCTSTR)lpParam, // folder path
TRUE,
FILE_NOTIFY_CHANGE_FILE_NAME);
if(INVALID_HANDLE_VALUE == hFileChange)
{
DWORD dwError = ::GetLastError();
return dwError;
}

while(TRUE)
{
::WaitForSingleObject(hFileChange, INFINITE);
// do my job
::FindNextChangeNotification(hFileChange);
}
return 0;
}

// somewhere …
::CreateThread(NULL, 0, &CMyClass::FilesWatchDogThread, _T("c:\\temp"), 0, NULL);

but how can Itell the thread when a folder to watch has been changed (I mean _T("c:\\temp") change to _T("e:\\whatever")) ? What should I use for that ? CreateEvent or what ?

Victor Nijegorodov

Yes, you can use CreateEvent, change the folder, set its value in lpParam, then SetEvent to let the thread to reread lpParam and continue to work with a new folder. Note, that thread must now wait for multiple objects!

_Flaviu

Thank you Victor, can you elaborate a little bit ? Even some pseudo-code will be good :)

Victor Nijegorodov

Have a look at the [WorkerThreads](http://www.flounder.com/workerthreads.htm) essay of Joe Newcomer. It is MFC issue, but the main concept is the same as for plain Win32 API. Just look at the sections: Pausing a Thread and Thread Shutdown Shutting down a thread from a view or main frame Thread Shutdown Without Polling (Well, you would need something similar to "Pausing" event.

_Flaviu

I have two questions: 1. Why should I use CreateEvent, and if I should, where ? 2. Why should I use WaitForMultipleObjects instead of WaitForSingleObject ?

_Flaviu

There will be ideal a little code :)

Victor Nijegorodov

1. to inform the worker thread that it must do something (to change the folder in your case). CreateEvent/SetEvent - in the main thread. 2. Because there is already one object: hFileChange. However the thread should react also to the second object created/set by CreateEvent/SetEvent in the main thread.

leon de boer

Ask and you shall receive .. that is it straight on the windows API. GitHub - LdB-ECM/FolderWatcher[^] There is both the detailed and basic thread monitor code. There is a compiled exe in release/debug if you just want to run and see it.

In vino veritas

_Flaviu

Here is what I've done:

// TestFileView.h : interface of the CTestFileView class

protected:
CEvent m_event;
CString m_sFolder;
CWinThread* m_pWinThread;

/*static*/UINT CTestFileView::FilesWatchDogThread(LPVOID lpParam)
{
CTestFileView* pView = (CTestFileView*)lpParam;

if (NULL == pView->GetSafeHwnd())
	return 1;

HANDLE hChange = FindFirstChangeNotification(pView->GetFolder(), FALSE,	FILE\_NOTIFY\_CHANGE\_FILE\_NAME);

if (INVALID\_HANDLE\_VALUE == hChange)
	return 1;

HANDLE aHandles\[2\];
aHandles\[0\] = hChange;
aHandles\[1\] = pView->m\_event.m\_hObject;

BOOL bContinue = TRUE;
while (bContinue)
{
	DWORD dwResult = WaitForMultipleObjects(2, aHandles, FALSE, INFINITE);
	switch (dwResult)
	{
	case WAIT\_OBJECT\_0:
		::PostMessage(pView->GetSafeHwnd(), WMU\_NOTIFY\_FOLDERCHANGED, 0, (LPARAM)0);
		FindNextChangeNotification(hChange);
		break;
	case WAIT\_OBJECT\_0 + 1:
		bContinue = FALSE;
		break;
	}
}

FindCloseChangeNotification(hChange);

return 0;

}

and

void CTestFileView::StopThread()
{
m_event.SetEvent();
WaitForSingleObject(&m_event, INFINITE);
}

and

void CTestFileView::OnEditStartthread()
{
// TODO: Add your command handler code here

m\_event.ResetEvent();
m\_sFolder.Format(\_T("D:\\\\Tempx\\\\"));
m\_pWinThread = AfxBeginThread(&CTestFileView::FilesWatchDogThread, (LPVOID)this, THREAD\_PRIORITY\_NORMAL, CREATE\_SUSPENDED, 0, NULL);

}

there is remain one thing to do: to change the folder (m_sFolder) how can I complete this with CreateEvent/SetEvent ?

_Flaviu

"Go to ParentYes, you can use CreateEvent, change the folder, set its value in lpParam, then SetEvent to let the thread to reread lpParam and continue to work with a new folder" To create a third event which stop the thread and restart FilesWatchDogThread method with new folder value ?

Victor Nijegorodov

yes, you create the third event and NOT to stop thread, but to change the folder! Something like:

case WAIT\_OBJECT\_0 + 2:
	FindCloseChangeNotification(hChange);
	hChange = FindFirstChangeNotification(pView->GetFolder(), FALSE,	FILE\_NOTIFY\_CHANGE\_FILE\_NAME);
	aHandles\[1\] = pView->m\_event.m\_hObject;

And don't forget to set the new folder from the main thread BEFORE setting this event!

_Flaviu

Yes, has been done. Thank you all of you.

_Flaviu

Do you meant

aHandles[0] = hChange;

instead of

aHandles[1] = pView->m_event.m_hObject;

?

Victor Nijegorodov

Yes! Sorry for the typo! :sigh: