C / C++ / MFC

About FindFirstChangeNotification and ReadDirectoryChangeW

1-Jun-05 19:18

Sreekanth Muralidharan wrote:
I hope the thread is created with real time priority

I Believe, Real time Priority Thread is not good solution to you problem!. AFAIK, you are developin gyou application using worker thread, my suggesstion will be create UserInterface thread and handle different POWER EVENT and Windows Event within UI thread!

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow

cheers,
Alok Gupta

a_du1-Jun-05 16:31

a_du

1-Jun-05 16:31

hi,
i am developing a folder watching application. i want to use the fuction FindFirstChangeNotification and ReadDirectoryChangeW.
The problem here is when i create a MFC project using Dialog approach
i inlcude the sample code MSDN provide in a button handler function, and if i run it, the application will hung and i have to use task manager to close it.
the same thing happen if i use ReadDirectoryChangeW.
however if i put the same code in a win32 application project( using MFC shared .dll ), it can works well.
Pls help, thanks, the msdn sample code is below:

DWORD dwWaitStatus;
HANDLE dwChangeHandles[2];

// Watch the C:\WINDOWS directory for file creation and
// deletion.

dwChangeHandles[0] = FindFirstChangeNotification(
"C:\\WINDOWS", // directory to watch
FALSE, // do not watch the subtree
FILE_NOTIFY_CHANGE_FILE_NAME); // watch file name changes

if (dwChangeHandles[0] == INVALID_HANDLE_VALUE)
ExitProcess(GetLastError());

// Watch the C:\ subtree for directory creation and
// deletion.

dwChangeHandles[1] = FindFirstChangeNotification(
"C:\\", // directory to watch
TRUE, // watch the subtree
FILE_NOTIFY_CHANGE_DIR_NAME); // watch dir. name changes

if (dwChangeHandles[1] == INVALID_HANDLE_VALUE)
ExitProcess(GetLastError());

// Change notification is set. Now wait on both notification
// handles and refresh accordingly.

while (TRUE)
{

// Wait for notification.

dwWaitStatus = WaitForMultipleObjects(2, dwChangeHandles,
FALSE, INFINITE);

switch (dwWaitStatus)
{
case WAIT_OBJECT_0:

// A file was created or deleted in C:\WINDOWS.
// Refresh this directory and restart the
// change notification. RefreshDirectory is an
// application-defined function.

RefreshDirectory("C:\\WINDOWS")
if ( FindNextChangeNotification(
dwChangeHandles[0]) == FALSE )
ExitProcess(GetLastError());
break;

case WAIT_OBJECT_0 + 1:

// A directory was created or deleted in C:\.
// Refresh the directory tree and restart the
// change notification. RefreshTree is an
// application-defined function.

RefreshTree("C:\\");
if (FindNextChangeNotification(
dwChangeHandles[1]) == FALSE)
ExitProcess(GetLastError());
break;

default:
ExitProcess(GetLastError());
}
}

Re: About FindFirstChangeNotification and ReadDirectoryChangeW

ThatsAlok1-Jun-05 18:51

Re: About FindFirstChangeNotification and ReadDirectoryChangeW

1-Jun-05 18:51

a_du wrote:
dwWaitStatus = WaitForMultipleObjects(2, dwChangeHandles,
FALSE, INFINITE);

Actaully this is BLOCKING CALL,this will block all Message Coming to your Dialog Box.

Only Solution to above problem is to create multithread Program!

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow

cheers,
Alok Gupta

a_du1-Jun-05 23:41

a_du

1-Jun-05 23:41

Thanx alot. I have solved that problem by using multithread.

BTW, can some one tell me the differences between these two functions and whether it is possible to retrieve file name informations from the FindFirstChangeNotification approach.

Re: About FindFirstChangeNotification and ReadDirectoryChangeW

ThatsAlok1-Jun-05 23:55

Re: About FindFirstChangeNotification and ReadDirectoryChangeW

1-Jun-05 23:55

a_du wrote:
it is possible to retrieve file name informations from the FindFirstChangeNotification approach.

Nope, It not Possible to Retrieve File Information using FindFirstChangeNotification approach!, only ReadDirectoryChangeW will help in this regard!

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow

cheers,
Alok Gupta

S. Senthil Kumar1-Jun-05 23:45

S. Senthil Kumar

1-Jun-05 23:45

As Alok said, you're blocking the UI thread and waiting for directory changes. You need to move that code to a separate worker thread and inform the UI when you get modifications.

Regards
Senthil
_____________________________
My Blog | My Articles | WinMacro

Re: About FindFirstChangeNotification and ReadDirectoryChangeW

ThatsAlok1-Jun-05 23:57

PostMessage causing hang...

1-Jun-05 23:57

S. Senthil Kumar wrote:
and inform the UI when you get modifications.

Oops, I forget to mention that, Thanks for correcting me! Smile | :)

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow

cheers,
Alok Gupta

Ian Bowler1-Jun-05 16:25

1-Jun-05 16:25

Hello!

Here's my situation. I have a dialog with a CEdit-derived object and CListCtrl-derived object. When someone is typing in the edit control and they hit Enter, I want the text to be copied to the CListCtrl.

The CDialog class that contains these controls has a method that gets the text from the edit control and inserts it into the CListCtrl.

My problem is trapping the Enter key in the edit box and getting the dialog to call it's method that copies from the edit box to the list control.

I think I'm going about this all wrong. What I'm currently trying to do in the CEdit-derived class is this:

void CMyEdit::OnKeyUp(UINT nChar, UINT nRepCnt, UINT nFlags)
{
// If Enter has been pressed, we need to post this message to the dialog.

if(nChar == 13)
{
// Get a handle to our dialog window
CWnd* pDialogWnd = GetParent();

// Post the message to our dialog window
pDialogWnd->PostMessage(WM_KEYUP, VK_RETURN, 0);
}

CEdit::OnKeyUp(nChar, nRepCnt, nFlags);
}

I implement the WM_KEYUP message in the parent dialog class as follows:

void CMyDlg::OnKeyUp(UINT nChar, UINT nRepCnt, UINT nFlags)
{
// If Enter has been pressed, we need to update the rules list
if(nChar == 13)
{
UpdateRulesList();
}

CDialog::OnKeyUp(nChar, nRepCnt, nFlags);
}

But I end up hanging somewhere after my UpdateRulesList method executes (the problem is not in UpdateRulesList (at least I can call it from the dialog object and it works just fine)). I must be generating some kind of endless message loop or something...

Any help would be greatly appreciated!

Thanks!

-Ian

Christian Graus1-Jun-05 16:43

Christian Graus

1-Jun-05 16:43

Ian Bowler wrote:
I must be generating some kind of endless message loop or something...

Does this mean you've put a breakpoint in to see if this is the case ?

Christian Graus - Microsoft MVP - C++

Ian Bowler1-Jun-05 16:48

1-Jun-05 16:48

I put a break point where my dialog class calls the UpdateRegRules method. I step through that method and everything seems copacetic. If I hit f5, I seem to hang.

I'm totally guessing on the endless looping. I'm actually stumped. I've never used PostMessage before and I'd be willing to bet that that's the root of the problem...

Christian Graus1-Jun-05 16:51

Christian Graus

1-Jun-05 16:51

If you hit F5 and you're creating an endless message loop, you should find yourself hitting the same spot over and over. You can also override PreTranslateMessage to see if this is in fact occuring.

The difference between SendMessage and PostMessage is just that PostMessage sends the message when the current function ends, SendMessage does it right away.

Christian Graus - Microsoft MVP - C++

S. Senthil Kumar1-Jun-05 23:50

S. Senthil Kumar

1-Jun-05 23:50

Not really, PostMessage posts the message to the message queue and returns immediately, SendMessage sends the message to the message pump and waits till it gets processed.

Regards
Senthil
_____________________________
My Blog | My Articles | WinMacro

Nilesh K.1-Jun-05 16:49

1-Jun-05 16:49

Try out with SendMessage instead of PostMessage

<bold>- Nilesh
<italics>"Reading made Don Quixote a gentleman. Believing what he read made him mad" -George Bernard Shaw

Ian Bowler1-Jun-05 16:57

1-Jun-05 16:57

You guys ROCK! Thanks so much! SendMessage fixed the problem. I don't think I understand why sending the message right away fixed the problem though. Some crazy timing thing?

I actually thought the difference was that PostMessage added the message to the message queue and SendMessage bypassed the queue (at least that's how someone else explained it to me).

Thanks again, to both of you for your excellent help!

-Ian

Nilesh K.1-Jun-05 17:02

1-Jun-05 17:02

Well you are right about the difference between SendMessage and PostMessage but there is one more thing to be noted that PostMessage as it goes in queue is a blocking call, but it is not the case with SendMessage Smile | :)

<bold>- Nilesh
<italics>
"Reading made Don Quixote a gentleman. Believing what he read made him mad" -George Bernard Shaw

Ian Bowler1-Jun-05 17:04

1-Jun-05 17:04

You've been so helpful I hesitate to ask another question... Smile | :)

What is a blocking call?

Nilesh K.1-Jun-05 17:14

1-Jun-05 17:14

<code>
void function1()
{
PostMessage();
//this message box would get displayed only when the lengthy operation of
//ReceiverOfPostMessage function gets over, This is called a blocking call
AfxMessageBox("");

SendMessage();
//this message box would immediately get displayed and would not wait for the
//operation of ReceiverOfPostMessage to get over
AfxMessageBox("");
}

void ReceiverOfPostMessage()
{
//execute a lengthy operation
}
</code>
Hope that's clear Smile | :)

<bold>- Nilesh</bold>
<italics>
"Reading made Don Quixote a gentleman. Believing what he read made him mad" -George Bernard Shaw
</italics>

Ian Bowler1-Jun-05 17:19

1-Jun-05 17:19

Ahhh. I didn't realize you were talking about synchronous VS asynchronous. You explained it perfectly.

Thanks again for all your help! I hope I get the chance to return the favor someday...

-Ian

Nilesh K.1-Jun-05 17:31

1-Jun-05 17:31

Thanks

Glad to be of help Smile | :)

<bold>- Nilesh
<italics>
"Reading made Don Quixote a gentleman. Believing what he read made him mad" -George Bernard Shaw

ThatsAlok1-Jun-05 18:38

1-Jun-05 18:38

Nilesh K. wrote:
PostMessage();
//this message box would get displayed only when the lengthy operation of
//ReceiverOfPostMessage function gets over, This is called a blocking call
AfxMessageBox("");

SendMessage();
//this message box would immediately get displayed and would not wait for the
//operation of ReceiverOfPostMessage to get over
AfxMessageBox("");

Are you jokin!, have you consulted MSDN before writing difference between PostMessage and SendMessage Here! or try to run above code in your compiler!

The Defination is actually reverse!, SendMesssage Block the call and PostMessage Return immedietly.

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow

cheers,
Alok Gupta

Nilesh K.1-Jun-05 19:15