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The error message indicates that the database has been opened by another application with exclusive access or the user hasn't sufficient privileges to access the database file.
When using the CDaoDatabase class, pass FALSE as second parameter to the Open() function.
When using Access, it must be configured to open files in shared mode (this is the default).
The database file must be readable and writable for the users.
See also Ways to share an Access database[^].
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Hi,
Is it 'thread safe' to use API'slike
::SetWinowText(...) and
::ShowWindow(hWnd,nCmdShow) across threads, or should I use e.g.
::SendMessage(hWnd,WM_SHOWWINDOW,fShow,bShow) I want to Hide and Show and set aspects of a Progress Dialog box during the run of the worker thread.
Regards,
Bram van Kampen
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It is always recommended to use SendMessage to the UI thread so that the UI thread controls/manipulates its UI elements.
In some cases it may not be necessary like, for example, the documentation for SetWindowText says that a WM_SETTEXT message is send by the API.
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«_Superman_» wrote: It is always recommended to use SendMessage to the UI thread...
But what about deadlock?
"One man's wage rise is another man's price increase." - Harold Wilson
"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons
"Show me a community that obeys the Ten Commandments and I'll show you a less crowded prison system." - Anonymous
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SendMessage is the right way to avoid deadlocks.
This way all requests from any thread is always queued in the UI thread message loop.
So keeping all locks local to the message handlers will help in avoiding deadlocks.
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«_Superman_» wrote: SendMessage is the right way to avoid deadlocks.
This article would tend to disagree.
"One man's wage rise is another man's price increase." - Harold Wilson
"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons
"Show me a community that obeys the Ten Commandments and I'll show you a less crowded prison system." - Anonymous
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hi,
i have to implement one method to get all domain names from adsi forest in c++ anyone tell that how can do that in c++.
thanks
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I think my past sins and assumptions are coming home to haunt me. I have a fairly large/complex application that began its past life in another product. Consider it a dialog application with a lot of dialogs. The dialogs can become 3 or more deep. On rare and random occasions, the application will crash with a stack overflow exception - only when leaving a dialog. The exception happens somewhere after EndDialog processes.
Yes, I have checked stack usage - I'm well under the specified amount. The problem is very rare. My debug efforts have led me to the path of suspecting some sort of race condition in message processing or some other undetected data corruption.
The application itself uses your standard windows messages. However, I have a number of worker threads that post messages into the application message queues. My approach is what I need a basic sanity check on
In the InitInstance, I create a worker thread using ::CreateThread() - system timer, whose sole responsibility is to notify active dialogs that a time event has occurred. This thread:
- acquires a pointer to CWnd via AfxGetApp().
- posts the message to the window using PosteMessage()
- there is no stack allocated data passed of any kind whatsoever in these messages.
Based on what I have read about this approach, it's legal and the only appropriate way for a worker thread to send data to the GUI thread. Opinions?
My next question has to do with what I consider to be a possible race condition. Given that my worker thread is firing off messages irrespective of what is going on elsewhere in the application, is it possible for the worker thread to send a message to a window AND have the user press the "Exit" button before the message arrives? If so, I would expect the dialog to be in some sort of tear down state such that some of it's processing is indeterminate. Is my concern reasonable?
Charlie Gilley
<italic>You're going to tell me what I want to know, or I'm going to beat you to death in your own house.
"Where liberty dwells, there is my country." B. Franklin, 1783
“They who can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety.” BF, 1759
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Imagine the gui thread as a special thread that has a job queue (because it has one). The main loop of the application (in the gui thread) is spinning and gets/processes messages that it retrievees with PeekMessage() or GetMessage() from its per-thread message queue. Putting a message to this message queue with PostMessage() for later processing is OK. Putting a message to the queue with SendMessage() is also OK. The difference between the two is that SendMessage() waits till the end of message processing so you can safely pass even pointers that point to your stack, and you can process the return value immediately. To tell the exact reason of your problem you need to give more info:
Which CWnd? Main wnd? Dialog wnd? What is the scope/lifetime of the CWnd? What is the scope/lifetime of the thread? Which Exit button? The one that terminates the whole application or just one of your dialog boxes?
There are several other things you should consider: MFC itself is not thread safe. If you use for example CWnd retriever methods than you are accessing a window map that is sometimes modified by the gui thread. Maybe your only problem is that you should get a HWND on the gui thread and pass that as a message target to the worker thread when you create it. Another possible problem is that on the main thread you should ask the worker thread to terminate and wait for it to terminate at some point. For example when the main window is destructed.
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All good points. I know about SendMessage / PostMessage, and I've read many articles documenting why you need to be very careful sending messages in mfc. I'll see if I can come up with a description that isn't worse than I've already written . Note that this basic architecture was inherited, so I make no claim as to its validity. It's an approach, and it has worked. However in writing this out, I think I've discovered one issue...
- The worker thread is a child of the CWinApp application. It exists for the lifetime of the application.
- Window structure: CWinApp -> CFrameWnd -> Main Dialog -> many more dialogs. The frame object serves as the persistent object of the application. There are times when the main dialog may go away, so the frame object maintains application context. Normally, the CFrameWnd will instantiate the Main Dialog.
The frame object is created like so, its instance is saved away for the application:
CFrameWnd* pFrameWnd = new CFrameWnd;
m_pMainWnd = pFrameWnd;
- Every dialog created registers with the frame object its instance (using this). This is done so that when a general message is posted to the frame, the frame then echoes it out to all existing dialogs.
- The timer thread posts the timer message indirectly by invoking a public helper method from the frame object: pFrame->BroadcastTimerEvent(); The fact that a worker thread is using an object from the UI side of application CANNOT be a good thing. Comments?
In this BroadcastTimerEvent method, we finally post a message into the message queue:
<pre>Wnd * pWindow;
pWindow = AfxGetApp()->m_pMainWnd;
if (NULL != pWindow) pWindow->PostMessage(WM_DATA_TIMER);
This effectively queues the message to arrive and be handled by the frame's OnDataTimer handler. Note from above that I mentioned I keep track of all active dialogs. This is done to ease pushing out the timer events to all existing dialogs:
<pre>LRESULT CFrameWnd::OnPanelDataTimer(WPARAM wParam, LPARAM lParam)
{
if (NULL != m_pClientWnd) {
if (NULL != m_pClientWnd->m_hWnd)
{
for(SHORT j = m_pDialogsInUse.GetSize() - 1; j >= 0; j--)
if (m_pDialogsInUse[j]->m_hWnd != NULL)
m_pDialogsInUse[j]->PostMessage(WM_DATA_TIMER);
}
}
return 0;
}
So, I finally come back to my race condition issue - the timer thread fires and starts to push messages out as described. The user happens to press the dialog exit button. Standard button, invokes EndDialog() and terminates. Windows now goes about tearing down that dialog. This dialog has been created on the stack, so it's still out there for a brief period of time. This is where I think there might be a problem...
Charlie Gilley
<italic>You're going to tell me what I want to know, or I'm going to beat you to death in your own house.
"Where liberty dwells, there is my country." B. Franklin, 1783
“They who can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety.” BF, 1759
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You described a lot of things but the 2 main things you have to make sure are still the same:
1. From the worker thread you shouldn't use any data structures and mfc calls that are not thread safe. You can provide this by posting messages directly to a HWND, but calling object methods is also OK if you can guarantee thread safety.
2. Make sure that the thread is terminated before the objects it interacts with are destroyed. I recommended sending an exit request signal to the thread and the waiting for it to terminate from the destroy of the main window.
From your comments none of these can be checked.
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Item 2 - got it. But remember, it's not the application that's terminating, it's a single dialog.
Item 1 - you see how I am invoking PostMessage, from the CWnd object. This object has an m_hWnd variable, but how would I post a message using this? Based on what you have said, I'm not thread safe.
I'm tired, so might be a brain fart question....
Charlie Gilley
<italic>You're going to tell me what I want to know, or I'm going to beat you to death in your own house.
"Where liberty dwells, there is my country." B. Franklin, 1783
“They who can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety.” BF, 1759
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charlieg wrote: Item 1 - you see how I am invoking PostMessage, from the CWnd object. This
object has an m_hWnd variable, but how would I post a message using this? Based
on what you have said, I'm not thread safe.
Do not use CWnd::PostMessage, use the windows API ::PostMessage instead. It allows you to specify the HWND that is to recieve the message.
When you create the worker thread, pass the main window's HWND to it via the LPVOID parameter of the CreateThread function:
CreateThread(NULL, 0, MyWorkerThreadFunc, (LPVOID)m_pMainWnd->GetSafeHwnd(), 0, NULL) Then from the worker thread you can directly post your message to the main window
DWORD MyWorkerThreadFunc(LPVOID pParam)
{
HWND MainWnd = (HWND)pParam;
...
::PostMessage(MainWnd, WM_MYMESSAGE, 0, 0);
...
}
Independent ACN Business OwnerNeed a new cell phone? We supply most of the major carriers. Telus in Canada. Flash, Verizon, T-Mobile and Sprint in the USA. O 2, talkmobile, tmobile, orange, three, and vodafone in Europe. See my website for details.
Within you lies the power for good - Use it!
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oh, I truly appreciate the comments...
Charlie Gilley
<italic>You're going to tell me what I want to know, or I'm going to beat you to death in your own house.
"Where liberty dwells, there is my country." B. Franklin, 1783
“They who can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety.” BF, 1759
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No worries, but you should still fix the problems I described and post up the modifications for checking.
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Post a stack trace after the problem has occurred.
Steve
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Steve,
I would if I could. So far, the application crashes in extremely rare circumstances and only in retail. Working hard on the debug version crashing. We shall see. Changes in the works per the comments above.
thanks all
Charlie Gilley
<italic>You're going to tell me what I want to know, or I'm going to beat you to death in your own house.
"Where liberty dwells, there is my country." B. Franklin, 1783
“They who can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety.” BF, 1759
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Hi,
I want to learn about Namespaces. I know how includes work, I have used them for many years. When I google the subject, I get explanations about 'include',(which I already know), and a statement that Name Spaces are a Compiler Construct.
How does this 'Compiler Construct' work in terms of the "Compiler's Symbol Table" Compiler Action and "Linker".
Bram van Kampen
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It is a compiler construct because it affects only the source code and the name of the symbols your compiler outputs to object files. A simple linker works simply with the object files and their imported/exported symbols that are basically just strings (symbol names) and it doesn't care whether the name of your variable/function(/any other symbol) is prefixed with one or more namespaces or not. If we want to describe the job of the linker in simple terms then all the linker does is building up two big lists of strings (all imported symbol names and all exported symbol names from all linkable object files) and then finding all of the imported strings in the exported string list. (of course the strings have additional info attached in both lists - like object file, ...)
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So, what's the difference! Is it that the compiler now takes part in the linking? I understand intimately how the compiler and linker work together in the #include model. How does it work with namespaces.
Viz,
In the #include model, the Compiler builds one object file,out of each cpp file. Each object needs to be declared, before an instance can be instanciated)
Classes and Manifests are defined in a header file. This gives the compiler the confidence to produce an object file, containing Inward and Outward references. The compiler is happy to produce these, based on promises in the Header file.
Now, the task of the Linker is to knit this lot together, into an executable ( an exe or dll). It tries to resolve the Links, (i.e. called Named locations in the executable text in One Obj File, to actual Locations of where the code ends up in the final executable). If names cannot be resolved within the 'Project Object Files', further searches are carred out in folders, specified in the project documentation.(paraphrased as spaces where 'Lib Files'are held.) Where this process fails, we get 'Linker errors.'
Now, how does this process take place differently with 'namespace',
and in what way is'namespace' substantially different.
Bram van Kampen
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Hello Every one.
how one can hide file or folder from the explorer even it should not be visible when some one click on show
hidden file and folder .
is there any shell api for this ....
Regards
sarfaraz
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You could mark it as a system folder, which will hide it until the "Hode protected operating system files" option is unchecked.
You can use the SetFileAttributes[^] function to set the folder attributes to FILE_ATTRIBUTE_SYSTEM.
0100000101101110011001000111001011101001
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Hi,
Well, Ultimately you can not. You can hide it from casual 'browsers'. The situation is still that ultimately a designated administrator for a machine is considered 'the owner' of the contents of a drive, who has as such un restricted access to all files, stored on that machine.
Anything short of that would leave the 'data Controller' unable to control the data.
I think you refer to the MS Section of the disk (invisible to WinExplorer) from which the OS can be recovered. Part of the game is, that those sectors can 'NEVER'be written to by Users, hence it cannot contain any reportable personal data.
Bram van Kampen
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At the very least, anyone with admin rights will always be able to see it. If nothing else, this is necessary for virus scanners and backup programs! Why is the standard hide flag not sufficient for your purpose? Maybe your problem can be solved in another way.
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when I use following function get the web info,it always takes a long time,My application can not do anything ,so how to create a thread to get the info .
void CMainFrame::GetXMLInfo(string& strweb,wstring& strErrorMsg)
{
CString strURL = _T("http://xxxxx.com");
CInternetSession Session(_T("MyWeb"));
CHttpFile *lpFile = NULL;
try
{
lpFile = (CHttpFile*)Session.OpenURL(strURL,1,INTERNET_FLAG_TRANSFER_ASCII|INTERNET_FLAG_RELOAD|INTERNET_FLAG_DONT_CACHE);
if (lpFile != NULL)
{
while(TRUE)
{
char szBuff[1024] = {0};
if ( lpFile->Read(szBuff, 1024) <= 0)
{
break;
}
strweb += szBuff;
}
}
}catch(CInternetException* pEx)
{
TCHAR pszError[64];
pEx->GetErrorMessage(pszError, 64);
strErrorMsg = pszError;
pEx->Delete();
}
delete lpFile;
lpFile = NULL;
Session.Close();
}
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