0xc0000005: an access conflict occurred while reading location 0x00000020

0x in.exe… Unhandled exception at: 0xC0000005: Access conflict occurred while reading position 0x00000020

Error: : PostMessage (AfxGetMainWnd () – & gt; M_hWnd, UM_XX, 0, 0);

or: : sendMessage (
AfxGetMainWnd()-> M_hWnd, UM_XX, 0, 0);

How to solve it?
Verify that afxGetMainWnd () returns NULL

int k = (int)(& (((CWnd*)NULL)-> m_hWnd));
The value of k is 0x20

In general: afXGetApp ()-> GetMainWnd (); AfxGetMainWnd() is used only on the main thread;

How: foreground thread and background thread, afxGetApp ->; GetMainWnd() differs from AFXGetMainWnd ()

AFXGetMainWnd () is an example of a window handle that can be used to retrieve a window handle. AFXGetMainWnd () is an example of a window handle that can be used to retrieve a window handle. Clever you must have already understand, if you have been tracking AfxGetMainWnd () call, will find that it earned a AFX_MODULE_THREAD_STATE threading module save active threads in the window handle, and a background thread since no window, then obtains the window handle is you from??(maybe someone’s understanding of the background is not display window, even with all the window, as long as it doesn’t show is a background thread, strictly speaking is not the case, the window is mainly used to interact with the user, a window is hard to avoid jam, And the background thread is often used to carry out some behind the operation or processing, is through the foreground information or data to carry out the corresponding operation, if the window is hidden to talk about what information transfer?The case can not be done in the background like other threads. The key is to know the difference and how to use it.)
AfxGetApp()-> GetMainWnd () is the main window handle, no matter in that thread calls are no problem, because it is first obtained the main thread handle, to achieve the main thread of the active window (such as switch view can lead to replacement, I am not sure this kind of circumstance), if there is no active window is the main window, any program must have a main window, so it calls there is no problem, if you want to achieve the program’s main window suggested AfxGetApp () – & gt; GetMainWnd().
Note that the console program has no window and its window handle is always 0. Second, the background thread is actually a desktop control program, just not the main thread. Also, threads created using functions such as API CreateThread cannot generate CWinThread objects. So if you want to use CWinThread object function, and some of the global function, as AfxGetMainWnd (), you must use CWinThread object CreateThread function, or use the AfxBeginThread () function creates a thread, or may be an error, because the MFC to the management of the thread is done through the CWinThread object, it is not hard to see through the tracking code below you. I remember that there was a predecessor once mentioned this problem, but I do not know whether it understands the reason
Here is the trace code:
{ CWinThread* pThread = AfxGetThread();
return pThread! = NULL ?pThread-> GetMainWnd() : NULL; }

CWinThread* AFXAPI AfxGetThread()
// check for current thread in module thread state
AFX_MODULE_THREAD_STATE* pState = AfxGetModuleThreadState();
CWinThread* pThread = pState-> m_pCurrentWinThread;
// if no CwinThread for the module, then use the global app
if (pThread == NULL)
hread = afxGetApp ();
return pThread;
You can see that calling afxgetMainwnd () on the main thread is not a problem (except for console programs) because it returns the main thread handle if the fetched thread handle is empty.
{ return afxCurrentWinApp; }

CWnd* CWinThread::GetMainWnd()
if (m_pActiveWnd ! = NULL)
return m_pActiveWnd;// probably – place in active
// when not inplace active, just return main window
if (m_pMainWnd ! = NULL)
return m_pMainWnd;
return CWnd::GetActiveWindow();
I don’t need to worry about AFXCurrentInApp.
Hey hey, all finished, if your program exists similar to the above problems can be sure to change back oh.

Read More: