MFC Debug Assertation Failed!! wincore.cpp Line 972 - c++

Question

I have created an MFC Dialog box in a DLL for use in multiple projects and it has functionalities such as:
Getting Listbox data from the main application using the DLL. I can push string data through the main application to the MFC Dialog box but I am getting Assertation errors after compilation.
This process happens in a thread where one thread keeps the Dialog box active and another pushes data as shown in the code below.
void dbox(CDialogClass *dlg)
{
dlg->ShowDlg();
}
void input(CDialogClass *dlg)
{
string str1= "";
while (1)
{
getline(cin, str1);
//cin >> str1;
dlg->SetData(str1);
}
}
int main()
{
HMODULE hModule = ::GetModuleHandle(NULL);
if (hModule != NULL)
{
// initialize MFC and print and error on failure
if (!AfxWinInit(hModule, NULL, ::GetCommandLine(), 0))
{
_tprintf(_T("Fatal Error: MFC initialization failed\n"));
}
else
{
CDialogClass *dlg = new CDialogClass("Title Dbox",300.0f, 300.0f, 0);
thread t1(input, dlg);
thread t2(dbox, dlg);
t1.join();
t2.join();
}
}
return 0;
}
Here dbox() invokes a ShowDlg function which is in an MFC DLL as below:
void CDialogClass::ShowDlg()
{
dlgg->title = title;
dlgg->dialogWidth = D_width;
dlgg->dialogHeight = D_height;
dlgg->pos = pos;
dlgg->Create(IDD_DIALOG1);
dlgg->ShowWindow(SW_SHOWNORMAL);
dlgg->RunModalLoop();
//dlgg->DoModal();
}
SetData() is called by thread input() and it has the below code in the DLL:
void CDialogClass::SetData(string data)
{
p_text = data;
dlgg->calldata(data);
}
Below is the code for my Dialog class in the DLL just for reference if needed-
#include "stdafx.h"
#include "DlgDisp.h"
#include "afxdialogex.h"
#include "Resource.h"
#include <fstream>
#include <Thread>
IMPLEMENT_DYNAMIC(CDlgDisp, CDialogEx)
CDlgDisp::CDlgDisp(CWnd* pParent /*=NULL*/)
: CDialogEx(CDlgDisp::IDD, pParent)
{
}
CDlgDisp::~CDlgDisp()
{
}
void CDlgDisp::DoDataExchange(CDataExchange* pDX)
{
CDialogEx::DoDataExchange(pDX);
DDX_Control(pDX, IDC_LIST1, m_listbox);
}
BOOL CDlgDisp::OnInitDialog()
{
//Title manipulations
char *str_title;
str_title = &title[0];
SetWindowText((CAtlString)str_title);
//Size manipulations
CWnd* pctrl = GetDlgItem(IDC_LIST1);
CRect rectctrl;
SetWindowPos(NULL, 0, 0, dialogWidth, dialogHeight, SWP_NOMOVE | SWP_NOZORDER);
pctrl->GetWindowRect(rectctrl);
pctrl->SetWindowPos(NULL, 20, 20, dialogWidth-120, dialogHeight-80, SWP_NOMOVE | SWP_NOZORDER);
UpdateData(FALSE);
return TRUE; // return TRUE unless you set the focus to a control
}
BEGIN_MESSAGE_MAP(CDlgDisp, CDialogEx)
END_MESSAGE_MAP()
void CDlgDisp::calldata(string strdata)
{
char *str_parameter;
str_parameter = &strdata[0];
CString param = _T("");
param.Format(_T("%s"), (CAtlString)str_parameter);
if (pos == 0)
{
m_listbox.InsertString(0, param);
}
else
m_listbox.AddString(param);
UpdateData(FALSE);
}
the flow of the program for references:
CDlgDisp class is the Dialog class derived from CDialogEx class.
CDialogClass is for interaction with external applications which is derived from CDialog class.
CDialogClass has a public member variable of CDlgDisp class.
external application -> object.CdialogClass -> object.CDlgdisp class
when I execute the program it runs well, and I get an error when I try to input data through the console. It does get printed in the Listbox dynamically but then it shows the Assertation Error.
Here is an image after execution
[enter image description here][1]
and here is the image after I enter the data in console and press enter
[enter image description here][2]
what do you guys think the problem is?
[1]: https://i.stack.imgur.com/pXFMD.png
[2]: https://i.stack.imgur.com/eUXZ7.png

Answer 1

Look into the source where the ASSERT ion take place
You find this comment:
// Note: if either of the above asserts fire and you are
// writing a multithreaded application, it is likely that
// you have passed a C++ object from one thread to another
// and have used that object in a way that was not intended.
// (only simple inline wrapper functions should be used)
//
// In general, CWnd objects should be passed by HWND from
// one thread to another. The receiving thread can wrap
// the HWND with a CWnd object by using CWnd::FromHandle.
//
// It is dangerous to pass C++ objects from one thread to
// another, unless the objects are designed to be used in
// such a manner.
MFC window objects use handle maps per thread. This usually doesn't allow you to use the objects for some functions from other threads. It is possible but this is discussed in many other threads.
The way you want to use the MFC isn't possible. Think about synchronisation. If the functions are thread safe that you want to use with the other window you may use SendMessage and the m_hWnd handle directly.

Answer 2

Thank you guys for being first responders to my problem. All the comments were useful and helped me in understanding the problem.
Problem: Since MFC is thread-safe therefore using an object to SetData was creating a memory sharing conflict between both the threads.
Solution: I used a custom message to pass information to be displayed dynamically. Below Links helped completely-
https://blog.csdn.net/xust999/article/details/6267216
On sending end:
::PostMessage(HWND_BROADCAST, WM_UPDATE_DATA, 0, 0);
On receiving end in the header file:
const UINT WM_UPDATE_DATA = ::RegisterWindowMessage(_T("UpdateData"));
Also, in the header file in the Dialog class:
afx_msg LRESULT OnUpdateData(WPARAM wParam, LPARAM lParam);
The above function will be called when the message is posted and all functionalities to be added to it such as-
LRESULT CDlgDisp::OnUpdateData(WPARAM wParam, LPARAM lParam)
{
char *str_parameter;
str_parameter = &parameter[0];
CString param = _T("");
param.Format(_T("%s"), (CAtlString)str_parameter);
if (pos == 0)
{
m_listbox.InsertString(0, param);
}
else
m_listbox.AddString(param);
return 1;
}
Thank you, Everyone.