Question

I get this weird error:

MyView:OnInitialUpdate()
{
    int* my_int;
    *(my_int) = 1;
    AfxBeginThread(MyThread,my_int);
}

UINT MyThread(LPVOID param)
{  
    int* my_int = reinterpret_cast<int*>(param);
    message(*(my_int));
        return 0;
}

void message(int value)
{
    CString txt;
    txt.Format(_T("%d"),value);
    AfxMessageBox(txt);
}

The message box output is 4250636.

Now if I just add another message box before passing the value to the thread:

MyView:OnInitialUpdate()
{
    int* my_int;
    *(my_int) = 1;
    message(*(my_int));
    AfxBeginThread(MyThread,my_int);
}

Both message box outputs are 1.

Answer 1

int* my_int;
*(my_int) = 1;

This is undefined behavior. You didn’t initialize my_int, so you’re dereferencing an invalid pointer.

Instead, create a member x in your MyView class, and change to:

int* my_int = &x;