Access violating reading location

[zongul]

I have this code

	old_evhndl =  *(FARPROC*)(obj->old_eventhandler());

	return CallWindowProc(old_evhndl, hwnd, msg, (WPARAM)wParam, (LPARAM)lParam);

The exception points to the last line and shows me this:

Exception thrown at 0x00007FF9F23F5260 (user32.dll) in filetool.exe: 0xC0000005: Access violation reading location 0xFFFFFFFFFFFFFFFF.

I know that there is a problem with pointer conversions. What I could observe is that when I comment the first line out there is no exception. So I guess there is a problem with this (FARPROC)obj->old_eventhandler();

old_eventhandler() has a void* as a return type. It is defined like this:

#ifndef _GwGUI_MOTIF
virtual void* old_eventhandler()    { return NULL;  }

old_evhndl is defined like this:

static FARPROC old_evhndl = NULL;
STRICT is not defined so CallWindowProc is expecting a FARPROC as the first argument.

[salem c]

Since FARPROC is already hiding a pointer (it is just a function pointer), then perhaps you meant

old_evhndl = static_cast<FARPROC>(obj->old_eventhandler());

[zongul]

It does not work. I still get the same exception

[salem c]

Pull it apart into multiple statements and examine with the debugger.

[deleted account xyzzy]

https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-callwindowproca

[helios]

Where does the pointer that's returned by old_eventhandler() come from? Does the function have the correct signature and calling convention?

[jonnin]

any chance you are mixing 64 and 32 bit?

[matt112]

I don't think mixing of 64 and 32 bit. It will created mess.