Access violation when invoking a C++ DLL from Delphi - c++

Question

I write a Unicode DLL in Visual C++ 6.0. Then try to invoke the DLL function from Delphi XE3.
When I debug in Delphi, when step over the line to invoke the DLL function, I will always get an Access violation exception.
However, when I debug in Visual C++, I can see all parameters passed from Delphi are correct and I can step over all codelines without any exceptions.
If running outside the debugger, then I will not see any "access violation exceptions.
I try many methods but still cannot figure out how to eliminate the exception when debuggin in Delphi.
Below is the code in Visual C++ 6.0 part:
TestDLL.cpp:
extern "C" VOID WINAPI Test(CONST MESSAGEPROC lpMessageProc, LPVOID lParam)
{
if (lpMessageProc != NULL)
(*lpMessageProc)(1500, (const LPVOID)(LPCTSTR)CString((LPCSTR)IDS_MYTEST), lParam);
/*
if (lpMessageProc != NULL)
(*lpMessageProc)(1500, (const LPVOID)(LPCTSTR)CString(_T("Test")), lParam);*/
}
TestDLL.h:
// TestDLL.h : main header file for the TESTDLL DLL
//
#if !defined(AFX_TESTDLL_H__38054A53_5CEE_4ABF_9BA8_BCE427FCB8E1__INCLUDED_)
#define AFX_TESTDLL_H__38054A53_5CEE_4ABF_9BA8_BCE427FCB8E1__INCLUDED_
#if _MSC_VER > 1000
#pragma once
#endif // _MSC_VER > 1000
#ifndef __AFXWIN_H__
#error include 'stdafx.h' before including this file for PCH
#endif
#include "resource.h" // main symbols
#ifdef __cplusplus
extern "C" {
#endif /* __cplusplus */
typedef BOOL (CALLBACK* MESSAGEPROC)(CONST DWORD dwMessageId, CONST LPVOID lp, LPVOID lParam);
VOID WINAPI Test(CONST MESSAGEPROC lpMessageProc, LPVOID lParam);
#ifdef __cplusplus
}
#endif
/////////////////////////////////////////////////////////////////////////////
//{{AFX_INSERT_LOCATION}}
// Microsoft Visual C++ will insert additional declarations immediately before the previous line.
#endif // !defined(AFX_TESTDLL_H__38054A53_5CEE_4ABF_9BA8_BCE427FCB8E1__INCLUDED_)
Below is the codes in Delphi XE3 part:
unit Unit1;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;
type
TForm1 = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
public
{ Public declarations }
end;
PForm1 = ^TForm1;
TMessageProc = function (const dwMessageId: DWORD; const lp: Pointer; lParam: Pointer): BOOL; stdcall;
{$EXTERNALSYM TMessageProc}
var
Form1: TForm1;
procedure Test(const lpMessageProc: TMessageProc; lParam: Pointer); stdcall;
implementation
{$R *.dfm}
procedure Test; external 'TestDLL.dll' index 2;
function MessageProc(const dwMessageId: DWORD; const lp: Pointer; lParam: Pointer): BOOL; stdcall;
begin
Result := True;
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
Test(MessageProc, #Self); // <---- This code line will cause "access violation
end;
I belive the problem occurs in DLL test function, when it try to load string from resource using CString((LPCSTR)IDS_MYTEST). If I change the code to CString(_T("Test")), then problem disappears.
Thanks

Answer 1

As you surmised, this statement will not work:
CString((LPCSTR)IDS_MYTEST)
Although this constructor of CString does allow you to pass it a resource ID, it will try to find the resource in the calling process's (ie, the Delphi EXE's) resources, not in the DLL's resources. You need to use the HINSTANCE of the DLL, as provided by the DLL's DllMain(), when loading strings from the DLL's resources. You can use the CString::LoadString() method for that, eg:
HINSTANCE hInst;
BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved)
{
hInst = hinstDLL;
return TRUE;
}
extern "C" VOID WINAPI Test(CONST MESSAGEPROC lpMessageProc, LPVOID lParam)
{
if (lpMessageProc != NULL)
{
CString str;
str.LoadString(hInst, IDS_MYTEST);
(*lpMessageProc)(1500, (LPCTSTR)str, lParam);
}
}

Answer 2

I finally figure out this is a bug of MFC codes(VC6.0 version).
I don't know if I can post MFC source codes so I will just paste the function headers and related parts only.
In Microsoft Visual Studio\VC98\MFC\SRC\STRCORE.CPP, we can see the following 3 functions:
//////////////////////////////////////////////////////////////////////////////
// More sophisticated construction
CString::CString(LPCTSTR lpsz) // Function 1
{
Init();
if (lpsz != NULL && HIWORD(lpsz) == NULL)
{
UINT nID = LOWORD((DWORD)lpsz);
if (!LoadString(nID))
TRACE1("Warning: implicit LoadString(%u) failed\n", nID);
}
else
{
// Construct string normally
}
}
/////////////////////////////////////////////////////////////////////////////
// Special conversion constructors
#ifdef _UNICODE
CString::CString(LPCSTR lpsz) // Function 2
{
// Construct string normally
}
#else //_UNICODE
CString::CString(LPCWSTR lpsz) // Function 3
{
// Construct string normally
}
#endif //!_UNICODE
As we can see in the above code snippet, only function 1 contains codes that will take special process on lpsz and check if it is a string resource ID, if yes, then load the string from the resource. Both function 2 & 3 have no such special processes.
When we create a project in VS6, the default settings for a project is _MBCS, in such a case, function 1 will become
CString::CString(LPCSTR lpsz)
so CString((LPCSTR)nResID) will actually invoke function 1 and load string resource properly.
Function 2 will be disabled since _UNICODE is not defined. And function 3 works with wide char strings.
Therefore, for _MBCS project, everything works perfectly and consistently with the MSDN document.
However, when I change _MBCS to _UNICODE, function 1 will become
CString::CString(LPCWSTR lpsz)
Fucntion 2 will be enabled and function 3 will be disabled.
So CString((LPCSTR)nResID) will actually invoke function 2, which does NOT have special process to load string resource, which makes the problem.
There are two solutions for this problem:
Always use CString((LPCTSTR)nResID) instead of CString((LPCSTR)nResID) to load a string from resource. However, this usage is inconsistent with MSDN document so we have to call it as an undocumented usage.
Always use LoadString to load a string resource.
Though solution 1 is a little simpler, it is an undocumented usage so I finally opt solution 2 to solve my problem.
Many thanks to all your helps in solving this issue.