How to fix invalid characters which shown in MessageBox in C++ DLL when calling it from Delphi app?

Question

I have used Code::Blocks IDE for DLL and Delphi 10.3 Rio for Delphi app.

Here are my C++ DLL codes (CPP File):

#include "main.h"
#include "string"
#include "wchar2string.h"
using namespace std;
// a sample exported function
void DLL_EXPORT SomeFunction(wchar_t* sometext)
{
    string str = wchar2string(sometext);
    const char* cch = str.c_str();
    MessageBox(0, cch, "DLL Message", MB_OK | MB_ICONINFORMATION);
}

extern "C" DLL_EXPORT BOOL APIENTRY DllMain(HINSTANCE hinstDLL, DWORD     fdwReason, LPVOID lpvReserved)
{
    switch (fdwReason)
    {
        case DLL_PROCESS_ATTACH:
            // attach to process
            // return FALSE to fail DLL load
            break;

        case DLL_PROCESS_DETACH:
            // detach from process
            break;

        case DLL_THREAD_ATTACH:
            // attach to thread
            break;

        case DLL_THREAD_DETACH:
            // detach from thread
            break;
    }
    return TRUE; // succesful
}

Here is my .H File:

#ifndef __MAIN_H__
#define __MAIN_H__

#include <windows.h>

/*  To use this exported function of dll, include this header
 *  in your project.
 */

#ifdef BUILD_DLL
    #define DLL_EXPORT __declspec(dllexport)
#else
    #define DLL_EXPORT __declspec(dllimport)
#endif


#ifdef __cplusplus
extern "C"
{
#endif

void DLL_EXPORT SomeFunction(wchar_t* sometext);

#ifdef __cplusplus
}
#endif

#endif // __MAIN_H__

And here are my Delphi codes:

const
  DLL = 'deneme dll.dll';

procedure MyProcedure(sometext: PWideChar); external DLL name 'SomeFunction';

procedure TForm1.Button1Click(Sender: TObject);
var
  MyString: String;
begin
  MyString := Edit1.Text;
  MyProcedure(PWideChar(MyString));
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  SetErrorMode(0);
end;

end.

According to this website PWideChar is Delphi equivalent of wchar_t* in C++: http://rvelthuis.de/articles/articles-dlls.html

So, when I clicked to Button1; I got this message:

And if DLL is not found, Delphi app throws this ('Application Stopped Working' Message):

So, SetErrorMode(0); is not working.

What I mean, I don't know anything about DLL programming and there is not any guideline about that in any website.

So, what should I do in order to make this working correctly?

Answer 1

On the C++ side, the conversion of wchar_t* to std::string is unnecessary. Just use the Unicode version of MessageBox() instead, eg:

void DLL_EXPORT SomeFunction(wchar_t* sometext)
{
    MessageBoxW(0, sometext, L"DLL Message", MB_OK | MB_ICONINFORMATION);
}

However, the main reason for your trouble is a calling convention mismatch. On the Delphi side, the default calling convention is register, which is very different than the default of __cdecl used in C and C++. The Delphi declaration of the DLL function needs to specify the correct calling convention, eg:

procedure MyProcedure(sometext: PWideChar); cdecl; external DLL name 'SomeFunction';

procedure TForm1.Button1Click(Sender: TObject);
var
  MyString: UnicodeString;
begin
  MyString := Edit1.Text;
  MyProcedure(PWideChar(MyString));
end;