Call C++ dll from with Delphi

Question

My goal is to call a function written in C++ (with a C interface) from within Delphi (RAD Studio XE6). In the end, the dll will be generated by Visual Studio 2013, but I've tried to start with generating the Dll from RAD Studio XE6.

So, I have created a Dll project in Rad Studio (Using VC++ style Dll). The file is here

#include <vcl.h>
#include <windows.h>

#pragma hdrstop
#pragma argsused

int WINAPI DllEntryPoint(HINSTANCE hinst, unsigned long reason, void* lpReserved)
{
    return 1;
}

extern "C" int next(int n) {
    return n + 1;
}

It compiles as a Dll. On the Delphi side, the code is as below:

program Project1;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  System.SysUtils, Windows;
function next(a: Int32): Int32; cdecl;
  external 'Project3.dll';
var
  a: Int32;
begin
  try
    a := 3;
    Writeln('Hello world!', next(a));
    sleep(3000);
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

The project does compile but does not run. It gives the following error:

*** A stack buffer overrun occurred in "C:\...\Debug\Project1.exe"

It fails on the following assembly line

mov eax,[edi]

I have tried to change cdecl to std call, pascal or register. But nothing works.

Solution: Thanks to Rudy, the following code does work. First the C++ side:

#include <vcl.h>
#include <windows.h>

#pragma hdrstop
#pragma argsused

int WINAPI DllEntryPoint(HINSTANCE hinst, unsigned long reason, void* lpReserved)
{
    return 1;
}

extern "C" __declspec(dllexport) int __stdcall next(int n) {
    return n + 1;
}

and then the Delphi side.

program Project1;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  System.SysUtils, Windows;
function next(a: Int32): Int32; stdcall;
  external 'Project3.dll';
var
  a: Int32;
begin
  try
    a := 3;
    Writeln('Hello world!', next(a));
    sleep(3000);
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

Answer 1

You aren't exporting the function. Do that like this:

extern "C" 
{
    __declspec(dllexport) int next(int n)
    {
        return n + 1;
    }
}

Using extern "C", __declspec(dllexport) and the default calling convention of __cdecl will lean to the function being exported undecorated. Any other choice leads to decoration.

On the Delphi side it is:

function next(a: Integer): Integer; cdecl;
  external '...';

Decoration is not such a bad thing but if you want to use stdcall, and avoid decoration, then you should use a .def file to export the function.