Using a C/C++ DLL with FreePascal

Question

First of all I am using Mingw 4.8 as the compiler of the C++ DLL in Code:blocks 13.12 and Lazarus 1.4.2 for working with the pascal code.(windows 7)

I need to generete a dll in c++ or c that can be called from a pascal program.

The problem is that my knowlegde about pascal is null, It dosn't look really complicate to make a simple program but I can't find good information about how to import and use a C/C++ DLL.

The only thing that moreless worked was this: http://www.drbob42.com/delphi/headconv.htm My real code:

Pascal:

funtion hello():Integer; external 'function' index 1;
...
Label1.Caption:=IntToStr(hello());

C++ DLL header:

#ifndef function_H
#define function_H
#ifdef __cplusplus
extern "C" {
#endif
#ifdef BUILDING_DLL
#define FUNCTION_DLL __declspec(dllexport)
#else
#define FUNCTION_DLL __declspec(dllimport)
#endif
 int __stdcall FUNCTION_DLL hello( );

#ifdef __cplusplus
}
#endif
#endif

C++ file:

    #include <stdio.h>

#include "function.h"
  __stdcall int hello( )
{
return 8;
}

But when try to pass any argument or do something complicated with the function,starts to give randoms numbers.

This is the new code: Pascal:

function function1(t1:Integer):Integer; external 'function' index 1;  
...
entero:=8;
Label1.Caption:=IntToStr(function1(entero2));

Also I update the c++ code to this:

C++:

#include <stdio.h>

#include "function.h"
  __stdcall int function1(int t1)
{
return t1*2;
}

Header:

#ifndef funtion_H
#define funtion_H
#ifdef __cplusplus
extern "C" {
#endif
#ifdef BUILDING_DLL
#define FUNCTION_DLL __declspec(dllexport)
#else
#define FUNCTION_DLL __declspec(dllimport)
#endif
 int __stdcall FUNCTION_DLL function1(int t1);

#ifdef __cplusplus
}
#endif
#endif

I aslo read this other info:http://www.jrsoftware.org/ishelp/index.php?topic=scriptdll. And tried to implement the dll call like this:

Pascal:

function function1(t1: Integer): Integer; external 'function1@files:function.dll';

But I receive an error saying that:

The procedure entry point function1 could not be located in the dynamic link library function.dll

I'm looking for an example that works or a online tutorial or something to continue working because I am very stuck with this. Thank you in advance.

David Heffernan David Heffernan · Accepted Answer · 2015-07-16T09:46:49

You need to make the calling conventions match. Your C++ code uses __stdcall. The Pascal code does not specify a calling convention and so defaults to register.

Declare the Pascal import like this:

function function1(t1:Integer):Integer; stdcall; external 'function' index 1;

Are you quite sure that you need to use an index when importing? It is much more common to import by name than by ordinal. I'd expect to see the import looking like this:

function function1(t1:Integer):Integer; stdcall; external 'function';

The reason why the function with no parameters succeeds is that for a parameterless function, the differences between calling convention do not matter. Once you start passing an argument, stdcall means that the argument is passed via the stack, and register means it is passed in a register. This mismatch explains the behaviour you observe.

Using a C/C++ DLL with FreePascal

1 Answers