C++ access function from C DLL/lib - c++

Question

I'm desperately trying to access a DLL supplied with some hardware. I have the DLL, the LIB file and the headers for the DLL.
I first tried using C# but failed due to the large data structures that are passed around. I got the functions called but the values in the structures that are modified are not correct.
I then thought about writing a wrapper class in C++ and use this as a library for C#. But again here I can call the function but if I tested an signed long that is passed it is "1072955392l" instead of "-1l".
I then just renamed the cpp file to ".c" and compiled it again. Now I get the correct value.
Are there some differences in the datatypes from C to C++?
The functions for the LIb in the supplied include file are declared like that:
_declspec (dllimport) long ResetControl(Registers* regs);
I compile using VS2013:
cl test.cpp /link test.lib
cl test.c /link test.lib
The cpp file and c file are the same unless I needed to include #include for the cpp and wrap the dll include header in
extern "C"
{
#include "test.h"
}
The test.c file looks like:
//#include <windows.h>
#include <stdlib.h>
#include <malloc.h>
#include <stdio.h>
#include <math.h>
#include "test.h"
int main (int argc, char **argv)
{
Registers Regs;
Reset (&Regs);
printf ("Value: %dl\n\r", Regs.Product);
return 0;
}
The C++ file:
#include <windows.h>
#include <stdlib.h>
#include <malloc.h>
#include <stdio.h>
#include <math.h>
extern "C"
{
#include "test.h"
}
int main (int argc, char **argv)
{
Registers Regs;
Reset (&Regs);
printf ("Value: %dl\n\r", Regs.Product);
return 0;
}
Both compile, but the result of printing Regs.Products is different:
C: -1l
C++: 1072955392l
What am I doing wrong here?

Answer 1

I saw somewhere that there is a matter of aligning to 4 bytes while compiling the client of the DLL.
However have a look to those pages:
http://www.codeproject.com/Articles/21/Beginner-s-Tutorial-Calling-Visual-Basic-ActiveX-D
http://www.codeproject.com/Articles/6242/Step-by-Step-Calling-C-DLLs-from-VC-and-VB-Part
Alexandre