Calling a c dll from C++, C# and ruby

Question

Hi I have a DLL with a function that I need to call. The signature is:

const char* callMethod(const char* key, const char* inParams);

If I use ruby everything works fine:

attach_function :callMethod, [:string, :string], :string

If I use C++ or C# I get stack overflow!?

C#:

[DllImport("DeviceHub.dll", CallingConvention = CallingConvention.Cdecl)]
private unsafe static extern IntPtr callMethod(
    [MarshalAs(UnmanagedType.LPArray)] byte[]  key,
    [MarshalAs(UnmanagedType.LPArray)] byte[] inParams
);

System.Text.UTF8Encoding encoding = new UTF8Encoding();
IntPtr p = callMethod(encoding.GetBytes(key), encoding.GetBytes(args)); // <- stack overflow here

c++:

extern "C"
{
typedef  DllImport const char*  (  *pICFUNC) (const char*, const char*); 
}
HINSTANCE hGetProcIDDLL = LoadLibrary(TEXT("C:\\JOAO\\Temp\\testedll\\Debug\\DeviceHub.dll"));  
FARPROC lpfnGetProcessID = GetProcAddress(HMODULE (hGetProcIDDLL),"callMethod");*      pICFUNC callMethod;
callMethod = (pICFUNC) lpfnGetProcessID;
const char * ptr = callMethod("c", "{}");

I have tried lots of variations for function calling : WINAPI, PASCAL, stdcall, fastcall,... nothing works.

The DLL has not been made by me and I have no control on it.

Can anyone help me with any suggestion!?

Answer 1

Make sure that the declaration in your header is enclosed in an extern "C" block:

extern "C" {

const char* callMethod(const char* key, const char* inParams);

}

See http://www.parashift.com/c++-faq-lite/mixing-c-and-cpp.html#faq-32.3

Answer 2

This is just a idea but AFAIK this might be a issue with null-terminated strings, const char* myvar is null-terminated but a byte array isn't. Everything you need to to is to change the call to ...(String a, String b) and marshal them as LPStr.