JNA cannot find function

Question

JNA and DLLs are completely new territory for me... I have a custom DLL that has a function with this declaration:

int myfunc (const char*);

The dll compiles fine under MinGW with the following command:

>gcc -shared -omydll.dll mydll.c -lgdi32

However, loading it with JNA fails because it can't find the function within the DLL.

public interface mydll extends StdCallLibrary {
  mydll INSTANCE = (mydll)Native.loadLibrary("mydll", mydll.class);
  int myfunc (String arg);
  }

I did some research and it seems that this particular error has something to do with the calling procedure of the DLL functions. I've seen the __stdcall and the __cdecl procedures. I also saw that many DLL functions put __declspec(dllexport) in front of their function declarations/implementations (i have no idea what this means or what it does). So, since JNA seems to like the __stdcall procedure better, now my function looks like this:

__declspec(dllexport) int __stdcall myfunc (const char*);

Which looks super-complicated, but does no better than anything else i've tried. Using a HashMap to add the underscore prefix and the @4 suffix didn't work either:

mydll INSTANCE = (mydll)Native.loadLibrary("mydll", mydll.class, new HashMap () {{
  add("myfunc", "_myfunc@4");
  }});

The JNA documentation has been absolutely no help. I honestly have no idea what i'm doing anymore.

Answer 1

Turns out that i was building my DLL just fine, and JNA was finding my DLL just fine as well; i made an error in determining how the compiler mangled my symbols. Functions i named like myfunc were exported as myfunc@8 or myfunc@32 depending on how many bytes they took as arguments. Here's the code i used in my JNA project:

import java.util.*;
import com.sun.jna.*;
import com.sun.jna.win32.*;
//
public class Test
  {
  public interface mydll extends StdCallLibrary
    {
    mydll INSTANCE = Native.loadLibrary("mydll", mydll.class, new HashMap {{
      put("myfunc", "myfunc@8");
      //Other functions
      }});
    public int myfunc (long arg);
    //Other functions
    }
  //
  public static void main (String[] args)
    {
    System.out.println
      (mydll.INSTANCE.myfunc((long)0x23A3920F)); //Or whatever
    return 0;
    }
  }

My c code:

#include <windows.h>
#include <stdio.h>

__declspec(dllexport) int __stdcall myfunc (__int64);
/* Other functions */

__declspec(dllexport) int __stdcall myfunc (__int64 arg)
  {
  /* Whatever */
  return return_value;
  }

GCC was happy with just the -shared switch and linking against the proper libraries, like in my original question. I highly recommend downloading this tool so you can find out exactly what your function names are.