Calling C++ dll from Java

[Tamas]

Hi, I'd set up my JNA "environment" and successfully run the printf() C Demo code.

Now, I've created a class in MS Studio and built with the setting can be seen in the picture.

Class looks like

class __declspec(dllexport) CMyClass

{

public:

    void welcome() { ... Hello world ...};

};

And I want to call this welcome method from Java. 

I saw somewhere an example code, but I could find it now. I've found several example for simple C files. But this is different, because I've a .lib too and I don't know whether it has to be used or not.

Thanks for your help!

Tamás

[L Will Ahonen]

Hi,

You'll save yourself a lot of headaches if you use dumpbin /exports yourdll.dll now, to make sure you're exporting the symbols you expect, without mangling.

Generally, it's Tricky™ to call C++ with JNA. 

A typical pattern is to write all your code in C++, then create a C wrapper api like so:

extern "C" {

int c_declared_function(void* actuallyObject)

{

    cxx_object* o=(cxx_object*)actuallyObject;

    return o->wrappedfunction

}

void* c_declared_factory(parameters)

{

    return new cxx_object(parameters);

}

void c_declared_destructor(void* actuallyObject)

{

    cxx_object* o=(cxx_object*)actuallyObject;

    delete o;

}

}


This lets you have the convenience of writing C++ instead of C, and still avoid the headaches of passing this-pointers etc.

Cheers!

Will

[Tamas]

Hi, thanks!

I followed your instructions, but firstly I've decided to try a pure C library.

My hello.c file in MS Studio 2015:

#include <stdio.h>

extern void hello();

void hello()

{

    printf("Hello from C");

}

My Java file in eclipse:

import com.sun.jna.Library;

import com.sun.jna.Native;

import com.sun.jna.Platform;

public class PaMMTE {

   public interface CPaMMTE_testDLL extends Library {

   public void welcome();

   }

   public static void main(String[] args) {

   CPaMMTE_testDLL lib = (CPaMMTE_testDLL) Native.loadLibrary("TestCallingFromJava", CPaMMTE_testDLL.class);

   lib.welcome();

       System.out.println("Hello, World");

   }

}

The name of the dll is CPaMMTE_testDLL.

Anyway, I've got the following error message:

Exception in thread "main" java.lang.UnsatisfiedLinkError: %1 is not a valid Win32 application.

at com.sun.jna.Native.open(Native Method)

at com.sun.jna.NativeLibrary.loadLibrary(NativeLibrary.java:288)

at com.sun.jna.NativeLibrary.getInstance(NativeLibrary.java:427)

at com.sun.jna.Library$Handler.<init>(Library.java:179)

at com.sun.jna.Native.loadLibrary(Native.java:569)

at com.sun.jna.Native.loadLibrary(Native.java:544)

at PaMMTE.main(PaMMTE.java:18)

I'm pretty sure that the project configuration is wrong in MS Studio project.

I've tried several different configurations without any good result.

The last one I've tried is:

​

If I could run this demo, I would summarize and write here my experience if anyone else needs to solve a similar issue.

Thanks for your help!

Mentes a vírusoktól. www.avast.com

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[Tamas]

Hi,

I could solve my problem!

The MS Studio 2015 C dll source is:

#include <stdio.h>


__declspec(dllexport) void hello()

{
    printf("Hello from C");
}

(Maybe hello -> welcome is also could cause a problem, but I tried several examples and implemented lots of dummy stuff and I think I just copy the wrong sample.)

And the Java is still the same:

import com.sun.jna.Library;
import com.sun.jna.Native;
import com.sun.jna.Platform;


public class PaMMTE {


    public interface CPaMMTE_testDLL extends Library {

    public void hello();

    }


    public static void main(String[] args) {
    CPaMMTE_testDLL lib = (CPaMMTE_testDLL) Native.loadLibrary("TestCallingFromJava", CPaMMTE_testDLL.class);

    lib.hello();
        System.out.println("Hello, World");
    }
}

And the most important is the MS Studio config:

I don't understand why I shouldn't have had to use extern. Anyway, the next step is to try a class and complex C++ libraries.

Thanks Will for the help, now I'm going to use your code.

Nice day!