Return byte array from C# to Java using UnmanagedExports and JNA

Question

I recently found the library UnmanagedExports that allowed me to access C#-methods directly from Java using JNA.

Has anyone an idea about what is wrong with my attempt to return a byte array from C# to Java?

Here is my example:

C# code:

using System;
using RGiesecke.DllExport;
namespace JnaTestLibrary
{
  public class JnaTest
  {
    [DllExport]
    public static byte[] returnT1()
    {
        byte[] t1 = {1,2,3,4,5};
        return t1;
    }
  }
}

Java code:

package me.mt.test;
import com.sun.jna.Library;
import com.sun.jna.Native;
import com.sun.jna.Platform;
public class JnaTest {
  public interface JnaTestInterface extends Library{
      byte[] returnT1();
  }
  static JnaTestInterface jnaTest = null;

  static{       
        if(Platform.is64Bit()){
            jnaTest = (JnaTestInterface)Native.loadLibrary("JnaTestLibrary64", JnaTestInterface.class);
        }
        else{
            jnaTest = (JnaTestInterface)Native.loadLibrary("JnaTestLibrary86", JnaTestInterface.class);
        }
    }

  public byte[] returnT1(){
      return jnaTest.returnT1();
  }
}

Java exception:

Exception in thread "main" java.lang.IllegalArgumentException: Unsupported return type class [I in function returnT1

I do not know a lot about jna, but I believe that ByteBuffers (specifically the direct variant) can be used to pass byte arrays across native boundaries. — Brett Okken
– Brett Okken, Commented Dec 30, 2014 at 21:54

20058 · Accepted Answer · 2014-12-31 11:12:27Z

1

I solved the problem by using pointers.

C# code:

using System;
using RGiesecke.DllExport;
namespace JnaTestLibrary
{
  public class JnaTest
  {
    [DllExport]
    public unsafe static byte* returnT1()
    {
        byte[] t1 = {1,2,3,4,5};
        fixed (byte* p1 = t1)
        {
          return p1;
        }
    }
  }
}

Java code:

package me.mt.test;
import com.sun.jna.Library;
import com.sun.jna.Native;
import com.sun.jna.Platform;
import com.sun.jna.Pointer;
public class JnaTest {
  public interface JnaTestInterface extends Library{
      Pointer returnT1();
  }
  static JnaTestInterface jnaTest = null;

  static{       
        if(Platform.is64Bit()){
            jnaTest = (JnaTestInterface)Native.loadLibrary("JnaTestLibrary64", JnaTestInterface.class);
        }
        else{
            jnaTest = (JnaTestInterface)Native.loadLibrary("JnaTestLibrary86", JnaTestInterface.class);
        }
    }

  public byte[] returnT1(){
      Pointer p1 = jnaTest.returnT1();
      return p1.getByteArray(0, 5);
  }
}

answered Dec 31, 2014 at 11:12

20058

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3 Comments

Tom Blodget Over a year ago

I think the pointed-to data could get clobbered by a CLR garbage collection between jnaTest.returnT1() and p1.getByteArray(0, 5).

20058 Over a year ago

@TomBlodget The unsafe keyword doesn't change the semantics of ordinary code at all. Until now everything works fine.

Tom Blodget Over a year ago

You're banking on luck. See this answer. The basic problem is memory ownership and, depending on the technique, transfer of ownership. You are using memory after giving ownership back to the CLR—outside the scope of the fixed statement.

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