Structure JNA returns wrong value

Question

From C++ code I am getting the structure.But all the values I am getting is default value.Below is my c++ code

extern "C" 
{ 
PDFCONTENTDATA GetPDFContentData(LPTSTR lptszS3FileURL)
{
PDFCONTENTDATA pdfContentData;
pdfContentData.m_uiRasterDPI = 100;

return pdfContentData;
}
}; 

Below is my java/scala code

@Structure.FieldOrder({ "m_uiRasterDPI"})
public class tagPDFContentData extends Structure {

    public static class ByValue extends tagPDFContentData implements Structure.ByValue { }
    public static class ByReference extends tagPDFContentData implements Structure.ByReference { }
    public volatile int m_uiRasterDPI;
}


trait CDocuLinkCoreServices extends Library{
  def GetPDFContentData(value: Pointer):  tagPDFContentData.ByValue
}

But I am getting

m_uiRasterDPI= 0

where it should return 100.

Answer 1

Here you go: https://github.com/mkowsiak/jnicookbook/tree/master/recipes/recipeNo057

To make a smooth call to JNA, there is a Java class recipeNo057.Data. This class is super simple

package recipeNo057;

import com.sun.jna.Library;
import com.sun.jna.WString;
import com.sun.jna.Native;
import com.sun.jna.Structure;

@Structure.FieldOrder({ "field" })
public class Data extends Structure {

    public static class ByValue extends Data implements Structure.ByValue { }
    public static class ByReference extends Data implements Structure.ByReference { }
    public volatile int field;

}

on the other side (C++ side) we have "the same" structure.

typedef struct data {
  int field;
} 

Scala binds all that stuff together by "linking" Java based class, JNA based call to native code, and native code in C++ itself.

trait HelloWorld extends Library {

  def GetData(m: WString) : Data.ByValue;

}

object HelloJNA {

  def main(args:Array[String]):Unit = {

    val libc   = Native.load( "HelloWorld", classOf[HelloWorld] )

    var result = libc.GetData( new WString("I am passing String!") )
    println("Result: " + result.field);

  }
}

Note

It's important to pay attention to type match between Java structures and C++ structures.