Unable to access Memory reference with help of JNA library

Question

I am trying to access this piece of c++ code through JNA library that is written in .dll file

bool Services::ReverseString(const std::wstring &strIn)
{

    return true;
} 

My scala code is written below

trait CoreServices extends Library{

  def ReverseString(m:WString):Boolean

}
  val librarypath = "somepath"
    System.setProperty("jna.library.path", librarypath)
    val libc = Native.load("Services", classOf[CDocuLinkCoreServices])
    val x=libc.ReverseString(new WString("dddd"))

But I am getting the below error

java.lang.error invalid memory access

I am pretty new to JNA. ANy help will be appreciated.

Answer 1

I doubt you can call C++ code from Scala without using C style naming convention. I guess, the only way to go here is via extern "C" wrapper

For the following project tree

.
|-- Makefile
|-- c
|   `-- Services.cc
|-- jar
|   `-- jna-5.4.0.jar
|-- lib
|   |-- libServices.dylib
|   `-- libServices.dylib.dSYM
|       `-- Contents
|           |-- Info.plist
|           `-- Resources
|               `-- DWARF
|                   `-- libServices.dylib
`-- scala
|   `-- jna_call.scala
`-- target

and following code

Scala

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

trait Services extends Library {

  def ReverseStringWrapper(m:WString) : Boolean

}

object JNA {
  def main(args:Array[String]):Unit = {
    println("Testin JNA!! ")
    val librarypath = "./lib"
    System.setProperty("jna.library.path", librarypath)
    val libc = Native.load("Services", classOf[Services])
    val x=libc.ReverseStringWrapper(new WString("dddd"))
    println("Result: " + x);
  }
}

where native code looks like this

C++

#include <string>

class Services {
  public:
    bool ReverseString(const std::wstring &strIn);
};

bool Services::ReverseString(const std::wstring &strIn)
{
  return true;
}

extern "C" {

bool ReverseStringWrapper(const std::wstring &strIn)
{
  Services s;
  return s.ReverseString(strIn);
}

}

and library being built following way

> c++ -std=c++11 -g -shared \
  -fpic -I${JAVA_HOME}/include \
  -I${JAVA_HOME}/include/$(ARCH) \
  c/Services.cc -o lib/libServices.dylib

while Scala code is compiled following way

> scalac -d target -classpath "jar/jna-5.4.0.jar" scala/jna_call.scala

and executed this way

> scala -classpath "./target:jar/jna-5.4.0.jar" JNA
Testin JNA!!
Result: true

everything works as expected.

You can find full sample code here: https://github.com/mkowsiak/jnicookbook/tree/master/recipes/recipeNo054