Can I someone point to me what I did wrong? Trying to map VB to Java using JNA to access the library

Question

Original Working VB_Code

Private Declare Function ConnectReader Lib "rfidhid.dll" () As Integer
Private Declare Function DisconnectReader Lib "rfidhid.dll" () As Integer
Private Declare Function SetAntenna Lib "rfidhid.dll" (ByVal mode As Integer) As Integer
Private Declare Function Inventory Lib "rfidhid.dll" (ByRef tagdata As Byte, ByVal mode As Integer, ByRef taglen As Integer) As Integer

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    Dim desc As String
    desc = "1. Click ""Connect"" to talk to reader." & vbCr & vbCr
    desc &= "2. Click ""RF On"" to wake up the TAG." & vbCr & vbCr
    desc &= "3. Click ""Read Tag"" to get tag PCEPC."
    lblDesc.Text = desc
End Sub

Private Sub cmdConnect_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdConnect.Click
    If cmdConnect.Text = "Connect" Then
        If ConnectReader() Then
            cmdConnect.Text = "Disconnect"
        Else
            MsgBox("Unable to connect to RFID Reader. Please check reader connection.")
        End If
    Else
        If DisconnectReader() Then
            cmdConnect.Text = "Connect"
        End If
    End If
End Sub

Private Sub cmdRF_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdRF.Click
    If cmdRF.Text = "RF On" Then
        If SetAntenna(&HFF) Then
            cmdRF.Text = "RF Off"
        End If
    Else
        If SetAntenna(&H0) Then
            cmdRF.Text = "RF On"
        End If
    End If
End Sub

Private Sub cmdReadTag_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdReadTag.Click
    Dim tagdata(64) As Byte
    Dim taglen As Integer, cnt As Integer
    Dim pcepc As String
    pcepc = ""
    If Inventory(tagdata(0), 1, taglen) Then
        For cnt = 0 To taglen - 1
            pcepc &= tagdata(cnt).ToString("X2")
        Next
        txtPCEPC.Text = pcepc
    Else
        txtPCEPC.Text = "ReadError"
    End If

End Sub

Java Code (Simplified)

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

public class HelloWorld {
public interface MyLibrary extends Library {
   public int ConnectReader();  
   public int SetAntenna (int mode);
   public int Inventory (byte tagdata, int mode, int taglen); 

 }

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

System.out.println(lib.ConnectReader());

System.out.println(lib.SetAntenna(255));
byte[] tagdata = new byte[64];

int taglen = 0;
int cnt;
String pcepc;
pcepc = "";

if (lib.Inventory(tagdata[0], 1, taglen) == 1) {
    for (cnt = 0; cnt < taglen; cnt++) 
        pcepc += String.valueOf(tagdata[cnt]);           
} 
}
}

The error happens when lib.Inventory is run. lib.Inventory is used to get the tag from the RFID reader. If there is no tag, no error.

The error code

 An unexpected error has been detected by Java Runtime Environment:

 EXCEPTION_ACCESS_VIOLATION (0xc0000005) at pc=0x0b1d41ab, pid=5744, tid=4584

Java VM: Java HotSpot(TM) Client VM (11.2-b01 mixed mode windows-x86)
Problematic frame:
C  [rfidhid.dll+0x141ab]

An error report file with more information is saved as: C:\eclipse\workspace\FelmiReader\hs_err_pid5744.log

Answer 1

At first blush, I suspect the problem is an incorrect type signature in the Inventory declaration.

If the parameter is ByRef, that implies a pointer (in C this would be a *byte instead of a byte). JNA has a DoubleByReference class type that you probably should be using. You'll have a similar issue with the 3rd parameter (which is really an int*, not an int as you have it coded).

As an FYI, the native crash that you are getting should leave a dump file in the application directory. If you open that up, you should find the native call that was in place when the failure occurred - I'll bet money that it was the lib.Inventory(tagdata[0], 1, taglen) call.

Figuring out exactly which call is causing the crash will be key to tracking it down.