Java JNA Invalid Memory Access

Question

I have a DLL which is written in Delphi (do not have the source code, but only the API). Here is the DLL function I am trying to invoke through Java using the JNA (version 5.4.0)

The following shows how my Java code looks like.

public interface FPrintDLL extends StdCallLibrary {
    FPrintDLL INSTANCE = Native.load("FPrintDLL", FPrintDLL.class);
    int OPEN_TCPIP(WString ipAddress, int port, int deviceIndex, WString serialKey);
}

public static void main(String[] args) {
    FPrintDLL fPrintDLL = FPrintDLL.INSTANCE;
    WString ipAddress = new WString("192.170.1.3");
    WString serialKey = new WString("12345678");
    int deviceIndex = 4004;
    int port = 9100;

    int connectResult = fPrintDLL.OPEN_TCPIP(ipAddress, port, deviceIndex, serialKey); // Line 81
}

Everything loads fine, the problem is I am getting the following exception,

Exception in thread "main" java.lang.Error: Invalid memory access
at com.sun.jna.Native.invokeInt(Native Method)
at com.sun.jna.Function.invoke(Function.java:426)
at com.sun.jna.Function.invoke(Function.java:361)
at com.sun.jna.Library$Handler.invoke(Library.java:265)
at com.sun.proxy.$Proxy0.OPEN_TCPIP(Unknown Source)
at HelloJNA.main(HelloJNA.java:81)

According to the exception log the problem occurred because of the ints. According to the method signature it doesn't use any pointers/references. So I am not sure what exactly the problem is.

Note - I managed to successfully run this function under the same conditions (the OS, JAVA, DLL architectures are listed below) using the following C# the code,

[DllImport(FPRINT_DLL, CallingConvention = CallingConvention.StdCall)]
public static extern int OPEN_TCPIP([MarshalAs(UnmanagedType.BStr)] string ipAddress,
        int tcpPort,
        int deviceIndex,
        [MarshalAs(UnmanagedType.BStr)] string serialKey);

I mentioned this because to confirm that the given API for OPEN_TCPIP is correct. Now I want to make a direct interaction between Java and the DLL. (not using some kind of a wrapper class)

The DLL is a 32-bit DLL and I am trying it on a 32-bit JVM, Windows10 64bit. FYI I tried the following sources (listed few) but couldn't work this out.

DLL issues and Invalid Memory Access

Java Native Access code Error: “Invalid memory access”

Function call returns java.lang.Error: Invalid memory access

JNA: java.lang.Error: Invalid memory access

Java use JNA call dll error:Invalid memory access (All the required DLLs are in the same directory)

JNA Exception in thread “main” java.lang.Error: Invalid memory access

Does anyone have an idea how to fix this issue?

Answer 1

The method signature requires a BSTR which requires memory allocation on the C side. You are passing a WString which is causing the Invalid memory access.

Allocate memory for the BSTR arguments like this:

BSTR ipAddress = OleAuto.INSTANCE.SysAllocString("192.170.1.3");
BSTR serialKey = OleAuto.INSTANCE.SysAllocString("12345678");

Once you are finished with the BSTR s you must release the memory:

OleAuto.INSTANCE.SysFreeString(ipAddress);
OleAuto.INSTANCE.SysFreeString(serialKey);