[FIXED] Invalid memory access in JNA

Issue

When using JNA, I am receiving an Invalid Memory Access error when invoking the QLConnect method.

Here is my interface mapping the DLL:

import com.sun.jna.Native;
import com.sun.jna.win32.StdCallLibrary;

public interface QuikLimit extends StdCallLibrary {
    
    QuikLimit INSTANCE = (QuikLimit) Native.load(new File("").getAbsolutePath() + "\\QuikLimit.dll", QuikLimit.class);
    
    int QLConnect(String userName, String userPassword, byte[] desc);
    
    //more methods from dll
}

I have another class with constants:

public class Const {
    public static final int DESC_SIZE = 1024;
    
    public enum Status {
        //enum values

        int code;

        private Status(int code) {
            this.code = code;
        }
        public int getCode() {
            return code;
        }
    };
}

And here is the place of occurrence in the code:

@Component
public class ConnectionManager {
    
    @Value("${quik.login}")
    private String login;
    @Value("${quik.password}")
    private String password;

    private Logger logger = LoggerFactory.getLogger(this.getClass());

    private boolean connected;

    public synchronized BaseResponse connect() {
        BaseResponse result = new BaseResponse();
        byte[] desc = new byte[Const.DESC_SIZE]; 
        if (connected) {
        //logic if connected
        logger.info("call QLConnect");
        int retVal = QuikLimit.INSTANCE.QLConnect(login, password, desc);
        if (retVal == Const.Status.QL_ACTIVE.getCode()) {       
        //logic from response
        } else if (retVal == Const.Status.QL_CONNECTEDNOTACTIVE.getCode()) {
        //logic from response
        }
        result.setCode(retVal);
        result.setDescription(Util.descToStr(desc));
        QuikLimit.INSTANCE.QLDisconnect();
        return result;
    }
}

I understand that 1 assumption immediately falls on the fact that the type does not match in the dll. But the header file clearly states that the return type is int:

typedef __int32             ql_long;
ql_long _stdcall QLConnect(const char* lpszUserName, const char* lpszUserPassword, char* lpszError);

lpszUserName is a pointer to an ASCIIZ string containing the username.

pszUserPassword is a pointer to an ASCIIZ string containing the user's password.

lpszDesc is a pointer to a buffer into which, in case of an error, the line with its description. The minimum buffer size is 512 bytes.

What else could be the cause of the error?

Solution

The first thing to check when you get Invalid Memory Access errors is your type mapping, which you correctly started with looking at the int return value. Turns out the String mapping for the other arguments, which works fine on *nix systems, needs a little special treatment on Windows.

By default Windows uses UTF 16 encoding, and JNA's default type mapping under-the-hood tries to convert Java String objects to UTF-16 encoded native strings (wchar*). This works fine for Windows API DLLs.

However, your custom DLL specified that the User Name and Password arguments should be in ASCIIZ format (null-terminated C strings).

You can force JNA to use an ASCII String type mapper by including W32APIOptions.ASCII_OPTIONS as a third argument when loading the DLL, e.g.,

QuikLimit INSTANCE = (QuikLimit) Native.load(new File("").getAbsolutePath() + "\\QuikLimit.dll", QuikLimit.class, W32APIOptions.ASCII_OPTIONS);

There are other finer grained approaches dealing just with String type mapping, that don't try to also do Function mapping (if any of your mapped functions end in W or A this might break!). You can dig into the W32APIOptions class to see how to create your own custom options to pass.

Answered By - Daniel Widdis
Answer Checked By - Clifford M. (JavaFixing Volunteer)