PointerByReference not returning value

Question

I'm trying to call a C++ function from Java via JNA. I want to pass in a string, and get a string back. This is done by using an in parameter and an out parameter. I use PointerByReference to represent the char** out parameter. The call to C++ works, but the PointerByReference is null after the call.

I based my code on the PointerByReference docs. Any ideas what I'm doing wrong?

I've added print statements to the C++ to make sure it's not not setting the pointer to null, and it's definitely not. So some in my use of JNA must be wrong. But what?

C++ code

void processRequest(char* input, char** output, int* outputLength)
{

    // Variable output size from processInput

    std::string sOutput = processInput(input);


    char* results = new char[sOutput.length() + 1];

    strncpy(results, sOutput.c_str(), sOutput.length());

    results[sOutput.length()] = '\0';


    output = &results;

    outputLength = new int(strlen(results) + 1);

}

Java code

Which is called by java like so:

public class RunRequestT1 {
    private static final Logger log = LoggerFactory.getLogger(MyLibWrapperImpl.class);

    static boolean envVarsSetupDone = false;

    static interface MyLib extends Library {

        MyLib INSTANCE = (MyLib) Native.loadLibrary("N:\\sys1\\sys1_dist\\MyLib\\MyLib9.8.Q.P3.G15.T\\bin\\MyLib.dll", MyLib.class);

        public void processRequest(String request, PointerByReference bufp, IntByReference lenp);

        public void clearMemoryPtr(PointerByReference bufp, IntByReference lenp);
    }

    public static void main(String[] args) throws IOException {

        System.out.println("sys1: Reading request");
        String request = FileUtils.readFileToString(new File("C:\\dev\\prj\\single-request.xml"), "UTF-8");

        System.out.println("sys1: Pricing request");
        String response = processRequest(request);

        System.out.println(response);

    }

    public static String processRequest(String request) {

        System.out.println("sys1: prepare args");
        // code based on https://github.com/twall/jna/blob/master/www/ByRefArguments.md
        PointerByReference bufp = new PointerByReference();
        IntByReference lenp = new IntByReference();

        System.out.println("sys1: making call");
        MyLib.INSTANCE.processRequest(request, bufp, lenp);

        System.out.println("reading response");
        System.out.println("bufp: " + bufp.getValue());
        System.out.println("lenp: " + lenp.getValue());
        Pointer p = bufp.getValue();
        byte[] buffer = p.getByteArray(0, lenp.getValue());
        String response = Native.toString(buffer);
        //String response = p.getString(0);

        // de-allocate memory buffer
        System.out.println("cleaning memory");
        MyLib.INSTANCE.clearMemoryPtr(bufp, lenp);
        return response;
    }
}

Stdout

And when I call it, I get results like this:

sys1: Reading request
sys1: Pricing request
sys1: prepare args
sys1: making call
reading response
bufp: null
lenp: 0
Exception in thread "main" java.lang.NullPointerException
    at com.calyon.gcm.MyLibwrapper.main.RunRequestT1.priceRequest()
    at com.calyon.gcm.MyLibwrapper.main.RunRequestT1.main()

I've tried to keep it minimal, and indeed there is not a lot going on.

I've tested with JNA versions 3.4 and 3.5

Answer 1

Instead of allocating a new pointer for your length "return", you need to write to the existing pointer, e.g.

*outputLength = strlen(results) + 1;

You also need to write to the the buffer pointer that was given, rather than just assigning a new value to your local arg:

*output = results;