Pointer to pointer to structure as argument in JNA

[Luis Cardozo]

Hello!

I have posted this question in StakOverflow, but I think that here is where it belongs:

A 3rd party DLL have a function that expects a pointer to pointer to structure as argument:

__declspec(dllimport) int __stdcall
SegmentImages( unsigned char* imageData, int imageWidth, int imageHeight,
               int* numOfFinger, SlapInfo** slapInfo, const char* outFilename );

It segments a "slap" image of numOfFinger fingerprints (usually 4) into individual fingerprints (outFilename + number of finger).

The definition of SlapInfo is:

struct SlapInfo {
    int fingerType; 
    Point fingerPosition[4];
    int imageQuality;
    int rotation;
    int reserved[3];
};

struct Point {
    int x;
    int y;
};

A snippet of the example in C:

unsigned char* imageData;
int imageWidth, imageHeight;
//obtain imageData, imageWidth and imageHeight from scanner...
int numOfFinger;
SlapInfo* slapInfo;

int result = SegmentImages( imageData, imageWidth, imageHeight, &numOfFinger,
                            &slapInfo, FINGER_IMG_PATH);




JNA

According to JNA FAQ, In my case I should use "Structure.ByReference[]":

void myfunc(simplestruct** data_array, int count); // use Structure.ByReference[]

So, I mapped in Java/JNA like this:

Structures

public class Point extends Structure {
    public int x;
    public int y;

    public Point(){
        super();
    }

    @Override
    protected List<String> getFieldOrder() {
        return Arrays.asList("x", "y");
    }
}


public class SlapInfo extends Structure {    
    public int fingerType;
    public Point[] fingerPosition = new Point[4];
    public int imageQuality;
    public int rotation;
    public int[] reserved = new int[3];

    public SlapInfo() {
        super();
    }

    public SlapInfo(Pointer pointer){
        super(pointer);
    }

    @Override
    protected List<String> getFieldOrder() {
        return Arrays.asList("fingerType", "fingerPosition", "imageQuality",
                "rotation", "reserved");
    }

    public static class ByReference extends SlapInfo implements Structure.ByReference { }
}

Function

int SegmentImages(byte[] imageData, int imageWidth, int imageHeight, 
                  IntByReference numOfFinger, SlapInfo.ByReference[] slapInfo,
                  String outFileName);

Usage

//imageData, width and height are with valid values
IntByReference nrOfFingers = new IntByReference();
SlapInfo.ByReference[] slapInfoArray = (SlapInfo.ByReference[]) 
                new SlapInfo.ByReference().toArray(4);
                //Usually, there are 4 fingers in a slap, but the correct
                //value should be the returned in nrOfFingers

int result = lib.SegmentImages(imageData, width, height,
                nrOfFingers, slapInfoArray, FINGER_IMG_PATH);

But with this method, I get the error:

java.lang.Error: Invalid memory access
        at com.sun.jna.Native.invokeInt(Native Method)
        at com.sun.jna.Function.invoke(Function.java:419)
        at com.sun.jna.Function.invoke(Function.java:354)
        at com.sun.jna.Library$Handler.invoke(Library.java:244)
        at com.sun.proxy.$Proxy0.SegmentImages(Unknown Source)
        at scanners.JNAScanner.segmentToFile(JNAScanner.java:366)

I also tried with:

• PointerByReference instead of SlapInfo.ByReference[] (as explained in Pointer to array of structures as JNA method arguments)
  
• A "Pointer" class, as explained in JNA: pointers to array of pointers
  
• SlapInfo[] (only to try)
  

In all the cases, I got the error "Invalid memory access".

What am I doing wrong?




Thanks,

[Douglas Lewis]

Luis,

Did you get this problem solved?

Doug

[Timothy Wall]

See https://stackoverflow.com/questions/47420070/pointer-to-pointer-to-structure-as-argument-in-jna/48348568#48348568

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