java - 指向结构指针的指针作为 JNA 中的参数

Question

第 3 方 DLL 有一个函数，它需要一个指向结构指针的指针作为参数:

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

它将 numOfFinger 个指纹(通常是 4 个)的“拍打”图像分割成单个指纹(输出文件名 + 手指数)。

SlapInfo 的定义是:

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

struct Point {
    int x;
    int y;
};

C 中示例的片段:

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

根据 JNA FAQ ，在我的例子中，我应该使用“Structure.ByReference[]”:

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

所以，我像这样映射到 Java/JNA 中:

结构

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 { }
}

函数

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

用法

//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);

但是用这个方法，我得到了错误:

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)

我也尝试过:

• PointerByReference 而不是 SlapInfo.ByReference[](如 Pointer to array of structures as JNA method arguments 中所述)
• “指针”类，如 JNA: pointers to array of pointers 中所述
• SlapInfo[](仅供尝试)

在所有情况下，我都会收到“无效内存访问”错误。

我做错了什么？

Answer 1

如果 SegmentImages 返回 slapInfo 中一个或多个结构的地址，您需要使用 PointerByReference。实际的 native 类型是 struct **，但您在 slapInfo 中返回的值是一个指针。

IntegerByReference iref = new IntegerByReference();
PointerByReference pref = new PointerByReference();
int result = SegmentImages(..., iref, pref, ...);

如果结构在单个 block 中返回，您可以这样做:

SlapInfo s = new SlapInfo(pref.getValue());
SlapInfo[] slaps = s.toArray(iref.getValue());