JNA从无效转换为结构**

Question

I have to call a C library that receives as parameter a void** ; it converts the void** into a known structure (depending on function internal information) then allocates memory and fills it with data.

I don't know how to call it from Java.

Below are some details:

The struct:

typedef struct  { 
uint32_t   Size[64];  
uint16_t   *Datas[64]; 
} MyStruct_t;

The C function

int Cfunc( void **Evt) {
  int i;
  MyStruct_t *s;
  s = (MyStruct_t *) malloc(sizeof(MyStruct_t));
  for (i=0; i<8; i++) {
    (s)->Size[i] = 512;
    (s)->Datas[i] = malloc(512 * sizeof (short));
  }
  for (i=8; i<63; i++) {
    (s)->Size[i] = 0;
    (s)->Datas[i] = NULL;
  }
*Evt = s;
}

My JNA wrapper class:

public class MyStruct_t extends Structure<MyStruct_t, MyStruct_t.ByValue, MyStruct_t.ByReference > {
/// the number of samples stored in Datas array
public int[] Size = new int[64];
/// the array of Size samples
public ShortByReference[] Datas = new ShortByReference[64];
    public MyStruct_t() {
    super();
    initFieldOrder();
}
protected void initFieldOrder() {
    setFieldOrder(new String[]{"Size", "Datas"});
}
public MyStruct_t(int Size[], ShortByReference Datas[]) {
    super();
    if (Size.length != this.Size.length) 
        throw new IllegalArgumentException("Wrong array size !");
    this.Size = Size;
    if (Datas.length != this.Datas.length) 
        throw new IllegalArgumentException("Wrong array size !");
    this.Datas = Datas;
    initFieldOrder();
}
@Override protected ByReference newByReference() { return new ByReference(); }
@Override protected ByValue newByValue() { return new ByValue(); }
@Override protected MyStruct_t newInstance() { return new MyStruct_t(); }
public static MyStruct_t[] newArray(int arrayLength) {
    return Structure.newArray(MyStruct_t.class, arrayLength);
}
public static class ByReference extends MyStruct_t implements Structure.ByReference {};
public static class ByValue extends MyStruct_t implements Structure.ByValue {};
}

The Java function declaration:

public static native int Cfunc(MyStruct_t.ByReference Evt);
public static native int Cfunc(PointerByReference Evt);

If I call the Cfunc passing a PointerByReference I can see that memory is allocated and something is present but I don't know how to cast (?) it to the real structure (MyStruct_t).

If I call the Cfunc passing a MyStruct_t.ByReference , I find the structure not well allocated and values inside are not the expected. What am I doing wrong?

Answer 1

To be clear, you are providing the address of a pointer. The callee sets the pointer value to the address of a struct which it allocates.

First, you pass the address of the pointer (aka PointerByReference ):

public static native int Cfunc(PointerByReference Evt);

Then, you extract the "returned" pointer value:

PointerByReference pref = new PointerByReference();
lib.CFunc(pref);
Pointer p = pref.getValue();

Then, you can create a new instance of your structure based on the "returned" pointer value:

MyStruct s = new MyStruct(p);

This, of course, requires that you implement the Pointer -based ctor for your structure:

public MyStruct(Pointer p) {
    super(p);
    read();
}

Answer 2

I found the mistake! I was using Structure class from jnaerator, I removed the jnaerator Structure class and used JNA Structure and now it works.

here is the modified and working java class:

import com.sun.jna.Pointer;
import com.sun.jna.Structure;
import com.sun.jna.ptr.ShortByReference;

public class t_t extends Structure {
public int[] lsize = new int[64];
public Pointer[] ldata = new Pointer[64];
public t_t() {
    super();
    initFieldOrder();
}
protected void initFieldOrder() {
    setFieldOrder(new String[]{"size", "data"});
}
public t_t(int size[], Pointer data[]) {
    super();
    if (size.length != this.lsize.length) 
        throw new IllegalArgumentException("Wrong array size !");
    this.lsize = size;
    if (data.length != this.ldata.length) 
        throw new IllegalArgumentException("Wrong array size !");
    this.ldata = data;
    initFieldOrder();
}
    public t_t(Pointer p){
       super(p);
       read();
    }
    public short[] getData(int i) {
        if (this.lsize[i] == 0) return null;
        return this.ldata[i].getShortArray(0, this.lsize[i]);
    }
    public int getSize(int i) {
        return this.lsize[i];
    }
public static class ByReference extends t_t implements Structure.ByReference {

};
public static class ByValue extends t_t implements Structure.ByValue {

};
}

Thanks technomage for the tip! p1906