JNA返回结构数组作为参数

Question

I have a C function, which creates an array of structs

int createArray(myStruct** data_array);

From C I call the function like this:

myStruct* data_array;
int size = createArray(&data_array);

I want to call createArray in a similar way with JNA. I've generated the wrapper using JNAerator.

int createArray(myStruct.ByReference[] data_array);

I don't know what to pass to the JNA method createArray since data_array is actually a output parameter. I actually think the generated JNA method is wrong since I don't want to pass an array of myStruct , I want to receive one.

Shouldn't the parameter of the JNA method be PointerByReference ?

Here are some tries that didn't worked for me

PointerByReference ptr = new PointerByReference();
int size = api.createArray(ptr); //Invalid memory access

myStruct.ByReference[] array = new myStruct.ByReference[10];
Arrays.fill(stLinks, new myStruct.ByReference()); //without this I get NullpointerException
int size = api.createArray(array); //Invalid memory access

Answer 1

Shouldn't the parameter of the JNA method be PointerByReference ?

Well it could be. Should it? Depends on whether you want type safety or not.

The important point here is that native arrays are contiguous blocks of memory, so an array of fixed size structures is just a pointer with offsets for each element of the array.

If you pass an array as an argument, it will receive the pointer to the first element ( myStruct[0] ) and auto-write the native memory to fill it.

Alternately you could pass a PointerByReference and instantiate an array from it. A little lower-level interface and you have more direct control over when memory is read and written.

The most important thing here is just to realize who is allocating the memory for the array, and making sure it's released. The rest is mostly up to coding style and readability and type safety.

Answer 2

Seems like the problem with Invalid memory access was caused by other side effects. I had to call other functions from the API before I can use createArray.

The working code is now:

PointerByReference ptr = new PointerByReference();
            
int size = api.createArray(ptr);
            
Pointer ptrToFirst = ptr.getValue();
myStruct firstElement = new myStruct(ptrToFirst);
myStruct[] array = (myStruct []) firstElement.toArray(size);

//Work with Array

api.deleteArray();