JNA how do I go from a C int* to a JNA struct

Question

I have the following C code that I am trying to map into java:

int32 Query( void* qryBuffer, 
            uint32_t qryLength)
{
  static int8_t  sendBuffer[MAX_REQUEST_SIZE];
  message*  queryMessage;
...
  queryMessage = (message*)sendBuffer;

  memcpy( &sendBuffer[0], &qryLength, sizeof(qryLength) );
  memcpy( &sendBuffer[sizeof(qryLength)], qryBuffer, qryLength );
...
  query_ex(queryMessage);
}

typedef struct
{                               
    uint32      length_u;       
} message;

The JNA equivalent is for the message struct is:

public static class message extends Structure {
    public int length_u;
    public message() {
        super();
    }
    protected List getFieldOrder() {
        return Arrays.asList("length_u");
    }
    public message(int length_u) {
        super();
        this.length_u = length_u;
    }
    public static class ByReference extends omni_message implements Structure.ByReference {
    };
    public static class ByValue extends omni_message implements Structure.ByValue {
    };
};

I know what they are doing in the C code, but I cannot figure out how to do this on the Java side. I need to have the Java message class contain the length and the array of the parameters that were passed in

Please help!

Update: I have created my own copy of the message struct that holds the byte[]:

    public static class messageEx extends Structure {
        public byte[] buffer;
...
    }

I am going to try and pass this struct in as oppose to the other one, but I cannot test until tomorrow.

int size = struct.size();
byte[] buffer = new byte[size];
ByteBuffer bb = ByteBuffer.wrap(buffer);
bb.putInt(size);
bb.put(struct.getPointer().getByteArray(0, size));
messageEx msg = new messageEx(buffer);
query_ex(msg); // call to native world

Answer 1

You don't need the complication of structures here, given a buffer pointer you can explicitly write what you need. All the native code is doing is writing a buffer's length followed by its contents into another buffer:

byte[] buffer;
int length;
Pointer msg_buffer = new Memory(buffer.length + 4);

msg_buffer.setInt(0, length);
msg_buffer.setByteArray(4, buffer, buffer.length);

Of course, you can pre-allocate a fixed buffer instead of creating a new one every time (which is what the native code does).

Answer 2

Solution:

public int query(Structure struct) {
    struct.write(); // this makes sure all my data is being ref'd by the pointer
    int size = struct.size();
    Pointer p = new Memory(size + 4);
    p.setInt(0, size);
    p.write(4, struct.getPointer().getByteArray(0, size), 0, size);
    message msg = new message(p, size);
    query_ex(msg); // call to native world

I had to create a new constructor for the message class so I can have the struct use my pointer. Note: the size is meaningless:

public message(Pointer p, int size) {
    super(p);
    this.length_u = size;
}