JNA错误的结构字段值

Question

I'm dancing around JNA adapter to "twain.h" - almost did it, but still have some problems with getting scanner capabilities. There is an original enity:

typedef unsigned short TW_UINT16, FAR *pTW_UINT16;
typedef unsigned long  TW_UINT32, FAR *pTW_UINT32;

typedef struct {
   TW_UINT16  ItemType;
   TW_UINT32  MinValue;     /* Starting value in the range.           */
   TW_UINT32  MaxValue;     /* Final value in the range.              */
   TW_UINT32  StepSize;     /* Increment from MinValue to MaxValue.   */
   TW_UINT32  DefaultValue; /* Power-up value.                        */
   TW_UINT32  CurrentValue; /* The value that is currently in effect. */
} TW_RANGE, FAR * pTW_RANGE;

This is my mapping (thnx JNAerator)

public static class TW_RANGE extends Structure {
        /** C type : TW_UINT16 */
        public short ItemType;
        /** C type : TW_UINT32 */
        public NativeLong MinValue;
        /** C type : TW_UINT32 */
        public NativeLong MaxValue;
        /** C type : TW_UINT32 */
        public NativeLong StepSize;
        /** C type : TW_UINT32 */
        public NativeLong DefaultValue;
        /** C type : TW_UINT32 */
        public NativeLong CurrentValue;
        public TW_RANGE() {
            super();
        }
        protected List<? > getFieldOrder() {
            return Arrays.asList("ItemType", "MinValue", "MaxValue", "StepSize", "DefaultValue", "CurrentValue");
        }
}

When I'm asking scanner for this entity, scanner sends me back filled object, but when I create it through

new TW_RANGE(pointer)

it returns me some wired stuff

TwaindsmLibrary$TW_RANGE(native@0x157c000c) (22 bytes) {
  short ItemType@0=870
  NativeLong MinValue@2=1572916
  NativeLong MaxValue@6=5500
  NativeLong StepSize@a=2097152
  NativeLong DefaultValue@e=5500
  NativeLong CurrentValue@12=2621440
}

1. How can I interpret this values?
2. Can I dump native structure and analyze it with some tool? Have seen Pointer.dump, but it requires the size of dumped value, how can i get size of structure under pointer?

Answer 1

Structure.read() must be called in order to copy the native memory into your JNA Structure fields.

The easiest way to do this is to ensure Structure.read() is called from your TW_RANGE(Pointer) constructor as the very last statement.

JNA normally calls Structure.read()/write() automatically when making function calls. It intentionally does not do so in Structure constructors, leaving it up to the programmer to decide the best point at which to perform that synchronization (if at all).

Answer 2

I got it! After little debugging C++ code, I found that (TW_RANGE*)pointer makes the same mess as Java does. But (TW_RANGE**)pointer works like a charm. So

 
 
 
  
  new TW_RANGE(pointer.getPointer(0))
 
  

[APPLAUSE] This solution lead me to random memory access errors. I should work with Twain native MemAllocate/Lock/Free functions which returns right values from given pointer.