JNA return char*

Question

how to work with Check type char*? (Check fill in function and contains text >500 characters or pointer on memory with text)

I have my_dll.dll. Description dll

int my_function(char* param1, struct answer* ans);

#pragma pack(1)
struct answer{
  int TType;                   //IN
  unsigned long Amount;   //IN
  char          Rcode   [2+1];  //OUT
  char          AMessage[16 ]; //OUT
  int           CType;              //OUT
  char*         Check;            //OUT
};

In Java I have code:

 public interface My_Dll extends Library {
        public static class answer extends Structure {
                    public static class ByReference extends answer 
                             implements Structure.ByReference {}

              public int  TType      = 0;                   
              public int  Amount     = 0;        
              public byte Rcode[]    = new byte[3]; //OUT: 
              public byte AMessage[] = new byte[16]; //OUT: 
              public int  CType      = 0;         //OUT: 
              public ??? Check;         //OUT: 
              protected List getFieldOrder() {
                      return Arrays.asList(new String[] {"TType", "Amount", 
                             "Rcode","AMessage", "CType","Check"});
              }
        }
        public int my_function(byte track2[], answer.ByReference ans);
  }

  public static void Start() {
      My_Dll test_dll = (My_Dll) Native.loadLibrary("my_dll", My_Dll.class);
      My_Dll.answer.ByReference aa = new My_Dll.answer.ByReference();
      //  In
      aa.Amount = 100;
      aa.TType =3;

      int result = test_dll.my_function(null,aa);
      //  OUT
      System.out.println("Result: "  + result);
      System.out.println("Rcode: "  + new String(aa.Rcode));
      System.out.println("Amessage: " + new String(aa.AMessage));
}

Answer 1

One thing I've tried is to use a pre-constructed array of byte, and then use Native.toString(...) to convert it to a Java String. For example,

byte[] windowText = new byte[SOME_CONSTANT];
user32.GetWindowTextA(hWnd, windowText, SOME_CONSTANT);
String wText = Native.toString(windowText).trim();

In the code above, 512 worked well as my SOME_CONSTANT, but you will probably need to use a larger constant.

Answer 2

Your Check field must be of pointer type. If you declare it as Pointer , you can use Pointer.getString(0) to extract the String value.

If it is up to the caller to allocate the memory, you can use com.sun.jna.Memory to initialize it; if not, you will need to release the memory returned in the struct to avoid a leak.