java jna - get byte array by reference java.lang.IndexOutOfBoundsException

Question

I'm using JNA and I get a strange error getting a byte array.

I use this code:

PointerByReference mac=new PointerByReference();
NativeInterface.getMac(mac);
mac.getPointer().getByteArray(0,8)

And it throws a IndexOutOfBoundsException: Bounds exceeds available space : size=4, offset=8 also if I'm sure thate the byte returned is a 8byte length. I tried to get that array as String:

mac.getPointer().getString(0)

And here I get successfully a String 8 chars lenght. Can you understand why?

Thank you.

technomage · Accepted Answer · 2012-05-23 11:26:24Z

3

PointerByReference.getValue() returns the Pointer you're looking for. PointerByReference.getPointer() returns its address.

mac.getPointer().getByteArray(0, 8) is attempting to read 8 bytes from the PointerByReference allocated memory (which is a pointer), and put those bytes into a Java primitive array. You're asking for 8 bytes but there are only 4 allocated, thus the corresponding error.

mac.getPointer().getString(0) is attempting to read a C string from the memory allocated for a pointer value (as if it were const char *, and convert that C string into a Java String. It only bounds-checks the start of the string on the Java side, so it will keep reading memory (even if it is technically out of bounds) until it finds a zero value.

EDIT

mac.getValue().getByteArray(0, 8) will give you what you were originally trying to obtain (an array of 8 bytes).

EDIT

If your called function is supposed to be writing to a buffer (and not writing the address of a buffer), then you should change its signature to accept byte[] instead, e.g.

byte[] buffer = new byte[8];
getMac(buffer);

edited May 23, 2012 at 11:26

answered May 22, 2012 at 11:07

technomage

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5 Comments

Tobia Over a year ago

Ok, but than how to read the bytearray values and not the pointer?

Tobia Over a year ago

Using getValue() my jvm goes in ACCESS_VIOLATION error and crashes... :-( I can not understand why with getString(0) it works...

technomage Over a year ago

Take a moment and read how pointers work. If you don't understand that, you'll only be guessing.

technomage Over a year ago

I'm betting your native code doesn't do what you're implying it does. What is the actual declaration? Does the native code write a pointer to the supplied address, or does it write data to the supplied address?

Tobia Over a year ago

Your last edit solved my trouble... it was so simple, but I did not try that because i thought to use some ByReference object. Thank you.

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