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javaarrayspass-by-referencejna

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

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.

Solution

  • 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);