GetDirectBufferAddress: Address out by 4

Question

I am attempting to pass a bytebuffer from Java to native code in android. However i am consistantly getting the returned memory address starting at index 4.

I have tested across various devices and the array is the same size across Java and JNI when calling GetDirectBufferCapacity.

Why is this offset occuring?

Java:

inputBuffer = ByteBuffer.allocateDirect(inputBufferSize);

JNI:

uint8_t* inputBuffer = (uint8_t*) env->GetDirectBufferAddress(inputByteBuffer);

The given memory address is offset by 4 bytes and only by moving the pointer does the array match (as shown below).

uint8_t* inputBuffer = (uint8_t*) env->GetDirectBufferAddress(inputByteBuffer) - 4;

Answer 1

The address returned is the effective address, based upon the alignment of the buffer. The code (Copyright AOSP, all rights reserved) looks like this:

public static ByteBuffer allocateDirect(int capacity) {
    if (capacity < 0) {
        throw new IllegalArgumentException("capacity < 0: " + capacity);
    }
    // Ensure alignment by 8.
    MemoryBlock memoryBlock = MemoryBlock.allocate(capacity + 7);
    long address = memoryBlock.toLong();
    long alignedAddress = (address + 7) & ~(long)7;
    return new DirectByteBuffer(memoryBlock, capacity, (int)(alignedAddress - address), false, null);
}

When you get the direct address, you get the effective address, which is aligned, while the actual buffer is not aligned.