Truncating ByteArray Does Not Dispose Contents, Free Up Memory

http://www.ghostwire.com/blog/archives/as3-truncating-bytearray-does-not-dispose-contents-free-up-memory/

When targeting Flash Player 10 or AIR 1.5, you can use the clear() method of the ByteArray class to explicitly clear the contents of the byte array and free up the memory otherwise used by the bytes. The length andposition properties are reset to zero after calling the clear() method.

Unfortunately, when targeting Flash Player 9, this clear() method is not available. If you are using a ByteArrayobject as a data store, keeping a reference to the object and therefore not allowing the object to be garbage collected, do take note that there is no way to clear the contents. This means that the size ofByteArray objects can only be enlarged and never shrunk.

It is important to note that while you can truncate the ByteArray to zero byte by setting its length property to zero, this will not dispose the contents or free up the memory used.

This is actually a rather odd implementation. After you truncate a ByteArray, the bytes stay in memory and is in fact restorable simply by setting the length property back to a non-zero value.

var bytes:ByteArray = new ByteArray();
bytes.writeUTFBytes("ABCDE");
trace(bytes[0]); // 65
trace(bytes[1]); // 66
trace(bytes[2]); // 67
trace(bytes[3]); // 68
trace(bytes[4]); // 69
bytes.length = 0;
trace(bytes[0]); // undefined
trace(bytes[1]); // undefined
trace(bytes[2]); // undefined
trace(bytes[3]); // undefined
trace(bytes[4]); // undefined
bytes.length = 5;
trace(bytes[0]); // 65 ** what the ??? necromancy at work ??? **
trace(bytes[1]); // 66
trace(bytes[2]); // 67
trace(bytes[3]); // 68
trace(bytes[4]); // 69

In my opinion, this implementation is flawed. The Flash Player really should clear the data once the array of bytes is truncated; this should not require a new API method clear() to accomplish. To introduce a new API method that does what setting the length property to zero should have done in the first place really baffles me.

Anyway, rant aside, so that is that – when you truncate a ByteArray, remember that the data is not lost. When targeting Flash Player 10 or AIR 1.5, do remember to call the clear() method instead of attempting to clear the contents by truncating.

Zerofying Bytes
Since the data stays in memory, I reckon it could sometimes be useful to erase the data explicitly, especially if it contains sensitive data:

function zerofy(bytes:ByteArray):void
{
	if (bytes)
	{
		var j:int = bytes.length;
		while (j--)
		{
			bytes[j] = 0;
		}
	}
}

I am not sure if the clear() method in FP10 API does this (or something similar to erase the data in memory) – I sure hope it does.

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Truncating ByteArray Does Not Dispose Contents, Free Up Memory

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