[foreign-abi] RFR 8232628: foreign-abi usability issues

[Maurizio Cimadamore]

On 18/10/2019 18:36, John Rose wrote:
> On Oct 18, 2019, at 10:23 AM, Maurizio Cimadamore 
> <maurizio.cimadamore at oracle.com 
> <mailto:maurizio.cimadamore at oracle.com>> wrote:
>>
>> which means we should probably create a NOTHING memory segment to 
>> prevent slicing
>
> Maybe it’s enough to have a special zero-length segment.
>
> (Can I make such a segment by slicing?  Answer should probably be “yes”.)

You can indeed slice your way up into a zero-length segment.

MemorySegment zero = MemorySegment.ofNative(10).slice(0, 0);

But... a segment also has a 'base' object and a 'min' offset attached to it.

A NULL address is a specific address which can be thought of as the base 
address of a segment whose base is null and whose min offset is zero

And obtaining such a segment is 'unsafe' (because that's the EVERYTHING 
segment).

So... "everything" is unsafe, but "nothing" is trivially safe (even 
though it can be derived from "everything").

But "nothing" != "zero length"

In fact you might have _infinitely many_ distinct "zero length" segment, 
but only one true "nothing" one.

Makes sense?

Maurizio