Ability to extend a MemorySegment

[Maurizio Cimadamore]

A memory segment has a fixed size - its boundaries are set on 
construction (pretty much like in a ByteBuffer). Think of a 
MemorySegment as the result of a 'malloc' - if you want something 
fancier than that, it's likely you're trying to use a memory segment in 
a way it's not intended to.

So, when you use a layout to do MemorySegment::allocateNative, the 
layout is used to determine the size (and alignment) of the chunk of 
memory to be allocated - so the size has to be known at segment creation.

How would an array list be implemented in C? Or, how is it implemented 
even in Java? You will see that the 'creating new array and copying 
contents' is really how things are done at the low level. So, if you 
want a variable-sized data structure based on MemorySegment, yes, you 
will have to take into account the fact that the MemorySegment will have 
to be thrown away and re-created with a new size (**)

(*) Actually there is another possibility when working with malloc/free 
which is "realloc" - it is theoretically possible to provide a re-alloc 
API point to memory segment which would:

- kill the current segment
- realloc the underlying native memory to new size
- create a new segment with new size

But we will have to evaluate this possibility in conjunction with the 
fact that, one day, memory segment might not exactly map to malloc/free 
and instead use a different allocator (which might or might not have 
ability to "reallocate"). Something to keep in mind though.

Maurizio




On 20/01/2020 21:53, Ty Young wrote:
> Hi,
>
>
> Would it be possible to add the ability to extend a memory segment(in 
> other words, increase its limit)? Right now it's not possible to 
> implement dynamic arrays(read: ArrayList like arrays) as it's not 
> possible to calculate the length of a SequenceLayout that has no 
> specified length. You can work around this by creating a new array and 
> copying the contents of the old array to it but that seems inefficient...
>