New candidate JEP: 419: Foreign Function & Memory API (Second Incubator)

[Maurizio Cimadamore]

On 12/10/2021 19:20, leerho wrote:
>
>     Except that allocateNative does zero the memory :-) There are some
>     obscure flags by which you can disable this, but in general that's the
>     lay of the land.
>
>
> Where are these "obscure flags" documented?
> In general, where can I find documentation of all possible flags, say 
> for JDK 17?
I don't think there's a general documentation for all possible flag. The 
one in question is "-Djdk.internal.foreign.skipZeroMemory=true"
>
>     If a more optimized allocator is needed, a
>     SegmentAllocator is probably the way to go, in which there's more
>     freedom as to how memory is actually allocated/inited.
>
>
> Could you give an example of how to implement the 
> /SegmentAllocator::allocate(long,long)/ method to return a new segment 
> that does not do memory zeroing? (presumably without having to use 
> "obscure flags" option)

you can:

1. setup the method handles to call "malloc" and "free" using the linker API
2. when you receive an allocation request, you call malloc
3. that gives you back a MemoryAddress
4. you attach the memory address to a given ResourceScope (the client 
will have given that to you), to form a MemorySegment, using 
MemorySegment::allocateNative
5. you register a close action on the scope, so that when the scope is 
closed, "free" is called with that address
6. you return the segment to the user

It's really simpler than it sounds. All the pieces are there. (I might 
follow up with a Gist to show something along these lines).

Maurizio

>
> Thanks & Regards,
>
> Lee.
>
>
>
> On Tue, Oct 12, 2021 at 5:06 AM Sebastian Stenzel 
> <sebastian.stenzel at gmail.com <mailto:sebastian.stenzel at gmail.com>> wrote:
>
>
>     > On 12. Oct 2021, at 13:13, Maurizio Cimadamore
>     <maurizio.cimadamore at oracle.com
>     <mailto:maurizio.cimadamore at oracle.com>> wrote:
>     >
>     >
>     > On 12/10/2021 11:54, Sebastian Stenzel wrote:
>     >> Oh well... good to know! :D
>     >>
>     >> While my assumption was wrong, the conclusion stays the same:
>     It should be mentioned whether we can rely on one or the other
>     behaviour.
>     >>
>     > For the records, in the current javadoc for
>     MemorySegment::allocateNative I see this sentence:
>     >
>     >> The block of off-heap memory associated with the returned
>     native memory segment is initialized to zero.
>     >
>     > Which is repeated across all the allocateNative factories.
>     >
>     > Maurizio
>     >
>
>     My bad, I must have missed this. ��
>