Bug: MappedMemorySegmentImpl uses internal FileChannelImpl

[Maurizio Cimadamore]

On 29/06/2020 18:49, Ty Young wrote:
>
> On 6/29/20 11:32 AM, Maurizio Cimadamore wrote:
>> Note that to be able to map a file we need a file descriptor. The 
>> file system you provided has support for channels, etc. but note that 
>> the channels implementation do not support mapping using 
>> MappedByteBuffer:
>>
>> https://github.com/google/jimfs/blob/master/jimfs/src/main/java/com/google/common/jimfs/JimfsFileChannel.java#L592 
>>
>>
>> So I don't really see how we could make this work. At the end of the 
>> day, MemorySegment.mapFromPath has to end up calling `mmap` - and we 
>> need to be able to retrieve the parameters of the mmap call from 
>> somewhere.
>
>
> Would FMA having its own platform independant mmap bindings be a 
> possible solution? I guess I could just do it myself either way.

I think I showed an example of how to create a mapped segment manually, 
by relying on FFI:

https://gist.github.com/mcimadamore/128ee904157bb6c729a10596e69edffd

If you happen to have a file descriptor for the thing you want to map, 
then an approach like this would be feasible

Maurizio

>
>
>>
>> Maurizio
>>
>>
>> On 29/06/2020 16:55, Ty Young wrote:
>>> Tried using Google's in-memory filesystem:
>>>
>>>
>>> https://github.com/google/jimfs
>>>
>>>
>>> for storing mmap files but ran into an issue because 
>>> MappedMemorySegmentImpl uses internal FileChannelImpl method:
>>>
>>>
>>> https://github.com/openjdk/panama-foreign/blob/acc11115db42cc0e17efdf361d58c65f4e922f39/src/jdk.incubator.foreign/share/classes/jdk/internal/foreign/MappedMemorySegmentImpl.java#L105 
>>>
>>>
>>>
>>> which causes a class casting exception when using Google's in-memory 
>>> Java filesystem and probably other implementations as well.
>>>
>>>
>>>