dart:io - is there a support for memory-mapped files?

[isoos]

Is there any plan to support memory-mapped files in dart:io? It would be nice for server apps that want to keep an index on disk but still access it efficiently.

[kevmoo]

@a-siva ?

[a-siva]

At this point we don't have any plans for supporting memory-mapped files in dart:io, we can consider adding it at sometime depending on use case and requirements.

[jwinarske]

@a-siva This would be useful when dealing with Isolate/FFI and zero copy. I could use this right now.

[mraleph]

Duplicate of #49810

[mraleph]

@jwinarske you can implement memory mapped IO directly through FFI, no need for dart:io support. See https://github.com/dart-lang/sdk/blob/main/pkg/compiler/lib/src/io/mapped_file.dart for example.

[jwinarske]

@mraleph the example references:

Regenerate by running dart mmap.dart gen

Where do I find mmap.dart?

[mraleph]

@jwinarske you can find this code here: https://gist.github.com/mraleph/d8ffb32cd419e08e15b8fc245dbbeff7#file-test-dart-L214

[jwinarske]

@mraleph Thank you!

[renatoathaydes]

The link that was supposed to show an example of using FFI for mmap unfortunately linked to the main branch which currently uses the mmap package instead of FFI.
The mmap package does use FFI and only works on Unix: https://github.com/farshed/mmap/blob/main/lib/src/unix.dart

[mraleph]

@renatoathaydes mmap package is here (and it is not published): https://github.com/dart-lang/sdk/tree/main/pkg/mmap, and yes it currently only supports Linux, though it would be fairly trivial to update it to support Windows and Mac.