Comment by pjmlp
7 months ago
Android has two official languages for userspace Java and Kotlin.
NDK is only for writing native methods, reuse C and C++ libraries, and better performance for 3D and real time audio.
Anything else, is not officially supported by the Android team.
fopen(), fwrite(), etc. are a part of libc and as such are officially supported [1], but they become somewhat useless with SAF. (It makes sense that you'd at least have to use some Java to open a file picker and request access, but why not provide native access as well when a file is picked?)
[1] https://developer.android.com/ndk/guides/stable_apis
And they can be used the same as java.io.File, inside the application sandbox.
Other than that, it boils down to:
" - Squeeze extra performance out of a device to achieve low latency or run computationally intensive applications, such as games or physics simulations.
- Reuse your own or other developers' C or C++ libraries. "
From https://developer.android.com/ndk/guides
And most likely safety, not having C and C++ dealing directly with random bytes from untrusted files.
Those are vague examples of what the NDK could be used for, not an exhaustive list of all supported use cases. And Syncthing is really just trying to re-use their existing code on Android (example use case #2). It doesn't even require low-level syscalls, it just needs to be able to call fopen() on files which the user has explicitly granted to the app, which you'd expect would be available within the application sandbox.
> And most likely safety, not having C and C++ dealing directly with random bytes from untrusted files
You might as well just axe the entire NDK if you're worried about unsafe code.
Yeah, I personally think Scoped Storage on Android was a bit misdesigned and it would probably be much easier for all app developers if they (ab)used FUSE to present allowlisted directories inside app-specific mounts.