std::filesystem support

[chenxiaolong]

Description

The libc++ libraries (both shared and static) seem to be missing symbols needed to use <experimental/filesystem>. For example, trying to compile a simple program that calls std::experimental::filesystem::remove_all(path) will fail at link time with:

/stuff/Applications/Linux/android-ndk/sources/cxx-stl/llvm-libc++/include/experimental/filesystem:1695: error: undefined reference to 'std::experimental::filesystem::v1::__remove_all(std::experimental::filesystem::v1::path const&, std::__ndk1::error_code*)'
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

Sample project: https://github.com/chenxiaolong/ndk_filesystem_test (run ./test.sh to reproduce issue)

Is this expected to happen? I couldn't find any details as to whether the filesystem library should work with the NDK.

Environment Details

• NDK Version: r16b (16.1.4479499)
• Build sytem: any
• Host OS: Fedora 27 x86_64
• Compiler: clang
• ABI: any
• STL: libc++
• NDK API level: 21
• Device API level: n/a

[jmgao]

libc++ compiles its experimental parts into libc++experimental, a library that you need to link in separately. Unfortunately, we don't seem to build it as part of the NDK. Retitling the bug to look into shipping it.

In the mean time, you might be able to just add the required files directly into your project (from $ANDROID_NDK_ROOT/sources/cxx-stl/llvm-libc++/src/experimental/filesystem) to work around this on your end, but I haven't tried it (and this is guaranteed to break in the future, when the files are moved into the standard or abandoned).

[chenxiaolong]

Ah okay, that makes perfect sense. I didn't realize libc++ also had a separate library like gcc's libstdc++fs.

[DanAlbert]

The tests for std::filesystem are not going to be easy to run on Android (they require a python helper). Going to find a way to work around that, but it's not likely to happen during r17.

[DanAlbert]

Unlikely to fit into r18. I want to get this done for the platform before the NDK since it should be a smaller problem to solve, and haven't done that yet.