Android compilation with per-arch C libraries: Is there a prepared/expected way of doing it?

[ig-garcia]

Hello, I am trying to compile for Android a C library that needs to be compiled separately for every Android architecture.
This C library is a dependency of a rust library which in turn is also dependency of another rust library which is dependency of the hub crate.
The rust library hub is depending on has a build.rs file where I build the C library. But in order for it to properly build for Android it seems like I have to pinpoint myself the right compilers in the NDK path. This is not a problem, just I wonder if I am messing with rinf's expected behavior or usage.
Because rinf compilation for Android is creating a libhub.so file, which is a dynamic library, I cannot link in build time.
Then I am adding CMakeLists.txt file in Android for the linking.
Is there a related or different expected way to handle this situation?

Thanks in advance.

[temeddix]

Hello :)

Looks like you're it doing the right way. I'm not an expert in the field of linking C libraries to Rust libraries, but your method does align with the guides written in the Rust docs:

• https://docs.rust-embedded.org/book/interoperability/c-with-rust.html#building-your-cc-code

Perhaps using CMakeLists.txt is optional becuase the docs states that using shell commands from build.rs is possible, but not 100% sure it would be ideal:

• https://docs.rust-embedded.org/book/interoperability/c-with-rust.html#triggering-external-build-systems

FYI, Rinf is using Cargokit under the hood to automatically build native Rust libraries. If I'm correct, it produces fat libraries(for multiple targets), but I think you should handle building C libraries for multiple targets manually, as you stated.

• https://github.com/irondash/cargokit/blob/d56f1f269efeb7a7b2919dc7d8302ef6bddf47f0/build_tool/lib/src/target.dart#L98

[ig-garcia]

I see something weird in the env variables I get from rust, which are set from rinf's cargokit:

CC_aarch64-linux-android: /Users/ignaciogarcia/Library/Android/sdk/ndk/25.1.8937393/toolchains/llvm/prebuilt/darwin-x86_64/bin/clang

Somehow the target arch is not included in these vars... like I would be expecting something like

/Users/ignaciogarcia/Library/Android/sdk/ndk/25.1.8937393/toolchains/llvm/prebuilt/darwin-x86_64/bin/aarch64-linux-android21-clang

Please see the difference: the second path has included aarch64-linux-android21- which are the target triplet + the min android version.
Then my questions are:

• Are these env variables supposed to be ignored in rust ndk builds?
• Are these env variables supposed to be used in a specific way in rust ndk builds?
• Any more info is welcome.
  Thanks in advance.

EDIT:
This is in the context that the C library is building fine, and on startup I get this error:

E/flutter (23507): [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: Invalid argument(s): Failed to load dynamic library 'libhub.so': dlopen failed: cannot locate symbol "_ZNSt6__ndk112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEED1Ev" referenced by "/data/app/~~J4JqwUrdz54b5jQTjobuHg==/security.planck.phoenix-uCymzZHJOoinr6TH7XUveg==/base.apk!/lib/arm64-v8a/libhub.so"...
E/flutter (23507): #0      _open (dart:ffi-patch/ffi_dynamic_library_patch.dart:11:43)
E/flutter (23507): #1      new DynamicLibrary.open (dart:ffi-patch/ffi_dynamic_library_patch.dart:22:12)
E/flutter (23507): #2      loadRustLibrary (package:rinf/src/load_os.dart:25:27)
E/flutter (23507): #3      rustLibrary (package:rinf/src/load_os.dart:5:21)
E/flutter (23507): #4      rustLibrary (package:rinf/src/load_os.dart)
E/flutter (23507): #5      prepareInterfaceReal (package:rinf/src/interface_os.dart:21:24)
E/flutter (23507): #6      initializeRust (package:rinf/rinf.dart:17:9)

[temeddix]

I'm sorry to say that I cannot be a big help here, as this is an issue directly related to Cargokit. Rinf is simply using its build system and does not handle any build process.

I do suggest to go to the Cargokit repo and ask for additional infos:

• https://github.com/irondash/cargokit

Thank you for your participation :)

[ig-garcia]

Finally I fixed the build after some playing with rinf and understanding the things my library needed.