flutter ffi can't find .so file

I tried to execute .so file (golang file build for android) on android via flutter dart FFI. Everything works just fine when I do the same with .dll file on windows.

FFI exits with error:

E/flutter (12713): [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: Invalid argument(s): Failed to load dynamic library 'assets/lib.so': dlopen failed: library "assets/lib.so" not found
E/flutter (12713): #0      _open (dart:ffi-patch/ffi_dynamic_library_patch.dart:11:43)
E/flutter (12713): #1      new DynamicLibrary.open (dart:ffi-patch/ffi_dynamic_library_patch.dart:22:12)
E/flutter (12713): #2      dylib (package:proof2/fficheck.dart:7:34)
E/flutter (12713): #3      dylib (package:proof2/fficheck.dart)
E/flutter (12713): #4      runServer (package:proof2/fficheck.dart:10:5)
E/flutter (12713): #5      runServer (package:proof2/fficheck.dart)
E/flutter (12713): #6      main (package:proof2/main.dart:11:3)
E/flutter (12713): #7      _runMain.<anonymous closure> (dart:ui/hooks.dart:301:23)
E/flutter (12713): #8      _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:297:19)
E/flutter (12713): #9      _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)

The code snippet. lib is in /assets/ directory

import 'dart:ffi' as ffi;
import 'package:ffi/ffi.dart';
import 'package:ffi/src/utf8.dart';

typedef run_server = ffi.Pointer<Utf8> Function(); // FFI fn signature
typedef RunServer = ffi.Pointer<Utf8> Function(); // Dart fn signature
final dylib = ffi.DynamicLibrary.open('assets/lib.so');

final RunServer runServer =
    dylib.lookup<ffi.NativeFunction<run_server>>('RunServer').asFunction();

Solution

In Android, you can't access assets as normal file like in Windows, since it is packaged in a binary form.

To bundle prebuilt 3rd party library, you might want to add .so files to android/src/main/jniLibs/[abi]. Then you should be able to load the library via DynamicLibrary.open('libfoo.so'). This is doucmented here.

You need to provide a library for every ABI, so as a result, your project structure should look like this.

.
└── android/
    └── src/
        └── main/
            └── jniLibs/
                ├── arm64-v8a/
                │   └── libfoo.so
                ├── armeabi-v7a/
                │   └── libfoo.so
                ├── x86/
                │   └── libfoo.so
                └── x86_64/
                    └── libfoo.so

Else, as Richard Heap mentioned, you can copy .so from assets to application storage as file with following code snippet.

But I think it's less of a headache to let Gradle manage the libraries for you via jniLibs than it is to decide which .so files to load for each ABI yourself.

Future<File> getFileFromAssets(String assetPath, String filename) async {
  final Directory docDir = await getApplicationDocumentsDirectory();
  final String localPath = docDir.path;
  final String targetFilePath = '$localPath/$filename';
  File targetFile = File(targetFilePath);
  final asset = await rootBundle.load(assetPath);
  final buffer = asset.buffer;
  return targetFile.writeAsBytes(
      buffer.asUint8List(asset.offsetInBytes, asset.lengthInBytes));
}