Master Dart FFI: Step‑by‑Step Guide to Call C/C++ from Flutter and Back

What is FFI

FFI (Foreign Function Interface) is a mechanism that lets code written in one programming language call functions or services written in another language, similar to Java JNI.

Create a sample project

Generate a Flutter plugin project for macOS using the plugin_ffi template and run it to produce a macOS executable.

flutter --version

flutter create --template=plugin_ffi --platforms=macos plugin_ffi_sample

cd plugin_ffi_sample/example
flutter run

Inspect the plugin project

The example directory demonstrates how to use the plugin. The pubspec.yaml declares a path dependency on the plugin itself.

dependencies:
  flutter:
    sdk: flutter
  plugin_ffi_sample:
    path: ../

Calling native functions from Dart

Import the generated bindings and call sum and sumAsync from Dart.

import 'package:plugin_ffi_sample/plugin_ffi_sample.dart' as plugin_ffi_sample;

class _MyAppState extends State<MyApp> {
  late int sumResult;
  late Future<int> sumAsyncResult;

  @override
  void initState() {
    super.initState();
    sumResult = plugin_ffi_sample.sum(1, 2);
    sumAsyncResult = plugin_ffi_sample.sumAsync(3, 4);
  }
}

Generated binding file

The file plugin_ffi_sample_bindings_generated.dart is auto‑generated by ffigen and maps Dart calls to the native symbols.

// AUTO GENERATED FILE, DO NOT EDIT.
import 'dart:ffi' as ffi;

class PluginFfiSampleBindings {
  final ffi.Pointer<ffi.NativeFunction<ffi.Int32 Function(ffi.Int32, ffi.Int32)>> _sum;
  PluginFfiSampleBindings(ffi.DynamicLibrary dylib)
      : _sum = dylib.lookup<ffi.NativeFunction<ffi.Int32 Function(ffi.Int32, ffi.Int32)>>('sum');

  int sum(int a, int b) => _sum.asFunction<int Function(int, int)>()(a, b);
}

Adding a C/C++ callback

Define a function pointer type pong and a ping function that invokes the callback.

typedef void (*pong)(void);
FFI_PLUGIN_EXPORT void ping(pong callback) {
  printf("ping
");
  callback();
}

Regenerate bindings after changing the header.

flutter pub run ffigen --config ffigen.yaml

Asynchronous thread callbacks

When using a C++ thread, wrap the callback with Dart_PostCObject_DL to send a message to a Dart SendPort instead of calling Dart code directly.

void entry_point(Dart_Port_DL port) {
  printf("entry_point
");
  Dart_CObject obj;
  obj.type = Dart_CObject_kString;
  obj.value.as_string = "pong";
  Dart_PostCObject_DL(port, &obj);
}

FFI_PLUGIN_EXPORT void ping(Dart_Port_DL port) {
  printf("ping
");
  std::thread* t = new std::thread(entry_point, port);
}

Integrating with Flutter

In Dart, create a ReceivePort to listen for messages from native code and pass its nativePort to ping. Also initialize the dynamically linked Dart API.

void initializeApiDL() => _bindings.ffi_Dart_InitializeApiDL(NativeApi.initializeApiDLData);

void ping() {
  final receivePort = ReceivePort()..listen((msg) => print('pong $msg'));
  _bindings.ping(receivePort.sendPort.nativePort);
}

@override
void initState() {
  super.initState();
  initializeApiDL();
  ping();
  sumResult = plugin_ffi_sample.sum(1, 2);
  sumAsyncResult = plugin_ffi_sample.sumAsync(3, 4);
}

Key take‑aways

Use toNativeUtf8 with an allocator and free the memory after the call. NativeFinalizer can bind Dart objects to C++ objects and run cleanup code when the Dart object is garbage‑collected.

By following these steps you can successfully call C/C++ code from Dart, handle synchronous and asynchronous callbacks, and communicate between native threads and Dart isolates.