wasm_ffi

wasm_ffi intends to be a drop-in replacement for dart:ffi on the web platform using wasm. wasm_ffi is built on top of web_ffi. The general idea is to expose an API that is compatible with dart:ffi but translates all calls through dart:js to a browser running WebAssembly. Wasm with js helper as well as standalone wasm is supported. For testing emcc is used.

To simplify the usage, universal_ffi is provided, which uses wasm_ffi on web and dart:ffi on other platforms.

Differences to dart:ffi

While wasm_ffi tries to mimic the dart:ffi API as close as possible, there are some differences. The list below documents the most importent ones, make sure to read it. For more insight, take a look at the API documentation.

• The DynamicLibrary open method is asynchronous. It also accepts some additional optional parameters.
• If more than one library is loaded, the memory will continue to refer to the first library. This breaks calls to later loaded libraries! One workaround is to specify the correct library.allocator for each usage of using.
• Each library has its own memory, so objects cannot be shared between libraries.
• Some advanced types are still unsupported.
• There are some classes and functions that are present in wasm_ffi but not in dart:ffi; such things are annotated with @extra.
• There is a new class Memory which is IMPORTANT and explained in deepth below.
• If you extend the Opaque class, you must register the extended class using @extra registerOpaqueType<T>() before using it! Also, your class MUST NOT have type arguments (what should not be a problem).
• There are some rules concerning interacting with native functions, as listed below.

Rules for functions

There are some rules and things to notice when working with functions:

• When looking up a function using DynamicLibrary.lookup<NativeFunction<NF>>() (or DynamicLibraryExtension.lookupFunction<T extends Function, F extends Function>()) the actuall type argument NF (or T respectively) of is not used: There is no type checking, if the function exported from WebAssembly has the same signature or amount of parameters, only the name is looked up.
• There are special constraints on the return type (not on parameter types) of functions DF (or F ) if you call NativeFunctionPointer.asFunction<DF>() (or DynamicLibraryExtension.lookupFunction<T extends Function, F extends Function>() what uses the former internally):
  • You may nest the pointer type up to two times but not more:
    • e.g. Pointer<Int32> and Pointer<Pointer<Int32>> are allowed but Pointer<Pointer<Pointer<Int32>>> is not.
  • If the return type is Pointer<NativeFunction> you MUST use Pointer<NativeFunction<dynamic>>, everything else will fail. You can restore the type arguments afterwards yourself using casting. On the other hand, as stated above, type arguments for NativeFunctions are just ignored anyway.
  • To concretize the things above, return_types.md lists what may be used as return type, everyhing else will cause a runtime error.
  • WORKAROUND: If you need something else (e.g. Pointer<Pointer<Pointer<Double>>>), use Pointer<IntPtr> and cast it yourselfe afterwards using Pointer.cast().

Memory

NOTE: While most of this section is still correct, some of it is now automated. The first call you sould do when you want to use wasm_ffi is Memory.init(). It has an optional parameter where you can adjust your pointer size. The argument defaults to 4 to represent 32bit pointers, if you use wasm64, call Memory.init(8). Contraty to dart:ffi where the dart process shares all the memory, on WebAssembly, each instance is bound to a WebAssembly.Memory object. For now, we assume that every WebAssembly module you use has it's own memory. If you think we should change that, open a issue on GitHub and report your usecase. Every pointer you use is bound to a memory object. This memory object is accessible using the @extra Pointer.boundMemory field. If you want to create a Pointer using the Pointer.fromAddress() constructor, you may notice the optional bindTo parameter. Since each pointer must be bound to a memory object, you can explicitly speficy a memory object here. To match the dart:ffi API, the bindTo parameter is optional. Because it is optional, there has to be a fallback mechanism if no bindTo is specified: The static Memory.global field. If that field is also not set, an exception is thrown when invoking the Pointer.fromAddress() constructor. Also, each DynamicLibrary is bound to a memory object, which is again accessible with @extra DynamicLibrary.boundMemory. This might come in handy, since Memory implements the Allocator class.

Usage

Install

dart pub add wasm_ffi

or

flutter pub add wasm_ffi

Usage without ffi bindings

import 'package:wasm_ffi/ffi.dart' as ffi;

Future<void> main() async {
    final library = await DynamicLibrary.open('path to wasm or js'); // NOTE: It is async
    final func = library.lookupFunction<int Function(), int Function()>('functionName');
    print(func());
}

Usage with ffi bindings

Generates ffi bindings using package:ffigen on the header file. In the generated bindings file, replace import 'dart:ffi' as ffi; with import 'package:wasm_ffi/ffi.dart' as ffi;

import 'package:wasm_ffi/ffi.dart';
import 'package:wasm_ffi/ffi_utils.dart';
import 'native_example_bindings.dart';

...
  final library = await DynamicLibrary.open(libName);
  final bindings = NativeExampleBindings(library);

  // assuming that native library is has a function `hello` which takes a name and returns a string `Hello name!`
  using((Arena arena) {
    final cString = name.toNativeUtf8(allocator: arena).cast<Char>();
    return bindings.hello(cString).cast<Utf8>().toDartString();
  }, library.allocator); // library.allocator is optional if only one module is loaded
...

build wasm

The generated wasm file needs all exported function. To ensure that, one of the two can be done:

• Use EMSCRIPTEN_KEEPALIVE annotation on all exported functions
• Define EXPORTED_FUNCTIONS when compiling the wasm

---

Contributions are welcome! 🚀