WebAssembly C and C++ API

Work in progress! No docs yet.

Design Goals

• Provide a "black box" API for embedding a Wasm engine in other C/C++ applications.

  • Be completely agnostic to VM specifics.

  • Non-goal: "white box" interoperability with embedder (such as combined GC instead of mere finalisation) -- much more difficult to achieve.

• Allow creation of bindings for other languages through typical C foreign function interfaces.

  • Support a plain C API.

  • Stick to mostly manual memory management of interface objects.

• Avoid language features that raise barrier to use.

  • E.g., no exceptions or post-C++11 features in C++ API.

  • E.g., no passing of structs by-value or post-C99 features in C API.

• Achieve link-time compatibility between different implementations.

  • All implementation-dependent API classes are abstract and can be instantiated through factory methods only.

Interfaces

• C++ API:

  • See include/wasm.hh for interface.

  • See example/*.cc for example usages.

• C API:

  • See include/wasm.h for interface.

  • See example/*.c for example usages.

Some random explanations:

• The VM must be initialised by creating an instance of an engine (wasm::Engine/wasm_engine_t) and is shut down by deleting it. Such an instance may only be created once per process.

• All runtime objects are tied to a specific store (wasm::Store/wasm_store_t). Multiple stores can be created, but their objects cannot interact. Every store and its objects must only be accessed in a single thread.

• To exchange module objects between threads, create a shared module (wasm::Shared<Module>/wasm_shared_module_t). Other objects cannot be shared in current Wasm.

• Vector structures (wasm::vec<X>/wasm_x_vec_t) are lightweight abstractions of a pair of a plain array and its length. The C++ API does not use std::vector because that does not support adopting pre-existing arrays.

• References point to runtime objects, but may involve internal indirections, which may or may not be cached. Thus, pointer equality on Ref* or subclasses cannot be used to compare identity of the underlying objects (Ref::eq may be added later). However, nullptr/NULL uniquely represents null references.

• The API already encompasses current proposals like multiple return values and reference types, but not yet threads.

Prototype Implementation

• This repo contains a prototype implementation based on V8 is in src.

  • Note that this requires adding a module to V8, so it patches V8's build file.

• The C API is implemented on top of the C++ API.

• See Makefile for build recipe. Canonical steps to run examples:

  1. make v8-checkout
  2. make v8
  3. make all

Limitations

V8 implementation:

• Currently requires patching V8 by adding a module.

• Host functions (Func::make) create a JavaScript function internally, since V8 cannot handle raw C imports yet.

• As a consequence, does not support multiple results in external calls or host functions.

• Host functions and host globals are created through auxiliary modules constructed on the fly, to work around limitations in JS API.

• Shared<Module> is currently implemented via serialisation, since V8 does not currently have direct support for cross-isolate sharing.

Other Implementations

Currently, known implementations of this API are included in

• V8 natively (both C and C++)
• Wabt (only C?)
• Wasmtime (only C?)
• Wasmer (only C, C++ coming soon)

TODO

Possible API tweaks:

• Add Ref::eq (or better, a subclass EqRef::eq) for reference equality?

• Add a way to return error messages from Module::make and Module::validate.

• Use restrict in C API?

• Find a way to perform C callbacks through C++ without extra wrapper?

• Add iterators to vec class?