What is Wasm/k?

Wasm/k (WebAssembly continuations) is an extension of WebAssembly with additional primitive instructions that offer support for full first-class continuations, very similar to call/cc in Scheme / Racket. These instructions allow for significantly more efficient implementations of high-level language features such as green threads. Wasm/k consists of three parts:

  1. A fork of Wasmtime, a JIT implementation of WebAssembly, which implements the additional instructions of Wasm/k.
  2. C/k, a wrapper around Emscripten which exposes first-class continuations to C, and compiles them to Wasm/k.
  3. Our paper, which includes the full formal definition of Wasm/k and proofs of safety.


Wasmtime is a JIT for WebAssembly implemented in Rust. We extended Wasmtime in a fork to support Wasm/k. Building our fork of Wasmtime also depends on our forks of a few other Rust WebAssembly toolchain libraries. Please see the instructions for building our fork of Wasmtime.


C/k can be used to compile C/C++ which uses first class continuations to Wasm/k code, which can then be run in the modified Wasmtime JIT. C/k is implemented as a set of wrapper scripts around Emscripten / WASI-SDK. Please see the instructions for installing C/k.