Skip to content


Switch branches/tags

Latest commit

Since #144, we've specified that `memory.atomic.notify` is allowed to be executed on unshared memories, but always returns 0 since it is impossible for there to be anything waiting on an unshared memory since all variations of the wait instructions in Wasm and JS trap or throw exceptions on unshared memories. However, I realized recently that there is no technical reason why [`Atomics.waitAsync`]( shouldn't be able to wait on an unshared memory. Because `Atomics.waitAsync` returns a promise rather than blocking, it can in principle be used to synchronize between multiple asynchronous tasks in flight concurrently in the same JS context, and now that [JSPI]( is becoming more real, this actually might be a useful thing to do. In particular `Atomics.waitAsync` could be used to build suspending versions of mutexes and condition variables to synchronize between threads running as different asynchronous tasks within a single JS context.

Actually realizing this vision of asynchronous threading with JSPI on top of `Atomics.waitAsync` requires changes on the JS side to allow `Atomics.waitAsync` and `Atomics.notify` to be used on unshared ArrayBuffers, but in the meantime we can prepare for that change on the Wasm side by softening the language stating that `memory.atomic.notify` unconditionally returns 0 for unshared memories.

Git stats


Failed to load latest commit information.

Build Status

Threads Proposal for WebAssembly

This repository is a clone of It is meant for discussion, prototype specification and implementation of a proposal to threads support to WebAssembly.

See the overview for a summary of the proposal.

A formatted version of the spec with proposed changes is available here:

Original README from upstream repository follows...


This repository holds the sources for the WebAssembly draft specification (to seed a future WebAssembly Working Group), a reference implementation, and the official testsuite.

A formatted version of the spec is available here:,

Participation is welcome. Discussions about new features, significant semantic changes, or any specification change likely to generate substantial discussion should take place in the WebAssembly design repository first, so that this spec repository can remain focused. And please follow the guidelines for contributing.


For citing WebAssembly in LaTeX, use this bibtex file.