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wasm_thread

License Cargo Documentation

An std::thread replacement for wasm32 target.

This crate tries to closely replicate std::thread API. Namely, it doesn't require you to bundle worker scripts and resolves wasm-bindgen shim URL automatically.

Note that some API is still missing and may be even impossible to implement given wasm limitations.

Using as a library

  • Add wasm_thread to your Cargo.toml.
  • This project supports wasm-pack targets web and no-modules. es_modules feature is enabled by default, if building for no-modules, use default-features = false when specifying dependency.
  • Replace use std::thread with use wasm_thread as thread. Note that some API might be missing.
  • Build normally using wasm-pack or adapt build_wasm.sh to your project.

Notes on wasm limitations

  • In order for multiple wasm instances to share the same memory, SharedArrayBuffer is required. This means that the COOP and COEP security headers for the webpage will need to be set (see Mozilla's documentation). These may be enabled by adjusting webserver settings or using a service worker.
  • Any blocking API (thread.join(), futures::block_on(), etc) on the main thread will freeze the browser for as long as lock is maintained. This also freezes any proxied functions, which means that worker spawning, network fetches and other similar asynchronous APIs will block also and can cause a deadlock. To avoid this, either run your main() in a worker thread or use async futures.
  • Atomic locks (i32.atomic.wait to be specific) will panic on the main thread. This means that mutex.lock() will likely crash. Solution is the same as above.
  • Web workers are normally spawned by providing a script URL, however, to avoid bundling scripts this library uses URL encoded blob web_worker.js to avoid HTTP fetch. wasm_bindgen generated .js shim script is still needed and a hack is used to obtain its URL. If this for some reason does not work in your setup, please report an issue or use Builder::wasm_bindgen_shim_url() to specify explicit URL.
  • For additional information on wasm threading look at this blogpost or raytrace-parallel example.

Alternatives

For a higher-level threading solution, see wasm-bindgen-rayon, which allows one to utilize a fixed-size threadpool in web browsers.

Running examples

Simple

Native

  • Just cargo run --example simple

wasm-bindgen

  • Install nightly toolchain and dependencies:
rustup toolchain install nightly
rustup component add rust-src --toolchain nightly
cargo install wasm-bindgen-cli
  • Build with ./build_wasm.sh (bash) or ./build_wasm.ps1 (PowerShell). This custom build step is required because prebuilt standard library does not have support for atomics yet. Read more about this here.
  • Serve examples directory over HTTP with cross-origin isolation enabled and open simple.html in the browser. Inspect console output. You can use cargo install sfz as a basic HTTP server and serve with sfz examples --coi.

wasm-pack

  • Install wasm-pack:
cargo install wasm-pack
  • Build with ./examples-wasm-pack/web-build.sh for an example targeting web, and ./examples-wasm-pack/web-build-no-module.sh for an example targeting no-modules.
  • Serve ./examples-wasm-pack/module or ./examples-wasm-pack/no-module, respectively, over HTTP and open simple.html in browser. Inspect console output.

Example output

Native:

hi number 1 from the spawned thread ThreadId(2)!
hi number 1 from the main thread ThreadId(1)!
hi number 1 from the spawned thread ThreadId(3)!
hi number 2 from the main thread ThreadId(1)!
hi number 2 from the spawned thread ThreadId(2)!
hi number 2 from the spawned thread ThreadId(3)!

Wasm:

hi number 1 from the main thread ThreadId(1)!
hi number 2 from the main thread ThreadId(1)!
hi number 1 from the spawned thread ThreadId(2)!
hi number 1 from the spawned thread ThreadId(3)!
hi number 2 from the spawned thread ThreadId(2)!
hi number 2 from the spawned thread ThreadId(3)!

As you can see wasm threads are only spawned after main() returns, because browser event loop cannot continue while main thread is blocked.

License

Licensed under either of

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.