Pthreads with WebAssembly

Emscripten has support for compiling with pthreads, using asm.js, Web Workers, and SharedArrayBuffer. That support is documented here. WebAssembly has a proposed spec for threads (or more properly, for a shared linear memory and atomic memory instructions), with an overview here and spec here. This spec has experimental implementations in Firefox Nightly and Chrome Canary.

Emscripten now also has experimental support for the wasm threading spec. Currently you need to use the head of the incoming branch of emscripten. To build a simple threaded program: emcc program.c -s USE_PTHREADS=1 -s WASM=1 -o program.html (it may also be desirable to use -s PTHREAD_POOL_SIZE as documented in the emscripten threading doc). To test with Chrome Canary, use the following flags: --js-flags=--experimental-wasm-threads --enable-features=WebAssembly,SharedArrayBuffer --disable-features=WebAssemblyTrapHandler. Firefox nightly has all experimental features turned on by default, except SharedArrayBuffer.

Using pthreads also requires use of a memory initializer file (e.g. as if you had specified --mem-init-file 1 instead of using the data section of the wasm file.