nix-language-atlas emscripten

18 mar 2018


in the nix-language-atlas series on i want to discuss how well programming languages, for which i’m familiar with, integrate with nix. today, let’s revisit emscripten, as there also had been improvements since i wrote about it last time.

projects we have done:

what’s new

  • emscripten toolchain:
    • refactored to force a common revision (for example 1.37.16) on emscripten, emscripten-fastcomp and emscripten-fastcomp-clang called emscriptenVersion
    • added a unit test in emscripten to verify a small part of the toolchain
    • initial emscripten documentation in nixpkgs (not on yet)
  • added 2 more unit tests & repaired all builds

for details see the PR

want to give this a shot from nixos?

git clone
cd nixpkgs
git checkout f41a3e7d7d327ea66459d17bfbe4a751b2496cb1
nix-env -f default.nix -I nixpkgs=. -iA emscriptenPackages
installing ‘emscripten-json-c-0.13’
installing ‘emscripten-libxml2-2.9.7’
installing ‘emscripten-xmlmirror’
installing ‘emscripten-zlib-1.2.11’

dynamic/static libraries

in the above toolchain we are using libraries in .so format, not the .a format and in the end we link them together using emcc. this has some advantages:

  • building .so files is best practice on linux
  • thus easy to do
  • license wise smart, some packages can’t be statically linked legally IIRC


nix emscripten toolchain is now supported from:

especially the microsoft WSL release with the creators update is a very interesting audience as it makes it so easy to use nix on windows. no mingw, no cygwin! took me 10 minutes to install nix on windows!

for testing i’ve been compiling this very toolchain on my windows 10 computer. IO is slow but it works and it is easy to deploy.


i’ve sumbled on this questions in

How do I change the currently active SDK version?

How do I build multiple projects with different SDK versions in parallel?

How do I use Emscripten SDK with a custom version of python, java, node.js or some other tool?

this are very interesting questions and all get easy once one uses this nixpkgs based toolchain as pointed out. i’ve been using the emsdk in the past but now that we have the ‘bits’ automated in nixpkgs i’m happy to not have to work statefully anymore!


during my last two days of work on the toolchain update, i had these ideas and motivations for the future:

  • more documentation:

    • nix-shell -A usage
    • cross platform nix usage
    • section how to (fast) write good and lasting unit tests
    • how to use older nixpkgs vs. newer ones (slightly modify a very old project for instance which has a bug)
  • packaging some common targts, what would these be?

  • package the caching of emscripten in ~/.emscripten properly, so that build artefacts can be reused over builds (time save) and remove the HOME=$TMPDIR requirement (ugly)

      DEBUG:root:adding object /home/joachim/.emscripten_cache/asmjs/dlmalloc.bc to link
      DEBUG:root:adding object /home/joachim/.emscripten_cache/asmjs/libc.bc to link
  • update nix-instantiate, which we use in the ‘tour of nix’, to a more recent version and also package it into the toolchain as an example

  • hydra builds

  • be more transparent on the license, ideally we could generate a list of licenses in the final folder

@nixos community: i’d love to get some feedback on this, so send me an email to if you have some interesting input.


thanks to kripken (emscripten author) for his help! i’d love to put more effort into this, i think it’s really worth it so if you have any funding for general development or want to have something special realized, let me know!

another interesting project i’ve learned about lately would be from

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