Emscripten: An LLVM-to-JavaScript Compiler

This section of the manual covers how to use emscripten in nixpkgs.

Minimal requirements:

  • nix
  • nixpkgs

Modes of use of emscripten:

  • Imperative usage (on the command line):

    If you want to work with emcc, emconfigure and emmake as you are used to from Ubuntu and similar distributions you can use these commands:

    • nix-env -f "<nixpkgs>" -iA emscripten
    • nix-shell -p emscripten
  • Declarative usage:

    This mode is far more power full since this makes use of nix for dependency management of emscripten libraries and targets by using the mkDerivation which is implemented by pkgs.emscriptenStdenv and pkgs.buildEmscriptenPackage. The source for the packages is in pkgs/top-level/emscripten-packages.nix and the abstraction behind it in pkgs/development/em-modules/generic/default.nix. From the root of the nixpkgs repository:

    • build and install all packages:

      • nix-env -iA emscriptenPackages
    • dev-shell for zlib implementation hacking:

      • nix-shell -A emscriptenPackages.zlib

Imperative usage

A few things to note:

  • export EMCC_DEBUG=2 is nice for debugging
  • ~/.emscripten, the build artifact cache sometimes creates issues and needs to be removed from time to time

Declarative usage

Let's see two different examples from pkgs/top-level/emscripten-packages.nix:

  • pkgs.zlib.override
  • pkgs.buildEmscriptenPackage

Both are interesting concepts.

A special requirement of the pkgs.buildEmscriptenPackage is the doCheck = true is a default meaning that each emscriptenPackage requires a checkPhase implemented.

  • Use export EMCC_DEBUG=2 from within a emscriptenPackage's phase to get more detailed debug output what is going wrong.
  • ~/.emscripten cache is requiring us to set HOME=$TMPDIR in individual phases. This makes compilation slower but also makes it more deterministic.

Usage 1: pkgs.zlib.override

This example uses zlib from nixpkgs but instead of compiling C to ELF it compiles C to JS since we were using pkgs.zlib.override and changed stdenv to pkgs.emscriptenStdenv. A few adaptions and hacks were set in place to make it working. One advantage is that when pkgs.zlib is updated, it will automatically update this package as well. However, this can also be the downside...

See the zlib example:

zlib = (pkgs.zlib.override {
  stdenv = pkgs.emscriptenStdenv;
(old: rec {
  buildInputs = old.buildInputs ++ [ pkg-config ];
  # we need to reset this setting!
  configurePhase = ''
    # FIXME: Some tests require writing at $HOME
    runHook preConfigure

    #export EMCC_DEBUG=2
    emconfigure ./configure --prefix=$out --shared

    runHook postConfigure
  dontStrip = true;
  outputs = [ "out" ];
  buildPhase = ''
    emmake make
  installPhase = ''
    emmake make install
  checkPhase = ''
    echo "================= testing zlib using node ================="

    echo "Compiling a custom test"
    set -x
    emcc -O2 -s EMULATE_FUNCTION_POINTER_CASTS=1 test/example.c -DZ_SOLO \
    libz.so.${old.version} -I . -o example.js

    echo "Using node to execute the test"
    ${pkgs.nodejs}/bin/node ./example.js

    set +x
    if [ $? -ne 0 ]; then
      echo "test failed for some reason"
      exit 1;
      echo "it seems to work! very good."
    echo "================= /testing zlib using node ================="

  postPatch = pkgs.lib.optionalString pkgs.stdenv.isDarwin ''
    substituteInPlace configure \
      --replace '/usr/bin/libtool' 'ar' \
      --replace 'AR="libtool"' 'AR="ar"' \
      --replace 'ARFLAGS="-o"' 'ARFLAGS="-r"'

Usage 2: pkgs.buildEmscriptenPackage

This xmlmirror example features a emscriptenPackage which is defined completely from this context and no pkgs.zlib.override is used.

xmlmirror = pkgs.buildEmscriptenPackage rec {
  name = "xmlmirror";

  buildInputs = [ pkg-config autoconf automake libtool gnumake libxml2 nodejs openjdk json_c ];
  nativeBuildInputs = [ pkg-config zlib ];

  src = pkgs.fetchgit {
    url = "https://gitlab.com/odfplugfest/xmlmirror.git";
    rev = "4fd7e86f7c9526b8f4c1733e5c8b45175860a8fd";
    sha256 = "1jasdqnbdnb83wbcnyrp32f36w3xwhwp0wq8lwwmhqagxrij1r4b";

  configurePhase = ''
    rm -f fastXmlLint.js*
    # a fix for ERROR:root:For asm.js, TOTAL_MEMORY must be a multiple of 16MB, was 234217728
    # https://gitlab.com/odfplugfest/xmlmirror/issues/8
    sed -e "s/TOTAL_MEMORY=234217728/TOTAL_MEMORY=268435456/g" -i Makefile.emEnv
    # https://github.com/kripken/emscripten/issues/6344
    # https://gitlab.com/odfplugfest/xmlmirror/issues/9
    # https://gitlab.com/odfplugfest/xmlmirror/issues/11
    sed -e "s/-o fastXmlLint.js/-s EXTRA_EXPORTED_RUNTIME_METHODS='[\"ccall\", \"cwrap\"]' -o fastXmlLint.js/g" -i Makefile.emEnv

  buildPhase = ''
    make -f Makefile.emEnv

  outputs = [ "out" "doc" ];

  installPhase = ''
    mkdir -p $out/share
    mkdir -p $doc/share/${name}

    cp Demo* $out/share
    cp -R codemirror-5.12 $out/share
    cp fastXmlLint.js* $out/share
    cp *.xsd $out/share
    cp *.js $out/share
    cp *.xhtml $out/share
    cp *.html $out/share
    cp *.json $out/share
    cp *.rng $out/share
    cp README.md $doc/share/${name}
  checkPhase = ''


Declarative debugging

Use nix-shell -I nixpkgs=/some/dir/nixpkgs -A emscriptenPackages.libz and from there you can go trough the individual steps. This makes it easy to build a good unit test or list the files of the project.

  1. nix-shell -I nixpkgs=/some/dir/nixpkgs -A emscriptenPackages.libz
  2. cd /tmp/
  3. unpackPhase
  4. cd libz-1.2.3
  5. configurePhase
  6. buildPhase
  7. ... happy hacking...


Using this toolchain makes it easy to leverage nix from NixOS, MacOSX or even Windows (WSL+ubuntu+nix). This toolchain is reproducible, behaves like the rest of the packages from nixpkgs and contains a set of well working examples to learn and adapt from.

If in trouble, ask the maintainers.