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# Emscripten
[Emscripten](https://github.com/kripken/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 -i 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`.
* 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;
}).overrideDerivation
(old: rec {
buildInputs = old.buildInputs ++ [ pkgconfig ];
# we need to reset this setting!
NIX_CFLAGS_COMPILE="";
configurePhase = ''
# FIXME: Some tests require writing at $HOME
HOME=$TMPDIR
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 [ $? -ne 0 ]; then
echo "test failed for some reason"
exit 1;
else
echo "it seems to work! very good."
fi
echo "================= /testing zlib using node ================="
'';
postPatch = pkgs.stdenv.lib.optionalString pkgs.stdenv.isDarwin ''
substituteInPlace configure \
--replace '/usr/bin/libtool' 'ar' \
--replace 'AR="libtool"' 'AR="ar"' \
--replace 'ARFLAGS="-o"' 'ARFLAGS="-r"'
'';
});
### Usage 2: pkgs.buildEmscriptenPackage
This `xmlmirror` example features a emscriptenPackage which is defined completely from this context and no `pkgs.zlib.override` is used.
xmlmirror = pkgs.buildEmscriptenPackage rec {
name = "xmlmirror";
buildInputs = [ pkgconfig autoconf automake libtool gnumake libxml2 nodejs openjdk json_c ];
nativeBuildInputs = [ pkgconfig zlib ];
src = pkgs.fetchgit {
url = "https://gitlab.com/odfplugfest/xmlmirror.git";
rev = "4fd7e86f7c9526b8f4c1733e5c8b45175860a8fd";
sha256 = "1jasdqnbdnb83wbcnyrp32f36w3xwhwp0wq8lwwmhqagxrij1r4b";
};
configurePhase = ''
rm -f fastXmlLint.js*
# a fix for ERROR:root:For asm.js, TOTAL_MEMORY must be a multiple of 16MB, was 234217728
# https://gitlab.com/odfplugfest/xmlmirror/issues/8
sed -e "s/TOTAL_MEMORY=234217728/TOTAL_MEMORY=268435456/g" -i Makefile.emEnv
# https://github.com/kripken/emscripten/issues/6344
# https://gitlab.com/odfplugfest/xmlmirror/issues/9
sed -e "s/\$(JSONC_LDFLAGS) \$(ZLIB_LDFLAGS) \$(LIBXML20_LDFLAGS)/\$(JSONC_LDFLAGS) \$(LIBXML20_LDFLAGS) \$(ZLIB_LDFLAGS) /g" -i Makefile.emEnv
# https://gitlab.com/odfplugfest/xmlmirror/issues/11
sed -e "s/-o fastXmlLint.js/-s EXTRA_EXPORTED_RUNTIME_METHODS='[\"ccall\", \"cwrap\"]' -o fastXmlLint.js/g" -i Makefile.emEnv
'';
buildPhase = ''
HOME=$TMPDIR
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...
## Summary
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.
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