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# Emscripten {#emscripten}
[Emscripten](https://github.com/kripken/emscripten): An LLVM-to-JavaScript Compiler
If you want to work with `emcc`, `emconfigure` and `emmake` as you are used to from Ubuntu and similar distributions,
```console
nix-shell -p emscripten
```
A few things to note:
* `export EMCC_DEBUG=2` is nice for debugging
* The build artifact cache in `~/.emscripten` sometimes creates issues and needs to be removed from time to time
## Examples {#declarative-usage}
Let's see two different examples from `pkgs/top-level/emscripten-packages.nix`:
* `pkgs.zlib.override`
* `pkgs.buildEmscriptenPackage`
A special requirement of the `pkgs.buildEmscriptenPackage` is the `doCheck = true`.
This means each Emscripten package requires that a [`checkPhase`](#ssec-check-phase) is implemented.
* Use `export EMCC_DEBUG=2` from within a phase to get more detailed debug output what is going wrong.
* The cache at `~/.emscripten` requires to set `HOME=$TMPDIR` in individual phases.
This makes compilation slower but also more deterministic.
::: {.example #usage-1-pkgs.zlib.override}
# Using `pkgs.zlib.override {}`
This example uses `zlib` from Nixpkgs, but instead of compiling **C** to **ELF** it compiles **C** to **JavaScript** since we were using `pkgs.zlib.override` and changed `stdenv` to `pkgs.emscriptenStdenv`.
A few adaptions and hacks were put in place to make it work.
One advantage is that when `pkgs.zlib` is updated, it will automatically update this package as well.
```nix
(pkgs.zlib.override {
stdenv = pkgs.emscriptenStdenv;
}).overrideAttrs
(old: rec {
buildInputs = old.buildInputs ++ [ pkg-config ];
# we need to reset this setting!
env = (old.env or { }) // { 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 [ $? -ne 0 ]; then
echo "test failed for some reason"
exit 1;
else
echo "it seems to work! very good."
fi
echo "================= /testing zlib using node ================="
'';
postPatch = pkgs.lib.optionalString pkgs.stdenv.isDarwin ''
substituteInPlace configure \
--replace-fail '/usr/bin/libtool' 'ar' \
--replace-fail 'AR="libtool"' 'AR="ar"' \
--replace-fail 'ARFLAGS="-o"' 'ARFLAGS="-r"'
'';
})
```
:::{.example #usage-2-pkgs.buildemscriptenpackage}
# Using `pkgs.buildEmscriptenPackage {}`
This `xmlmirror` example features an Emscripten package that is defined completely from this context and no `pkgs.zlib.override` is used.
```nix
pkgs.buildEmscriptenPackage rec {
name = "xmlmirror";
buildInputs = [ pkg-config autoconf automake libtool gnumake libxml2 nodejs openjdk json_c ];
nativeBuildInputs = [ pkg-config zlib ];
src = pkgs.fetchgit {
url = "https://gitlab.com/odfplugfest/xmlmirror.git";
rev = "4fd7e86f7c9526b8f4c1733e5c8b45175860a8fd";
hash = "sha256-i+QgY+5PYVg5pwhzcDnkfXAznBg3e8sWH2jZtixuWsk=";
};
configurePhase = ''
rm -f fastXmlLint.js*
# a fix for ERROR:root:For asm.js, TOTAL_MEMORY must be a multiple of 16MB, was 234217728
# https://gitlab.com/odfplugfest/xmlmirror/issues/8
sed -e "s/TOTAL_MEMORY=234217728/TOTAL_MEMORY=268435456/g" -i Makefile.emEnv
# https://github.com/kripken/emscripten/issues/6344
# https://gitlab.com/odfplugfest/xmlmirror/issues/9
sed -e "s/\$(JSONC_LDFLAGS) \$(ZLIB_LDFLAGS) \$(LIBXML20_LDFLAGS)/\$(JSONC_LDFLAGS) \$(LIBXML20_LDFLAGS) \$(ZLIB_LDFLAGS) /g" -i Makefile.emEnv
# https://gitlab.com/odfplugfest/xmlmirror/issues/11
sed -e "s/-o fastXmlLint.js/-s EXTRA_EXPORTED_RUNTIME_METHODS='[\"ccall\", \"cwrap\"]' -o fastXmlLint.js/g" -i Makefile.emEnv
'';
buildPhase = ''
HOME=$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 = ''
'';
}
```
:::
## Debugging {#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...
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