# Build Instructions (experimental GN build)

Follow the guidelines below for building Electron with the experimental GN
build.

> **NOTE**: The GN build system is in _experimental_ status.

## Platform prerequisites

Check the build prerequisites for your platform before proceeding

  * [macOS](build-instructions-osx.md#prerequisites)
  * [Linux](build-instructions-linux.md#prerequisites)
  * [Windows](build-instructions-windows.md#prerequisites)

## GN prerequisites

You'll need to install [`depot_tools`][depot-tools], the toolset
used for fetching Chromium and its dependencies.

Also, on Windows, you'll need to set the environment variable
`DEPOT_TOOLS_WIN_TOOLCHAIN=0`. To do so, open `Control Panel` → `System and
Security` → `System` → `Advanced system settings` and add a system variable
`DEPOT_TOOLS_WIN_TOOLCHAIN` with value `0`.  This tells `depot_tools` to use
your locally installed version of Visual Studio (by default, `depot_tools` will
try to download a Google-internal version that only Googlers have access to).

[depot-tools]: http://commondatastorage.googleapis.com/chrome-infra-docs/flat/depot_tools/docs/html/depot_tools_tutorial.html#_setting_up

## Cached builds (optional step)

### GIT_CACHE_PATH

If you plan on building Electron more than once, adding a git cache will
speed up subsequent calls to `gclient`. To do this, set a `GIT_CACHE_PATH`
environment variable:

```sh
$ export GIT_CACHE_PATH="${HOME}/.git_cache"
$ mkdir -p "${GIT_CACHE_PATH}"
# This will use about 16G.
```

### sccache

Thousands of files must be compiled to build Chromium and Electron.
You can avoid much of the wait by reusing Electron CI's build output via
[sccache](https://github.com/mozilla/sccache). This requires some
optional steps (listed below) and these two environment variables:

```sh
export SCCACHE_BUCKET="electronjs-sccache"
export SCCACHE_TWO_TIER=true
```

## Getting the code

```sh
$ mkdir electron-gn && cd electron-gn
$ gclient config \
    --name "src/electron" \
    --unmanaged \
    https://github.com/electron/electron
$ gclient sync --with_branch_heads --with_tags
# This will take a while, go get a coffee.
```

## Building

```sh
$ cd src
$ export CHROMIUM_BUILDTOOLS_PATH=`pwd`/buildtools
# this next line is needed only if building with sccache
$ export GN_EXTRA_ARGS="${GN_EXTRA_ARGS} cc_wrapper=\"${PWD}/electron/external_binaries/sccache\""
$ gn gen out/Default --args="import(\"//electron/build/args/debug.gn\") $GN_EXTRA_ARGS"
```

This will generate a build directory `out/Default` under `src/` with
debug build configuration. You can replace `Default` with another name,
but it should be a subdirectory of `out`.
Also you shouldn't have to run `gn gen` again—if you want to change the
build arguments, you can run `gn args out/Default` to bring up an editor.

To see the list of available build configuration options, run `gn args
out/Default --list`.

**For generating Debug (aka "component" or "shared") build config of
Electron:**

```sh
$ gn gen out/Default --args="import(\"//electron/build/args/debug.gn\") $GN_EXTRA_ARGS"
```

**For generating Release (aka "non-component" or "static") build config of
Electron:**

```sh
$ gn gen out/Default --args="import(\"//electron/build/args/release.gn\") $GN_EXTRA_ARGS"
```

**To build, run `ninja` with the `electron:electron_app` target:**

```sh
$ ninja -C out/Default electron:electron_app
# This will also take a while and probably heat up your lap.
```

This will build all of what was previously 'libchromiumcontent' (i.e. the
`content/` directory of `chromium` and its dependencies, incl. WebKit and V8),
so it will take a while.

To speed up subsequent builds, you can use [sccache][sccache]. Add the GN arg
`cc_wrapper = "sccache"` by running `gn args out/Default` to bring up an
editor and adding a line to the end of the file.

[sccache]: https://github.com/mozilla/sccache

The built executable will be under `./out/Default`:

```sh
$ ./out/Default/Electron.app/Contents/MacOS/Electron
# or, on Windows
$ ./out/Default/electron.exe
# or, on Linux
$ ./out/Default/electron
```

### Cross-compiling

To compile for a platform that isn't the same as the one you're building on,
set the `target_cpu` and `target_os` GN arguments. For example, to compile an
x86 target from an x64 host, specify `target_cpu = "x86"` in `gn args`.

```sh
$ gn gen out/Default-x86 --args='... target_cpu = "x86"'
```

Not all combinations of source and target CPU/OS are supported by Chromium.
Only cross-compiling Windows 32-bit from Windows 64-bit and Linux 32-bit from
Linux 64-bit have been tested in Electron. If you test other combinations and
find them to work, please update this document :)

See the GN reference for allowable values of [`target_os`][target_os values]
and [`target_cpu`][target_cpu values]

[target_os values]: https://gn.googlesource.com/gn/+/master/docs/reference.md#built_in-predefined-variables-target_os_the-desired-operating-system-for-the-build-possible-values
[target_cpu values]: https://gn.googlesource.com/gn/+/master/docs/reference.md#built_in-predefined-variables-target_cpu_the-desired-cpu-architecture-for-the-build-possible-values

## Tests

To run the tests, you'll first need to build the test modules against the
same version of Node.js that was built as part of the build process. To
generate build headers for the modules to compile against, run the following
under `src/` directory.

```sh
$ ninja -C out/Default third_party/electron_node:headers
# Install the test modules with the generated headers
$ (cd electron/spec && npm i --nodedir=../../out/Default/gen/node_headers)
```

Then, run Electron with `electron/spec` as the argument:

```sh
# on Mac:
$ ./out/Default/Electron.app/Contents/MacOS/Electron electron/spec
# on Windows:
$ ./out/Default/electron.exe electron/spec
# on Linux:
$ ./out/Default/electron electron/spec
```

If you're debugging something, it can be helpful to pass some extra flags to
the Electron binary:

```sh
$ ./out/Default/Electron.app/Contents/MacOS/Electron electron/spec \
  --ci --enable-logging -g 'BrowserWindow module'
```