# How to build Skia

Make sure you have first followed the
[instructions to download Skia](./download).

Skia uses [GN](https://chromium.googlesource.com/chromium/src/tools/gn/) to
configure its builds.

-   [`is_official_build` and Third-party Dependencies](#third-party)
-   [Supported and Preferred Compilers](#compilers)
-   [Quickstart](#quick)
-   [Android](#android)
-   [ChromeOS](#cros)
-   [Mac](#macos)
-   [iOS](#ios)
-   [Windows](#windows)
-   [Windows ARM64](#win-arm64)
-   [CMake](#cmake)

## <span id="third-party">`is_official_build` and Third-party Dependencies</span>

Most users of Skia should set `is_official_build=true`, and most developers
should leave it to its `false` default.

This mode configures Skia in a way that's suitable to ship: an optimized build
with no debug symbols, dynamically linked against its third-party dependencies
using the ordinary library search path.

In contrast, the developer-oriented default is an unoptimized build with full
debug symbols and all third-party dependencies built from source and embedded
into libskia. This is how we do all our manual and automated testing.

Skia offers several features that make use of third-party libraries, like
libpng, libwebp, or libjpeg-turbo to decode images, or ICU and sftnly to subset
fonts. All these third-party dependencies are optional and can be controlled by
a GN argument that looks something like `skia_use_foo` for appropriate `foo`.

If `skia_use_foo` is enabled, enabling `skia_use_system_foo` will build and link
Skia against the headers and libaries found on the system paths.
`is_official_build=true` enables all `skia_use_system_foo` by default. You can
use `extra_cflags` and `extra_ldflags` to add include or library paths if
needed.

## <span id="compilers">Supported and Preferred Compilers</span>

While Skia should compile with GCC, MSVC, and other compilers, a number of
routines in Skia's software backend have been written to run fastest when
compiled with Clang. If you depend on software rasterization, image decoding, or
color space conversion and compile Skia with a compiler other than Clang, you
will see dramatically worse performance. This choice was only a matter of
prioritization; there is nothing fundamentally wrong with non-Clang compilers.
So if this is a serious issue for you, please let us know on the mailing list.

Skia makes use of C++17 language features (compiles with `-std=c++17` flag) and
thus requires a C++17 compatible compiler. Clang 5 and later implement all of
the features of the c++17 standard. Older compilers that lack C++17 support may
produce non-obvious compilation errors. You can configure your build to use
specific executables for `cc` and `cxx` invocations using e.g.
`--args='cc="clang-6.0" cxx="clang++6.0"'` GN build arguments, as illustrated in
[Quickstart](#quick). This can be useful for building Skia without needing to
modify your machine's default compiler toolchain.

## <span id="quick">Quickstart</span>

Run `gn gen` to generate your build files. As arguments to `gn gen`, pass a name
for your build directory, and optionally `--args=` to configure the build type.

To build Skia as a static library in a build directory named `out/Static`:

```
bin/gn gen out/Static --args='is_official_build=true'
```

To build Skia as a shared library (DLL) in a build directory named `out/Shared`:

```
bin/gn gen out/Shared --args='is_official_build=true is_component_build=true'
```

If you find that you don't have `bin/gn`, make sure you've run:

```
python2 tools/git-sync-deps
```

For a list of available build arguments, take a look at `gn/skia.gni`, or run:

```
bin/gn args out/Debug --list
```

GN allows multiple build folders to coexist; each build can be configured
separately as desired. For example:

```
bin/gn gen out/Debug
bin/gn gen out/Release  --args='is_debug=false'
bin/gn gen out/Clang    --args='cc="clang" cxx="clang++"'
bin/gn gen out/Cached   --args='cc_wrapper="ccache"'
bin/gn gen out/RTTI     --args='extra_cflags_cc=["-frtti"]'
```

Once you have generated your build files, run Ninja to compile and link Skia:

```
ninja -C out/Static
```

If some header files are missing, install the corresponding dependencies:

```
tools/install_dependencies.sh
```

To pull new changes and rebuild:

```
git pull
python tools/git-sync-deps
ninja -C out/Static
```

## <span id="android">Android</span>

To build Skia for Android you need an
[Android NDK](https://developer.android.com/ndk/index.html).

If you do not have an NDK and have access to CIPD, you can use one of these
commands to fetch the NDK our bots use:

```
python2 infra/bots/assets/android_ndk_linux/download.py -t /tmp/ndk
python2 infra/bots/assets/android_ndk_darwin/download.py -t /tmp/ndk
python2 infra/bots/assets/android_ndk_windows/download.py -t C:/ndk
```

When generating your GN build files, pass the path to your `ndk` and your
desired `target_cpu`:

```
bin/gn gen out/arm   --args='ndk="/tmp/ndk" target_cpu="arm"'
bin/gn gen out/arm64 --args='ndk="/tmp/ndk" target_cpu="arm64"'
bin/gn gen out/x64   --args='ndk="/tmp/ndk" target_cpu="x64"'
bin/gn gen out/x86   --args='ndk="/tmp/ndk" target_cpu="x86"'
```

Other arguments like `is_debug` and `is_component_build` continue to work.
Tweaking `ndk_api` gives you access to newer Android features like Vulkan.

To test on an Android device, push the binary and `resources` over, and run it
as normal. You may find `bin/droid` convenient.

```
ninja -C out/arm64
adb push out/arm64/dm /data/local/tmp
adb push resources /data/local/tmp
adb shell "cd /data/local/tmp; ./dm --src gm --config gl"
```

## <span id="cros">ChromeOS</span>

To cross-compile Skia for arm ChromeOS devices the following is needed:

-   Clang 4 or newer
-   An armhf sysroot
-   The (E)GL lib files on the arm chromebook to link against.

To compile Skia for an x86 ChromeOS device, one only needs Clang and the lib
files.

If you have access to CIPD, you can fetch all of these as follows:

```
python2 infra/bots/assets/clang_linux/download.py  -t /opt/clang
python2 infra/bots/assets/armhf_sysroot/download.py -t /opt/armhf_sysroot
python2 infra/bots/assets/chromebook_arm_gles/download.py -t /opt/chromebook_arm_gles
python2 infra/bots/assets/chromebook_x86_64_gles/download.py -t /opt/chromebook_x86_64_gles
```

If you don't have authorization to use those assets, then see the README.md
files for
[armhf_sysroot](https://skia.googlesource.com/skia/+/master/infra/bots/assets/armhf_sysroot/README.md),
[chromebook_arm_gles](https://skia.googlesource.com/skia/+/master/infra/bots/assets/chromebook_arm_gles/README.md),
and
[chromebook_x86_64_gles](https://skia.googlesource.com/skia/+/master/infra/bots/assets/chromebook_x86_64_gles/README.md)
for instructions on creating those assets.

Once those files are in place, generate the GN args that resemble the following:

```
#ARM
cc= "/opt/clang/bin/clang"
cxx = "/opt/clang/bin/clang++"

extra_asmflags = [
    "--target=armv7a-linux-gnueabihf",
    "--sysroot=/opt/armhf_sysroot/",
    "-march=armv7-a",
    "-mfpu=neon",
    "-mthumb",
]
extra_cflags=[
    "--target=armv7a-linux-gnueabihf",
    "--sysroot=/opt/armhf_sysroot",
    "-I/opt/chromebook_arm_gles/include",
    "-I/opt/armhf_sysroot/include/",
    "-I/opt/armhf_sysroot/include/c++/4.8.4/",
    "-I/opt/armhf_sysroot/include/c++/4.8.4/arm-linux-gnueabihf/",
    "-DMESA_EGL_NO_X11_HEADERS",
    "-funwind-tables",
]
extra_ldflags=[
    "--sysroot=/opt/armhf_sysroot",
    "-B/opt/armhf_sysroot/bin",
    "-B/opt/armhf_sysroot/gcc-cross",
    "-L/opt/armhf_sysroot/gcc-cross",
    "-L/opt/armhf_sysroot/lib",
    "-L/opt/chromebook_arm_gles/lib",
    "--target=armv7a-linux-gnueabihf",
]
target_cpu="arm"
skia_use_fontconfig = false
skia_use_system_freetype2 = false
skia_use_egl = true


# x86_64
cc= "/opt/clang/bin/clang"
cxx = "/opt/clang/bin/clang++"
extra_cflags=[
    "-I/opt/clang/include/c++/v1/",
    "-I/opt/chromebook_x86_64_gles/include",
    "-DMESA_EGL_NO_X11_HEADERS",
    "-DEGL_NO_IMAGE_EXTERNAL",
]
extra_ldflags=[
    "-stdlib=libc++",
    "-fuse-ld=lld",
    "-L/opt/chromebook_x86_64_gles/lib",
]
target_cpu="x64"
skia_use_fontconfig = false
skia_use_system_freetype2 = false
skia_use_egl = true
```

Compile dm (or another executable of your choice) with ninja, as per usual.

Push the binary to a chromebook via ssh and
[run dm as normal](https://skia.org/dev/testing/tests) using the gles GPU
config.

Most chromebooks by default have their home directory partition marked as
noexec. To avoid "permission denied" errors, remember to run something like:

```
sudo mount -i -o remount,exec /home/chronos
```

## <span id="macos">Mac</span>

Mac users may want to pass `--ide=xcode` to `bin/gn gen` to generate an Xcode
project.

## <span id="ios">iOS</span>

Run GN to generate your build files. Set `target_os="ios"` to build for iOS.
This defaults to `target_cpu="arm64"`. Choosing `x64` targets the iOS simulator.

```
bin/gn gen out/ios64  --args='target_os="ios"'
bin/gn gen out/ios32  --args='target_os="ios" target_cpu="arm"'
bin/gn gen out/iossim --args='target_os="ios" target_cpu="x64"'
```

This will also package (and for devices, sign) iOS test binaries. This defaults
to a Google signing identity and provisioning profile. To use a different one
set the GN args `skia_ios_identity` to match your code signing identity and
`skia_ios_profile` to the name of your provisioning profile, e.g.

```
skia_ios_identity=".*Jane Doe.*"
skia_ios_profile="iPad Profile"`
```

A list of identities can be found by typing `security find-identity` on the
command line. The name of the provisioning profile should be available on the
Apple Developer site. Alternatively, `skia_ios_profile` can be the absolute path
to the mobileprovision file.

If you find yourself missing a Google signing identity or provisioning profile,
you'll want to have a read through go/appledev.

For signed packages `ios-deploy` makes installing and running them on a device
easy:

```
ios-deploy -b out/Debug/dm.app -d --args "--match foo"
```

Alternatively you can generate an Xcode project by passing `--ide=xcode` to
`bin/gn gen`. If you are using Xcode version 10 or later, you may need to go to
`Project Settings...` and verify that `Build System:` is set to `Legacy Build System`.

Deploying to a device with an OS older than the current SDK can be done by setting the
`ios_min_target` arg:

```
ios_min_target = "<major>.<minor>"
```

where `<major>.<minor>` is the iOS version on the device, e.g., 12.0 or 11.4.

## <span id="windows">Windows</span>

Skia can build on Windows with Visual Studio 2017 or 2019. If GN is unable to
locate either of those, it will print an error message. In that case, you can
pass your `VC` path to GN via `win_vc`.

Skia can be compiled with the free
[Build Tools for Visual Studio 2017 or 2019](https://www.visualstudio.com/downloads/#build-tools-for-visual-studio-2019).

The bots use a packaged 2019 toolchain, which Googlers can download like this:

```
python2 infra/bots/assets/win_toolchain/download.py -t C:/toolchain
```

You can then pass the VC and SDK paths to GN by setting your GN args:

```
win_vc = "C:\toolchain\VC"
win_sdk = "C:\toolchain\win_sdk"
```

This toolchain is the only way we support 32-bit builds, by also setting
`target_cpu="x86"`.

The Skia build assumes that the PATHEXT environment variable contains ".EXE".

### **Highly Recommended**: Build with clang-cl

Skia uses generated code that is only optimized when Skia is built with clang.
Other compilers get generic unoptimized code.

Setting the `cc` and `cxx` gn args is _not_ sufficient to build with clang-cl.
These variables are ignored on Windows. Instead set the variable `clang_win` to
your LLVM installation directory. If you installed the prebuilt LLVM downloaded
from [here](https://releases.llvm.org/download.html "LLVM Download") in the
default location that would be:

```
clang_win = "C:\Program Files\LLVM"
```

Follow the standard Windows path specification and not MinGW convention (e.g.
`C:\Program Files\LLVM` not ~~`/c/Program Files/LLVM`~~).

### Visual Studio Solutions

If you use Visual Studio, you may want to pass `--ide=vs` to `bin/gn gen` to
generate `all.sln`. That solution will exist within the GN directory for the
specific configuration, and will only build/run that configuration.

If you want a Visual Studio Solution that supports multiple GN configurations,
there is a helper script. It requires that all of your GN directories be inside
the `out` directory. First, create all of your GN configurations as usual. Pass
`--ide=vs` when running `bin/gn gen` for each one. Then:

```
python2 gn/gn_meta_sln.py
```

This creates a new dedicated output directory and solution file
`out/sln/skia.sln`. It has one solution configuration for each GN configuration,
and supports building and running any of them. It also adjusts syntax
highlighting of inactive code blocks based on preprocessor definitions from the
selected solution configuration.

## <span id="win-arm64">Windows ARM64</span>

There is early, experimental support for
[Windows 10 on ARM](https://docs.microsoft.com/en-us/windows/arm/). This
currently requires (a recent version of) MSVC, and the `Visual C++ compilers and
libraries for ARM64` individual component in the Visual Studio Installer. For
Googlers, the win_toolchain asset includes the ARM64 compiler.

To use that toolchain, set the `target_cpu` GN argument to `"arm64"`. Note that
OpenGL is not supported by Windows 10 on ARM, so Skia's GL backends are stubbed
out, and will not work. ANGLE is supported:

```
bin/gn gen out/win-arm64 --args='target_cpu="arm64" skia_use_angle=true'
```

This will produce a build of Skia that can use the software or ANGLE backends,
in DM. Viewer only works when launched with `--backend angle`, because the
software backend tries to use OpenGL to display the window contents.

## <span id="cmake">CMake</span>

We have added a GN-to-CMake translator mainly for use with IDEs that like CMake
project descriptions. This is not meant for any purpose beyond development.

```
bin/gn gen out/config --ide=json --json-ide-script=../../gn/gn_to_cmake.py
```