How to build Skia

Make sure you have first followed the instructions to download Skia.

Skia uses GN to configure its builds.

is_official_build and Third-party Dependencies

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 libraries 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.

Supported and Preferred Compilers

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" cxx="clang++"' GN build arguments, as illustrated in Quickstart. This can be useful for building Skia without needing to modify your machine’s default compiler toolchain.

If you do not specify cc and cxx in your gn arguments, Skia will default to cc and c++. This is often GCC by default on many platforms, not Clang.

Quickstart

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:

python3 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 all of Skia:

ninja -C out/Static

To avoid building everything, include the target or targets after the ninja command. For example:

ninja -C out/Debug skia
ninja -C out/Debug viewer dm

Not all targets are available for all sets of build arguments. For a list of all available targets for a given build directory, run:

gn ls out/Debug

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

tools/install_dependencies.sh

To pull new changes and rebuild:

git pull
python3 tools/git-sync-deps
ninja -C out/Static

Android

To build Skia for Android you need a recent version of Java and a recent Android NDK.

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:

./bin/fetch-sk
./bin/sk asset download android_ndk_linux /tmp/ndk     # on Linux
./bin/sk asset download android_ndk_darwin /tmp/ndk    # on Mac
./bin/sk.exe asset download android_ndk_windows C:/ndk # on Windows

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"

ChromeOS

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:

./bin/sk asset download clang_linux /opt/clang
./bin/sk asset download armhf_sysroot /opt/armhf_sysroot
./bin/sk asset download chromebook_arm_gles /opt/chromebook_arm_gles
./bin/sk asset download chromebook_x86_64_gles /opt/chromebook_x86_64_gles

If you don’t have authorization to use those assets, then see the README.md files for armhf_sysroot, chromebook_arm_gles, and chromebook_x86_64_gles 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 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

Mac

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

Mac GN builds assume an Intel CPU by default. If you are building for Apple Silicon (M1 and newer) instead, add a gn arg to set target_cpu="arm64":

bin/gn gen out/AppleSilicon --args='target_cpu="arm64"'

Googlers should see go/skia-corp-xcode for instructions on setting up Xcode on a corp machine.

Python

The version of Python supplied by Apple is a few versions out of date, and it is known to interact poorly with our build system. We recommend installing the latest official version of Python from https://www.python.org/downloads/. Then run “Applications/Python 3.11/Install Certificates.command”.

iOS

Run GN to generate your build files. Set target_os="ios" to build for iOS. This defaults to target_cpu="arm64". To use the iOS simulator, set ios_use_simulator=true and set target_cpu to your Mac’s architecture. On an Intel Mac, setting target_cpu="x64" alone will also target 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-apple --args='target_os="ios" target_cpu="arm64" ios_use_simulator=true'
bin/gn gen out/iossim-intel --args='target_os="ios" target_cpu="x64"'

By default this will also package (and for non-simulator devices, sign) iOS test binaries. If you wish to skip signing (for testing compilation alone, for example), you can disable it by setting skia_ios_use_signing to false.

When signing, the build 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, you can examine the installed provisioning profile files in the Finder by going to ~/Library/MobileDevice/Provisioning Profiles, selecting a .mobileprovision file, and hitting space. The value of skia_ios_profile can either be the string given at the top of that file or on the Developer site, or the absolute path to the file.

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

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"

If you wish to deploy through Xcode you can generate a 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.

Windows

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.

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

./bin/sk.exe asset download win_toolchain 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 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).

If you will be compiling the rest of your program with a compiler other than Clang, add this GN argument as well:

is_trivial_abi = false

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:

python3 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.

Windows ARM64

There is early, experimental support for Windows 10 on 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.

CMake

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