Using Bazel


Skia is currently migrating towards using Bazel as a build system, due to the ability to more tightly control what and how gets built.

When referring to a file in this doc, we use Bazel label notation, so to refer the file located at $SKIA_ROOT/docs/examples/Arc.cpp, we would say //docs/examples/Arc.cpp.

Learning more about Bazel

The Bazel docs are quite good. Suggested reading order if you are new to Bazel:

Googlers, check out go/bazel-bites for more tips.

Building with Bazel

All this assumes you have downloaded Skia, especially having synced the third_party deps using ./tools/git-sync-deps.

Linux Hosts (you are running Bazel on a Linux machine)

You can run a command like:

bazel build //example:hello_world_gl

This uses a hermetic C++ toolchain we put together to compile Skia on a Linux host (implementation is in //toolchain. It builds the target defined in //examples/BUILD.bazel named “hello_world_gl”, which uses the sk_app framework we designed to make simple applications using Skia.

Bazel will put this executable in //bazel-bin/example/hello_world_gl and tell you it did so in the logs. You can run this executable yourself, or have Bazel run it by modifying the command to be:

bazel run //example:hello_world_gl

If you want to pass one or more flags to bazel run, add them on the end after a -- like:

bazel run //example:hello_world_gl -- --flag_one=apple --flag_two=cherry

Mac Hosts (you are running Bazel on a Mac machine)

You can run a command like:

bazel build //example:bazel_test_exe

When building for Mac, we require the user to have Xcode installed on their device so that we can use system headers and Mac-specific includes when compiling. Googlers, as per usual, follow the instructions at go/skia-corp-xcode to install Xcode.

Our Bazel toolchain assumes you have xcode-select in your path so that we may symlink the user’s current Xcode directory in the toolchain’s cache. Make sure xcode-select -p returns a valid path.

.bazelrc Tips

You should make a .bazelrc file in your home directory where you can specify settings that apply only to you. These can augment or replace the ones we define in the //.bazelrc configuration file.

Skia defines some configs, that is, group of settings and features in //bazel/buildrc. This file contains configs for builds that we use regularly (for example, in our continuous integration system).

If you want to define Skia-specific configs (and options which do not conflict with other Bazel projects), you make a file in //bazel/user/buildrc which will automatically be read in. This file is covered by a .gitignore rule and should not be checked in.

You may want some or all of the following entries in your ~/.bazelrc or //bazel/user/buildrc file.

Build Skia faster locally

Many Linux machines have a RAM disk mounted at /dev/shm and using this as the location for the Bazel sandbox can dramatically improve compile times because sandboxing has been observed to be I/O intensive.

Add the following to ~/.bazelrc if you have a /dev/shm partition that is 4+ GB big.

build --sandbox_base=/dev/shm

Mac users should probably bypass sandboxing as it is known to be slow.

build --spawn_strategy=local

Authenticate to RBE on a Linux VM

We are in the process of setting up Remote Build Execution (RBE) for Bazel. Some users have reported errors when trying to use RBE (via --config=linux_rbe) on Linux VMs such as:

ERROR: Failed to query remote execution capabilities:
Error code 404 trying to get security access token from Compute Engine metadata for the default
service account. This may be because the virtual machine instance does not have permission
scopes specified. It is possible to skip checking for Compute Engine metadata by specifying the
environment variable NO_GCE_CHECK=true.

For instances where it is not possible to set the cloud-platform scope on the VM, one can directly link to their GCP credentials by adding the following to ~/.bazelrc (substituting their username for <user>) after logging in via gcloud auth login:

build:remote --google_credentials=/usr/local/google/home/<user>/.config/gcloud/application_default_credentials.json

Make local builds compatible with remote builds (e.g. better caching)

Add the following to //bazel/user/buildrc if you are on a Linux x64 box and want to be able to share cached build results between things you build locally and build with --config=linux_rbe.

build --host_platform=//bazel/platform:linux_x64_hermetic

For example, if you are on a laptop, using --config=linux_rbe will speed up builds when you have access to Internet, but then if you need to go offline, you can still build locally and use the previous build results from the remote builds.