GN in Chrome OS (go/chromeos-gn)

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https://chromium.googlesource.com/chromiumos/docs/+/HEAD/chromeos_gn.md

New packages should use GN instead of GYP.

See the official step-by-step introduction for the GN basics. This article discusses Chrome OS specific stuff.

How to build your package with GN

Example: arc/adbd/BUILD.gn

  • Put BUILD.gn in your package directory, which is determined by PLATFORM_SUBDIR in your ebuild. Existence of BUILD.gn indicates to the platform2 build system that GN should be used to build this package.
  • Have a target named "all" in the BUILD.gn. It‘s the root target built by the platform2 build system. Typically it’s a group target depends on all the targets to be built.
  • common-mk/ contains common templates or common settings, which your build files can utilize.
    • pkg_config.gni defines the pkg_config rule to generate configs for package dependencies.
    • BUILDCONFIG.gn defines the default configs for each target type (e.g. executable). You can remove default configs in individual target with configs -= if needed. (example)

How to write ebuilds

Example: arc-adbd-9999.ebuild

Note we should add .gn in CROS_WORKON_SUBTREE so that the platform2 build system can access the file.

Packages outside platform2

For packages outside platform2, use CROS_WORKON_DESTDIR to copy the package under <workspace>/platform2/ while building it. (example)

How to check USE flags in GN

In GN files, USE flags can be referred as use.foo. (example)

Only whitelisted USE flags can be used. If you need to use new USE flags, update:

  • _IUSE constant in platform2/common-mk/platform2.py (example)
  • IUSE variable in your ebuild file (example)

How to write unit tests

use.test flag is set to true on unit testing. Enclose test only targets with if (use.test) {} to reduce compile time on production. The test targets are typically executables which depend on //common-mk:test to use gtest and gmock.

How to run unit tests doesn‘t change: In chroot, run cros_workon --board=$BOARD start $package_name if you haven’t. Then run

FEATURES=test emerge-$BOARD $package_name

Prepend VERBOSE=1 to see GN and ninja commands (plus other logs).

FAQ

How to create standalone static library?

Static libraries are compiled with thin archive enabled by default i.e. not standalone. Remove use_thin_archive from configs and add nouse_thin_archive to generate a standalone static library (example).

TODO(crbug.com/991440): consider making standalone library the default and replace static_library with source_set whenever possible.