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gn build for Ladybird

Warning! The GN build is experimental and best-effort. It might not work, and if you use it you're expected to feel comfortable to unbreak it if necessary. Ladybird's official build system is CMake, if in doubt use that. If you add files, you're expected to update the CMake build but you don't need to update GN build files. Reviewers should not ask authors to update GN build files. Keeping the GN build files up-to-date is on the people who use the GN build.

GN is a metabuild system. It always creates ninja files, but it can create some IDE projects (MSVC, Xcode, ...) which then shell out to ninja for the actual build.

This is a good overview of GN.

For more information, motivation, philosophy, and inspiration, see the LLVM documentation on its GN build

Creating a gn build

To create a GN build, you need to have GN installed. You can install it via homebrew on macOS, or via your package manager on Linux. On Ubuntu 22.04, the main package repos do not have an up to date enough package for GN, so you will need to build it from source or get a binary from Google.

The easiest way to build GN from source is to use our Toolchain/BuildGN.sh script, which will drop the built binary into the Toolchain/Local/gn/bin directory. The instructions for downloading a prebuilt binary from Google are here.

Once you have GN installed, you can create a build directory by running the following commands:

gn gen out

gn gen creates a ninja build in the out directory. You can then build the project with ninja:

ninja -C out

If GN or ninja report a bunch of errors, it's likely that you need to create an args.gn that points to all the required tools. args.gn belongs at the root of the build directory, and can be placed there before running gn gen, or modified with gn args <build dir>. See the section below for a typical args.gn.

If you modify args.gn outside of gn args, be sure to run gn gen again to regenerate the ninja files.

Typical gn args

On macOS, the default args should work out of the box. For compiling Ladybird there won't be any tailoring needed if you have Qt6 installed via homebrew and the Xcode tools installed.

On Ubuntu, it's likely that the default cc and c++ will not be able to compile the project. For compiling Ladybird, a typical args.gn might look like the below:

args.gn

# Set build arguments here. See `gn help buildargs`.
# Chosen clang must be >= version 15.0.0
host_cc="clang"
host_cxx="clang++"
is_clang=true
use_lld=true
qt_install_headers="/usr/include/x86_64-linux-gnu/qt6/"
qt_install_lib="/usr/lib/x86_64-linux-gnu"
qt_install_libexec="/usr/lib/qt6/libexec/"

As with any gn project, gn args <build dir> --list is your best friend.

Running binaries from the GN build

Targets in the gn build are prefixed by the directory they are declared in. For example, to build the default target in the Ladybird/ directory and LibWeb, you would run:

ninja -C out Ladybird
ninja -C out Userland/Libraries/LibWeb

Binaries are placed in the out/bin directory, and can be run from there.

./out/bin/Ladybird
# or on macOS
open -W --stdout $(tty) --stderr $(tty) ./out/bin/Ladybird.app --args https://ladybird.dev