The Fuchsia build system


The Fuchsia build system aims at building both boot images and updatable packages for various devices. To do so, it uses GN, a meta-build system that generates build files consumed by Ninja, which executes the actual build.

Note that Zircon uses a different build system, though still using GN and Ninja.

Getting started

If you are unfamiliar with Fuchsia's build system and GN, see Using GN build which outlines the basic principles of the GN build system.

Boards and Products

The contents of the generated image are controlled by a combination of a board and a product that are the minimal starting configuration of a Fuchsia build. Boards and products define dependency sets that define the packages that are included in images, updates, and package repositories. boards and products documents the structure and usage of these build configurations.


A bundle is a grouping of related packages within a part of the source tree, such as all tools or all tests. An overview of bundles is provided in bundles. A set of top-level bundles are defined in //bundles.

Build targets

Build targets are defined in files scattered all over the source tree. These files use a Python-like syntax to declare buildable objects:


my_template("foo") {
  name = "foo"
  extra_options = "//my/foo/options"
  deps = [

Available commands (invoked using gn cli tool) and constructs (built-in target declaration types) are defined in the GN reference. There are also a handful of custom templates in .gni files in the //build project.

These custom templates mostly define custom target declaration types, such as the package declaration type.

TODO(pylaligand): list available templates

Executing a build

The simplest way to this is through the fx tool, as described in fx workflows. Read on to see what fx does under the hood.

The rest of this document assumes that gn and ninja commands are available in your PATH. These commands can be found in prebuilt/third_party/gn/<platform> and prebuilt/third_party/ninja/<platform> respectively. Alternatively, if you want to avoid modifying your PATH, you can prefix all invocations with fx, i.e. fx gn or fx ninja.

Gen step

First configure the primary build artifacts by choosing the board and product to build:

$ gn gen out/default --args='import("//boards/x64.gni") import("//products/core.gni")'

This will create an out/default directory containing Ninja files.

The equivalent fx set command is:

$ fx set core.x64

For a list of all GN build arguments, run gn args out/default --list. For documentation on the select_variant argument, see Variants.

Build step

The next step is to run the actual build with Ninja:

$ ninja -C out/default

This is what gets run under the hood by fx build.


In order to rebuild the tree after modifying some sources, just rerun Build step. This holds true even if you modify files as GN adds Ninja targets to update Ninja targets if build files are changed! The same holds true for other files used to configure the build. Any change of source that requires a manual re-invocation of the Gen step is a build bug and should be reported.

Tips and tricks

Inspecting the content of a GN target

$ fx gn desc out/default //path/to/my:target

Finding references to a GN target

$ gn refs out/default //path/to/my:target

Referencing targets for the build host

Various host tools (some used in the build itself) need to be built along with the final image.

To reference a build target for the host toolchain from a module file:


To reference a build target for the host toolchain from within a file:


Building only a specific target

If a target is defined in a GN build file as //foo/bar/blah:dash, that target (and its dependencies) can be built with:

$ ninja -C out/default -j64 foo/bar/blah:dash

Note that this only works for targets in the default toolchain.

Note: Building package targets does not result in an updated package repository, because the package repository is updated by the updates group target. In order for updated package changes to be made available via fx serve, users must build the updates group.

Exploring Ninja targets

GN extensively documents which Ninja targets it generates. The documentation is accessible with:

$ gn help ninja_rules

You can also browse the set of Ninja targets currently defined in your output directory with:

$ ninja -C out/default -t browse

Note that the presence of a Ninja target does not mean it will be built - for that it needs to depend on the “default” target.

Understanding why Ninja does what it does

Add -d explain to your Ninja command to have it explain every step of its execution.

Debugging build timing issues

When running a build, Ninja keeps logs that can be used to generate visualizations of the build process:

  1. Delete your output directory - this is to ensure the logs represent only the build iteration you’re about to run;
  2. Run a build as you would normally do;
  3. Get;
  4. Run ninjatracing <output directory>/.ninja_log > trace.json;
  5. Load the resulting json file in Chrome in about:tracing.


My GN target is not being built!

Make sure it rolls up to a label defined in a module file, otherwise the build system will ignore it.

GN complains about missing sysroot.

You likely forgot to run both commands of Build step.

TODO(pylaligand): command showing path to default target

Internal GN setup

TODO(pylaligand): .gn, default target, GN labels insertion