The build system


The Fuchsia build system aims at building both boot images and installable 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. Using GN build is a good intro to GN.

Note that Zircon uses a different build system, though still using GN and Ninja. The rest of the build relies on Zircon being built ahead of time.


The contents of the generated image are controlled by a set of top level products. Products define sets of packages that are included in boot and system update images, preinstalled in paver images, and installable using the update system. products documents the structure and usage of fields in product definitions.


The contents of products are packages, which may aggregate or reference other packages and GN labels that are to be built. See packages for more information.


A bundle is a grouping of related packages within a part of the source tree, such as all tools or all tests. 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][packages-source].

TODO(pylaligand): list available templates

Executing a build

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


The first step is to build Zircon which uses its own build system:

$ scripts/

This is what gets run under the hood by fx build-zircon, which is run by fx build.

For a list of all options, run -h. See Zircon's Getting started and GN options for details.


Then configure the content of the generated image by choosing the top level product to build:

$ buildtools/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:

$ scripts/fx set core.x64

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


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

$ buildtools/ninja -C out/<arch> -j 64

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


After modifying non-Zircon files

In order to rebuild the tree after modifying some sources, just rerun step C. 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 package files used to configure the build.

After modifying Zircon files

You will want to rerun A and C.

After syncing sources

You’ll most likely need to run A once if anything in the Zircon tree was changed. After that, run C again.

Tips and tricks

Visualizing the hierarchy of build packages

$ scripts/ >
$ dot -Tpng -o tree.png

Inspecting the content of a GN target

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

Finding references to a GN target

$ buildtools/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:

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

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

Exploring Ninja targets

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

$ buildtools/gn help ninja_rules

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

$ buildtools/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 a missing sysroot.

You likely forgot to run A before running B.

TODO(pylaligand): command showing path to default target

Internal GN setup

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