gn/docs/standalone.md

# Introduction

This page is about how to design a project that can build independently
with GN but also be brought into the Chrome build.

GN is in principle no different than GYP in that there is some core
configuration that must be the same between both the standalone build
and the Chrome build. However, GN is much more explicit in its naming
and configuration, so the similarities between the two builds are also
much more explicit and there is less flexibility in how things are
configured.

# What you need for a minimal GN build

Requirements:

  * A master build config file. Chrome's is `//build/config/BUILDCONFIG.gn`
  * A separate build file for the toolchain definition. It's not a good idea
    to put these in a BUILD.gn file shared with any target definitions for
    complex reasons. Chrome's are in `//build/toolchain/<platform>/BUILD.gn`.
  * A `BUILD.gn` file in the root directory. This will be loaded after the
    build config file to start the build.

You may want a `.gn` file in the root directory. When you run GN it
recursively looks up the directory tree until it finds this file, and it
treats the containing directory as the "source root". This file also
defines the location of the master build config file:

  * See Chrome's `src/.gn` file.
  * Unlike Chrome, you probably don't need to define a secondary root.
  * see `gn help dotfile` for more.

Adding a `.gn` file in a repository that is pulled into Chrome means
that then running GN in your subdirectory will configure a build for
your subproject rather than for all of Chrome. This could be an
advantage or a disadvantage.

If you are in a directory with such a file and you want to not use it
(e.g., to do the full Chrome build instead), you can use the command-line
flags `--root` and `--dotfile` to set the values you want.

If you want a completely standalone build that has nothing to do with Chrome
and doesn't use Chrome's `//build` files, you can look at an example in
[//tools/gn/example](../tools/gn/example).