3 GN is a meta-build system that generates build files for
4 [Ninja](https://ninja-build.org).
8 * Documentation in [docs/](https://gn.googlesource.com/gn/+/master/docs/). In
9 particular [GN Quick Start
10 guide](https://gn.googlesource.com/gn/+/master/docs/quick_start.md)
11 and the [reference](https://gn.googlesource.com/gn/+/master/docs/reference.md)
12 (the latter is all builtin help converted to a single file).
13 * An introductory [presentation](https://docs.google.com/presentation/d/15Zwb53JcncHfEwHpnG_PoIbbzQ3GQi_cpujYwbpcbZo/edit?usp=sharing).
14 * The [mailing list](https://groups.google.com/a/chromium.org/forum/#!forum/gn-dev).
18 GN is currently used as the build system for Chromium, Fuchsia, and related
19 projects. Some strengths of GN are:
21 * It is designed for large projects and large teams. It scales efficiently to
22 many thousands of build files and tens of thousands of source files.
24 * It has a readable, clean syntax. Once a build is set-up, it is generally
25 easy for people with no backround in GN to make basic edits to the build.
27 * It is designed for multi-platform projects. It can cleanly express many
28 complicated build variants across different platforms. A single build
29 invocation can target multiple platforms.
31 * It supports multiple parallel output directories, each with their own
32 configuration. This allows a developer to maintain builds targeting debug,
33 release, or different platforms in parallel without forced rebuilds when
36 * It has a focus on correctness. GN checks for the correct dependencies,
37 inputs, and outputs to the extent possible, and has a number of tools to
38 allow developers to ensure the build evolves as desired (for example, `gn
39 check`, `testonly`, `assert_no_deps`).
41 * It has comprehensive build-in help available from the command-line.
43 Although small projects successfully use GN, the focus on large projects has
46 * GN has the goal of being minimally expressive. Although it can be quite
47 flexible, a design goal is to direct members of a large team (who may not
48 have much knowledge about the build) down an easy-to-understand, well-lit
49 path. This isn't necessarily the correct trade-off for smaller projects.
51 * The minimal build configuration is relatively heavyweight. There are several
52 files required and the exact way all compilers are linkers are run must be
53 specified in the configuration (see "Examples" below). There is no default
54 compiler configuration.
56 * It is not easily composable. GN is designed to compile a single large
57 project with relatively uniform settings and rules. Projects like Chromium
58 do bring together multiple repositories from multiple teams, but the
59 projects must agree on some conventions in the build files to allow this to
62 * GN is designed with the expectation that the developers building a project
63 want to compile an identical configuration. So while builds can integrate
64 with the user's environment like the CXX and CFLAGS variables if they want,
65 this is not the default and most project's builds do not do this. The result
66 is that many GN projects do not integrate well with other systems like
69 * There is no simple release scheme (see "Versioning and distribution" below).
70 Projects are expected to manage the version of GN they require. Getting an
71 appropriate GN binary can be a hurdle for new contributors to a project.
72 Since it is relatively uncommon, it can be more difficult to find
73 information and examples.
75 GN can generate Ninja build files for C, C++, Rust, Objective C, and Swift
76 source on most popular platforms. Other languages can be compiled using the
77 general "action" rules which are executed by Python or another scripting
78 language (Google does this to compile Java and Go). But because this is not as
79 clean, generally GN is only used when the bulk of the build is in one of the
80 main built-in languages.
84 You can download the latest version of GN binary for
85 [Linux](https://chrome-infra-packages.appspot.com/dl/gn/gn/linux-amd64/+/latest),
86 [macOS](https://chrome-infra-packages.appspot.com/dl/gn/gn/mac-amd64/+/latest) and
87 [Windows](https://chrome-infra-packages.appspot.com/dl/gn/gn/windows-amd64/+/latest)
88 from Google's build infrastructure (see "Versioning and distribution" below for
89 how this is expected to work).
91 Alternatively, you can build GN from source with a C++17 compiler:
93 git clone https://gn.googlesource.com/gn
100 On Windows, it is expected that `cl.exe`, `link.exe`, and `lib.exe` can be found
101 in `PATH`, so you'll want to run from a Visual Studio command prompt, or
104 On Linux and Mac, the default compiler is `clang++`, a recent version is
105 expected to be found in `PATH`. This can be overridden by setting `CC`, `CXX`,
110 There is a simple example in [examples/simple_build](examples/simple_build)
111 directory that is a good place to get started with the minimal configuration.
113 To build and run the simple example with the default gcc compiler:
115 cd examples/simple_build
116 ../../out/gn gen -C out
120 For a maximal configuration see the Chromium setup:
121 * [.gn](https://cs.chromium.org/chromium/src/.gn)
122 * [BUILDCONFIG.gn](https://cs.chromium.org/chromium/src/build/config/BUILDCONFIG.gn)
123 * [Toolchain setup](https://cs.chromium.org/chromium/src/build/toolchain/)
124 * [Compiler setup](https://cs.chromium.org/chromium/src/build/config/compiler/BUILD.gn)
126 and the Fuchsia setup:
127 * [.gn](https://fuchsia.googlesource.com/fuchsia/+/refs/heads/master/.gn)
128 * [BUILDCONFIG.gn](https://fuchsia.googlesource.com/fuchsia/+/refs/heads/master/build/config/BUILDCONFIG.gn)
129 * [Toolchain setup](https://fuchsia.googlesource.com/fuchsia/+/refs/heads/master/build/toolchain/)
130 * [Compiler setup](https://fuchsia.googlesource.com/fuchsia/+/refs/heads/master/build/config/BUILD.gn)
134 If you find a bug, you can see if it is known or report it in the [bug
135 database](https://bugs.chromium.org/p/gn/issues/list).
139 GN uses [Gerrit](https://www.gerritcodereview.com/) for code review. The short
140 version of how to patch is:
142 Register at https://gn-review.googlesource.com.
145 ninja -C out && out/gn_unittests
147 Then, to upload a change for review:
150 git push origin HEAD:refs/for/master
152 The first time you do this you'll get an error from the server about a missing
153 change-ID. Follow the directions in the error message to install the change-ID
154 hook and run `git commit --amend` to apply the hook to the current commit.
156 When revising a change, use:
159 git push origin HEAD:refs/for/master
161 which will add the new changes to the existing code review, rather than creating
164 We ask that all contributors
165 [sign Google's Contributor License Agreement](https://cla.developers.google.com/)
166 (either individual or corporate as appropriate, select 'any other Google
171 You may ask questions and follow along with GN's development on Chromium's
172 [gn-dev@](https://groups.google.com/a/chromium.org/forum/#!forum/gn-dev)
175 ## Versioning and distribution
177 Most open-source projects are designed to use the developer's computer's current
178 toolchain such as compiler, linker, and build tool. But the large
179 centrally controlled projects that GN is designed for typically want a more
180 hermetic environment. They will ensure that developers are using a specific
181 compatible toolchain that is versioned with the code
183 As a result, GN expects that the project choose the appropriate version of GN
184 that will work with each version of the project. There is no "current stable
185 version" of GN that is expected to work for all projects.
187 As a result, the GN developers to not maintain any packages in any of the
188 various packaging systems (Debian, RedHat, HomeBrew, etc.). Some of these
189 systems to have GN packages, but they are maintained by third parties and you
190 should use at your own risk. Instead, we recommend you refer your checkout
191 tooling to download binaries for a specific hash from [Google's build
192 infrastructure](https://chrome-infra-packages.appspot.com/p/gn/gn) or compile
195 GN does not guarantee the backwards-compatibility of new versions and has no
196 branches or versioning scheme beyond the sequence of commits to the master git
197 branch (which is expected to be stable).
199 In practice, however, GN is very backwards-compatible. The core functionality
200 has been stable for many years and there is enough GN code at Google alone to
201 make non-backwards-compatible changes very difficult, even if they were
204 There have been discussions about adding a versioning scheme with some
205 guarantees about backwards-compatibility, but nothing has yet been implemented.