Support `format --read-tree=json`

Allows roundtripping to json and back with `gn format --dump-tree=json` and then `gn format --read-tree=json`. The preexisting
.json format is not the most pleasant to work with, but this does at
least allow for programmatic editing of the gn files with knowledge of
structure, going beyond naive string replacement.

For example, a python script to change `deps` inside `source_sets` from
`=` to `+=` to support a change:

The .json format was extended slightly, mostly to include location
information which is necessary to faithfully roundtrip including
whitespace. If the input .gn is formatted by `gn format`, then it can
roundtrip to json without change. This was confirmed by running
--dump-tree=json => --read-tree=json on all .gn and .gni files in both
the Fuchsia and Chromium trees
(, as
well as by adding a roundtrip step to the formatter tests in-repo.

Change-Id: Ie10bbbcb32b8184400ca9bb1f6ab63e5ca9c89e7
Reviewed-by: Brett Wilson <>
Commit-Queue: Scott Graham <>
11 files changed
tree: 33667120ee5c5dd5657d718594269c9ca9547df5
  1. build/
  2. docs/
  3. examples/
  4. infra/
  5. misc/
  6. src/
  7. tools/
  8. .clang-format
  9. .editorconfig
  10. .gitignore
  11. .style.yapf
  14. OWNERS


GN is a meta-build system that generates build files for Ninja.

Related resources:

Getting a binary

You can download the latest version of GN binary for Linux, macOS and Windows.

Alternatively, you can build GN from source:

git clone
cd gn
python build/
ninja -C out
# To run tests:

On Windows, it is expected that cl.exe, link.exe, and lib.exe can be found in PATH, so you'll want to run from a Visual Studio command prompt, or similar.

On Linux and Mac, the default compiler is clang++, a recent version is expected to be found in PATH. This can be overridden by setting CC, CXX, and AR.


There is a simple example in examples/simple_build directory that is a good place to get started with the minimal configuration.

To build and run the simple example with the default gcc compiler:

cd examples/simple_build
../../out/gn gen -C out
ninja -C out

For a maximal configuration see the Chromium setup:

and the Fuchsia setup:

Reporting bugs

If you find a bug, you can see if it is known or report it in the bug database.

Sending patches

GN uses Gerrit for code review. The short version of how to patch is:

Register at

... edit code ...
ninja -C out && out/gn_unittests

Then, to upload a change for review:

git commit
git push origin HEAD:refs/for/master

The first time you do this you'll get an error from the server about a missing change-ID. Follow the directions in the error message to install the change-ID hook and run git commit --amend to apply the hook to the current commit.

When revising a change, use:

git commit --amend
git push origin HEAD:refs/for/master

which will add the new changes to the existing code review, rather than creating a new one.

We ask that all contributors sign Google's Contributor License Agreement (either individual or corporate as appropriate, select ‘any other Google project’).


You may ask questions and follow along with GN‘s development on Chromium’s gn-dev@ Google Group.