Restat after gen

As of version 1.8, ninja caches mtimes in the .ninja_log file and will
not restat a file if it has an entry in the .ninja_log. This becomes
problematic for generator files such as In a scenario where
the .ninja_log has an entry for (ie, after ninja has had to
trigger a re-gen) and a manual run of of `gn gen` has updated the file, a subsequent invocation of ninja will trigger yet
another re-gen of

This can be reproduced like this:

  # Initial build
  gn gen out && ninja -C out
  # Trigger ninja re-gen so there's a log entry
  touch && ninja -C out
  # Manual re-gen
  gn gen out
  # The following command will re-gen again before building
  ninja -C out

To work around this, the recommended approach from ninja is to use the
ninja restat tool to get ninja to update the mtime in the .ninja_log.
This CL does that by attempting to invoke `ninja -t restat`
immediately after the is generated by `gn gen`. Because this
tool was only introduced in 1.10, the usage is gated on having a ninja
binary of at least that version. The ninja binary can be provided via
the --ninja-executable switch.

Bug: 136
Change-Id: If40391aa13e6ef71c4e8ab26aff57d66a8137b8e
Commit-Queue: RJ Ascani <>
Reviewed-by: Brett Wilson <>
8 files changed
tree: f094107d1e05a2a0648ca1db3bfd66ba0e5a97bb
  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.