Add weak_frameworks linking option

The -weak_framework linker flag adds a dynamic library dependency to the
output, but it instructs the loader to skip loading the library if it is
not present and leave the external symbols unresolved. This is typically
used when a framework is present in a new version of an SDK that is not
present on an older OS. See
https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html
for details.

Currently users of gn implement this using a config, but due to the way
configs propagate, the -weak_link ldflags get lost at static_library
boundaries. Having first-class support for weakly linked frameworks
simplifies that.

Bug: chromium:1082896
Change-Id: Ia93bf1b67e5b9205073b8f2bf3ed9b96e3af79e4
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/8464
Commit-Queue: Brett Wilson <brettw@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Brett Wilson <brettw@chromium.org>
16 files changed
tree: 30b0c28e7a18989dc99aacef8398a711cdfaba7d
  1. .clang-format
  2. .editorconfig
  3. .gitignore
  4. .style.yapf
  5. AUTHORS
  6. LICENSE
  7. OWNERS
  8. README.md
  9. build/
  10. docs/
  11. examples/
  12. infra/
  13. misc/
  14. src/
  15. tools/
README.md

GN

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 https://gn.googlesource.com/gn
cd gn
python build/gen.py
ninja -C out
# To run tests:
out/gn_unittests

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.

Examples

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
./out/hello

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 https://gn-review.googlesource.com.

... 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’).

Community

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