Contributing to libc++

This file contains notes about various tasks and processes specific to contributing to libc++. If this is your first time contributing, please also read this document on general rules for contributing to LLVM.

For libc++, please make sure you follow these instructions for submitting a code review from the command-line using arc, since we have some automation (e.g. CI) that depends on the review being submitted that way.

Looking for pre-existing reviews

Before you start working on any feature, please take a look at the open reviews to avoid duplicating someone else’s work. You can do that by going to the website where code reviews are held, Differential, and clicking on Libc++ Open Reviews in the sidebar to the left. If you see that your feature is already being worked on, please consider chiming in instead of duplicating work!

Pre-commit check list

Before committing or creating a review, please go through this check-list to make sure you don’t forget anything:

  • Do you have tests for every public class and/or function you’re adding or modifying?
  • Did you update the synopsis of the relevant headers?
  • Did you update the relevant files to track implementation status (in docs/Status/)?
  • Did you mark all functions and type declarations with the proper visibility macro?
  • If you added a header:
    • Did you add it to include/module.modulemap?
    • Did you add it to include/CMakeLists.txt?
    • If it’s a public header, did you add a test under test/libcxx that the new header defines _LIBCPP_VERSION? See test/libcxx/algorithms/version.pass.cpp for an example. NOTE: This should be automated.
    • If it’s a public header, did you update utils/generate_header_inclusion_tests.py?
  • Did you add the relevant feature test macro(s) for your feature? Did you update the generate_feature_test_macro_components.py script with it?
  • Did you run the libcxx-generate-files target and verify its output?

Post-release check list

After branching for an LLVM release:

  1. Update _LIBCPP_VERSION in include/__config
  2. Update the include/__libcpp_version file
  3. Update the version number in docs/conf.py

Exporting new symbols from the library

When exporting new symbols from libc++, you must update the ABI lists located in lib/abi. To test whether the lists are up-to-date, please run the target check-cxx-abilist. To regenerate the lists, use the target generate-cxx-abilist. The ABI lists must be updated for all supported platforms; currently Linux and Apple. If you don’t have access to one of these platforms, you can download an updated list from the failed build at Buildkite. Look for the failed build and select the artifacts tab. There, download the abilist for the platform, e.g.:

  • C++20 for the Linux platform.
  • MacOS C++20 for the Apple platform.