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.

If you plan on contributing to libc++, it can be useful to join the #libcxx channel on LLVM’s Discord server.

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/

  • Did you add the relevant feature test macro(s) for your feature? Did you update the script with it?

  • Did you run the libcxx-generate-files target and verify its output?

The review process

After uploading your patch, you should see that the “libc++” review group is automatically added as a reviewer for your patch. Once the group is marked as having approved your patch, you can commit it. However, if you get an approval very quickly for a significant patch, please try to wait a couple of business days before committing to give the opportunity for other reviewers to chime in. If you need someone else to commit the patch for you, please mention it and provide your Name <email@domain> for us to attribute the commit properly.

Note that the rule for accepting as the “libc++” review group is to wait for two members of the group to have approved the patch, excluding the patch author. This is not a hard rule – for very simple patches, use your judgement. The “libc++” review group consists of frequent libc++ contributors with a good understanding of the project’s guidelines – if you would like to be added to it, please reach out on Discord.

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/

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.

Working on large features

Libc++ makes no guarantees about the implementation status or the ABI stability of features that have not yet been ratified in the C++ Standard. After the C++ Standard is ratified libc++ promises a conforming and ABI-stable implementation. When working on a large new feature in the ratified version of the C++ Standard that can’t be finished before the next release branch is created, we can’t honor this promise. Another reason for not being able to promise ABI stability happens when the C++ Standard committee retroactively accepts ABI breaking papers as defect reports against the ratified C++ Standard.

When working on these features it should be possible for libc++ vendors to disable these incomplete features, so they can promise ABI stability to their customers. This is done by the CMake option LIBCXX_ENABLE_INCOMPLETE_FEATURES. When start working on a large feature the following steps are required to guard the new library with the CMake option.

  • libcxx/CMakeLists.txt: Add

  • libcxx/include/ Add

  • libcxx/include/foo: The contents of the file should be guarded in an ifdef and always include <version>

    #ifndef _LIBCPP_FOO
    #define _LIBCPP_FOO
    // Make sure all feature-test macros are available.
    #include <version>
    // Enable the contents of the header only when libc++ was built with LIBCXX_ENABLE_INCOMPLETE_FEATURES.
    #endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_FO0)
    #endif // _LIBCPP_FOO
  • libcxx/src/CMakeLists.txt: When the library has a file foo.cpp it should only be added when LIBCXX_ENABLE_INCOMPLETE_FEATURES is enabled

  • libcxx/utils/ Add to lit_markup

    "foo": ["UNSUPPORTED: libcpp-has-no-incomplete-foo"],
  • libcxx/utils/ Add to lit_markup

    "foo": ["UNSUPPORTED: libcpp-has-no-incomplete-foo"],
  • libcxx/utils/ Add to header_markup

    "foo": ["ifndef _LIBCPP_HAS_NO_INCOMPLETE_FOO"],
  • libcxx/utils/libcxx/test/ Add to macros

    '_LIBCPP_HAS_NO_INCOMPLETE_FOO': 'libcpp-has-no-incomplete-foo',
  • All tests that include <foo> should contain

    // UNSUPPORTED: libcpp-has-no-incomplete-foo

Once the library is complete these changes and guards should be removed.