33 lines · plain
1 2====================3Libc++ ABI stability4====================5 6Libc++ aims to preserve a stable ABI to avoid subtle bugs when code built under the old ABI7is linked with code built under the new ABI. At the same time, libc++ wants to make8ABI-breaking improvements and bugfixes in scenarios where the user doesn't mind ABI breaks.9 10To support both cases, libc++ allows specifying an ABI version at11build time. The version is defined with CMake option ``LIBCXX_ABI_VERSION``.12Currently supported values are ``1`` (the stable default)13and ``2`` (the unstable "next" version). At some point "ABI version 2" will be14frozen and new ABI-breaking changes will start being applied to version ``3``;15but this has not happened yet.16 17To always use the most cutting-edge, most unstable ABI (which is currently ``2``18but at some point will become ``3``), set the CMake option ``LIBCXX_ABI_UNSTABLE``.19 20Internally, each ABI-changing feature is placed under its own C++ macro,21``_LIBCPP_ABI_XXX``. These macros' definitions are controlled by the C++ macro22``_LIBCPP_ABI_VERSION``, which is controlled by the ``LIBCXX_ABI_VERSION`` set23at build time. Libc++ does not intend users to interact with these C++ macros24directly.25 26-----------------27MSVC environments28-----------------29 30The exception to this is MSVC environments. Libc++ does not currently have users31that require a stable ABI in MSVC environments, so MSVC-only changes may be32applied unconditionally.33