33 lines
1.4 KiB
ReStructuredText
33 lines
1.4 KiB
ReStructuredText
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====================
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Libc++ ABI stability
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====================
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Libc++ aims to preserve a stable ABI to avoid subtle bugs when code built under the old ABI
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is linked with code built under the new ABI. At the same time, libc++ wants to make
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ABI-breaking improvements and bugfixes in scenarios where the user doesn't mind ABI breaks.
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To support both cases, libc++ allows specifying an ABI version at
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build time. The version is defined with CMake option ``LIBCXX_ABI_VERSION``.
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Currently supported values are ``1`` (the stable default)
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and ``2`` (the unstable "next" version). At some point "ABI version 2" will be
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frozen and new ABI-breaking changes will start being applied to version ``3``;
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but this has not happened yet.
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To always use the most cutting-edge, most unstable ABI (which is currently ``2``
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but at some point will become ``3``), set the CMake option ``LIBCXX_ABI_UNSTABLE``.
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Internally, each ABI-changing feature is placed under its own C++ macro,
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``_LIBCPP_ABI_XXX``. These macros' definitions are controlled by the C++ macro
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``_LIBCPP_ABI_VERSION``, which is controlled by the ``LIBCXX_ABI_VERSION`` set
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at build time. Libc++ does not intend users to interact with these C++ macros
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directly.
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-----------------
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MSVC environments
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-----------------
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The exception to this is MSVC environments. Libc++ does not currently have users
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that require a stable ABI in MSVC environments, so MSVC-only changes may be
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applied unconditionally.
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