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+=====================
+Threading Support API
+=====================
+
+.. contents::
+   :local:
+
+Overview
+========
+
+Libc++ supports using multiple different threading models and configurations
+to implement the threading parts of libc++, including ``<thread>`` and ``<mutex>``.
+These different models provide entirely different interfaces from each
+other. To address this libc++ wraps the underlying threading API in a new and
+consistent API, which it uses internally to implement threading primitives.
+
+The ``<__threading_support>`` header is where libc++ defines its internal
+threading interface. It contains forward declarations of the internal threading
+interface as well as definitions for the interface.
+
+External Threading API and the ``<__external_threading>`` header
+================================================================
+
+In order to support vendors with custom threading API's libc++ allows the
+entire internal threading interface to be provided by an external,
+vendor provided, header.
+
+When ``_LIBCPP_HAS_THREAD_API_EXTERNAL`` is defined the ``<__threading_support>``
+header simply forwards to the ``<__external_threading>`` header (which must exist).
+It is expected that the ``<__external_threading>`` header provide the exact
+interface normally provided by ``<__threading_support>``.
+
+External Threading Library
+==========================
+
+libc++ can be compiled with its internal threading API delegating to an external
+library. Such a configuration is useful for library vendors who wish to
+distribute a thread-agnostic libc++ library, where the users of the library are
+expected to provide the implementation of the libc++ internal threading API.
+
+On a production setting, this would be achieved through a custom
+``<__external_threading>`` header, which declares the libc++ internal threading
+API but leaves out the implementation.
+
+The ``-DLIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY`` option allows building libc++ in
+such a configuration while allowing it to be tested on a platform that supports
+any of the threading systems (e.g. pthread) supported in ``__threading_support``
+header. Therefore, the main purpose of this option is to allow testing of this
+particular configuration of the library without being tied to a vendor-specific
+threading system. This option is only meant to be used by libc++ library
+developers.
+
+Threading Configuration Macros
+==============================
+
+**_LIBCPP_HAS_NO_THREADS**
+  This macro is defined when libc++ is built without threading support. It
+  should not be manually defined by the user.
+
+**_LIBCPP_HAS_THREAD_API_EXTERNAL**
+  This macro is defined when libc++ should use the ``<__external_threading>``
+  header to provide the internal threading API. This macro overrides
+  ``_LIBCPP_HAS_THREAD_API_PTHREAD``.
+
+**_LIBCPP_HAS_THREAD_API_PTHREAD**
+  This macro is defined when libc++ should use POSIX threads to implement the
+  internal threading API.
+
+**_LIBCPP_HAS_THREAD_API_WIN32**
+  This macro is defined when libc++ should use Win32 threads to implement the
+  internal threading API.
+
+**_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL**
+  This macro is defined when libc++ expects the definitions of the internal
+  threading API to be provided by an external library. When defined
+  ``<__threading_support>`` will only provide the forward declarations and
+  typedefs for the internal threading API.
+
+**_LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL**
+  This macro is used to build an external threading library using the
+  ``<__threading_support>``. Specifically it exposes the threading API
+  definitions in ``<__threading_support>`` as non-inline definitions meant to
+  be compiled into a library.