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