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Summary: This patch attempts to clean up the macro configuration mess in `<__threading_support>`, specifically the mess involving external threading variants. Additionally this patch adds design documentation for `<__threading_support>` and the configuration macros it uses. The primary change in this patch is separating the idea of an "external API" provided by `<__external_threading>` and the idea of having an external threading library. Now `_LIBCPP_HAS_THREAD_API_EXTERNAL` means that libc++ should use `<__external_threading>` and that the header is expected to exist. Additionally the new macro `_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL` is now used to configure for using an "external library" with the default threading API. Reviewers: compnerd, rmaprath Subscribers: smeenai, cfe-commits, mgorny Differential Revision: https://reviews.llvm.org/D28316 llvm-svn: 291275
71 lines
2.8 KiB
ReStructuredText
71 lines
2.8 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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Normally ``<__threading_support>`` provides inline definitions to each internal
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threading API function it declares. However libc++ also supports using an
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external library to provide the definitions.
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When ``_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL`` libc++ does not provide inline
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definitions for the internal API, instead assuming the definitions will be
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provided by an external library.
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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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