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[libcxx] Improve design documentation for the external-thread-library

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configuration

NFC.

Differential revision: https://reviews.llvm.org/D28610

Reviewers: EricWF
llvm-svn: 292108
This commit is contained in:
Asiri Rathnayake 2017-01-16 12:44:08 +00:00
parent af762e91b2
commit e3d832a3e8
1 changed files with 15 additions and 6 deletions
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21
libcxx/docs/DesignDocs/ThreadingSupportAPI.rst
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@ -33,13 +33,22 @@ interface normally provided by ``<__threading_support>``.
External Threading Library
==========================
Normally ``<__threading_support>`` provides inline definitions to each internal
threading API function it declares. However libc++ also supports using an
external library to provide the definitions.
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.
When ``_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL`` libc++ does not provide inline
definitions for the internal API, instead assuming the definitions will be
provided by an external library.
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
==============================