Threading Support API

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.