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 <__thread/support.h> header is where libc++ defines its internal threading interface. It documents the functions and declarations required to fullfil the internal threading 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 <__thread/support.h> 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 <__thread/support.h>.

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.

Threading Configuration Macros¶

_LIBCPP_HAS_THREADS

: This macro is set to 1 when libc++ is built with threading support. Otherwise it is set to 0. 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_C11

: This macro is defined when libc++ should use C11 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.