Platform Support
****************

The "asyncio" module is designed to be portable, but some platforms
have subtle differences and limitations due to the platforms'
underlying architecture and capabilities.


All Platforms
=============

* "loop.add_reader()" and "loop.add_writer()" cannot be used to
  monitor file I/O.


Windows
=======

**Source code:** Lib/asyncio/proactor_events.py,
Lib/asyncio/windows_events.py, Lib/asyncio/windows_utils.py

======================================================================

Changed in version 3.8: On Windows, "ProactorEventLoop" is now the
default event loop.

All event loops on Windows do not support the following methods:

* "loop.create_unix_connection()" and "loop.create_unix_server()" are
  not supported. The "socket.AF_UNIX" socket family is specific to
  Unix.

* "loop.add_signal_handler()" and "loop.remove_signal_handler()" are
  not supported.

"SelectorEventLoop" has the following limitations:

* "SelectSelector" is used to wait on socket events: it supports
  sockets and is limited to 512 sockets.

* "loop.add_reader()" and "loop.add_writer()" only accept socket
  handles (e.g. pipe file descriptors are not supported).

* Pipes are not supported, so the "loop.connect_read_pipe()" and
  "loop.connect_write_pipe()" methods are not implemented.

* Subprocesses are not supported, i.e. "loop.subprocess_exec()" and
  "loop.subprocess_shell()" methods are not implemented.

"ProactorEventLoop" has the following limitations:

* The "loop.add_reader()" and "loop.add_writer()" methods are not
  supported.

The resolution of the monotonic clock on Windows is usually around
15.6 milliseconds.  The best resolution is 0.5 milliseconds. The
resolution depends on the hardware (availability of HPET) and on the
Windows configuration.


Subprocess Support on Windows
-----------------------------

On Windows, the default event loop "ProactorEventLoop" supports
subprocesses, whereas "SelectorEventLoop" does not.

The "policy.set_child_watcher()" function is also not supported, as
"ProactorEventLoop" has a different mechanism to watch child
processes.


macOS
=====

Modern macOS versions are fully supported.

-[ macOS <= 10.8 ]-

On macOS 10.6, 10.7 and 10.8, the default event loop uses
"selectors.KqueueSelector", which does not support character devices
on these versions.  The "SelectorEventLoop" can be manually configured
to use "SelectSelector" or "PollSelector" to support character devices
on these older versions of macOS.  Example:

   import asyncio
   import selectors

   selector = selectors.SelectSelector()
   loop = asyncio.SelectorEventLoop(selector)
   asyncio.set_event_loop(loop)
