# 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)