18.5.1. Base Event Loop

Source code: Lib/asyncio/events.py

The event loop is the central execution device provided by asyncio. It provides multiple facilities, including:

  • Registering, executing and cancelling delayed calls (timeouts).

  • Creating client and server transports for various kinds of communication.

  • Launching subprocesses and the associated transports for communication with an external program.

  • Delegating costly function calls to a pool of threads.

class asyncio.BaseEventLoop

This class is an implementation detail. It is a subclass of AbstractEventLoop and may be a base class of concrete event loop implementations found in asyncio. It should not be used directly; use AbstractEventLoop instead. BaseEventLoop should not be subclassed by third-party code; the internal interface is not stable.

class asyncio.AbstractEventLoop

Abstract base class of event loops.

This class is not thread safe.

18.5.1.1. Run an event loop

AbstractEventLoop.run_forever()

Run until stop() is called. If stop() is called before run_forever() is called, this polls the I/O selector once with a timeout of zero, runs all callbacks scheduled in response to I/O events (and those that were already scheduled), and then exits. If stop() is called while run_forever() is running, this will run the current batch of callbacks and then exit. Note that callbacks scheduled by callbacks will not run in that case; they will run the next time run_forever() is called.

在 3.5.1 版更改.

AbstractEventLoop.run_until_complete(future)

Run until the Future is done.

If the argument is a coroutine object, it is wrapped by ensure_future().

Return the Future’s result, or raise its exception.

AbstractEventLoop.is_running()

Returns running status of event loop.

AbstractEventLoop.stop()

Stop running the event loop.

This causes run_forever() to exit at the next suitable opportunity (see there for more details).

在 3.5.1 版更改.

AbstractEventLoop.is_closed()

Returns True if the event loop was closed.

3.4.2 新版功能.

AbstractEventLoop.close()

Close the event loop. The loop must not be running. Pending callbacks will be lost.

This clears the queues and shuts down the executor, but does not wait for the executor to finish.

This is idempotent and irreversible. No other methods should be called after this one.

coroutine AbstractEventLoop.shutdown_asyncgens()

Schedule all currently open asynchronous generator objects to close with an aclose() call. After calling this method, the event loop will issue a warning whenever a new asynchronous generator is iterated. Should be used to finalize all scheduled asynchronous generators reliably. Example:

try:
    loop.run_forever()
finally:
    loop.run_until_complete(loop.shutdown_asyncgens())
    loop.close()

3.6 新版功能.

18.5.1.2. Calls

Most asyncio functions don’t accept keywords. If you want to pass keywords to your callback, use functools.partial(). For example, loop.call_soon(functools.partial(print, "Hello", flush=True)) will call print("Hello", flush=True).

注解

functools.partial() is better than lambda functions, because asyncio can inspect functools.partial() object to display parameters in debug mode, whereas lambda functions have a poor representation.

AbstractEventLoop.call_soon(callback, *args)

Arrange for a callback to be called as soon as possible. The callback is called after call_soon() returns, when control returns to the event loop.

This operates as a FIFO queue, callbacks are called in the order in which they are registered. Each callback will be called exactly once.

Any positional arguments after the callback will be passed to the callback when it is called.

An instance of asyncio.Handle is returned, which can be used to cancel the callback.

Use functools.partial to pass keywords to the callback.

AbstractEventLoop.call_soon_threadsafe(callback, *args)

Like call_soon(), but thread safe.

参见 concurrency and multithreading 部分的文档。

18.5.1.3. Delayed calls

The event loop has its own internal clock for computing timeouts. Which clock is used depends on the (platform-specific) event loop implementation; ideally it is a monotonic clock. This will generally be a different clock than time.time().

注解

Timeouts (relative delay or absolute when) should not exceed one day.

AbstractEventLoop.call_later(delay, callback, *args)

Arrange for the callback to be called after the given delay seconds (either an int or float).

An instance of asyncio.Handle is returned, which can be used to cancel the callback.

callback will be called exactly once per call to call_later(). If two callbacks are scheduled for exactly the same time, it is undefined which will be called first.

The optional positional args will be passed to the callback when it is called. If you want the callback to be called with some named arguments, use a closure or functools.partial().

Use functools.partial to pass keywords to the callback.

AbstractEventLoop.call_at(when, callback, *args)

Arrange for the callback to be called at the given absolute timestamp when (an int or float), using the same time reference as AbstractEventLoop.time().

本方法的行为和 call_later() 方法相同。

An instance of asyncio.Handle is returned, which can be used to cancel the callback.

Use functools.partial to pass keywords to the callback.

AbstractEventLoop.time()

Return the current time, as a float value, according to the event loop’s internal clock.

参见

asyncio.sleep() 函数。

18.5.1.4. Futures

AbstractEventLoop.create_future()

Create an asyncio.Future object attached to the loop.

This is a preferred way to create futures in asyncio, as event loop implementations can provide alternative implementations of the Future class (with better performance or instrumentation).

3.5.2 新版功能.

18.5.1.5. Tasks

AbstractEventLoop.create_task(coro)

Schedule the execution of a coroutine object: wrap it in a future. Return a Task object.

第三方的事件循环可以使用它们自己的 Task 子类来满足互操作性。这种情况下结果类型是一个 Task 的子类。

This method was added in Python 3.4.2. Use the async() function to support also older Python versions.

3.4.2 新版功能.

AbstractEventLoop.set_task_factory(factory)

Set a task factory that will be used by AbstractEventLoop.create_task().

If factory is None the default task factory will be set.

If factory is a callable, it should have a signature matching (loop, coro), where loop will be a reference to the active event loop, coro will be a coroutine object. The callable must return an asyncio.Future compatible object.

3.4.4 新版功能.

AbstractEventLoop.get_task_factory()

Return a task factory, or None if the default one is in use.

3.4.4 新版功能.

18.5.1.6. Creating connections

coroutine AbstractEventLoop.create_connection(protocol_factory, host=None, port=None, *, ssl=None, family=0, proto=0, flags=0, sock=None, local_addr=None, server_hostname=None)

Create a streaming transport connection to a given Internet host and port: socket family AF_INET or AF_INET6 depending on host (or family if specified), socket type SOCK_STREAM. protocol_factory must be a callable returning a protocol instance.

This method is a coroutine which will try to establish the connection in the background. When successful, the coroutine returns a (transport, protocol) pair.

底层操作的大致的执行顺序是这样的：

  1. The connection is established, and a transport is created to represent it.

  2. protocol_factory is called without arguments and must return a protocol instance.

  3. The protocol instance is tied to the transport, and its connection_made() method is called.

  4. The coroutine returns successfully with the (transport, protocol) pair.

被创建的传输对象是一个实现相关的双向流。

注解

protocol_factory can be any kind of callable, not necessarily a class. For example, if you want to use a pre-created protocol instance, you can pass lambda: my_protocol.

Options that change how the connection is created:

  • ssl: if given and not false, a SSL/TLS transport is created (by default a plain TCP transport is created). If ssl is a ssl.SSLContext object, this context is used to create the transport; if ssl is True, a context with some unspecified default settings is used.

    参见

    SSL/TLS security considerations

  • server_hostname, is only for use together with ssl, and sets or overrides the hostname that the target server’s certificate will be matched against. By default the value of the host argument is used. If host is empty, there is no default and you must pass a value for server_hostname. If server_hostname is an empty string, hostname matching is disabled (which is a serious security risk, allowing for man-in-the-middle-attacks).

  • family, proto, flags 是可选的地址族、协议和标志，它们会被传递给 getaddrinfo() 来对 host 进行解析。如果要指定的话，这些都应该是来自于 socket 模块的对应常量。

  • sock，如果指定的话，其应该是一个已经存在，并且已经处于连接状态的 socket.socket 对象，其会被传输对象使用。如果指定了 sock ，那么 host, port, family, proto, flags 和 local_addr 就都不应该被指定了。

  • local_addr, if given, is a (local_host, local_port) tuple used to bind the socket to locally. The local_host and local_port are looked up using getaddrinfo(), similarly to host and port.

在 3.5 版更改: On Windows with ProactorEventLoop, SSL/TLS is now supported.

参见

The open_connection() function can be used to get a pair of (StreamReader, StreamWriter) instead of a protocol.

coroutine AbstractEventLoop.create_datagram_endpoint(protocol_factory, local_addr=None, remote_addr=None, *, family=0, proto=0, flags=0, reuse_address=None, reuse_port=None, allow_broadcast=None, sock=None)

注解

形参 reuse_address 已不再受支持，因为使用 SO_REUSEADDR 会对 UDP 造成显著的安全问题。 显式地传入 reuse_address=True 将会引发异常。

当具有不同 UID 的多个进程将套接字赋给具有 SO_REUSEADDR 的相同 UDP 套接字地址时，传入的数据包可能会在套接字间随机分配。

对于受支持的平台，reuse_port 可以被用作类似功能的替代。 通过 reuse_port 将改用 SO_REUSEPORT，它能够防止具有不同 UID 的进程将套接字赋给相同的套接字地址。

创建一个数据报连接。

Create datagram connection: socket family AF_INET or AF_INET6 depending on host (or family if specified), socket type SOCK_DGRAM. protocol_factory must be a callable returning a protocol instance.

This method is a coroutine which will try to establish the connection in the background. When successful, the coroutine returns a (transport, protocol) pair.

Options changing how the connection is created:

  • local_addr，如果指定的话，就是一个 (local_host, local_port) 元组，用于在本地绑定套接字。 local_host 和 local_port 是使用 getaddrinfo() 来查找的。

  • remote_addr，如果指定的话，就是一个 (remote_host, remote_port) 元组，用于同一个远程地址连接。remote_host 和 remote_port 是使用 getaddrinfo() 来查找的。

  • family, proto, flags 是可选的地址族，协议和标志，其会被传递给 getaddrinfo() 来完成 host 的解析。如果要指定的话，这些都应该是来自于 socket 模块的对应常量。

  • reuse_port tells the kernel to allow this endpoint to be bound to the same port as other existing endpoints are bound to, so long as they all set this flag when being created. This option is not supported on Windows and some UNIX’s. If the SO_REUSEPORT constant is not defined then this capability is unsupported.

  • allow_broadcast 告知内核允许此端点向广播地址发送消息。

  • sock 可选择通过指定此值用于使用一个预先存在的，已经处于连接状态的 socket.socket 对象，并将其提供给此传输对象使用。如果指定了这个值， local_addr 和 remote_addr 就应该被忽略 (必须为 None)。

On Windows with ProactorEventLoop, this method is not supported.

参见 UDP echo 客户端协议 和 UDP echo 服务端协议 的例子。

在 3.4.4 版更改: 添加了 family, proto, flags, reuse_address, reuse_port, allow_broadcast 和 sock 等参数。

在 3.6.10 版更改: The reuse_address parameter is no longer supporter due to security concerns

coroutine AbstractEventLoop.create_unix_connection(protocol_factory, path, *, ssl=None, sock=None, server_hostname=None)

Create UNIX connection: socket family AF_UNIX, socket type SOCK_STREAM. The AF_UNIX socket family is used to communicate between processes on the same machine efficiently.

This method is a coroutine which will try to establish the connection in the background. When successful, the coroutine returns a (transport, protocol) pair.

path is the name of a UNIX domain socket, and is required unless a sock parameter is specified. Abstract UNIX sockets, str, and bytes paths are supported.

See the AbstractEventLoop.create_connection() method for parameters.

Availability: UNIX.

18.5.1.7. Creating listening connections

coroutine AbstractEventLoop.create_server(protocol_factory, host=None, port=None, *, family=socket.AF_UNSPEC, flags=socket.AI_PASSIVE, sock=None, backlog=100, ssl=None, reuse_address=None, reuse_port=None)

Create a TCP server (socket type SOCK_STREAM) bound to host and port.

Return a Server object, its sockets attribute contains created sockets. Use the Server.close() method to stop the server: close listening sockets.

参数：

  • The host parameter can be a string, in that case the TCP server is bound to host and port. The host parameter can also be a sequence of strings and in that case the TCP server is bound to all hosts of the sequence. If host is an empty string or None, all interfaces are assumed and a list of multiple sockets will be returned (most likely one for IPv4 and another one for IPv6).

  • family can be set to either socket.AF_INET or AF_INET6 to force the socket to use IPv4 or IPv6. If not set it will be determined from host (defaults to socket.AF_UNSPEC).

  • flags 是用于 getaddrinfo() 的位掩码。

  • sock can optionally be specified in order to use a preexisting socket object. If specified, host and port should be omitted (must be None).

  • backlog 是传递给 listen() 的最大排队连接的数量（默认为100）。

  • ssl 可被设置为一个 SSLContext 以在接受的连接上启用 SSL。

  • reuse_address tells the kernel to reuse a local socket in TIME_WAIT state, without waiting for its natural timeout to expire. If not specified will automatically be set to True on UNIX.

  • reuse_port 告知内核，只要在创建的时候都设置了这个标志，就允许此端点绑定到其它端点列表所绑定的同样的端口上。这个选项在 Windows 上是不支持的。

This method is a coroutine.

在 3.5 版更改: On Windows with ProactorEventLoop, SSL/TLS is now supported.

参见

The function start_server() creates a (StreamReader, StreamWriter) pair and calls back a function with this pair.

在 3.5.1 版更改: The host parameter can now be a sequence of strings.

coroutine AbstractEventLoop.create_unix_server(protocol_factory, path=None, *, sock=None, backlog=100, ssl=None)

Similar to AbstractEventLoop.create_server(), but specific to the socket family AF_UNIX.

This method is a coroutine.

Availability: UNIX.

coroutine BaseEventLoop.connect_accepted_socket(protocol_factory, sock, *, ssl=None)

Handle an accepted connection.

This is used by servers that accept connections outside of asyncio but that use asyncio to handle them.

参数：

  • sock is a preexisting socket object returned from an accept call.

  • ssl 可被设置为一个 SSLContext 以在接受的连接上启用 SSL。

This method is a coroutine. When completed, the coroutine returns a (transport, protocol) pair.

3.5.3 新版功能.

18.5.1.8. Watch file descriptors

On Windows with SelectorEventLoop, only socket handles are supported (ex: pipe file descriptors are not supported).

On Windows with ProactorEventLoop, these methods are not supported.

AbstractEventLoop.add_reader(fd, callback, *args)

Start watching the file descriptor for read availability and then call the callback with specified arguments.

Use functools.partial to pass keywords to the callback.

AbstractEventLoop.remove_reader(fd)

Stop watching the file descriptor for read availability.

AbstractEventLoop.add_writer(fd, callback, *args)

Start watching the file descriptor for write availability and then call the callback with specified arguments.

Use functools.partial to pass keywords to the callback.

AbstractEventLoop.remove_writer(fd)

Stop watching the file descriptor for write availability.

The watch a file descriptor for read events example uses the low-level AbstractEventLoop.add_reader() method to register the file descriptor of a socket.

18.5.1.9. Low-level socket operations

coroutine AbstractEventLoop.sock_recv(sock, nbytes)

Receive data from the socket. Modeled after blocking socket.socket.recv() method.

The return value is a bytes object representing the data received. The maximum amount of data to be received at once is specified by nbytes.

With SelectorEventLoop event loop, the socket sock must be non-blocking.

This method is a coroutine.

coroutine AbstractEventLoop.sock_sendall(sock, data)

Send data to the socket. Modeled after blocking socket.socket.sendall() method.

The socket must be connected to a remote socket. This method continues to send data from data until either all data has been sent or an error occurs. None is returned on success. On error, an exception is raised, and there is no way to determine how much data, if any, was successfully processed by the receiving end of the connection.

With SelectorEventLoop event loop, the socket sock must be non-blocking.

This method is a coroutine.

coroutine AbstractEventLoop.sock_connect(sock, address)

Connect to a remote socket at address. Modeled after blocking socket.socket.connect() method.

With SelectorEventLoop event loop, the socket sock must be non-blocking.

This method is a coroutine.

在 3.5.2 版更改: address no longer needs to be resolved. sock_connect will try to check if the address is already resolved by calling socket.inet_pton(). If not, AbstractEventLoop.getaddrinfo() will be used to resolve the address.

参见

AbstractEventLoop.create_connection() and asyncio.open_connection().

coroutine AbstractEventLoop.sock_accept(sock)

Accept a connection. Modeled after blocking socket.socket.accept().

此 scoket 必须绑定到一个地址上并且监听连接。返回值是一个 (conn, address) 对，其中 conn 是一个 新*的套接字对象，用于在此连接上收发数据，*address 是连接的另一端的套接字所绑定的地址。

The socket sock must be non-blocking.

This method is a coroutine.

参见

AbstractEventLoop.create_server() and start_server().

18.5.1.10. Resolve host name

coroutine AbstractEventLoop.getaddrinfo(host, port, *, family=0, type=0, proto=0, flags=0)

This method is a coroutine, similar to socket.getaddrinfo() function but non-blocking.

coroutine AbstractEventLoop.getnameinfo(sockaddr, flags=0)

This method is a coroutine, similar to socket.getnameinfo() function but non-blocking.

18.5.1.11. Connect pipes

On Windows with SelectorEventLoop, these methods are not supported. Use ProactorEventLoop to support pipes on Windows.

coroutine AbstractEventLoop.connect_read_pipe(protocol_factory, pipe)

Register read pipe in eventloop.

protocol_factory should instantiate object with Protocol interface. pipe is a file-like object. Return pair (transport, protocol), where transport supports the ReadTransport interface.

使用 SelectorEventLoop 事件循环， pipe 被设置为非阻塞模式。

This method is a coroutine.

coroutine AbstractEventLoop.connect_write_pipe(protocol_factory, pipe)

Register write pipe in eventloop.

protocol_factory should instantiate object with BaseProtocol interface. pipe is file-like object. Return pair (transport, protocol), where transport supports WriteTransport interface.

使用 SelectorEventLoop 事件循环， pipe 被设置为非阻塞模式。

This method is a coroutine.

参见

The AbstractEventLoop.subprocess_exec() and AbstractEventLoop.subprocess_shell() methods.

18.5.1.12. UNIX signals

Availability: UNIX only.

AbstractEventLoop.add_signal_handler(signum, callback, *args)

Add a handler for a signal.

如果信号数字非法或者不可捕获，就抛出一个 ValueError 。如果建立处理器的过程中出现问题，会抛出一个 RuntimeError 。

Use functools.partial to pass keywords to the callback.

AbstractEventLoop.remove_signal_handler(sig)

Remove a handler for a signal.

Return True if a signal handler was removed, False if not.

参见

signal 模块。

18.5.1.13. Executor

Call a function in an Executor (pool of threads or pool of processes). By default, an event loop uses a thread pool executor (ThreadPoolExecutor).

coroutine AbstractEventLoop.run_in_executor(executor, func, *args)

Arrange for a func to be called in the specified executor.

The executor argument should be an Executor instance. The default executor is used if executor is None.

Use functools.partial to pass keywords to the *func*.

This method is a coroutine.

在 3.5.3 版更改: BaseEventLoop.run_in_executor() no longer configures the max_workers of the thread pool executor it creates, instead leaving it up to the thread pool executor (ThreadPoolExecutor) to set the default.

AbstractEventLoop.set_default_executor(executor)

Set the default executor used by run_in_executor().

18.5.1.14. 错误处理API

允许自定义事件循环中如何去处理异常。

AbstractEventLoop.set_exception_handler(handler)

将 handler 设置为新的事件循环异常处理器。

If handler is None, the default exception handler will be set.

If handler is a callable object, it should have a matching signature to (loop, context), where loop will be a reference to the active event loop, context will be a dict object (see call_exception_handler() documentation for details about context).

AbstractEventLoop.get_exception_handler()

Return the exception handler, or None if the default one is in use.

3.5.2 新版功能.

AbstractEventLoop.default_exception_handler(context)

默认的异常处理器。

This is called when an exception occurs and no exception handler is set, and can be called by a custom exception handler that wants to defer to the default behavior.

context 参数和 call_exception_handler() 中的同名参数完全相同。

AbstractEventLoop.call_exception_handler(context)

调用当前事件循环的异常处理器。

context is a dict object containing the following keys (new keys may be introduced later):

  • ‘message’: 错误消息；

  • ‘exception’ （可选）: 异常对象；

  • ‘future’ （可选）: asyncio.Future 实例；

  • ‘handle’ （可选）: asyncio.Handle 实例；

  • ‘protocol’ （可选）: Protocol 实例；

  • ‘transport’ （可选）: Transport 实例；

  • ‘socket’ （可选）： socket.socket 实例。

注解

Note: this method should not be overloaded in subclassed event loops. For any custom exception handling, use set_exception_handler() method.

18.5.1.15. Debug mode

AbstractEventLoop.get_debug()

获取事件循环调试模式设置（bool）。

如果环境变量 PYTHONASYNCIODEBUG 是一个非空字符串，就返回 True ，否则就返回 False 。

3.4.2 新版功能.

AbstractEventLoop.set_debug(enabled: bool)

设置事件循环的调试模式。

3.4.2 新版功能.

参见

debug mode of asyncio.

18.5.1.16. Server

class asyncio.Server

Server listening on sockets.

Object created by the AbstractEventLoop.create_server() method and the start_server() function. Don’t instantiate the class directly.

close()

停止服务：关闭监听的套接字并且设置 sockets 属性为 None 。

用于表示已经连进来的客户端连接会保持打开的状态。

服务器是被异步关闭的，使用 wait_closed() 协程来等待服务器关闭。

coroutine wait_closed()

等待 close() 方法执行完毕。

This method is a coroutine.

sockets

List of socket.socket objects the server is listening to, or None if the server is closed.

18.5.1.17. Handle

class asyncio.Handle

A callback wrapper object returned by AbstractEventLoop.call_soon(), AbstractEventLoop.call_soon_threadsafe(), AbstractEventLoop.call_later(), and AbstractEventLoop.call_at().

cancel()

Cancel the call. If the callback is already canceled or executed, this method has no effect.

18.5.1.18. Event loop examples

18.5.1.18.1. call_soon() 的 Hello World 示例。

Example using the AbstractEventLoop.call_soon() method to schedule a callback. The callback displays "Hello World" and then stops the event loop:

import asyncio

def hello_world(loop):
    print('Hello World')
    loop.stop()

loop = asyncio.get_event_loop()

# Schedule a call to hello_world()
loop.call_soon(hello_world, loop)

# Blocking call interrupted by loop.stop()
loop.run_forever()
loop.close()

参见

The Hello World coroutine example uses a coroutine.

18.5.1.18.2. 使用 call_later() 来展示当前的日期

Example of callback displaying the current date every second. The callback uses the AbstractEventLoop.call_later() method to reschedule itself during 5 seconds, and then stops the event loop:

import asyncio
import datetime

def display_date(end_time, loop):
    print(datetime.datetime.now())
    if (loop.time() + 1.0) < end_time:
        loop.call_later(1, display_date, end_time, loop)
    else:
        loop.stop()

loop = asyncio.get_event_loop()

# Schedule the first call to display_date()
end_time = loop.time() + 5.0
loop.call_soon(display_date, end_time, loop)

# Blocking call interrupted by loop.stop()
loop.run_forever()
loop.close()

参见

The coroutine displaying the current date example uses a coroutine.

18.5.1.18.3. 监控一个文件描述符的读事件

Wait until a file descriptor received some data using the AbstractEventLoop.add_reader() method and then close the event loop:

import asyncio
try:
    from socket import socketpair
except ImportError:
    from asyncio.windows_utils import socketpair

# Create a pair of connected file descriptors
rsock, wsock = socketpair()
loop = asyncio.get_event_loop()

def reader():
    data = rsock.recv(100)
    print("Received:", data.decode())
    # We are done: unregister the file descriptor
    loop.remove_reader(rsock)
    # Stop the event loop
    loop.stop()

# Register the file descriptor for read event
loop.add_reader(rsock, reader)

# Simulate the reception of data from the network
loop.call_soon(wsock.send, 'abc'.encode())

# Run the event loop
loop.run_forever()

# We are done, close sockets and the event loop
rsock.close()
wsock.close()
loop.close()

参见

The register an open socket to wait for data using a protocol example uses a low-level protocol created by the AbstractEventLoop.create_connection() method.

The register an open socket to wait for data using streams example uses high-level streams created by the open_connection() function in a coroutine.

18.5.1.18.4. 为SIGINT和SIGTERM设置信号处理器

Register handlers for signals SIGINT and SIGTERM using the AbstractEventLoop.add_signal_handler() method:

import asyncio
import functools
import os
import signal

def ask_exit(signame):
    print("got signal %s: exit" % signame)
    loop.stop()

loop = asyncio.get_event_loop()
for signame in ('SIGINT', 'SIGTERM'):
    loop.add_signal_handler(getattr(signal, signame),
                            functools.partial(ask_exit, signame))

print("Event loop running forever, press Ctrl+C to interrupt.")
print("pid %s: send SIGINT or SIGTERM to exit." % os.getpid())
try:
    loop.run_forever()
finally:
    loop.close()

This example only works on UNIX.