"select" --- 等待 I/O 完成
**************************

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

This module provides access to the "select()" and "poll()" functions
available in most operating systems, "devpoll()" available on Solaris
and derivatives, "epoll()" available on Linux 2.5+ and "kqueue()"
available on most BSD. Note that on Windows, it only works for
sockets; on other operating systems, it also works for other file
types (in particular, on Unix, it works on pipes). It cannot be used
on regular files to determine whether a file has grown since it was
last read.

注解:

  The "selectors" module allows high-level and efficient I/O
  multiplexing, built upon the "select" module primitives. Users are
  encouraged to use the "selectors" module instead, unless they want
  precise control over the OS-level primitives used.

该模块定义以下内容：

exception select.error

   一个被弃用的 "OSError" 的别名。

   在 3.3 版更改: 根据 **PEP 3151**，这个类是 "OSError" 的别名。

select.devpoll()

   (Only supported on Solaris and derivatives.)  Returns a "/dev/poll"
   polling object; see section /dev/poll Polling Objects below for the
   methods supported by devpoll objects.

   "devpoll()" objects are linked to the number of file descriptors
   allowed at the time of instantiation. If your program reduces this
   value, "devpoll()" will fail. If your program increases this value,
   "devpoll()" may return an incomplete list of active file
   descriptors.

   新的文件描述符 non-inheritable.

   3.3 新版功能.

   在 3.4 版更改: 新的文件描述符现在是不可继承的。

select.epoll(sizehint=-1, flags=0)

   (Only supported on Linux 2.5.44 and newer.) Return an edge polling
   object, which can be used as Edge or Level Triggered interface for
   I/O events.

   *sizehint* informs epoll about the expected number of events to be
   registered.  It must be positive, or *-1* to use the default. It is
   only used on older systems where "epoll_create1()" is not
   available; otherwise it has no effect (though its value is still
   checked).

   *flags* is deprecated and completely ignored.  However, when
   supplied, its value must be "0" or "select.EPOLL_CLOEXEC",
   otherwise "OSError" is raised.

   See the Edge and Level Trigger Polling (epoll) Objects section
   below for the methods supported by epolling objects.

   "epoll" objects support the context management protocol: when used
   in a "with" statement, the new file descriptor is automatically
   closed at the end of the block.

   新的文件描述符 non-inheritable.

   在 3.3 版更改: 增加了 *flags* 参数。

   在 3.4 版更改: Support for the "with" statement was added. The new
   file descriptor is now non-inheritable.

   3.4 版后已移除: The *flags* parameter.  "select.EPOLL_CLOEXEC" is
   used by default now. Use "os.set_inheritable()" to make the file
   descriptor inheritable.

select.poll()

   (Not supported by all operating systems.)  Returns a polling
   object, which supports registering and unregistering file
   descriptors, and then polling them for I/O events; see section
   Polling Objects below for the methods supported by polling objects.

select.kqueue()

   (Only supported on BSD.)  Returns a kernel queue object; see
   section Kqueue 对象 below for the methods supported by kqueue
   objects.

   新的文件描述符 non-inheritable.

   在 3.4 版更改: 新的文件描述符现在是不可继承的。

select.kevent(ident, filter=KQ_FILTER_READ, flags=KQ_EV_ADD, fflags=0, data=0, udata=0)

   (Only supported on BSD.)  Returns a kernel event object; see
   section Kevent 对象 below for the methods supported by kevent
   objects.

select.select(rlist, wlist, xlist[, timeout])

   This is a straightforward interface to the Unix "select()" system
   call. The first three arguments are sequences of 'waitable
   objects': either integers representing file descriptors or objects
   with a parameterless method named "fileno()" returning such an
   integer:

   * *rlist*: wait until ready for reading

   * *wlist*: wait until ready for writing

   * *xlist*: wait for an "exceptional condition" (see the manual page
     for what your system considers such a condition)

   Empty sequences are allowed, but acceptance of three empty
   sequences is platform-dependent. (It is known to work on Unix but
   not on Windows.)  The optional *timeout* argument specifies a time-
   out as a floating point number in seconds.  When the *timeout*
   argument is omitted the function blocks until at least one file
   descriptor is ready.  A time-out value of zero specifies a poll and
   never blocks.

   The return value is a triple of lists of objects that are ready:
   subsets of the first three arguments.  When the time-out is reached
   without a file descriptor becoming ready, three empty lists are
   returned.

   Among the acceptable object types in the sequences are Python *file
   objects* (e.g. "sys.stdin", or objects returned by "open()" or
   "os.popen()"), socket objects returned by "socket.socket()".  You
   may also define a *wrapper* class yourself, as long as it has an
   appropriate "fileno()" method (that really returns a file
   descriptor, not just a random integer).

   注解:

     File objects on Windows are not acceptable, but sockets are.  On
     Windows, the underlying "select()" function is provided by the
     WinSock library, and does not handle file descriptors that don't
     originate from WinSock.

   在 3.5 版更改: The function is now retried with a recomputed
   timeout when interrupted by a signal, except if the signal handler
   raises an exception (see **PEP 475** for the rationale), instead of
   raising "InterruptedError".

select.PIPE_BUF

   The minimum number of bytes which can be written without blocking
   to a pipe when the pipe has been reported as ready for writing by
   "select()", "poll()" or another interface in this module.  This
   doesn't apply to other kind of file-like objects such as sockets.

   This value is guaranteed by POSIX to be at least 512.

   Availability: Unix

   3.2 新版功能.


"/dev/poll" Polling Objects
===========================

Solaris and derivatives have "/dev/poll". While "select()" is
O(highest file descriptor) and "poll()" is O(number of file
descriptors), "/dev/poll" is O(active file descriptors).

"/dev/poll" behaviour is very close to the standard "poll()" object.

devpoll.close()

   Close the file descriptor of the polling object.

   3.4 新版功能.

devpoll.closed

   "True" if the polling object is closed.

   3.4 新版功能.

devpoll.fileno()

   Return the file descriptor number of the polling object.

   3.4 新版功能.

devpoll.register(fd[, eventmask])

   Register a file descriptor with the polling object.  Future calls
   to the "poll()" method will then check whether the file descriptor
   has any pending I/O events.  *fd* can be either an integer, or an
   object with a "fileno()" method that returns an integer.  File
   objects implement "fileno()", so they can also be used as the
   argument.

   *eventmask* is an optional bitmask describing the type of events
   you want to check for. The constants are the same that with
   "poll()" object. The default value is a combination of the
   constants "POLLIN", "POLLPRI", and "POLLOUT".

   警告:

     Registering a file descriptor that's already registered is not an
     error, but the result is undefined. The appropriate action is to
     unregister or modify it first. This is an important difference
     compared with "poll()".

devpoll.modify(fd[, eventmask])

   This method does an "unregister()" followed by a "register()". It
   is (a bit) more efficient that doing the same explicitly.

devpoll.unregister(fd)

   Remove a file descriptor being tracked by a polling object.  Just
   like the "register()" method, *fd* can be an integer or an object
   with a "fileno()" method that returns an integer.

   Attempting to remove a file descriptor that was never registered is
   safely ignored.

devpoll.poll([timeout])

   Polls the set of registered file descriptors, and returns a
   possibly-empty list containing "(fd, event)" 2-tuples for the
   descriptors that have events or errors to report. *fd* is the file
   descriptor, and *event* is a bitmask with bits set for the reported
   events for that descriptor --- "POLLIN" for waiting input,
   "POLLOUT" to indicate that the descriptor can be written to, and so
   forth. An empty list indicates that the call timed out and no file
   descriptors had any events to report. If *timeout* is given, it
   specifies the length of time in milliseconds which the system will
   wait for events before returning. If *timeout* is omitted, -1, or
   "None", the call will block until there is an event for this poll
   object.

   在 3.5 版更改: The function is now retried with a recomputed
   timeout when interrupted by a signal, except if the signal handler
   raises an exception (see **PEP 475** for the rationale), instead of
   raising "InterruptedError".


Edge and Level Trigger Polling (epoll) Objects
==============================================

   https://linux.die.net/man/4/epoll

   *屏蔽事件*

   +---------------------------+-------------------------------------------------+
   | 常数                      | 意义                                            |
   |===========================|=================================================|
   | "EPOLLIN"                 | 可读                                            |
   +---------------------------+-------------------------------------------------+
   | "EPOLLOUT"                | 可写                                            |
   +---------------------------+-------------------------------------------------+
   | "EPOLLPRI"                | 紧急数据读取                                    |
   +---------------------------+-------------------------------------------------+
   | "EPOLLERR"                | Error condition happened on the assoc. fd       |
   +---------------------------+-------------------------------------------------+
   | "EPOLLHUP"                | Hang up happened on the assoc. fd               |
   +---------------------------+-------------------------------------------------+
   | "EPOLLET"                 | Set Edge Trigger behavior, the default is Level |
   |                           | Trigger behavior                                |
   +---------------------------+-------------------------------------------------+
   | "EPOLLONESHOT"            | Set one-shot behavior. After one event is       |
   |                           | pulled out, the fd is internally disabled       |
   +---------------------------+-------------------------------------------------+
   | "EPOLLEXCLUSIVE"          | Wake only one epoll object when the associated  |
   |                           | fd has an event. The default (if this flag is   |
   |                           | not set) is to wake all epoll objects polling   |
   |                           | on a fd.                                        |
   +---------------------------+-------------------------------------------------+
   | "EPOLLRDHUP"              | Stream socket peer closed connection or shut    |
   |                           | down writing half of connection.                |
   +---------------------------+-------------------------------------------------+
   | "EPOLLRDNORM"             | Equivalent to "EPOLLIN"                         |
   +---------------------------+-------------------------------------------------+
   | "EPOLLRDBAND"             | 可以读取优先数据带。                            |
   +---------------------------+-------------------------------------------------+
   | "EPOLLWRNORM"             | Equivalent to "EPOLLOUT"                        |
   +---------------------------+-------------------------------------------------+
   | "EPOLLWRBAND"             | 可以写入优先级数据。                            |
   +---------------------------+-------------------------------------------------+
   | "EPOLLMSG"                | 忽略                                            |
   +---------------------------+-------------------------------------------------+

   3.6 新版功能: "EPOLLEXCLUSIVE" was added.  It's only supported by
   Linux Kernel 4.5 or later.

epoll.close()

   Close the control file descriptor of the epoll object.

epoll.closed

   "True" if the epoll object is closed.

epoll.fileno()

   Return the file descriptor number of the control fd.

epoll.fromfd(fd)

   Create an epoll object from a given file descriptor.

epoll.register(fd[, eventmask])

   Register a fd descriptor with the epoll object.

epoll.modify(fd, eventmask)

   Modify a registered file descriptor.

epoll.unregister(fd)

   Remove a registered file descriptor from the epoll object.

   在 3.9 版更改: The method no longer ignores the "EBADF" error.

epoll.poll(timeout=None, maxevents=-1)

   Wait for events. timeout in seconds (float)

   在 3.5 版更改: The function is now retried with a recomputed
   timeout when interrupted by a signal, except if the signal handler
   raises an exception (see **PEP 475** for the rationale), instead of
   raising "InterruptedError".


Polling Objects
===============

The "poll()" system call, supported on most Unix systems, provides
better scalability for network servers that service many, many clients
at the same time. "poll()" scales better because the system call only
requires listing the file descriptors of interest, while "select()"
builds a bitmap, turns on bits for the fds of interest, and then
afterward the whole bitmap has to be linearly scanned again.
"select()" is O(highest file descriptor), while "poll()" is O(number
of file descriptors).

poll.register(fd[, eventmask])

   Register a file descriptor with the polling object.  Future calls
   to the "poll()" method will then check whether the file descriptor
   has any pending I/O events.  *fd* can be either an integer, or an
   object with a "fileno()" method that returns an integer.  File
   objects implement "fileno()", so they can also be used as the
   argument.

   *eventmask* is an optional bitmask describing the type of events
   you want to check for, and can be a combination of the constants
   "POLLIN", "POLLPRI", and "POLLOUT", described in the table below.
   If not specified, the default value used will check for all 3 types
   of events.

   +---------------------+--------------------------------------------+
   | 常数                | 意义                                       |
   |=====================|============================================|
   | "POLLIN"            | 有要读取的数据                             |
   +---------------------+--------------------------------------------+
   | "POLLPRI"           | 有紧急数据需要读取                         |
   +---------------------+--------------------------------------------+
   | "POLLOUT"           | 准备输出：写不会阻塞                       |
   +---------------------+--------------------------------------------+
   | "POLLERR"           | 某种错误条件                               |
   +---------------------+--------------------------------------------+
   | "POLLHUP"           | 挂起                                       |
   +---------------------+--------------------------------------------+
   | "POLLRDHUP"         | 流套接字对等体关闭连接，或关闭写入一半连接 |
   +---------------------+--------------------------------------------+
   | "POLLNVAL"          | 无效的请求：描述符未打开                   |
   +---------------------+--------------------------------------------+

   Registering a file descriptor that's already registered is not an
   error, and has the same effect as registering the descriptor
   exactly once.

poll.modify(fd, eventmask)

   Modifies an already registered fd. This has the same effect as
   "register(fd, eventmask)".  Attempting to modify a file descriptor
   that was never registered causes an "OSError" exception with errno
   "ENOENT" to be raised.

poll.unregister(fd)

   Remove a file descriptor being tracked by a polling object.  Just
   like the "register()" method, *fd* can be an integer or an object
   with a "fileno()" method that returns an integer.

   Attempting to remove a file descriptor that was never registered
   causes a "KeyError" exception to be raised.

poll.poll([timeout])

   Polls the set of registered file descriptors, and returns a
   possibly-empty list containing "(fd, event)" 2-tuples for the
   descriptors that have events or errors to report. *fd* is the file
   descriptor, and *event* is a bitmask with bits set for the reported
   events for that descriptor --- "POLLIN" for waiting input,
   "POLLOUT" to indicate that the descriptor can be written to, and so
   forth. An empty list indicates that the call timed out and no file
   descriptors had any events to report. If *timeout* is given, it
   specifies the length of time in milliseconds which the system will
   wait for events before returning. If *timeout* is omitted,
   negative, or "None", the call will block until there is an event
   for this poll object.

   在 3.5 版更改: The function is now retried with a recomputed
   timeout when interrupted by a signal, except if the signal handler
   raises an exception (see **PEP 475** for the rationale), instead of
   raising "InterruptedError".


Kqueue 对象
===========

kqueue.close()

   Close the control file descriptor of the kqueue object.

kqueue.closed

   "True" if the kqueue object is closed.

kqueue.fileno()

   Return the file descriptor number of the control fd.

kqueue.fromfd(fd)

   Create a kqueue object from a given file descriptor.

kqueue.control(changelist, max_events[, timeout]) -> eventlist

   Low level interface to kevent

   * changelist must be an iterable of kevent objects or "None"

   * max_events must be 0 or a positive integer

   * timeout in seconds (floats possible); the default is "None", to
     wait forever

   在 3.5 版更改: The function is now retried with a recomputed
   timeout when interrupted by a signal, except if the signal handler
   raises an exception (see **PEP 475** for the rationale), instead of
   raising "InterruptedError".


Kevent 对象
===========

https://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2

kevent.ident

   Value used to identify the event. The interpretation depends on the
   filter but it's usually the file descriptor. In the constructor
   ident can either be an int or an object with a "fileno()" method.
   kevent stores the integer internally.

kevent.filter

   内核过滤器的名称。

   +-----------------------------+-----------------------------------------------+
   | 常数                        | 意义                                          |
   |=============================|===============================================|
   | "KQ_FILTER_READ"            | 获取描述符，并在有数据可读时返回              |
   +-----------------------------+-----------------------------------------------+
   | "KQ_FILTER_WRITE"           | 获取描述符，并在有数据可写时返回              |
   +-----------------------------+-----------------------------------------------+
   | "KQ_FILTER_AIO"             | AIO 请求                                      |
   +-----------------------------+-----------------------------------------------+
   | "KQ_FILTER_VNODE"           | 当在 *fflag* 中监视的一个或多个请求事件发生时 |
   |                             | 返回                                          |
   +-----------------------------+-----------------------------------------------+
   | "KQ_FILTER_PROC"            | 监视进程ID上的事件                            |
   +-----------------------------+-----------------------------------------------+
   | "KQ_FILTER_NETDEV"          | 观察网络设备上的事件[在Mac OS X上不可用]      |
   +-----------------------------+-----------------------------------------------+
   | "KQ_FILTER_SIGNAL"          | 每当监视的信号传递到进程时返回                |
   +-----------------------------+-----------------------------------------------+
   | "KQ_FILTER_TIMER"           | 建立一个任意的计时器                          |
   +-----------------------------+-----------------------------------------------+

kevent.flags

   筛选器操作。

   +-----------------------------+-----------------------------------------------+
   | 常数                        | 意义                                          |
   |=============================|===============================================|
   | "KQ_EV_ADD"                 | 添加或修改事件                                |
   +-----------------------------+-----------------------------------------------+
   | "KQ_EV_DELETE"              | 从队列中删除事件                              |
   +-----------------------------+-----------------------------------------------+
   | "KQ_EV_ENABLE"              | Permitscontrol() 返回事件                     |
   +-----------------------------+-----------------------------------------------+
   | "KQ_EV_DISABLE"             | 禁用事件                                      |
   +-----------------------------+-----------------------------------------------+
   | "KQ_EV_ONESHOT"             | 在第一次发生后删除事件                        |
   +-----------------------------+-----------------------------------------------+
   | "KQ_EV_CLEAR"               | 检索事件后重置状态                            |
   +-----------------------------+-----------------------------------------------+
   | "KQ_EV_SYSFLAGS"            | 内部事件                                      |
   +-----------------------------+-----------------------------------------------+
   | "KQ_EV_FLAG1"               | 内部事件                                      |
   +-----------------------------+-----------------------------------------------+
   | "KQ_EV_EOF"                 | 筛选特定EOF条件                               |
   +-----------------------------+-----------------------------------------------+
   | "KQ_EV_ERROR"               | 请参阅返回值                                  |
   +-----------------------------+-----------------------------------------------+

kevent.fflags

   筛选特定标志。

   "KQ_FILTER_READ" 和  "KQ_FILTER_WRITE" 过滤标志：

   +------------------------------+----------------------------------------------+
   | 常数                         | 意义                                         |
   |==============================|==============================================|
   | "KQ_NOTE_LOWAT"              | 套接字缓冲区的低水线                         |
   +------------------------------+----------------------------------------------+

   "KQ_FILTER_VNODE" 过滤标志：

   +------------------------------+----------------------------------------------+
   | 常数                         | 意义                                         |
   |==============================|==============================================|
   | "KQ_NOTE_DELETE"             | 已调用 *unlink()*                            |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_WRITE"              | 发生写入                                     |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_EXTEND"             | 文件已扩展                                   |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_ATTRIB"             | 属性已更改                                   |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_LINK"               | 链接计数已更改                               |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_RENAME"             | 文件已重命名                                 |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_REVOKE"             | 对文件的访问权限已被撤销                     |
   +------------------------------+----------------------------------------------+

   "KQ_FILTER_PROC" filter flags:

   +------------------------------+----------------------------------------------+
   | 常数                         | 意义                                         |
   |==============================|==============================================|
   | "KQ_NOTE_EXIT"               | 进程已退出                                   |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_FORK"               | 该进程调用了 *fork()*                        |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_EXEC"               | 进程已执行新进程                             |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_PCTRLMASK"          | 内部过滤器标志                               |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_PDATAMASK"          | 内部过滤器标志                               |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_TRACK"              | 跨  *fork()* 执行进程                        |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_CHILD"              | 在  *NOTE_TRACK* 的子进程上返回              |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_TRACKERR"           | 无法附加到子对象                             |
   +------------------------------+----------------------------------------------+

   "KQ_FILTER_NETDEV" 过滤器标志（在Mac OS X上不可用）：

   +------------------------------+----------------------------------------------+
   | 常数                         | 意义                                         |
   |==============================|==============================================|
   | "KQ_NOTE_LINKUP"             | 链接已建立                                   |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_LINKDOWN"           | 链接已断开                                   |
   +------------------------------+----------------------------------------------+
   | "KQ_NOTE_LINKINV"            | 链接状态无效                                 |
   +------------------------------+----------------------------------------------+

kevent.data

   过滤特定数据。

kevent.udata

   用户定义的值。
