# `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.

備註

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。

  3.3 版更變: 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 版新加入.

  3.4 版更變: 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.

  3.3 版更變: 增加了 flags 参数。

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

  3.4 版後已棄用: 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.

  3.4 版更變: 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).

  備註

  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.

  3.5 版更變: 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 版新加入.

## `/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 版新加入.

`devpoll.``closed`

  `True` if the polling object is closed.

  3.4 版新加入.

`devpoll.``fileno`()

  Return the file descriptor number of the polling object.

  3.4 版新加入.

`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.

  3.5 版更變: 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 版新加入: `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.

  3.9 版更變: The method no longer ignores the `EBADF` error.

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

  Wait for events. timeout in seconds (float)

  3.5 版更變: 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.

  3.5 版更變: 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

  3.5 版更變: 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。