451 lines
20 KiB
ReStructuredText
451 lines
20 KiB
ReStructuredText
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:mod:`select` --- Waiting for I/O completion
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============================================
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.. module:: select
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:synopsis: Wait for I/O completion on multiple streams.
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This module provides access to the :cfunc:`select` and :cfunc:`poll` functions
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available in most operating systems, :cfunc:`epoll` available on Linux 2.5+ and
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:cfunc:`kqueue` available on most BSD.
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Note that on Windows, it only works for sockets; on other operating systems,
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it also works for other file types (in particular, on Unix, it works on pipes).
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It cannot be used on regular files to determine whether a file has grown since
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it was last read.
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The module defines the following:
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.. exception:: error
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The exception raised when an error occurs. The accompanying value is a pair
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containing the numeric error code from :cdata:`errno` and the corresponding
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string, as would be printed by the C function :cfunc:`perror`.
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.. function:: epoll([sizehint=-1])
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(Only supported on Linux 2.5.44 and newer.) Returns an edge polling object,
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which can be used as Edge or Level Triggered interface for I/O events; see
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section :ref:`epoll-objects` below for the methods supported by epolling
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objects.
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.. versionadded:: 2.6
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.. function:: poll()
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(Not supported by all operating systems.) Returns a polling object, which
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supports registering and unregistering file descriptors, and then polling them
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for I/O events; see section :ref:`poll-objects` below for the methods supported
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by polling objects.
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.. function:: kqueue()
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(Only supported on BSD.) Returns a kernel queue object object; see section
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:ref:`kqueue-objects` below for the methods supported by kqueue objects.
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.. versionadded:: 2.6
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.. function:: kevent(ident, filter=KQ_FILTER_READ, flags=KQ_ADD, fflags=0, data=0, udata=0)
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(Only supported on BSD.) Returns a kernel event object object; see section
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:ref:`kevent-objects` below for the methods supported by kqueue objects.
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.. versionadded:: 2.6
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.. function:: select(rlist, wlist, xlist[, timeout])
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This is a straightforward interface to the Unix :cfunc:`select` system call.
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The first three arguments are sequences of 'waitable objects': either
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integers representing file descriptors or objects with a parameterless method
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named :meth:`fileno` returning such an integer:
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* *rlist*: wait until ready for reading
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* *wlist*: wait until ready for writing
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* *xlist*: wait for an "exceptional condition" (see the manual page for what
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your system considers such a condition)
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Empty sequences are allowed, but acceptance of three empty sequences is
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platform-dependent. (It is known to work on Unix but not on Windows.) The
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optional *timeout* argument specifies a time-out as a floating point number
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in seconds. When the *timeout* argument is omitted the function blocks until
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at least one file descriptor is ready. A time-out value of zero specifies a
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poll and never blocks.
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The return value is a triple of lists of objects that are ready: subsets of the
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first three arguments. When the time-out is reached without a file descriptor
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becoming ready, three empty lists are returned.
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.. index::
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single: socket() (in module socket)
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single: popen() (in module os)
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Among the acceptable object types in the sequences are Python file objects (e.g.
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``sys.stdin``, or objects returned by :func:`open` or :func:`os.popen`), socket
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objects returned by :func:`socket.socket`. You may also define a :dfn:`wrapper`
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class yourself, as long as it has an appropriate :meth:`fileno` method (that
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really returns a file descriptor, not just a random integer).
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.. note::
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.. index:: single: WinSock
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File objects on Windows are not acceptable, but sockets are. On Windows,
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the underlying :cfunc:`select` function is provided by the WinSock
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library, and does not handle file descriptors that don't originate from
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WinSock.
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.. attribute:: select.PIPE_BUF
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Files reported as ready for writing by :func:`select`, :func:`poll` or
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similar interfaces in this module are guaranteed to not block on a write
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of up to :const:`PIPE_BUF` bytes.
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This value is guaranteed by POSIX to be at least 512. Availability: Unix.
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.. versionadded:: 2.7
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.. _epoll-objects:
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Edge and Level Trigger Polling (epoll) Objects
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----------------------------------------------
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http://linux.die.net/man/4/epoll
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*eventmask*
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+-----------------------+-----------------------------------------------+
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| Constant | Meaning |
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+=======================+===============================================+
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| :const:`EPOLLIN` | Available for read |
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+-----------------------+-----------------------------------------------+
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| :const:`EPOLLOUT` | Available for write |
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+-----------------------+-----------------------------------------------+
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| :const:`EPOLLPRI` | Urgent data for read |
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+-----------------------+-----------------------------------------------+
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| :const:`EPOLLERR` | Error condition happened on the assoc. fd |
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+-----------------------+-----------------------------------------------+
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| :const:`EPOLLHUP` | Hang up happened on the assoc. fd |
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+-----------------------+-----------------------------------------------+
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| :const:`EPOLLET` | Set Edge Trigger behavior, the default is |
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| | Level Trigger behavior |
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+-----------------------+-----------------------------------------------+
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| :const:`EPOLLONESHOT` | Set one-shot behavior. After one event is |
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| | pulled out, the fd is internally disabled |
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+-----------------------+-----------------------------------------------+
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| :const:`EPOLLRDNORM` | ??? |
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+-----------------------+-----------------------------------------------+
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| :const:`EPOLLRDBAND` | ??? |
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+-----------------------+-----------------------------------------------+
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| :const:`EPOLLWRNORM` | ??? |
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+-----------------------+-----------------------------------------------+
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| :const:`EPOLLWRBAND` | ??? |
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+-----------------------+-----------------------------------------------+
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| :const:`EPOLLMSG` | ??? |
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+-----------------------+-----------------------------------------------+
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.. method:: epoll.close()
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Close the control file descriptor of the epoll object.
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.. method:: epoll.fileno()
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Return the file descriptor number of the control fd.
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.. method:: epoll.fromfd(fd)
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Create an epoll object from a given file descriptor.
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.. method:: epoll.register(fd[, eventmask])
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Register a fd descriptor with the epoll object.
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.. note::
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Registering a file descriptor that's already registered raises an
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IOError -- contrary to :ref:`poll-objects`'s register.
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.. method:: epoll.modify(fd, eventmask)
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Modify a register file descriptor.
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.. method:: epoll.unregister(fd)
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Remove a registered file descriptor from the epoll object.
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.. method:: epoll.poll([timeout=-1[, maxevents=-1]])
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Wait for events. timeout in seconds (float)
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.. _poll-objects:
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Polling Objects
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---------------
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The :cfunc:`poll` system call, supported on most Unix systems, provides better
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scalability for network servers that service many, many clients at the same
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time. :cfunc:`poll` scales better because the system call only requires listing
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the file descriptors of interest, while :cfunc:`select` builds a bitmap, turns
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on bits for the fds of interest, and then afterward the whole bitmap has to be
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linearly scanned again. :cfunc:`select` is O(highest file descriptor), while
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:cfunc:`poll` is O(number of file descriptors).
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.. method:: poll.register(fd[, eventmask])
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Register a file descriptor with the polling object. Future calls to the
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:meth:`poll` method will then check whether the file descriptor has any pending
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I/O events. *fd* can be either an integer, or an object with a :meth:`fileno`
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method that returns an integer. File objects implement :meth:`fileno`, so they
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can also be used as the argument.
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*eventmask* is an optional bitmask describing the type of events you want to
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check for, and can be a combination of the constants :const:`POLLIN`,
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:const:`POLLPRI`, and :const:`POLLOUT`, described in the table below. If not
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specified, the default value used will check for all 3 types of events.
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+-------------------+------------------------------------------+
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| Constant | Meaning |
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+===================+==========================================+
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| :const:`POLLIN` | There is data to read |
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+-------------------+------------------------------------------+
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| :const:`POLLPRI` | There is urgent data to read |
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+-------------------+------------------------------------------+
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| :const:`POLLOUT` | Ready for output: writing will not block |
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+-------------------+------------------------------------------+
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| :const:`POLLERR` | Error condition of some sort |
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+-------------------+------------------------------------------+
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| :const:`POLLHUP` | Hung up |
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+-------------------+------------------------------------------+
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| :const:`POLLNVAL` | Invalid request: descriptor not open |
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+-------------------+------------------------------------------+
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Registering a file descriptor that's already registered is not an error, and has
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the same effect as registering the descriptor exactly once.
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.. method:: poll.modify(fd, eventmask)
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Modifies an already registered fd. This has the same effect as
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:meth:`register(fd, eventmask)`. Attempting to modify a file descriptor
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that was never registered causes an :exc:`IOError` exception with errno
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:const:`ENOENT` to be raised.
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.. versionadded:: 2.6
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.. method:: poll.unregister(fd)
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Remove a file descriptor being tracked by a polling object. Just like the
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:meth:`register` method, *fd* can be an integer or an object with a
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:meth:`fileno` method that returns an integer.
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Attempting to remove a file descriptor that was never registered causes a
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:exc:`KeyError` exception to be raised.
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.. method:: poll.poll([timeout])
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Polls the set of registered file descriptors, and returns a possibly-empty list
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containing ``(fd, event)`` 2-tuples for the descriptors that have events or
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errors to report. *fd* is the file descriptor, and *event* is a bitmask with
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bits set for the reported events for that descriptor --- :const:`POLLIN` for
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waiting input, :const:`POLLOUT` to indicate that the descriptor can be written
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to, and so forth. An empty list indicates that the call timed out and no file
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descriptors had any events to report. If *timeout* is given, it specifies the
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length of time in milliseconds which the system will wait for events before
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returning. If *timeout* is omitted, negative, or :const:`None`, the call will
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block until there is an event for this poll object.
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.. _kqueue-objects:
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Kqueue Objects
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--------------
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.. method:: kqueue.close()
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Close the control file descriptor of the kqueue object.
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.. method:: kqueue.fileno()
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Return the file descriptor number of the control fd.
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.. method:: kqueue.fromfd(fd)
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Create a kqueue object from a given file descriptor.
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.. method:: kqueue.control(changelist, max_events[, timeout=None]) -> eventlist
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Low level interface to kevent
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- changelist must be an iterable of kevent object or None
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- max_events must be 0 or a positive integer
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- timeout in seconds (floats possible)
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.. _kevent-objects:
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Kevent Objects
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--------------
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http://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2
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.. attribute:: kevent.ident
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Value used to identify the event. The interpretation depends on the filter
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but it's usually the file descriptor. In the constructor ident can either
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be an int or an object with a fileno() function. kevent stores the integer
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internally.
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.. attribute:: kevent.filter
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Name of the kernel filter.
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+---------------------------+---------------------------------------------+
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| Constant | Meaning |
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+===========================+=============================================+
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| :const:`KQ_FILTER_READ` | Takes a descriptor and returns whenever |
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| | there is data available to read |
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+---------------------------+---------------------------------------------+
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| :const:`KQ_FILTER_WRITE` | Takes a descriptor and returns whenever |
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| | there is data available to write |
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+---------------------------+---------------------------------------------+
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| :const:`KQ_FILTER_AIO` | AIO requests |
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+---------------------------+---------------------------------------------+
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| :const:`KQ_FILTER_VNODE` | Returns when one or more of the requested |
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| | events watched in *fflag* occurs |
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+---------------------------+---------------------------------------------+
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| :const:`KQ_FILTER_PROC` | Watch for events on a process id |
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+---------------------------+---------------------------------------------+
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| :const:`KQ_FILTER_NETDEV` | Watch for events on a network device |
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| | [not available on Mac OS X] |
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+---------------------------+---------------------------------------------+
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| :const:`KQ_FILTER_SIGNAL` | Returns whenever the watched signal is |
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| | delivered to the process |
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+---------------------------+---------------------------------------------+
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| :const:`KQ_FILTER_TIMER` | Establishes an arbitrary timer |
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+---------------------------+---------------------------------------------+
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.. attribute:: kevent.flags
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Filter action.
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+---------------------------+---------------------------------------------+
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| Constant | Meaning |
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+===========================+=============================================+
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| :const:`KQ_EV_ADD` | Adds or modifies an event |
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+---------------------------+---------------------------------------------+
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| :const:`KQ_EV_DELETE` | Removes an event from the queue |
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+---------------------------+---------------------------------------------+
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| :const:`KQ_EV_ENABLE` | Permitscontrol() to returns the event |
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+---------------------------+---------------------------------------------+
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| :const:`KQ_EV_DISABLE` | Disablesevent |
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+---------------------------+---------------------------------------------+
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| :const:`KQ_EV_ONESHOT` | Removes event after first occurrence |
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+---------------------------+---------------------------------------------+
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| :const:`KQ_EV_CLEAR` | Reset the state after an event is retrieved |
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+---------------------------+---------------------------------------------+
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| :const:`KQ_EV_SYSFLAGS` | internal event |
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+---------------------------+---------------------------------------------+
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| :const:`KQ_EV_FLAG1` | internal event |
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+---------------------------+---------------------------------------------+
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| :const:`KQ_EV_EOF` | Filter specific EOF condition |
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+---------------------------+---------------------------------------------+
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| :const:`KQ_EV_ERROR` | See return values |
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+---------------------------+---------------------------------------------+
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.. attribute:: kevent.fflags
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Filter specific flags.
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:const:`KQ_FILTER_READ` and :const:`KQ_FILTER_WRITE` filter flags:
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+----------------------------+--------------------------------------------+
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| Constant | Meaning |
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+============================+============================================+
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| :const:`KQ_NOTE_LOWAT` | low water mark of a socket buffer |
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+----------------------------+--------------------------------------------+
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:const:`KQ_FILTER_VNODE` filter flags:
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+----------------------------+--------------------------------------------+
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| Constant | Meaning |
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+============================+============================================+
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| :const:`KQ_NOTE_DELETE` | *unlink()* was called |
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+----------------------------+--------------------------------------------+
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| :const:`KQ_NOTE_WRITE` | a write occurred |
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+----------------------------+--------------------------------------------+
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| :const:`KQ_NOTE_EXTEND` | the file was extended |
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+----------------------------+--------------------------------------------+
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| :const:`KQ_NOTE_ATTRIB` | an attribute was changed |
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+----------------------------+--------------------------------------------+
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| :const:`KQ_NOTE_LINK` | the link count has changed |
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+----------------------------+--------------------------------------------+
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| :const:`KQ_NOTE_RENAME` | the file was renamed |
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+----------------------------+--------------------------------------------+
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| :const:`KQ_NOTE_REVOKE` | access to the file was revoked |
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+----------------------------+--------------------------------------------+
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:const:`KQ_FILTER_PROC` filter flags:
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+----------------------------+--------------------------------------------+
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| Constant | Meaning |
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+============================+============================================+
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| :const:`KQ_NOTE_EXIT` | the process has exited |
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+----------------------------+--------------------------------------------+
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| :const:`KQ_NOTE_FORK` | the process has called *fork()* |
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+----------------------------+--------------------------------------------+
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| :const:`KQ_NOTE_EXEC` | the process has executed a new process |
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+----------------------------+--------------------------------------------+
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| :const:`KQ_NOTE_PCTRLMASK` | internal filter flag |
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+----------------------------+--------------------------------------------+
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| :const:`KQ_NOTE_PDATAMASK` | internal filter flag |
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+----------------------------+--------------------------------------------+
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| :const:`KQ_NOTE_TRACK` | follow a process across *fork()* |
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+----------------------------+--------------------------------------------+
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| :const:`KQ_NOTE_CHILD` | returned on the child process for |
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| | *NOTE_TRACK* |
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+----------------------------+--------------------------------------------+
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| :const:`KQ_NOTE_TRACKERR` | unable to attach to a child |
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+----------------------------+--------------------------------------------+
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:const:`KQ_FILTER_NETDEV` filter flags (not available on Mac OS X):
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+----------------------------+--------------------------------------------+
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| Constant | Meaning |
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+============================+============================================+
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| :const:`KQ_NOTE_LINKUP` | link is up |
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+----------------------------+--------------------------------------------+
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| :const:`KQ_NOTE_LINKDOWN` | link is down |
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+----------------------------+--------------------------------------------+
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| :const:`KQ_NOTE_LINKINV` | link state is invalid |
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+----------------------------+--------------------------------------------+
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.. attribute:: kevent.data
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Filter specific data.
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.. attribute:: kevent.udata
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User defined value.
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