647 lines
28 KiB
ReStructuredText
647 lines
28 KiB
ReStructuredText
: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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--------------
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This module provides access to the :c:func:`select` and :c:func:`poll` functions
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available in most operating systems, :c:func:`devpoll` available on
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Solaris and derivatives, :c:func:`epoll` available on Linux 2.5+ and
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:c:func:`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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.. note::
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The :mod:`selectors` module allows high-level and efficient I/O
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multiplexing, built upon the :mod:`select` module primitives. Users are
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encouraged to use the :mod:`selectors` module instead, unless they want
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precise control over the OS-level primitives used.
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The module defines the following:
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.. exception:: error
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A deprecated alias of :exc:`OSError`.
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.. versionchanged:: 3.3
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Following :pep:`3151`, this class was made an alias of :exc:`OSError`.
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.. function:: devpoll()
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(Only supported on Solaris and derivatives.) Returns a ``/dev/poll``
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polling object; see section :ref:`devpoll-objects` below for the
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methods supported by devpoll objects.
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:c:func:`devpoll` objects are linked to the number of file
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descriptors allowed at the time of instantiation. If your program
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reduces this value, :c:func:`devpoll` will fail. If your program
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increases this value, :c:func:`devpoll` may return an
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incomplete list of active file descriptors.
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The new file descriptor is :ref:`non-inheritable <fd_inheritance>`.
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.. versionadded:: 3.3
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.. versionchanged:: 3.4
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The new file descriptor is now non-inheritable.
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.. function:: epoll(sizehint=-1, flags=0)
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(Only supported on Linux 2.5.44 and newer.) Return an edge polling object,
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which can be used as Edge or Level Triggered interface for I/O
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events.
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*sizehint* informs epoll about the expected number of events to be
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registered. It must be positive, or `-1` to use the default. It is only
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used on older systems where :c:func:`epoll_create1` is not available;
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otherwise it has no effect (though its value is still checked).
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*flags* is deprecated and completely ignored. However, when supplied, its
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value must be ``0`` or ``select.EPOLL_CLOEXEC``, otherwise ``OSError`` is
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raised.
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See the :ref:`epoll-objects` section below for the methods supported by
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epolling objects.
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``epoll`` objects support the context management protocol: when used in a
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:keyword:`with` statement, the new file descriptor is automatically closed
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at the end of the block.
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The new file descriptor is :ref:`non-inheritable <fd_inheritance>`.
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.. versionchanged:: 3.3
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Added the *flags* parameter.
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.. versionchanged:: 3.4
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Support for the :keyword:`with` statement was added.
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The new file descriptor is now non-inheritable.
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.. deprecated:: 3.4
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The *flags* parameter. ``select.EPOLL_CLOEXEC`` is used by default now.
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Use :func:`os.set_inheritable` to make the file descriptor inheritable.
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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; see section
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:ref:`kqueue-objects` below for the methods supported by kqueue objects.
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The new file descriptor is :ref:`non-inheritable <fd_inheritance>`.
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.. versionchanged:: 3.4
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The new file descriptor is now non-inheritable.
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.. function:: kevent(ident, filter=KQ_FILTER_READ, flags=KQ_EV_ADD, fflags=0, data=0, udata=0)
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(Only supported on BSD.) Returns a kernel event object; see section
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:ref:`kevent-objects` below for the methods supported by kevent objects.
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.. function:: select(rlist, wlist, xlist[, timeout])
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This is a straightforward interface to the Unix :c:func:`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:`~io.IOBase.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 :term:`file
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objects <file object>` (e.g. ``sys.stdin``, or objects returned by
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:func:`open` or :func:`os.popen`), socket objects returned by
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:func:`socket.socket`. You may also define a :dfn:`wrapper` class yourself,
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as long as it has an appropriate :meth:`~io.IOBase.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 :c:func:`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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.. versionchanged:: 3.5
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The function is now retried with a recomputed timeout when interrupted by
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a signal, except if the signal handler raises an exception (see
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:pep:`475` for the rationale), instead of raising
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:exc:`InterruptedError`.
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.. attribute:: PIPE_BUF
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The minimum number of bytes which can be written without blocking to a pipe
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when the pipe has been reported as ready for writing by :func:`~select.select`,
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:func:`poll` or another interface in this module. This doesn't apply
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to other kind of file-like objects such as sockets.
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This value is guaranteed by POSIX to be at least 512.
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.. availability:: Unix
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.. versionadded:: 3.2
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.. _devpoll-objects:
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``/dev/poll`` Polling Objects
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-----------------------------
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Solaris and derivatives have ``/dev/poll``. While :c:func:`select` is
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O(highest file descriptor) and :c:func:`poll` is O(number of file
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descriptors), ``/dev/poll`` is O(active file descriptors).
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``/dev/poll`` behaviour is very close to the standard :c:func:`poll`
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object.
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.. method:: devpoll.close()
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Close the file descriptor of the polling object.
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.. versionadded:: 3.4
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.. attribute:: devpoll.closed
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``True`` if the polling object is closed.
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.. versionadded:: 3.4
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.. method:: devpoll.fileno()
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Return the file descriptor number of the polling object.
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.. versionadded:: 3.4
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.. method:: devpoll.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
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pending I/O events. *fd* can be either an integer, or an object with a
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:meth:`~io.IOBase.fileno` method that returns an integer. File objects
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implement :meth:`!fileno`, so they 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. The constants are the same that with :c:func:`poll`
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object. The default value is a combination of the constants :const:`POLLIN`,
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:const:`POLLPRI`, and :const:`POLLOUT`.
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.. warning::
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Registering a file descriptor that's already registered is not an
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error, but the result is undefined. The appropriate action is to
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unregister or modify it first. This is an important difference
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compared with :c:func:`poll`.
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.. method:: devpoll.modify(fd[, eventmask])
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This method does an :meth:`unregister` followed by a
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:meth:`register`. It is (a bit) more efficient that doing the same
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explicitly.
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.. method:: devpoll.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:`~io.IOBase.fileno` method that returns an integer.
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Attempting to remove a file descriptor that was never registered is
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safely ignored.
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.. method:: devpoll.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, -1, or :const:`None`, the call will
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block until there is an event for this poll object.
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.. versionchanged:: 3.5
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The function is now retried with a recomputed timeout when interrupted by
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a signal, except if the signal handler raises an exception (see
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:pep:`475` for the rationale), instead of raising
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:exc:`InterruptedError`.
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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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https://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:`EPOLLEXCLUSIVE` | Wake only one epoll object when the |
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| | associated fd has an event. The default (if |
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| | this flag is not set) is to wake all epoll |
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| | objects polling on a fd. |
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+-------------------------+-----------------------------------------------+
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| :const:`EPOLLRDHUP` | Stream socket peer closed connection or shut |
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| | down writing half of connection. |
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+-------------------------+-----------------------------------------------+
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| :const:`EPOLLRDNORM` | Equivalent to :const:`EPOLLIN` |
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+-------------------------+-----------------------------------------------+
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| :const:`EPOLLRDBAND` | Priority data band can be read. |
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+-------------------------+-----------------------------------------------+
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| :const:`EPOLLWRNORM` | Equivalent to :const:`EPOLLOUT` |
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+-------------------------+-----------------------------------------------+
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| :const:`EPOLLWRBAND` | Priority data may be written. |
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+-------------------------+-----------------------------------------------+
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| :const:`EPOLLMSG` | Ignored. |
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+-------------------------+-----------------------------------------------+
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.. versionadded:: 3.6
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:const:`EPOLLEXCLUSIVE` was added. It's only supported by Linux Kernel 4.5
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or later.
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.. method:: epoll.close()
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Close the control file descriptor of the epoll object.
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.. attribute:: epoll.closed
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``True`` if the epoll object is closed.
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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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.. method:: epoll.modify(fd, eventmask)
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Modify a registered 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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.. versionchanged:: 3.5
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The function is now retried with a recomputed timeout when interrupted by
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a signal, except if the signal handler raises an exception (see
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:pep:`475` for the rationale), instead of raising
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:exc:`InterruptedError`.
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.. _poll-objects:
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Polling Objects
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---------------
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The :c:func:`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. :c:func:`poll` scales better because the system call only requires listing
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the file descriptors of interest, while :c:func:`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. :c:func:`select` is O(highest file descriptor), while
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:c:func:`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
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pending I/O events. *fd* can be either an integer, or an object with a
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:meth:`~io.IOBase.fileno` method that returns an integer. File objects
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implement :meth:`!fileno`, so they 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:`POLLRDHUP`| Stream socket peer closed connection, or |
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| | shut down writing half of connection |
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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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``register(fd, eventmask)``. Attempting to modify a file descriptor
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that was never registered causes an :exc:`OSError` exception with errno
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:const:`ENOENT` to be raised.
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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:`~io.IOBase.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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|
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.. versionchanged:: 3.5
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The function is now retried with a recomputed timeout when interrupted by
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a signal, except if the signal handler raises an exception (see
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:pep:`475` for the rationale), instead of raising
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:exc:`InterruptedError`.
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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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.. attribute:: kqueue.closed
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``True`` if the kqueue object is closed.
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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]) -> eventlist
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Low level interface to kevent
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- changelist must be an iterable of kevent objects or ``None``
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- max_events must be 0 or a positive integer
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- timeout in seconds (floats possible); the default is ``None``,
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to wait forever
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.. versionchanged:: 3.5
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The function is now retried with a recomputed timeout when interrupted by
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a signal, except if the signal handler raises an exception (see
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:pep:`475` for the rationale), instead of raising
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:exc:`InterruptedError`.
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.. _kevent-objects:
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Kevent Objects
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--------------
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https://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 :meth:`~io.IOBase.fileno` method. kevent
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stores the integer 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 |
|
|
| | there is data available to write |
|
|
+---------------------------+---------------------------------------------+
|
|
| :const:`KQ_FILTER_AIO` | AIO requests |
|
|
+---------------------------+---------------------------------------------+
|
|
| :const:`KQ_FILTER_VNODE` | Returns when one or more of the requested |
|
|
| | events watched in *fflag* occurs |
|
|
+---------------------------+---------------------------------------------+
|
|
| :const:`KQ_FILTER_PROC` | Watch for events on a process id |
|
|
+---------------------------+---------------------------------------------+
|
|
| :const:`KQ_FILTER_NETDEV` | Watch for events on a network device |
|
|
| | [not available on Mac OS X] |
|
|
+---------------------------+---------------------------------------------+
|
|
| :const:`KQ_FILTER_SIGNAL` | Returns whenever the watched signal is |
|
|
| | delivered to the process |
|
|
+---------------------------+---------------------------------------------+
|
|
| :const:`KQ_FILTER_TIMER` | Establishes an arbitrary timer |
|
|
+---------------------------+---------------------------------------------+
|
|
|
|
.. attribute:: kevent.flags
|
|
|
|
Filter action.
|
|
|
|
+---------------------------+---------------------------------------------+
|
|
| Constant | Meaning |
|
|
+===========================+=============================================+
|
|
| :const:`KQ_EV_ADD` | Adds or modifies an event |
|
|
+---------------------------+---------------------------------------------+
|
|
| :const:`KQ_EV_DELETE` | Removes an event from the queue |
|
|
+---------------------------+---------------------------------------------+
|
|
| :const:`KQ_EV_ENABLE` | Permitscontrol() to returns the event |
|
|
+---------------------------+---------------------------------------------+
|
|
| :const:`KQ_EV_DISABLE` | Disablesevent |
|
|
+---------------------------+---------------------------------------------+
|
|
| :const:`KQ_EV_ONESHOT` | Removes event after first occurrence |
|
|
+---------------------------+---------------------------------------------+
|
|
| :const:`KQ_EV_CLEAR` | Reset the state after an event is retrieved |
|
|
+---------------------------+---------------------------------------------+
|
|
| :const:`KQ_EV_SYSFLAGS` | internal event |
|
|
+---------------------------+---------------------------------------------+
|
|
| :const:`KQ_EV_FLAG1` | internal event |
|
|
+---------------------------+---------------------------------------------+
|
|
| :const:`KQ_EV_EOF` | Filter specific EOF condition |
|
|
+---------------------------+---------------------------------------------+
|
|
| :const:`KQ_EV_ERROR` | See return values |
|
|
+---------------------------+---------------------------------------------+
|
|
|
|
|
|
.. attribute:: kevent.fflags
|
|
|
|
Filter specific flags.
|
|
|
|
:const:`KQ_FILTER_READ` and :const:`KQ_FILTER_WRITE` filter flags:
|
|
|
|
+----------------------------+--------------------------------------------+
|
|
| Constant | Meaning |
|
|
+============================+============================================+
|
|
| :const:`KQ_NOTE_LOWAT` | low water mark of a socket buffer |
|
|
+----------------------------+--------------------------------------------+
|
|
|
|
:const:`KQ_FILTER_VNODE` filter flags:
|
|
|
|
+----------------------------+--------------------------------------------+
|
|
| Constant | Meaning |
|
|
+============================+============================================+
|
|
| :const:`KQ_NOTE_DELETE` | *unlink()* was called |
|
|
+----------------------------+--------------------------------------------+
|
|
| :const:`KQ_NOTE_WRITE` | a write occurred |
|
|
+----------------------------+--------------------------------------------+
|
|
| :const:`KQ_NOTE_EXTEND` | the file was extended |
|
|
+----------------------------+--------------------------------------------+
|
|
| :const:`KQ_NOTE_ATTRIB` | an attribute was changed |
|
|
+----------------------------+--------------------------------------------+
|
|
| :const:`KQ_NOTE_LINK` | the link count has changed |
|
|
+----------------------------+--------------------------------------------+
|
|
| :const:`KQ_NOTE_RENAME` | the file was renamed |
|
|
+----------------------------+--------------------------------------------+
|
|
| :const:`KQ_NOTE_REVOKE` | access to the file was revoked |
|
|
+----------------------------+--------------------------------------------+
|
|
|
|
:const:`KQ_FILTER_PROC` filter flags:
|
|
|
|
+----------------------------+--------------------------------------------+
|
|
| Constant | Meaning |
|
|
+============================+============================================+
|
|
| :const:`KQ_NOTE_EXIT` | the process has exited |
|
|
+----------------------------+--------------------------------------------+
|
|
| :const:`KQ_NOTE_FORK` | the process has called *fork()* |
|
|
+----------------------------+--------------------------------------------+
|
|
| :const:`KQ_NOTE_EXEC` | the process has executed a new process |
|
|
+----------------------------+--------------------------------------------+
|
|
| :const:`KQ_NOTE_PCTRLMASK` | internal filter flag |
|
|
+----------------------------+--------------------------------------------+
|
|
| :const:`KQ_NOTE_PDATAMASK` | internal filter flag |
|
|
+----------------------------+--------------------------------------------+
|
|
| :const:`KQ_NOTE_TRACK` | follow a process across *fork()* |
|
|
+----------------------------+--------------------------------------------+
|
|
| :const:`KQ_NOTE_CHILD` | returned on the child process for |
|
|
| | *NOTE_TRACK* |
|
|
+----------------------------+--------------------------------------------+
|
|
| :const:`KQ_NOTE_TRACKERR` | unable to attach to a child |
|
|
+----------------------------+--------------------------------------------+
|
|
|
|
:const:`KQ_FILTER_NETDEV` filter flags (not available on Mac OS X):
|
|
|
|
+----------------------------+--------------------------------------------+
|
|
| Constant | Meaning |
|
|
+============================+============================================+
|
|
| :const:`KQ_NOTE_LINKUP` | link is up |
|
|
+----------------------------+--------------------------------------------+
|
|
| :const:`KQ_NOTE_LINKDOWN` | link is down |
|
|
+----------------------------+--------------------------------------------+
|
|
| :const:`KQ_NOTE_LINKINV` | link state is invalid |
|
|
+----------------------------+--------------------------------------------+
|
|
|
|
|
|
.. attribute:: kevent.data
|
|
|
|
Filter specific data.
|
|
|
|
|
|
.. attribute:: kevent.udata
|
|
|
|
User defined value.
|