
``select`` --- Waiting for I/O completion
*****************************************

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.

The module defines the following:

exception exception select.error

   A deprecated alias of ``OSError``.

   Changed in version 3.3: Following **PEP 3151**, this class was made
   an alias of ``OSError``.

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.

   New in version 3.3.

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* is deprecated and completely ignored.
   *flags* can be set to ``EPOLL_CLOEXEC``, which causes the epoll
   descriptor to be closed automatically when ``os.execve()`` is
   called. See section *Edge and Level Trigger Polling (epoll)
   Objects* below for the methods supported by epolling objects.

   Changed in version 3.3: Added the *flags* parameter.

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 Objects* below for the methods supported by kqueue
   objects.

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 Objects* 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).

   Note: 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.

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.

   New in version 3.2.


``/dev/poll`` Polling Objects
=============================

   http://developers.sun.com/solaris/articles/using_devpoll.html
   http://developers.sun.com/solaris/articles/polling_efficient.html

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.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``.

   Warning: Registering a file descriptor that's already registered is not an
     error, but the result is undefined. The appropiate 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.


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

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

   *eventmask*

   +-------------------------+-------------------------------------------------+
   | Constant                | Meaning                                         |
   +=========================+=================================================+
   | ``EPOLLIN``             | Available for read                              |
   +-------------------------+-------------------------------------------------+
   | ``EPOLLOUT``            | Available for write                             |
   +-------------------------+-------------------------------------------------+
   | ``EPOLLPRI``            | Urgent data for read                            |
   +-------------------------+-------------------------------------------------+
   | ``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       |
   +-------------------------+-------------------------------------------------+
   | ``EPOLLRDNORM``         | Equivalent to ``EPOLLIN``                       |
   +-------------------------+-------------------------------------------------+
   | ``EPOLLRDBAND``         | Priority data band can be read.                 |
   +-------------------------+-------------------------------------------------+
   | ``EPOLLWRNORM``         | Equivalent to ``EPOLLOUT``                      |
   +-------------------------+-------------------------------------------------+
   | ``EPOLLWRBAND``         | Priority data may be written.                   |
   +-------------------------+-------------------------------------------------+
   | ``EPOLLMSG``            | Ignored.                                        |
   +-------------------------+-------------------------------------------------+

epoll.close()

   Close the control file descriptor of the epoll object.

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 register file descriptor.

epoll.unregister(fd)

   Remove a registered file descriptor from the epoll object.

epoll.poll(timeout=-1, maxevents=-1)

   Wait for events. timeout in seconds (float)


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.

   +---------------------+--------------------------------------------+
   | Constant            | Meaning                                    |
   +=====================+============================================+
   | ``POLLIN``          | There is data to read                      |
   +---------------------+--------------------------------------------+
   | ``POLLPRI``         | There is urgent data to read               |
   +---------------------+--------------------------------------------+
   | ``POLLOUT``         | Ready for output: writing will not block   |
   +---------------------+--------------------------------------------+
   | ``POLLERR``         | Error condition of some sort               |
   +---------------------+--------------------------------------------+
   | ``POLLHUP``         | Hung up                                    |
   +---------------------+--------------------------------------------+
   | ``POLLNVAL``        | Invalid request: descriptor not open       |
   +---------------------+--------------------------------------------+

   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 ``IOError``
   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.


Kqueue Objects
==============

kqueue.close()

   Close the control file descriptor of the kqueue object.

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=None]) -> eventlist

   Low level interface to kevent

   * changelist must be an iterable of kevent object or None

   * max_events must be 0 or a positive integer

   * timeout in seconds (floats possible)


Kevent Objects
==============

http://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() function.
   kevent stores the integer internally.

kevent.filter

   Name of the kernel filter.

   +-----------------------------+-----------------------------------------------+
   | Constant                    | Meaning                                       |
   +=============================+===============================================+
   | ``KQ_FILTER_READ``          | Takes a descriptor and returns whenever there |
   |                             | is data available to read                     |
   +-----------------------------+-----------------------------------------------+
   | ``KQ_FILTER_WRITE``         | Takes a descriptor and returns whenever there |
   |                             | is data available to write                    |
   +-----------------------------+-----------------------------------------------+
   | ``KQ_FILTER_AIO``           | AIO requests                                  |
   +-----------------------------+-----------------------------------------------+
   | ``KQ_FILTER_VNODE``         | Returns when one or more of the requested     |
   |                             | events watched in *fflag* occurs              |
   +-----------------------------+-----------------------------------------------+
   | ``KQ_FILTER_PROC``          | Watch for events on a process id              |
   +-----------------------------+-----------------------------------------------+
   | ``KQ_FILTER_NETDEV``        | Watch for events on a network device [not     |
   |                             | available on Mac OS X]                        |
   +-----------------------------+-----------------------------------------------+
   | ``KQ_FILTER_SIGNAL``        | Returns whenever the watched signal is        |
   |                             | delivered to the process                      |
   +-----------------------------+-----------------------------------------------+
   | ``KQ_FILTER_TIMER``         | Establishes an arbitrary timer                |
   +-----------------------------+-----------------------------------------------+

kevent.flags

   Filter action.

   +-----------------------------+-----------------------------------------------+
   | Constant                    | Meaning                                       |
   +=============================+===============================================+
   | ``KQ_EV_ADD``               | Adds or modifies an event                     |
   +-----------------------------+-----------------------------------------------+
   | ``KQ_EV_DELETE``            | Removes an event from the queue               |
   +-----------------------------+-----------------------------------------------+
   | ``KQ_EV_ENABLE``            | Permitscontrol() to returns the event         |
   +-----------------------------+-----------------------------------------------+
   | ``KQ_EV_DISABLE``           | Disablesevent                                 |
   +-----------------------------+-----------------------------------------------+
   | ``KQ_EV_ONESHOT``           | Removes event after first occurrence          |
   +-----------------------------+-----------------------------------------------+
   | ``KQ_EV_CLEAR``             | Reset the state after an event is retrieved   |
   +-----------------------------+-----------------------------------------------+
   | ``KQ_EV_SYSFLAGS``          | internal event                                |
   +-----------------------------+-----------------------------------------------+
   | ``KQ_EV_FLAG1``             | internal event                                |
   +-----------------------------+-----------------------------------------------+
   | ``KQ_EV_EOF``               | Filter specific EOF condition                 |
   +-----------------------------+-----------------------------------------------+
   | ``KQ_EV_ERROR``             | See return values                             |
   +-----------------------------+-----------------------------------------------+

kevent.fflags

   Filter specific flags.

   ``KQ_FILTER_READ`` and  ``KQ_FILTER_WRITE`` filter flags:

   +------------------------------+----------------------------------------------+
   | Constant                     | Meaning                                      |
   +==============================+==============================================+
   | ``KQ_NOTE_LOWAT``            | low water mark of a socket buffer            |
   +------------------------------+----------------------------------------------+

   ``KQ_FILTER_VNODE`` filter flags:

   +------------------------------+----------------------------------------------+
   | Constant                     | Meaning                                      |
   +==============================+==============================================+
   | ``KQ_NOTE_DELETE``           | *unlink()* was called                        |
   +------------------------------+----------------------------------------------+
   | ``KQ_NOTE_WRITE``            | a write occurred                             |
   +------------------------------+----------------------------------------------+
   | ``KQ_NOTE_EXTEND``           | the file was extended                        |
   +------------------------------+----------------------------------------------+
   | ``KQ_NOTE_ATTRIB``           | an attribute was changed                     |
   +------------------------------+----------------------------------------------+
   | ``KQ_NOTE_LINK``             | the link count has changed                   |
   +------------------------------+----------------------------------------------+
   | ``KQ_NOTE_RENAME``           | the file was renamed                         |
   +------------------------------+----------------------------------------------+
   | ``KQ_NOTE_REVOKE``           | access to the file was revoked               |
   +------------------------------+----------------------------------------------+

   ``KQ_FILTER_PROC`` filter flags:

   +------------------------------+----------------------------------------------+
   | Constant                     | Meaning                                      |
   +==============================+==============================================+
   | ``KQ_NOTE_EXIT``             | the process has exited                       |
   +------------------------------+----------------------------------------------+
   | ``KQ_NOTE_FORK``             | the process has called *fork()*              |
   +------------------------------+----------------------------------------------+
   | ``KQ_NOTE_EXEC``             | the process has executed a new process       |
   +------------------------------+----------------------------------------------+
   | ``KQ_NOTE_PCTRLMASK``        | internal filter flag                         |
   +------------------------------+----------------------------------------------+
   | ``KQ_NOTE_PDATAMASK``        | internal filter flag                         |
   +------------------------------+----------------------------------------------+
   | ``KQ_NOTE_TRACK``            | follow a process across *fork()*             |
   +------------------------------+----------------------------------------------+
   | ``KQ_NOTE_CHILD``            | returned on the child process for            |
   |                              | *NOTE_TRACK*                                 |
   +------------------------------+----------------------------------------------+
   | ``KQ_NOTE_TRACKERR``         | unable to attach to a child                  |
   +------------------------------+----------------------------------------------+

   ``KQ_FILTER_NETDEV`` filter flags (not available on Mac OS X):

   +------------------------------+----------------------------------------------+
   | Constant                     | Meaning                                      |
   +==============================+==============================================+
   | ``KQ_NOTE_LINKUP``           | link is up                                   |
   +------------------------------+----------------------------------------------+
   | ``KQ_NOTE_LINKDOWN``         | link is down                                 |
   +------------------------------+----------------------------------------------+
   | ``KQ_NOTE_LINKINV``          | link state is invalid                        |
   +------------------------------+----------------------------------------------+

kevent.data

   Filter specific data.

kevent.udata

   User defined value.
