"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.

Note:

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

The module defines the following:

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.

   The new file descriptor is non-inheritable.

   New in version 3.3.

   Changed in version 3.4: The new file descriptor is now non-
   inheritable.

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

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

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

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

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

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

   The new file descriptor is non-inheritable.

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

   Changed in version 3.4: Support for the "with" statement was added.
   The new file descriptor is now non-inheritable.

   Deprecated since version 3.4: The *flags* parameter.
   "select.EPOLL_CLOEXEC" is used by default now. Use
   "os.set_inheritable()" to make the file descriptor inheritable.

select.poll()

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

select.kqueue()

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

   The new file descriptor is non-inheritable.

   Changed in version 3.4: The new file descriptor is now non-
   inheritable.

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 iterables 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 iterables are allowed, but acceptance of three empty
   iterables 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 iterables 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.

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

select.PIPE_BUF

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

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

   Availability: Unix

   New in version 3.2.


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

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

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

devpoll.close()

   Close the file descriptor of the polling object.

   New in version 3.4.

devpoll.closed

   "True" if the polling object is closed.

   New in version 3.4.

devpoll.fileno()

   Return the file descriptor number of the polling object.

   New in version 3.4.

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 appropriate action is to
     unregister or modify it first. This is an important difference
     compared with "poll()".

devpoll.modify(fd[, eventmask])

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

devpoll.unregister(fd)

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

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

devpoll.poll([timeout])

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

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


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

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

   *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       |
   +---------------------------+-------------------------------------------------+
   | "EPOLLEXCLUSIVE"          | Wake only one epoll object when the associated  |
   |                           | fd has an event. The default (if this flag is   |
   |                           | not set) is to wake all epoll objects polling   |
   |                           | on a fd.                                        |
   +---------------------------+-------------------------------------------------+
   | "EPOLLRDHUP"              | Stream socket peer closed connection or shut    |
   |                           | down writing half of connection.                |
   +---------------------------+-------------------------------------------------+
   | "EPOLLRDNORM"             | Equivalent to "EPOLLIN"                         |
   +---------------------------+-------------------------------------------------+
   | "EPOLLRDBAND"             | Priority data band can be read.                 |
   +---------------------------+-------------------------------------------------+
   | "EPOLLWRNORM"             | Equivalent to "EPOLLOUT"                        |
   +---------------------------+-------------------------------------------------+
   | "EPOLLWRBAND"             | Priority data may be written.                   |
   +---------------------------+-------------------------------------------------+
   | "EPOLLMSG"                | Ignored.                                        |
   +---------------------------+-------------------------------------------------+

   New in version 3.6: "EPOLLEXCLUSIVE" was added.  It’s only
   supported by Linux Kernel 4.5 or later.

epoll.close()

   Close the control file descriptor of the epoll object.

epoll.closed

   "True" if the epoll object is closed.

epoll.fileno()

   Return the file descriptor number of the control fd.

epoll.fromfd(fd)

   Create an epoll object from a given file descriptor.

epoll.register(fd[, eventmask])

   Register a fd descriptor with the epoll object.

epoll.modify(fd, eventmask)

   Modify a registered file descriptor.

epoll.unregister(fd)

   Remove a registered file descriptor from the epoll object.

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

   Wait for events. timeout in seconds (float)

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


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

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

poll.register(fd[, eventmask])

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

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

   +---------------------+--------------------------------------------+
   | 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                                    |
   +---------------------+--------------------------------------------+
   | "POLLRDHUP"         | Stream socket peer closed connection, or   |
   |                     | shut down writing half of connection       |
   +---------------------+--------------------------------------------+
   | "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 "OSError" exception with errno
   "ENOENT" to be raised.

poll.unregister(fd)

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

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

poll.poll([timeout])

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

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


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

kqueue.close()

   Close the control file descriptor of the kqueue object.

kqueue.closed

   "True" if the kqueue object is closed.

kqueue.fileno()

   Return the file descriptor number of the control fd.

kqueue.fromfd(fd)

   Create a kqueue object from a given file descriptor.

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

   Low level interface to kevent

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

   * max_events must be 0 or a positive integer

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

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


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

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

kevent.ident

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

kevent.filter

   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.
