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

======================================================================

Este módulo fornece acesso às funções "select()" e "poll()"
disponíveis na maioria dos sistemas operacionais, "devpoll()"
disponível no Solaris e derivados, "epoll()" disponível no Linux 2.5+
e "kqueue()" disponível na maioria dos BSD. Observe que no Windows,
ele funciona apenas para sockets; em outros sistemas operacionais, ele
também funciona para outros tipos de arquivo (em particular, no Unix,
ele funciona em pipes). Ele não pode ser usado em arquivos regulares
para determinar se um arquivo cresceu desde a última leitura.

Nota:

  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.

Disponibilidade: not WASI.

Este módulo não funciona ou não está disponível em WebAssembly. Veja
Plataformas WebAssembly para mais informações.

O módulo define o seguinte:

exception select.error

   Um apelido descontinuado de "OSError".

   Alterado na versão 3.3: Seguindo a **PEP 3151**, esta classe foi
   transformada em um apelido de "OSError".

select.devpoll()

   Returns a "/dev/poll" polling object; see section /dev/poll polling
   objects below for the methods supported by devpoll objects.

   Objetos "devpoll()" são vinculados ao número de descritores de
   arquivo permitidos no momento da instanciação. Se seu programa
   reduzir esse valor, "devpoll()" falhará. Se seu programa aumentar
   esse valor, "devpoll()" pode retornar uma lista incompleta de
   descritores de arquivo ativos.

   O novo descritor de arquivo é não herdável.

   Adicionado na versão 3.3.

   Alterado na versão 3.4: O novo descritor de arquivo agora é não-
   hereditário.

   Disponibilidade: Solaris.

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

   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(2)* 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.

   O novo descritor de arquivo é não herdável.

   Alterado na versão 3.3: Added the *flags* parameter.

   Alterado na versão 3.4: Support for the "with" statement was added.
   The new file descriptor is now non-inheritable.

   Descontinuado desde a versão 3.4: The *flags* parameter.
   "select.EPOLL_CLOEXEC" is used by default now. Use
   "os.set_inheritable()" to make the file descriptor inheritable.

   Alterado na versão 3.15: When CPython is built, this function may
   be disabled using "--disable-epoll".

   Disponibilidade: Linux >= 2.5.44.

select.poll()

   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.

   Disponibilidade: Unix.

select.kqueue()

   Returns a kernel queue object; see section Kqueue objects below for
   the methods supported by kqueue objects.

   O novo descritor de arquivo é não herdável.

   Alterado na versão 3.4: O novo descritor de arquivo agora é não-
   hereditário.

   Disponibilidade: BSD, macOS.

select.kevent(ident, filter=KQ_FILTER_READ, flags=KQ_EV_ADD, fflags=0, data=0, udata=0)

   Returns a kernel event object; see section Kevent objects below for
   the methods supported by kevent objects.

   Disponibilidade: BSD, macOS.

select.select(rlist, wlist, xlist, timeout=None)

   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 in seconds; it may be a non-integer to specify fractions of
   seconds. When the *timeout* argument is omitted or "None", 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).

   Nota:

     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.

   Alterado na versão 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".

   Alterado na versão 3.15: Accepts any real number as *timeout*, not
   only integer or float.

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 kinds of file-like objects such as sockets.

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

   Disponibilidade: Unix

   Adicionado na versão 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.

   Adicionado na versão 3.4.

devpoll.closed

   "True" if the polling object is closed.

   Adicionado na versão 3.4.

devpoll.fileno()

   Return the file descriptor number of the polling object.

   Adicionado na versão 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 as with "poll()"
   object. The default value is a combination of the constants
   "POLLIN", "POLLPRI", and "POLLOUT".

   Aviso:

     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 than 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.

   Alterado na versão 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".

   Alterado na versão 3.15: Accepts any real number as *timeout*, not
   only integer or float.


Edge and level trigger polling (epoll) objects
==============================================

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

   The *eventmask* is a bit mask using the following constants:

   +---------------------------+--------------------------------------------------+
   | Constante                 | Significado                                      |
   |===========================|==================================================|
   | "EPOLLIN"                 | Available for read.                              |
   +---------------------------+--------------------------------------------------+
   | "EPOLLOUT"                | Available for write.                             |
   +---------------------------+--------------------------------------------------+
   | "EPOLLPRI"                | Urgent data for read.                            |
   +---------------------------+--------------------------------------------------+
   | "EPOLLERR"                | Error condition happened on the associated fd.   |
   +---------------------------+--------------------------------------------------+
   | "EPOLLHUP"                | Hang up happened on the associated 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 |
   |                           | an 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"                | Ignorado.                                        |
   +---------------------------+--------------------------------------------------+
   | "EPOLLWAKEUP"             | Prevents sleep during event waiting.             |
   +---------------------------+--------------------------------------------------+

   Adicionado na versão 3.6: "EPOLLEXCLUSIVE" was added.  It's only
   supported by Linux Kernel 4.5 or later.

   Adicionado na versão 3.14: "EPOLLWAKEUP" was added. It's only
   supported by Linux Kernel 3.5 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 file descriptor *fd* with the epoll object.

epoll.modify(fd, eventmask)

   Modify a registered file descriptor *fd*.

epoll.unregister(fd)

   Remove a registered file descriptor from the epoll object.

   Alterado na versão 3.9: The method no longer ignores the "EBADF"
   error.

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

   Wait for events. If *timeout* is given, it specifies the length of
   time in seconds (may be non-integer) which the system will wait for
   events before returning.

   Alterado na versão 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".

   Alterado na versão 3.15: Accepts any real number as *timeout*, not
   only integer or 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.

   +---------------------+---------------------------------------------+
   | Constante           | Significado                                 |
   |=====================|=============================================|
   | "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.

   Alterado na versão 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".

   Alterado na versão 3.15: Accepts any real number as *timeout*, not
   only integer or float. If "ppoll()" function is available,
   *timeout* has a resolution of "1" ns ("1e-6" ms) instead of "1" ms.


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 (non-integers are possible); the default is
     "None", to wait forever

   Alterado na versão 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".

   Alterado na versão 3.15: Accepts any real number as *timeout*, not
   only integer or float.


Kevent objects
==============

https://man.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.

   +-----------------------------+-----------------------------------------------+
   | Constante                   | Significado                                   |
   |=============================|===============================================|
   | "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 macOS).                          |
   +-----------------------------+-----------------------------------------------+
   | "KQ_FILTER_SIGNAL"          | Returns whenever the watched signal is        |
   |                             | delivered to the process.                     |
   +-----------------------------+-----------------------------------------------+
   | "KQ_FILTER_TIMER"           | Establishes an arbitrary timer.               |
   +-----------------------------+-----------------------------------------------+

kevent.flags

   Filter action.

   +-----------------------------+------------------------------------------------+
   | Constante                   | Significado                                    |
   |=============================|================================================|
   | "KQ_EV_ADD"                 | Adds or modifies an event.                     |
   +-----------------------------+------------------------------------------------+
   | "KQ_EV_DELETE"              | Removes an event from the queue.               |
   +-----------------------------+------------------------------------------------+
   | "KQ_EV_ENABLE"              | Permits control() to return the event.         |
   +-----------------------------+------------------------------------------------+
   | "KQ_EV_DISABLE"             | Disables event.                                |
   +-----------------------------+------------------------------------------------+
   | "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:

   +------------------------------+----------------------------------------------+
   | Constante                    | Significado                                  |
   |==============================|==============================================|
   | "KQ_NOTE_LOWAT"              | Low water mark of a socket buffer.           |
   +------------------------------+----------------------------------------------+

   "KQ_FILTER_VNODE" filter flags:

   +------------------------------+----------------------------------------------+
   | Constante                    | Significado                                  |
   |==============================|==============================================|
   | "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:

   +------------------------------+----------------------------------------------+
   | Constante                    | Significado                                  |
   |==============================|==============================================|
   | "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 macOS):

   +------------------------------+----------------------------------------------+
   | Constante                    | Significado                                  |
   |==============================|==============================================|
   | "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.
