# `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

O módulo `selectors` permite multiplexação de E/S de alto nível e eficiente, construída sobre as primitivas do módulo `select`. Os usuários são encorajados a usar o módulo `selectors` em vez disso, a menos que queiram controle preciso sobre as primitivas de nível de SO usadas.

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()

  (Suportado somente no Solaris e derivados.) Retorna um objeto de pesquisa `/dev/poll`; consulte a seção /dev/poll Polling Objects abaixo para os métodos suportados pelos objetos devpoll.

  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.

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.

  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.

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.

  O novo descritor de arquivo é não herdável.

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

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=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 as a floating-point number in 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`.

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.

  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 that 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 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.

  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`.

## Edge and Level Trigger Polling (epoll) Objects

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

> eventmask

> Constante | Significado
> --- | ---
> `EPOLLIN` | Disponível para leitura
> `EPOLLOUT` | Disponível para escrita
> `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` | Equivalente a `EPOLLOUT`
> `EPOLLWRBAND` | Priority data may be written.
> `EPOLLMSG` | Ignorado.

> Adicionado na versão 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.

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

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

  Wait for events. timeout in seconds (float)

  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`.

## 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`.

## 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

  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`.

## 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` | 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:

  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` | estado do link é inválido

kevent.data

  Filter specific data.

kevent.udata

  User defined value.