# 16.1. `select` — Waiting for I/O completion

This module provides access to the `select()` and `poll()` functions available in most operating systems, `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 `select.``error`

  The exception raised when an error occurs. The accompanying value is a pair containing the numeric error code from `errno` and the corresponding string, as would be printed by the C function `perror()`.

`select.``epoll`([sizehint=-1])

  (Only supported on Linux 2.5.44 and newer.) Returns an edge polling object, which can be used as Edge or Level Triggered interface for I/O events; see section Edge and Level Trigger Polling (epoll) Objects below for the methods supported by epolling objects.

  New in version 2.6.

`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.

  New in version 2.6.

`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.

  New in version 2.6.

`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`

  Files reported as ready for writing by `select()`, `poll()` or similar interfaces in this module are guaranteed to not block on a write of up to `PIPE_BUF` bytes. This value is guaranteed by POSIX to be at least 512. Availability: Unix.

  New in version 2.7.

## 16.1.1. 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.

  Note

  Registering a file descriptor that’s already registered raises an IOError – contrary to Polling Objects’s register.

`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)

## 16.1.2. 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.

  New in version 2.6.

`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.

## 16.1.3. 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)

## 16.1.4. 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() 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.