summaryrefslogtreecommitdiffstats
path: root/Doc/library/select.rst
blob: 4a971792019b783a6d4773e9b953fb8cd376bb28 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139

:mod:`select` --- Waiting for I/O completion
============================================

.. module:: select
   :synopsis: Wait for I/O completion on multiple streams.


This module provides access to the :cfunc:`select` and :cfunc:`poll` functions
available in most operating systems.  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:: error

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


.. function:: 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 :ref:`poll-objects` below for the methods supported
   by polling objects.


.. function:: select(iwtd, owtd, ewtd[, timeout])

   This is a straightforward interface to the Unix :cfunc:`select` system call.
   The first three arguments are sequences of 'waitable objects': either
   integers representing file descriptors or objects with a parameterless method
   named :meth:`fileno` returning such an integer.  The three sequences of
   waitable objects are for input, output and 'exceptional conditions',
   respectively.  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.

   .. index::
      single: socket() (in module socket)
      single: popen() (in module os)

   Among the acceptable object types in the sequences are Python file objects (e.g.
   ``sys.stdin``, or objects returned by :func:`open` or :func:`os.popen`), socket
   objects returned by :func:`socket.socket`.  You may also define a :dfn:`wrapper`
   class yourself, as long as it has an appropriate :meth:`fileno` method (that
   really returns a file descriptor, not just a random integer).

   .. note::

      .. index:: single: WinSock

      File objects on Windows are not acceptable, but sockets are.  On Windows, the
      underlying :cfunc:`select` function is provided by the WinSock library, and does
      not handle file descriptors that don't originate from WinSock.


.. _poll-objects:

Polling Objects
---------------

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


.. method:: poll.register(fd[, eventmask])

   Register a file descriptor with the polling object.  Future calls to the
   :meth:`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 :meth:`fileno`
   method that returns an integer.  File objects implement :meth:`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 :const:`POLLIN`,
   :const:`POLLPRI`, and :const:`POLLOUT`, described in the table below.  If not
   specified, the default value used will check for all 3 types of events.

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


.. method:: poll.unregister(fd)

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

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


.. method:: 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 --- :const:`POLLIN` for
   waiting input, :const:`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 :const:`None`, the call will
   block until there is an event for this poll object.