source: python/trunk/Doc/library/select.rst

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[2]1
2:mod:`select` --- Waiting for I/O completion
3============================================
4
5.. module:: select
6 :synopsis: Wait for I/O completion on multiple streams.
7
8
[391]9This module provides access to the :c:func:`select` and :c:func:`poll` functions
10available in most operating systems, :c:func:`epoll` available on Linux 2.5+ and
11:c:func:`kqueue` available on most BSD.
[2]12Note that on Windows, it only works for sockets; on other operating systems,
13it also works for other file types (in particular, on Unix, it works on pipes).
14It cannot be used on regular files to determine whether a file has grown since
15it was last read.
16
17The module defines the following:
18
19
20.. exception:: error
21
22 The exception raised when an error occurs. The accompanying value is a pair
[391]23 containing the numeric error code from :c:data:`errno` and the corresponding
24 string, as would be printed by the C function :c:func:`perror`.
[2]25
26
27.. function:: epoll([sizehint=-1])
28
29 (Only supported on Linux 2.5.44 and newer.) Returns an edge polling object,
30 which can be used as Edge or Level Triggered interface for I/O events; see
31 section :ref:`epoll-objects` below for the methods supported by epolling
32 objects.
33
34 .. versionadded:: 2.6
35
36
37.. function:: poll()
38
39 (Not supported by all operating systems.) Returns a polling object, which
40 supports registering and unregistering file descriptors, and then polling them
41 for I/O events; see section :ref:`poll-objects` below for the methods supported
42 by polling objects.
43
44
45.. function:: kqueue()
46
[391]47 (Only supported on BSD.) Returns a kernel queue object; see section
[2]48 :ref:`kqueue-objects` below for the methods supported by kqueue objects.
49
50 .. versionadded:: 2.6
51
52
[391]53.. function:: kevent(ident, filter=KQ_FILTER_READ, flags=KQ_EV_ADD, fflags=0, data=0, udata=0)
[2]54
[391]55 (Only supported on BSD.) Returns a kernel event object; see section
56 :ref:`kevent-objects` below for the methods supported by kevent objects.
[2]57
58 .. versionadded:: 2.6
59
60
61.. function:: select(rlist, wlist, xlist[, timeout])
62
[391]63 This is a straightforward interface to the Unix :c:func:`select` system call.
[2]64 The first three arguments are sequences of 'waitable objects': either
65 integers representing file descriptors or objects with a parameterless method
[391]66 named :meth:`~io.IOBase.fileno` returning such an integer:
[2]67
68 * *rlist*: wait until ready for reading
69 * *wlist*: wait until ready for writing
70 * *xlist*: wait for an "exceptional condition" (see the manual page for what
71 your system considers such a condition)
72
73 Empty sequences are allowed, but acceptance of three empty sequences is
74 platform-dependent. (It is known to work on Unix but not on Windows.) The
75 optional *timeout* argument specifies a time-out as a floating point number
76 in seconds. When the *timeout* argument is omitted the function blocks until
77 at least one file descriptor is ready. A time-out value of zero specifies a
78 poll and never blocks.
79
80 The return value is a triple of lists of objects that are ready: subsets of the
81 first three arguments. When the time-out is reached without a file descriptor
82 becoming ready, three empty lists are returned.
83
84 .. index::
85 single: socket() (in module socket)
86 single: popen() (in module os)
87
88 Among the acceptable object types in the sequences are Python file objects (e.g.
89 ``sys.stdin``, or objects returned by :func:`open` or :func:`os.popen`), socket
90 objects returned by :func:`socket.socket`. You may also define a :dfn:`wrapper`
[391]91 class yourself, as long as it has an appropriate :meth:`~io.IOBase.fileno`
92 method (that really returns a file descriptor, not just a random integer).
[2]93
94 .. note::
95
96 .. index:: single: WinSock
97
98 File objects on Windows are not acceptable, but sockets are. On Windows,
[391]99 the underlying :c:func:`select` function is provided by the WinSock
[2]100 library, and does not handle file descriptors that don't originate from
101 WinSock.
102
[391]103.. attribute:: select.PIPE_BUF
[2]104
[391]105 Files reported as ready for writing by :func:`select`, :func:`poll` or
106 similar interfaces in this module are guaranteed to not block on a write
107 of up to :const:`PIPE_BUF` bytes.
108 This value is guaranteed by POSIX to be at least 512. Availability: Unix.
109
110 .. versionadded:: 2.7
111
112
[2]113.. _epoll-objects:
114
115Edge and Level Trigger Polling (epoll) Objects
116----------------------------------------------
117
118 http://linux.die.net/man/4/epoll
119
120 *eventmask*
121
122 +-----------------------+-----------------------------------------------+
123 | Constant | Meaning |
124 +=======================+===============================================+
125 | :const:`EPOLLIN` | Available for read |
126 +-----------------------+-----------------------------------------------+
127 | :const:`EPOLLOUT` | Available for write |
128 +-----------------------+-----------------------------------------------+
129 | :const:`EPOLLPRI` | Urgent data for read |
130 +-----------------------+-----------------------------------------------+
131 | :const:`EPOLLERR` | Error condition happened on the assoc. fd |
132 +-----------------------+-----------------------------------------------+
133 | :const:`EPOLLHUP` | Hang up happened on the assoc. fd |
134 +-----------------------+-----------------------------------------------+
135 | :const:`EPOLLET` | Set Edge Trigger behavior, the default is |
136 | | Level Trigger behavior |
137 +-----------------------+-----------------------------------------------+
138 | :const:`EPOLLONESHOT` | Set one-shot behavior. After one event is |
139 | | pulled out, the fd is internally disabled |
140 +-----------------------+-----------------------------------------------+
[391]141 | :const:`EPOLLRDNORM` | Equivalent to :const:`EPOLLIN` |
[2]142 +-----------------------+-----------------------------------------------+
[391]143 | :const:`EPOLLRDBAND` | Priority data band can be read. |
[2]144 +-----------------------+-----------------------------------------------+
[391]145 | :const:`EPOLLWRNORM` | Equivalent to :const:`EPOLLOUT` |
[2]146 +-----------------------+-----------------------------------------------+
[391]147 | :const:`EPOLLWRBAND` | Priority data may be written. |
[2]148 +-----------------------+-----------------------------------------------+
[391]149 | :const:`EPOLLMSG` | Ignored. |
[2]150 +-----------------------+-----------------------------------------------+
151
152
153.. method:: epoll.close()
154
155 Close the control file descriptor of the epoll object.
156
157
158.. method:: epoll.fileno()
159
160 Return the file descriptor number of the control fd.
161
162
163.. method:: epoll.fromfd(fd)
164
165 Create an epoll object from a given file descriptor.
166
167
168.. method:: epoll.register(fd[, eventmask])
169
170 Register a fd descriptor with the epoll object.
171
172 .. note::
173
174 Registering a file descriptor that's already registered raises an
175 IOError -- contrary to :ref:`poll-objects`'s register.
176
177
178.. method:: epoll.modify(fd, eventmask)
179
180 Modify a register file descriptor.
181
182
183.. method:: epoll.unregister(fd)
184
185 Remove a registered file descriptor from the epoll object.
186
187
188.. method:: epoll.poll([timeout=-1[, maxevents=-1]])
189
190 Wait for events. timeout in seconds (float)
191
192
193.. _poll-objects:
194
195Polling Objects
196---------------
197
[391]198The :c:func:`poll` system call, supported on most Unix systems, provides better
[2]199scalability for network servers that service many, many clients at the same
[391]200time. :c:func:`poll` scales better because the system call only requires listing
201the file descriptors of interest, while :c:func:`select` builds a bitmap, turns
[2]202on bits for the fds of interest, and then afterward the whole bitmap has to be
[391]203linearly scanned again. :c:func:`select` is O(highest file descriptor), while
204:c:func:`poll` is O(number of file descriptors).
[2]205
206
207.. method:: poll.register(fd[, eventmask])
208
209 Register a file descriptor with the polling object. Future calls to the
[391]210 :meth:`poll` method will then check whether the file descriptor has any
211 pending I/O events. *fd* can be either an integer, or an object with a
212 :meth:`~io.IOBase.fileno` method that returns an integer. File objects
213 implement :meth:`!fileno`, so they can also be used as the argument.
[2]214
215 *eventmask* is an optional bitmask describing the type of events you want to
216 check for, and can be a combination of the constants :const:`POLLIN`,
217 :const:`POLLPRI`, and :const:`POLLOUT`, described in the table below. If not
218 specified, the default value used will check for all 3 types of events.
219
220 +-------------------+------------------------------------------+
221 | Constant | Meaning |
222 +===================+==========================================+
223 | :const:`POLLIN` | There is data to read |
224 +-------------------+------------------------------------------+
225 | :const:`POLLPRI` | There is urgent data to read |
226 +-------------------+------------------------------------------+
227 | :const:`POLLOUT` | Ready for output: writing will not block |
228 +-------------------+------------------------------------------+
229 | :const:`POLLERR` | Error condition of some sort |
230 +-------------------+------------------------------------------+
231 | :const:`POLLHUP` | Hung up |
232 +-------------------+------------------------------------------+
233 | :const:`POLLNVAL` | Invalid request: descriptor not open |
234 +-------------------+------------------------------------------+
235
236 Registering a file descriptor that's already registered is not an error, and has
237 the same effect as registering the descriptor exactly once.
238
239
240.. method:: poll.modify(fd, eventmask)
241
242 Modifies an already registered fd. This has the same effect as
[391]243 ``register(fd, eventmask)``. Attempting to modify a file descriptor
[2]244 that was never registered causes an :exc:`IOError` exception with errno
245 :const:`ENOENT` to be raised.
246
247 .. versionadded:: 2.6
248
249
250.. method:: poll.unregister(fd)
251
252 Remove a file descriptor being tracked by a polling object. Just like the
253 :meth:`register` method, *fd* can be an integer or an object with a
[391]254 :meth:`~io.IOBase.fileno` method that returns an integer.
[2]255
256 Attempting to remove a file descriptor that was never registered causes a
257 :exc:`KeyError` exception to be raised.
258
259
260.. method:: poll.poll([timeout])
261
262 Polls the set of registered file descriptors, and returns a possibly-empty list
263 containing ``(fd, event)`` 2-tuples for the descriptors that have events or
264 errors to report. *fd* is the file descriptor, and *event* is a bitmask with
265 bits set for the reported events for that descriptor --- :const:`POLLIN` for
266 waiting input, :const:`POLLOUT` to indicate that the descriptor can be written
267 to, and so forth. An empty list indicates that the call timed out and no file
268 descriptors had any events to report. If *timeout* is given, it specifies the
269 length of time in milliseconds which the system will wait for events before
270 returning. If *timeout* is omitted, negative, or :const:`None`, the call will
271 block until there is an event for this poll object.
272
273
274.. _kqueue-objects:
275
276Kqueue Objects
277--------------
278
279.. method:: kqueue.close()
280
281 Close the control file descriptor of the kqueue object.
282
283
284.. method:: kqueue.fileno()
285
286 Return the file descriptor number of the control fd.
287
288
289.. method:: kqueue.fromfd(fd)
290
291 Create a kqueue object from a given file descriptor.
292
293
294.. method:: kqueue.control(changelist, max_events[, timeout=None]) -> eventlist
295
296 Low level interface to kevent
297
298 - changelist must be an iterable of kevent object or None
299 - max_events must be 0 or a positive integer
300 - timeout in seconds (floats possible)
301
302
303.. _kevent-objects:
304
305Kevent Objects
306--------------
307
308http://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2
309
310.. attribute:: kevent.ident
311
312 Value used to identify the event. The interpretation depends on the filter
313 but it's usually the file descriptor. In the constructor ident can either
314 be an int or an object with a fileno() function. kevent stores the integer
315 internally.
316
317.. attribute:: kevent.filter
318
319 Name of the kernel filter.
320
321 +---------------------------+---------------------------------------------+
322 | Constant | Meaning |
323 +===========================+=============================================+
324 | :const:`KQ_FILTER_READ` | Takes a descriptor and returns whenever |
325 | | there is data available to read |
326 +---------------------------+---------------------------------------------+
327 | :const:`KQ_FILTER_WRITE` | Takes a descriptor and returns whenever |
328 | | there is data available to write |
329 +---------------------------+---------------------------------------------+
330 | :const:`KQ_FILTER_AIO` | AIO requests |
331 +---------------------------+---------------------------------------------+
332 | :const:`KQ_FILTER_VNODE` | Returns when one or more of the requested |
333 | | events watched in *fflag* occurs |
334 +---------------------------+---------------------------------------------+
335 | :const:`KQ_FILTER_PROC` | Watch for events on a process id |
336 +---------------------------+---------------------------------------------+
337 | :const:`KQ_FILTER_NETDEV` | Watch for events on a network device |
338 | | [not available on Mac OS X] |
339 +---------------------------+---------------------------------------------+
340 | :const:`KQ_FILTER_SIGNAL` | Returns whenever the watched signal is |
341 | | delivered to the process |
342 +---------------------------+---------------------------------------------+
343 | :const:`KQ_FILTER_TIMER` | Establishes an arbitrary timer |
344 +---------------------------+---------------------------------------------+
345
346.. attribute:: kevent.flags
347
348 Filter action.
349
350 +---------------------------+---------------------------------------------+
351 | Constant | Meaning |
352 +===========================+=============================================+
353 | :const:`KQ_EV_ADD` | Adds or modifies an event |
354 +---------------------------+---------------------------------------------+
355 | :const:`KQ_EV_DELETE` | Removes an event from the queue |
356 +---------------------------+---------------------------------------------+
357 | :const:`KQ_EV_ENABLE` | Permitscontrol() to returns the event |
358 +---------------------------+---------------------------------------------+
359 | :const:`KQ_EV_DISABLE` | Disablesevent |
360 +---------------------------+---------------------------------------------+
361 | :const:`KQ_EV_ONESHOT` | Removes event after first occurrence |
362 +---------------------------+---------------------------------------------+
363 | :const:`KQ_EV_CLEAR` | Reset the state after an event is retrieved |
364 +---------------------------+---------------------------------------------+
365 | :const:`KQ_EV_SYSFLAGS` | internal event |
366 +---------------------------+---------------------------------------------+
367 | :const:`KQ_EV_FLAG1` | internal event |
368 +---------------------------+---------------------------------------------+
369 | :const:`KQ_EV_EOF` | Filter specific EOF condition |
370 +---------------------------+---------------------------------------------+
371 | :const:`KQ_EV_ERROR` | See return values |
372 +---------------------------+---------------------------------------------+
373
374
375.. attribute:: kevent.fflags
376
377 Filter specific flags.
378
379 :const:`KQ_FILTER_READ` and :const:`KQ_FILTER_WRITE` filter flags:
380
381 +----------------------------+--------------------------------------------+
382 | Constant | Meaning |
383 +============================+============================================+
384 | :const:`KQ_NOTE_LOWAT` | low water mark of a socket buffer |
385 +----------------------------+--------------------------------------------+
386
387 :const:`KQ_FILTER_VNODE` filter flags:
388
389 +----------------------------+--------------------------------------------+
390 | Constant | Meaning |
391 +============================+============================================+
392 | :const:`KQ_NOTE_DELETE` | *unlink()* was called |
393 +----------------------------+--------------------------------------------+
394 | :const:`KQ_NOTE_WRITE` | a write occurred |
395 +----------------------------+--------------------------------------------+
396 | :const:`KQ_NOTE_EXTEND` | the file was extended |
397 +----------------------------+--------------------------------------------+
398 | :const:`KQ_NOTE_ATTRIB` | an attribute was changed |
399 +----------------------------+--------------------------------------------+
400 | :const:`KQ_NOTE_LINK` | the link count has changed |
401 +----------------------------+--------------------------------------------+
402 | :const:`KQ_NOTE_RENAME` | the file was renamed |
403 +----------------------------+--------------------------------------------+
404 | :const:`KQ_NOTE_REVOKE` | access to the file was revoked |
405 +----------------------------+--------------------------------------------+
406
407 :const:`KQ_FILTER_PROC` filter flags:
408
409 +----------------------------+--------------------------------------------+
410 | Constant | Meaning |
411 +============================+============================================+
412 | :const:`KQ_NOTE_EXIT` | the process has exited |
413 +----------------------------+--------------------------------------------+
414 | :const:`KQ_NOTE_FORK` | the process has called *fork()* |
415 +----------------------------+--------------------------------------------+
416 | :const:`KQ_NOTE_EXEC` | the process has executed a new process |
417 +----------------------------+--------------------------------------------+
418 | :const:`KQ_NOTE_PCTRLMASK` | internal filter flag |
419 +----------------------------+--------------------------------------------+
420 | :const:`KQ_NOTE_PDATAMASK` | internal filter flag |
421 +----------------------------+--------------------------------------------+
422 | :const:`KQ_NOTE_TRACK` | follow a process across *fork()* |
423 +----------------------------+--------------------------------------------+
424 | :const:`KQ_NOTE_CHILD` | returned on the child process for |
425 | | *NOTE_TRACK* |
426 +----------------------------+--------------------------------------------+
427 | :const:`KQ_NOTE_TRACKERR` | unable to attach to a child |
428 +----------------------------+--------------------------------------------+
429
430 :const:`KQ_FILTER_NETDEV` filter flags (not available on Mac OS X):
431
432 +----------------------------+--------------------------------------------+
433 | Constant | Meaning |
434 +============================+============================================+
435 | :const:`KQ_NOTE_LINKUP` | link is up |
436 +----------------------------+--------------------------------------------+
437 | :const:`KQ_NOTE_LINKDOWN` | link is down |
438 +----------------------------+--------------------------------------------+
439 | :const:`KQ_NOTE_LINKINV` | link state is invalid |
440 +----------------------------+--------------------------------------------+
441
442
443.. attribute:: kevent.data
444
445 Filter specific data.
446
447
448.. attribute:: kevent.udata
449
450 User defined value.
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