2 :mod:`select` --- Waiting for I/O completion
3 ============================================
6 :synopsis: Wait for I/O completion on multiple streams.
9 This module provides access to the :cfunc:`select` and :cfunc:`poll` functions
10 available in most operating systems, :cfunc:`epoll` available on Linux 2.5+ and
11 :cfunc:`kqueue` available on most BSD.
12 Note that on Windows, it only works for sockets; on other operating systems,
13 it also works for other file types (in particular, on Unix, it works on pipes).
14 It cannot be used on regular files to determine whether a file has grown since
17 The module defines the following:
22 The exception raised when an error occurs. The accompanying value is a pair
23 containing the numeric error code from :cdata:`errno` and the corresponding
24 string, as would be printed by the C function :cfunc:`perror`.
27 .. function:: epoll([sizehint=-1])
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
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
45 .. function:: kqueue()
47 (Only supported on BSD.) Returns a kernel queue object object; see section
48 :ref:`kqueue-objects` below for the methods supported by kqueue objects.
53 .. function:: kevent(ident, filter=KQ_FILTER_READ, flags=KQ_ADD, fflags=0, data=0, udata=0)
55 (Only supported on BSD.) Returns a kernel event object object; see section
56 :ref:`kevent-objects` below for the methods supported by kqueue objects.
61 .. function:: select(rlist, wlist, xlist[, timeout])
63 This is a straightforward interface to the Unix :cfunc:`select` system call.
64 The first three arguments are sequences of 'waitable objects': either
65 integers representing file descriptors or objects with a parameterless method
66 named :meth:`fileno` returning such an integer:
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)
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.
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.
85 single: socket() (in module socket)
86 single: popen() (in module os)
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`
91 class yourself, as long as it has an appropriate :meth:`fileno` method (that
92 really returns a file descriptor, not just a random integer).
96 .. index:: single: WinSock
98 File objects on Windows are not acceptable, but sockets are. On Windows,
99 the underlying :cfunc:`select` function is provided by the WinSock
100 library, and does not handle file descriptors that don't originate from
103 .. attribute:: select.PIPE_BUF
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.
110 .. versionadded:: 2.7
115 Edge and Level Trigger Polling (epoll) Objects
116 ----------------------------------------------
118 http://linux.die.net/man/4/epoll
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 +-----------------------+-----------------------------------------------+
141 | :const:`EPOLLRDNORM` | ??? |
142 +-----------------------+-----------------------------------------------+
143 | :const:`EPOLLRDBAND` | ??? |
144 +-----------------------+-----------------------------------------------+
145 | :const:`EPOLLWRNORM` | ??? |
146 +-----------------------+-----------------------------------------------+
147 | :const:`EPOLLWRBAND` | ??? |
148 +-----------------------+-----------------------------------------------+
149 | :const:`EPOLLMSG` | ??? |
150 +-----------------------+-----------------------------------------------+
153 .. method:: epoll.close()
155 Close the control file descriptor of the epoll object.
158 .. method:: epoll.fileno()
160 Return the file descriptor number of the control fd.
163 .. method:: epoll.fromfd(fd)
165 Create an epoll object from a given file descriptor.
168 .. method:: epoll.register(fd[, eventmask])
170 Register a fd descriptor with the epoll object.
174 Registering a file descriptor that's already registered raises an
175 IOError -- contrary to :ref:`poll-objects`'s register.
178 .. method:: epoll.modify(fd, eventmask)
180 Modify a register file descriptor.
183 .. method:: epoll.unregister(fd)
185 Remove a registered file descriptor from the epoll object.
188 .. method:: epoll.poll([timeout=-1[, maxevents=-1]])
190 Wait for events. timeout in seconds (float)
198 The :cfunc:`poll` system call, supported on most Unix systems, provides better
199 scalability for network servers that service many, many clients at the same
200 time. :cfunc:`poll` scales better because the system call only requires listing
201 the file descriptors of interest, while :cfunc:`select` builds a bitmap, turns
202 on bits for the fds of interest, and then afterward the whole bitmap has to be
203 linearly scanned again. :cfunc:`select` is O(highest file descriptor), while
204 :cfunc:`poll` is O(number of file descriptors).
207 .. method:: poll.register(fd[, eventmask])
209 Register a file descriptor with the polling object. Future calls to the
210 :meth:`poll` method will then check whether the file descriptor has any pending
211 I/O events. *fd* can be either an integer, or an object with a :meth:`fileno`
212 method that returns an integer. File objects implement :meth:`fileno`, so they
213 can also be used as the argument.
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.
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 +-------------------+------------------------------------------+
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.
240 .. method:: poll.modify(fd, eventmask)
242 Modifies an already registered fd. This has the same effect as
243 :meth:`register(fd, eventmask)`. Attempting to modify a file descriptor
244 that was never registered causes an :exc:`IOError` exception with errno
245 :const:`ENOENT` to be raised.
247 .. versionadded:: 2.6
250 .. method:: poll.unregister(fd)
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
254 :meth:`fileno` method that returns an integer.
256 Attempting to remove a file descriptor that was never registered causes a
257 :exc:`KeyError` exception to be raised.
260 .. method:: poll.poll([timeout])
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.
279 .. method:: kqueue.close()
281 Close the control file descriptor of the kqueue object.
284 .. method:: kqueue.fileno()
286 Return the file descriptor number of the control fd.
289 .. method:: kqueue.fromfd(fd)
291 Create a kqueue object from a given file descriptor.
294 .. method:: kqueue.control(changelist, max_events[, timeout=None]) -> eventlist
296 Low level interface to kevent
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)
308 http://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2
310 .. attribute:: kevent.ident
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
317 .. attribute:: kevent.filter
319 Name of the kernel filter.
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 +---------------------------+---------------------------------------------+
346 .. attribute:: kevent.flags
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 +---------------------------+---------------------------------------------+
375 .. attribute:: kevent.fflags
377 Filter specific flags.
379 :const:`KQ_FILTER_READ` and :const:`KQ_FILTER_WRITE` filter flags:
381 +----------------------------+--------------------------------------------+
382 | Constant | Meaning |
383 +============================+============================================+
384 | :const:`KQ_NOTE_LOWAT` | low water mark of a socket buffer |
385 +----------------------------+--------------------------------------------+
387 :const:`KQ_FILTER_VNODE` filter flags:
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 +----------------------------+--------------------------------------------+
407 :const:`KQ_FILTER_PROC` filter flags:
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 |
426 +----------------------------+--------------------------------------------+
427 | :const:`KQ_NOTE_TRACKERR` | unable to attach to a child |
428 +----------------------------+--------------------------------------------+
430 :const:`KQ_FILTER_NETDEV` filter flags (not available on Mac OS X):
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 +----------------------------+--------------------------------------------+
443 .. attribute:: kevent.data
445 Filter specific data.
448 .. attribute:: kevent.udata