18.3. select
— Waiting for I/O completion¶
This module provides access to the select()
and poll()
functions
available in most operating systems, devpoll()
available on
Solaris and derivatives, 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.
Nota
The selectors
module allows high-level and efficient I/O
multiplexing, built upon the select
module primitives. Users are
encouraged to use the selectors
module instead, unless they want
precise control over the OS-level primitives used.
The module defines the following:
-
select.
devpoll
()¶ (Only supported on Solaris and derivatives.) Returns a
/dev/poll
polling object; see section /dev/poll Polling Objects below for the methods supported by devpoll objects.devpoll()
objects are linked to the number of file descriptors allowed at the time of instantiation. If your program reduces this value,devpoll()
will fail. If your program increases this value,devpoll()
may return an incomplete list of active file descriptors.The new file descriptor is non-inheritable.
Nuevo en la versión 3.3.
Distinto en la versión 3.4: The new file descriptor is now non-inheritable.
-
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 and flags are deprecated and completely ignored.
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 awith
statement, the new file descriptor is automatically closed at the end of the block.The new file descriptor is non-inheritable.
Distinto en la versión 3.3: Added the flags parameter.
Distinto en la versión 3.4: Support for the
with
statement was added. The new file descriptor is now non-inheritable.Obsoleto desde la versión 3.4: The flags parameter.
select.EPOLL_CLOEXEC
is used by default now. Useos.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.
The new file descriptor is non-inheritable.
Distinto en la versión 3.4: The new file descriptor is now non-inheritable.
-
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])¶ 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 namedfileno()
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 byopen()
oros.popen()
), socket objects returned bysocket.socket()
. You may also define a wrapper class yourself, as long as it has an appropriatefileno()
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.Distinto en la versión 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. Availability: Unix.
Nuevo en la versión 3.2.
18.3.1. /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.
Nuevo en la versión 3.4.
-
devpoll.
closed
¶ True
if the polling object is closed.Nuevo en la versión 3.4.
-
devpoll.
fileno
()¶ Return the file descriptor number of the polling object.
Nuevo en la versión 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 afileno()
method that returns an integer. File objects implementfileno()
, 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 constantsPOLLIN
,POLLPRI
, andPOLLOUT
.Advertencia
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 aregister()
. 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 afileno()
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, orNone
, the call will block until there is an event for this poll object.Distinto en la versión 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
.
18.3.2. 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.
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.
-
epoll.
poll
(timeout=-1, maxevents=-1)¶ Wait for events. timeout in seconds (float)
Distinto en la versión 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
.
18.3.3. 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 afileno()
method that returns an integer. File objects implementfileno()
, 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
, andPOLLOUT
, 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 anOSError
exception with errnoENOENT
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 afileno()
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, orNone
, the call will block until there is an event for this poll object.Distinto en la versión 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
.
18.3.4. 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=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)
Distinto en la versión 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
.- changelist must be an iterable of kevent object or
18.3.5. 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()
method. 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
andKQ_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.