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.
Note
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.
New in version 3.3.
Changed in version 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 informs epoll about the expected number of events to be registered. It must be positive, or -1 to use the default. It is only used on older systems where
epoll_create1()
is not available; otherwise it has no effect (though its value is still checked).flags is deprecated and completely ignored. However, when supplied, its value must be
0
orselect.EPOLL_CLOEXEC
, otherwiseOSError
is raised.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.
Changed in version 3.3: Added the flags parameter.
Changed in version 3.4: Support for the
with
statement was added. The new file descriptor is now non-inheritable.Deprecated since version 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.
Changed in version 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 iterables 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 iterables are allowed, but acceptance of three empty iterables 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 iterables 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).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.Changed in version 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
New in version 3.2.
/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.
New in version 3.4.
- devpoll.closed¶
True
if the polling object is closed.New in version 3.4.
- devpoll.fileno()¶
Return the file descriptor number of the polling object.
New in version 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
.Warning
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.Changed in version 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
.
Edge and Level Trigger Polling (epoll) Objects¶
https://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
EPOLLEXCLUSIVE
Wake only one epoll object when the associated fd has an event. The default (if this flag is not set) is to wake all epoll objects polling on a fd.
EPOLLRDHUP
Stream socket peer closed connection or shut down writing half of connection.
EPOLLRDNORM
Equivalent to
EPOLLIN
EPOLLRDBAND
Priority data band can be read.
EPOLLWRNORM
Equivalent to
EPOLLOUT
EPOLLWRBAND
Priority data may be written.
EPOLLMSG
Ignored.
New in version 3.6:
EPOLLEXCLUSIVE
was added. It’s only supported by Linux Kernel 4.5 or later.
- 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.
Changed in version 3.9: The method no longer ignores the
EBADF
error.
- epoll.poll(timeout=None, maxevents=- 1)¶
Wait for events. timeout in seconds (float)
Changed in version 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
.
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
POLLRDHUP
Stream socket peer closed connection, or shut down writing half of connection
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.Changed in version 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
.
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]) → eventlist¶
Low level interface to kevent
changelist must be an iterable of kevent objects or
None
max_events must be 0 or a positive integer
timeout in seconds (floats possible); the default is
None
, to wait forever
Changed in version 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
.
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.