select — Waiting for I/O completion¶

/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 a fileno() method that returns an integer. File objects implement 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. The constants are the same that with poll() object. The default value is a combination of the constants POLLIN, POLLPRI, and POLLOUT.

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 a register(). 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 a fileno() 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, or None, 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 a fileno() method that returns an integer. File objects implement 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 POLLIN, POLLPRI, and POLLOUT, 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 an OSError exception with errno ENOENT 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 a fileno() 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, or None, 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 macOS]
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 and KQ_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 macOS):

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.