uselect – Wait for events

This module provides functions to efficiently wait for events on multiple streams.


This module is not available on the BOOST Move hub.


Data is available for reading.


More data can be written.


Error condition happened on the associated stream.


Hang up happened on the associated stream.

poll() Poll

Creates an instance of the Poll class.

class Poll
register(obj: IO) None
register(obj: IO, eventmask: int) None

Register stream obj for polling. eventmask is logical OR of:

Note that flags like POLLHUP and POLLERR are not valid as input eventmask (these are unsolicited events which will be returned from poll() regardless of whether they are asked for). This semantics is per POSIX.

eventmask defaults to POLLIN | POLLOUT.

It is OK to call this function multiple times for the same obj. Successive calls will update obj’s eventmask to the value of eventmask (i.e. will behave as modify()).

unregister(obj: IO) None

Unregister obj from polling.

modify(obj: IO, eventmask: int) None

Modify the eventmask for obj. If obj is not registered, OSError is raised with error of ENOENT.

poll() List[Tuple[IO, int]]
poll(timeout: int) List[Tuple[IO, int]]

Wait for at least one of the registered objects to become ready or have an exceptional condition, with optional timeout in milliseconds (if timeout arg is not specified or -1, there is no timeout).

Returns list of (obj, event, …) tuples. There may be other elements in tuple, depending on a platform and version, so don’t assume that its size is 2. The event element specifies which events happened with a stream and is a combination of POLL* constants described above. Note that flags POLLHUP and POLLERR can be returned at any time (even if were not asked for), and must be acted on accordingly (the corresponding stream unregistered from poll and likely closed), because otherwise all further invocations of poll() may return immediately with these flags set for this stream again.

In case of timeout, an empty list is returned.

ipoll() Iterator[Tuple[IO, int]]
ipoll(timeout: int) Iterator[Tuple[IO, int]]
ipoll(timeout: int, flags: int) Iterator[Tuple[IO, int]]

Like poll(), but instead returns an iterator which yields a callee-owned tuple. This function provides an efficient, allocation-free way to poll on streams.

If flags is 1, one-shot behavior for events is employed: streams for which events happened will have their event masks automatically reset (equivalent to poll.modify(obj, 0)), so new events for such a stream won’t be processed until new mask is set with modify(). This behavior is useful for asynchronous I/O schedulers.