1. Nick Coghlan
  2. cpython_sandbox


cpython_sandbox / Doc / library / select.rst

:mod:`select` --- Waiting for I/O completion

This module provides access to the :c:func:`select` and :c:func:`poll` functions available in most operating systems, :c:func:`epoll` available on Linux 2.5+ and :c:func:`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.

The module defines the following:

Edge and Level Trigger Polling (epoll) Objects



Constant Meaning
:const:`EPOLLIN` Available for read
:const:`EPOLLOUT` Available for write
:const:`EPOLLPRI` Urgent data for read
:const:`EPOLLERR` Error condition happened on the assoc. fd
:const:`EPOLLHUP` Hang up happened on the assoc. fd
:const:`EPOLLET` Set Edge Trigger behavior, the default is Level Trigger behavior
:const:`EPOLLONESHOT` Set one-shot behavior. After one event is pulled out, the fd is internally disabled
:const:`EPOLLRDNORM` Equivalent to :const:`EPOLLIN`
:const:`EPOLLRDBAND` Priority data band can be read.
:const:`EPOLLWRNORM` Equivalent to :const:`EPOLLOUT`
:const:`EPOLLWRBAND` Priority data may be written.
:const:`EPOLLMSG` Ignored.

Polling Objects

The :c:func:`poll` system call, supported on most Unix systems, provides better scalability for network servers that service many, many clients at the same time. :c:func:`poll` scales better because the system call only requires listing the file descriptors of interest, while :c:func:`select` builds a bitmap, turns on bits for the fds of interest, and then afterward the whole bitmap has to be linearly scanned again. :c:func:`select` is O(highest file descriptor), while :c:func:`poll` is O(number of file descriptors).

Kqueue Objects