sam-t2/devtools/v9/html/python/lib/module-select.html

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<link rel="STYLESHEET" href="lib.css" type='text/css' />
<link rel="SHORTCUT ICON" href="../icons/pyfav.png" type="image/png" />
<link rel='start' href='../index.html' title='Python Documentation Index' />
<link rel="first" href="lib.html" title='Python Library Reference' />
<link rel='contents' href='contents.html' title="Contents" />
<link rel='index' href='genindex.html' title='Index' />
<link rel='last' href='about.html' title='About this document...' />
<link rel='help' href='about.html' title='About this document...' />
<link rel="next" href="module-thread.html" />
<link rel="prev" href="module-socket.html" />
<link rel="parent" href="someos.html" />
<link rel="next" href="poll-objects.html" />
<meta name='aesop' content='information' />
<title>7.3 select -- Waiting for I/O completion</title>
</head>
<body>
<DIV CLASS="navigation">
<div id='top-navigation-panel' xml:id='top-navigation-panel'>
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td class='online-navigation'><a rel="prev" title="7.2.3 Example"
  href="socket-example.html"><img src='../icons/previous.png'
  border='0' height='32'  alt='Previous Page' width='32' /></A></td>
<td class='online-navigation'><a rel="parent" title="7. Optional Operating System"
  href="someos.html"><img src='../icons/up.png'
  border='0' height='32'  alt='Up One Level' width='32' /></A></td>
<td class='online-navigation'><a rel="next" title="7.3.1 Polling Objects"
  href="poll-objects.html"><img src='../icons/next.png'
  border='0' height='32'  alt='Next Page' width='32' /></A></td>
<td align="center" width="100%">Python Library Reference</td>
<td class='online-navigation'><a rel="contents" title="Table of Contents"
  href="contents.html"><img src='../icons/contents.png'
  border='0' height='32'  alt='Contents' width='32' /></A></td>
<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
  border='0' height='32'  alt='Module Index' width='32' /></a></td>
<td class='online-navigation'><a rel="index" title="Index"
  href="genindex.html"><img src='../icons/index.png'
  border='0' height='32'  alt='Index' width='32' /></A></td>
</tr></table>
<div class='online-navigation'>
<b class="navlabel">Previous:</b>
<a class="sectref" rel="prev" href="socket-example.html">7.2.3 Example</A>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="someos.html">7. Optional Operating System</A>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="poll-objects.html">7.3.1 Polling Objects</A>
</div>
<hr /></div>
</DIV>
<!--End of Navigation Panel-->

<H1><A NAME="SECTION009300000000000000000">
7.3 <tt class="module">select</tt> --
         Waiting for I/O completion</A>
</H1>

<P>
<A NAME="module-select"></A>

<P>
This module provides access to the <tt class="cfunction">select()</tt>
and <tt class="cfunction">poll()</tt> functions
available in most operating systems.  Note that on Windows, it only
works for sockets; on other operating systems, it also works for other
file types (in particular, on <span class="Unix">Unix</span>, it works on pipes).  It cannot
be used on regular files to determine whether a file has grown since
it was last read.

<P>
The module defines the following:

<P>
<dl><dt><b><span class="typelabel">exception</span>&nbsp;<tt id='l2h-2676' xml:id='l2h-2676' class="exception">error</tt></b></dt>
<dd>
The exception raised when an error occurs.  The accompanying value is
a pair containing the numeric error code from <tt class="cdata">errno</tt> and the
corresponding string, as would be printed by the C function
<tt class="cfunction">perror()</tt>.
</dd></dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2677' xml:id='l2h-2677' class="function">poll</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
(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&nbsp;<A href="poll-objects.html#poll-objects">7.3.1</A> below for the methods supported by
polling objects.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2678' xml:id='l2h-2678' class="function">select</tt></b>(</nobr></td>
  <td><var>iwtd, owtd, ewtd</var><big>[</big><var>, timeout</var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
This is a straightforward interface to the <span class="Unix">Unix</span> <tt class="cfunction">select()</tt>
system call.  The first three arguments are sequences of `waitable
objects': either integers representing file descriptors or
objects with a parameterless method named <tt class="method">fileno()</tt> returning
such an integer.  The three sequences of waitable objects are for input,
output and `exceptional conditions', respectively.  Empty sequences are
allowed, but acceptance of three empty sequences is platform-dependent.
(It is known to work on <span class="Unix">Unix</span> but not on Windows.)  The optional
<var>timeout</var> argument specifies a time-out as a floating point number
in seconds.  When the <var>timeout</var> 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.

<P>
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.

<P>
Among the acceptable object types in the sequences are Python file
objects (e.g. <code>sys.stdin</code>, or objects returned by
<tt class="function">open()</tt> or <tt class="function">os.popen()</tt>), socket objects
returned by <tt class="function">socket.socket()</tt>.<a id='l2h-2680' xml:id='l2h-2680'></a><a id='l2h-2682' xml:id='l2h-2682'></a>You may also define a <i class="dfn">wrapper</i> class yourself, as long as it has
an appropriate <tt class="method">fileno()</tt> method (that really returns a file
descriptor, not just a random integer).
<span class="note"><b class="label">Note:</b>
File objects on Windows are not acceptable, but sockets
are.<a id='l2h-2683' xml:id='l2h-2683'></a>  On Windows, the underlying <tt class="cfunction">select()</tt>
function is provided by the WinSock library, and does not handle file
descriptors that don't originate from WinSock.</span>
</dl>

<P>

<p><br /></p><hr class='online-navigation' />
<div class='online-navigation'>
<!--Table of Child-Links-->
<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></a>

<UL CLASS="ChildLinks">
<LI><A href="poll-objects.html">7.3.1 Polling Objects</a>
</ul>
<!--End of Table of Child-Links-->
</div>

<DIV CLASS="navigation">
<div class='online-navigation'>
<p></p><hr />
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td class='online-navigation'><a rel="prev" title="7.2.3 Example"
  href="socket-example.html"><img src='../icons/previous.png'
  border='0' height='32'  alt='Previous Page' width='32' /></A></td>
<td class='online-navigation'><a rel="parent" title="7. Optional Operating System"
  href="someos.html"><img src='../icons/up.png'
  border='0' height='32'  alt='Up One Level' width='32' /></A></td>
<td class='online-navigation'><a rel="next" title="7.3.1 Polling Objects"
  href="poll-objects.html"><img src='../icons/next.png'
  border='0' height='32'  alt='Next Page' width='32' /></A></td>
<td align="center" width="100%">Python Library Reference</td>
<td class='online-navigation'><a rel="contents" title="Table of Contents"
  href="contents.html"><img src='../icons/contents.png'
  border='0' height='32'  alt='Contents' width='32' /></A></td>
<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
  border='0' height='32'  alt='Module Index' width='32' /></a></td>
<td class='online-navigation'><a rel="index" title="Index"
  href="genindex.html"><img src='../icons/index.png'
  border='0' height='32'  alt='Index' width='32' /></A></td>
</tr></table>
<div class='online-navigation'>
<b class="navlabel">Previous:</b>
<a class="sectref" rel="prev" href="socket-example.html">7.2.3 Example</A>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="someos.html">7. Optional Operating System</A>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="poll-objects.html">7.3.1 Polling Objects</A>
</div>
</div>
<hr />
<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
</DIV>
<!--End of Navigation Panel-->
<ADDRESS>
See <i><a href="about.html">About this document...</a></i> for information on suggesting changes.
</ADDRESS>
</BODY>
</HTML>