<!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.gif" /> <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 http-equiv="Content-Type" content="text/html; charset=iso-8859-1" /> <meta name='aesop' content='information' /> <META name="description" content="select -- Waiting for I/O completion"> <META name="keywords" content="lib"> <META name="resource-type" content="document"> <META name="distribution" content="global"> <title>7.3 select -- Waiting for I/O completion</title> </head> <body> <DIV CLASS="navigation"> <div 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"><!--z--></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 <font style="font-variant: small-caps;">Unix</font>, 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> <tt id='l2h-2481' 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-2482' class="function">poll</tt></b>(</nobr></td> <td>)</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 <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-2483' class="function">select</tt></b>(</nobr></td> <td><var>iwtd, owtd, ewtd</var><big>[</big><var>, timeout</var><big>]</big>)</td></tr></table></dt> <dd> This is a straightforward interface to the <font style="font-variant: small-caps;">Unix</font> <tt class="cfunction">select()</tt> system call. The first three arguments are lists 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 lists of waitable objects are for input, output and `exceptional conditions', respectively. Empty lists are allowed, but acceptance of three empty lists is platform-dependent. (It is known to work on <font style="font-variant: small-caps;">Unix</font> 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 lists 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-2485'><!--x--></a><a id='l2h-2487'><!--x--></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-2488'><!--x--></a> On Windows, the underlying <tt class="cfunction">select()</tt> function is provided by the WinSock library, and does not handle file desciptors 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'><hr /> <table align="center" width="100%" cellpadding="0" cellspacing="2"> <tr> <td class='online-navigation'><a rel="prev" title="7.2.3 Example" 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" 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" 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" 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" 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 2.3.4, documentation updated on May 20, 2004.</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>