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| 54 | ||
| 55 | <H1><A NAME="SECTION009300000000000000000"> | |
| 56 | 7.3 <tt class="module">select</tt> -- | |
| 57 | Waiting for I/O completion</A> | |
| 58 | </H1> | |
| 59 | ||
| 60 | <P> | |
| 61 | <A NAME="module-select"></A> | |
| 62 | ||
| 63 | <P> | |
| 64 | This module provides access to the <tt class="cfunction">select()</tt> | |
| 65 | and <tt class="cfunction">poll()</tt> functions | |
| 66 | available in most operating systems. Note that on Windows, it only | |
| 67 | works for sockets; on other operating systems, it also works for other | |
| 68 | file types (in particular, on <span class="Unix">Unix</span>, it works on pipes). It cannot | |
| 69 | be used on regular files to determine whether a file has grown since | |
| 70 | it was last read. | |
| 71 | ||
| 72 | <P> | |
| 73 | The module defines the following: | |
| 74 | ||
| 75 | <P> | |
| 76 | <dl><dt><b><span class="typelabel">exception</span> <tt id='l2h-2676' xml:id='l2h-2676' class="exception">error</tt></b></dt> | |
| 77 | <dd> | |
| 78 | The exception raised when an error occurs. The accompanying value is | |
| 79 | a pair containing the numeric error code from <tt class="cdata">errno</tt> and the | |
| 80 | corresponding string, as would be printed by the C function | |
| 81 | <tt class="cfunction">perror()</tt>. | |
| 82 | </dd></dl> | |
| 83 | ||
| 84 | <P> | |
| 85 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
| 86 | <td><nobr><b><tt id='l2h-2677' xml:id='l2h-2677' class="function">poll</tt></b>(</nobr></td> | |
| 87 | <td><var></var>)</td></tr></table></dt> | |
| 88 | <dd> | |
| 89 | (Not supported by all operating systems.) Returns a polling object, | |
| 90 | which supports registering and unregistering file descriptors, and | |
| 91 | then polling them for I/O events; | |
| 92 | see section <A href="poll-objects.html#poll-objects">7.3.1</A> below for the methods supported by | |
| 93 | polling objects. | |
| 94 | </dl> | |
| 95 | ||
| 96 | <P> | |
| 97 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
| 98 | <td><nobr><b><tt id='l2h-2678' xml:id='l2h-2678' class="function">select</tt></b>(</nobr></td> | |
| 99 | <td><var>iwtd, owtd, ewtd</var><big>[</big><var>, timeout</var><big>]</big><var></var>)</td></tr></table></dt> | |
| 100 | <dd> | |
| 101 | This is a straightforward interface to the <span class="Unix">Unix</span> <tt class="cfunction">select()</tt> | |
| 102 | system call. The first three arguments are sequences of `waitable | |
| 103 | objects': either integers representing file descriptors or | |
| 104 | objects with a parameterless method named <tt class="method">fileno()</tt> returning | |
| 105 | such an integer. The three sequences of waitable objects are for input, | |
| 106 | output and `exceptional conditions', respectively. Empty sequences are | |
| 107 | allowed, but acceptance of three empty sequences is platform-dependent. | |
| 108 | (It is known to work on <span class="Unix">Unix</span> but not on Windows.) The optional | |
| 109 | <var>timeout</var> argument specifies a time-out as a floating point number | |
| 110 | in seconds. When the <var>timeout</var> argument is omitted the function | |
| 111 | blocks until at least one file descriptor is ready. A time-out value | |
| 112 | of zero specifies a poll and never blocks. | |
| 113 | ||
| 114 | <P> | |
| 115 | The return value is a triple of lists of objects that are ready: | |
| 116 | subsets of the first three arguments. When the time-out is reached | |
| 117 | without a file descriptor becoming ready, three empty lists are | |
| 118 | returned. | |
| 119 | ||
| 120 | <P> | |
| 121 | Among the acceptable object types in the sequences are Python file | |
| 122 | objects (e.g. <code>sys.stdin</code>, or objects returned by | |
| 123 | <tt class="function">open()</tt> or <tt class="function">os.popen()</tt>), socket objects | |
| 124 | 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 | |
| 125 | an appropriate <tt class="method">fileno()</tt> method (that really returns a file | |
| 126 | descriptor, not just a random integer). | |
| 127 | <span class="note"><b class="label">Note:</b> | |
| 128 | File objects on Windows are not acceptable, but sockets | |
| 129 | are.<a id='l2h-2683' xml:id='l2h-2683'></a> On Windows, the underlying <tt class="cfunction">select()</tt> | |
| 130 | function is provided by the WinSock library, and does not handle file | |
| 131 | descriptors that don't originate from WinSock.</span> | |
| 132 | </dl> | |
| 133 | ||
| 134 | <P> | |
| 135 | ||
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| 138 | <!--Table of Child-Links--> | |
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| 140 | ||
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| 142 | <LI><A href="poll-objects.html">7.3.1 Polling Objects</a> | |
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| 181 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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