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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>18.3. select — Waiting for I/O completion — Python 3.6.7 documentation</title> <link rel="stylesheet" href="../_static/pydoctheme.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 3.6.7 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="18.4. selectors — High-level I/O multiplexing" href="selectors.html" /> <link rel="prev" title="18.2. ssl — TLS/SSL wrapper for socket objects" href="ssl.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/3/library/select.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="selectors.html" title="18.4. selectors — High-level I/O multiplexing" accesskey="N">next</a> |</li> <li class="right" > <a href="ssl.html" title="18.2. ssl — TLS/SSL wrapper for socket objects" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">3.6.7 Documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> »</li> <li class="nav-item nav-item-2"><a href="ipc.html" accesskey="U">18. Interprocess Communication and Networking</a> »</li> <li class="right"> <div class="inline-search" style="display: none" role="search"> <form class="inline-search" action="../search.html" method="get"> <input placeholder="Quick search" type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> <script type="text/javascript">$('.inline-search').show(0);</script> | </li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="module-select"> <span id="select-waiting-for-i-o-completion"></span><h1>18.3. <a class="reference internal" href="#module-select" title="select: Wait for I/O completion on multiple streams."><code class="xref py py-mod docutils literal notranslate"><span class="pre">select</span></code></a> — Waiting for I/O completion<a class="headerlink" href="#module-select" title="Permalink to this headline">¶</a></h1> <hr class="docutils" /> <p>This module provides access to the <code class="xref c c-func docutils literal notranslate"><span class="pre">select()</span></code> and <code class="xref c c-func docutils literal notranslate"><span class="pre">poll()</span></code> functions available in most operating systems, <code class="xref c c-func docutils literal notranslate"><span class="pre">devpoll()</span></code> available on Solaris and derivatives, <code class="xref c c-func docutils literal notranslate"><span class="pre">epoll()</span></code> available on Linux 2.5+ and <code class="xref c c-func docutils literal notranslate"><span class="pre">kqueue()</span></code> 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.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">The <a class="reference internal" href="selectors.html#module-selectors" title="selectors: High-level I/O multiplexing."><code class="xref py py-mod docutils literal notranslate"><span class="pre">selectors</span></code></a> module allows high-level and efficient I/O multiplexing, built upon the <a class="reference internal" href="#module-select" title="select: Wait for I/O completion on multiple streams."><code class="xref py py-mod docutils literal notranslate"><span class="pre">select</span></code></a> module primitives. Users are encouraged to use the <a class="reference internal" href="selectors.html#module-selectors" title="selectors: High-level I/O multiplexing."><code class="xref py py-mod docutils literal notranslate"><span class="pre">selectors</span></code></a> module instead, unless they want precise control over the OS-level primitives used.</p> </div> <p>The module defines the following:</p> <dl class="exception"> <dt id="select.error"> <em class="property">exception </em><code class="descclassname">select.</code><code class="descname">error</code><a class="headerlink" href="#select.error" title="Permalink to this definition">¶</a></dt> <dd><p>A deprecated alias of <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">OSError</span></code></a>.</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.3: </span>Following <span class="target" id="index-0"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3151"><strong>PEP 3151</strong></a>, this class was made an alias of <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">OSError</span></code></a>.</p> </div> </dd></dl> <dl class="function"> <dt id="select.devpoll"> <code class="descclassname">select.</code><code class="descname">devpoll</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#select.devpoll" title="Permalink to this definition">¶</a></dt> <dd><p>(Only supported on Solaris and derivatives.) Returns a <code class="docutils literal notranslate"><span class="pre">/dev/poll</span></code> polling object; see section <a class="reference internal" href="#devpoll-objects"><span class="std std-ref">/dev/poll Polling Objects</span></a> below for the methods supported by devpoll objects.</p> <p><code class="xref c c-func docutils literal notranslate"><span class="pre">devpoll()</span></code> objects are linked to the number of file descriptors allowed at the time of instantiation. If your program reduces this value, <code class="xref c c-func docutils literal notranslate"><span class="pre">devpoll()</span></code> will fail. If your program increases this value, <code class="xref c c-func docutils literal notranslate"><span class="pre">devpoll()</span></code> may return an incomplete list of active file descriptors.</p> <p>The new file descriptor is <a class="reference internal" href="os.html#fd-inheritance"><span class="std std-ref">non-inheritable</span></a>.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 3.3.</span></p> </div> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.4: </span>The new file descriptor is now non-inheritable.</p> </div> </dd></dl> <dl class="function"> <dt id="select.epoll"> <code class="descclassname">select.</code><code class="descname">epoll</code><span class="sig-paren">(</span><em>sizehint=-1</em>, <em>flags=0</em><span class="sig-paren">)</span><a class="headerlink" href="#select.epoll" title="Permalink to this definition">¶</a></dt> <dd><p>(Only supported on Linux 2.5.44 and newer.) Return an edge polling object, which can be used as Edge or Level Triggered interface for I/O events.</p> <p><em>sizehint</em> informs epoll about the expected number of events to be registered. It must be positive, or <cite>-1</cite> to use the default. It is only used on older systems where <code class="xref c c-func docutils literal notranslate"><span class="pre">epoll_create1()</span></code> is not available; otherwise it has no effect (though its value is still checked).</p> <p><em>flags</em> is deprecated and completely ignored. However, when supplied, its value must be <code class="docutils literal notranslate"><span class="pre">0</span></code> or <code class="docutils literal notranslate"><span class="pre">select.EPOLL_CLOEXEC</span></code>, otherwise <code class="docutils literal notranslate"><span class="pre">OSError</span></code> is raised.</p> <p>See the <a class="reference internal" href="#epoll-objects"><span class="std std-ref">Edge and Level Trigger Polling (epoll) Objects</span></a> section below for the methods supported by epolling objects.</p> <p><code class="docutils literal notranslate"><span class="pre">epoll</span></code> objects support the context management protocol: when used in a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement, the new file descriptor is automatically closed at the end of the block.</p> <p>The new file descriptor is <a class="reference internal" href="os.html#fd-inheritance"><span class="std std-ref">non-inheritable</span></a>.</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.3: </span>Added the <em>flags</em> parameter.</p> </div> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.4: </span>Support for the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement was added. The new file descriptor is now non-inheritable.</p> </div> <div class="deprecated"> <p><span class="versionmodified">Deprecated since version 3.4: </span>The <em>flags</em> parameter. <code class="docutils literal notranslate"><span class="pre">select.EPOLL_CLOEXEC</span></code> is used by default now. Use <a class="reference internal" href="os.html#os.set_inheritable" title="os.set_inheritable"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.set_inheritable()</span></code></a> to make the file descriptor inheritable.</p> </div> </dd></dl> <dl class="function"> <dt id="select.poll"> <code class="descclassname">select.</code><code class="descname">poll</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#select.poll" title="Permalink to this definition">¶</a></dt> <dd><p>(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 class="reference internal" href="#poll-objects"><span class="std std-ref">Polling Objects</span></a> below for the methods supported by polling objects.</p> </dd></dl> <dl class="function"> <dt id="select.kqueue"> <code class="descclassname">select.</code><code class="descname">kqueue</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#select.kqueue" title="Permalink to this definition">¶</a></dt> <dd><p>(Only supported on BSD.) Returns a kernel queue object; see section <a class="reference internal" href="#kqueue-objects"><span class="std std-ref">Kqueue Objects</span></a> below for the methods supported by kqueue objects.</p> <p>The new file descriptor is <a class="reference internal" href="os.html#fd-inheritance"><span class="std std-ref">non-inheritable</span></a>.</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.4: </span>The new file descriptor is now non-inheritable.</p> </div> </dd></dl> <dl class="function"> <dt id="select.kevent"> <code class="descclassname">select.</code><code class="descname">kevent</code><span class="sig-paren">(</span><em>ident</em>, <em>filter=KQ_FILTER_READ</em>, <em>flags=KQ_EV_ADD</em>, <em>fflags=0</em>, <em>data=0</em>, <em>udata=0</em><span class="sig-paren">)</span><a class="headerlink" href="#select.kevent" title="Permalink to this definition">¶</a></dt> <dd><p>(Only supported on BSD.) Returns a kernel event object; see section <a class="reference internal" href="#kevent-objects"><span class="std std-ref">Kevent Objects</span></a> below for the methods supported by kevent objects.</p> </dd></dl> <dl class="function"> <dt id="select.select"> <code class="descclassname">select.</code><code class="descname">select</code><span class="sig-paren">(</span><em>rlist</em>, <em>wlist</em>, <em>xlist</em><span class="optional">[</span>, <em>timeout</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#select.select" title="Permalink to this definition">¶</a></dt> <dd><p>This is a straightforward interface to the Unix <code class="xref c c-func docutils literal notranslate"><span class="pre">select()</span></code> system call. The first three arguments are sequences of ‘waitable objects’: either integers representing file descriptors or objects with a parameterless method named <a class="reference internal" href="io.html#io.IOBase.fileno" title="io.IOBase.fileno"><code class="xref py py-meth docutils literal notranslate"><span class="pre">fileno()</span></code></a> returning such an integer:</p> <ul class="simple"> <li><em>rlist</em>: wait until ready for reading</li> <li><em>wlist</em>: wait until ready for writing</li> <li><em>xlist</em>: wait for an “exceptional condition” (see the manual page for what your system considers such a condition)</li> </ul> <p>Empty sequences are allowed, but acceptance of three empty sequences is platform-dependent. (It is known to work on Unix but not on Windows.) The optional <em>timeout</em> argument specifies a time-out as a floating point number in seconds. When the <em>timeout</em> 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> <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> <p id="index-1">Among the acceptable object types in the sequences are Python <a class="reference internal" href="../glossary.html#term-file-object"><span class="xref std std-term">file objects</span></a> (e.g. <code class="docutils literal notranslate"><span class="pre">sys.stdin</span></code>, or objects returned by <a class="reference internal" href="functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate"><span class="pre">open()</span></code></a> or <a class="reference internal" href="os.html#os.popen" title="os.popen"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.popen()</span></code></a>), socket objects returned by <a class="reference internal" href="socket.html#socket.socket" title="socket.socket"><code class="xref py py-func docutils literal notranslate"><span class="pre">socket.socket()</span></code></a>. You may also define a <em class="dfn">wrapper</em> class yourself, as long as it has an appropriate <a class="reference internal" href="io.html#io.IOBase.fileno" title="io.IOBase.fileno"><code class="xref py py-meth docutils literal notranslate"><span class="pre">fileno()</span></code></a> method (that really returns a file descriptor, not just a random integer).</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last" id="index-2">File objects on Windows are not acceptable, but sockets are. On Windows, the underlying <code class="xref c c-func docutils literal notranslate"><span class="pre">select()</span></code> function is provided by the WinSock library, and does not handle file descriptors that don’t originate from WinSock.</p> </div> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.5: </span>The function is now retried with a recomputed timeout when interrupted by a signal, except if the signal handler raises an exception (see <span class="target" id="index-3"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0475"><strong>PEP 475</strong></a> for the rationale), instead of raising <a class="reference internal" href="exceptions.html#InterruptedError" title="InterruptedError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">InterruptedError</span></code></a>.</p> </div> </dd></dl> <dl class="attribute"> <dt id="select.PIPE_BUF"> <code class="descclassname">select.</code><code class="descname">PIPE_BUF</code><a class="headerlink" href="#select.PIPE_BUF" title="Permalink to this definition">¶</a></dt> <dd><p>The minimum number of bytes which can be written without blocking to a pipe when the pipe has been reported as ready for writing by <a class="reference internal" href="#select.select" title="select.select"><code class="xref py py-func docutils literal notranslate"><span class="pre">select()</span></code></a>, <a class="reference internal" href="#select.poll" title="select.poll"><code class="xref py py-func docutils literal notranslate"><span class="pre">poll()</span></code></a> or another interface in this module. This doesn’t apply to other kind of file-like objects such as sockets.</p> <p>This value is guaranteed by POSIX to be at least 512. Availability: Unix.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 3.2.</span></p> </div> </dd></dl> <div class="section" id="dev-poll-polling-objects"> <span id="devpoll-objects"></span><h2>18.3.1. <code class="docutils literal notranslate"><span class="pre">/dev/poll</span></code> Polling Objects<a class="headerlink" href="#dev-poll-polling-objects" title="Permalink to this headline">¶</a></h2> <p>Solaris and derivatives have <code class="docutils literal notranslate"><span class="pre">/dev/poll</span></code>. While <code class="xref c c-func docutils literal notranslate"><span class="pre">select()</span></code> is O(highest file descriptor) and <code class="xref c c-func docutils literal notranslate"><span class="pre">poll()</span></code> is O(number of file descriptors), <code class="docutils literal notranslate"><span class="pre">/dev/poll</span></code> is O(active file descriptors).</p> <p><code class="docutils literal notranslate"><span class="pre">/dev/poll</span></code> behaviour is very close to the standard <code class="xref c c-func docutils literal notranslate"><span class="pre">poll()</span></code> object.</p> <dl class="method"> <dt id="select.devpoll.close"> <code class="descclassname">devpoll.</code><code class="descname">close</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#select.devpoll.close" title="Permalink to this definition">¶</a></dt> <dd><p>Close the file descriptor of the polling object.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 3.4.</span></p> </div> </dd></dl> <dl class="attribute"> <dt id="select.devpoll.closed"> <code class="descclassname">devpoll.</code><code class="descname">closed</code><a class="headerlink" href="#select.devpoll.closed" title="Permalink to this definition">¶</a></dt> <dd><p><code class="docutils literal notranslate"><span class="pre">True</span></code> if the polling object is closed.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 3.4.</span></p> </div> </dd></dl> <dl class="method"> <dt id="select.devpoll.fileno"> <code class="descclassname">devpoll.</code><code class="descname">fileno</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#select.devpoll.fileno" title="Permalink to this definition">¶</a></dt> <dd><p>Return the file descriptor number of the polling object.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 3.4.</span></p> </div> </dd></dl> <dl class="method"> <dt id="select.devpoll.register"> <code class="descclassname">devpoll.</code><code class="descname">register</code><span class="sig-paren">(</span><em>fd</em><span class="optional">[</span>, <em>eventmask</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#select.devpoll.register" title="Permalink to this definition">¶</a></dt> <dd><p>Register a file descriptor with the polling object. Future calls to the <a class="reference internal" href="#select.poll" title="select.poll"><code class="xref py py-meth docutils literal notranslate"><span class="pre">poll()</span></code></a> method will then check whether the file descriptor has any pending I/O events. <em>fd</em> can be either an integer, or an object with a <a class="reference internal" href="io.html#io.IOBase.fileno" title="io.IOBase.fileno"><code class="xref py py-meth docutils literal notranslate"><span class="pre">fileno()</span></code></a> method that returns an integer. File objects implement <code class="xref py py-meth docutils literal notranslate"><span class="pre">fileno()</span></code>, so they can also be used as the argument.</p> <p><em>eventmask</em> is an optional bitmask describing the type of events you want to check for. The constants are the same that with <code class="xref c c-func docutils literal notranslate"><span class="pre">poll()</span></code> object. The default value is a combination of the constants <code class="xref py py-const docutils literal notranslate"><span class="pre">POLLIN</span></code>, <code class="xref py py-const docutils literal notranslate"><span class="pre">POLLPRI</span></code>, and <code class="xref py py-const docutils literal notranslate"><span class="pre">POLLOUT</span></code>.</p> <div class="admonition warning"> <p class="first admonition-title">Warning</p> <p class="last">Registering a file descriptor that’s already registered is not an error, but the result is undefined. The appropriate action is to unregister or modify it first. This is an important difference compared with <code class="xref c c-func docutils literal notranslate"><span class="pre">poll()</span></code>.</p> </div> </dd></dl> <dl class="method"> <dt id="select.devpoll.modify"> <code class="descclassname">devpoll.</code><code class="descname">modify</code><span class="sig-paren">(</span><em>fd</em><span class="optional">[</span>, <em>eventmask</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#select.devpoll.modify" title="Permalink to this definition">¶</a></dt> <dd><p>This method does an <a class="reference internal" href="#select.devpoll.unregister" title="select.devpoll.unregister"><code class="xref py py-meth docutils literal notranslate"><span class="pre">unregister()</span></code></a> followed by a <a class="reference internal" href="#select.devpoll.register" title="select.devpoll.register"><code class="xref py py-meth docutils literal notranslate"><span class="pre">register()</span></code></a>. It is (a bit) more efficient that doing the same explicitly.</p> </dd></dl> <dl class="method"> <dt id="select.devpoll.unregister"> <code class="descclassname">devpoll.</code><code class="descname">unregister</code><span class="sig-paren">(</span><em>fd</em><span class="sig-paren">)</span><a class="headerlink" href="#select.devpoll.unregister" title="Permalink to this definition">¶</a></dt> <dd><p>Remove a file descriptor being tracked by a polling object. Just like the <a class="reference internal" href="#select.devpoll.register" title="select.devpoll.register"><code class="xref py py-meth docutils literal notranslate"><span class="pre">register()</span></code></a> method, <em>fd</em> can be an integer or an object with a <a class="reference internal" href="io.html#io.IOBase.fileno" title="io.IOBase.fileno"><code class="xref py py-meth docutils literal notranslate"><span class="pre">fileno()</span></code></a> method that returns an integer.</p> <p>Attempting to remove a file descriptor that was never registered is safely ignored.</p> </dd></dl> <dl class="method"> <dt id="select.devpoll.poll"> <code class="descclassname">devpoll.</code><code class="descname">poll</code><span class="sig-paren">(</span><span class="optional">[</span><em>timeout</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#select.devpoll.poll" title="Permalink to this definition">¶</a></dt> <dd><p>Polls the set of registered file descriptors, and returns a possibly-empty list containing <code class="docutils literal notranslate"><span class="pre">(fd,</span> <span class="pre">event)</span></code> 2-tuples for the descriptors that have events or errors to report. <em>fd</em> is the file descriptor, and <em>event</em> is a bitmask with bits set for the reported events for that descriptor — <code class="xref py py-const docutils literal notranslate"><span class="pre">POLLIN</span></code> for waiting input, <code class="xref py py-const docutils literal notranslate"><span class="pre">POLLOUT</span></code> to indicate that the descriptor can be written to, and so forth. An empty list indicates that the call timed out and no file descriptors had any events to report. If <em>timeout</em> is given, it specifies the length of time in milliseconds which the system will wait for events before returning. If <em>timeout</em> is omitted, -1, or <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a>, the call will block until there is an event for this poll object.</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.5: </span>The function is now retried with a recomputed timeout when interrupted by a signal, except if the signal handler raises an exception (see <span class="target" id="index-4"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0475"><strong>PEP 475</strong></a> for the rationale), instead of raising <a class="reference internal" href="exceptions.html#InterruptedError" title="InterruptedError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">InterruptedError</span></code></a>.</p> </div> </dd></dl> </div> <div class="section" id="edge-and-level-trigger-polling-epoll-objects"> <span id="epoll-objects"></span><h2>18.3.2. Edge and Level Trigger Polling (epoll) Objects<a class="headerlink" href="#edge-and-level-trigger-polling-epoll-objects" title="Permalink to this headline">¶</a></h2> <blockquote> <div><p><a class="reference external" href="http://linux.die.net/man/4/epoll">http://linux.die.net/man/4/epoll</a></p> <p><em>eventmask</em></p> <table border="1" class="docutils"> <colgroup> <col width="35%" /> <col width="65%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Constant</th> <th class="head">Meaning</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLIN</span></code></td> <td>Available for read</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLOUT</span></code></td> <td>Available for write</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLPRI</span></code></td> <td>Urgent data for read</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLERR</span></code></td> <td>Error condition happened on the assoc. fd</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLHUP</span></code></td> <td>Hang up happened on the assoc. fd</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLET</span></code></td> <td>Set Edge Trigger behavior, the default is Level Trigger behavior</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLONESHOT</span></code></td> <td>Set one-shot behavior. After one event is pulled out, the fd is internally disabled</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLEXCLUSIVE</span></code></td> <td>Wake only one epoll object when the associated fd has an event. The default (if this flag is not set) is to wake all epoll objects polling on a fd.</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLRDHUP</span></code></td> <td>Stream socket peer closed connection or shut down writing half of connection.</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLRDNORM</span></code></td> <td>Equivalent to <code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLIN</span></code></td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLRDBAND</span></code></td> <td>Priority data band can be read.</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLWRNORM</span></code></td> <td>Equivalent to <code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLOUT</span></code></td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLWRBAND</span></code></td> <td>Priority data may be written.</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLMSG</span></code></td> <td>Ignored.</td> </tr> </tbody> </table> </div></blockquote> <dl class="method"> <dt id="select.epoll.close"> <code class="descclassname">epoll.</code><code class="descname">close</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#select.epoll.close" title="Permalink to this definition">¶</a></dt> <dd><p>Close the control file descriptor of the epoll object.</p> </dd></dl> <dl class="attribute"> <dt id="select.epoll.closed"> <code class="descclassname">epoll.</code><code class="descname">closed</code><a class="headerlink" href="#select.epoll.closed" title="Permalink to this definition">¶</a></dt> <dd><p><code class="docutils literal notranslate"><span class="pre">True</span></code> if the epoll object is closed.</p> </dd></dl> <dl class="method"> <dt id="select.epoll.fileno"> <code class="descclassname">epoll.</code><code class="descname">fileno</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#select.epoll.fileno" title="Permalink to this definition">¶</a></dt> <dd><p>Return the file descriptor number of the control fd.</p> </dd></dl> <dl class="method"> <dt id="select.epoll.fromfd"> <code class="descclassname">epoll.</code><code class="descname">fromfd</code><span class="sig-paren">(</span><em>fd</em><span class="sig-paren">)</span><a class="headerlink" href="#select.epoll.fromfd" title="Permalink to this definition">¶</a></dt> <dd><p>Create an epoll object from a given file descriptor.</p> </dd></dl> <dl class="method"> <dt id="select.epoll.register"> <code class="descclassname">epoll.</code><code class="descname">register</code><span class="sig-paren">(</span><em>fd</em><span class="optional">[</span>, <em>eventmask</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#select.epoll.register" title="Permalink to this definition">¶</a></dt> <dd><p>Register a fd descriptor with the epoll object.</p> </dd></dl> <dl class="method"> <dt id="select.epoll.modify"> <code class="descclassname">epoll.</code><code class="descname">modify</code><span class="sig-paren">(</span><em>fd</em>, <em>eventmask</em><span class="sig-paren">)</span><a class="headerlink" href="#select.epoll.modify" title="Permalink to this definition">¶</a></dt> <dd><p>Modify a registered file descriptor.</p> </dd></dl> <dl class="method"> <dt id="select.epoll.unregister"> <code class="descclassname">epoll.</code><code class="descname">unregister</code><span class="sig-paren">(</span><em>fd</em><span class="sig-paren">)</span><a class="headerlink" href="#select.epoll.unregister" title="Permalink to this definition">¶</a></dt> <dd><p>Remove a registered file descriptor from the epoll object.</p> </dd></dl> <dl class="method"> <dt id="select.epoll.poll"> <code class="descclassname">epoll.</code><code class="descname">poll</code><span class="sig-paren">(</span><em>timeout=-1</em>, <em>maxevents=-1</em><span class="sig-paren">)</span><a class="headerlink" href="#select.epoll.poll" title="Permalink to this definition">¶</a></dt> <dd><p>Wait for events. timeout in seconds (float)</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.5: </span>The function is now retried with a recomputed timeout when interrupted by a signal, except if the signal handler raises an exception (see <span class="target" id="index-5"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0475"><strong>PEP 475</strong></a> for the rationale), instead of raising <a class="reference internal" href="exceptions.html#InterruptedError" title="InterruptedError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">InterruptedError</span></code></a>.</p> </div> </dd></dl> </div> <div class="section" id="polling-objects"> <span id="poll-objects"></span><h2>18.3.3. Polling Objects<a class="headerlink" href="#polling-objects" title="Permalink to this headline">¶</a></h2> <p>The <code class="xref c c-func docutils literal notranslate"><span class="pre">poll()</span></code> system call, supported on most Unix systems, provides better scalability for network servers that service many, many clients at the same time. <code class="xref c c-func docutils literal notranslate"><span class="pre">poll()</span></code> scales better because the system call only requires listing the file descriptors of interest, while <code class="xref c c-func docutils literal notranslate"><span class="pre">select()</span></code> builds a bitmap, turns on bits for the fds of interest, and then afterward the whole bitmap has to be linearly scanned again. <code class="xref c c-func docutils literal notranslate"><span class="pre">select()</span></code> is O(highest file descriptor), while <code class="xref c c-func docutils literal notranslate"><span class="pre">poll()</span></code> is O(number of file descriptors).</p> <dl class="method"> <dt id="select.poll.register"> <code class="descclassname">poll.</code><code class="descname">register</code><span class="sig-paren">(</span><em>fd</em><span class="optional">[</span>, <em>eventmask</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#select.poll.register" title="Permalink to this definition">¶</a></dt> <dd><p>Register a file descriptor with the polling object. Future calls to the <a class="reference internal" href="#select.poll" title="select.poll"><code class="xref py py-meth docutils literal notranslate"><span class="pre">poll()</span></code></a> method will then check whether the file descriptor has any pending I/O events. <em>fd</em> can be either an integer, or an object with a <a class="reference internal" href="io.html#io.IOBase.fileno" title="io.IOBase.fileno"><code class="xref py py-meth docutils literal notranslate"><span class="pre">fileno()</span></code></a> method that returns an integer. File objects implement <code class="xref py py-meth docutils literal notranslate"><span class="pre">fileno()</span></code>, so they can also be used as the argument.</p> <p><em>eventmask</em> is an optional bitmask describing the type of events you want to check for, and can be a combination of the constants <code class="xref py py-const docutils literal notranslate"><span class="pre">POLLIN</span></code>, <code class="xref py py-const docutils literal notranslate"><span class="pre">POLLPRI</span></code>, and <code class="xref py py-const docutils literal notranslate"><span class="pre">POLLOUT</span></code>, described in the table below. If not specified, the default value used will check for all 3 types of events.</p> <table border="1" class="docutils"> <colgroup> <col width="31%" /> <col width="69%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Constant</th> <th class="head">Meaning</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">POLLIN</span></code></td> <td>There is data to read</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">POLLPRI</span></code></td> <td>There is urgent data to read</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">POLLOUT</span></code></td> <td>Ready for output: writing will not block</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">POLLERR</span></code></td> <td>Error condition of some sort</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">POLLHUP</span></code></td> <td>Hung up</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">POLLRDHUP</span></code></td> <td>Stream socket peer closed connection, or shut down writing half of connection</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">POLLNVAL</span></code></td> <td>Invalid request: descriptor not open</td> </tr> </tbody> </table> <p>Registering a file descriptor that’s already registered is not an error, and has the same effect as registering the descriptor exactly once.</p> </dd></dl> <dl class="method"> <dt id="select.poll.modify"> <code class="descclassname">poll.</code><code class="descname">modify</code><span class="sig-paren">(</span><em>fd</em>, <em>eventmask</em><span class="sig-paren">)</span><a class="headerlink" href="#select.poll.modify" title="Permalink to this definition">¶</a></dt> <dd><p>Modifies an already registered fd. This has the same effect as <code class="docutils literal notranslate"><span class="pre">register(fd,</span> <span class="pre">eventmask)</span></code>. Attempting to modify a file descriptor that was never registered causes an <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">OSError</span></code></a> exception with errno <code class="xref py py-const docutils literal notranslate"><span class="pre">ENOENT</span></code> to be raised.</p> </dd></dl> <dl class="method"> <dt id="select.poll.unregister"> <code class="descclassname">poll.</code><code class="descname">unregister</code><span class="sig-paren">(</span><em>fd</em><span class="sig-paren">)</span><a class="headerlink" href="#select.poll.unregister" title="Permalink to this definition">¶</a></dt> <dd><p>Remove a file descriptor being tracked by a polling object. Just like the <a class="reference internal" href="#select.poll.register" title="select.poll.register"><code class="xref py py-meth docutils literal notranslate"><span class="pre">register()</span></code></a> method, <em>fd</em> can be an integer or an object with a <a class="reference internal" href="io.html#io.IOBase.fileno" title="io.IOBase.fileno"><code class="xref py py-meth docutils literal notranslate"><span class="pre">fileno()</span></code></a> method that returns an integer.</p> <p>Attempting to remove a file descriptor that was never registered causes a <a class="reference internal" href="exceptions.html#KeyError" title="KeyError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyError</span></code></a> exception to be raised.</p> </dd></dl> <dl class="method"> <dt id="select.poll.poll"> <code class="descclassname">poll.</code><code class="descname">poll</code><span class="sig-paren">(</span><span class="optional">[</span><em>timeout</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#select.poll.poll" title="Permalink to this definition">¶</a></dt> <dd><p>Polls the set of registered file descriptors, and returns a possibly-empty list containing <code class="docutils literal notranslate"><span class="pre">(fd,</span> <span class="pre">event)</span></code> 2-tuples for the descriptors that have events or errors to report. <em>fd</em> is the file descriptor, and <em>event</em> is a bitmask with bits set for the reported events for that descriptor — <code class="xref py py-const docutils literal notranslate"><span class="pre">POLLIN</span></code> for waiting input, <code class="xref py py-const docutils literal notranslate"><span class="pre">POLLOUT</span></code> to indicate that the descriptor can be written to, and so forth. An empty list indicates that the call timed out and no file descriptors had any events to report. If <em>timeout</em> is given, it specifies the length of time in milliseconds which the system will wait for events before returning. If <em>timeout</em> is omitted, negative, or <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a>, the call will block until there is an event for this poll object.</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.5: </span>The function is now retried with a recomputed timeout when interrupted by a signal, except if the signal handler raises an exception (see <span class="target" id="index-6"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0475"><strong>PEP 475</strong></a> for the rationale), instead of raising <a class="reference internal" href="exceptions.html#InterruptedError" title="InterruptedError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">InterruptedError</span></code></a>.</p> </div> </dd></dl> </div> <div class="section" id="kqueue-objects"> <span id="id1"></span><h2>18.3.4. Kqueue Objects<a class="headerlink" href="#kqueue-objects" title="Permalink to this headline">¶</a></h2> <dl class="method"> <dt id="select.kqueue.close"> <code class="descclassname">kqueue.</code><code class="descname">close</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#select.kqueue.close" title="Permalink to this definition">¶</a></dt> <dd><p>Close the control file descriptor of the kqueue object.</p> </dd></dl> <dl class="attribute"> <dt id="select.kqueue.closed"> <code class="descclassname">kqueue.</code><code class="descname">closed</code><a class="headerlink" href="#select.kqueue.closed" title="Permalink to this definition">¶</a></dt> <dd><p><code class="docutils literal notranslate"><span class="pre">True</span></code> if the kqueue object is closed.</p> </dd></dl> <dl class="method"> <dt id="select.kqueue.fileno"> <code class="descclassname">kqueue.</code><code class="descname">fileno</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#select.kqueue.fileno" title="Permalink to this definition">¶</a></dt> <dd><p>Return the file descriptor number of the control fd.</p> </dd></dl> <dl class="method"> <dt id="select.kqueue.fromfd"> <code class="descclassname">kqueue.</code><code class="descname">fromfd</code><span class="sig-paren">(</span><em>fd</em><span class="sig-paren">)</span><a class="headerlink" href="#select.kqueue.fromfd" title="Permalink to this definition">¶</a></dt> <dd><p>Create a kqueue object from a given file descriptor.</p> </dd></dl> <dl class="method"> <dt id="select.kqueue.control"> <code class="descclassname">kqueue.</code><code class="descname">control</code><span class="sig-paren">(</span><em>changelist</em>, <em>max_events</em><span class="optional">[</span>, <em>timeout=None</em><span class="optional">]</span><span class="sig-paren">)</span> → eventlist<a class="headerlink" href="#select.kqueue.control" title="Permalink to this definition">¶</a></dt> <dd><p>Low level interface to kevent</p> <ul class="simple"> <li>changelist must be an iterable of kevent object or <code class="docutils literal notranslate"><span class="pre">None</span></code></li> <li>max_events must be 0 or a positive integer</li> <li>timeout in seconds (floats possible)</li> </ul> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.5: </span>The function is now retried with a recomputed timeout when interrupted by a signal, except if the signal handler raises an exception (see <span class="target" id="index-7"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0475"><strong>PEP 475</strong></a> for the rationale), instead of raising <a class="reference internal" href="exceptions.html#InterruptedError" title="InterruptedError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">InterruptedError</span></code></a>.</p> </div> </dd></dl> </div> <div class="section" id="kevent-objects"> <span id="id2"></span><h2>18.3.5. Kevent Objects<a class="headerlink" href="#kevent-objects" title="Permalink to this headline">¶</a></h2> <p><a class="reference external" href="https://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2">https://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2</a></p> <dl class="attribute"> <dt id="select.kevent.ident"> <code class="descclassname">kevent.</code><code class="descname">ident</code><a class="headerlink" href="#select.kevent.ident" title="Permalink to this definition">¶</a></dt> <dd><p>Value used to identify the event. The interpretation depends on the filter but it’s usually the file descriptor. In the constructor ident can either be an int or an object with a <a class="reference internal" href="io.html#io.IOBase.fileno" title="io.IOBase.fileno"><code class="xref py py-meth docutils literal notranslate"><span class="pre">fileno()</span></code></a> method. kevent stores the integer internally.</p> </dd></dl> <dl class="attribute"> <dt id="select.kevent.filter"> <code class="descclassname">kevent.</code><code class="descname">filter</code><a class="headerlink" href="#select.kevent.filter" title="Permalink to this definition">¶</a></dt> <dd><p>Name of the kernel filter.</p> <table border="1" class="docutils"> <colgroup> <col width="38%" /> <col width="63%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Constant</th> <th class="head">Meaning</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_READ</span></code></td> <td>Takes a descriptor and returns whenever there is data available to read</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_WRITE</span></code></td> <td>Takes a descriptor and returns whenever there is data available to write</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_AIO</span></code></td> <td>AIO requests</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_VNODE</span></code></td> <td>Returns when one or more of the requested events watched in <em>fflag</em> occurs</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_PROC</span></code></td> <td>Watch for events on a process id</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_NETDEV</span></code></td> <td>Watch for events on a network device [not available on Mac OS X]</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_SIGNAL</span></code></td> <td>Returns whenever the watched signal is delivered to the process</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_TIMER</span></code></td> <td>Establishes an arbitrary timer</td> </tr> </tbody> </table> </dd></dl> <dl class="attribute"> <dt id="select.kevent.flags"> <code class="descclassname">kevent.</code><code class="descname">flags</code><a class="headerlink" href="#select.kevent.flags" title="Permalink to this definition">¶</a></dt> <dd><p>Filter action.</p> <table border="1" class="docutils"> <colgroup> <col width="38%" /> <col width="63%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Constant</th> <th class="head">Meaning</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_ADD</span></code></td> <td>Adds or modifies an event</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_DELETE</span></code></td> <td>Removes an event from the queue</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_ENABLE</span></code></td> <td>Permitscontrol() to returns the event</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_DISABLE</span></code></td> <td>Disablesevent</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_ONESHOT</span></code></td> <td>Removes event after first occurrence</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_CLEAR</span></code></td> <td>Reset the state after an event is retrieved</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_SYSFLAGS</span></code></td> <td>internal event</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_FLAG1</span></code></td> <td>internal event</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_EOF</span></code></td> <td>Filter specific EOF condition</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_ERROR</span></code></td> <td>See return values</td> </tr> </tbody> </table> </dd></dl> <dl class="attribute"> <dt id="select.kevent.fflags"> <code class="descclassname">kevent.</code><code class="descname">fflags</code><a class="headerlink" href="#select.kevent.fflags" title="Permalink to this definition">¶</a></dt> <dd><p>Filter specific flags.</p> <p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_READ</span></code> and <code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_WRITE</span></code> filter flags:</p> <table border="1" class="docutils"> <colgroup> <col width="39%" /> <col width="61%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Constant</th> <th class="head">Meaning</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_LOWAT</span></code></td> <td>low water mark of a socket buffer</td> </tr> </tbody> </table> <p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_VNODE</span></code> filter flags:</p> <table border="1" class="docutils"> <colgroup> <col width="39%" /> <col width="61%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Constant</th> <th class="head">Meaning</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_DELETE</span></code></td> <td><em>unlink()</em> was called</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_WRITE</span></code></td> <td>a write occurred</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_EXTEND</span></code></td> <td>the file was extended</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_ATTRIB</span></code></td> <td>an attribute was changed</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_LINK</span></code></td> <td>the link count has changed</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_RENAME</span></code></td> <td>the file was renamed</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_REVOKE</span></code></td> <td>access to the file was revoked</td> </tr> </tbody> </table> <p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_PROC</span></code> filter flags:</p> <table border="1" class="docutils"> <colgroup> <col width="39%" /> <col width="61%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Constant</th> <th class="head">Meaning</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_EXIT</span></code></td> <td>the process has exited</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_FORK</span></code></td> <td>the process has called <em>fork()</em></td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_EXEC</span></code></td> <td>the process has executed a new process</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_PCTRLMASK</span></code></td> <td>internal filter flag</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_PDATAMASK</span></code></td> <td>internal filter flag</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_TRACK</span></code></td> <td>follow a process across <em>fork()</em></td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_CHILD</span></code></td> <td>returned on the child process for <em>NOTE_TRACK</em></td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_TRACKERR</span></code></td> <td>unable to attach to a child</td> </tr> </tbody> </table> <p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_NETDEV</span></code> filter flags (not available on Mac OS X):</p> <table border="1" class="docutils"> <colgroup> <col width="39%" /> <col width="61%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Constant</th> <th class="head">Meaning</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_LINKUP</span></code></td> <td>link is up</td> </tr> <tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_LINKDOWN</span></code></td> <td>link is down</td> </tr> <tr class="row-even"><td><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_LINKINV</span></code></td> <td>link state is invalid</td> </tr> </tbody> </table> </dd></dl> <dl class="attribute"> <dt id="select.kevent.data"> <code class="descclassname">kevent.</code><code class="descname">data</code><a class="headerlink" href="#select.kevent.data" title="Permalink to this definition">¶</a></dt> <dd><p>Filter specific data.</p> </dd></dl> <dl class="attribute"> <dt id="select.kevent.udata"> <code class="descclassname">kevent.</code><code class="descname">udata</code><a class="headerlink" href="#select.kevent.udata" title="Permalink to this definition">¶</a></dt> <dd><p>User defined value.</p> </dd></dl> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table Of Contents</a></h3> <ul> <li><a class="reference internal" href="#">18.3. <code class="docutils literal notranslate"><span class="pre">select</span></code> — Waiting for I/O completion</a><ul> <li><a class="reference internal" href="#dev-poll-polling-objects">18.3.1. <code class="docutils literal notranslate"><span class="pre">/dev/poll</span></code> Polling Objects</a></li> <li><a class="reference internal" href="#edge-and-level-trigger-polling-epoll-objects">18.3.2. Edge and Level Trigger Polling (epoll) Objects</a></li> <li><a class="reference internal" href="#polling-objects">18.3.3. Polling Objects</a></li> <li><a class="reference internal" href="#kqueue-objects">18.3.4. Kqueue Objects</a></li> <li><a class="reference internal" href="#kevent-objects">18.3.5. Kevent Objects</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="ssl.html" title="previous chapter">18.2. <code class="docutils literal notranslate"><span class="pre">ssl</span></code> — TLS/SSL wrapper for socket objects</a></p> <h4>Next topic</h4> <p class="topless"><a href="selectors.html" title="next chapter">18.4. <code class="docutils literal notranslate"><span class="pre">selectors</span></code> — High-level I/O multiplexing</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li> <a href="https://github.com/python/cpython/blob/3.6/Doc/library/select.rst" rel="nofollow">Show Source </a> </li> </ul> </div> </div> </div> <div class="clearer"></div> </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="selectors.html" title="18.4. selectors — High-level I/O multiplexing" >next</a> |</li> <li class="right" > <a href="ssl.html" title="18.2. ssl — TLS/SSL wrapper for socket objects" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">3.6.7 Documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> »</li> <li class="nav-item nav-item-2"><a href="ipc.html" >18. 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