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<div class="section" id="module-select">
<span id="select-waiting-for-i-o-completion"></span><h1>16.1. <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>
<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">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>
<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>The exception raised when an error occurs. The accompanying value is a pair
containing the numeric error code from <code class="xref c c-data docutils literal notranslate"><span class="pre">errno</span></code> and the corresponding
string, as would be printed by the C function <code class="xref c c-func docutils literal notranslate"><span class="pre">perror()</span></code>.</p>
</dd></dl>
<dl class="function">
<dt id="select.epoll">
<code class="descclassname">select.</code><code class="descname">epoll</code><span class="sig-paren">(</span><span class="optional">[</span><em>sizehint=-1</em><span class="optional">]</span><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.) Returns an edge polling object,
which can be used as Edge or Level Triggered interface for I/O events; see
section <a class="reference internal" href="#epoll-objects"><span class="std std-ref">Edge and Level Trigger Polling (epoll) Objects</span></a> below for the methods supported by epolling
objects.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 2.6.</span></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>
<div class="versionadded">
<p><span class="versionmodified added">New in version 2.6.</span></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>
<div class="versionadded">
<p><span class="versionmodified added">New in version 2.6.</span></p>
</div>
</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><p><em>rlist</em>: wait until ready for reading</p></li>
<li><p><em>wlist</em>: wait until ready for writing</p></li>
<li><p><em>xlist</em>: wait for an “exceptional condition” (see the manual page for what
your system considers such a condition)</p></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-0">Among the acceptable object types in the sequences are Python file objects (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="admonition-title">Note</p>
<p id="index-1">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>
</dd></dl>
<dl class="attribute">
<dt id="select.select.PIPE_BUF">
<code class="descclassname">select.</code><code class="descname">PIPE_BUF</code><a class="headerlink" href="#select.select.PIPE_BUF" title="Permalink to this definition">¶</a></dt>
<dd><p>Files reported as ready for writing by <a class="reference internal" href="#module-select" title="select: Wait for I/O completion on multiple streams."><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
similar interfaces in this module are guaranteed to not block on a write
of up to <a class="reference internal" href="#select.select.PIPE_BUF" title="select.select.PIPE_BUF"><code class="xref py py-const docutils literal notranslate"><span class="pre">PIPE_BUF</span></code></a> bytes.
This value is guaranteed by POSIX to be at least 512. Availability: Unix.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 2.7.</span></p>
</div>
</dd></dl>
<div class="section" id="edge-and-level-trigger-polling-epoll-objects">
<span id="epoll-objects"></span><h2>16.1.1. 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 class="docutils align-center">
<colgroup>
<col style="width: 33%" />
<col style="width: 67%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Constant</p></th>
<th class="head"><p>Meaning</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLIN</span></code></p></td>
<td><p>Available for read</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLOUT</span></code></p></td>
<td><p>Available for write</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLPRI</span></code></p></td>
<td><p>Urgent data for read</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLERR</span></code></p></td>
<td><p>Error condition happened on the assoc. fd</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLHUP</span></code></p></td>
<td><p>Hang up happened on the assoc. fd</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLET</span></code></p></td>
<td><p>Set Edge Trigger behavior, the default is
Level Trigger behavior</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLONESHOT</span></code></p></td>
<td><p>Set one-shot behavior. After one event is
pulled out, the fd is internally disabled</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLRDNORM</span></code></p></td>
<td><p>Equivalent to <code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLIN</span></code></p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLRDBAND</span></code></p></td>
<td><p>Priority data band can be read.</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLWRNORM</span></code></p></td>
<td><p>Equivalent to <code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLOUT</span></code></p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLWRBAND</span></code></p></td>
<td><p>Priority data may be written.</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">EPOLLMSG</span></code></p></td>
<td><p>Ignored.</p></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="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>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>Registering a file descriptor that’s already registered raises an
IOError – contrary to <a class="reference internal" href="#poll-objects"><span class="std std-ref">Polling Objects</span></a>’s register.</p>
</div>
</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 register 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><span class="optional">[</span><em>timeout=-1</em><span class="optional">[</span>, <em>maxevents=-1</em><span class="optional">]</span><span class="optional">]</span><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>
</dd></dl>
</div>
<div class="section" id="polling-objects">
<span id="poll-objects"></span><h2>16.1.2. 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 class="docutils align-center">
<colgroup>
<col style="width: 31%" />
<col style="width: 69%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Constant</p></th>
<th class="head"><p>Meaning</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">POLLIN</span></code></p></td>
<td><p>There is data to read</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">POLLPRI</span></code></p></td>
<td><p>There is urgent data to read</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">POLLOUT</span></code></p></td>
<td><p>Ready for output: writing will not block</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">POLLERR</span></code></p></td>
<td><p>Error condition of some sort</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">POLLHUP</span></code></p></td>
<td><p>Hung up</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">POLLNVAL</span></code></p></td>
<td><p>Invalid request: descriptor not open</p></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#exceptions.IOError" title="exceptions.IOError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">IOError</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>
<div class="versionadded">
<p><span class="versionmodified added">New in version 2.6.</span></p>
</div>
</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#exceptions.KeyError" title="exceptions.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>
</dd></dl>
</div>
<div class="section" id="kqueue-objects">
<span id="id1"></span><h2>16.1.3. 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="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</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><p>changelist must be an iterable of kevent objects or <code class="docutils literal notranslate"><span class="pre">None</span></code></p></li>
<li><p>max_events must be 0 or a positive integer</p></li>
<li><p>timeout in seconds (floats possible); the default is <code class="docutils literal notranslate"><span class="pre">None</span></code>,
to wait forever</p></li>
</ul>
</dd></dl>
</div>
<div class="section" id="kevent-objects">
<span id="id2"></span><h2>16.1.4. 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 fileno() function. 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 class="docutils align-center">
<colgroup>
<col style="width: 38%" />
<col style="width: 63%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Constant</p></th>
<th class="head"><p>Meaning</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_READ</span></code></p></td>
<td><p>Takes a descriptor and returns whenever
there is data available to read</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_WRITE</span></code></p></td>
<td><p>Takes a descriptor and returns whenever
there is data available to write</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_AIO</span></code></p></td>
<td><p>AIO requests</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_VNODE</span></code></p></td>
<td><p>Returns when one or more of the requested
events watched in <em>fflag</em> occurs</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_PROC</span></code></p></td>
<td><p>Watch for events on a process id</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_NETDEV</span></code></p></td>
<td><p>Watch for events on a network device
[not available on Mac OS X]</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_SIGNAL</span></code></p></td>
<td><p>Returns whenever the watched signal is
delivered to the process</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_FILTER_TIMER</span></code></p></td>
<td><p>Establishes an arbitrary timer</p></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 class="docutils align-center">
<colgroup>
<col style="width: 38%" />
<col style="width: 63%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Constant</p></th>
<th class="head"><p>Meaning</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_ADD</span></code></p></td>
<td><p>Adds or modifies an event</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_DELETE</span></code></p></td>
<td><p>Removes an event from the queue</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_ENABLE</span></code></p></td>
<td><p>Permitscontrol() to returns the event</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_DISABLE</span></code></p></td>
<td><p>Disablesevent</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_ONESHOT</span></code></p></td>
<td><p>Removes event after first occurrence</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_CLEAR</span></code></p></td>
<td><p>Reset the state after an event is retrieved</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_SYSFLAGS</span></code></p></td>
<td><p>internal event</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_FLAG1</span></code></p></td>
<td><p>internal event</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_EOF</span></code></p></td>
<td><p>Filter specific EOF condition</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_EV_ERROR</span></code></p></td>
<td><p>See return values</p></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 class="docutils align-center">
<colgroup>
<col style="width: 39%" />
<col style="width: 61%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Constant</p></th>
<th class="head"><p>Meaning</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_LOWAT</span></code></p></td>
<td><p>low water mark of a socket buffer</p></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 class="docutils align-center">
<colgroup>
<col style="width: 39%" />
<col style="width: 61%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Constant</p></th>
<th class="head"><p>Meaning</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_DELETE</span></code></p></td>
<td><p><em>unlink()</em> was called</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_WRITE</span></code></p></td>
<td><p>a write occurred</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_EXTEND</span></code></p></td>
<td><p>the file was extended</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_ATTRIB</span></code></p></td>
<td><p>an attribute was changed</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_LINK</span></code></p></td>
<td><p>the link count has changed</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_RENAME</span></code></p></td>
<td><p>the file was renamed</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_REVOKE</span></code></p></td>
<td><p>access to the file was revoked</p></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 class="docutils align-center">
<colgroup>
<col style="width: 39%" />
<col style="width: 61%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Constant</p></th>
<th class="head"><p>Meaning</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_EXIT</span></code></p></td>
<td><p>the process has exited</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_FORK</span></code></p></td>
<td><p>the process has called <em>fork()</em></p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_EXEC</span></code></p></td>
<td><p>the process has executed a new process</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_PCTRLMASK</span></code></p></td>
<td><p>internal filter flag</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_PDATAMASK</span></code></p></td>
<td><p>internal filter flag</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_TRACK</span></code></p></td>
<td><p>follow a process across <em>fork()</em></p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_CHILD</span></code></p></td>
<td><p>returned on the child process for
<em>NOTE_TRACK</em></p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_TRACKERR</span></code></p></td>
<td><p>unable to attach to a child</p></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 class="docutils align-center">
<colgroup>
<col style="width: 39%" />
<col style="width: 61%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Constant</p></th>
<th class="head"><p>Meaning</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_LINKUP</span></code></p></td>
<td><p>link is up</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_LINKDOWN</span></code></p></td>
<td><p>link is down</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">KQ_NOTE_LINKINV</span></code></p></td>
<td><p>link state is invalid</p></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>
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<h3><a href="../contents.html">Table of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">16.1. <code class="xref py py-mod docutils literal notranslate"><span class="pre">select</span></code> — Waiting for I/O completion</a><ul>
<li><a class="reference internal" href="#edge-and-level-trigger-polling-epoll-objects">16.1.1. Edge and Level Trigger Polling (epoll) Objects</a></li>
<li><a class="reference internal" href="#polling-objects">16.1.2. Polling Objects</a></li>
<li><a class="reference internal" href="#kqueue-objects">16.1.3. Kqueue Objects</a></li>
<li><a class="reference internal" href="#kevent-objects">16.1.4. Kevent Objects</a></li>
</ul>
</li>
</ul>
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