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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."><tt class="xref py py-mod docutils literal"><span class="pre">select</span></tt></a> &#8212; 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 <tt class="xref c c-func docutils literal"><span class="pre">select()</span></tt> and <tt class="xref c c-func docutils literal"><span class="pre">poll()</span></tt> functions
available in most operating systems, <tt class="xref c c-func docutils literal"><span class="pre">epoll()</span></tt> available on Linux 2.5+ and
<tt class="xref c c-func docutils literal"><span class="pre">kqueue()</span></tt> 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><tt class="descclassname">select.</tt><tt class="descname">error</tt><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 <tt class="xref c c-data docutils literal"><span class="pre">errno</span></tt> and the corresponding
string, as would be printed by the C function <tt class="xref c c-func docutils literal"><span class="pre">perror()</span></tt>.</p>
</dd></dl>

<dl class="function">
<dt id="select.epoll">
<tt class="descclassname">select.</tt><tt class="descname">epoll</tt><big>(</big><span class="optional">[</span><em>sizehint=-1</em><span class="optional">]</span><big>)</big><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"><em>Edge and Level Trigger Polling (epoll) Objects</em></a> below for the methods supported by epolling
objects.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.6.</span></p>
</dd></dl>

<dl class="function">
<dt id="select.poll">
<tt class="descclassname">select.</tt><tt class="descname">poll</tt><big>(</big><big>)</big><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"><em>Polling Objects</em></a> below for the methods supported
by polling objects.</p>
</dd></dl>

<dl class="function">
<dt id="select.kqueue">
<tt class="descclassname">select.</tt><tt class="descname">kqueue</tt><big>(</big><big>)</big><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"><em>Kqueue Objects</em></a> below for the methods supported by kqueue objects.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.6.</span></p>
</dd></dl>

<dl class="function">
<dt id="select.kevent">
<tt class="descclassname">select.</tt><tt class="descname">kevent</tt><big>(</big><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><big>)</big><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"><em>Kevent Objects</em></a> below for the methods supported by kevent objects.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.6.</span></p>
</dd></dl>

<dl class="function">
<dt id="select.select">
<tt class="descclassname">select.</tt><tt class="descname">select</tt><big>(</big><em>rlist</em>, <em>wlist</em>, <em>xlist</em><span class="optional">[</span>, <em>timeout</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#select.select" title="Permalink to this definition">¶</a></dt>
<dd><p>This is a straightforward interface to the Unix <tt class="xref c c-func docutils literal"><span class="pre">select()</span></tt> system call.
The first three arguments are sequences of &#8216;waitable objects&#8217;: either
integers representing file descriptors or objects with a parameterless method
named <tt class="xref py py-meth docutils literal"><span class="pre">fileno()</span></tt> 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 &#8220;exceptional condition&#8221; (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-0">Among the acceptable object types in the sequences are Python file objects (e.g.
<tt class="docutils literal"><span class="pre">sys.stdin</span></tt>, or objects returned by <a class="reference internal" href="functions.html#open" title="open"><tt class="xref py py-func docutils literal"><span class="pre">open()</span></tt></a> or <a class="reference internal" href="os.html#os.popen" title="os.popen"><tt class="xref py py-func docutils literal"><span class="pre">os.popen()</span></tt></a>), socket
objects returned by <a class="reference internal" href="socket.html#socket.socket" title="socket.socket"><tt class="xref py py-func docutils literal"><span class="pre">socket.socket()</span></tt></a>.  You may also define a <em class="dfn">wrapper</em>
class yourself, as long as it has an appropriate <tt class="xref py py-meth docutils literal"><span class="pre">fileno()</span></tt> 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-1">File objects on Windows are not acceptable, but sockets are.  On Windows,
the underlying <tt class="xref c c-func docutils literal"><span class="pre">select()</span></tt> function is provided by the WinSock
library, and does not handle file descriptors that don&#8217;t originate from
WinSock.</p>
</div>
</dd></dl>

<dl class="attribute">
<dt id="select.select.PIPE_BUF">
<tt class="descclassname">select.</tt><tt class="descname">PIPE_BUF</tt><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."><tt class="xref py py-func docutils literal"><span class="pre">select()</span></tt></a>, <a class="reference internal" href="#select.poll" title="select.poll"><tt class="xref py py-func docutils literal"><span class="pre">poll()</span></tt></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"><tt class="xref py py-const docutils literal"><span class="pre">PIPE_BUF</span></tt></a> bytes.
This value is guaranteed by POSIX to be at least 512.  Availability: Unix.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.7.</span></p>
</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 border="1" class="docutils">
<colgroup>
<col width="33%" />
<col width="67%" />
</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><tt class="xref py py-const docutils literal"><span class="pre">EPOLLIN</span></tt></td>
<td>Available for read</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLOUT</span></tt></td>
<td>Available for write</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLPRI</span></tt></td>
<td>Urgent data for read</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLERR</span></tt></td>
<td>Error condition happened on the assoc. fd</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLHUP</span></tt></td>
<td>Hang up happened on the assoc. fd</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLET</span></tt></td>
<td>Set Edge Trigger behavior, the default is
Level Trigger behavior</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLONESHOT</span></tt></td>
<td>Set one-shot behavior. After one event is
pulled out, the fd is internally disabled</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLRDNORM</span></tt></td>
<td>Equivalent to <tt class="xref py py-const docutils literal"><span class="pre">EPOLLIN</span></tt></td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLRDBAND</span></tt></td>
<td>Priority data band can be read.</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLWRNORM</span></tt></td>
<td>Equivalent to <tt class="xref py py-const docutils literal"><span class="pre">EPOLLOUT</span></tt></td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLWRBAND</span></tt></td>
<td>Priority data may be written.</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">EPOLLMSG</span></tt></td>
<td>Ignored.</td>
</tr>
</tbody>
</table>
</div></blockquote>
<dl class="method">
<dt id="select.epoll.close">
<tt class="descclassname">epoll.</tt><tt class="descname">close</tt><big>(</big><big>)</big><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">
<tt class="descclassname">epoll.</tt><tt class="descname">fileno</tt><big>(</big><big>)</big><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">
<tt class="descclassname">epoll.</tt><tt class="descname">fromfd</tt><big>(</big><em>fd</em><big>)</big><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">
<tt class="descclassname">epoll.</tt><tt class="descname">register</tt><big>(</big><em>fd</em><span class="optional">[</span>, <em>eventmask</em><span class="optional">]</span><big>)</big><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="first admonition-title">Note</p>
<p class="last">Registering a file descriptor that&#8217;s already registered raises an
IOError &#8211; contrary to <a class="reference internal" href="#poll-objects"><em>Polling Objects</em></a>&#8216;s register.</p>
</div>
</dd></dl>

<dl class="method">
<dt id="select.epoll.modify">
<tt class="descclassname">epoll.</tt><tt class="descname">modify</tt><big>(</big><em>fd</em>, <em>eventmask</em><big>)</big><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">
<tt class="descclassname">epoll.</tt><tt class="descname">unregister</tt><big>(</big><em>fd</em><big>)</big><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">
<tt class="descclassname">epoll.</tt><tt class="descname">poll</tt><big>(</big><span class="optional">[</span><em>timeout=-1</em><span class="optional">[</span>, <em>maxevents=-1</em><span class="optional">]</span><span class="optional">]</span><big>)</big><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 <tt class="xref c c-func docutils literal"><span class="pre">poll()</span></tt> system call, supported on most Unix systems, provides better
scalability for network servers that service many, many clients at the same
time. <tt class="xref c c-func docutils literal"><span class="pre">poll()</span></tt> scales better because the system call only requires listing
the file descriptors of interest, while <tt class="xref c c-func docutils literal"><span class="pre">select()</span></tt> builds a bitmap, turns
on bits for the fds of interest, and then afterward the whole bitmap has to be
linearly scanned again. <tt class="xref c c-func docutils literal"><span class="pre">select()</span></tt> is O(highest file descriptor), while
<tt class="xref c c-func docutils literal"><span class="pre">poll()</span></tt> is O(number of file descriptors).</p>
<dl class="method">
<dt id="select.poll.register">
<tt class="descclassname">poll.</tt><tt class="descname">register</tt><big>(</big><em>fd</em><span class="optional">[</span>, <em>eventmask</em><span class="optional">]</span><big>)</big><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"><tt class="xref py py-meth docutils literal"><span class="pre">poll()</span></tt></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 <tt class="xref py py-meth docutils literal"><span class="pre">fileno()</span></tt>
method that returns an integer.  File objects implement <tt class="xref py py-meth docutils literal"><span class="pre">fileno()</span></tt>, 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 <tt class="xref py py-const docutils literal"><span class="pre">POLLIN</span></tt>,
<tt class="xref py py-const docutils literal"><span class="pre">POLLPRI</span></tt>, and <tt class="xref py py-const docutils literal"><span class="pre">POLLOUT</span></tt>, 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><tt class="xref py py-const docutils literal"><span class="pre">POLLIN</span></tt></td>
<td>There is data to read</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">POLLPRI</span></tt></td>
<td>There is urgent data to read</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">POLLOUT</span></tt></td>
<td>Ready for output: writing will not block</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">POLLERR</span></tt></td>
<td>Error condition of some sort</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">POLLHUP</span></tt></td>
<td>Hung up</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">POLLNVAL</span></tt></td>
<td>Invalid request: descriptor not open</td>
</tr>
</tbody>
</table>
<p>Registering a file descriptor that&#8217;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">
<tt class="descclassname">poll.</tt><tt class="descname">modify</tt><big>(</big><em>fd</em>, <em>eventmask</em><big>)</big><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
<tt class="docutils literal"><span class="pre">register(fd,</span> <span class="pre">eventmask)</span></tt>.  Attempting to modify a file descriptor
that was never registered causes an <a class="reference internal" href="exceptions.html#exceptions.IOError" title="exceptions.IOError"><tt class="xref py py-exc docutils literal"><span class="pre">IOError</span></tt></a> exception with errno
<tt class="xref py py-const docutils literal"><span class="pre">ENOENT</span></tt> to be raised.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.6.</span></p>
</dd></dl>

<dl class="method">
<dt id="select.poll.unregister">
<tt class="descclassname">poll.</tt><tt class="descname">unregister</tt><big>(</big><em>fd</em><big>)</big><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"><tt class="xref py py-meth docutils literal"><span class="pre">register()</span></tt></a> method, <em>fd</em> can be an integer or an object with a
<tt class="xref py py-meth docutils literal"><span class="pre">fileno()</span></tt> 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"><tt class="xref py py-exc docutils literal"><span class="pre">KeyError</span></tt></a> exception to be raised.</p>
</dd></dl>

<dl class="method">
<dt id="select.poll.poll">
<tt class="descclassname">poll.</tt><tt class="descname">poll</tt><big>(</big><span class="optional">[</span><em>timeout</em><span class="optional">]</span><big>)</big><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 <tt class="docutils literal"><span class="pre">(fd,</span> <span class="pre">event)</span></tt> 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 &#8212; <tt class="xref py py-const docutils literal"><span class="pre">POLLIN</span></tt> for
waiting input, <tt class="xref py py-const docutils literal"><span class="pre">POLLOUT</span></tt> 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"><tt class="xref py py-const docutils literal"><span class="pre">None</span></tt></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">
<tt class="descclassname">kqueue.</tt><tt class="descname">close</tt><big>(</big><big>)</big><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">
<tt class="descclassname">kqueue.</tt><tt class="descname">fileno</tt><big>(</big><big>)</big><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">
<tt class="descclassname">kqueue.</tt><tt class="descname">fromfd</tt><big>(</big><em>fd</em><big>)</big><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">
<tt class="descclassname">kqueue.</tt><tt class="descname">control</tt><big>(</big><em>changelist</em>, <em>max_events</em><span class="optional">[</span>, <em>timeout=None</em><span class="optional">]</span><big>)</big> &rarr; 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 None</li>
<li>max_events must be 0 or a positive integer</li>
<li>timeout in seconds (floats possible)</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="http://www.freebsd.org/cgi/man.cgi?query=kqueue&amp;sektion=2">http://www.freebsd.org/cgi/man.cgi?query=kqueue&amp;sektion=2</a></p>
<dl class="attribute">
<dt id="select.kevent.ident">
<tt class="descclassname">kevent.</tt><tt class="descname">ident</tt><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&#8217;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">
<tt class="descclassname">kevent.</tt><tt class="descname">filter</tt><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><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_READ</span></tt></td>
<td>Takes a descriptor and returns whenever
there is data available to read</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_WRITE</span></tt></td>
<td>Takes a descriptor and returns whenever
there is data available to write</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_AIO</span></tt></td>
<td>AIO requests</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_VNODE</span></tt></td>
<td>Returns when one or more of the requested
events watched in <em>fflag</em> occurs</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_PROC</span></tt></td>
<td>Watch for events on a process id</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_NETDEV</span></tt></td>
<td>Watch for events on a network device
[not available on Mac OS X]</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_SIGNAL</span></tt></td>
<td>Returns whenever the watched signal is
delivered to the process</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_TIMER</span></tt></td>
<td>Establishes an arbitrary timer</td>
</tr>
</tbody>
</table>
</dd></dl>

<dl class="attribute">
<dt id="select.kevent.flags">
<tt class="descclassname">kevent.</tt><tt class="descname">flags</tt><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><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_ADD</span></tt></td>
<td>Adds or modifies an event</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_DELETE</span></tt></td>
<td>Removes an event from the queue</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_ENABLE</span></tt></td>
<td>Permitscontrol() to returns the event</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_DISABLE</span></tt></td>
<td>Disablesevent</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_ONESHOT</span></tt></td>
<td>Removes event after first occurrence</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_CLEAR</span></tt></td>
<td>Reset the state after an event is retrieved</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_SYSFLAGS</span></tt></td>
<td>internal event</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_FLAG1</span></tt></td>
<td>internal event</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_EOF</span></tt></td>
<td>Filter specific EOF condition</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_EV_ERROR</span></tt></td>
<td>See return values</td>
</tr>
</tbody>
</table>
</dd></dl>

<dl class="attribute">
<dt id="select.kevent.fflags">
<tt class="descclassname">kevent.</tt><tt class="descname">fflags</tt><a class="headerlink" href="#select.kevent.fflags" title="Permalink to this definition">¶</a></dt>
<dd><p>Filter specific flags.</p>
<p><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_READ</span></tt> and  <tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_WRITE</span></tt> 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><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_LOWAT</span></tt></td>
<td>low water mark of a socket buffer</td>
</tr>
</tbody>
</table>
<p><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_VNODE</span></tt> 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><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_DELETE</span></tt></td>
<td><em>unlink()</em> was called</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_WRITE</span></tt></td>
<td>a write occurred</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_EXTEND</span></tt></td>
<td>the file was extended</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_ATTRIB</span></tt></td>
<td>an attribute was changed</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_LINK</span></tt></td>
<td>the link count has changed</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_RENAME</span></tt></td>
<td>the file was renamed</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_REVOKE</span></tt></td>
<td>access to the file was revoked</td>
</tr>
</tbody>
</table>
<p><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_PROC</span></tt> 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><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_EXIT</span></tt></td>
<td>the process has exited</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_FORK</span></tt></td>
<td>the process has called <em>fork()</em></td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_EXEC</span></tt></td>
<td>the process has executed a new process</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_PCTRLMASK</span></tt></td>
<td>internal filter flag</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_PDATAMASK</span></tt></td>
<td>internal filter flag</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_TRACK</span></tt></td>
<td>follow a process across <em>fork()</em></td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_CHILD</span></tt></td>
<td>returned on the child process for
<em>NOTE_TRACK</em></td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_TRACKERR</span></tt></td>
<td>unable to attach to a child</td>
</tr>
</tbody>
</table>
<p><tt class="xref py py-const docutils literal"><span class="pre">KQ_FILTER_NETDEV</span></tt> 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><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_LINKUP</span></tt></td>
<td>link is up</td>
</tr>
<tr class="row-odd"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_LINKDOWN</span></tt></td>
<td>link is down</td>
</tr>
<tr class="row-even"><td><tt class="xref py py-const docutils literal"><span class="pre">KQ_NOTE_LINKINV</span></tt></td>
<td>link state is invalid</td>
</tr>
</tbody>
</table>
</dd></dl>

<dl class="attribute">
<dt id="select.kevent.data">
<tt class="descclassname">kevent.</tt><tt class="descname">data</tt><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">
<tt class="descclassname">kevent.</tt><tt class="descname">udata</tt><a class="headerlink" href="#select.kevent.udata" title="Permalink to this definition">¶</a></dt>
<dd><p>User defined value.</p>
</dd></dl>

</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. <tt class="docutils literal"><span class="pre">select</span></tt> &#8212; 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>
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