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Code Editor : select.html

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 <section id="module-select">
<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">select</code></a> — Waiting for I/O completion<a class="headerlink" href="#module-select" title="Permalink to this headline">¶</a></h1>
This module provides access to the <code class="xref c c-func docutils literal notranslate">select()</code> and <code class="xref c c-func docutils literal notranslate">poll()</code> functions
available in most operating systems, <code class="xref c c-func docutils literal notranslate">epoll()</code> available on Linux 2.5+ and
<code class="xref c c-func docutils literal notranslate">kqueue()</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.
The module defines the following:
<dl class="py exception">
<dt class="sig sig-object py" id="select.error">
exception select.error<a class="headerlink" href="#select.error" title="Permalink to this definition">¶</a></dt>
<dd>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">errno</code> and the corresponding
string, as would be printed by the C function <code class="xref c c-func docutils literal notranslate">perror()</code>.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="select.epoll">
select.epoll([sizehint=-1])<a class="headerlink" href="#select.epoll" title="Permalink to this definition">¶</a></dt>
<dd>(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">Edge and Level Trigger Polling (epoll) Objects</a> below for the methods supported by epolling
objects.
<div class="versionadded">
New in version 2.6.
</div>
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="select.poll">
select.poll()<a class="headerlink" href="#select.poll" title="Permalink to this definition">¶</a></dt>
<dd>(Not supported by all operating systems.) Returns a polling object, which
supports registering and unregistering file descriptors, and then polling them
for I/O events; see section <a class="reference internal" href="#poll-objects">Polling Objects</a> below for the methods supported
by polling objects.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="select.kqueue">
select.kqueue()<a class="headerlink" href="#select.kqueue" title="Permalink to this definition">¶</a></dt>
<dd>(Only supported on BSD.) Returns a kernel queue object; see section
<a class="reference internal" href="#kqueue-objects">Kqueue Objects</a> below for the methods supported by kqueue objects.
<div class="versionadded">
New in version 2.6.
</div>
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="select.kevent">
select.kevent(ident, filter=KQ_FILTER_READ, flags=KQ_EV_ADD, fflags=0, data=0, udata=0)<a class="headerlink" href="#select.kevent" title="Permalink to this definition">¶</a></dt>
<dd>(Only supported on BSD.) Returns a kernel event object; see section
<a class="reference internal" href="#kevent-objects">Kevent Objects</a> below for the methods supported by kevent objects.
<div class="versionadded">
New in version 2.6.
</div>
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="select.select">
select.select(rlist, wlist, xlist[, timeout])<a class="headerlink" href="#select.select" title="Permalink to this definition">¶</a></dt>
<dd>This is a straightforward interface to the Unix <code class="xref c c-func docutils literal notranslate">select()</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">fileno()</code></a> returning such an integer:
<ul class="simple">
<li>rlist: wait until ready for reading</li>
<li>wlist: wait until ready for writing</li>
<li>xlist: wait for an “exceptional condition” (see the manual page for what
your system considers such a condition)</li>
</ul>
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 timeout argument specifies a time-out as a floating point number
in seconds. When the timeout 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.
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.
Among the acceptable object types in the sequences are Python file objects (e.g.
<code class="docutils literal notranslate">sys.stdin</code>, or objects returned by <a class="reference internal" href="functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate">open()</code></a> or <a class="reference internal" href="os.html#os.popen" title="os.popen"><code class="xref py py-func docutils literal notranslate">os.popen()</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">socket.socket()</code></a>. You may also define a wrapper
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">fileno()</code></a>
method (that really returns a file descriptor, not just a random integer).
<div class="admonition note">
Note
File objects on Windows are not acceptable, but sockets are. On Windows,
the underlying <code class="xref c c-func docutils literal notranslate">select()</code> function is provided by the WinSock
library, and does not handle file descriptors that don’t originate from
WinSock.
</div>
</dd></dl>

<dl class="py attribute">
<dt class="sig sig-object py" id="select.select.PIPE_BUF">
select.PIPE_BUF<a class="headerlink" href="#select.select.PIPE_BUF" title="Permalink to this definition">¶</a></dt>
<dd>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">select()</code></a>, <a class="reference internal" href="#select.poll" title="select.poll"><code class="xref py py-func docutils literal notranslate">poll()</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">PIPE_BUF</code></a> bytes.
This value is guaranteed by POSIX to be at least 512. Availability: Unix.
<div class="versionadded">
New in version 2.7.
</div>
</dd></dl>

<section id="edge-and-level-trigger-polling-epoll-objects">
<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><a class="reference external" href="http://linux.die.net/man/4/epoll">http://linux.die.net/man/4/epoll</a>
eventmask
<table class="docutils align-default">
<colgroup>
<col style="width: 33%" />
<col style="width: 67%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head">Constant</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">EPOLLIN</code></td>
<td>Available for read</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">EPOLLOUT</code></td>
<td>Available for write</td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">EPOLLPRI</code></td>
<td>Urgent data for read</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">EPOLLERR</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">EPOLLHUP</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">EPOLLET</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">EPOLLONESHOT</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">EPOLLRDNORM</code></td>
<td>Equivalent to <code class="xref py py-const docutils literal notranslate">EPOLLIN</code></td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">EPOLLRDBAND</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">EPOLLWRNORM</code></td>
<td>Equivalent to <code class="xref py py-const docutils literal notranslate">EPOLLOUT</code></td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">EPOLLWRBAND</code></td>
<td>Priority data may be written.</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">EPOLLMSG</code></td>
<td>Ignored.</td>
</tr>
</tbody>
</table>
</div></blockquote>
<dl class="py method">
<dt class="sig sig-object py" id="select.epoll.close">
epoll.close()<a class="headerlink" href="#select.epoll.close" title="Permalink to this definition">¶</a></dt>
<dd>Close the control file descriptor of the epoll object.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="select.epoll.fileno">
epoll.fileno()<a class="headerlink" href="#select.epoll.fileno" title="Permalink to this definition">¶</a></dt>
<dd>Return the file descriptor number of the control fd.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="select.epoll.fromfd">
epoll.fromfd(fd)<a class="headerlink" href="#select.epoll.fromfd" title="Permalink to this definition">¶</a></dt>
<dd>Create an epoll object from a given file descriptor.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="select.epoll.register">
epoll.register(fd[, eventmask])<a class="headerlink" href="#select.epoll.register" title="Permalink to this definition">¶</a></dt>
<dd>Register a fd descriptor with the epoll object.
<div class="admonition note">
Note
Registering a file descriptor that’s already registered raises an
IOError – contrary to <a class="reference internal" href="#poll-objects">Polling Objects</a>’s register.
</div>
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="select.epoll.modify">
epoll.modify(fd, eventmask)<a class="headerlink" href="#select.epoll.modify" title="Permalink to this definition">¶</a></dt>
<dd>Modify a register file descriptor.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="select.epoll.unregister">
epoll.unregister(fd)<a class="headerlink" href="#select.epoll.unregister" title="Permalink to this definition">¶</a></dt>
<dd>Remove a registered file descriptor from the epoll object.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="select.epoll.poll">
epoll.poll([timeout=-1[, maxevents=-1]])<a class="headerlink" href="#select.epoll.poll" title="Permalink to this definition">¶</a></dt>
<dd>Wait for events. timeout in seconds (float)
</dd></dl>

</section>
<section id="polling-objects">
<h2>16.1.2. Polling Objects<a class="headerlink" href="#polling-objects" title="Permalink to this headline">¶</a></h2>
The <code class="xref c c-func docutils literal notranslate">poll()</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">poll()</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">select()</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">select()</code> is O(highest file descriptor), while
<code class="xref c c-func docutils literal notranslate">poll()</code> is O(number of file descriptors).
<dl class="py method">
<dt class="sig sig-object py" id="select.poll.register">
poll.register(fd[, eventmask])<a class="headerlink" href="#select.poll.register" title="Permalink to this definition">¶</a></dt>
<dd>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">poll()</code></a> method will then check whether the file descriptor has any
pending I/O events. fd 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">fileno()</code></a> method that returns an integer. File objects
implement <code class="xref py py-meth docutils literal notranslate">fileno()</code>, so they can also be used as the argument.
eventmask 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">POLLIN</code>,
<code class="xref py py-const docutils literal notranslate">POLLPRI</code>, and <code class="xref py py-const docutils literal notranslate">POLLOUT</code>, described in the table below. If not
specified, the default value used will check for all 3 types of events.
<table class="docutils align-default">
<colgroup>
<col style="width: 31%" />
<col style="width: 69%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head">Constant</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">POLLIN</code></td>
<td>There is data to read</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">POLLPRI</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">POLLOUT</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">POLLERR</code></td>
<td>Error condition of some sort</td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">POLLHUP</code></td>
<td>Hung up</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">POLLNVAL</code></td>
<td>Invalid request: descriptor not open</td>
</tr>
</tbody>
</table>
Registering a file descriptor that’s already registered is not an error, and has
the same effect as registering the descriptor exactly once.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="select.poll.modify">
poll.modify(fd, eventmask)<a class="headerlink" href="#select.poll.modify" title="Permalink to this definition">¶</a></dt>
<dd>Modifies an already registered fd. This has the same effect as
<code class="docutils literal notranslate">register(fd, eventmask)</code>. Attempting to modify a file descriptor
that was never registered causes an <code class="xref py py-exc docutils literal notranslate">IOError</code> exception with errno
<code class="xref py py-const docutils literal notranslate">ENOENT</code> to be raised.
<div class="versionadded">
New in version 2.6.
</div>
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="select.poll.unregister">
poll.unregister(fd)<a class="headerlink" href="#select.poll.unregister" title="Permalink to this definition">¶</a></dt>
<dd>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">register()</code></a> method, fd 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">fileno()</code></a> method that returns an integer.
Attempting to remove a file descriptor that was never registered causes a
<code class="xref py py-exc docutils literal notranslate">KeyError</code> exception to be raised.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="select.poll.poll">
poll.poll([timeout])<a class="headerlink" href="#select.poll.poll" title="Permalink to this definition">¶</a></dt>
<dd>Polls the set of registered file descriptors, and returns a possibly-empty list
containing <code class="docutils literal notranslate">(fd, event)</code> 2-tuples for the descriptors that have events or
errors to report. fd is the file descriptor, and event is a bitmask with
bits set for the reported events for that descriptor — <code class="xref py py-const docutils literal notranslate">POLLIN</code> for
waiting input, <code class="xref py py-const docutils literal notranslate">POLLOUT</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 timeout is given, it specifies the
length of time in milliseconds which the system will wait for events before
returning. If timeout is omitted, negative, or <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a>, the call will
block until there is an event for this poll object.
</dd></dl>

</section>
<section id="kqueue-objects">
<h2>16.1.3. Kqueue Objects<a class="headerlink" href="#kqueue-objects" title="Permalink to this headline">¶</a></h2>
<dl class="py method">
<dt class="sig sig-object py" id="select.kqueue.close">
kqueue.close()<a class="headerlink" href="#select.kqueue.close" title="Permalink to this definition">¶</a></dt>
<dd>Close the control file descriptor of the kqueue object.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="select.kqueue.fileno">
kqueue.fileno()<a class="headerlink" href="#select.kqueue.fileno" title="Permalink to this definition">¶</a></dt>
<dd>Return the file descriptor number of the control fd.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="select.kqueue.fromfd">
kqueue.fromfd(fd)<a class="headerlink" href="#select.kqueue.fromfd" title="Permalink to this definition">¶</a></dt>
<dd>Create a kqueue object from a given file descriptor.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="select.kqueue.control">
kqueue.control(changelist, max_events[, timeout]) &#x2192; eventlist<a class="headerlink" href="#select.kqueue.control" title="Permalink to this definition">¶</a></dt>
<dd>Low level interface to kevent
<ul class="simple">
<li>changelist must be an iterable of kevent objects or <code class="docutils literal notranslate">None</code></li>
<li>max_events must be 0 or a positive integer</li>
<li>timeout in seconds (floats possible); the default is <code class="docutils literal notranslate">None</code>,
to wait forever</li>
</ul>
</dd></dl>

</section>
<section id="kevent-objects">
<h2>16.1.4. Kevent Objects<a class="headerlink" href="#kevent-objects" title="Permalink to this headline">¶</a></h2>
<a class="reference external" href="https://www.freebsd.org/cgi/man.cgi?query=kqueue&amp;sektion=2">https://www.freebsd.org/cgi/man.cgi?query=kqueue&amp;sektion=2</a>
<dl class="py attribute">
<dt class="sig sig-object py" id="select.kevent.ident">
kevent.ident<a class="headerlink" href="#select.kevent.ident" title="Permalink to this definition">¶</a></dt>
<dd>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.
</dd></dl>

<dl class="py attribute">
<dt class="sig sig-object py" id="select.kevent.filter">
kevent.filter<a class="headerlink" href="#select.kevent.filter" title="Permalink to this definition">¶</a></dt>
<dd>Name of the kernel filter.
<table class="docutils align-default">
<colgroup>
<col style="width: 38%" />
<col style="width: 63%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head">Constant</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">KQ_FILTER_READ</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">KQ_FILTER_WRITE</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">KQ_FILTER_AIO</code></td>
<td>AIO requests</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">KQ_FILTER_VNODE</code></td>
<td>Returns when one or more of the requested
events watched in fflag occurs</td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">KQ_FILTER_PROC</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">KQ_FILTER_NETDEV</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">KQ_FILTER_SIGNAL</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">KQ_FILTER_TIMER</code></td>
<td>Establishes an arbitrary timer</td>
</tr>
</tbody>
</table>
</dd></dl>

<dl class="py attribute">
<dt class="sig sig-object py" id="select.kevent.flags">
kevent.flags<a class="headerlink" href="#select.kevent.flags" title="Permalink to this definition">¶</a></dt>
<dd>Filter action.
<table class="docutils align-default">
<colgroup>
<col style="width: 38%" />
<col style="width: 63%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head">Constant</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">KQ_EV_ADD</code></td>
<td>Adds or modifies an event</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">KQ_EV_DELETE</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">KQ_EV_ENABLE</code></td>
<td>Permitscontrol() to returns the event</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">KQ_EV_DISABLE</code></td>
<td>Disablesevent</td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">KQ_EV_ONESHOT</code></td>
<td>Removes event after first occurrence</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">KQ_EV_CLEAR</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">KQ_EV_SYSFLAGS</code></td>
<td>internal event</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">KQ_EV_FLAG1</code></td>
<td>internal event</td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">KQ_EV_EOF</code></td>
<td>Filter specific EOF condition</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">KQ_EV_ERROR</code></td>
<td>See return values</td>
</tr>
</tbody>
</table>
</dd></dl>

<dl class="py attribute">
<dt class="sig sig-object py" id="select.kevent.fflags">
kevent.fflags<a class="headerlink" href="#select.kevent.fflags" title="Permalink to this definition">¶</a></dt>
<dd>Filter specific flags.
<code class="xref py py-const docutils literal notranslate">KQ_FILTER_READ</code> and <code class="xref py py-const docutils literal notranslate">KQ_FILTER_WRITE</code> filter flags:
<table class="docutils align-default">
<colgroup>
<col style="width: 39%" />
<col style="width: 61%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head">Constant</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">KQ_NOTE_LOWAT</code></td>
<td>low water mark of a socket buffer</td>
</tr>
</tbody>
</table>
<code class="xref py py-const docutils literal notranslate">KQ_FILTER_VNODE</code> filter flags:
<table class="docutils align-default">
<colgroup>
<col style="width: 39%" />
<col style="width: 61%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head">Constant</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">KQ_NOTE_DELETE</code></td>
<td>unlink() was called</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">KQ_NOTE_WRITE</code></td>
<td>a write occurred</td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">KQ_NOTE_EXTEND</code></td>
<td>the file was extended</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">KQ_NOTE_ATTRIB</code></td>
<td>an attribute was changed</td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">KQ_NOTE_LINK</code></td>
<td>the link count has changed</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">KQ_NOTE_RENAME</code></td>
<td>the file was renamed</td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">KQ_NOTE_REVOKE</code></td>
<td>access to the file was revoked</td>
</tr>
</tbody>
</table>
<code class="xref py py-const docutils literal notranslate">KQ_FILTER_PROC</code> filter flags:
<table class="docutils align-default">
<colgroup>
<col style="width: 39%" />
<col style="width: 61%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head">Constant</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">KQ_NOTE_EXIT</code></td>
<td>the process has exited</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">KQ_NOTE_FORK</code></td>
<td>the process has called fork()</td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">KQ_NOTE_EXEC</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">KQ_NOTE_PCTRLMASK</code></td>
<td>internal filter flag</td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">KQ_NOTE_PDATAMASK</code></td>
<td>internal filter flag</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">KQ_NOTE_TRACK</code></td>
<td>follow a process across fork()</td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">KQ_NOTE_CHILD</code></td>
<td>returned on the child process for
NOTE_TRACK</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">KQ_NOTE_TRACKERR</code></td>
<td>unable to attach to a child</td>
</tr>
</tbody>
</table>
<code class="xref py py-const docutils literal notranslate">KQ_FILTER_NETDEV</code> filter flags (not available on Mac OS X):
<table class="docutils align-default">
<colgroup>
<col style="width: 39%" />
<col style="width: 61%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head">Constant</th>
<th class="head">Meaning</th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">KQ_NOTE_LINKUP</code></td>
<td>link is up</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">KQ_NOTE_LINKDOWN</code></td>
<td>link is down</td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">KQ_NOTE_LINKINV</code></td>
<td>link state is invalid</td>
</tr>
</tbody>
</table>
</dd></dl>

<dl class="py attribute">
<dt class="sig sig-object py" id="select.kevent.data">
kevent.data<a class="headerlink" href="#select.kevent.data" title="Permalink to this definition">¶</a></dt>
<dd>Filter specific data.
</dd></dl>

<dl class="py attribute">
<dt class="sig sig-object py" id="select.kevent.udata">
kevent.udata<a class="headerlink" href="#select.kevent.udata" title="Permalink to this definition">¶</a></dt>
<dd>User defined value.
</dd></dl>

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<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>
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