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<title>7.3.1 Polling Objects
</title>
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<H2><A NAME=
"SECTION009310000000000000000"></A><A NAME=
"poll-objects"></A>
The
<tt class=
"cfunction">poll()
</tt> system call, supported on most
<span class=
"Unix">Unix
</span> systems,
provides better scalability for network servers that service many,
many clients at the same time.
<tt class=
"cfunction">poll()
</tt> scales better because the system call only
requires listing the file descriptors of interest, while
<tt class=
"cfunction">select()
</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=
"cfunction">select()
</tt> is O(highest file descriptor), while
<tt class=
"cfunction">poll()
</tt> is O(number of file descriptors).
<dl><dt><table cellpadding=
"0" cellspacing=
"0"><tr valign=
"baseline">
<td><nobr><b><tt id='l2h-
2684' xml:id='l2h-
2684'
class=
"method">register
</tt></b>(
</nobr></td>
<td><var>fd
</var><big>[
</big><var>, eventmask
</var><big>]
</big><var></var>)
</td></tr></table></dt>
Register a file descriptor with the polling object. Future calls to
the
<tt class=
"method">poll()
</tt> method will then check whether the file descriptor
has any pending I/O events.
<var>fd
</var> can be either an integer, or an
object with a
<tt class=
"method">fileno()
</tt> method that returns an integer. File
<tt class=
"method">fileno()
</tt>, so they can also be used as the argument.
<var>eventmask
</var> is an optional bitmask describing the type of events you
want to check for, and can be a combination of the constants
<tt class=
"constant">POLLIN
</tt>,
<tt class=
"constant">POLLPRI
</tt>, and
<tt class=
"constant">POLLOUT
</tt>,
described in the table below. If not specified, the default value
used will check for all
3 types of events.
<div class=
"center"><table class=
"realtable">
<th class=
"left" >Constant
</th>
<th class=
"left" >Meaning
</th>
<tr><td class=
"left" valign=
"baseline"><tt class=
"constant">POLLIN
</tt></td>
<td class=
"left" >There is data to read
</td></tr>
<tr><td class=
"left" valign=
"baseline"><tt class=
"constant">POLLPRI
</tt></td>
<td class=
"left" >There is urgent data to read
</td></tr>
<tr><td class=
"left" valign=
"baseline"><tt class=
"constant">POLLOUT
</tt></td>
<td class=
"left" >Ready for output: writing will not block
</td></tr>
<tr><td class=
"left" valign=
"baseline"><tt class=
"constant">POLLERR
</tt></td>
<td class=
"left" >Error condition of some sort
</td></tr>
<tr><td class=
"left" valign=
"baseline"><tt class=
"constant">POLLHUP
</tt></td>
<td class=
"left" >Hung up
</td></tr>
<tr><td class=
"left" valign=
"baseline"><tt class=
"constant">POLLNVAL
</tt></td>
<td class=
"left" >Invalid request: descriptor not open
</td></tr></tbody>
Registering a file descriptor that's already registered is not an
error, and has the same effect as registering the descriptor exactly
<dl><dt><table cellpadding=
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<td><nobr><b><tt id='l2h-
2685' xml:id='l2h-
2685'
class=
"method">unregister
</tt></b>(
</nobr></td>
<td><var>fd
</var>)
</td></tr></table></dt>
Remove a file descriptor being tracked by a polling object. Just like
the
<tt class=
"method">register()
</tt> method,
<var>fd
</var> can be an integer or an
object with a
<tt class=
"method">fileno()
</tt> method that returns an integer.
Attempting to remove a file descriptor that was never registered
causes a
<tt class=
"exception">KeyError
</tt> exception to be raised.
<dl><dt><table cellpadding=
"0" cellspacing=
"0"><tr valign=
"baseline">
<td><nobr><b><tt id='l2h-
2686' xml:id='l2h-
2686'
class=
"method">poll
</tt></b>(
</nobr></td>
<td><var></var><big>[
</big><var>timeout
</var><big>]
</big><var></var>)
</td></tr></table></dt>
Polls the set of registered file descriptors, and returns a
possibly-empty list containing
<code>(
<var>fd
</var>,
<var>event
</var>)
</code> 2-tuples
for the descriptors that have events or errors to report.
<var>fd
</var> is the file descriptor, and
<var>event
</var> is a bitmask
with bits set for the reported events for that descriptor
--
<tt class=
"constant">POLLIN
</tt> for waiting input,
<tt class=
"constant">POLLOUT
</tt> to indicate that the descriptor can be written to, and
An empty list indicates that the call timed out and no file
descriptors had any events to report.
If
<var>timeout
</var> is given, it specifies the length of time in
milliseconds which the system will wait for events before returning.
If
<var>timeout
</var> is omitted, negative, or
<tt class=
"constant">None
</tt>, the call will
block until there is an event for this poll object.
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<span class=
"release-info">Release
2.4.2, documentation updated on
28 September
2005.
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