sam-t2/devtools/v8plus/html/python/lib/semaphore-examples.html

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<link rel="STYLESHEET" href="lib.css" type='text/css' />
<link rel="SHORTCUT ICON" href="../icons/pyfav.png" type="image/png" />
<link rel='start' href='../index.html' title='Python Documentation Index' />
<link rel="first" href="lib.html" title='Python Library Reference' />
<link rel='contents' href='contents.html' title="Contents" />
<link rel='index' href='genindex.html' title='Index' />
<link rel='last' href='about.html' title='About this document...' />
<link rel='help' href='about.html' title='About this document...' />
<link rel="prev" href="semaphore-objects.html" />
<link rel="parent" href="semaphore-objects.html" />
<link rel="next" href="event-objects.html" />
<meta name='aesop' content='information' />
<title>7.5.4.1 Semaphore Example </title>
</head>
<body>
<DIV CLASS="navigation">
<div id='top-navigation-panel' xml:id='top-navigation-panel'>
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td class='online-navigation'><a rel="prev" title="7.5.4 Semaphore Objects"
  href="semaphore-objects.html"><img src='../icons/previous.png'
  border='0' height='32'  alt='Previous Page' width='32' /></A></td>
<td class='online-navigation'><a rel="parent" title="7.5.4 Semaphore Objects"
  href="semaphore-objects.html"><img src='../icons/up.png'
  border='0' height='32'  alt='Up One Level' width='32' /></A></td>
<td class='online-navigation'><a rel="next" title="7.5.5 Event Objects"
  href="event-objects.html"><img src='../icons/next.png'
  border='0' height='32'  alt='Next Page' width='32' /></A></td>
<td align="center" width="100%">Python Library Reference</td>
<td class='online-navigation'><a rel="contents" title="Table of Contents"
  href="contents.html"><img src='../icons/contents.png'
  border='0' height='32'  alt='Contents' width='32' /></A></td>
<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
  border='0' height='32'  alt='Module Index' width='32' /></a></td>
<td class='online-navigation'><a rel="index" title="Index"
  href="genindex.html"><img src='../icons/index.png'
  border='0' height='32'  alt='Index' width='32' /></A></td>
</tr></table>
<div class='online-navigation'>
<b class="navlabel">Previous:</b>
<a class="sectref" rel="prev" href="semaphore-objects.html">7.5.4 Semaphore Objects</A>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="semaphore-objects.html">7.5.4 Semaphore Objects</A>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="event-objects.html">7.5.5 Event Objects</A>
</div>
<hr /></div>
</DIV>
<!--End of Navigation Panel-->

<H3><A NAME="SECTION009541000000000000000"></A><A NAME="semaphore-examples"></A>
<BR>
7.5.4.1 <tt class="class">Semaphore</tt> Example
</H3>

<P>
Semaphores are often used to guard resources with limited capacity, for
example, a database server.  In any situation where the size of the resource
size is fixed, you should use a bounded semaphore.  Before spawning any
worker threads, your main thread would initialize the semaphore:

<P>
<div class="verbatim"><pre>
maxconnections = 5
...
pool_sema = BoundedSemaphore(value=maxconnections)
</pre></div>

<P>
Once spawned, worker threads call the semaphore's acquire and release
methods when they need to connect to the server:

<P>
<div class="verbatim"><pre>
pool_sema.acquire()
conn = connectdb()
... use connection ...
conn.close()
pool_sema.release()
</pre></div>

<P>
The use of a bounded semaphore reduces the chance that a programming error
which causes the semaphore to be released more than it's acquired will go
undetected.

<P>

<DIV CLASS="navigation">
<div class='online-navigation'>
<p></p><hr />
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td class='online-navigation'><a rel="prev" title="7.5.4 Semaphore Objects"
  href="semaphore-objects.html"><img src='../icons/previous.png'
  border='0' height='32'  alt='Previous Page' width='32' /></A></td>
<td class='online-navigation'><a rel="parent" title="7.5.4 Semaphore Objects"
  href="semaphore-objects.html"><img src='../icons/up.png'
  border='0' height='32'  alt='Up One Level' width='32' /></A></td>
<td class='online-navigation'><a rel="next" title="7.5.5 Event Objects"
  href="event-objects.html"><img src='../icons/next.png'
  border='0' height='32'  alt='Next Page' width='32' /></A></td>
<td align="center" width="100%">Python Library Reference</td>
<td class='online-navigation'><a rel="contents" title="Table of Contents"
  href="contents.html"><img src='../icons/contents.png'
  border='0' height='32'  alt='Contents' width='32' /></A></td>
<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
  border='0' height='32'  alt='Module Index' width='32' /></a></td>
<td class='online-navigation'><a rel="index" title="Index"
  href="genindex.html"><img src='../icons/index.png'
  border='0' height='32'  alt='Index' width='32' /></A></td>
</tr></table>
<div class='online-navigation'>
<b class="navlabel">Previous:</b>
<a class="sectref" rel="prev" href="semaphore-objects.html">7.5.4 Semaphore Objects</A>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="semaphore-objects.html">7.5.4 Semaphore Objects</A>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="event-objects.html">7.5.5 Event Objects</A>
</div>
</div>
<hr />
<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
</DIV>
<!--End of Navigation Panel-->
<ADDRESS>
See <i><a href="about.html">About this document...</a></i> for information on suggesting changes.
</ADDRESS>
</BODY>
</HTML>