sam-t2/devtools/v9/html/python/lib/module-fpectl.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="next" href="module-atexit.html" />
<link rel="prev" href="module-weakref.html" />
<link rel="parent" href="python.html" />
<link rel="next" href="fpectl-example.html" />
<meta name='aesop' content='information' />
<title>3.4 fpectl -- Floating point exception control</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="3.3.3 Weak References in"
  href="weakref-extension.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="3. Python Runtime Services"
  href="python.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="3.4.1 Example"
  href="fpectl-example.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="weakref-extension.html">3.3.3 Weak References in</A>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="python.html">3. Python Runtime Services</A>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="fpectl-example.html">3.4.1 Example</A>
</div>
<hr /></div>
</DIV>
<!--End of Navigation Panel-->

<H1><A NAME="SECTION005400000000000000000">
3.4 <tt class="module">fpectl</tt> --
         Floating point exception control</A>
</H1>

<P>
<A NAME="module-fpectl"></A>
<p class="availability">Availability: <span
 class="platform">Unix</span>.</p>

<P>
Most computers carry out floating point operations<a id='l2h-434' xml:id='l2h-434'></a>
in conformance with the so-called IEEE-754 standard.
On any real computer,
some floating point operations produce results that cannot
be expressed as a normal floating point value.
For example, try

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; import math
&gt;&gt;&gt; math.exp(1000)
inf
&gt;&gt;&gt; math.exp(1000) / math.exp(1000)
nan
</pre></div>

<P>
(The example above will work on many platforms.
DEC Alpha may be one exception.)
"Inf" is a special, non-numeric value in IEEE-754 that
stands for "infinity", and "nan" means "not a number."
Note that,
other than the non-numeric results,
nothing special happened when you asked Python
to carry out those calculations.
That is in fact the default behaviour prescribed in the IEEE-754 standard,
and if it works for you,
stop reading now.

<P>
In some circumstances,
it would be better to raise an exception and stop processing
at the point where the faulty operation was attempted.
The <tt class="module">fpectl</tt> module
is for use in that situation.
It provides control over floating point
units from several hardware manufacturers,
allowing the user to turn on the generation
of <tt class="constant">SIGFPE</tt> whenever any of the
IEEE-754 exceptions Division by Zero, Overflow, or
Invalid Operation occurs.
In tandem with a pair of wrapper macros that are inserted
into the C code comprising your python system,
<tt class="constant">SIGFPE</tt> is trapped and converted into the Python
<tt class="exception">FloatingPointError</tt> exception.

<P>
The <tt class="module">fpectl</tt> module defines the following functions and
may raise the given exception:

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-431' xml:id='l2h-431' class="function">turnon_sigfpe</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
Turn on the generation of <tt class="constant">SIGFPE</tt>,
and set up an appropriate signal handler.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-432' xml:id='l2h-432' class="function">turnoff_sigfpe</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
Reset default handling of floating point exceptions.
</dl>

<P>
<dl><dt><b><span class="typelabel">exception</span>&nbsp;<tt id='l2h-433' xml:id='l2h-433' class="exception">FloatingPointError</tt></b></dt>
<dd>
After <tt class="function">turnon_sigfpe()</tt> has been executed,
a floating point operation that raises one of the
IEEE-754 exceptions
Division by Zero, Overflow, or Invalid operation
will in turn raise this standard Python exception.
</dd></dl>

<P>

<p><br /></p><hr class='online-navigation' />
<div class='online-navigation'>
<!--Table of Child-Links-->
<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></a>

<UL CLASS="ChildLinks">
<LI><A href="fpectl-example.html">3.4.1 Example</a>
<LI><A href="node45.html">3.4.2 Limitations and other considerations</a>
</ul>
<!--End of Table of Child-Links-->
</div>

<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="3.3.3 Weak References in"
  href="weakref-extension.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="3. Python Runtime Services"
  href="python.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="3.4.1 Example"
  href="fpectl-example.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="weakref-extension.html">3.3.3 Weak References in</A>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="python.html">3. Python Runtime Services</A>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="fpectl-example.html">3.4.1 Example</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>