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<title>3.4 fpectl -- Floating point exception control
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<H1><A NAME=
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3.4 <tt class=
"module">fpectl
</tt> --
Floating point exception control
</A>
<A NAME=
"module-fpectl"></A>
<p class=
"availability">Availability:
<span
class=
"platform">Unix
</span>.
</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.
some floating point operations produce results that cannot
be expressed as a normal floating point value.
<div class=
"verbatim"><pre>
>>> math.exp(
1000)
>>> math.exp(
1000) / math.exp(
1000)
(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."
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,
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.
The
<tt class=
"module">fpectl
</tt> module defines the following functions and
may raise the given exception:
<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>
Turn on the generation of
<tt class=
"constant">SIGFPE
</tt>,
and set up an appropriate signal handler.
<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>
Reset default handling of floating point exceptions.
<dl><dt><b><span class=
"typelabel">exception
</span> <tt id='l2h-
433' xml:id='l2h-
433'
class=
"exception">FloatingPointError
</tt></b></dt>
After
<tt class=
"function">turnon_sigfpe()
</tt> has been executed,
a floating point operation that raises one of the
Division by Zero, Overflow, or Invalid operation
will in turn raise this standard Python exception.
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<LI><A href=
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<LI><A href=
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<span class=
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2.4.2, documentation updated on
28 September
2005.
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