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123 lines
3.9 KiB
TeX
123 lines
3.9 KiB
TeX
\section{\module{fpectl} ---
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Floating point exception control}
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\declaremodule{extension}{fpectl}
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\platform{Unix}
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\moduleauthor{Lee Busby}{busby1@llnl.gov}
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\sectionauthor{Lee Busby}{busby1@llnl.gov}
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\modulesynopsis{Provide control for floating point exception handling.}
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Most computers carry out floating point operations\index{IEEE-754}
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in conformance with the so-called IEEE-754 standard.
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On any real computer,
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some floating point operations produce results that cannot
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be expressed as a normal floating point value.
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For example, try
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\begin{verbatim}
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>>> import math
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>>> math.exp(1000)
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inf
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>>> math.exp(1000) / math.exp(1000)
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nan
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\end{verbatim}
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(The example above will work on many platforms.
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DEC Alpha may be one exception.)
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"Inf" is a special, non-numeric value in IEEE-754 that
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stands for "infinity", and "nan" means "not a number."
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Note that,
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other than the non-numeric results,
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nothing special happened when you asked Python
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to carry out those calculations.
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That is in fact the default behaviour prescribed in the IEEE-754 standard,
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and if it works for you,
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stop reading now.
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In some circumstances,
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it would be better to raise an exception and stop processing
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at the point where the faulty operation was attempted.
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The \module{fpectl} module
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is for use in that situation.
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It provides control over floating point
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units from several hardware manufacturers,
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allowing the user to turn on the generation
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of \constant{SIGFPE} whenever any of the
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IEEE-754 exceptions Division by Zero, Overflow, or
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Invalid Operation occurs.
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In tandem with a pair of wrapper macros that are inserted
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into the C code comprising your python system,
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\constant{SIGFPE} is trapped and converted into the Python
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\exception{FloatingPointError} exception.
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The \module{fpectl} module defines the following functions and
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may raise the given exception:
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\begin{funcdesc}{turnon_sigfpe}{}
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Turn on the generation of \constant{SIGFPE},
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and set up an appropriate signal handler.
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\end{funcdesc}
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\begin{funcdesc}{turnoff_sigfpe}{}
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Reset default handling of floating point exceptions.
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\end{funcdesc}
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\begin{excdesc}{FloatingPointError}
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After \function{turnon_sigfpe()} has been executed,
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a floating point operation that raises one of the
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IEEE-754 exceptions
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Division by Zero, Overflow, or Invalid operation
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will in turn raise this standard Python exception.
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\end{excdesc}
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\subsection{Example \label{fpectl-example}}
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The following example demonstrates how to start up and test operation of
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the \module{fpectl} module.
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\begin{verbatim}
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>>> import fpectl
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>>> import fpetest
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>>> fpectl.turnon_sigfpe()
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>>> fpetest.test()
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overflow PASS
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FloatingPointError: Overflow
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div by 0 PASS
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FloatingPointError: Division by zero
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[ more output from test elided ]
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>>> import math
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>>> math.exp(1000)
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Traceback (most recent call last):
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File "<stdin>", line 1, in ?
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FloatingPointError: in math_1
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\end{verbatim}
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\subsection{Limitations and other considerations}
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Setting up a given processor to trap IEEE-754 floating point
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errors currently requires custom code on a per-architecture basis.
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You may have to modify \module{fpectl} to control your particular hardware.
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Conversion of an IEEE-754 exception to a Python exception requires
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that the wrapper macros \code{PyFPE_START_PROTECT} and
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\code{PyFPE_END_PROTECT} be inserted into your code in an appropriate
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fashion. Python itself has been modified to support the
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\module{fpectl} module, but many other codes of interest to numerical
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analysts have not.
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The \module{fpectl} module is not thread-safe.
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\begin{seealso}
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\seetext{Some files in the source distribution may be interesting in
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learning more about how this module operates.
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The include file \file{Include/pyfpe.h} discusses the
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implementation of this module at some length.
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\file{Modules/fpetestmodule.c} gives several examples of
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use.
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Many additional examples can be found in
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\file{Objects/floatobject.c}.}
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\end{seealso}
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