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121 lines
4.1 KiB
121 lines
4.1 KiB
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:mod:`fpectl` --- Floating point exception control
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==================================================
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.. module:: fpectl
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:platform: Unix
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:synopsis: Provide control for floating point exception handling.
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.. moduleauthor:: Lee Busby <busby1@llnl.gov>
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.. sectionauthor:: Lee Busby <busby1@llnl.gov>
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.. note::
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The :mod:`fpectl` module is not built by default, and its usage is discouraged
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and may be dangerous except in the hands of experts. See also the section
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:ref:`fpectl-limitations` on limitations for more details.
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.. index:: single: IEEE-754
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Most computers carry out floating point operations in conformance with the
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so-called IEEE-754 standard. On any real computer, some floating point
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operations produce results that cannot be expressed as a normal floating point
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value. For example, try ::
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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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(The example above will work on many platforms. DEC Alpha may be one exception.)
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"Inf" is a special, non-numeric value in IEEE-754 that stands for "infinity",
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and "nan" means "not a number." Note that, other than the non-numeric results,
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nothing special happened when you asked Python to carry out those calculations.
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That is in fact the default behaviour prescribed in the IEEE-754 standard, and
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if it works for you, stop reading now.
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In some circumstances, it would be better to raise an exception and stop
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processing at the point where the faulty operation was attempted. The
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:mod:`fpectl` module is for use in that situation. It provides control over
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floating point units from several hardware manufacturers, allowing the user to
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turn on the generation of :const:`SIGFPE` whenever any of the IEEE-754
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exceptions Division by Zero, Overflow, or Invalid Operation occurs. In tandem
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with a pair of wrapper macros that are inserted into the C code comprising your
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python system, :const:`SIGFPE` is trapped and converted into the Python
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:exc:`FloatingPointError` exception.
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The :mod:`fpectl` module defines the following functions and may raise the given
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exception:
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.. function:: turnon_sigfpe()
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Turn on the generation of :const:`SIGFPE`, and set up an appropriate signal
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handler.
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.. function:: turnoff_sigfpe()
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Reset default handling of floating point exceptions.
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.. exception:: FloatingPointError
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After :func:`turnon_sigfpe` has been executed, a floating point operation that
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raises one of the IEEE-754 exceptions Division by Zero, Overflow, or Invalid
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operation will in turn raise this standard Python exception.
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.. _fpectl-example:
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Example
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-------
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The following example demonstrates how to start up and test operation of the
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:mod:`fpectl` module. ::
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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 <module>
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FloatingPointError: in math_1
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.. _fpectl-limitations:
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Limitations and other considerations
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------------------------------------
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Setting up a given processor to trap IEEE-754 floating point errors currently
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requires custom code on a per-architecture basis. You may have to modify
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:mod:`fpectl` to control your particular hardware.
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Conversion of an IEEE-754 exception to a Python exception requires that the
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wrapper macros ``PyFPE_START_PROTECT`` and ``PyFPE_END_PROTECT`` be inserted
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into your code in an appropriate fashion. Python itself has been modified to
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support the :mod:`fpectl` module, but many other codes of interest to numerical
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analysts have not.
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The :mod:`fpectl` module is not thread-safe.
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.. seealso::
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Some files in the source distribution may be interesting in learning more about
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how this module operates. The include file :source:`Include/pyfpe.h` discusses the
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implementation of this module at some length. :source:`Modules/fpetestmodule.c`
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gives several examples of use. Many additional examples can be found in
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:source:`Objects/floatobject.c`.
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