mirror of
https://github.com/python/cpython.git
synced 2024-12-02 22:35:26 +08:00
120 lines
4.1 KiB
ReStructuredText
120 lines
4.1 KiB
ReStructuredText
:mod:`fpectl` --- Floating point exception control
|
|
==================================================
|
|
|
|
.. module:: fpectl
|
|
:platform: Unix
|
|
:synopsis: Provide control for floating point exception handling.
|
|
.. moduleauthor:: Lee Busby <busby1@llnl.gov>
|
|
.. sectionauthor:: Lee Busby <busby1@llnl.gov>
|
|
|
|
|
|
.. note::
|
|
|
|
The :mod:`fpectl` module is not built by default, and its usage is discouraged
|
|
and may be dangerous except in the hands of experts. See also the section
|
|
:ref:`fpectl-limitations` on limitations for more details.
|
|
|
|
.. index:: single: IEEE-754
|
|
|
|
Most computers carry out floating point operations 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 ::
|
|
|
|
>>> import math
|
|
>>> math.exp(1000)
|
|
inf
|
|
>>> math.exp(1000) / math.exp(1000)
|
|
nan
|
|
|
|
(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.
|
|
|
|
In some circumstances, it would be better to raise an exception and stop
|
|
processing at the point where the faulty operation was attempted. The
|
|
:mod:`fpectl` 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 :const:`SIGFPE` 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, :const:`SIGFPE` is trapped and converted into the Python
|
|
:exc:`FloatingPointError` exception.
|
|
|
|
The :mod:`fpectl` module defines the following functions and may raise the given
|
|
exception:
|
|
|
|
|
|
.. function:: turnon_sigfpe()
|
|
|
|
Turn on the generation of :const:`SIGFPE`, and set up an appropriate signal
|
|
handler.
|
|
|
|
|
|
.. function:: turnoff_sigfpe()
|
|
|
|
Reset default handling of floating point exceptions.
|
|
|
|
|
|
.. exception:: FloatingPointError
|
|
|
|
After :func:`turnon_sigfpe` 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.
|
|
|
|
|
|
.. _fpectl-example:
|
|
|
|
Example
|
|
-------
|
|
|
|
The following example demonstrates how to start up and test operation of the
|
|
:mod:`fpectl` module. ::
|
|
|
|
>>> import fpectl
|
|
>>> import fpetest
|
|
>>> fpectl.turnon_sigfpe()
|
|
>>> fpetest.test()
|
|
overflow PASS
|
|
FloatingPointError: Overflow
|
|
|
|
div by 0 PASS
|
|
FloatingPointError: Division by zero
|
|
[ more output from test elided ]
|
|
>>> import math
|
|
>>> math.exp(1000)
|
|
Traceback (most recent call last):
|
|
File "<stdin>", line 1, in ?
|
|
FloatingPointError: in math_1
|
|
|
|
|
|
.. _fpectl-limitations:
|
|
|
|
Limitations and other considerations
|
|
------------------------------------
|
|
|
|
Setting up a given processor to trap IEEE-754 floating point errors currently
|
|
requires custom code on a per-architecture basis. You may have to modify
|
|
:mod:`fpectl` to control your particular hardware.
|
|
|
|
Conversion of an IEEE-754 exception to a Python exception requires that the
|
|
wrapper macros ``PyFPE_START_PROTECT`` and ``PyFPE_END_PROTECT`` be inserted
|
|
into your code in an appropriate fashion. Python itself has been modified to
|
|
support the :mod:`fpectl` module, but many other codes of interest to numerical
|
|
analysts have not.
|
|
|
|
The :mod:`fpectl` module is not thread-safe.
|
|
|
|
|
|
.. seealso::
|
|
|
|
Some files in the source distribution may be interesting in learning more about
|
|
how this module operates. The include file :file:`Include/pyfpe.h` discusses the
|
|
implementation of this module at some length. :file:`Modules/fpetestmodule.c`
|
|
gives several examples of use. Many additional examples can be found in
|
|
:file:`Objects/floatobject.c`.
|
|
|