PSSE and Floating point exceptions

PSSE crashes hard when it runs across certain floating point numbers. This can be a particularly hard problem to debug, because there may be no traceback or error message at all depending on how you are using PSSE.

The trivial case to reproduce:

  1. Start PSSE
  2. In the Python command line enter float('NaN')
  3. Result: PSSE Crashes

This issue can also cause crashes deep inside 3rd party libraries which are hard to debug. I first saw this issue in pandas and pytables.

Quick Fix

Set the environment variable FPMASK=59

PSSE uses this environment variable to set the floating point control word for the math coprocessor [1].

x87 Control Word

The control word is a special 16-bit register maintained by the math coprocessor. By setting and clearing bit fields in the control word, you can exercise control of certain aspects of coprocessor operation.

x87 control word

The environment variable is in base 10. So we convert to hex then binary.

53 (base 10) = 0x35 = 0011 1001

Fix inside python

You can set the environment variable inside your python script. This way the script will work on other systems.

import os
os.setenv('FPMASK') = 93
import psspy

# Make sure no exceptions are thrown.
x = float(NaN)
x = 1.0/0