HP-UX Floating-Point Guide

108 Chapter 4
HP-UX Math Libraries on HP 9000 Systems
Math Library Basics
Figure 4-4 Fortran 77 Math Library Error Handling
The ON EXTERNAL ERROR Statement (HP
FORTRAN/9000 only)
NOTE HP Fortran 90 does not support the ON EXTERNAL ERROR statement,
although it does support the ON statement for other kinds of error
handling.
If you use +T to enable a trap for invalid operations, what happens next
depends on whether your program contains an ON EXTERNAL ERROR
statement. If it does not, the function merely returns the default result.