HP-UX Floating-Point Guide

158 Chapter 6

Floating-Point Trap Handling

Handling Traps

The ON statement allows you to specify a particular action to be taken

when a particular exception arises. The action may be any of the

following:

• ABORT (the default action). Specifying ABORT allows you to get the

address where the error occurred, which may be useful in debugging.

• IGNORE (usually not a good idea).

• CALL sub (call a subroutine). If you call a subroutine to handle an

IEEE exception, you must pass it one argument, which is of the same

type as the type associated with the exception. If the subroutine

returns, the program uses the assigned value of the argument as the

result value of the operation that caused the handler to be invoked.

NOTE A subroutine that handles an IEEE exception takes a different number

of arguments from a subroutine that handles a math library error (as

described in “Math Library Error Handling for Fortran” on page 106).

See the HP FORTRAN/9000 Programmer’s Guide for details. HP

discourages the use of the ON statement to handle library errors in HP

Fortran 90, but using it to handle arithmetic errors poses no problems.

You can use the ON statement to handle any of the following exceptions:

DIV 0

OVERFLOW

UNDERFLOW

INEXACT

ILLEGAL

For example, the following program calls the subroutine HANDLE_OFL to

handle a DOUBLE PRECISION overﬂow. The subroutine prints a message

describing the error, prints the biased value passed to the subroutine,

then the correct value, and exits.