HP Fortran Programmer Guide (766160-001, March 2014)

5 Using the ON statement
Whenever a runtime error occurs, the default action of your program depends on the type of the
error. If the error results from a floating-point exception, the program will continue to execute.
Other errors will cause it to abort.
As described in “Handling runtime exceptions” (page 77), the +fp_exception and +FPoptions
provide control over how a program behaves when a runtime error occurs. The ON statement
provides an additional level of control by enabling your program to handle floating-point and
integer exceptions and +Ctrl-C interrupts. Before an exception can be handled, the flow of
control must pass through an ON statement that specifies:
• The type of the exception
• One of the following actions:
❏ Execute a trap procedure
❏ Ignore the interrupt
❏ Abort the program
The action specified by the ON statement can only be changed by another ON statement that
specifies the same exception.
This chapter describes how to use the ON statement. The syntax of the ON statement is described
in the HPFortran Programmer’s Reference. For detailed information about trapping math errors,
see the HP-UX Floating-Point Guide.
NOTE: If you include theON statement in a program that you optimize at level 2 or higher and
the program takes an exception, the results may vary from those you would get from an unoptimized
program or from a program that didn’t have the ONstatement.
Exceptions handled by the ON statement
Like the +fp_exceptionoption, the ONstatement enables traps for floating-point exceptions (by
default, traps for floating-point exceptions are disabled on HP 9000 computers). When traps are
enabled, an executing program that takes any of the following exceptions will abort, unless an
ONstatement specifies a different action:
• Division by zero
• Overflow
• Underflow
• Invalid (or illegal) operation
These exceptions are defined by the IEEE standard for floating-point operations. The ONstatement
enables traps for these exceptions, regardless of whether the exception is taken by user code or
by a call to a library routine. In addition, the ON statement also enables traps for integer division
by zero, integer overflow, and +Ctrl-Cinterrupts. The +Ctrl-Cinterrupt occurs when the user
presses +Ctrl-Cduring program execution.
Table5-1 on page131 lists the exceptions handled by the ONstatement and gives the keywords
that must be specified in the ONstatement to indicate the exception being handled. The first column
indicates the type of exception. The second column gives the keywords that must appear in the
ON statement, immediately following the word ON. The third column gives alternate keywords you
can specify instead of those in the second column.
For example, the following ONstatement will trap attempts to divide by zero with 8-byte floating-point
operands:
ON REAL(8) DIV 0 CALL div_zero_trap
Exceptions handled by the ON statement 81