HP Fortran Programmer's Guide (B3908-90031; September 2011)

Debugging
Handling runtime exceptions
Chapter 4 123
Handling runtime exceptions
Broadly defined, an exception is an error or fault condition that affects a program’s results. Exceptions can
range from the relatively benign inexact result condition that occurs in certain floating-point operations to
the more severe segmentation violation that occurs when a runaway program attempts to access
unallocated memory.
Exceptions that threaten the integrity of the operating system can cause HP-UX to raise an exception signal
(for example, SIGSEGV for a segmentation violation) so that the process can take appropriate action to
recover from the exception. Such exceptions may cause the program that took the exception to abort, but not
necessarily. By trapping an exception—that is, by catching the signal—a program may handle the
exception, if only by aborting when it occurs.
There are also a well-defined set of floating-point conditions that, although they pose no threat to the
operating system, can also cause an exception—for example, dividing a floating-point number by zero. By
default, traps for floating-point exceptions are disabled on HP 9000 computers, but they can be enabled by
+fp_exception and +FP options. (You can also use the ON statement to enable traps for floating-point
exceptions.)
Programs that have been compiled with the +fp_exception option can trap the exceptions listed in
Table 4-1. Any of the exceptions listed in the second column will cause the operating system to generate the
signal listed in the first column. Programs compiled with +FP can trap specific floating-point exceptions
(SIGFPE).
When a program compiled with +fp_exception takes an exception, the following events occur:
• The program traps the exception.
• A procedure traceback is displayed on standard error. A procedure traceback lists routine names and
their offsets that are helpful in locating the code that triggered the exception.
Table 4-1 Signals recognized by +fp_exception
Signal Exception
SIGBUS Bus error instruction
SIGFPE Floating-point exceptions
SIGILL Illegal instruction
SIGSEGV Segmentation violation or memory fault
SIGSYS Bad argument to a kernel system call