XL Fortran Enterprise Edition for AIX : User's Guide - IBM

The XL Fortran compiler and the operating system provide facilities for working
with floating-point exception conditions. The compiler facilities indicate the
presence of exceptions by generating SIGTRAP signals. The operating-system
facilities generate SIGFPE signals. Do not mix these different facilities within a
single program.
Compiler Features for Trapping Floating-Point Exceptions
To turn on XL Fortran exception trapping, compile the program with the -qflttrap
option and some combination of suboptions that includes enable. This option uses
trap operations to detect floating-point exceptions and generates SIGTRAP signals
when exceptions occur.
-qflttrap also has suboptions that correspond to the names of the exception
conditions. For example, if you are only concerned with handling overflow and
underflow exceptions, you could specify something similar to the following:
xlf95 -qflttrap=overflow:underflow:enable compute_pi.f
You only need enable when you are compiling the main program. However, it is
very important and does not cause any problems if you specify it for other files, so
always include it when you use -qflttrap.
An advantage of this approach is that performance impact is relatively low. To
further reduce performance impact, you can include the imprecise suboption of the
-qflttrap option. This suboption delays any trapping until the program reaches the
start or end of a subprogram.
The disadvantages of this approach include the following:
v It only traps exceptions that occur in code that you compiled with -qflttrap,
which does not include system library routines.
v It is generally not possible for a handler to substitute results for failed
calculations if you use the imprecise suboption of -qflttrap.
Notes:
1. If your program depends on floating-point exceptions occurring for particular
operations, also specify -qfloat suboptions that include nofold and nohssngl.
Otherwise, the compiler might replace an exception-producing calculation with
a constant NaN or infinity value, or it might eliminate an overflow in a
single-precision operation.
2. The suboptions of the -qflttrap option replace an earlier technique that required
you to modify your code with calls to the fpsets and fpgets procedures. You no
longer require these calls for exception handling if you use the appropriate
-qflttrap settings.
Attention: If your code contains fpsets calls that enable checking for
floating-point exceptions and you do not use the -qflttrap option when
compiling the whole program, the program will produce unexpected results if
exceptions occur, as explained in Table 17 on page 290.
Operating System Features for Trapping Floating-Point
Exceptions
A direct way to turn on exception trapping is to call the operating system routine
fp_trap. It uses the system hardware to detect floating-point exceptions and
generates SIGFPE signals when exceptions occur. Fortran definitions for the values
needed to call it are in the files /usr/include/fp_fort_c.f, fp_fort_t.f, or the
xlf_fp_util module.
XL Fortran Floating-Point Processing 297