Detecting an Exception With ieee_flags

The following example demonstrates how to determine which floating-point exceptions have been raised by earlier computations. Bit masks defined in the system include file f77_floatingpoint.h are applied to the value returned by ieee_flags.

In this example, DetExcFlg.F, the include file is introduced using the #include preprocessor directive, which requires us to name the source file with a .F suffix. Underflow is caused by dividing the smallest double-precision number by 2.

Example: Detect an exception using ieee_flags and decode it:

#include "f77_floatingpoint.h"
         CHARACTER*16 out
         DOUBLE PRECISION d_max_subnormal, x
         INTEGER div, flgs, inv, inx, over, under

       x = d_max_subnormal() / 2.0                ! Cause underflow

       flgs=ieee_flags('get','exception','',out)  ! Which are raised?

       inx   = and(rshift(flgs, fp_inexact)  , 1)  ! Decode
       div   = and(rshift(flgs, fp_division) , 1)   ! the value
       under = and(rshift(flgs, fp_underflow), 1)    ! returned
       over  = and(rshift(flgs, fp_overflow) , 1)     ! by
       inv   = and(rshift(flgs, fp_invalid)  , 1)      ! ieee_flags

       PRINT *, "Highest priority exception is: ", out
       PRINT *, ' invalid  divide  overflo underflo inexact'
       PRINT '(5i8)', inv, div, over, under, inx
       PRINT *, '(1 = exception is raised; 0 = it is not)'
       i = ieee_flags('clear', 'exception', 'all', out)    ! Clear all
       END

Example: Compile and run the preceding example (DetExcFlg.F):

demo% f77 -silent DetExcFlg.F 
demo% a.out 
 Highest priority exception is: underflow
  invalid  divide  overflo underflo inexact
       0       0       0       1       1
 (1 = exception is raised; 0 = it is not)
demo%