INFNAN(3M)                                                          INFNAN(3M)



NAME
       infnan - signals invalid floating-point operations on a VAX (temporary)

SYNOPSIS
       #include <math.h>

       double infnan(iarg)
       int iarg;

DESCRIPTION
       At some time in the future,  some  of  the  useful  properties  of  the
       Infinities  and _N_a_Ns in the IEEE standard 754 for Binary Floating-Point
       Arithmetic will be simulated in UNIX  on  the  DEC  VAX  by  using  its
       Reserved Operands.  Meanwhile, the Invalid, Overflow and Divide-by-Zero
       exceptions of the IEEE standard are being  approximated  on  a  VAX  by
       calls to a procedure _i_n_f_n_a_n in appropriate places in _l_i_b_m.  When better
       exception-handling is implemented in UNIX, only _i_n_f_n_a_n among the  codes
       in  _l_i_b_m  will  have to be changed.  And users of _l_i_b_m can design their
       own _i_n_f_n_a_n now to insulate themselves from future changes.

       Whenever an elementary function code in _l_i_b_m has to simulate one of the
       aforementioned IEEE exceptions, it calls infnan(iarg) with an appropri‐
       ate value of _i_a_r_g.  Then a reserved operand  fault  stops  computation.
       But  _i_n_f_n_a_n  could  be  replaced  by a function with the same name that
       returns some plausible value, assigns an apt value to the global  vari‐
       able  _e_r_r_n_o,  and  allows  computation  to  resume.  Alternatively, the
       Reserved Operand Fault Handler could be changed to respond by returning
       that plausible value, etc.  instead of aborting.

       In  the table below, the first two columns show various exceptions sig‐
       naled by the IEEE standard, and the default result it prescribes.   The
       third  column  shows  what  value is given to _i_a_r_g by functions in _l_i_b_m
       when they invoke infnan(iarg) under analogous circumstances on  a  VAX.
       Currently  _i_n_f_n_a_n stops computation under all those circumstances.  The
       last two columns offer an alternative; they suggest a setting for _e_r_r_n_o
       and  a value for a revised _i_n_f_n_a_n to return.  And a C program to imple‐
       ment that suggestion follows.
              IEEE      IEEE
              Signal    Default   _i_a_r_g    _e_r_r_n_o             _i_n_f_n_a_n
              __________________________________________________
              Invalid   _N_a_N       EDOM    EDOM              0
              Overflow  ±Infinity ERANGE  ERANGE            HUGE
              Div-by-0  ±Infinity ±ERANGE ERANGE or EDOM    ±HUGE
                        (HUGE = 1.7e38 ... nearly  2.0**127)

       ALTERNATIVE  _i_n_f_n_a_n:
       #include   <math.h>
       #include   <errno.h>
       extern int errno ;
       double     infnan(iarg)
       int        iarg ;
       {
                  switch(iarg) {
                  case      ERANGE: errno = ERANGE; return(HUGE);
                  case     -ERANGE: errno = EDOM;   return(-HUGE);
                  default:          errno = EDOM;   return(0);
                  }
       }

SEE ALSO
       math(3M), intro(2), signal(3).

       ERANGE and EDOM are defined in <errno.h>.  See intro(2) for explanation
       of EDOM and ERANGE.



4.3 Berkeley Distribution        May 27, 1986                       INFNAN(3M)