2.11BSD#431 - /usr/man/cat3/infnan.0

INFNAN(3M)	    UNIX Programmer's Manual	       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 NaNs 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 pro-
     cedure infnan in appropriate places in libm.  When better
     exception-handling is implemented in UNIX, only infnan among
     the codes in libm will have to be changed.  And users of
     libm can design their own infnan now to insulate themselves
     from future changes.

     Whenever an elementary function code in libm has to simulate
     one of the aforementioned IEEE exceptions, it calls
     infnan(iarg) with an appropriate value of iarg.  Then a
     reserved operand fault stops computation.	But infnan could
     be replaced by a function with the same name that returns
     some plausible value, assigns an apt value to the global
     variable errno, and allows computation to resume.	Alterna-
     tively, 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 signaled by the IEEE standard, and the default
     result it prescribes.  The third column shows what value is
     given to iarg by functions in libm when they invoke
     infnan(iarg) under analogous circumstances on a VAX.
     Currently infnan stops computation under all those cir-
     cumstances.  The last two columns offer an alternative; they
     suggest a setting for errno and a value for a revised infnan
     to return.  And a C program to implement that suggestion
     follows.
	  IEEE	    IEEE
	  Signal    Default   iarg    errno		infnan
	  __________________________________________________
	  Invalid   NaN       EDOM    EDOM		0
	  Overflow  +Infinity ERANGE  ERANGE		HUGE
	  Div-by-0  +Infinity +ERANGE ERANGE or EDOM	+HUGE
		    (HUGE = 1.7e38 ... nearly  2.0**127)


Printed 11/26/99	  May 27, 1986				1


INFNAN(3M)	    UNIX Programmer's Manual	       INFNAN(3M)


     ALTERNATIVE  infnan:
     #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.


Printed 11/26/99	  May 27, 1986				2