# 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

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