INFNAN(3)

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

SYNOPSIS
#include <math.h>
double
infnan(int iarg);

DESCRIPTION
At some time in the future, some of the useful properties of the Infinities and s 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 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. 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 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 circumstances. 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 Signal | IEEE Default | iarg | errno | infnan()
Invalid | | EDOM | EDOM | 0
Overflow | ± | ERANGE | ERANGE | HUGE
Div-by-0 | ±Infinity | +-ERANGE | ERANGE/EDOM | +-HUGE

    (HUGE = 1.7e38 ... nearly 2.0**127)

ALTERNATIVE infnan():

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

SEE ALSO
intro(2), signal(3)

HISTORY
The infnan() function appeared in 4.3BSD.