INFNAN(3) MachTen Programmer’s Manual 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 Infini-
ties and NaNs in the IEEE standard 754 for Binary Floating-Point Arith-
metic 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 proce-
dure infnan() in appropriate places in libm(3). When better excep-
tion-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 sig-
naled 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. Current-
ly 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 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)

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);
}
}