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 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 procedureinfnan()
in appropriate places in
libm(3). 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 | 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);
}
}
SEE ALSO
ERANGE and EDOM
are defined in ⟨errno.h⟩. (See
intro(2) for explanation of EDOM and
ERANGE.)
HISTORY
The infnan() function appeared in
4.3BSD.