INFNAN(3M)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 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 appropri‐
ate 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 vari‐
able 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.
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 imple‐
ment 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)
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 ALSOmath(3M), intro(2), signal(3).
ERANGE and EDOM are defined in <errno.h>. See intro(2) for explanation
of EDOM and ERANGE.
4.3 Berkeley Distribution May 27, 1986 INFNAN(3M)