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@(#)infnan.3 6.4 (Berkeley) 05/27/86

C 6 infnan - signals invalid floating-point operations on a VAX (temporary)

 #include <math.h> 

 double infnan(iarg)  int iarg; 
 DESCRIPTION
At some time in the future, some of the useful properties of
the Infinities and \*(nns 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 IEEE
Signal Default iarg errno infnan
.tc \(ru
 
.tc \}
\l'5i'
Invalid \*(nn EDOM EDOM 0
Overflow \(+-Infinity ERANGE ERANGE HUGE
Div-by-0 \(+-Infinity \(+-ERANGE ERANGE or EDOM \(+-HUGE \}
Overflow \(+-\(if ERANGE ERANGE HUGE
Div-by-0 \(+-\(if \(+-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 \0ERANGE: 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.