INFNAN

Section: C Library Functions (3)
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BSD mandoc
BSD 4.3

NAME

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

SYNOPSIS

Fd #include <math.h> Ft double Fn 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 Fn infnan in appropriate places in libm(3). When better exception-handling is implemented in UNIX , only Fn infnan among the codes in libm will have to be changed. And users of libm can design their own Fn 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 Fn infnan iarg with an appropriate value of Fa iarg . Then a reserved operand fault stops computation. But Fn 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 Fa iarg by functions in libm when they invoke Fn infnan iarg under analogous circumstances on a VAX . Currently Fn 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 Fn infnan to return. And a C program to implement that suggestion follows.

IEEE Signal IEEE Default Ta Fa iarg Ta errno Ta Fn infnan

Invalid Ta EDOM EDOM 0

Overflow± Ta ERANGEERANGEHUGE

Div-by-0±Infinity Ta ±ERANGEERANGE or EDOM±HUGE

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

ALTERNATIVE
Fn 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);
        }
}

HISTORY

The Fn infnan function appeared in BSD 4.3