#include <math.h> double infnan(iarg) int iarg;
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
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);
}
}
ERANGE and EDOM are defined in <errno.h>. See intro(2) for explanation of EDOM and ERANGE.