INFNAN(3)                 386BSD Programmer's Manual                 INFNAN(3)

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
     procedure infnan() 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
     math(3),  intro(2),  signal(3).

     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.

4.3 Berkeley Distribution       April 19, 1991                               2