ultrix man page for isnan

isnan(3)						     Library Functions Manual							  isnan(3)

Name
       isnan - test for NaN

Syntax
       #include <math.h>

       int isnan (x)
       double x;

Description
       The  function  returns 1 if x is NaN (the IEEE floating point reserved not-a-number value) and zero otherwise.  On VAX, the return value is
       always zero.

																	  isnan(3)