man(1) Manual page archive

     NAN(2)                                                     NAN(2)

          NaN, Inf, isNaN, isInf - not-a-number and infinity functions

          double NaN(void)

          long   Inf(int)

          int    isNaN(double)

          int    isInf(double, int)

          The IEEE floating point standard defines values called
          `not-a-number' and positive and negative `infinity'.  These
          values can be produced by such things as overflow and divi-
          sion by zero.  Also, the library functions sometimes return
          them when the arguments are not in the domain, or the result
          is out of range.

          NaN returns a double that is not-a-number.  IsNaN returns
          true if its argument is not-a-number.

          Inf(i) returns positive infinity if i is greater than or
          equal to zero, else negative infinity.  IsInf returns true
          if its first argument is infinity with the same sign as the
          second argument.