#include <stdio.h>

      int IsNaN(double x){
        int rc;   /* The value return */
        volatile double y = x;
        volatile double z = y;
        rc = (y!=z);
        return rc;
      }

      int main() {
        double tests[] = { 1.0, 3.333, 1.0/0.0, 0.0/0.0, -1.0/0.0, -0, 0, -123123123, 12.0E200 };
        for (int i = 0; i < sizeof(tests)/sizeof(double); i++)
          printf("%d - %f - %d\n", i, tests[i], IsNaN(tests[i]));
      }