#include <stdio.h>
    #define NaN	   0xffffffff
    #define plusINF    0x7f800000
    #define minusINF   0xff800000
    union f  {
      float 	  f;
      unsigned long   ul;
    };
       union f x;
    main()  {
        float a,b;
     l: printf("please input a, b:\n");
        scanf("%f  %f",&a,&b);
        x.f=a*b;
        if (x.ul==NaN)  {
          printf("NaN  error! \n"); goto l;
        }
        else
        if (x.ul==plusINF)  {
          printf("+INF error! \n"); goto l;
        }
        else
        if (x.ul==minusINF)  {
          printf("-INF error! \n"); goto l;
        }
        else
          printf("OK!  x.f=%e",x.f);printf("\n");
        while(1);
     }