VS2015 scanf(） scanf_s

今天用VS2015  编写

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#include <stdio.h>

int main(void)

{
float radius = 0.0f;
float diameter = 0.0f;
float circumference = 0.0f;
float area = 0.0f;
float Pi = 3.14159265f;

printf("Input the diameter of the table: ");
scanf("%f", &diameter);
circumference = diameter/2.0f;
circumference = 2.0f*Pi*radius;
area = Pi*radius*radius;
printf("\nThe circumference is %.2f", circumference);
printf("\nThe area is %.2f \n,area");

getchar();
return 0;

}

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开始用scanf（）始终报错

后来上网求助 ，然后改成scanf_s()  果然好了，但是再改成scanf（)依然不报错，还不知道原因