圆的周长和面积（指针）

如图所示，我想不通应该如何使用这个area函数的double *girth，麻烦大家提供点思路，感谢！

mj = area(r,&zc);


double area(double r,double *girth)
{
    double mj = 3.1415926 * r * r;
    *girth = 2 * 3.1415926 * r;
    return mj;
}

#include <stdio.h>
#include <time.h>
#include <stdlib.h>
double area(double r, double* girth); 
int main()
{
    double r, mj, zc; 
    scanf("%lf", &r);
    mj = area(r, &zc);
    printf("mj=%.4lf zc=%.2lf\n", mj, zc);
}
double area(double r, double* girth)
{
    *girth = 2 * 3.1415926 * r;
    return 3.1415926 * r * r;
}