代码如下，为什么两个变量的值还是没有变，求解答。
int sum(int *a,int *b)
{
        int *tmp;
        tmp=a;
        a=b;
        b=tmp;

}
void main()
{
        int x=1,y=9;
        int (*p)();
        p=sum;
        (*p)(&x,&y);
        printf("x=%d,y=%d\n",x,y);
}

int tmp = *a;
*a = *b;
*b = tmp;

值传递