#define T Stack_T
typedef struct T *T;

struct T {
int count;
struct elem {
void *x;
struct elem *link;
} *head;
}


const T stk   //指针常量, const修改struct T *。地址不能被改变。 
const struct T *stk //指着常量指针, const修改struct T。数据不能改变。