C 语言最大难点揭秘[3]

　　悬空指针比较棘手。当程序员在内存资源释放后使用资源时会发生悬空指针（请参见清单 5）：

　　清单 5. 悬空指针
               
       void f8()
       {
    struct x *xp;

    xp = (struct x *) malloc(sizeof (struct x));
    xp.q = 13;
    ...
    free(xp);
    ...
        /* Problem!  There's no guarantee that
    the memory block to which xp points
    hasn't been overwritten. */
    return xp.q;