这段是用链表和文件做的学生成绩管理系统按学号排序。。。看不懂。。。。。求讲解

struct stu  *sort_xuehao (struct stu  *head)  //     按学号排序
{
    struct stu  *p,*p1,*p2,*p3;
    struct stu  h, t;
    if (head == NULL) return NULL;
    h.next=head;
    p=&h;
    while (p->next!=NULL)
    {
        p=p->next;
    }
    p=p->next=&t;
    while (p!=h.next)
    {
        p3=&h;
        p1=p3->next;
        p2=p1->next;
        while (p2!=p)
        {
            if ((p1->num )>(p2->num))
            {
                p1->next=p2->next;
                p2->next=p1;
                p3->next=p2;

                p3=p2;
                p2=p1->next;

            }
           else
            {
                p3=p1;
                p1=p2;
                p2=p2->next;
            }
        }
  p=p1;
    }
    while (p->next!=&t)
    {
        p=p->next;
    }
    p->next=NULL;
    return h.next;
}

额。。请问是啥意思？？没看懂。。