程序代码：

下面的内容无法理解是什么意思，对于链表很迷糊。。。
刚学链表，老师讲课时写了一大堆，无法理解，帮帮我，该怎么读懂（理解）下面的内容啊？？？？小弟 先谢了！
# include <stdio.h>
# include <malloc.h>
# include <stdlib.h>

typedef struct Node
{
    int data;
    struct Node * pNext;
}NODE, *PNODE;

PNODE create_list(void);
void traverse_list(PNODE pHead);
bool is_empty(PNODE pHead);
int length_list(PNODE);
bool insert_list(PNODE, int, int);
bool delete_list(PNODE, int, int *);
void sort_list(PNODE);

int main(void)
{
    PNODE pHead =NULL;
   
    pHead = create_list();
    traverse_list(pHead);
    int len = length_list(pHead);
    printf ("链表的长度是%d\n", len);

    sort_list(pHead);
    traverse_list(pHead);

/*    if (is_empty(pHead))
        printf ("链表为空\n");
    else
        printf ("链表不空\n");
*/   
    return 0;
}

PNODE create_list(void)
{
    int len;
    int i;
    int val;
   
    PNODE pHead = (PNODE)malloc(sizeof(NODE));
    if (NULL == pHead)
    {
        printf ("分配失败，程序终止！\n");
        exit(-1);
    }
    PNODE pTail = pHead;
    pTail->pNext = NULL;
   
    printf ("请输入您需要生成的链表节点的个数: len = ");
    scanf ("%d", &len);
   
    for (i=0; i<len; ++i)
    {
        printf ("请输入第%d个节点的值：", i+1);
        scanf ("%d", &val);
       
        PNODE pNew = (PNODE)malloc(sizeof(NODE));
        if (NULL == pNew)
        {
            printf ("分配失败，程序终止！\n");
            exit(-1);
        }
        pNew->data = val;
        pTail->pNext = pNew;
        pNew->pNext = NULL;
        pTail = pNew;
    }
    return pHead;
}
void traverse_list(PNODE pHead)
{
    PNODE p = pHead->pNext;

    while (NULL != p)
    {
        printf ("%d     ", p->data);
        p = p->pNext;
    }
    printf ("\n");
    return;
}
bool is_empty(PNODE pHead)
{
    if (NULL == pHead->pNext)
        return true;
    else
        return false;
}
int length_list(PNODE pHead)
{
    PNODE p = pHead->pNext;
    int len = 0;
    while (NULL != p)
    {
        ++len;
        p = p->pNext;
    }
    return len;
}
void sort_list(PNODE pHead)
{
    int i, j, t;
    int len = length_list(pHead);
    PNODE p, q;
    for (i=0,p=pHead->pNext; i<len-1; ++i,p=p->pNext)
    {
        for (j=i+1,q=p->pNext; j<len; ++j,q=q->pNext)
        {   
            if (p->data > q->data)
            {
                t = p->data;
                p->data = q->data;
                q->data = t;
            }
        }
    }
    return;
}