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內(nèi)核list.h 用戶態(tài)使用
分類： linux

  仰慕kernel的list.h良久了,還記得以前o2的李大哥號稱list.h飄逸寫手...自己也曾經(jīng)牛刀小試過一把.今日得閑,暫探索性的分析一把.

1. 簡約而又不簡單的鏈表定義

于雙向鏈表而言，內(nèi)核中定義了如下簡單結(jié)構(gòu)：

  struct list_head {                     
    struct list_head *next, *prev;  
 };                                               
   
 這個不含任何數(shù)據(jù)項的結(jié)構(gòu)，注定了它的通用性和未來使用的靈活性，例如前面的例子就可以按如下方式定義：
 

struct my_list{ 
	void *mydata; 
	struct list_head list;
	};  
  在此，進一步說明幾點：
  1）list字段，隱藏了鏈表的指針特性，但正是它，把我們要鏈接的數(shù)據(jù)組織成了鏈表。
  2）struct list_head可以位于結(jié)構(gòu)的任何位置
  3）可以給struct list_head起任何名字。
  4）在一個結(jié)構(gòu)中可以有多個list

例如，我們對要完成的任務進行描述，而任務中又包含子任務，于是有如下結(jié)構(gòu)：

-------------------------------------------------------------------------------------------------------------------------
struct todo_tasks{
	char *task_name;
	unsigned int name_len;
	short int status;

	int sub_tasks;

	int subtasks_completed;
	struct list_head completed_subtasks;/* 已完成的子任務形成鏈表 */

	int subtasks_waiting;
	struct list_head waiting_subtasks; /* 待完成的子任務形成鏈表 */

	struct list_head todo_list;	/* 要完成的任務形成鏈表 */
	};
-----------------------------------------------------------------------
  簡約而又不簡單struct list_head，以此為基本對象，就衍生了對鏈表的插入、刪除、合并以及遍歷等各種操作：
2. 鏈表的聲明和初始化宏
   

   實際上， struct list_head只定義了鏈表節(jié)點，并沒有專門定義鏈表頭，那么一個鏈表結(jié)構(gòu)是如何建立起來的？讓我們來看看下面兩個宏：

#define LIST_HEAD_INIT(name) { &(name), &(name) }
#define LIST_HEAD(name) struct list_head name = LIST_HEAD_INIT(name)

   如果我們要申明并定義自己的鏈表頭mylist，直接調(diào)用LIST_HEAD：

    LIST_HEAD(mylist)

    則mylist的next、prev指針都初始化為指向自己，這樣，我們就有了一個空鏈表，如何判斷鏈表是否為空，自己寫一下這個簡單的函數(shù)list_empty ，也就是讓頭指針的next指向自己而已。


3. staitic inline函數(shù)－隱藏并展開
   在list.h中定義的函數(shù)大都是 staitic inline f()形式？為什么這樣定義？
   關(guān)鍵字“static”加在函數(shù)前，表示這個函數(shù)是靜態(tài)函數(shù)，所謂靜態(tài)函數(shù)，實際上是對函數(shù)作用域的限制，指該函數(shù)的作用域僅
 局限于本文件。所以說，static具有信息隱藏作用。
     而關(guān)鍵字"inline“加在函數(shù)前，說明這個函數(shù)對編譯程序是可見的，也就是說，編譯程序在調(diào)用這個函數(shù)時就立即展開該函數(shù)。所以，關(guān)鍵字inline 必須與函數(shù)定義體放在一起才能使函數(shù)成為內(nèi)聯(lián)。inline函數(shù)一般放在頭文件中。

4.  無處不在的隱藏特性
     我們分析一下在鏈表中增加一個節(jié)點的函數(shù)實現(xiàn)：
     有三個函數(shù)：
     static inline void __list_add();
     static inline void list_add();
     static inline void list_add_tail();


-------------------------------------------------------------------------------------------------
  /*
 * Insert a new entry between two known consecutive entries. 
 *
 * This is only for internal list manipulation where we know
 * the prev/next entries already!
 */
static inline void __list_add(struct list_head *new,
                  struct list_head *prev,
                  struct list_head *next)
{
    next->prev = new;
    new->next = next;
    new->prev = prev;
    prev->next = new;
}
--------------------------------------------------------------------------------------------------
/**
 * list_add - add a new entry
 * @new: new entry to be added
 * @head: list head to add it after
 *
 * Insert a new entry after the specified head.
 * This is good for implementing stacks.
 */
static inline void list_add(struct list_head *new, struct list_head *head)
{
    __list_add(new, head, head->next);
}
--------------------------------------------------------------------------------------------------
/**
 * list_add_tail - add a new entry
 * @new: new entry to be added
 * @head: list head to add it before
 *
 * Insert a new entry before the specified head.                                          

 * This is useful for implementing queues.
 */
static inline void list_add_tail(struct list_head *new, struct list_head *head)
{
    __list_add(new, head->prev, head);
}

--------------------------------------------------------------------------------------------------
   
  仔細體會其實現(xiàn)代碼，看起來簡單有效，但實際上也是一種抽象和封裝的體現(xiàn)。首先__list_add()函數(shù)做基本的操作，該函數(shù)僅僅是增加一個節(jié)點，至于這個節(jié)點加到何處，暫不考慮。list_add（）調(diào)用__list_add()這個內(nèi)部函數(shù)，在鏈表頭增加一個節(jié)點，實際上實現(xiàn)了棧在頭部增加節(jié)點的操作，而list_add_tail()在尾部增加一個節(jié)點，實際上實現(xiàn)了隊的操作。
 
至于鏈表的刪除、搬移和合并，比較簡單，不再此一一討論

5. 鏈表遍歷－似走過千山萬水
  遍歷鏈表本是簡單的，list.h中就定義了如下的宏：
--------------------------------------------------------------------------------------------------
  **
 * list_for_each    -    iterate over a list
 * @pos:    the &struct list_head to use as a loop counter.
 * @head:    the head for your list.
 */
#define list_for_each(pos, head) \
    for (pos = (head)->next; pos != (head); \
            pos = pos->next)

--------------------------------------------------------------------------------------------------
  這種遍歷僅僅是找到一個個節(jié)點在鏈表中的位置pos，難點在于，如何通過pos獲得節(jié)點的地址，從而可以使用節(jié)點中的數(shù)據(jù)？ 于是 list.h中定義了晦澀難懂的list_entry（）宏：


--------------------------------------------------------------------------------------------------
/**
 * list_entry - get the struct for this entry
 * @ptr:    the &struct list_head pointer.
 * @type:    the type of the struct this is embedded in.
 * @member:    the name of the list_struct within the struct.
 */
#define list_entry(ptr, type, member) \
    ((type *)((char *)(ptr)-(unsigned long)(&((type *)0)->member)))

--------------------------------------------------------------------------------------------------
指針ptr指向結(jié)構(gòu)體type中的成員member；通過指針ptr，返回結(jié)構(gòu)體type的起始地址，如圖2。
      
          type

          |----------|
          |              |
          |              |
          |----------|
ptr--> | member --|
          |----------|
          |              |
          |              |
          |----------| 

        圖2 list_entry（）宏的示意圖

為了便于理解，在此給予進一步說明。
 例如my_list結(jié)構(gòu)：
struct my_list{ 
	void *mydata; 
	struct list_head list;
	};
struct list_head *pos;  

則list_entry(pos, mylist, list)宏，就可以根據(jù)pos的值，獲取mylist的地址，也就是指向mylist的指針，這樣，我們就可以存取mylist->mydata字段了。

可為什么能夠達到這樣的效果？
list_entry(pos, mylist, list) 展開以后為：

  ((struct my_list *)((char *)(pos) - (unsigned long)(&((struct my_list *)0)->list)))

這看起來會使大多數(shù)人眩暈，但仔細分析一下，實際很簡單。
 ((size_t) &(type*)0)->member)把0地址轉(zhuǎn)化為type結(jié)構(gòu)的指針，然后獲取該結(jié)構(gòu)中member成員的指針，并將其強制轉(zhuǎn)換為size_t類型。于是，由于結(jié)構(gòu)從0地址開始定義，因此，這樣求出member的成員地址，實際上就是它在結(jié)構(gòu)中的偏移量。為了更好的理解這些，我們可以寫一段程序來驗證:
---------------------------------------------------------------------------------------

#include <stdio.h>
#include <stdlib.h>

struct foobar{
	unsigned int foo;
	char bar;
	char boo;
};

int main(int argc, char** argv){

	struct foobar tmp;

	printf("address of &tmp is= %p\n\n", &tmp);
	printf("address of tmp->foo= %p \t offset of tmp->foo= %lu\n", &tmp.foo, (unsigned long) &((struct foobar *)0)->foo);
	printf("address of tmp->bar= %p \t offset of tmp->bar= %lu\n", &tmp.bar, (unsigned long) &((struct foobar *)0)->bar);
	printf("address of tmp->boo= %p \t offset of tmp->boo= %lu\n\n", &tmp.boo, (unsigned long) &((struct foobar *)0)->boo);

	printf("computed address of &tmp using:\n");
	printf("\taddress and offset of tmp->foo= %p\n",
	(struct foobar *) (((char *) &tmp.foo) - ((unsigned long) &((struct foobar *)0)->foo)));
	printf("\taddress and offset of tmp->bar= %p\n",
	(struct foobar *) (((char *) &tmp.bar) - ((unsigned long) &((struct foobar *)0)->bar)));
	printf("\taddress and offset of tmp->boo= %p\n",
	(struct foobar *) (((char *) &tmp.boo) - ((unsigned long) &((struct foobar *)0)->boo)));

	return 0;
}
Output from this code is:
address of &tmp is= 0xbfffed00

address of tmp->foo= 0xbfffed00 offset of tmp->foo= 0
address of tmp->bar= 0xbfffed04 offset of tmp->bar= 4
address of tmp->boo= 0xbfffed05 offset of tmp->boo= 5

computed address of &tmp using:
address and offset of tmp->foo= 0xbfffed00
address and offset of tmp->bar= 0xbfffed00
address and offset of tmp->boo= 0xbfffed00

----------------------------------------------------------------------------------------

  到此，我們對鏈表的實現(xiàn)機制有所了解，但在此止步的話，我們依然無法領(lǐng)略這風景背后的韻味。
  盡管list.h是內(nèi)核代碼中的頭文件，但我們可以把它移植到用戶空間使用。且看下一講，鏈表接口之應用。


[root@mip-123456 list]# cat list.h
#ifndef _LINUX_LIST_H
#define _LINUX_LIST_H

#define LIST_POISON1 ((void *) 0x00100100)
#define LIST_POISON2 ((void *) 0x00200200)

#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
#define container_of(ptr, type, member) ({\
           const typeof( ((type *)0)->member ) *__mptr = (ptr);\
           (type *)( (char *)__mptr - offsetof(type,member) );})

struct list_head {
        struct list_head *next, *prev;
};

#define LIST_HEAD_INIT(name) { &(name), &(name) }

#define LIST_HEAD(name) \
        struct list_head name = LIST_HEAD_INIT(name)

static inline void INIT_LIST_HEAD(struct list_head *list)
{
        list->next = list;
        list->prev = list;
}


static inline void __list_add(struct list_head *new,
                              struct list_head *prev,
                              struct list_head *next)
{
        next->prev = new;
        new->next = next;
        new->prev = prev;
        prev->next = new;
}


static inline void list_add(struct list_head *new, struct list_head *head)
{
        __list_add(new, head, head->next);
}

static inline void list_add_tail(struct list_head *new, struct list_head *head)
{
        __list_add(new, head->prev, head);
}

static inline void __list_del(struct list_head * prev, struct list_head * next)
{
        next->prev = prev;
        prev->next = next;
}

static inline void list_del(struct list_head *entry)
{
        __list_del(entry->prev, entry->next);
        entry->next = LIST_POISON1;
        entry->prev = LIST_POISON2;
}

static inline void list_replace(struct list_head *old,
                                struct list_head *new)
{
        new->next = old->next;
        new->next->prev = new;
        new->prev = old->prev;
        new->prev->next = new;
}

static inline void list_replace_init(struct list_head *old,
                                        struct list_head *new)
{
        list_replace(old, new);
        INIT_LIST_HEAD(old);
}

static inline void list_del_init(struct list_head *entry)
{
        __list_del(entry->prev, entry->next);
        INIT_LIST_HEAD(entry);
}

static inline void list_move(struct list_head *list, struct list_head *head)
{
        __list_del(list->prev, list->next);
        list_add(list, head);
}

static inline void list_move_tail(struct list_head *list,
                                  struct list_head *head)
{
        __list_del(list->prev, list->next);
        list_add_tail(list, head);
}

static inline int list_is_last(const struct list_head *list,
                                const struct list_head *head)
{
        return list->next == head;
}

static inline int list_empty(const struct list_head *head)
{
        return head->next == head;
}

static inline int list_empty_careful(const struct list_head *head)
{
        struct list_head *next = head->next;
        return (next == head) && (next == head->prev);
}

static inline void __list_splice(struct list_head *list,
                                 struct list_head *head)
{
        struct list_head *first = list->next;
        struct list_head *last = list->prev;
        struct list_head *at = head->next;

        first->prev = head;
        head->next = first;

        last->next = at;
        at->prev = last;
}

static inline void list_splice(struct list_head *list, struct list_head *head)
{
        if (!list_empty(list))
                __list_splice(list, head);
}

static inline void list_splice_init(struct list_head *list,
                                    struct list_head *head)
{
        if (!list_empty(list)) {
                __list_splice(list, head);
                INIT_LIST_HEAD(list);
        }
}

#define list_entry(ptr, type, member) \
        container_of(ptr, type, member)

#define list_for_each(pos, head) \
        for (pos = (head)->next;pos != (head); \
                pos = pos->next)

#define __list_for_each(pos, head) \
        for (pos = (head)->next; pos != (head); pos = pos->next)

#define list_for_each_prev(pos, head) \
        for (pos = (head)->prev; pos != (head); \
                pos = pos->prev)

#define list_for_each_safe(pos, n, head) \
        for (pos = (head)->next, n = pos->next; pos != (head); \
                pos = n, n = pos->next)

#define list_for_each_entry(pos, head, member) \
        for (pos = list_entry((head)->next, typeof(*pos), member); \
             &pos->member != (head); \
             pos = list_entry(pos->member.next, typeof(*pos), member))

#define list_for_each_entry_reverse(pos, head, member) \
        for (pos = list_entry((head)->prev, typeof(*pos), member); \
             &pos->member != (head); \
             pos = list_entry(pos->member.prev, typeof(*pos), member))

#define list_prepare_entry(pos, head, member) \
        ((pos) ? : list_entry(head, typeof(*pos), member))


#define list_for_each_entry_continue(pos, head, member) \
        for (pos = list_entry(pos->member.next, typeof(*pos), member); \
             prefetch(pos->member.next), &pos->member != (head); \
             pos = list_entry(pos->member.next, typeof(*pos), member))

#define list_for_each_entry_from(pos, head, member) \
        for (; prefetch(pos->member.next), &pos->member != (head); \
             pos = list_entry(pos->member.next, typeof(*pos), member))


#define list_for_each_entry_safe(pos, n, head, member) \
        for (pos = list_entry((head)->next, typeof(*pos), member), \
                n = list_entry(pos->member.next, typeof(*pos), member); \
             &pos->member != (head); \
             pos = n, n = list_entry(n->member.next, typeof(*n), member))


#define list_for_each_entry_safe_continue(pos, n, head, member) \
        for (pos = list_entry(pos->member.next, typeof(*pos), member), \
                n = list_entry(pos->member.next, typeof(*pos), member); \
             &pos->member != (head); \
             pos = n, n = list_entry(n->member.next, typeof(*n), member))

#define list_for_each_entry_safe_from(pos, n, head, member) \
        for (n = list_entry(pos->member.next, typeof(*pos), member); \
             &pos->member != (head); \
             pos = n, n = list_entry(n->member.next, typeof(*n), member))

#define list_for_each_entry_safe_reverse(pos, n, head, member) \
        for (pos = list_entry((head)->prev, typeof(*pos), member), \
                n = list_entry(pos->member.prev, typeof(*pos), member); \
             &pos->member != (head); \
             pos = n, n = list_entry(n->member.prev, typeof(*n), member))

#endif

[root@mip-123456 list]# cat list.c
#include <stdio.h>
#include <stdlib.h>

#include "list.h"

struct jimmy_list{
        int id;
        struct list_head list;
        char name[10];
        };

int main(int argc, char **argv){

        struct jimmy_list *tmp;
        struct list_head *pos, *q;
        unsigned int i;

        struct jimmy_list mylist;
        INIT_LIST_HEAD(&mylist.list); /*初始化鏈表頭*/

        for(i=0; i<5; i++){
        tmp= (struct jimmy_list *)malloc(sizeof(struct jimmy_list));
        
        tmp->id = i+1;
        sprintf(tmp->name, "jimmy %d", i+1);

        /*list_add每次添加都是添加在list的第一個節(jié)點處*/
        list_add(&(tmp->list), &(mylist.list));
        }
        /*list 5->4->3->2->1*/

        /*這里添加在尾部list_add_tail*/
        for(i=5; i<7; i++){
        tmp= (struct jimmy_list *)malloc(sizeof(struct jimmy_list));
        
        tmp->id = i+1;
        sprintf(tmp->name, "kenthy %d", i+1);
        /*list_add每次添加都是添加在list的第一個節(jié)點處*/
        list_add_tail(&(tmp->list), &(mylist.list));
        }
        /*list 5->4->3->2->1->6->7*/


        /*兩種遍歷list的方法--正向遍歷*/
        printf("traversing the list using list_for_each()\n");
        list_for_each(pos, &mylist.list){
              tmp= list_entry(pos, struct jimmy_list, list);
              printf("id= %d name= %s\n", tmp->id, tmp->name);
        }
        printf("\n");

        /*這里添加一個替換玩玩*/
        #if 1
        tmp= (struct jimmy_list *)malloc(sizeof(struct jimmy_list));
        tmp->id = 55;
        sprintf(tmp->name, "kenthy %d", 55);
        INIT_LIST_HEAD(&(tmp->list));
        list_replace((&mylist.list)->next,&(tmp->list));
        #endif
        /*list 55->4->3->2->1->6->7*/

        printf("traversing the list using list_for_each_entry()\n");
        list_for_each_entry(tmp, &mylist.list, list)
          printf("id= %d name= %s\n", tmp->id, tmp->name);
        printf("\n");

        /*兩種遍歷list的方法--反向遍歷*/
        printf("reverse the list using list_for_each_prev()\n");
        list_for_each_prev(pos, &mylist.list){
              tmp= list_entry(pos, struct jimmy_list, list);
              printf("id= %d name= %s\n", tmp->id, tmp->name);
        }
        printf("\n");
        
        printf("reverse the list using list_for_each_entry()\n");
        list_for_each_entry_reverse(tmp, &mylist.list, list)
          printf("id= %d name= %s\n", tmp->id, tmp->name);
        printf("\n");


 

        printf("deleting the list using list_for_each_safe()\n");
        list_for_each_safe(pos, q, &mylist.list){
        tmp= list_entry(pos, struct jimmy_list, list);
        printf("freeing item id= %d name= %s\n", tmp->id, tmp->name);
        list_del(pos);
        free(tmp);
        }

        if(list_empty(&mylist.list))
          printf("now the list if empty\n");
        return 0;
}

[root@mip-123456 list]# ./list
traversing the list using list_for_each()
id= 5 name= jimmy 5
id= 4 name= jimmy 4
id= 3 name= jimmy 3
id= 2 name= jimmy 2
id= 1 name= jimmy 1
id= 6 name= kenthy 6
id= 7 name= kenthy 7

traversing the list using list_for_each_entry()
id= 55 name= kenthy 55
id= 4 name= jimmy 4
id= 3 name= jimmy 3
id= 2 name= jimmy 2
id= 1 name= jimmy 1
id= 6 name= kenthy 6
id= 7 name= kenthy 7

reverse the list using list_for_each_prev()
id= 7 name= kenthy 7
id= 6 name= kenthy 6
id= 1 name= jimmy 1
id= 2 name= jimmy 2
id= 3 name= jimmy 3
id= 4 name= jimmy 4
id= 55 name= kenthy 55

reverse the list using list_for_each_entry()
id= 7 name= kenthy 7
id= 6 name= kenthy 6
id= 1 name= jimmy 1
id= 2 name= jimmy 2
id= 3 name= jimmy 3
id= 4 name= jimmy 4
id= 55 name= kenthy 55

deleting the list using list_for_each_safe()
freeing item id= 55 name= kenthy 55
freeing item id= 4 name= jimmy 4
freeing item id= 3 name= jimmy 3
freeing item id= 2 name= jimmy 2
freeing item id= 1 name= jimmy 1
freeing item id= 6 name= kenthy 6
freeing item id= 7 name= kenthy 7
now the list if empty

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