freebsd free tel 所有 lis img 對象 processor 宏定義

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這兩天想看看memcached的實現,所以先學習了libevent,使用起來還是比較簡單的,其實是對select/poll/kqueue等的封裝,學習libevent過程中又遇到了linux下隊列的使用,簡單分析如下,權當做記錄:

libevent中的例子中使用的是FreeBSD下的queue.h,在linux的/usr/include/sys/queue.h也有該頭文件,但是是一個縮減版本,而且沒有看到queue 的access method,不知道是不是跟我們的linux服務器版本有關,沒辦法google了一下,找到了FreeBSD 下queue.h的定義,我們看一下tail queue的定義

C代碼

1. #define TAILQ_HEAD(name, type) \
2. struct name { \
3. struct type *tqh_first; /* first element */ \
4. struct type **tqh_last; /* addr of last next element */\
5. }
7. #define TAILQ_ENTRY(type) \
8. struct { \
9. struct type *tqe_next; /* next element */ \
10. struct type **tqe_prev;/* addr of previous next element*/ \
11. }
13. #define TAILQ_INIT(head) do { \
14. (head)->tqh_first = NULL; \
15. (head)->tqh_last = &(head)->tqh_first; \
16. } while (0)
18. #define TAILQ_INSERT_TAIL(head, elm, field) do { \
19. (elm)->field.tqe_next = NULL; \
20. (elm)->field.tqe_prev = (head)->tqh_last; \
21. *(head)->tqh_last = (elm); \
22. (head)->tqh_last = &(elm)->field.tqe_next; \
23. } while (0)
25. #define TAILQ_INSERT_BEFORE(listelm, elm, field) do { \
26. (elm)->field.tqe_prev = (listelm)->field.tqe_prev; \
27. (elm)->field.tqe_next = (listelm); \
28. *(listelm)->field.tqe_prev = (elm); \
29. (listelm)->field.tqe_prev = &(elm)->field.tqe_next; \
30. } while (0)
31. #define TAILQ_FIRST(head) ((head)->tqh_first)
33. #define TAILQ_NEXT(elm, field) ((elm)->field.tqe_next)
34. ....

我們就先分析上面的這些定義,先看個應用的例子

C代碼

1. #include <stdio.h>
2. #include <stdlib.h>
3. #include "queue.h"
5. struct QUEUE_ITEM{
6. int value;
7. TAILQ_ENTRY(QUEUE_ITEM) entries;
8. };
9. TAILQ_HEAD(,QUEUE_ITEM) queue_head;
10. int main(int argc,char **argv){
11. struct QUEUE_ITEM *item;
12. struct QUEUE_ITEM *tmp_item;
14. TAILQ_INIT(&queue_head);
15. int i=0;
16. for(i=5;i<10;i+=2){
17. item=malloc(sizeof(item));
18. item->value=i;
19. TAILQ_INSERT_TAIL(&queue_head, item, entries);
20. }
22. struct QUEUE_ITEM *ins_item;
23. ins_item=malloc(sizeof(ins_item));
25. ins_item->value=100;
26. TAILQ_INSERT_BEFORE(item,ins_item,entries);
29. tmp_item=TAILQ_FIRST(&queue_head);
30. printf("first element is %d\n",tmp_item->value);
32. tmp_item=TAILQ_NEXT(tmp_item,entries);
33. printf("next element is %d\n",tmp_item->value);
35. tmp_item=TAILQ_NEXT(tmp_item,entries);
36. printf("next element is %d\n",tmp_item->value);
38. tmp_item=TAILQ_NEXT(tmp_item,entries);
39. printf("next element is %d\n",tmp_item->value);
41. }

結果:
Java代碼

1. first element is 5
2. next element is 7
3. next element is 100
4. next element is 9

分析:
QUEUE_ITEM 是我們定義的存放在隊列裏的東東,簡單起見只包括一個int值
TAILQ_ENTRY(QUEUE_ITEM) entries 主要是存放下一個對象和前一個對象的指針,具體見 header

根據頭文件進行宏替換後,實際我們聲明的是這樣的結構:

C代碼

1. struct QUEUE_ITEM{
2. int value;
3. struct {
4. struct QUEUE_ITEM *tqe_next;
5. struct QUEUE_ITEM **tqe_prev;
6. }entries;
7. };

TAILQ_HEAD(,QUEUE_ITEM) queue_head; 實際是
C代碼

1. struct {
2. struct QUEUE_ITEM *tqh_first;
3. struct QUEUE_ITEM **tqh_last;
4. }queue_head;

接著我們定義了QUEUE_ITEM的兩個指針變量item和tmp_item

TAILQ_INIT(&queue_head); 相當於是
C代碼

1. do {
2. (&queue_head)->tqh_first = NULL;
3. (&queue_head)->tqh_last = &(&queue_head)->tqh_first;
4. } while (0);

head的初始化如 下圖1

接著我們通過循環分配了幾個元素,並賦值
C代碼

1. TAILQ_INSERT_TAIL(&queue_head, item, entries); 相當於執行
3. do {
4. (item)->entries.tqe_next = NULL;
5. (item)->entries.tqe_prev = (&queue_head)->tqh_last;
6. *(&queue_head)->tqh_last = (item);
7. (&queue_head)->tqh_last = &(item)->entries.tqe_next;
8. } while (0);

也就是我們的循環執行下面代碼段,結果分析見圖2,3
C代碼

1. for(i=5;i<10;i+=2){
2. item=malloc(sizeof(item));
3. item->value=i;
4. do {
5. (item)->entries.tqe_next = NULL;
6. //首次執行相當於item->entries.tqe_prev=&(&queue_head)->tqh_first
7. //以後執行相當於是(item)->entries.tqe_prev=&(前一個item)->entries.tqe_next;
8. (item)->entries.tqe_prev = (&queue_head)->tqh_last;
9. //首次執行相當於(&queue_head)->tqh_first=item
10. //以後執行相當於是(前一個item)->entries.tqe_next=當前item
11. *(&queue_head)->tqh_last = (item);
12. (&queue_head)->tqh_last = &(item)->entries.tqe_next;
13. } while (0);
14. }

C代碼

1. 最終建立的鏈表結構如圖,下面看一下insert操作,經過宏替換後代碼如下
3. struct QUEUE_ITEM *ins_item;
4. ins_item=malloc(sizeof(ins_item));
5. ins_item->value=100;
7. do {
8. (ins_item)->entries.tqe_prev = (item)->entries.tqe_prev;
9. (ins_item)->entries.tqe_next = (item);
10. //這句話體現了TAILQ的特色,tqe_prev是前一個元素的下個元素地址,
11. //所以正好應該是當前插入item的地址
12. *(item)->entries.tqe_prev = (ins_item);
13. (item)->entries.tqe_prev = &(ins_item)->entries.tqe_next;
14. } while (0);

總結:TAILQ的最大特點就是每個entry的二級指針tqe_prev其存放的是前一個元素的下個元素地址,呵呵,聽起來都很拗口
我現在就是不知道為什麽linux的queue.h只有建立tailq的宏定義而缺少所有的access method,初涉linux c編程,請大家指教

附經過宏替換後的所有代碼
C代碼

1. #include "stdio.h"
2. #include "stdlib.h"
3. struct QUEUE_ITEM{
4. int value;
5. struct {
6. struct QUEUE_ITEM *tqe_next;
7. struct QUEUE_ITEM **tqe_prev;
8. }entries;
9. };
10. struct {
11. struct QUEUE_ITEM *tqh_first;
12. struct QUEUE_ITEM **tqh_last;
13. }queue_head;
15. int main(int argc,char **argv){
16. struct QUEUE_ITEM *item;
17. struct QUEUE_ITEM *tmp_item;
19. do {
20. (&queue_head)->tqh_first = NULL;
21. (&queue_head)->tqh_last = &(&queue_head)->tqh_first;
22. } while (0);
24. int i=0;
25. for(i=5;i<10;i+=2){
26. item=malloc(sizeof(item));
27. item->value=i;
28. do {
29. (item)->entries.tqe_next = NULL;
30. //首次執行相當於item->entries.tqe_prev=&(&queue_head)->tqh_first
31. //以後執行相當於是(item)->entries.tqe_prev=&(前一個item)->entries.tqe_next;
32. (item)->entries.tqe_prev = (&queue_head)->tqh_last;
33. //首次執行相當於(&queue_head)->tqh_first=item
34. //以後執行相當於是(前一個item)->entries.tqe_next=當前item
35. *(&queue_head)->tqh_last = (item);
36. (&queue_head)->tqh_last = &(item)->entries.tqe_next;
37. } while (0);
38. }
40. struct QUEUE_ITEM *ins_item;
41. ins_item=malloc(sizeof(ins_item));
43. ins_item->value=100;
44. do {
45. (ins_item)->entries.tqe_prev = (item)->entries.tqe_prev;
46. (ins_item)->entries.tqe_next = (item);
47. *(item)->entries.tqe_prev = (ins_item);
48. (item)->entries.tqe_prev = &(ins_item)->entries.tqe_next;
49. } while (0);
51. tmp_item=((&queue_head)->tqh_first);
52. printf("first element is %d\n",tmp_item->value);
54. tmp_item=((tmp_item)->entries.tqe_next);
55. printf("next element is %d\n",tmp_item->value);
57. tmp_item=((tmp_item)->entries.tqe_next);
58. printf("next element is %d\n",tmp_item->value);
60. tmp_item=((tmp_item)->entries.tqe_next);
61. printf("next element is %d\n",tmp_item->value);
63. }

Linux queue.h之TAILQ隊列分析