文章目錄

• • 頭文件
  • 例子
  • 代碼分析
  • • CIRCLEQ_ENTRY 和 CIRCLEQ_HEAD
    • CIRCLEQ_HEAD_INITIALIZER
    • CIRCLEQ_FIRST 和 CIRCLEQ_LAST
    • CIRCLEQ_PREV 和 CIRCLEQ_NEXT
    • CIRCLEQ_INIT
    • CIRCLEQ_EMPTY
    • CIRCLEQ_INSERT_TAIL
    • CIRCLEQ_INSERT_HEAD
    • CIRCLEQ_INSERT_AFTER
    • CIRCLEQ_INSERT_BEFORE
    • CIRCLEQ_FOREACH
    • CIRCLEQ_FOREACH_REVERSE
    • CIRCLEQ_REMOVE

頭文件

頭文件來自我最近看的項(xiàng)目：apache-mynewt-core-1.9.0\sys\sys\include\sys\queue.h

僅截取部分。

/** Circular queue declarations.*/ #define CIRCLEQ_HEAD(name, type) \ struct name { \struct type *cqh_first; /* first element */ \struct type *cqh_last; /* last element */ \ }#define CIRCLEQ_HEAD_INITIALIZER(head) \{ (void *)&(head), (void *)&(head) }#define CIRCLEQ_ENTRY(type) \ struct { \struct type *cqe_next; /* next element */ \struct type *cqe_prev; /* previous element */ \ }/** Circular queue functions.*/ #define CIRCLEQ_EMPTY(head) ((head)->cqh_first == (void *)(head))#define CIRCLEQ_FIRST(head) ((head)->cqh_first)#define CIRCLEQ_FOREACH(var, head, field) \for ((var) = CIRCLEQ_FIRST((head)); \(var) != (void *)(head) || ((var) = NULL); \(var) = CIRCLEQ_NEXT((var), field))#define CIRCLEQ_FOREACH_REVERSE(var, head, field) \for ((var) = CIRCLEQ_LAST((head)); \(var) != (void *)(head) || ((var) = NULL); \(var) = CIRCLEQ_PREV((var), field))#define CIRCLEQ_INIT(head) do { \CIRCLEQ_FIRST((head)) = (void *)(head); \CIRCLEQ_LAST((head)) = (void *)(head); \ } while (0)#define CIRCLEQ_INSERT_AFTER(head, listelm, elm, field) do { \CIRCLEQ_NEXT((elm), field) = CIRCLEQ_NEXT((listelm), field); \CIRCLEQ_PREV((elm), field) = (listelm); \if (CIRCLEQ_NEXT((listelm), field) == (void *)(head)) \CIRCLEQ_LAST((head)) = (elm); \else \CIRCLEQ_PREV(CIRCLEQ_NEXT((listelm), field), field) = (elm);\CIRCLEQ_NEXT((listelm), field) = (elm); \ } while (0)#define CIRCLEQ_INSERT_BEFORE(head, listelm, elm, field) do { \CIRCLEQ_NEXT((elm), field) = (listelm); \CIRCLEQ_PREV((elm), field) = CIRCLEQ_PREV((listelm), field); \if (CIRCLEQ_PREV((listelm), field) == (void *)(head)) \CIRCLEQ_FIRST((head)) = (elm); \else \CIRCLEQ_NEXT(CIRCLEQ_PREV((listelm), field), field) = (elm);\CIRCLEQ_PREV((listelm), field) = (elm); \ } while (0)#define CIRCLEQ_INSERT_HEAD(head, elm, field) do { \CIRCLEQ_NEXT((elm), field) = CIRCLEQ_FIRST((head)); \CIRCLEQ_PREV((elm), field) = (void *)(head); \if (CIRCLEQ_LAST((head)) == (void *)(head)) \CIRCLEQ_LAST((head)) = (elm); \else \CIRCLEQ_PREV(CIRCLEQ_FIRST((head)), field) = (elm); \CIRCLEQ_FIRST((head)) = (elm); \ } while (0)#define CIRCLEQ_INSERT_TAIL(head, elm, field) do { \CIRCLEQ_NEXT((elm), field) = (void *)(head); \CIRCLEQ_PREV((elm), field) = CIRCLEQ_LAST((head)); \if (CIRCLEQ_FIRST((head)) == (void *)(head)) \CIRCLEQ_FIRST((head)) = (elm); \else \CIRCLEQ_NEXT(CIRCLEQ_LAST((head)), field) = (elm); \CIRCLEQ_LAST((head)) = (elm); \ } while (0)#define CIRCLEQ_LAST(head) ((head)->cqh_last)#define CIRCLEQ_NEXT(elm,field) ((elm)->field.cqe_next)#define CIRCLEQ_PREV(elm,field) ((elm)->field.cqe_prev)#define CIRCLEQ_REMOVE(head, elm, field) do { \if (CIRCLEQ_NEXT((elm), field) == (void *)(head)) \CIRCLEQ_LAST((head)) = CIRCLEQ_PREV((elm), field); \else \CIRCLEQ_PREV(CIRCLEQ_NEXT((elm), field), field) = \CIRCLEQ_PREV((elm), field); \if (CIRCLEQ_PREV((elm), field) == (void *)(head)) \CIRCLEQ_FIRST((head)) = CIRCLEQ_NEXT((elm), field); \else \CIRCLEQ_NEXT(CIRCLEQ_PREV((elm), field), field) = \CIRCLEQ_NEXT((elm), field); \ } while (0)

例子

#include <stddef.h> #include <stdio.h> #include <stdlib.h> #include "bsd_queue.h" // 就是我上面貼的代碼struct entry {int data;CIRCLEQ_ENTRY(entry) entries; /* Queue */ };CIRCLEQ_HEAD(circlehead, entry);int main(void) {struct entry *n1, *n2, *n3, *np;struct circlehead head; /* Queue head */int i;CIRCLEQ_INIT(&head); /* Initialize the queue */n1 = malloc(sizeof(struct entry)); /* Insert at the head */CIRCLEQ_INSERT_HEAD(&head, n1, entries);n1 = malloc(sizeof(struct entry)); /* Insert at the tail */CIRCLEQ_INSERT_TAIL(&head, n1, entries);n2 = malloc(sizeof(struct entry)); /* Insert after */CIRCLEQ_INSERT_AFTER(&head, n1, n2, entries);n3 = malloc(sizeof(struct entry)); /* Insert before */CIRCLEQ_INSERT_BEFORE(&head, n2, n3, entries);CIRCLEQ_REMOVE(&head, n2, entries); /* Deletion */free(n2);/* Forward traversal */i = 0;CIRCLEQ_FOREACH(np, &head, entries)np->data = i++;/* Reverse traversal */CIRCLEQ_FOREACH_REVERSE(np, &head, entries)printf("%i\n", np->data);/* Queue deletion */n1 = CIRCLEQ_FIRST(&head);while (n1 != (void *)&head) {n2 = CIRCLEQ_NEXT(n1, entries);free(n1);n1 = n2;}CIRCLEQ_INIT(&head);exit(EXIT_SUCCESS); }

代碼分析

我們一點(diǎn)一點(diǎn)看上面的例子。

先來一張我自己繪制的示意圖。紅色表示我覺得不太“正常”的地方，指針的類型不匹配，需要強(qiáng)制轉(zhuǎn)換。

CIRCLEQ_ENTRY 和 CIRCLEQ_HEAD

struct entry {int data;CIRCLEQ_ENTRY(entry) entries; /* Queue */ };CIRCLEQ_HEAD(circlehead, entry);

宏展開是：

struct entry {int data;struct { struct entry *cqe_next; struct entry *cqe_prev; } entries; }; struct circlehead { struct entry *cqh_first; struct entry *cqh_last; };

和其他鏈表不同，這里沒有用二級指針，cqe_prev 和 cqh_last 都是 struct entry * 類型。（為什么呢）

我前面寫的文章都是研究宏展開后的代碼，這次我們換一個思路，直接看宏的定義

CIRCLEQ_HEAD_INITIALIZER

#define CIRCLEQ_HEAD_INITIALIZER(head) \{ (void *)&(head), (void *)&(head) }

初始化一個鏈表后，表頭的兩個指針都指向自己。

CIRCLEQ_FIRST 和 CIRCLEQ_LAST

#define CIRCLEQ_FIRST(head) ((head)->cqh_first) #define CIRCLEQ_LAST(head) ((head)->cqh_last)

這兩個很好理解，不解釋了。

CIRCLEQ_PREV 和 CIRCLEQ_NEXT

#define CIRCLEQ_PREV(elm,field) ((elm)->field.cqe_prev) #define CIRCLEQ_NEXT(elm,field) ((elm)->field.cqe_next)

field 就是最開始定義的 entries

CIRCLEQ_INIT

#define CIRCLEQ_INIT(head) do { \CIRCLEQ_FIRST((head)) = (void *)(head); \CIRCLEQ_LAST((head)) = (void *)(head); \ } while (0)

初始化一個鏈表后，表頭的兩個指針都指向自己。如下圖

CIRCLEQ_EMPTY

#define CIRCLEQ_EMPTY(head) ((head)->cqh_first == (void *)(head))

CIRCLEQ_INSERT_TAIL

#define CIRCLEQ_INSERT_TAIL(head, elm, field) do { \CIRCLEQ_NEXT((elm), field) = (void *)(head); \CIRCLEQ_PREV((elm), field) = CIRCLEQ_LAST((head)); \if (CIRCLEQ_FIRST((head)) == (void *)(head)) \CIRCLEQ_FIRST((head)) = (elm); \else \CIRCLEQ_NEXT(CIRCLEQ_LAST((head)), field) = (elm); \CIRCLEQ_LAST((head)) = (elm); \ } while (0)

把 elm 插入到鏈表的尾部，都需要改變哪些指針呢？

elm 的 field.cqe_next，因?yàn)樗亲詈笠粋€，所以它應(yīng)該等于 (void *)(head)

elm 的 field.cqe_prev

插入后，elm 前面的那個節(jié)點(diǎn)的 field.cqe_next（如果前面的節(jié)點(diǎn)是表頭，那就修改CIRCLEQ_FIRST(head) ）

修改 (head)->cqh_last 指向 elm

代碼第 2、3 行分別解決了 1、2

代碼第 4、5 行解決了 3 的括號中的情況

第 7 行解決了 3，elm 前面的那個節(jié)點(diǎn)就是原來最后的節(jié)點(diǎn)，CIRCLEQ_LAST(head)，它的 cqe_next 就是 CIRCLEQ_NEXT(CIRCLEQ_LAST((head)), field)

第 8 行解決了 4

CIRCLEQ_INSERT_HEAD

#define CIRCLEQ_INSERT_HEAD(head, elm, field) do { \CIRCLEQ_NEXT((elm), field) = CIRCLEQ_FIRST((head)); \CIRCLEQ_PREV((elm), field) = (void *)(head); \if (CIRCLEQ_LAST((head)) == (void *)(head)) \CIRCLEQ_LAST((head)) = (elm); \else \CIRCLEQ_PREV(CIRCLEQ_FIRST((head)), field) = (elm); \CIRCLEQ_FIRST((head)) = (elm); \ } while (0)

把 elm 插入到鏈表的頭部，都需要改變哪些指針呢？

elm 的 field.cqe_next

elm 的 field.cqe_prev，它應(yīng)該等于 (void *)(head)

(head)->cqh_first

插入后，elm 后面的那個節(jié)點(diǎn)的 field.cqe_prev（如果插入 elm 前鏈表為空，那就修改CIRCLEQ_LAST(head) ）

第 2 行解決了 1

第 3 行解決了 2

第 4-5 行解決了 4 的括號中的情況

第 7 行解決了 4

第 8 行解決了 3

代碼 4-7 行說明，因?yàn)闆]有用二級指針，所以頭插的時候需要分情況考慮。但是不用二級指針也有好處，就是代碼更好理解。

CIRCLEQ_INSERT_AFTER

#define CIRCLEQ_INSERT_AFTER(head, listelm, elm, field) do { \CIRCLEQ_NEXT((elm), field) = CIRCLEQ_NEXT((listelm), field); \CIRCLEQ_PREV((elm), field) = (listelm); \if (CIRCLEQ_NEXT((listelm), field) == (void *)(head)) \CIRCLEQ_LAST((head)) = (elm); \else \CIRCLEQ_PREV(CIRCLEQ_NEXT((listelm), field), field) = (elm);\CIRCLEQ_NEXT((listelm), field) = (elm); \ } while (0)

要把 elm 插入到 listelm 元素的后面，要修改哪些指針呢？

elm 的 field.cqe_next

elm 的 field.cqe_prev

listelm 的 field.cqe_next

插入之前，listelm 后面的那個節(jié)點(diǎn)的 field.cqe_prev（如果 listelm 后面沒有節(jié)點(diǎn)了，就修改 CIRCLEQ_LAST(head)）

2-3 行分別解決了 1、2

4-5 行解決了 4 括號里的情況

第 7 行解決了 4

第 8 行解決了 3

CIRCLEQ_INSERT_BEFORE

#define CIRCLEQ_INSERT_BEFORE(head, listelm, elm, field) do { \CIRCLEQ_NEXT((elm), field) = (listelm); \CIRCLEQ_PREV((elm), field) = CIRCLEQ_PREV((listelm), field); \if (CIRCLEQ_PREV((listelm), field) == (void *)(head)) \CIRCLEQ_FIRST((head)) = (elm); \else \CIRCLEQ_NEXT(CIRCLEQ_PREV((listelm), field), field) = (elm);\CIRCLEQ_PREV((listelm), field) = (elm); \ } while (0)

要把 elm 插入到 listelm 元素的前面，要修改哪些指針呢？

elm 的 field.cqe_next

elm 的 field.cqe_prev

listelm 的 field.cqe_prev

插入之前，listelm 前面的那個節(jié)點(diǎn)的 field.cqe_next（如果 listelm 前面是表頭，就修改 CIRCLEQ_FIRST(head)）

2-3 行分別解決了 1、2

4-5 行解決了 4 括號里的情況

第 7 行解決了 4

第 8 行解決了 3

CIRCLEQ_FOREACH

#define CIRCLEQ_FOREACH(var, head, field) \for ((var) = CIRCLEQ_FIRST((head)); \(var) != (void *)(head) || ((var) = NULL); \(var) = CIRCLEQ_NEXT((var), field))

順序遍歷，只要 (var) != (void *)(head) 就會往下進(jìn)行，如果等于 head，根據(jù) || 運(yùn)算符的求值規(guī)則，會執(zhí)行 (var) = NULL，這個表達(dá)式的結(jié)果為 false，于是終止 for 循環(huán)

CIRCLEQ_FOREACH_REVERSE

#define CIRCLEQ_FOREACH_REVERSE(var, head, field) \for ((var) = CIRCLEQ_LAST((head)); \(var) != (void *)(head) || ((var) = NULL); \(var) = CIRCLEQ_PREV((var), field))

逆序遍歷，和順序遍歷類似，不解釋了。

CIRCLEQ_REMOVE

#define CIRCLEQ_REMOVE(head, elm, field) do { \if (CIRCLEQ_NEXT((elm), field) == (void *)(head)) \CIRCLEQ_LAST((head)) = CIRCLEQ_PREV((elm), field); \else \CIRCLEQ_PREV(CIRCLEQ_NEXT((elm), field), field) = \CIRCLEQ_PREV((elm), field); \if (CIRCLEQ_PREV((elm), field) == (void *)(head)) \CIRCLEQ_FIRST((head)) = CIRCLEQ_NEXT((elm), field); \else \CIRCLEQ_NEXT(CIRCLEQ_PREV((elm), field), field) = \CIRCLEQ_NEXT((elm), field); \ } while (0)

移除節(jié)點(diǎn) elm，需要修改那些指針呢？

elm 前面那個節(jié)點(diǎn)的 field.cqe_next

elm 后面那個節(jié)點(diǎn)的 field.cqe_prev

對于 1，如果 elm 前面的節(jié)點(diǎn)是 head，那就需要修改 CIRCLEQ_FIRST(head)

對于 2， 如果 elm 后面沒有節(jié)點(diǎn)了，那就需要修改 CIRCLEQ_LAST(head)

代碼 2-3 行解決了 4

5-6 行解決了 2

代碼 7-8 行解決了 3

10-11 解決了 1

【end】

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