# nginx数据结构_循环链表
# 主要思想:
链表中不存数据域
只定义循环链表的常见操作
链表节点作为数据结构体中的一个最后一个字段，起到串联数据的作用
使用linux的一个内核宏
offsetof(type, link)
获取type结构体中link字段的偏移量，相对于type开始的位置
比如ngx_qtest_t类型 offsetof(ngx_qtest_t, link) 的值应该是int 4字节 + name 32字节 = 36
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# queue.h文件
/*
* Copyright (C) Igor Sysoev
* Copyright (C) Nginx, Inc.
*/
#ifndef _NGX_QUEUE_H_INCLUDED_
#define _NGX_QUEUE_H_INCLUDED_
#define u_char char
#define ngx_int_t int
typedef struct ngx_queue_s ngx_queue_t;
struct ngx_queue_s {
ngx_queue_t *prev;
ngx_queue_t *next;
};
#define ngx_queue_init(q) \
(q)->prev = q; \
(q)->next = q
#define ngx_queue_empty(h) \
(h == (h)->prev)
#define ngx_queue_insert_head(h, x) \
(x)->next = (h)->next; \
(x)->next->prev = x; \
(x)->prev = h; \
(h)->next = x
#define ngx_queue_insert_after ngx_queue_insert_head
#define ngx_queue_insert_tail(h, x) \
(x)->prev = (h)->prev; \
(x)->prev->next = x; \
(x)->next = h; \
(h)->prev = x
#define ngx_queue_head(h) \
(h)->next
#define ngx_queue_last(h) \
(h)->prev
#define ngx_queue_sentinel(h) \
(h)
#define ngx_queue_next(q) \
(q)->next
#define ngx_queue_prev(q) \
(q)->prev
#if (NGX_DEBUG)
#define ngx_queue_remove(x) \
(x)->next->prev = (x)->prev; \
(x)->prev->next = (x)->next; \
(x)->prev = NULL; \
(x)->next = NULL
#else
#define ngx_queue_remove(x) \
(x)->next->prev = (x)->prev; \
(x)->prev->next = (x)->next
#endif
#define ngx_queue_split(h, q, n) \
(n)->prev = (h)->prev; \
(n)->prev->next = n; \
(n)->next = q; \
(h)->prev = (q)->prev; \
(h)->prev->next = h; \
(q)->prev = n;
#define ngx_queue_add(h, n) \
(h)->prev->next = (n)->next; \
(n)->next->prev = (h)->prev; \
(h)->prev = (n)->prev; \
(h)->prev->next = h;
#define ngx_queue_data(q, type, link) \
(type *) ((u_char *) q - offsetof(type, link))
#endif /* _NGX_QUEUE_H_INCLUDED_ */
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# main.c
#include<stdio.h>
#include"queue.h"
#pragma pack(1) // 规定对其字节数 ， gcc 64位默认是pack(8)
#define offsetof(TYPE, MEMBER) ((size_t)&((TYPE *)0)->MEMBER)
// 将0地址开始内存映射为结构体TYPE, 然后取MEMBER字段的地址，这个地址就是MEMBER相对于结构体开始的偏移量。
// 再转为int64类型size_t
typedef struct {
int key;
char name[32];
ngx_queue_t link;
}ngx_qtest_t;
int main(void)
{
ngx_qtest_t arr[5];
ngx_queue_t head;
ngx_queue_init(&head);
int i=0;
printf("====%d==111==\n", 0);
for(; i<5; ++i){
arr[i].key = i;
sprintf(arr[i].name, "my key is :%d", arr[i].key);
if(i%2){
printf("==1==%d====\n", i);
ngx_queue_insert_head(&head, &arr[i].link);
}else{
printf("==2==%d====\n", i);
ngx_queue_insert_tail(&head, &arr[i].link);
}
}
ngx_queue_t *tmp = ngx_queue_head(&head);
for(i=0; i<5;i++){
ngx_qtest_t *data = ngx_queue_data(tmp, ngx_qtest_t, link);
printf("====%d====\n", data->key);
tmp = ngx_queue_next(tmp);
}
printf("---offsetof link in ngx_qtest_t: %d \n", offsetof(ngx_qtest_t, link));
return 0;
}
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