Nginx advanced data structure source code analysis (3) ----- linked list

ngx_list_t is a linked list container encapsulated by Nginx and is used frequently. It has two structures, ngx_list_t describes the entire linked list, and ngx_list_part_t only describes one element of the linked list. For ease of understanding, we can call it a linked list of arrays. In other words, ngx_list_t is a linked list container, and the elements in the linked list are an array. In fact, the elements in the ngx_list_part_t array are what the user needs to store.

What are the benefits of such a structural expression:

(1) The elements stored in the linked list are flexible, it can be any kind of data structure;

(2) Linked list The memory that the elements need to occupy is managed by ngx_list_t, which has been allocated through the array;

(3) Using linked lists to access small pieces of memory is inefficient, but using arrays to access memory through offsets is much more efficient.

Definition of ngx_list_t structure:

typedef struct {
    ngx_list_part_t  *last;     //指向链表的最后一个数组元素

    ngx_list_part_t   part;     //链表的首个数组元素
    size_t            size;     //每一个用户要存储的一个数据必须小于或等于size
    ngx_uint_t        nalloc;   //表示数组元素的个数还是每个数组元素中的元素个数？？？问徐
    ngx_pool_t       *pool;     //内存池对象
} ngx_list_t;

Definition of ngx_list_part_t structure:

struct ngx_list_part_s {
    void             *elts;     //指向数组的起始地址
    ngx_uint_t        nelts;    //表示数组中已经用了多少个元素
    ngx_list_part_t  *next;     //下一个链表元素的地址
};

Initialization of array linked list:

static ngx_inline ngx_int_t
ngx_list_init(ngx_list_t *list, ngx_pool_t *pool, ngx_uint_t n, size_t size)//初始化链表
{
    list->part.elts = ngx_palloc(pool, n * size);//申请第一个数组元素的内存？？？应该是申请整个数组链表的                                                 //内存 问徐，弄清楚了
    if (list->part.elts == NULL) {
        return NGX_ERROR;
    }

    list->part.nelts = 0;    //开始数组元素中只有0个元素
    list->part.next = NULL;
    list->last = &list->part;//指向第一个节点
    list->size = size;
    list->nalloc = n;
    list->pool = pool;//内存池对象

    return NGX_OK;
}

Create array linked list:

ngx_list_t *
ngx_list_create(ngx_pool_t *pool, ngx_uint_t n, size_t size)//创建一个链表
{
    ngx_list_t  *list;

    list = ngx_palloc(pool, sizeof(ngx_list_t));//申请ngx_list_t的内存
    if (list == NULL) {
        return NULL;
    }

    if (ngx_list_init(list, pool, n, size) != NGX_OK) {//调用初始化函数对ngx_list_t初始化
        return NULL;
    }

    return list;
}

Add New elements:

void *
ngx_list_push(ngx_list_t *l)
{
    void             *elt;
    ngx_list_part_t  *last;

    last = l->last;

    if (last->nelts == l->nalloc) {//判断最后一个数组节点是否没有空间了

        /* the last part is full, allocate a new list part */

        last = ngx_palloc(l->pool, sizeof(ngx_list_part_t));//再申请一个节点的内存
        if (last == NULL) {
            return NULL;
        }

        last->elts = ngx_palloc(l->pool, l->nalloc * l->size);//申请节点的存储内存
        if (last->elts == NULL) {
            return NULL;
        }

        last->nelts = 0;
        last->next = NULL;

        l->last->next = last;//更新这两个变量
        l->last = last;
    }

    elt = (char *) last->elts + l->size * last->nelts;//返回可以插入元素的内存地址
    last->nelts++;

    return elt;
}

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