Nginx 链表结构 ngx_list_t
链表结构
ngx_list_t 是 Nginx 封装的链表容器，链表容器内存分配是基于内存池进行的，操作方便，效率高。Nginx 链表容器和普通链表类似，均有链表表头和链表节点，通过节点指针组成链表。其结构定义如下：
typedef struct ngx_list_part_s ngx_list_part_t;
struct ngx_list_part_s {
void *elts;
ngx_uint_t nelts;
ngx_list_part_t *next;
};
typedef struct {
ngx_list_part_t *last;
ngx_list_part_t part;
size_t size;
ngx_uint_t nalloc;
ngx_pool_t *pool;
} ngx_list_t;
链表数据结构如下图所示：
链表操作
Nginx 链表的操作只有两个：创建链表 和 添加元素。由于链表的内存分配是基于内存池，所有内存的销毁由内存池进行，即链表没有销毁操作。
创建链表
创建新的链表时，首先分配链表表头，再对该链表进行初始化，在初始化过程中分配头节点数据区内存。
ngx_list_t *
ngx_list_create(ngx_pool_t *pool, ngx_uint_t n, size_t size)
{
ngx_list_t *list;
list = ngx_palloc(pool, sizeof(ngx_list_t));
if (list == NULL) {
return NULL;
}
if (ngx_list_init(list, pool, n, size) != NGX_OK) {
return NULL;
}
return 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)
{
list->part.elts = ngx_palloc(pool, n * size);
if (list->part.elts == NULL) {
return NGX_ERROR;
}
list->part.nelts = 0;
list->part.next = NULL;
list->last = &list->part;
list->size = size;
list->nalloc = n;
list->pool = pool;
return NGX_OK;
}
添加元素
添加元素到链表时，都是从最后一个节点开始，首先判断最后一个节点的数据区是否由内存存放新增加的元素，若足以存储该新元素，则返回存储新元素内存的位置，若没有足够的内存存储新增加的元素，则分配一个新的节点，再把该新的节点连接到现有链表中，并返回存储新元素内存的位置。注意：添加的元素可以是整数，也可以是一个结构。
void *
ngx_list_push(ngx_list_t *l)
{
void *elt;
ngx_list_part_t *last;
last = l->last;
if (last->nelts == l->nalloc) {
last = ngx_palloc(l->pool, sizeof(ngx_list_part_t));
if (last == NULL) {
return NULL;
}
last->elts = ngx_palloc(l->pool, l->nalloc * l->size);
if (last->elts == NULL) {
return NULL;
}
last->nelts = 0;
last->next = NULL;
l->last->next = last;
l->last = last;
}
elt = (char *) last->elts + l->size * last->nelts;
last->nelts++;
return elt;
}
测试程序：
#include "ngx_config.h"
#include <stdio.h>
#include "ngx_conf_file.h"
#include "nginx.h"
#include "ngx_core.h"
#include "ngx_string.h"
#include "ngx_palloc.h"
#include "ngx_list.h"
volatile ngx_cycle_t *ngx_cycle;
void ngx_log_error_core(ngx_uint_t level, ngx_log_t *log, ngx_err_t err,
const char *fmt, ...)
{
}
void dump_list_part(ngx_list_t* list, ngx_list_part_t* part)
{
int *ptr = (int*)(part->elts);
int loop = 0;
printf(" .part = 0x%x\n", &(list->part));
printf(" .elts = 0x%x ", part->elts);
printf("(");
for (; loop < list->nalloc - 1; loop++)
{
printf("%d, ", ptr[loop]);
}
printf("%d)\n", ptr[loop]);
printf(" .nelts = %d\n", part->nelts);
printf(" .next = 0x%x", part->next);
if (part->next)
printf(" -->\n");
printf(" \n");
}
void dump_list(ngx_list_t* list)
{
if (list)
{
printf("list = 0x%x\n", list);
printf(" .last = 0x%x\n", list->last);
printf(" .part = %d\n", &(list->part));
printf(" .size = %d\n", list->size);
printf(" .nalloc = %d\n", list->nalloc);
printf(" .pool = 0x%x\n", list->pool);
printf("elements: \n");
ngx_list_part_t *part = &(list->part);
while(part)
{
dump_list_part(list, part);
part = part->next;
}
printf("\n");
}
}
int main()
{
ngx_pool_t *pool;
int i;
printf("--------------------------------\n");
printf("create a new pool:\n");
printf("--------------------------------\n");
pool = ngx_create_pool(1024, NULL);
printf("--------------------------------\n");
printf("alloc an list from the pool:\n");
printf("--------------------------------\n");
ngx_list_t *list = ngx_list_create(pool, 5, sizeof(int));
if(NULL == list)
{
return -1;
}
for (i = 0; i < 5; i++)
{
int *ptr = ngx_list_push(list);
*ptr = 2*i;
}
dump_list(list);
ngx_destroy_pool(pool);
return 0;
}输出结果：
$ ./list_test
--------------------------------
create a new pool:
--------------------------------
--------------------------------
alloc an list from the pool:
--------------------------------
list = 0x98ce048
.last = 0x98ce04c
.part = 160227404
.size = 4
.nalloc = 5
.pool = 0x98ce020
elements:
.part = 0x98ce04c
.elts = 0x98ce064 (0, 2, 4, 6, 8)
.nelts = 5
.next = 0x0
参考资料：
《深入理解 Nginx 》
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