nginx源码分析—数组结构ngx_array_t

数组实现文件：文件：./src/core/ngx_array.h/.c

1.数组结构

struct ngx_array_s {
void        *elts;        //数组数据区起始位置
ngx_uint_t   nelts;       //实际存放的元素个数
size_t       size;        //每个元素的大小
ngx_uint_t   nalloc;      //数组分配的空间个数，即实际饭分配的小空间个数
ngx_pool_t  *pool;        //该数组在内存中的分配
};

2.数组的基本操作（5个）

ngx_array_t *ngx_array_create(ngx_pool_t *p, ngx_uint_t n, size_t size);   //创建数组
//数组销毁
void ngx_array_destroy(ngx_array_t *a);
//往数组中添加元素
void *ngx_array_push(ngx_array_t *a);
void *ngx_array_push_n(ngx_array_t *a, ngx_uint_t n);
//数组的初始化操作
static ngx_inline ngx_int_t
ngx_array_init(ngx_array_t *array, ngx_pool_t *pool, ngx_uint_t n, size_t size)

2.1创建数组

创建数组的操作实现过程，首先分配数组头(20B)，然后分配数组数据区，两次分配均在传入的内存池(pool指向的内存池)中进行。然后简单初始化数组头并返回数组头的起始位置。（分配分两个阶段完成，注意和释放过程对应）

ngx_array_t *
ngx_array_create(ngx_pool_t *p, ngx_uint_t n, size_t size)
{
ngx_array_t *a;

a = ngx_palloc(p, sizeof(ngx_array_t));   //从内存中分配数组头
if (a == NULL) {
return NULL;
}

a->elts = ngx_palloc(p, n * size);        //分配大小为＊size的内存作为数组数据区
if (a->elts == NULL) {
return NULL;
}

a->nelts = 0;                         //初始化
a->size = size;
a->nalloc = n;
a->pool = p;

return a;                             //返回内存起始位置
}

2.2数组销毁

销毁数组的操作实现过程，包括销毁数组数据区和数组头。这里的销毁动作实际上就是修改内存池的last指针，并没有调用free等释放内存的操作，显然，这种维护效率是很高的。

void
ngx_array_destroy(ngx_array_t *a)
{
ngx_pool_t  *p;

p = a->pool;

if ((u_char *) a->elts + a->size * a->nalloc == p->d.last) {
p->d.last -= a->size * a->nalloc;               //释放数组的数据区，只是修改内存池的last指针
}

if ((u_char *) a + sizeof(ngx_array_t) == p->d.last) {
p->d.last = (u_char *) a;                       //释放数组头， 再次修改内存池的last指针
}
}

2.3添加元素到数组

向数组添加元素的操作有两个，ngx_array_push和ngx_array_push_n，分别添加一个和多个元素。

但实际的添加操作并不在这两个函数中完成，例如ngx_array_push返回可以在该数组数据区中添加这个元素的位置，ngx_array_push_n则返回可以在该数组数据区中添加n个元素的起始位置，而添加操作即在获得添加位置之后进行

void *
ngx_array_push(ngx_array_t *a)
{
void        *elt, *new;
size_t       size;
ngx_pool_t  *p;

if (a->nelts == a->nalloc) {            //数据区已满

/* the array is full */

size = a->size * a->nalloc;        //数组当前可用数据区大小

p = a->pool;             //指向当前内存池

if ((u_char *) a->elts + size == p->d.last    //条件:内存池的last指针指向数组的数据区末端
&& p->d.last + a->size <= p->d.end)    //当前内存池还有可分配一个元素大小的空间
{
/*
* the array allocation is the last in the pool
* and there is space for new allocation
*/

p->d.last += a->size;  //last指针后移一位
a->nalloc++;   //数组元素总数+1

} else {
/* allocate a new array */

new = ngx_palloc(p, 2 * size);     //重新分配2倍空间
if (new == NULL) {
return NULL;
}
ngx_memcpy(new, a->elts, size);     //拷贝原来的数据
a->elts = new;              //更新数组数据区指针
a->nalloc *= 2;       //更新总容量
}
}
elt = (u_char *) a->elts + a->size * a->nelts;//没满，直接后移指
a->nelts++;     //总数+1
return elt;
}

void *
ngx_array_push_n(ngx_array_t *a, ngx_uint_t n)
{
void        *elt, *new;
size_t       size;
ngx_uint_t   nalloc;
ngx_pool_t  *p;

size = n * a->size;

if (a->nelts + n > a->nalloc) {

/* the array is full */

p = a->pool;

if ((u_char *) a->elts + a->size * a->nalloc == p->d.last
&& p->d.last + size <= p->d.end)
{
/*
* the array allocation is the last in the pool
* and there is space for new allocation
*/

p->d.last += size;
a->nalloc += n;

} else {
/* allocate a new array */

nalloc = 2 * ((n >= a->nalloc) ? n : a->nalloc);

new = ngx_palloc(p, nalloc * a->size);
if (new == NULL) {
return NULL;
}

ngx_memcpy(new, a->elts, a->nelts * a->size);
a->elts = new;
a->nalloc = nalloc;
}
}

elt = (u_char *) a->elts + a->size * a->nelts;
a->nelts += n;

return elt;
}

测试demo

#include <stdio.h>
#include "ngx_config.h"
#include "ngx_conf_file.h"
#include "nginx.h"
#include "ngx_core.h"
#include "ngx_string.h"
#include "ngx_palloc.h"
#include "ngx_array.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_pool(ngx_pool_t* pool)
{
while(pool)
{
printf("pool = 0x%x\n", pool);
printf("  .d\n");
printf(".last = 0x%x\n", pool->d.last);
printf(".end = 0x%x\n", pool->d.end);
printf(".next = 0x%x\n", pool->d.next);
printf(".failed = %d\n", pool->d.failed);

printf(".max = %d\n", pool->max);
printf(".current = 0x%x\n", pool->current);
printf(".chain = 0x%x\n", pool->chain);
printf(".large = 0x%x\n", pool->large);
printf(".cleanup = 0x%x\n", pool->cleanup);
printf(".log = 0x%x\n", pool->log);
printf("available pool memory = 0x%x\n\n", pool->d.end - pool->d.last);
pool = pool->d.next;
}
}
void dump_array(ngx_array_t *a)
{
if(a)
{
printf("array = 0x%x\n", a);
printf("        .elts = 0x%x\n", a->elts);
printf("        .nelts = %d\n", a->nelts);
printf("        .size = %d\n", a->size);
printf("        .nalloc = %d\n", a->nalloc);
printf("        .pool = 0x%x\n", a->pool);

printf("elements:    ");
int* ptr = (int*)(a->elts);
for(; ptr < (int*)(a->elts + a->nalloc*a->size);)
{
printf("0x%x    ", *ptr++);
}
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);
dump_pool(pool);

printf("-------------------------------------------------\n");
printf("alloc an array from the pool:\n");
printf("-------------------------------------------------\n");

ngx_array_t     *a = ngx_array_create(pool, 10, sizeof(int));
dump_pool(pool);
for(i = 0; i < 1000; i++)
{
int *ptr = ngx_array_push(a);
*ptr = i + 1;
}
dump_array(a);

size_t  iLen = sizeof(ngx_array_t);
printf("sizeof(ngx_array_t) == == 0x%x\n", iLen);

ngx_array_destroy(a);
ngx_destroy_pool(pool);

return 0;
}

测试结果

*************************************************
create a new pool:
*************************************************
pool = 0x9b6b020
.d
.last = 0x9b6b048
.end = 0x9b6b420
.next = 0x0
.failed = 0
.max = 984
.current = 0x9b6b020
.chain = 0x0
.large = 0x0
.cleanup = 0x0
.log = 0x0
available pool memory = 0x3d8

-------------------------------------------------
alloc an array from the pool:
-------------------------------------------------
pool = 0x9b6b020
.d
.last = 0x9b6b084
.end = 0x9b6b420
.next = 0x0
.failed = 0
.max = 984
.current = 0x9b6b020
.chain = 0x0
.large = 0x0
.cleanup = 0x0
.log = 0x0
available pool memory = 0x39c

array = 0x9b6b048
.elts = 0x9b6b05c
.nelts = 10
.size = 4
.nalloc = 10
.pool = 0x9b6b020
elements:    0x1	0x2	0x3	0x4	0x5	0x6	0x7	0x8	0x9	0xa
sizeof(ngx_array_t) == == 0x14
sizeof(ngx_pool_t)  ===  == 0x28
sizeof(ngx_pool_data_t) ===  === 0x10