struct ngx_pool_large_s {
    ngx_pool_large_t     *next; //用链表组织，指向下一块较大内存
    void                 *alloc; //实际内存地址
};


typedef struct {
    u_char               *last; //当前内存分配结束位置，即下一段可分配内存的起始位置
    u_char               *end; //内存池结束位置
    ngx_pool_t           *next; //链接到下一个内存池，内存池的很多块内存就是通过该指针连成链表
    ngx_uint_t            failed; //内存池分配失败次数
} ngx_pool_data_t;


struct ngx_pool_s {
    ngx_pool_data_t       d; //内存池的数据块
    size_t                max; //数据块大小，小块内存的最大值
    ngx_pool_t           *current; //当前内存池的指针
    ngx_chain_t          *chain; //该指针挂接一个ngx_chain_t结构
    ngx_pool_large_t     *large; //指向大块内存分配，nginx中，大块内存分配直接采用标准系统接口malloc
    ngx_pool_cleanup_t   *cleanup; //释放内存的callback
    ngx_log_t            *log;
};


struct ngx_pool_cleanup_s {
    ngx_pool_cleanup_pt   handler;
    void                 *data;
    ngx_pool_cleanup_t   *next;
};

ngx_pool_t *
ngx_create_pool(size_t size, ngx_log_t *log)
{
    ngx_pool_t  *p;

    p = ngx_memalign(NGX_POOL_ALIGNMENT, size, log);
    if (p == NULL) {
        return NULL;
    }

    p->d.last = (u_char *) p + sizeof(ngx_pool_t);
    p->d.end = (u_char *) p + size;
    p->d.next = NULL;
    p->d.failed = 0;

    size = size - sizeof(ngx_pool_t);
    //最大不超过4095B
    p->max = (size < NGX_MAX_ALLOC_FROM_POOL) ? size : NGX_MAX_ALLOC_FROM_POOL;

    p->current = p;
    p->chain = NULL;
    p->large = NULL;
    p->cleanup = NULL;
    p->log = log;

    return p;
}

void
ngx_destroy_pool(ngx_pool_t *pool)
{
    ngx_pool_t          *p, *n;
    ngx_pool_large_t    *l;
    ngx_pool_cleanup_t  *c;

    /**
     * cleanup指向析构函数，用于执行相关的内存池销毁之前的清理工作，如文件的关闭等。
     * 对内存池中的析构函数遍历调用
     */
    for (c = pool->cleanup; c; c = c->next) {
        if (c->handler) {
            ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, pool->log, 0,
                           "run cleanup: %p", c);
            c->handler(c->data);
        }
    }

    for (l = pool->large; l; l = l->next) {

        ngx_log_debug1(NGX_LOG_DEBUG_ALLOC, pool->log, 0, "free: %p", l->alloc);

        if (l->alloc) {
            ngx_free(l->alloc);
        }
    }

#if (NGX_DEBUG)

    /*
     * we could allocate the pool->log from this pool
     * so we cannot use this log while free()ing the pool
     */

    for (p = pool, n = pool->d.next; /* void */; p = n, n = n->d.next) {
        ngx_log_debug2(NGX_LOG_DEBUG_ALLOC, pool->log, 0,
                       "free: %p, unused: %uz", p, p->d.end - p->d.last);

        if (n == NULL) {
            break;
        }
    }

#endif

    //彻底销毁内存池
    for (p = pool, n = pool->d.next; /* void */; p = n, n = n->d.next) {
        ngx_free(p);

        if (n == NULL) {
            break;
        }
    }
}

void *
ngx_palloc(ngx_pool_t *pool, size_t size)
{
    u_char      *m;
    ngx_pool_t  *p;

    //小于max值，则从current结点开始遍历pool链表
    if (size <= pool->max) {

        p = pool->current;

        do {
            //以last开始，计算以NGX_ALIGNMENT对齐的偏移位置指针
            m = ngx_align_ptr(p->d.last, NGX_ALIGNMENT);

            if ((size_t) (p->d.end - m) >= size) {
                p->d.last = m + size;

                return m;
            }

            //如果不满足，则查找下一个链
            p = p->d.next;

        } while (p);
        /**
         * 遍历完整个内存池链表均未找到合适大小的内存块供分配，则执行ngx_palloc_block()来分配
         * 为该内存池再分配一个block，该block的大小为链表中前面每一个block大小的值
         * 一个内存池是由多个block链接起来的。分配成功后，将该block链入该poll链的最后
         */
        return ngx_palloc_block(pool, size);
    }
    //大于max值，则执行大块内存分配的函数ngx_palloc_large，在large链表里分配内存
    return ngx_palloc_large(pool, size);
}


static void *
ngx_palloc_block(ngx_pool_t *pool, size_t size)
{
    u_char      *m;
    size_t       psize;
    ngx_pool_t  *p, *new;

    psize = (size_t) (pool->d.end - (u_char *) pool);

    m = ngx_memalign(NGX_POOL_ALIGNMENT, psize, pool->log);
    if (m == NULL) {
        return NULL;
    }

    //block初始化
    new = (ngx_pool_t *) m;

    new->d.end = m + psize;
    new->d.next = NULL;
    new->d.failed = 0;

    m += sizeof(ngx_pool_data_t);
    m = ngx_align_ptr(m, NGX_ALIGNMENT);
    new->d.last = m + size;

    for (p = pool->current; p->d.next; p = p->d.next) {
        //分配失败次数大于4,移动current指针
        if (p->d.failed++ > 4) {
            pool->current = p->d.next;
        }
    }

    ///将分配的block加入内存池
    p->d.next = new;

    return m;
}

static void *
ngx_palloc_large(ngx_pool_t *pool, size_t size)
{
    void              *p;
    ngx_uint_t         n;
    ngx_pool_large_t  *large;

    p = ngx_alloc(size, pool->log);
    if (p == NULL) {
        return NULL;
    }

    n = 0;

    for (large = pool->large; large; large = large->next) {
        if (large->alloc == NULL) {
            large->alloc = p;
            return p;
        }

        if (n++ > 3) {
            break;
        }
    }

    large = ngx_palloc(pool, sizeof(ngx_pool_large_t));
    if (large == NULL) {
        ngx_free(p);
        return NULL;
    }

    large->alloc = p;
    large->next = pool->large;
    pool->large = large;

    return p;
}