Nginx源码分析-数据结构之数组array

//array结构体定义
typedef struct {
    void        *elts;    //数组数据存储地址
    ngx_uint_t   nelts;    //数组存储个数
    size_t       size;    //数组元素大小
    ngx_uint_t   nalloc;    //array结构总共能容纳数据个数
    ngx_pool_t  *pool;    //内存池
} ngx_array_t;

//初始化array
static ngx_inline ngx_int_t
ngx_array_init(ngx_array_t *array, ngx_pool_t *pool, ngx_uint_t n, size_t size)
{
    /*
     * set "array->nelts" before "array->elts", otherwise MSVC thinks
     * that "array->nelts" may be used without having been initialized
     */

    array->nelts = 0;
    array->size = size;
    array->nalloc = n;
    array->pool = pool;

    array->elts = ngx_palloc(pool, n * size);
    if (array->elts == NULL) {
        return NGX_ERROR;
    }

    return NGX_OK;
}

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));
    if (a == NULL) {
        return NULL;
    }

    if (ngx_array_init(a, p, n, size) != NGX_OK) {
        return NULL;
    }

    return a;
}

//销毁ngx_array_t内存
void
ngx_array_destroy(ngx_array_t *a)
{
    ngx_pool_t  *p;

    p = a->pool;

    //判断a中数据部分是否为内存池最后申请部分，如果是，则归还内存池内存
    if ((u_char *) a->elts + a->size * a->nalloc == p->d.last) {
        p->d.last -= a->size * a->nalloc;
    }

    //判断a中array_t头部分是否为内存池最后申请部分，如果是，则归还内存池内存
    if ((u_char *) a + sizeof(ngx_array_t) == p->d.last) {
        p->d.last = (u_char *) a;
    }
}

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

    //当array中数据个数达到最大值时，申请新的内存
    if (a->nelts == a->nalloc) {

        /* the array is full */

        size = a->size * a->nalloc;

        p = a->pool;
        //申请新内存时会在内存池中判断，如果内存池数据最后部分正好存储的是array_t的数据，且还有空间可添加新元素，则直接将这部分内存池扩容给a->elt, 省去了重新申请的消耗
        if ((u_char *) a->elts + size == p->d.last
            && p->d.last + a->size <= p->d.end)
        {
            /*
             * the array allocation is the last in the pool
             * and there is space for new allocation
             */

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

        } else {//当内存池不满足情况时，则再申请一块内存来存储数据，每次容量*2
            /* allocate a new array */

            new = ngx_palloc(p, 2 * size);
            if (new == NULL) {
                return NULL;
            }

            ngx_memcpy(new, a->elts, size);
            a->elts = new;
            a->nalloc *= 2;
        }
    }

    //将要写入元素的地址返回，并将数组元素个数++
    elt = (u_char *) a->elts + a->size * a->nelts;
    a->nelts++;

    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;

        //这里与ngx_array_push相同，只是判断可容纳容量时变为了n*a->size，即n个元素
        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 {//再申请内存时会判断n与a->nalloc，取较大值
            /* 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;
        }
    }

    //将要写入元素的地址返回，并将数组元素个数+n
    elt = (u_char *) a->elts + a->size * a->nelts;
    a->nelts += n;

    return elt;
}