First reading of nginx source code (5) - Let the troubles start from main ngx_array

Structure definition of array:

<code><span>typedef</span><span>struct</span> ngx_array_s       ngx_array_t;
<span>struct</span> ngx_array_s {
    <span>void</span>        *elts;           <span>// 指向数组存储位置的首地址</span>
    ngx_uint_t   nelts;          <span>// 当前数组中已经存放的元素个数</span>
    size_t       size;           <span>// 数组中每个元素的大小</span>
    ngx_uint_t   nalloc;         <span>// 当前最多能容纳的元素个数，类似cpp中的Vector，当nelts大于nalloc时扩容</span>
    ngx_pool_t  *pool;           <span>// 该数组对应的内存池</span>
};</code>

The array operation function is introduced below:

<code>ngx_array_t *ngx_array_create(ngx_pool_t *p, ngx_uint_t n, size_t size);
<span>/* 从pool中申请array结构体内存，并调用init初始化（申请n*size内存，改变array内的属性），
 * 所以元素与结构体内存可能并不连续，但肯定在同一个pool里，失败返回NULL */</span><span>void</span> ngx_array_destroy(ngx_array_t *a);
<span>/* 依次销毁数组的数据区和结构体内存，将内存返还给pool（last-=）
 * if ((u_char *) a + sizeof(ngx_array_t) == p->d.last) {
       p->d.last = (u_char *) a;
   }
 * 销毁结构体的代码如上，因为这代码看起来很奇怪，它怎么知道数组肯定再pool的最后，没加过其他东西了？
   看了源码数组也并不是通过单独的pool来管理的，也就是说pool中还可能有很多其他的数据。
   在nginx整个代码中没有找到对ngx_array_destroy的引用
 */</span><span>void</span> *ngx_array_push(ngx_array_t *a);
<span>void</span> *ngx_array_push_n(ngx_array_t *a, ngx_uint_t n); 
<span>/* 在数组a上新追加元素，并返回指向新元素的指针。需要把返回的指针转换为具体类型，
   然后再给新元素本身或者是各字段（如果数组的元素是复杂类型）赋值。*/</span><span>static</span> ngx_inline ngx_int_t
ngx_array_init(ngx_array_t *<span>array</span>, ngx_pool_t *pool, ngx_uint_t n, size_t size)
<span>/* 如果一个数组对象是被分配在堆上的，那么当调用ngx_array_destroy销毁以后，如果想再次使用，就可以调用此函数。
   如果一个数组对象是被分配在栈上的，那么就需要调用此函数，进行初始化的工作以后，才可以使用。*/</span></code>

Have you noticed a serious problem from the above code? Whether it is destroy or expansion, the original memory location in the source code is not free. Yes, this will definitely cause a waste of memory. Why does the nginx author care about memory so much that there is such a problem? I really don’t understand it. I think it is easy to solve. But there is definitely a reason. When we use it, it is best to plan the size of the array in advance to avoid the waste caused by multiple expansions.

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