内存池的实现

古城微笑少年丶 2022-08-14 04:59 185阅读 0赞

C/C++下内存管理是让几乎每一个程序员头疼的问题，分配足够的内存、追踪内存的分配、在不需要的时候释放内存——这个任务相当复杂。而直接使用系统调用malloc/free、new/delete进行内存分配和释放，有以下弊端：

1.  调用malloc/new,系统需要根据“最先匹配”、“最优匹配”或其他算法在内存空闲块表中查找一块空闲内存，调用free/delete,系统可能需要合并空闲内存块，这些会产生额外开销  
    
2.  频繁使用时会产生大量内存碎片，从而降低程序运行效率
3.  容易造成内存泄漏

内存池（memory pool)是代替直接调用malloc/free、new/delete进行内存管理的常用方法，当我们申请内存空间时，首先到我们的内存池中查找合适的内存块，而不是直接向操作系统申请，优势在于：

1.  比malloc/free进行内存申请/释放的方式快
2.  不会产生或很少产生堆碎片
3.  可避免内存泄漏

** **

**内存池设计**

看到内存池好处这么多，是不是恨不能马上抛弃malloc/free，投奔内存池的怀抱呢？且慢，在我们自己动手实现内存池之前还需要明确以下几个问题：

1.  内存池的空间如何获得？是程序启动时分配一大块空间还是程序运行中按需求分配？
2.  内存池对到来的内存申请，有没有大小的限制？如果有，最小可申请的内存块为多大，最大的呢？  
    
3.  如何合理设计内存块结构，方便我们进行内存的申请、追踪和释放呢？  
    
4.  内存池占用越多空间，相对应其他程序能使用的内存就越少，是否要设定内存池空间的上限？设定为多少合适呢？

带着以上问题，我们来看以下一种内存池设计方案。

** **

**内存池实现方案一**

从[这里][Link 1]下载该内存池实现的源码。

首先给出该方案的整体架构，如下：

![内存池实现架构][r_1.jpg]

图1.内存池架构图

结构中主要包含block、list 和pool这三个结构体，block结构包含指向实际内存空间的指针，前向和后向指针让block能够组成双向链表；list结构中free指针指向空闲 内存块组成的链表，used指针指向程序使用中的内存块组成的链表，size值为内存块的大小，list之间组成单向链表；pool结构记录list链表的头和尾。

**内存跟踪策略**

该方案中，在进行内存分配时，将多申请12个字节，即实际申请的内存大小为所需内存大小+12。在多申请的12个字节中，分别存放对应的list指针（4字节）、used指针（4字节）和校验码（4字节）。通过这样设定，我们很容易得到该块内存所在的list和block，校验码起到粗略检查是否出错的作用。该结构图示如下：

![结点头12字节说明][12]

图2.内存块申请示意图

图中箭头指示的位置为内存块真正开始的位置。

** **

**内存申请和释放策略**

申请：根据所申请内存的大小，遍历list链表，查看是否存在相匹配的size；

存在匹配size：查看free时候为NULL

free为NULL：使用malloc/new申请内存，并将其置于used所指链表的尾部

free不为NULL：将free所指链表的头结点移除，放置于used所指链表的尾部

不存在匹配size：新建list，使用malloc/new申请内存，并将其置于该list的used所指链表尾部

返回内存空间指针

释放：根据内存跟踪策略，获取list指针和used指针，将其从used指针所指的链表中删除，放置于free指针所指向的链表

** **

**对方案一的分析**

对照“内存池设计”一节中提出的问题，我们的方案一有以下特点：

1.  程序启动后内存池并没有内存块，到程序真正进行内存申请和释放的时候才接管内存块管理；
2.  该内存池对到来的申请，对申请大小并不做限制，其为每个size值创建链表进行内存管理；
3.  该方案没有提供限定内存池大小的功能

结合分析，可以得出该方案应用场景如下：程序所申请的内存块大小比较固定（比如只申请/释放1024bytes或2048bytes的内存），申请和释放的频率基本保持一致（因申请多而释放少会占用过多内存，最终导致系统崩溃）。

这篇文章讲解了内存管理的基本知识，以一个简单的内存池实现例子作为敲门砖，引领大家认识内存池，下一篇为内存池进阶文章，讲解apache服务器中内存池的实现方法。

[《内存池的实现(一)》][Link 2]中，介绍了使用内存池的原因，设计内存池应该考虑的问题，最后给出一个简单的内存池实现例子。使用上一篇文章中介绍的内存池实现方案，要在一定的限定条件下，下面我们来看更通用的内存池实现——Apache服务器的内存池实现。

Apache服务器的开发人员将代码中可移植的部分整理出来，编辑成Apache可移植运行库（Apacheportable Run-timelibraries)，简称APR，该库可从[这里][Link 3]下载，其中包含这里要介绍的内存池的实现代码。下面将Apache服务器内存池简称为APR内存池。

# APR内存池结构 #

## 1.内存分配结点 ##

在了解整个内存池架构前，我们先来了解APR内存池中最基本的单元——内存分配结点。内存分配结点被用来描述每次分配的内存块，对应的结构名为apr\_memnode\_t，定义在文件apr\_allocator.h中，其定义如下：

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     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; background:none rgb(244,244,244)!important"> 
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      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; color:rgb(0,130,0)!important; background:none!important">/**&lt; next memnode */</code> 
     </div> 
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      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">apr_uint32_t&nbsp; index;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </code> 
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     </div> 
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      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; color:rgb(0,130,0)!important; background:none!important">/**&lt; pointer to end of free memory */</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; background:none rgb(244,244,244)!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">};</code> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

即使结构中的每个字段，源文件中都给出了注释，但对于每个字段的用途，还是很难让人理解。这里先给出每个字段的简略解析：

1.  next：指向下一个结点的指针；
2.  ref：指向上一结点的next结点，上一结点的next指向本结点，因此\*\*ref就是本结点自身；
3.  index：既指示了该结点的大小，同时指示了该结点所在链表的索引下标值；
4.  free\_index：所描述的内存块中未被占用的空间（这里和上面的index和它们字面意思有出入，理解的时候要留意）；
5.  first\_avail：指向可用空间开始位置的指针；
6.  endp：指向可用空间结尾位置的指针。

该结点示意图如下：

![结点示意图][r_tt.jpg]

## 2.内存分配器 ##

在ARP内存池中，使用内存分配器对内存分配结点进行管理，它在apr\_pools.c中定义如下：

<table style="border:1px solid silver; width:516.129px; border-collapse:collapse; word-break:break-word; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; font-size:12px!important; min-height:auto!important; background:none!important"> 
 <tbody style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; background:none!important"> 
  <tr style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; background:none!important"> 
   <td style="border:1px solid silver; padding:3px; border-collapse:collapse; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; margin:0px!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important"> 
    <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:relative!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; background:none!important"> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; background:none rgb(244,244,244)!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; color:rgb(0,0,255)!important; background:none!important">struct</code>&nbsp; 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">apr_allocator_t { </code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">apr_uint32_t&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; max_index;</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; background:none rgb(244,244,244)!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">apr_uint32_t&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; max_free_index;</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">apr_uint32_t&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; current_free_index;</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; background:none rgb(244,244,244)!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">apr_pool_t&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; *owner;</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">apr_memnode_t&nbsp; *</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(255,20,147)!important; background:none!important">free</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">[MAX_INDEX];</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; background:none rgb(244,244,244)!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">};</code> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

1.  max\_index：free指针数组的下标，free\[max\_index\]指向已有的最大内存块链表；
2.  max\_free\_index：内存分配器所能容纳的最大内存空间数值；
3.  current\_free\_index：内存分配器中还能接收的空间大小，与max\_free\_index结合使用，解决限制内存池空间大小的问题；
4.  owner：指示该分配器属于哪个内存池；
5.  free：指向一组链表的头结点，该链表中每个结点指向内存结点组成的链表，MAX\_INDEX为20。

内存分配器及其管理的内存结点图示如下：

![内存分配器][r_22.png]

从上图我们可以清晰地看出，free数组的下标从1到MAX\_INDEX-1，分别指向一条结点大小固定的链表，下标增加1，结点的大小增加4k，因此free\[MAX\_INDEX\]所指向的链表的结点大小为84k，这也是内存池使用者所能申请的最大”规则结点“，超过该大小的结点将下标0指向的链表进行管理。要明白free数组下标和结点大小的关系，我们需要知道宏定义APR\_ALIGN：

该宏所做的无非就是计算出最接近size的boundary的整数倍的整数。通常情况下size大小为整数即可，而boundary则必须保证为2的幂。比如APR\_ALIGN(7,4)为8；APR\_ALIGN(21,8)为24；APR\_ALIGN(21,16)则为32。

对于每次空间申请，APR先对齐空间大小：

结果是size的值变成4096（4k，2的12次方）的倍数，最后，通过左移与我们的下标对应起来：

# #

## 3.内存池结点 ##

APR的内存池结点，在apr\_pools.c文件中定义如下：

该结构中的字段比较多，我们主要关注以上列出的两个字段。

1.  allocator：指向相应的内存分配器；
2.  active：指向使用中内存链表的指针

内存池结点及其管理的内存结点如下图所示：

# ![内存池结点][r_33.png] #

需要留意的是虽然该结构字面意义上为“内存池结构”，但是它负责管理使用中的内存。

# #

## APR内存池的内存管理 ##

对APR内存池各个结构有了初步了解之后，我们来看APR中是如何利用这些结构进行内存管理的。

## ##

### 1.内存申请 ###

内存申请的核心函数是allocator\_alloc函数，参数为一个指向内存分配器的指针和所要申请空间的大小，内存分配就是对内存分配器进行操作（以下列出的字段参见“内存分配器”章节），其内存申请的策略如下：

1.  根据申请空间的大小size，生成索引index，如果索引数值在1～max\_index范围内，那就在index~max\_index范围内的链表中返回一块内存；
2.  如果索引数值index >max\_index，则在free\[0\]链表中查找一块合适的内存；
3.  经上两步仍未找到空闲内存块，则通过malloc(size)返回一块新生成的内存。

## ##

### 2.内存释放 ###

在内存申请中提到，假如内存分配器中没有合适的内存块，将会调用malloc获取一块，但是新分配的内存并不挂接到内存分配器链表中，而是在调用allocator\_free进行内存释放的时候，内存才可能挂到内存分配器链表上。内存释放策略如下：

1.  如果结点的大小超过了完全释放的阙值max\_free\_index，那么我们就不能将其简单的归还到索引链表中，而必须将其完全归还给操作系统；
2.  如果index< MAX\_INDEX，则意味着该结点属于“规则结点”的范围。因此可以将该结点返回到对应的“规则链表”中；
3.  如果结点超过了“规则结点”的范围，但是并没有超过阙值max\_free\_index，此时我们则可以将其置于“索引0”链表的首部中。

## ##

### 3.内存池结点管理的内存 ###

刚开始，由内存分配器管理的链表并没有挂接任何内存，也就是说内存池是空的，当我们申请内存时，必然进行“内存申请”中的第三步操作，新分配的内存就由我们的内存池结点进行管理。

先来看用于内存池结点管理的一个宏定义：

<table style="border:1px solid silver; width:516.129px; border-collapse:collapse; word-break:break-word; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; font-size:12px!important; min-height:auto!important; background:none!important"> 
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   <td style="border:1px solid silver; padding:3px; border-collapse:collapse; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; margin:0px!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important"> 
    <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:relative!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; background:none!important"> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; background:none rgb(244,244,244)!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; color:rgb(0,130,0)!important; background:none!important">/* Node list managementhelper macros; list_insert() inserts 'node'</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; color:rgb(0,130,0)!important; background:none!important">* before 'point'. */</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; background:none rgb(244,244,244)!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; color:gray!important; background:none!important">#define list_insert(node,point) do {&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; \</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">node-&gt;ref= point-&gt;ref;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; \</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; background:none rgb(244,244,244)!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">*node-&gt;ref= node;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; \</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">node-&gt;next= point;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; \</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; background:none rgb(244,244,244)!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">point-&gt;ref= &amp;node-&gt;next;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; \</code> 
     </div> 
     <div style="border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px 1em 0px 0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important"> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">} </code> 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; color:rgb(0,0,255)!important; background:none!important">while</code>&nbsp; 
      <code style="white-space:pre-wrap; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.8em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; min-height:auto!important; background:none!important">(0)</code> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

在内存池结点初次建立时，链表状态如下图：

![结点初次建立][r_mm.png]

再次申请一个结点，运行list\_insert(node,point)后，结果如下图：

![插入一个结点后][r_mmm.png]

我们可以通过allocator\_free调用或apr\_pool\_clear、apr\_pool\_destroy调用（它们内部调用allocator\_free）进行内存池结点的释放，所释放的内存将按照“内存释放”中的策略归还内存池或操作系统。

# #

## 小结 ##

free数组下标的双重含义（即是数组下标，又指示内存块大小）、通过阙值max\_free\_index限制内存池大小，这些都较难理解，但也是是APR内存池实现中出彩的部分，另外，APR内存池还涉及到父/子/兄弟内存池、内存池生命周期的概念，同学们可以通过文章末尾给出的参考文档，进行更深入地学习。

[Link 1]: http://download.csdn.net/source/3536291
[r_1.jpg]: /images/20220731/eee8b6b78917408695100f6eaeadc759.png
[12]: /images/20220731/0a9ddfa8e9bf406b8ba4673b834daaac.png
[Link 2]: http://www.cnblogs.com/bangerlee/archive/2011/08/31/2161421.html
[Link 3]: http://apr.apache.org/
[r_tt.jpg]: /images/20220731/29e585831f0f4031bb0b2f5846c50d90.png
[r_22.png]: /images/20220731/384498f299984b9cb2772def8c17738b.png
[r_33.png]: /images/20220731/531678ae1d084667b786a55bb47132b3.png
[r_mm.png]: /images/20220731/767eae32433d4ec0a5fc2d3e489858c0.png
[r_mmm.png]: /images/20220731/ac73e4e6aa5f4b0e85d5d99a5397c699.png