alloc_root

  此函数经由某些宏开关,被分成了两个部分.
  而实际,主要是从170行开始.之前的是另一个部分.
  这个函数主要做的是,从free链表里面获取lengt大小的内存块.
  如果没有合适大小的块,则用my_malloc申请新的块.
  
  得到块之后,并没有讲块从free里移除.依然在free链表里.
  内存块的大小也是不固定的.

alloc_root的代码

// mysys/my_alloc.c
void *alloc_root(MEM_ROOT *mem_root, size_t length)
{
#if defined(HAVE_purify) && defined(EXTRA_DEBUG)
  reg1 USED_MEM *next;
  DBUG_ENTER("alloc_root");
  DBUG_PRINT("enter",("root: 0x%lx", (long) mem_root));

  DBUG_ASSERT(alloc_root_inited(mem_root));

  length+=ALIGN_SIZE(sizeof(USED_MEM));
  if (!(next = (USED_MEM*) my_malloc(length,MYF(MY_WME))))
  {
    if (mem_root->error_handler)
      (*mem_root->error_handler)();
    DBUG_RETURN((uchar*) 0);   /* purecov: inspected */
  }
  next->next= mem_root->used;
  next->size= length;
  mem_root->used= next;  // 将next链接到root链表
  DBUG_PRINT("exit",("ptr: 0x%lx", (long) (((char*) next)+
                                           ALIGN_SIZE(sizeof(USED_MEM)))));
  DBUG_RETURN((uchar*) (((char*) next)+ALIGN_SIZE(sizeof(USED_MEM))));
#else
  size_t get_size, block_size;
  uchar* point;
  reg1 USED_MEM *next= 0;
  reg2 USED_MEM **prev;
  DBUG_ENTER("alloc_root");
  DBUG_PRINT("enter",("root: 0x%lx", (long) mem_root));
  DBUG_ASSERT(alloc_root_inited(mem_root));

  length= ALIGN_SIZE(length);
  if ((*(prev= &mem_root->free)) != NULL) //有可用内存区域,此时prev与free链表指向相同.
  {
    if ((*prev)->left < length &&
 mem_root->first_block_usage++ >= ALLOC_MAX_BLOCK_USAGE_BEFORE_DROP &&
 (*prev)->left < ALLOC_MAX_BLOCK_TO_DROP)
    {
      next= *prev;
      *prev= next->next;   /* Remove block from list */
      next->next= mem_root->used;
      mem_root->used= next;
      mem_root->first_block_usage= 0;
    }
    for (next= *prev ; next && next->left < length ; next= next->next)
      prev= &next->next;
  }
  if (! next) //上面的例程没找到合适大小的内存块,则申请新的内存区域
  {      /* Time to alloc new block */
    block_size= mem_root->block_size * (mem_root->block_num >> 2);
    get_size= length+ALIGN_SIZE(sizeof(USED_MEM));
    get_size= max(get_size, block_size);

    if (!(next = (USED_MEM*) my_malloc(get_size,MYF(MY_WME))))
    {
      if (mem_root->error_handler)
 (*mem_root->error_handler)();
      return((void*) 0);                      /* purecov: inspected */
    }
    mem_root->block_num++; 
    next->next= *prev;   //将新块添加到free链表头部
    next->size= get_size;
    next->left= get_size-ALIGN_SIZE(sizeof(USED_MEM));
    *prev=next; //重新让pre指向free链表头部
  }
  //获得内存块之后,减去链表数据结构的大小,得到实际可用内存块大小的起点.
  point= (uchar*) ((char*) next+ (next->size-next->left));
  /*TODO: next part may be unneded due to mem_root->first_block_usage counter*/
  if ((next->left-= length) < mem_root->min_malloc)
  {      /* Full block */
    *prev= next->next;    /* Remove block from list */
    next->next= mem_root->used;
    mem_root->used= next;
    mem_root->first_block_usage= 0;
  }
  DBUG_PRINT("exit",("ptr: 0x%lx", (ulong) point));
  DBUG_RETURN((void*) point);// 返回实际可用内存块的起始位置指针.此时内存块已经列入free链表里
#endif
}