init_alloc_root

  代码如下
  主要是预处理一下相关数据结构的内容.

init_alloc_root的代码

// my_sys/my_alloc.c
/*
  Initialize memory root

  SYNOPSIS
    init_alloc_root()
      mem_root       - memory root to initialize
      block_size     - size of chunks (blocks) used for memory allocation
                       (It is external size of chunk i.e. it should include
                        memory required for internal structures, thus it
                        should be no less than ALLOC_ROOT_MIN_BLOCK_SIZE)
      pre_alloc_size - if non-0, then size of block that should be
                       pre-allocated during memory root initialization.

  DESCRIPTION
    This function prepares memory root for further use, sets initial size of
    chunk for memory allocation and pre-allocates first block if specified.
    Altough error can happen during execution of this function if
    pre_alloc_size is non-0 it won't be reported. Instead it will be
    reported as error in first alloc_root() on this memory root.
*/

void init_alloc_root(MEM_ROOT *mem_root, size_t block_size,
       size_t pre_alloc_size __attribute__((unused)))
{
  DBUG_ENTER("init_alloc_root");
  DBUG_PRINT("enter",("root: 0x%lx", (long) mem_root));
  //为MEM_ROOT结构各部分置初始值.保持各链表为空.
  mem_root->free= mem_root->used= mem_root->pre_alloc= 0;
  mem_root->min_malloc= 32;
  mem_root->block_size= block_size - ALLOC_ROOT_MIN_BLOCK_SIZE;
  mem_root->error_handler= 0;
  mem_root->block_num= 4;   /* We shift this with >>2 */
  mem_root->first_block_usage= 0;

#if !(defined(HAVE_purify) && defined(EXTRA_DEBUG))
  if (pre_alloc_size)
  {
    if ((mem_root->free= mem_root->pre_alloc=
  (USED_MEM*) my_malloc(pre_alloc_size+ ALIGN_SIZE(sizeof(USED_MEM)),
          MYF(0))))
    {
      mem_root->free->size= pre_alloc_size+ALIGN_SIZE(sizeof(USED_MEM));
      mem_root->free->left= pre_alloc_size;
      mem_root->free->next= 0;
    }
  }
#endif
  DBUG_VOID_RETURN;
}

---

  相关的数据结构如下.
  USED_MEM代表的是一个内存块.
  而MEM_ROOT是一个内存链表.意义参见实现.
  这两个数据结构会被反复用到.
  参见相应的标签

USED_MEM和MEM_ROOT的代码

// include/my_alloc.h  
typedef struct st_used_mem
{       /* struct for once_alloc (block) */
  struct st_used_mem *next;    /* Next block in use */
  unsigned int left;     /* memory left in block  */
  unsigned int size;     /* size of block */
} USED_MEM;


typedef struct st_mem_root
{
  USED_MEM *free;                  /* blocks with free memory in it */
  USED_MEM *used;                  /* blocks almost without free memory */
  USED_MEM *pre_alloc;             /* preallocated block */
  /* if block have less memory it will be put in 'used' list */
  size_t min_malloc;
  size_t block_size;               /* initial block size */
  unsigned int block_num;          /* allocated blocks counter */
  /* 
     first free block in queue test counter (if it exceed 
     MAX_BLOCK_USAGE_BEFORE_DROP block will be dropped in 'used' list)
  */
  unsigned int first_block_usage;

  void (*error_handler)(void);
} MEM_ROOT;