#define MALLOC_OVERHEAD 8 //分配过程中，需要保留一部分额外的空间
#define ALLOC_MAX_BLOCK_TO_DROP 4096 //后续会继续分析该宏的用途
#define ALLOC_MAX_BLOCK_USAGE_BEFORE_DROP 10 //后续会继续分析该宏的用途

#define ALIGN_SIZE(A) MY_ALIGN((A),sizeof(double))
#define MY_ALIGN(A,L) (((A) + (L) - 1) & ~((L) - 1))

#define ALLOC_ROOT_MIN_BLOCK_SIZE (MALLOC_OVERHEAD + sizeof(USED_MEM) + 8)
/* Define some useful general macros (should be done after all headers). */
/*作者：www.manongjc.com */
#define MY_MAX(a, b) ((a) > (b) ? (a) : (b)) //求两个数值之间的最大值
#define MY_MIN(a, b) ((a) < (b) ? (a) : (b)) //求两个数值之间的最小值

typedef struct st_mem_root
{
 USED_MEM *free;         /* free block link list的链表头指针 */
 USED_MEM *used;         /* used block link list的链表头指针 */
 USED_MEM *pre_alloc;       /* 预先分配的block */
 size_t min_malloc;       /* 如果block剩下的可用空间小于该值，将会从free list移动到used list */
 size_t block_size;       /* 每次初始化的空间大小 */
 unsigned int block_num;       /* 记录实际的block数量,初始化为4 */
 unsigned int first_block_usage;   /* free list中的第一个block 测试不满足分配空间大小的次数 */
 void (*error_handler)( void );     /* 分配失败的错误处理函数 */
} MEM_ROOT;

typedef struct st_used_mem
{ 
  struct st_used_mem *next; //指向下一个分配的block
  unsigned int left; //该block剩余的空间大小
  unsigned int size; //该block的总大小
} USED_MEM;

void init_alloc_root( MEM_ROOT *mem_root, size_t block_size, size_t pre_alloc_size __attribute__( (unused) ) )
{
 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;
}

void *alloc_root( MEM_ROOT *mem_root, size_t length )
{
 size_t get_size, block_size;
 uchar * point;
 reg1 USED_MEM *next = 0;
 reg2 USED_MEM **prev;

 length = ALIGN_SIZE( length );
 if ( (*(prev = &mem_root->free) ) != NULL )
 {
 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 )
 {    /* 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 = MY_MAX( get_size, block_size );

 if ( !(next = (USED_MEM *) my_malloc( get_size, MYF( MY_WME | ME_FATALERROR ) ) ) )
 {
  if ( mem_root->error_handler )
  (*mem_root->error_handler)();
  DBUG_RETURN( (void *) 0 );               /* purecov: inspected */
 }
 mem_root->block_num++;
 next->next = *prev;
 next->size = get_size;
 next->left = get_size - ALIGN_SIZE( sizeof(USED_MEM) );  /* bug:如果该block是通过mem_root->block_size * (mem_root->block_num >> 2)计算出来的，则已经去掉了ALIGN_SIZE(sizeof(USED_MEM)，这里重复了。 */
 *prev = next;
 }

 point = (uchar *) ( (char *) next + (next->size - next->left) );
/*TODO: next part may be unneded due to mem_root->first_block_usage counter*/
/* 作者：www.manongjc.com */
 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;
 }
}

for (next= *prev ; next && next->left < length ; next= next->next)
prev= &next->next;
}