struct malloc_chunk {
  INTERNAL_SIZE_T prev_size; //size of prev_chunk
  INTERNAL_SIZE_T size;    // size of chunk

  /* Only use if free*/
  struct malloc_chunk *fd;  //point to nextchunk
  struct malloc_chunk *bk;  //point to backchunk

  /*Use for large chunk*/
  struct malloc_chunk *fd_nextsize;
  struct malloc_chunk *bk_nextsize;
};