In glibc malloc.c or dlmalloc It said "repositioning tricks"As in blew, and use this trick in bin_at. bins is a array,the space is allocated when av(struct malloc_state) is allocated.doesn't it? the sizeof(bin[i]) is less then sizeof(struct malloc_chunk*)? Who can describe this trick for me? I can't understand the bin_at macro.why they get the bins address use this method?how it works? Very thanks,and sorry for my poor English.

/*
     To simplify use in double-linked lists, each bin header acts
    as a malloc_chunk. This avoids special-casing for headers.
    But to conserve space and improve locality, we allocate
    only the fd/bk pointers of bins, and then use repositioning tricks
    to treat these as the fields of a malloc_chunk*.
*/

typedef struct malloc_chunk* mbinptr;

/* addressing -- note that bin_at(0) does not exist */
#define bin_at(m, i) \
  (mbinptr) (((char *) &((m)->bins[((i) - 1) * 2]))               \
         - offsetof (struct malloc_chunk, fd))

The malloc_chunk struct like this: struct malloc_chunk {

  INTERNAL_SIZE_T      prev_size;  /* Size of previous chunk (if free).  */
  INTERNAL_SIZE_T      size;       /* Size in bytes, including overhead. */

  struct malloc_chunk* fd;         /* double links -- used only if free. */
  struct malloc_chunk* bk;

  /* Only used for large blocks: pointer to next larger size.  */
  struct malloc_chunk* fd_nextsize; /* double links -- used only if free. */
  struct malloc_chunk* bk_nextsize;
};

And the bin type like this: typedef struct malloc_chunk* mbinptr;

struct malloc_state {
  /* Serialize access.  */
  mutex_t mutex;

  /* Flags (formerly in max_fast).  */
  int flags;

#if THREAD_STATS
  /* Statistics for locking.  Only used if THREAD_STATS is defined.  */
  long stat_lock_direct, stat_lock_loop, stat_lock_wait;
#endif

  /* Fastbins */
  mfastbinptr      fastbinsY[NFASTBINS];

  /* Base of the topmost chunk -- not otherwise kept in a bin */
  mchunkptr        top;

  /* The remainder from the most recent split of a small request */
  mchunkptr        last_remainder;

  /* Normal bins packed as described above */
  mchunkptr        bins[NBINS * 2 - 2];

  /* Bitmap of bins */
  unsigned int     binmap[BINMAPSIZE];

  /* Linked list */
  struct malloc_state *next;

#ifdef PER_THREAD
  /* Linked list for free arenas.  */
  struct malloc_state *next_free;
#endif

  /* Memory allocated from the system in this arena.  */
  INTERNAL_SIZE_T system_mem;
  INTERNAL_SIZE_T max_system_mem;
};