А я посмотрел сорцы realloc(), раз уж больше некому.

#if HAVE_MREMAP
      newp = mremap_chunk (oldp, nb);
      if (newp)
    {
      void *newmem = chunk2mem_tag (newp);
      /* Give the new block a different tag.  This helps to ensure
         that stale handles to the previous mapping are not
         reused.  There's a performance hit for both us and the
         caller for doing this, so we might want to
         reconsider.  */
      return tag_new_usable (newmem);
    }
#endif

Правда, это glibc.