以某个值作为对齐粒度向上求整的算法

static inline void *
ptr_align (void const *ptr, size_t alignment)
{
  char const *p0 = ptr;
  char const *p1 = p0 + alignment - 1;
  return (void *) (p1 - (size_t) p1 % alignment);
}

      对齐粒度  由 alignment 指定