5 RINGBUFF里镜像索引取反是为什么？

发布于 2021-10-16 22:45:30 浏览：1240 订阅该版

请教一下，环形队列里 为什么函数中让镜像索引按位取反不是一个很大的值吗？
比如 read_mirror =8,取反的值不是65527吗 这个值和镜像有什么关系？
```c
rt_size_t rt_ringbuffer_put_force(struct rt_ringbuffer *rb,
                            const rt_uint8_t     *ptr,
                            rt_uint16_t           length)
{
    rt_uint16_t space_length;

RT_ASSERT(rb != RT_NULL);

space_length = rt_ringbuffer_space_len(rb);

if (length > rb->buffer_size)
    {
        ptr = &ptr[length - rb->buffer_size];
        length = rb->buffer_size;
    }

if (rb->buffer_size - rb->write_index > length)
    {
        /* read_index - write_index = empty space */
        memcpy(&rb->buffer_ptr[rb->write_index], ptr, length);
        /* this should not cause overflow because there is enough space for
         * length of data in current mirror */
        rb->write_index += length;

if (length > space_length)
            rb->read_index = rb->write_index;

return length;
    }

memcpy(&rb->buffer_ptr[rb->write_index],
           &ptr[0],
           rb->buffer_size - rb->write_index);
    memcpy(&rb->buffer_ptr[0],
           &ptr[rb->buffer_size - rb->write_index],
           length - (rb->buffer_size - rb->write_index));

/* we are going into the other side of the mirror */
    rb->write_mirror = ~rb->write_mirror;
    rb->write_index = length - (rb->buffer_size - rb->write_index);

if (length > space_length)
    {
        rb->read_mirror = ~rb->read_mirror;
        rb->read_index = rb->write_index;
    }

return length;
}
```