TopSemic · 2021年12月17日

环形队列到底是怎么回事?

前两天看到一位朋友转发了公众号:嵌入式Linux 作者发哥的一篇C语言环形队列文章:C语言,环形队列,写的非常好。做过实际项目的朋友大概率都用到过队列,它可用于解决生产者消费者产出和消费不平衡的问题,举两个我之前项目中遇到的案例:

1) 我需要在一块屏幕上显示多种事件,而事件的来源很多、触发时间也很快,但是屏幕由于资源限制,没法把所有事件都同步显示。这时就可以使用队列,将事件插入到队列中,显示程序读取队列中的事件逐条显示。

2) 我在之前文章:一个蓝牙实战项目的掏肺总结 里提到的那个蓝牙收发器,蓝牙芯片一方面接收手机发过来的数据,另一方面要把该数据通过USB 发送出去,但是USB发送数据的间隔又要求比较长,这也可以通过队列来解决。

C++里有现成的队列函数,但是C语言需要自己来实现,我之前项目里用到了Github上的一个代码,https://github.com/kuaileguyu... ,简洁好用。那时就是用,因为大概知道原理,就没有认真的去分析代码的实现细节,这次刚好看到发哥文章下有很多关于环形队列的留言讨论,我就详细的去看了一下之前用的代码,发现收获还是不小的,在此分享给大家。

先上代码:

ringbuffer.h

#include <inttypes.h>

/**
 * @file
 * Prototypes and structures for the ring buffer module.
 */

#ifndef RINGBUFFER_H
#define RINGBUFFER_H

/**
 * The size of a ring buffer.
 * Due to the design only <tt> RING_BUFFER_SIZE-1 </tt> items
 * can be contained in the buffer.
 * The buffer size must be a power of two.
*/
#define RING_BUFFER_SIZE 128

#if (RING_BUFFER_SIZE & (RING_BUFFER_SIZE - 1)) != 0
#error "RING_BUFFER_SIZE must be a power of two"
#endif

/**
 * The type which is used to hold the size
 * and the indicies of the buffer.
 * Must be able to fit \c RING_BUFFER_SIZE .
 */
typedef uint8_t ring_buffer_size_t;

/**
 * Used as a modulo operator
 * as <tt> a % b = (a & (b − 1)) </tt>
 * where \c a is a positive index in the buffer and
 * \c b is the (power of two) size of the buffer.
 */
#define RING_BUFFER_MASK (RING_BUFFER_SIZE-1)

/**
 * Simplifies the use of <tt>struct ring_buffer_t</tt>.
 */
typedef struct ring_buffer_t ring_buffer_t;

/**
 * Structure which holds a ring buffer.
 * The buffer contains a buffer array
 * as well as metadata for the ring buffer.
 */
struct ring_buffer_t {
  /** Buffer memory. */
  char buffer[RING_BUFFER_SIZE];
  /** Index of tail. */
  ring_buffer_size_t tail_index;
  /** Index of head. */
  ring_buffer_size_t head_index;
};

/**
 * Initializes the ring buffer pointed to by <em>buffer</em>.
 * This function can also be used to empty/reset the buffer.
 * @param buffer The ring buffer to initialize.
 */
void ring_buffer_init(ring_buffer_t *buffer);

/**
 * Adds a byte to a ring buffer.
 * @param buffer The buffer in which the data should be placed.
 * @param data The byte to place.
 */
void ring_buffer_queue(ring_buffer_t *buffer, char data);

/**
 * Adds an array of bytes to a ring buffer.
 * @param buffer The buffer in which the data should be placed.
 * @param data A pointer to the array of bytes to place in the queue.
 * @param size The size of the array.
 */
void ring_buffer_queue_arr(ring_buffer_t *buffer, const char *data, ring_buffer_size_t size);

/**
 * Returns the oldest byte in a ring buffer.
 * @param buffer The buffer from which the data should be returned.
 * @param data A pointer to the location at which the data should be placed.
 * @return 1 if data was returned; 0 otherwise.
 */
uint8_t ring_buffer_dequeue(ring_buffer_t *buffer, char *data);

/**
 * Returns the <em>len</em> oldest bytes in a ring buffer.
 * @param buffer The buffer from which the data should be returned.
 * @param data A pointer to the array at which the data should be placed.
 * @param len The maximum number of bytes to return.
 * @return The number of bytes returned.
 */
uint8_t ring_buffer_dequeue_arr(ring_buffer_t *buffer, char *data, ring_buffer_size_t len);
/**
 * Peeks a ring buffer, i.e. returns an element without removing it.
 * @param buffer The buffer from which the data should be returned.
 * @param data A pointer to the location at which the data should be placed.
 * @param index The index to peek.
 * @return 1 if data was returned; 0 otherwise.
 */
uint8_t ring_buffer_peek(ring_buffer_t *buffer, char *data, ring_buffer_size_t index);


/**
 * Returns whether a ring buffer is empty.
 * @param buffer The buffer for which it should be returned whether it is empty.
 * @return 1 if empty; 0 otherwise.
 */
inline uint8_t ring_buffer_is_empty(ring_buffer_t *buffer) {
  return (buffer->head_index == buffer->tail_index);
}

/**
 * Returns whether a ring buffer is full.
 * @param buffer The buffer for which it should be returned whether it is full.
 * @return 1 if full; 0 otherwise.
 */
inline uint8_t ring_buffer_is_full(ring_buffer_t *buffer) {
  return ((buffer->head_index - buffer->tail_index) & RING_BUFFER_MASK) == RING_BUFFER_MASK;
}

/**
 * Returns the number of items in a ring buffer.
 * @param buffer The buffer for which the number of items should be returned.
 * @return The number of items in the ring buffer.
 */
inline ring_buffer_size_t ring_buffer_num_items(ring_buffer_t *buffer) {
  return ((buffer->head_index - buffer->tail_index) & RING_BUFFER_MASK);
}

#endif /* RINGBUFFER_H */

ringbuffer.c

#include "ringbuffer.h"

/**
 * @file
 * Implementation of ring buffer functions.
 */

void ring_buffer_init(ring_buffer_t *buffer) {
  buffer->tail_index = 0;
  buffer->head_index = 0;
}

void ring_buffer_queue(ring_buffer_t *buffer, char data) {
  /* Is buffer full? */
  if(ring_buffer_is_full(buffer)) {
    /* Is going to overwrite the oldest byte */
    /* Increase tail index */
    buffer->tail_index = ((buffer->tail_index + 1) & RING_BUFFER_MASK);
  }

  /* Place data in buffer */
  buffer->buffer[buffer->head_index] = data;
  buffer->head_index = ((buffer->head_index + 1) & RING_BUFFER_MASK);
}

void ring_buffer_queue_arr(ring_buffer_t *buffer, const char *data, ring_buffer_size_t size) {
  /* Add bytes; one by one */
  ring_buffer_size_t i;
  for(i = 0; i < size; i++) {
    ring_buffer_queue(buffer, data[i]);
  }
}

ring_buffer_size_t ring_buffer_dequeue(ring_buffer_t *buffer, char *data) {
  if(ring_buffer_is_empty(buffer)) {
    /* No items */
    return 0;
  }
  
  *data = buffer->buffer[buffer->tail_index];
  buffer->tail_index = ((buffer->tail_index + 1) & RING_BUFFER_MASK);
  return 1;
}

ring_buffer_size_t ring_buffer_dequeue_arr(ring_buffer_t *buffer, char *data, ring_buffer_size_t len) {
  if(ring_buffer_is_empty(buffer)) {
    /* No items */
    return 0;
  }

  char *data_ptr = data;
  ring_buffer_size_t cnt = 0;
  while((cnt < len) && ring_buffer_dequeue(buffer, data_ptr)) {
    cnt++;
    data_ptr++;
  }
  return cnt;
}

ring_buffer_size_t ring_buffer_peek(ring_buffer_t *buffer, char *data, ring_buffer_size_t index) {
  if(index >= ring_buffer_num_items(buffer)) {
    /* No items at index */
    return 0;
  }
  
  /* Add index to pointer */
  ring_buffer_size_t data_index = ((buffer->tail_index + index) & RING_BUFFER_MASK);
  *data = buffer->buffer[data_index];
  return 1;
}

extern inline uint8_t ring_buffer_is_empty(ring_buffer_t *buffer);
extern inline uint8_t ring_buffer_is_full(ring_buffer_t *buffer);
extern inline uint8_t ring_buffer_num_items(ring_buffer_t *buffer);

代码加起来只有200行,我就不逐条的去分析了,结合以下几个具体的问题重点讲解下。

问题1:如何初始化环形队列?

回答:只要定义一个ring_buffer_t类型的结构体,然后调用ring_buffer_init()函数初始化即可。

 ring_buffer_t ring_buffer;
  ring_buffer_init(&ring_buffer);

问题2:队列长度如何设置?

回答:在ringbuffer.h中下述宏定义设置,注意长度修改时要确认ring_buffer_size_t的类型uint8_t是否可以满足,否则应该修改此类型。

 #define RING_BUFFER_SIZE 128

问题3:为什么环形队列长度必须是2的n次方?

回答:因为在判断队列是否为满的时候,用到了RING_BUFFER_MASK,而RING_BUFFER_MASK的值为RING_BUFFER_SIZE-1,这个MASK为二进制全1,所以长度是2的n次方。

 inline uint8_t ring_buffer_is_full(ring_buffer_t *buffer) {
  return ((buffer->head_index - buffer->tail_index) & RING_BUFFER_MASK) == RING_BUFFER_MASK;
}

问题4:代码是如何判断队列为满的?满了以后会怎么样?

回答:在ring_buffer_is_full函数中,用head_index减去tail_index,根据这个值来判断。

head_index 的值是一直++的,从0一直加到RING_BUFFER_MASK,然后再回到0继续加(形成一个环形)。只在调用ring_buffer_queue()函数入队列的时候它才会变化,buffer->head_index = ((buffer->head_index + 1) & RING_BUFFER_MASK);

tail_index则不同,在调用ring_buffer_dequeue()函数出队列的时候会++,buffer->tail_index = ((buffer->tail_index + 1) & RING_BUFFER_MASK); 注意:在ring_buffer_queue()函数里,当判断队列为满的时候,它也会加。

  if(ring_buffer_is_full(buffer)) {
    /* Is going to overwrite the oldest byte */
    /* Increase tail index */
    buffer->tail_index = ((buffer->tail_index + 1) & RING_BUFFER_MASK);
  }

这个怎么理解呢?也就是说在入队列的时候,当判断到队列已满,head_index即将又回到tail_index位置时,这时就会把tail_index向前推一下,这样的效果是最初进入队列的那个数据被新入队列的数据覆盖掉了,再也没有机会被取出来了。

回过头再来看判断是否为满的这条语句,

 return ((buffer->head_index - buffer->tail_index) & RING_BUFFER_MASK) == RING_BUFFER_MASK;

可以体会到它的写法之高妙,head_index 是可能比 tail_index大,也可能比tail_index小的,

以一个长度为4的队列为例,假设一直入队列,没有出队列,整个过程如下:

a41b840e88306142afeb3460e76b85a5.png

2839b4a868c4bf28a7bc89a3c061e1f6.png

可以看到当head_index =3、tail_index =0或head_index =0、tail_index =1或head_index =1、tail_index =2或head_index =2、tail_index =3 这几种情况,都是队列为满的状态。

END

作者:wuyage
来源:https://mp.weixin.qq.com/s/v4DagsffP1nCOYlCk_1NXQ
aijishu_TopSemic_1.jpg

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