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这几天在做一些笔试题的时候,遇见了一个关于读写循环缓冲区的题目,然后通过学习,这篇文章大致总结一下对循环buffer的认识。
首先循环buffer是一个可以无限循环读写的缓冲区,上图是一个初始化的循环缓冲区。当然,当循环缓冲区满了的时候是不允许继续写的,应为里面的内容还没被读取,如果满了还继续写的话就会覆盖我们还没读取到的数据,造成数据丢失,然后下面是一个对我们循环缓冲区的初始化
typedef struct ring_buffer{
unsigned char* bufBase; //Base address
int bufLen; //Buffer Len
int writeIdx; //Write position
int readIdx; //Read position
} RING_BUFFER;
当我们对其进行读写的时候,相应的读写指针就会发生偏移,然后因为要求先写再读,就相当于是一个读指针在追着写指针走,因此这两个读写函数的内容应该是大致相同的,而对于如何让这个buffer循环起来,是个难点,这里直接给出我们的读写函数:
int ringBuf_write(RING_BUFFER* pRingBuf, unsigned char* pSrcBuf, unsigned int len)
{
int total;
int i;
total = rb_free_size(pRingBuf);
if(len > total)
len = total;
else
total = len;
i = pRingBuf->writeIdx; //i记录我们写指针的偏移
if(i + len > pRingBuf->bufLen) //判断buffer的容量是否满了
{
memcpy(pRingBuf->bufBase + i, pSrcBuf, pRingBuf->bufLen - i); //将我们的内容直接拷贝到偏移位置将我们的buffer填满
pSrcBuf += pRingBuf->bufLen - i; //获取要拷贝的buf地址
len -= pRingBuf->bufLen - i; //计算多出来的长度
i = 0;
}
memcpy(pRingBuf->bufBase + i, pSrcBuf, len); //将多出来的长度的内容搬到基址开始,从我们的buf中继续写过来,实现循环
pRingBuf->writeIdx = i + len; //移动我们的写指针
return total;
}
int ringBuf_read(RING_BUFFER* pRingBuf, unsigned char* pSrcBuf, unsigned int len)
{
int total;
int i;
total = rb_data_size(pRingBuf);
if(len > total)
len = total;
else
total = len;
i = pRingBuf->readIdx;
if(i + len > pRingBuf->bufLen)
{
memcpy(pSrcBuf, pRingBuf->bufBase + i, pRingBuf->bufLen - i); //将循环buffer中的内容读到我们的buf中。
pSrcBuf += pRingBuf->bufLen - i;
len -= pRingBuf->bufLen - i;
i = 0;
}
memcpy(pSrcBuf, pRingBuf->bufBase + i, len);
pRingBuf->readIdx = i + len;
return total;
}
下面给出完整的代码:
/*********************************************************************************
* Copyright: (C) 2017 tangyanjun<[email protected]>
* All rights reserved.
*
* Filename: ring_buffer.c
* Description: This file
*
* Version: 1.0.0(08/10/2017)
* Author: tangyanjun <[email protected]>
* ChangeLog: 1, Release initial version on "08/10/2017 01:25:49 PM"
*
********************************************************************************/
#include <stdio.h>
#include <string.h>
#include <assert.h>
#include <stdlib.h>
typedef struct ring_buffer{
unsigned char* bufBase; //Base address
int bufLen; //Buffer Len
int writeIdx; //Write position
int readIdx; //Read position
} RING_BUFFER;
void rb_init(RING_BUFFER *ring, unsigned char* buff, int size)
{
memset(ring, 0, sizeof(RING_BUFFER));
ring->readIdx = 0;
ring->writeIdx = 0;
ring->bufBase = buff;
ring->bufLen = size;
}
int rb_data_size(RING_BUFFER* rb)
{
return ((rb->writeIdx - rb->readIdx) & (rb->bufLen - 1));
}
int rb_free_size(RING_BUFFER* rb)
{
return (rb->bufLen - 1 - rb_data_size(rb));
}
int ringBuf_write(RING_BUFFER* pRingBuf, unsigned char* pSrcBuf, unsigned int len)
{
int total;
int i;
total = rb_free_size(pRingBuf);
if(len > total)
len = total;
else
total = len;
i = pRingBuf->writeIdx; //i记录我们写指针的偏移
if(i + len > pRingBuf->bufLen) //判断我们写的字节数是否大于整个buffer长度
{
memcpy(pRingBuf->bufBase + i, pSrcBuf, pRingBuf->bufLen - i); //将我们的内容直接拷贝到偏移位置
pSrcBuf += pRingBuf->bufLen - i;
len -= pRingBuf->bufLen - i;
i = 0;
}
memcpy(pRingBuf->bufBase + i, pSrcBuf, len);
pRingBuf->writeIdx = i + len;
return total;
}
int ringBuf_read(RING_BUFFER* pRingBuf, unsigned char* pSrcBuf, unsigned int len)
{
int total;
int i;
total = rb_data_size(pRingBuf);
if(len > total)
len = total;
else
total = len;
i = pRingBuf->readIdx;
if(i + len > pRingBuf->bufLen)
{
memcpy(pSrcBuf, pRingBuf->bufBase + i, pRingBuf->bufLen - i);
pSrcBuf += pRingBuf->bufLen - i;
len -= pRingBuf->bufLen - i;
i = 0;
}
memcpy(pSrcBuf, pRingBuf->bufBase + i, len);
pRingBuf->readIdx = i + len;
return total;
}
void rb_clear(RING_BUFFER *rb)
{
memset(rb->bufBase, 0, rb->bufLen);
rb->readIdx = 0;
rb->writeIdx = 0;
}
unsigned char rb_peek(RING_BUFFER* rb, int index)
{
assert(index < rb_data_size(rb));
return rb->bufBase[((rb->readIdx + index) % rb->bufLen)];
}
int main(int argc, char **argv)
{
RING_BUFFER* ring = (RING_BUFFER*)malloc(sizeof(RING_BUFFER));
unsigned char buff[16] = "";
unsigned char buff1[20] = "abcdefghijklmnopqrs";
unsigned char buff2[20] = "";
rb_init(ring, buff, sizeof(buff));
printf("初始化后的数据长度:%d\n", rb_data_size(ring));
printf("初始化后的剩余空间:%d\n", rb_free_size(ring));
printf("初始化后的写指针位置:%d\n", ring->writeIdx);
printf("初始化后的读指针位置:%d\n", ring->readIdx);
printf("----------------------------\n");
ringBuf_write(ring, buff1, 8);
printf("写进去的数据:%s\n", ring->bufBase);
printf("数据长度:%d\n", rb_data_size(ring));
printf("剩余空间:%d\n", rb_free_size(ring));
printf("写指针位置:%d\n", ring->writeIdx);
printf("读指针位置:%d\n", ring->readIdx);
printf("-------------------------\n");
ringBuf_write(ring, &buff1[8], 6);
printf("写进去的数据:%s\n", ring->bufBase);
printf("数据长度:%d\n", rb_data_size(ring));
printf("剩余空间:%d\n", rb_free_size(ring));
printf("写指针位置:%d\n", ring->writeIdx);
printf("读指针位置:%d\n", ring->readIdx);
printf("-------------------------\n");
ringBuf_read(ring, buff2, 7);
printf("读的数据:%s\n", buff2);
printf("数据长度:%d\n", rb_data_size(ring));
printf("剩余空间:%d\n", rb_free_size(ring));
printf("写指针位置:%d\n", ring->writeIdx);
printf("读指针位置:%d\n", ring->readIdx);
printf("------------------------\n");
ringBuf_write(ring, buff1, 5);
printf("数据长度:%d\n", rb_data_size(ring));
printf("剩余空间:%d\n", rb_free_size(ring));
printf("写指针位置:%d\n", ring->writeIdx);
printf("读指针位置:%d\n", ring->readIdx);
printf("--------------------\n");
ringBuf_read(ring, buff2, 7);
printf("读的数据:%s\n", buff2);
printf("数据长度:%d\n", rb_data_size(ring));
printf("剩余空间:%d\n", rb_free_size(ring));
printf("写指针位置:%d\n", ring->writeIdx);
printf("读指针位置:%d\n", ring->readIdx);
printf("------------------------\n");
printf("\n");
rb_clear(ring);
free(ring);
return 0;
}
[tangyanjun@VM_216_80_centos ring_buffer]$ a.out
初始化后的数据长度:0
初始化后的剩余空间:15
初始化后的写指针位置:0
初始化后的读指针位置:0
----------------------------
写进去的数据:abcdefgh
数据长度:8
剩余空间:7
写指针位置:8
读指针位置:0
-------------------------
写进去的数据:abcdefghijklmn
数据长度:14
剩余空间:1
写指针位置:14
读指针位置:0
-------------------------
读的数据:abcdefg
数据长度:7
剩余空间:8
写指针位置:14
读指针位置:7
------------------------
数据长度:12
剩余空间:3
写指针位置:3
读指针位置:7
--------------------
这个循环buffer这里只做测试用,根据读写指针的位置变化可以判断我们的循环是否完成。在实际的运用当中和这里不同,下篇文章将运用这个来学习一下PV操作以及 生产者——消费者的操作。