1、前言

最近项目中用到一个环形缓冲区（ring buffer），代码是由linux内核的kfifo改过来的。缓冲区在文件系统中经常用到，通过缓冲区缓解cpu读写内存和读写磁盘的速度。例如一个进程A产生数据发给另外一个进程B，进程B需要对进程A传的数据进行处理并写入文件，如果B没有处理完，则A要延迟发送。为了保证进程A减少等待时间，可以在A和B之间采用一个缓冲区，A每次将数据存放在缓冲区中，B每次冲缓冲区中取。这是典型的生产者和消费者模型，缓冲区中数据满足FIFO特性，因此可以采用队列进行实现。Linux内核的kfifo正好是一个环形队列，可以用来当作环形缓冲区。生产者与消费者使用缓冲区如下图所示：

环形缓冲区的详细介绍及实现方法可以参考
http://en.wikipedia.org/wiki/Circular_buffer，介绍的非常详细，列举了实现环形队列的几种方法。环形队列的不便之处在于如何判断队列是空还是满。维基百科上给三种实现方法。

2、linux 内核kfifo

kfifo设计的非常巧妙，代码很精简，对于入队和出对处理的出人意料。首先看一下kfifo的数据结构：

struct kfifo {
 unsigned char *buffer; /* the buffer holding the data */
 unsigned int size; /* the size of the allocated buffer */
 unsigned int in; /* data is added at offset (in % size) */
 unsigned int out; /* data is extracted from off. (out % size) */
 spinlock_t *lock; /* protects concurrent modifications */
};

kfifo提供的方法有：

//根据给定buffer创建一个kfifo
struct kfifo *kfifo_init(unsigned char *buffer, unsigned int size,
 gfp_t gfp_mask, spinlock_t *lock);
//给定size分配buffer和kfifo
struct kfifo *kfifo_alloc(unsigned int size, gfp_t gfp_mask,
 spinlock_t *lock);
//释放kfifo空间
void kfifo_free(struct kfifo *fifo)
//向kfifo中添加数据
unsigned int kfifo_put(struct kfifo *fifo,
 const unsigned char *buffer, unsigned int len)
//从kfifo中取数据
unsigned int kfifo_put(struct kfifo *fifo,
 const unsigned char *buffer, unsigned int len)
//获取kfifo中有数据的buffer大小
unsigned int kfifo_len(struct kfifo *fifo)

定义自旋锁的目的为了防止多进程/线程并发使用kfifo。因为in和out在每次get和out时，发生改变。初始化和创建kfifo的源代码如下：

struct kfifo *kfifo_init(unsigned char *buffer, unsigned int size,
 gfp_t gfp_mask, spinlock_t *lock)
{
 struct kfifo *fifo;
 /* size must be a power of 2 */
 BUG_ON(!is_power_of_2(size));
 fifo = kmalloc(sizeof(struct kfifo), gfp_mask);
 if (!fifo)
 return ERR_PTR(-ENOMEM);
 fifo->buffer = buffer;
 fifo->size = size;
 fifo->in = fifo->out = 0;
 fifo->lock = lock;
 return fifo;
}
struct kfifo *kfifo_alloc(unsigned int size, gfp_t gfp_mask, spinlock_t *lock)
{
 unsigned char *buffer;
 struct kfifo *ret;
 if (!is_power_of_2(size)) {
 BUG_ON(size > 0x80000000);
 size = roundup_pow_of_two(size);
 }
 buffer = kmalloc(size, gfp_mask);
 if (!buffer)
 return ERR_PTR(-ENOMEM);
 ret = kfifo_init(buffer, size, gfp_mask, lock);
 if (IS_ERR(ret))
 kfree(buffer);
 return ret;
}

在kfifo_init和kfifo_calloc中，kfifo->size的值总是在调用者传进来的size参数的基础上向2的幂扩展，这是内核一贯的做法。这样的好处不言而喻--对kfifo->size取模运算可以转化为与运算，如：kfifo->in % kfifo->size 可以转化为 kfifo->in & (kfifo->size – 1)

kfifo的巧妙之处在于in和out定义为无符号类型，在put和get时，in和out都是增加，当达到最大值时，产生溢出，使得从0开始，进行循环使用。put和get代码如下所示：

static inline unsigned int kfifo_put(struct kfifo *fifo,
 const unsigned char *buffer, unsigned int len)
{
 unsigned long flags;
 unsigned int ret;
 spin_lock_irqsave(fifo->lock, flags);
 ret = __kfifo_put(fifo, buffer, len);
 spin_unlock_irqrestore(fifo->lock, flags);
 return ret;
}
static inline unsigned int kfifo_get(struct kfifo *fifo,
 unsigned char *buffer, unsigned int len)
{
 unsigned long flags;
 unsigned int ret;
 spin_lock_irqsave(fifo->lock, flags);
 ret = __kfifo_get(fifo, buffer, len);
 //当fifo->in == fifo->out时，buufer为空
 if (fifo->in == fifo->out)
 fifo->in = fifo->out = 0;
 spin_unlock_irqrestore(fifo->lock, flags);
 return ret;
}
unsigned int __kfifo_put(struct kfifo *fifo,
 const unsigned char *buffer, unsigned int len)
{
 unsigned int l;
 //buffer中空的长度
 len = min(len, fifo->size - fifo->in + fifo->out);
 /*
 * Ensure that we sample the fifo->out index -before- we
 * start putting bytes into the kfifo.
 */
 smp_mb();
 /* first put the data starting from fifo->in to buffer end */
 l = min(len, fifo->size - (fifo->in & (fifo->size - 1)));
 memcpy(fifo->buffer + (fifo->in & (fifo->size - 1)), buffer, l);
 /* then put the rest (if any) at the beginning of the buffer */
 memcpy(fifo->buffer, buffer + l, len - l);
 /*
 * Ensure that we add the bytes to the kfifo -before-
 * we update the fifo->in index.
 */
 smp_wmb();
 fifo->in += len; //每次累加，到达最大值后溢出，自动转为0
 return len;
}
unsigned int __kfifo_get(struct kfifo *fifo,
 unsigned char *buffer, unsigned int len)
{
 unsigned int l;
 //有数据的缓冲区的长度
 len = min(len, fifo->in - fifo->out);
 /*
 * Ensure that we sample the fifo->in index -before- we
 * start removing bytes from the kfifo.
 */
 smp_rmb();
 /* first get the data from fifo->out until the end of the buffer */
 l = min(len, fifo->size - (fifo->out & (fifo->size - 1)));
 memcpy(buffer, fifo->buffer + (fifo->out & (fifo->size - 1)), l);
 /* then get the rest (if any) from the beginning of the buffer */
 memcpy(buffer + l, fifo->buffer, len - l);
 /*
 * Ensure that we remove the bytes from the kfifo -before-
 * we update the fifo->out index.
 */
 smp_mb();
 fifo->out += len; //每次累加，到达最大值后溢出，自动转为0
 return len;
}

put和get在调用__put和__get过程都进行加锁，防止并发。从代码中可以看出put和get都调用两次memcpy，这针对的是边界条件。例如下图：蓝色表示空闲，红色表示占用。

（1）空的kfifo，

（2）put一个buffer后

（3）get一个buffer后

（4）当此时put的buffer长度超出in到末尾长度时，则将剩下的移到头部去

3、测试程序

仿照kfifo编写一个ring_buffer，现有线程互斥量进行并发控制。设计的ring_buffer如下所示：

/**@brief 仿照linux kfifo写的ring buffer
 *@atuher Anker date:2013-12-18
* ring_buffer.h
 * */
#ifndef KFIFO_HEADER_H 
#define KFIFO_HEADER_H
#include <inttypes.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <assert.h>
//判断x是否是2的次方
#define is_power_of_2(x) ((x) != 0 && (((x) & ((x) - 1)) == 0))
//取a和b中最小值
#define min(a, b) (((a) < (b)) ? (a) : (b))
struct ring_buffer
{
 void *buffer; //缓冲区
 uint32_t size; //大小
 uint32_t in; //入口位置
 uint32_t out; //出口位置
 pthread_mutex_t *f_lock; //互斥锁
};
//初始化缓冲区
struct ring_buffer* ring_buffer_init(void *buffer, uint32_t size, pthread_mutex_t *f_lock)
{
 assert(buffer);
 struct ring_buffer *ring_buf = NULL;
 if (!is_power_of_2(size))
 {
 fprintf(stderr,"size must be power of 2.\n");
 return ring_buf;
 }
 ring_buf = (struct ring_buffer *)malloc(sizeof(struct ring_buffer));
 if (!ring_buf)
 {
 fprintf(stderr,"Failed to malloc memory,errno:%u,reason:%s",
 errno, strerror(errno));
 return ring_buf;
 }
 memset(ring_buf, 0, sizeof(struct ring_buffer));
 ring_buf->buffer = buffer;
 ring_buf->size = size;
 ring_buf->in = 0;
 ring_buf->out = 0;
 ring_buf->f_lock = f_lock;
 return ring_buf;
}
//释放缓冲区
void ring_buffer_free(struct ring_buffer *ring_buf)
{
 if (ring_buf)
 {
 if (ring_buf->buffer)
 {
 free(ring_buf->buffer);
 ring_buf->buffer = NULL;
 }
 free(ring_buf);
 ring_buf = NULL;
 }
}
//缓冲区的长度
uint32_t __ring_buffer_len(const struct ring_buffer *ring_buf)
{
 return (ring_buf->in - ring_buf->out);
}
//从缓冲区中取数据
uint32_t __ring_buffer_get(struct ring_buffer *ring_buf, void * buffer, uint32_t size)
{
 assert(ring_buf || buffer);
 uint32_t len = 0;
 size = min(size, ring_buf->in - ring_buf->out); 
 /* first get the data from fifo->out until the end of the buffer */
 len = min(size, ring_buf->size - (ring_buf->out & (ring_buf->size - 1)));
 memcpy(buffer, ring_buf->buffer + (ring_buf->out & (ring_buf->size - 1)), len);
 /* then get the rest (if any) from the beginning of the buffer */
 memcpy(buffer + len, ring_buf->buffer, size - len);
 ring_buf->out += size;
 return size;
}
//向缓冲区中存放数据
uint32_t __ring_buffer_put(struct ring_buffer *ring_buf, void *buffer, uint32_t size)
{
 assert(ring_buf || buffer);
 uint32_t len = 0;
 size = min(size, ring_buf->size - ring_buf->in + ring_buf->out);
 /* first put the data starting from fifo->in to buffer end */
 len = min(size, ring_buf->size - (ring_buf->in & (ring_buf->size - 1)));
 memcpy(ring_buf->buffer + (ring_buf->in & (ring_buf->size - 1)), buffer, len);
 /* then put the rest (if any) at the beginning of the buffer */
 memcpy(ring_buf->buffer, buffer + len, size - len);
 ring_buf->in += size;
 return size;
}
uint32_t ring_buffer_len(const struct ring_buffer *ring_buf)
{
 uint32_t len = 0;
 pthread_mutex_lock(ring_buf->f_lock);
 len = __ring_buffer_len(ring_buf);
 pthread_mutex_unlock(ring_buf->f_lock);
 return len;
}
uint32_t ring_buffer_get(struct ring_buffer *ring_buf, void *buffer, uint32_t size)
{
 uint32_t ret;
 pthread_mutex_lock(ring_buf->f_lock);
 ret = __ring_buffer_get(ring_buf, buffer, size);
 //buffer中没有数据
 if (ring_buf->in == ring_buf->out)
 ring_buf->in = ring_buf->out = 0;
 pthread_mutex_unlock(ring_buf->f_lock);
 return ret;
}
uint32_t ring_buffer_put(struct ring_buffer *ring_buf, void *buffer, uint32_t size)
{
 uint32_t ret;
 pthread_mutex_lock(ring_buf->f_lock);
 ret = __ring_buffer_put(ring_buf, buffer, size);
 pthread_mutex_unlock(ring_buf->f_lock);
 return ret;
}
#endif

采用多线程模拟生产者和消费者编写测试程序，如下所示：

/**@brief ring buffer测试程序，创建两个线程，一个生产者，一个消费者。
 * 生产者每隔1秒向buffer中投入数据，消费者每隔2秒去取数据。
 *@atuher Anker date:2013-12-18
 * */
#include "ring_buffer.h"
#include <pthread.h>
#include <time.h>
#define BUFFER_SIZE 1024 * 1024
typedef struct student_info
{
 uint64_t stu_id;
 uint32_t age;
 uint32_t score;
}student_info;
void print_student_info(const student_info *stu_info)
{
 assert(stu_info);
 printf("id:%lu\t",stu_info->stu_id);
 printf("age:%u\t",stu_info->age);
 printf("score:%u\n",stu_info->score);
}
student_info * get_student_info(time_t timer)
{
 student_info *stu_info = (student_info *)malloc(sizeof(student_info));
 if (!stu_info)
 {
 fprintf(stderr, "Failed to malloc memory.\n");
 return NULL;
 }
 srand(timer);
 stu_info->stu_id = 10000 + rand() % 9999;
 stu_info->age = rand() % 30;
 stu_info->score = rand() % 101;
 print_student_info(stu_info);
 return stu_info;
}
void * consumer_proc(void *arg)
{
 struct ring_buffer *ring_buf = (struct ring_buffer *)arg;
 student_info stu_info; 
 while(1)
 {
 sleep(2);
 printf("------------------------------------------\n");
 printf("get a student info from ring buffer.\n");
 ring_buffer_get(ring_buf, (void *)&stu_info, sizeof(student_info));
 printf("ring buffer length: %u\n", ring_buffer_len(ring_buf));
 print_student_info(&stu_info);
 printf("------------------------------------------\n");
 }
 return (void *)ring_buf;
}
void * producer_proc(void *arg)
{
 time_t cur_time;
 struct ring_buffer *ring_buf = (struct ring_buffer *)arg;
 while(1)
 {
 time(&cur_time);
 srand(cur_time);
 int seed = rand() % 11111;
 printf("******************************************\n");
 student_info *stu_info = get_student_info(cur_time + seed);
 printf("put a student info to ring buffer.\n");
 ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info));
 printf("ring buffer length: %u\n", ring_buffer_len(ring_buf));
 printf("******************************************\n");
 sleep(1);
 }
 return (void *)ring_buf;
}
int consumer_thread(void *arg)
{
 int err;
 pthread_t tid;
 err = pthread_create(&tid, NULL, consumer_proc, arg);
 if (err != 0)
 {
 fprintf(stderr, "Failed to create consumer thread.errno:%u, reason:%s\n",
 errno, strerror(errno));
 return -1;
 }
 return tid;
}
int producer_thread(void *arg)
{
 int err;
 pthread_t tid;
 err = pthread_create(&tid, NULL, producer_proc, arg);
 if (err != 0)
 {
 fprintf(stderr, "Failed to create consumer thread.errno:%u, reason:%s\n",
 errno, strerror(errno));
 return -1;
 }
 return tid;
}
int main()
{
 void * buffer = NULL;
 uint32_t size = 0;
 struct ring_buffer *ring_buf = NULL;
 pthread_t consume_pid, produce_pid;
 pthread_mutex_t *f_lock = (pthread_mutex_t *)malloc(sizeof(pthread_mutex_t));
 if (pthread_mutex_init(f_lock, NULL) != 0)
 {
 fprintf(stderr, "Failed init mutex,errno:%u,reason:%s\n",
 errno, strerror(errno));
 return -1;
 }
 buffer = (void *)malloc(BUFFER_SIZE);
 if (!buffer)
 {
 fprintf(stderr, "Failed to malloc memory.\n");
 return -1;
 }
 size = BUFFER_SIZE;
 ring_buf = ring_buffer_init(buffer, size, f_lock);
 if (!ring_buf)
 {
 fprintf(stderr, "Failed to init ring buffer.\n");
 return -1;
 }
#if 0
 student_info *stu_info = get_student_info(638946124);
 ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info));
 stu_info = get_student_info(976686464);
 ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info));
 ring_buffer_get(ring_buf, (void *)stu_info, sizeof(student_info));
 print_student_info(stu_info);
#endif
 printf("multi thread test.......\n");
 produce_pid = producer_thread((void*)ring_buf);
 consume_pid = consumer_thread((void*)ring_buf);
 pthread_join(produce_pid, NULL);
 pthread_join(consume_pid, NULL);
 ring_buffer_free(ring_buf);
 free(f_lock);
 return 0;
}

测试结果如下所示：