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kfifostruct kfifo *kfifo_init(unsigned char *buffer, unsigned int size, gfp_t gfp_mask, spinlock_t *lock);//给定size分配buffer和kfifostruct 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/**@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 * 1024typedef 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:%lut",stu_info->stu_ID); printf("age:%ut",stu_info->age); printf("score:%un",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: %un", 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: %un", 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:%sn", 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:%sn", 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:%sn", 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;}总结：
len = min(len, fifo->size - fifo->in + fifo->out);
在 len 和 (fifo->size - fifo->in + fifo->out) 之间取一个较小的值赋给len。注意，当 (fifo->in == fifo->out+fifo->size) 时，表示缓冲区已满，此时得到的较小值一定是0，后面实际写入的字节数也全为0。
另一种边界情况是当 len 很大时（因为len是无符号的，负数对它来说也是一个很大的正数），这一句也能保证len取到一个较小的值，因为 fifo->in 总是大于等于 fifo->out ，所以后面的那个表达式 l = min(len, fifo->size - (fifo->in & (fifo->size - 1))); 的值不会超过fifo->size的大小。
smp_mb(); smp_wmb(); 是加内存屏障，这里不是我们讨论的范围，你可以忽略它。
l = min(len, fifo->size - (fifo->in & (fifo->size - 1))); 是把上一步决定的要写入的字节数len “切开”，这里又使用了一个技巧。注意：实际分配给 fifo->buffer 的字节数 fifo->size，必须是2的幂，否则这里就会出错。既然 fifo->size 是2的幂，那么 (fifo->size-1) 也就是一个后面几位全为1的数，也就能保证(fifo->in & (fifo->size - 1)) 总为不超过 (fifo->size - 1) 的那一部分，和 (fifo->in)% (fifo->size - 1) 的效果一样。
这样后面的代码就不难理解了，它先向 fifo->in 到缓冲区末端这一块写数据，如果还没写完，在从缓冲区头开始写入剩下的，从而实现了循环缓冲。最后，把写指针后移 len 个字节，并返回len。
从上面可以看出，fifo->in的值可以从0变化到超过fifo->size的数值，fifo->out也如此，但它们的差不会超过fifo->size。
以上是内存溢出为你收集整理的Linux内核结构体--kfifo 环状缓冲区全部内容，希望文章能够帮你解决Linux内核结构体--kfifo 环状缓冲区所遇到的程序开发问题。
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