转自：http://www.cnblogs.com/Anker/p/3481373.html
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：
1 //根据给定buffer创建一个kfifo
2 struct kfifo *kfifo_init(unsigned char *buffer, unsigned int size, 3 gfp_t gfp_mask, spinlock_t *lock); 4 //给定size分配buffer和kfifo 5 struct kfifo *kfifo_alloc(unsigned int size, gfp_t gfp_mask, 6 spinlock_t *lock); 7 //释放kfifo空间 8 void kfifo_free(struct kfifo *fifo) 9 //向kfifo中添加数据 10 unsigned int kfifo_put(struct kfifo *fifo, 11 const unsigned char *buffer, unsigned int len) 12 //从kfifo中取数据 13 unsigned int kfifo_put(struct kfifo *fifo, 14 const unsigned char *buffer, unsigned int len) 15 //获取kfifo中有数据的buffer大小 16 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：
1 struct kfifo *kfifo_init(unsigned char *buffer, unsigned int size, 2 gfp_t gfp_mask, spinlock_t *lock) 3 { 4 struct kfifo *fifo; 6 /* size must be a power of 2 */ 7 BUG_ON(!is_power_of_2(size)); 9 fifo = kmalloc(sizeof(struct kfifo), gfp_mask); 10 if (!fifo) 11 return ERR_PTR(-ENOMEM); 13 fifo->buffer = buffer; 14 fifo->size = size; 15 fifo->in = fifo->out = 0; 16 fifo->lock = lock; 17 18 return fifo; 19 } 20 struct kfifo *kfifo_alloc(unsigned int size, gfp_t gfp_mask, spinlock_t *lock) 21 { 22 unsigned char *buffer; 23 struct kfifo *ret; 29 if (!is_power_of_2(size)) { 30 BUG_ON(size > 0x80000000); 31 size = roundup_pow_of_two(size); 32 } 34 buffer = kmalloc(size, gfp_mask); 35 if (!buffer) 36 return ERR_PTR(-ENOMEM); 38 ret = kfifo_init(buffer, size, gfp_mask, lock); 39 40 if (IS_ERR(ret)) 41 kfree(buffer); 43 return ret; 44 }
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：
1 static inline unsigned int kfifo_put(struct kfifo *fifo, 2 const unsigned char *buffer, unsigned int len) 3 { 4 unsigned long flags; 5 unsigned int ret; 6 spin_lock_irqsave(fifo->lock, flags); 7 ret = __kfifo_put(fifo, buffer, len); 8 spin_unlock_irqrestore(fifo->lock, flags); 9 return ret; 10 } 11 12 static inline unsigned int kfifo_get(struct kfifo *fifo, 13 unsigned char *buffer, unsigned int len) 14 { 15 unsigned long flags; 16 unsigned int ret; 17 spin_lock_irqsave(fifo->lock, flags); 18 ret = __kfifo_get(fifo, buffer, len); 19 //当fifo->in == fifo->out时，buufer为空 20 if (fifo->in == fifo->out) 21 fifo->in = fifo->out = 0; 22 spin_unlock_irqrestore(fifo->lock, flags); 23 return ret; 24 } 25 26 27 unsigned int __kfifo_put(struct kfifo *fifo, 28 const unsigned char *buffer, unsigned int len) 29 { 30 unsigned int l; 31 //buffer中空的长度 32 len = min(len, fifo->size - fifo->in + fifo->out); 34 /* 35 * Ensure that we sample the fifo->out index -before- we 36 * start putting bytes into the kfifo. 37 */ 39 smp_mb(); 41 /* first put the data starting from fifo->in to buffer end */ 42 l = min(len, fifo->size - (fifo->in & (fifo->size - 1))); 43 memcpy(fifo->buffer + (fifo->in & (fifo->size - 1)), buffer, l); 45 /* then put the rest (if any) at the beginning of the buffer */ 46 memcpy(fifo->buffer, buffer + l, len - l); 47 48 /* 49 * Ensure that we add the bytes to the kfifo -before- 50 * we update the fifo->in index. 51 */ 53 smp_wmb(); 55 fifo->in += len; //每次累加，到达最大值后溢出，自动转为0 57 return len; 58 } 59 60 unsigned int __kfifo_get(struct kfifo *fifo, 61 unsigned char *buffer, unsigned int len) 62 { 63 unsigned int l; 64 //有数据的缓冲区的长度 65 len = min(len, fifo->in - fifo->out); 67 /* 68 * Ensure that we sample the fifo->in index -before- we 69 * start removing bytes from the kfifo. 70 */ 72 smp_rmb(); 74 /* first get the data from fifo->out until the end of the buffer */ 75 l = min(len, fifo->size - (fifo->out & (fifo->size - 1))); 76 memcpy(buffer, fifo->buffer + (fifo->out & (fifo->size - 1)), l); 78 /* then get the rest (if any) from the beginning of the buffer */ 79 memcpy(buffer + l, fifo->buffer, len - l); 81 /* 82 * Ensure that we remove the bytes from the kfifo -before- 83 * we update the fifo->out index. 84 */ 86 smp_mb(); 88 fifo->out += len; //每次累加，到达最大值后溢出，自动转为0 90 return len; 91 }
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：
1 /**@brief 仿照linux kfifo写的ring buffer
2 *@atuher Anker date:2013-12-18 3 * ring_buffer.h 4 * */ 5 6 #ifndef KFIFO_HEADER_H 7 #define KFIFO_HEADER_H 8 9 #include <inttypes.h> 10 #include <string.h> 11 #include <stdlib.h> 12 #include <stdio.h> 13 #include <errno.h> 14 #include <assert.h> 15 16 //判断x是否是2的次方 17 #define is_power_of_2(x) ((x) != 0 && (((x) & ((x) - 1)) == 0)) 18 //取a和b中最小值 19 #define min(a, b) (((a) < (b)) ? (a) : (b)) 20 21 struct ring_buffer 22 { 23 void *buffer; //缓冲区 24 uint32_t size; //大小 25 uint32_t in; //入口位置 26 uint32_t out; //出口位置 27 pthread_mutex_t *f_lock; //互斥锁 28 }; 29 //初始化缓冲区 30 struct ring_buffer* ring_buffer_init(void *buffer, uint32_t size, pthread_mutex_t *f_lock) 31 { 32 assert(buffer); 33 struct ring_buffer *ring_buf = NULL; 34 if (!is_power_of_2(size)) 35 { 36 fprintf(stderr,"size must be power of 2.\n"); 37 return ring_buf; 38 } 39 ring_buf = (struct ring_buffer *)malloc(sizeof(struct ring_buffer)); 40 if (!ring_buf) 41 { 42 fprintf(stderr,"Failed to malloc memory,errno:%u,reason:%s", 43 errno, strerror(errno)); 44 return ring_buf; 45 } 46 memset(ring_buf, 0, sizeof(struct ring_buffer)); 47 ring_buf->buffer = buffer; 48 ring_buf->size = size; 49 ring_buf->in = 0; 50 ring_buf->out = 0; 51 ring_buf->f_lock = f_lock; 52 return ring_buf; 53 } 54 //释放缓冲区 55 void ring_buffer_free(struct ring_buffer *ring_buf) 56 { 57 if (ring_buf) 58 { 59 if (ring_buf->buffer) 60 { 61 free(ring_buf->buffer); 62 ring_buf->buffer = NULL; 63 } 64 free(ring_buf); 65 ring_buf = NULL; 66 } 67 } 68 69 //缓冲区的长度 70 uint32_t __ring_buffer_len(const struct ring_buffer *ring_buf) 71 { 72 return (ring_buf->in - ring_buf->out); 73 } 74 75 //从缓冲区中取数据 76 uint32_t __ring_buffer_get(struct ring_buffer *ring_buf, void * buffer, uint32_t size) 77 { 78 assert(ring_buf || buffer); 79 uint32_t len = 0; 80 size = min(size, ring_buf->in - ring_buf->out); 81 /* first get the data from fifo->out until the end of the buffer */ 82 len = min(size, ring_buf->size - (ring_buf->out & (ring_buf->size - 1))); 83 memcpy(buffer, ring_buf->buffer + (ring_buf->out & (ring_buf->size - 1)), len); 84 /* then get the rest (if any) from the beginning of the buffer */ 85 memcpy(buffer + len, ring_buf->buffer, size - len); 86 ring_buf->out += size; 87 return size; 88 } 89 //向缓冲区中存放数据 90 uint32_t __ring_buffer_put(struct ring_buffer *ring_buf, void *buffer, uint32_t size) 91 { 92 assert(ring_buf || buffer); 93 uint32_t len = 0; 94 size = min(size, ring_buf->size - ring_buf->in + ring_buf->out); 95 /* first put the data starting from fifo->in to buffer end */ 96 len = min(size, ring_buf->size - (ring_buf->in & (ring_buf->size - 1))); 97 memcpy(ring_buf->buffer + (ring_buf->in & (ring_buf->size - 1)), buffer, len); 98 /* then put the rest (if any) at the beginning of the buffer */ 99 memcpy(ring_buf->buffer, buffer + len, size - len); 100 ring_buf->in += size; 101 return size; 102 } 103 104 uint32_t ring_buffer_len(const struct ring_buffer *ring_buf) 105 { 106 uint32_t len = 0; 107 pthread_mutex_lock(ring_buf->f_lock); 108 len = __ring_buffer_len(ring_buf); 109 pthread_mutex_unlock(ring_buf->f_lock); 110 return len; 111 } 112 113 uint32_t ring_buffer_get(struct ring_buffer *ring_buf, void *buffer, uint32_t size) 114 { 115 uint32_t ret; 116 pthread_mutex_lock(ring_buf->f_lock); 117 ret = __ring_buffer_get(ring_buf, buffer, size); 118 //buffer中没有数据 119 if (ring_buf->in == ring_buf->out) 120 ring_buf->in = ring_buf->out = 0; 121 pthread_mutex_unlock(ring_buf->f_lock); 122 return ret; 123 } 124 125 uint32_t ring_buffer_put(struct ring_buffer *ring_buf, void *buffer, uint32_t size) 126 { 127 uint32_t ret; 128 pthread_mutex_lock(ring_buf->f_lock); 129 ret = __ring_buffer_put(ring_buf, buffer, size); 130 pthread_mutex_unlock(ring_buf->f_lock); 131 return ret; 132 } 133 #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：
1 /**@brief ring buffer测试程序，创建两个线程，一个生产者，一个消费者。
2 * 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。 3 *@atuher Anker date:2013-12-18 4 * */ 5 #include "ring_buffer.h" 6 #include <pthread.h> 7 #include <time.h> 8 9 #define BUFFER_SIZE 1024 * 1024 10 11 typedef struct student_info 12 { 13 uint64_t stu_id; 14 uint32_t age; 15 uint32_t score; 16 }student_info; 17 18 19 void print_student_info(const student_info *stu_info) 20 { 21 assert(stu_info); 22 printf("id:%lu\t",stu_info->stu_id); 23 printf("age:%u\t",stu_info->age); 24 printf("score:%u\n",stu_info->score); 25 } 26 27 student_info * get_student_info(time_t timer) 28 { 29 student_info *stu_info = (student_info *)malloc(sizeof(student_info)); 30 if (!stu_info) 31 { 32 fprintf(stderr, "Failed to malloc memory.\n"); 33 return NULL; 34 } 35 srand(timer); 36 stu_info->stu_id = 10000 + rand() % 9999; 37 stu_info->age = rand() % 30; 38 stu_info->score = rand() % 101; 39 print_student_info(stu_info); 40 return stu_info; 41 } 42 43 void * consumer_proc(void *arg) 44 { 45 struct ring_buffer *ring_buf = (struct ring_buffer *)arg; 46 student_info stu_info; 47 while(1) 48 { 49 sleep(2); 50 printf("------------------------------------------\n"); 51 printf("get a student info from ring buffer.\n"); 52 ring_buffer_get(ring_buf, (void *)&stu_info, sizeof(student_info)); 53 printf("ring buffer length: %u\n", ring_buffer_len(ring_buf)); 54 print_student_info(&stu_info); 55 printf("------------------------------------------\n"); 56 } 57 return (void *)ring_buf; 58 } 59 60 void * producer_proc(void *arg) 61 { 62 time_t cur_time; 63 struct ring_buffer *ring_buf = (struct ring_buffer *)arg; 64 while(1) 65 { 66 time(&cur_time); 67 srand(cur_time); 68 int seed = rand() % 11111; 69 printf("******************************************\n"); 70 student_info *stu_info = get_student_info(cur_time + seed); 71 printf("put a student info to ring buffer.\n"); 72 ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info)); 73 printf("ring buffer length: %u\n", ring_buffer_len(ring_buf)); 74 printf("******************************************\n"); 75 sleep(1); 76 } 77 return (void *)ring_buf; 78 } 79 80 int consumer_thread(void *arg) 81 { 82 int err; 83 pthread_t tid; 84 err = pthread_create(&tid, NULL, consumer_proc, arg); 85 if (err != 0) 86 { 87 fprintf(stderr, "Failed to create consumer thread.errno:%u, reason:%s\n", 88 errno, strerror(errno)); 89 return -1; 90 } 91 return tid; 92 } 93 int producer_thread(void *arg) 94 { 95 int err; 96 pthread_t tid; 97 err = pthread_create(&tid, NULL, producer_proc, arg); 98 if (err != 0) 99 { 100 fprintf(stderr, "Failed to create consumer thread.errno:%u, reason:%s\n", 101 errno, strerror(errno)); 102 return -1; 103 } 104 return tid; 105 } 106 107 108 int main() 109 { 110 void * buffer = NULL; 111 uint32_t size = 0; 112 struct ring_buffer *ring_buf = NULL; 113 pthread_t consume_pid, produce_pid; 114 115 pthread_mutex_t *f_lock = (pthread_mutex_t *)malloc(sizeof(pthread_mutex_t)); 116 if (pthread_mutex_init(f_lock, NULL) != 0) 117 { 118 fprintf(stderr, "Failed init mutex,errno:%u,reason:%s\n", 119 errno, strerror(errno)); 120 return -1; 121 } 122 buffer = (void *)malloc(BUFFER_SIZE); 123 if (!buffer) 124 { 125 fprintf(stderr, "Failed to malloc memory.\n"); 126 return -1; 127 } 128 size = BUFFER_SIZE; 129 ring_buf = ring_buffer_init(buffer, size, f_lock); 130 if (!ring_buf) 131 { 132 fprintf(stderr, "Failed to init ring buffer.\n"); 133 return -1; 134 } 135 #if 0 136 student_info *stu_info = get_student_info(638946124); 137 ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info)); 138 stu_info = get_student_info(976686464); 139 ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info)); 140 ring_buffer_get(ring_buf, (void *)stu_info, sizeof(student_info)); 141 print_student_info(stu_info); 142 #endif 143 printf("multi thread test.......\n"); 144 produce_pid = producer_thread((void*)ring_buf); 145 consume_pid = consumer_thread((void*)ring_buf); 146 pthread_join(produce_pid, NULL); 147 pthread_join(consume_pid, NULL); 148 ring_buffer_free(ring_buf); 149 free(f_lock); 150 return 0; 151 }
测试结果如下所示：
4、参考资料
http://blog.csdn.net/linyt/article/details/5764312
