使用无锁队列（环形缓冲区）注意事项
环形缓冲区是生产者和消费者模型中常用的数据结构。生产者将数据放入数组的尾端，而消费者从数组的另一端移走数据，当达到数组的尾部时，生产者绕回到数组的头部。如果只有一个生产者和一个消费者，那么就可以做到免锁访问环形缓冲区（Ring Buffer）。写入索引只允许生产者访问并修改，只要写入者在更新索引之前将新的值保存到缓冲区中，则读者将始终看到一致的数据结构。同理，读取索引也只允许消费者访问并修改。
环形缓冲区实现原理图
如图所示，当读者和写者指针相等时，表明缓冲区是空的，而只要写入指针在读取指针后面时，表明缓冲区已满。
清单 9. 2.6.10 环形缓冲区实现代码
/* * __kfifo_put - puts some data into the FIFO, no locking version * Note that with only one concurrent reader and one concurrent * writer, you don't need extra locking to use these functions. */ unsigned int __kfifo_put(struct kfifo *fifo, unsigned char *buffer, unsigned int len) { unsigned int l; len = min(len, fifo->size - fifo->in + fifo->out); /* 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); fifo->in += len; return len; } /* * __kfifo_get - gets some data from the FIFO, no locking version * Note that with only one concurrent reader and one concurrent * writer, you don't need extra locking to use these functions. */ unsigned int __kfifo_get(struct kfifo *fifo, unsigned char *buffer, unsigned int len) { unsigned int l; len = min(len, fifo->in - fifo->out); /* 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); fifo->out += len; return len; }
需要注意的是
使用ring_buffer_get（kfifo_get）或者ring_buffer_put（kfifo_put）时，如果返回参数与传入参数len不相等时，则操作失败
我们定义一个
//注意student_info 共17字节 按照内存排列占24字节
typedef struct student_info
{
uint64_t stu_id; //8个字节
uint32_t age; //4字节
uint32_t score;//4字节
char sex;//1字节
}student_info;
我们建立一个环形缓冲区，里面只有64字节大小（虽然我们实际使用时大小远大于此），向里面多次存入24字节student_info，看有什么反应
//打印学生信息 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("sex:%d\t",stu_info->sex); 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)); srand(timer); stu_info->stu_id = 10000 + rand() % 9999; stu_info->age = rand() % 30; stu_info->score = rand() % 101; stu_info->sex=rand() % 2; print_student_info(stu_info); return stu_info; } void print_ring_buffer_len(struct ring_buffer *ring_buf) { //用于打印缓冲区长度 uint32_t ring_buf_len = 0; //取得已经使用缓冲区长度 size-ring_buf_len为未使用缓冲区的长度 ring_buf_len=ring_buffer_len(ring_buf); printf("no use ring_buf_len:%d\n",(ring_buf->size-ring_buf_len)); } int main(int argc, char *argv[]) { uint32_t size = 0; //用于判断存储或者取得数据的字节数 uint32_t oklen = 0; struct ring_buffer *ring_buf = NULL; //64字节 size=BUFFER_SIZE; ring_buf = ring_buffer_alloc(size); printf("input student\n"); { student_info *stu_info; student_info stu_temp; uint32_t student_len=sizeof(student_info); printf("ring_buf_len:%d\n",ring_buf->size); printf("student_len:%d\n",student_len); //此时环形缓冲区没有数据我们去取数据当然为空 memset(&stu_temp,0,student_len); oklen=ring_buffer_get(ring_buf, (void *)(&stu_temp), student_len); if(oklen==student_len) { printf("get student data\n"); } else { printf("no student data\n"); } printf("\n"); //第一次调用时用字节结束后还有64-24 =40字节 stu_info = get_student_info(976686458); oklen = ring_buffer_put(ring_buf, (void *)stu_info, student_len); if(oklen==student_len) { printf("1 put student data success\n"); } else { printf("1 put student data failure\n"); } print_ring_buffer_len(ring_buf); printf("\n"); //第二次调用时用字节结束后还有64-48 =16字节 stu_info = get_student_info(976686464); oklen= ring_buffer_put(ring_buf, (void *)stu_info, student_len); if(oklen==student_len) { printf("2 put student data success\n"); } else { printf("2 put student data failure\n"); } print_ring_buffer_len(ring_buf); printf("\n"); //第三次调用时需要用字节但只有字节失败 //把字节都写满了 //验证了在调用__kfifo_put函数或者__kfifo_get函数时，如果返回参数与传入参数len不相等时，则操作失败 stu_info = get_student_info(976686445); oklen= ring_buffer_put(ring_buf, (void *)stu_info, student_len); if(oklen==student_len) { printf("3 put student data success\n"); } else { printf("3 put student data failure\n"); } print_ring_buffer_len(ring_buf); printf("\n"); //第四次调用时需要用字节但无字节 ////验证了在调用__kfifo_put函数或者__kfifo_get函数时，如果返回参数与传入参数len不相等时，则操作失败 stu_info = get_student_info(976686421); oklen= ring_buffer_put(ring_buf, (void *)stu_info, student_len); if(oklen==student_len) { printf("4 put student data success\n"); } else { printf("4 put student data failure\n"); } print_ring_buffer_len(ring_buf); printf("\n"); //现在开始取学生数据里面保存了个学生数据我们取三次看效果 printf("output student\n"); printf("\n"); //第一次取得数据并打印 memset(stu_info,0,student_len); oklen=ring_buffer_get(ring_buf, (void *)stu_info, student_len); if(oklen==student_len) { print_student_info(stu_info); printf("1 get student data success\n"); } else { printf("1 get student data failure\n"); } print_ring_buffer_len(ring_buf); printf("\n"); ////第二次取得数据并打印 memset(stu_info,0,student_len); oklen=ring_buffer_get(ring_buf, (void *)stu_info, student_len); if(oklen==student_len) { print_student_info(stu_info); printf("2 get student data success\n"); } else { printf("2 get student data failure\n"); } print_ring_buffer_len(ring_buf); printf("\n"); //第三次取得数据失败 memset(stu_info,0,student_len); oklen=ring_buffer_get(ring_buf, (void *)stu_info, student_len); if(oklen==student_len) { print_student_info(stu_info); printf("3 get student data success\n"); } else { printf("3 get student data failure\n"); } print_ring_buffer_len(ring_buf); } return 1; }
结论：在使用ring_buffer_get（kfifo_get）或者ring_buffer_put（kfifo_put）时，如果返回参数与传入参数len不相等时，则操作失败。代码下载：tessc.rar（https://files.cnblogs.com/dragonsuc/tessc.rar）
需要注意的地方：
1.只有一个线程负责读，另一个线程负责写的时候，数据是线程安全的。上面的实现是基于这个原理实现的，当有多个线程读或者多个线程写的时候，不保证数据的正确性。
所以使用的时候，一个线程写，一个线程读。网络应用中比较常用，就是开一个线程接口数据，然后把数据写入队列。然后开一个调度线程读取网络数据，然后分发到处理线程。
2.数据长度默认宏定义了一个长度，超过这个长度的时候，后续的数据会写入失败。
本文参考文章：
http://blog.csdn.net/mergerly/article/details/39009473
http://www.cnblogs.com/Anker/p/3481373.html
