环形队列数据的循环读写

在内存中建立一个环形队列的数据结构，此数据结构遵循先进先出的原则，它是用于对交通灯驱动程序的上半部和下半部之间缓冲数据，其中ring_buf_read()函数用于对环形队列进行读取数据。（本算法来自蒋静《操作系统原理。技术与编程》一书P392页）

#define R_SIGNATURE 0x860510
typedef struct{
long signature;
int *head;
int *tail;
int *end;
int *begin;
int buffer[64];
}ring_buf; //定义一个环形队列数据结构

#include <unistd.h>
#include <linux/kernel.h>
#include "ring.h"
int tag=0;
int * psudo_head=NULL;

void ring_buf_init(ring_buf *round)
{
round->signature=R_SIGNATURE;
round->head=round->buffer;
round->tail=round->buffer;
round->begin=round->buffer;
round->end=&round->buffer[sizeof(round->buffer)-1];
}

/** read the user_defined value from the ring_buffer,
** if no value available 0 is returned, on error -1 is
** returned, on success 1 is returned **/

int ring_buf_read(ring_buf *round,int *val)
{
int* tmp=round->tail;//每次调用该函数时都会将round->tail指针的值赋给tmp，因为round->tail始终指向环形队列中未读数据的地址，这是下面round->tail=tmp语句来实现的

if(round->signature !=R_SIGNATURE){
printk("ring_buf_read:signature fake/n");
return(-1);
}

if((tmp<round->begin)||(tmp>round->end)){
printk("ring_buf_read:tail bad/n");
return(-1);
}

if(tmp==round->head){
return(0);
}

*val=*tmp++;//将*tmp指向的值赋给val,实现数据的读出操作

if(tmp>round->end)
tmp=round->begin;
round->tail=tmp;//保证round->tail始终指向环形队列中未读数据的地址，从而实现循环读取
if(tag){
round->head=psudo_head;
tag=0;
}

return(1);
}

int ring_buf_write(ring_buf *round,int val)
{
int* tmp=round->head;

if(round->signature !=R_SIGNATURE){
printk("ring_buf_write:signature fake/n");
return(-1);
}
if((tmp<round->begin)||(tmp>round->end)){
printk("ring_buf_write:head bad/n");
return(-1);
}

if(tag)
return 0;
(*tmp)=val;
tmp++;

if(tmp>round->end)
tmp = round->begin;

if(tmp==round->tail){
tag=1;
psudo_head=tmp;
return(1);
}
round->head=tmp;
return(1);
}