Linux内核开发之异步通知与异步I/O(二)

“曾经有一份真挚的爱情摆在面前，我却不懂珍惜；曾经有一个承诺，我却倍感珍惜，今天一定要好好讲讲..”

讲讲啥，讲讲上节说的那个异步通知的例子呗，大家喜欢看代码，咋们就先上代码:

struct globalfifo_dev                                     
{                                                        
  struct cdev cdev; /*cdev结构体*/                       
  unsigned int current_len;    /*fifo有效数据长度*/
  unsigned char mem[GLOBALFIFO_SIZE]; /*全局内存*/        
  struct semaphore sem; /*并发控制用的信号量*/           
  wait_queue_head_t r_wait; /*阻塞读用的等待队列头*/     
  wait_queue_head_t w_wait; /*阻塞写用的等待队列头*/    
  struct fasync_struct *async_queue; /* 异步结构体指针，用于读 */ 
};
/*文件释放函数*/
int globalfifo_release(struct inode *inode, struct file *filp)
{
	/* 将文件从异步通知列表中删除 */
  globalmem_fasync( - 1, filp, 0);
  return 0;
}

static int globalfifo_fasync(int fd, struct file *filp, int mode)
{
	struct globalfifo_dev *dev = filp->private_data; 
	return fasync_helper(fd, filp, mode, &dev->async_queue);
}

/*globalfifo写操作*/
static ssize_t globalfifo_write(struct file *filp, const char __user *buf,
  size_t count, loff_t *ppos)
{
  struct globalfifo_dev *dev = filp->private_data; //获得设备结构体指针
  int ret;
  DECLARE_WAITQUEUE(wait, current); //定义等待队列

  down(&dev->sem); //获取信号量
  add_wait_queue(&dev->w_wait, &wait); //进入写等待队列头

  /* 等待FIFO非满 */
  if (dev->current_len == GLOBALFIFO_SIZE)
  {
    if (filp->f_flags &O_NONBLOCK)
    //如果是非阻塞访问
    {
      ret =  - EAGAIN;
      goto out;
    } 
    __set_current_state(TASK_INTERRUPTIBLE); //改变进程状态为睡眠
    up(&dev->sem);

    schedule(); //调度其他进程执行
    if (signal_pending(current))
    //如果是因为信号唤醒
    {
      ret =  - ERESTARTSYS;
      goto out2;
    }

    down(&dev->sem); //获得信号量
  }

  /*从用户空间拷贝到内核空间*/
  if (count > GLOBALFIFO_SIZE - dev->current_len)
    count = GLOBALFIFO_SIZE - dev->current_len;

  if (copy_from_user(dev->mem + dev->current_len, buf, count))
  {
    ret =  - EFAULT;
    goto out;
  }
  else
  {
    dev->current_len += count;
    printk(KERN_INFO "written %d bytes(s),current_len:%d\n", count, dev
      ->current_len);

    wake_up_interruptible(&dev->r_wait); //唤醒读等待队列
    /* 产生异步读信号 */
    if (dev->async_queue)
       kill_fasync(&dev->async_queue, SIGIO, POLL_IN);
    
    ret = count;
  }

  out: up(&dev->sem); //释放信号量
  out2:remove_wait_queue(&dev->w_wait, &wait); //从附属的等待队列头移除
  set_current_state(TASK_RUNNING);
  return ret;
}

下面再给出测试程序：

#include ...
//接收到异步读信号的动作
void input_handler(int signum)
{
    printf("Receive a signal from globalfifo,signalnum:%d\n",signum);
}

int main()
{
  int fd, oflags;
  fd = open("/dev/globalfifo", O_RDWR, S_IRUSR | S_IWUSR);
  if (fd !=  - 1)
  {
       //启动信号驱动机制
    signal(SIGIO, input_handler); //让input_handler()处理SIGIO信号
    fcntl(fd, F_SETOWN, getpid());
    oflags = fcntl(fd, F_GETFL);
    fcntl(fd, F_SETFL, oflags | FASYNC);
    while(1)
    {
    	sleep(100);
    }
  }
  else
  {
    printf("device open failure\n");
  }
}

当我们加载完驱动并创建完设备节点后，运行上述程序，每当通过echo向/dev/globalfilfo写入新的数据后，input_handler将会被调用。如下所示：

echo 0>/dev/globalfifo

receive a signal from globalfifo ,signalnum:29

 

echo 0>/dev/globalfifo

receive a signal from globalfifo ,signalnum:29

 

echo 0>/dev/globalfifo

receive a signal from globalfifo ,signalnum:29

通过上边实际的例子，小王，明白了吧，我的承诺也兑现了，下次咱们可要开始更高级的东西了..