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..