高级字符驱动之堵塞与非堵塞IO

/**
*此实例涉及到线程的挂起与竞态，字符IO的堵塞与非堵塞
*/

struct scull_pipe {
wait_queue_head_t inp, outp;
char *buffer, *end;
char *rp, wp;
//int buffersize;
int nreaders, nwriters;
struct fasync_struct *async_queue;
struct semaphore sem;
struct cdev cdev;
};

/**
* container_of - cast a member of a structure out to the containing structure
* @ptr:    the pointer to the member.
* @type:   the type of the container struct this is embedded in.
* @member: the name of the member within the struct.
* /inlcude/linux/kernel.h
*
*/
/*#define container_of(ptr, type, member) ({            \
const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
(type *)( (char *)__mptr - offsetof(type,member) );})
*/

static ssize_t scull_p_read (struct file *filp, char __user *buffer, size_t count, loff_t *f_pos)
{
/*得到指向结构体scull_pipe的指针*/
/*
(void *private_data)private_data指针是file结构体中的成员之一，
它是跨系统挑用是保存状态信息的非常有用的资源,这里我们
在open阶段使用container_of函数得到指向结构体scull_pipe的指针并将private_data指向它
记住要在release方法中释放其内存
*/

struct scull_pipe *dev = filp->private_data;

/*使用信号量限制在同一时刻只有一个线程执行*/
/*若被中断则返回非零值，若无法撤销任何用户可见的修改return -    EINTR*/
if (down_interruptible(&dev->sem))
return -ERESTARTSYS;

while (dev->rp == dev->wp)
{
up_interruptible(&dev->sem);

/*判断用户是否设置了非堵塞IO*/
if (filp->f_flags & O_NONBLOCK)
return -EAGAIN;

/*将线程挂起*/
if (wait_event_interruptible(&dev->inp, dev->rp != dev->wp))
return -ERESTARTSYS;

if (down_interruptible(&dev->sem))
return -ERESTARTSYS;
}

/*min函数在Linux内核/include/linux/kernel.h中*/
if (dev->wp > dev->rp)
count = min(count, (size_t) (dev->wp - dev->rp));
else
count = min(count, (size_t) (dev->end - dev->rp));

if (copy_to_user(buffer, dev->rp, count))
{
up_interruptible(&dev->sem);
return -EFAULT;
}

dev->rp += count;
if (dev->rp == dev->end)
dev->rp = dev->buffer

up_interruptible(&dev->sem);
wake_up_interruptible(&dev->outp);
return count;
}

/*个人仿照上方写的管道写操作代码*/
static ssize_t scull_p_write (struct file *filp, const char __user *buffer, size_t count, loff_t *f_pos)
{
struct scull_pipe *dev = filp->private_data;

if (down_interruptible(&dev->sem))
return -ERESTARTSYS;

/*判断条件有所不同*/
while (strlen(dev->buffer) && dev->rp == dev->wp)
{
up_interruptible(&dev->sem);

if (filp->f_flags & O_NONBLOCK)
return -EAGAIN;

if (wait_event_interruptible(&dev->outp))
return -ERESTARTSYS;

down_interruptible(&dev->sem);
}

if (dev->wp > dev->rp)
count = min(count, (size_t) (dev->end - dev->wp));
else
count = min(count, (size_t) (dev->rp - dev->wp - 1));

if (copy_form_user(dev->wp, buffer, count))
{
up_interruptible(&dev->sem);
return -EFAULT;
}

dev->wp += count;
if (dev->wp == dev->end)
dev->wp = dev->buffer;

up_interruptible(&dev->sem);
wake_up_interruptible(&dev->inp);
return count;
}