static unsigned int XXX_poll(struct file *filp, poll_table *wait)  
{  
    unsigned int mask = 0;  
    struct XXX_dev *dev = filp->private_data;     //获得设备结构指针 
    ...  
    poll_wait(filp, &dev->r_wait, wait);    //加读等待对列头  
    poll_wait(filp ,&dev->w_wait, wait);    //加写等待队列头  
    if(...)//可读
    { 
       POLLIN | POLLRDNORM;    //标识数据可获得  
    }  
    if(...)//可写  
    {
          mask |= POLLOUT | POLLWRNORM;    //标识数据可写入  
    }  
    ..  
    return mask;  
}          

#include <linux/module.h>  
#include <linux/types.h>  
#include <linux/fs.h>  
#include <linux/errno.h>  
#include <linux/mm.h>  
#include <linux/sched.h>  
#include <linux/init.h>  
#include <linux/cdev.h>  
#include <asm/io.h>  
#include <asm/system.h>  
#include <asm/uaccess.h>  
  
#include <linux/poll.h>  
#include "memdev.h"  
  
static mem_major = MEMDEV_MAJOR;  
bool have_data = false; /*表明设备有足够数据可供读*/  
  
module_param(mem_major, int, S_IRUGO);  
  
struct mem_dev *mem_devp; /*设备结构体指针*/  
  
struct cdev cdev;   
  
/*文件打开函数*/  
int mem_open(struct inode *inode, struct file *filp)  
{  
    struct mem_dev *dev;  
      
    /*获取次设备号*/  
    int num = MINOR(inode->i_rdev);  
  
    if (num >= MEMDEV_NR_DEVS)   
            return -ENODEV;  
    dev = &mem_devp[num];  
      
    /*将设备描述结构指针赋值给文件私有数据指针*/  
    filp->private_data = dev;  
      
    return 0;   
}  
  
/*文件释放函数*/  
int mem_release(struct inode *inode, struct file *filp)  
{  
  return 0;  
}  
  
/*读函数*/  
static ssize_t mem_read(struct file *filp, char __user *buf, size_t size, loff_t *ppos)  
{  
  unsigned long p =  *ppos;  
  unsigned int count = size;  
  int ret = 0;  
  struct mem_dev *dev = filp->private_data; /*获得设备结构体指针*/  
  
  /*判断读位置是否有效*/  
  if (p >= MEMDEV_SIZE)  
    return 0;  
  if (count > MEMDEV_SIZE - p)  
    count = MEMDEV_SIZE - p;  
      
  while (!have_data) /* 没有数据可读，考虑为什么不用if，而用while */  
  {  
        if (filp->f_flags & O_NONBLOCK)  
            return -EAGAIN;  
      
    wait_event_interruptible(dev->inq,have_data);  
  }  
  
  /*读数据到用户空间*/  
  if (copy_to_user(buf, (void*)(dev->data + p), count))  
  {  
    ret =  - EFAULT;  
  }  
  else  
  {  
    *ppos += count;  
    ret = count;  
     
    printk(KERN_INFO "read %d bytes(s) from %d\n", count, p);  
  }  
    
  have_data = false; /* 表明不再有数据可读 */  
  /* 唤醒写进程 */  
  return ret;  
}  
  
/*写函数*/  
static ssize_t mem_write(struct file *filp, const char __user *buf, size_t size, loff_t *ppos)  
{  
  unsigned long p =  *ppos;  
  unsigned int count = size;  
  int ret = 0;  
  struct mem_dev *dev = filp->private_data; /*获得设备结构体指针*/  
    
  /*分析和获取有效的写长度*/  
  if (p >= MEMDEV_SIZE)  
    return 0;  
  if (count > MEMDEV_SIZE - p)  
    count = MEMDEV_SIZE - p;  
  
  /*从用户空间写入数据*/  
  if (copy_from_user(dev->data + p, buf, count))  
    ret =  - EFAULT;  
  else  
  {  
    *ppos += count;  
    ret = count;  
      
    printk(KERN_INFO "written %d bytes(s) from %d\n", count, p);  
  }  
    
  have_data = true; /* 有新的数据可读 */  
      
    /* 唤醒读进程 */  
    wake_up(&(dev->inq));  
  
  return ret;  
}  
  
/* seek文件定位函数 */  
static loff_t mem_llseek(struct file *filp, loff_t offset, int whence)  
{   
    loff_t newpos;  
  
    switch(whence) {  
      case 0: /* SEEK_SET */  
        newpos = offset;  
        break;  
  
      case 1: /* SEEK_CUR */  
        newpos = filp->f_pos + offset;  
        break;  
  
      case 2: /* SEEK_END */  
        newpos = MEMDEV_SIZE -1 + offset;  
        break;  
  
      default: /* can‘t happen */  
        return -EINVAL;  
    }  
    if ((newpos<0) || (newpos>MEMDEV_SIZE))  
        return -EINVAL;  
          
    filp->f_pos = newpos;  
    return newpos;  
  
}  
unsigned int mem_poll(struct file *filp, poll_table *wait)  
{  
    struct mem_dev  *dev = filp->private_data;   
    unsigned int mask = 0;  
      
   /*将等待队列添加到poll_table */  
    poll_wait(filp, &dev->inq,  wait);  
   
      
    if (have_data)         mask |= POLLIN | POLLRDNORM;  /* readable */  
  
    return mask;  
}  
  
  
/*文件操作结构体*/  
static const struct file_operations mem_fops =  
{  
  .owner = THIS_MODULE,  
  .llseek = mem_llseek,  
  .read = mem_read,  
  .write = mem_write,  
  .open = mem_open,  
  .release = mem_release,  
  .poll = mem_poll,  
};  
  
/*设备驱动模块加载函数*/  
static int memdev_init(void)  
{  
  int result;  
  int i;  
  
  dev_t devno = MKDEV(mem_major, 0);  
  
  /* 静态申请设备号*/  
  if (mem_major)  
    result = register_chrdev_region(devno, 2, "memdev");  
  else  /* 动态分配设备号 */  
  {  
    result = alloc_chrdev_region(&devno, 0, 2, "memdev");  
    mem_major = MAJOR(devno);  
  }    
    
  if (result < 0)  
    return result;  
  
  /*初始化cdev结构*/  
  cdev_init(&cdev, &mem_fops);  
  cdev.owner = THIS_MODULE;  
  cdev.ops = &mem_fops;  
    
  /* 注册字符设备 */  
  cdev_add(&cdev, MKDEV(mem_major, 0), MEMDEV_NR_DEVS);  
     
  /* 为设备描述结构分配内存*/  
  mem_devp = kmalloc(MEMDEV_NR_DEVS * sizeof(struct mem_dev), GFP_KERNEL);  
  if (!mem_devp)    /*申请失败*/  
  {  
    result =  - ENOMEM;  
    goto fail_malloc;  
  }  
  memset(mem_devp, 0, sizeof(struct mem_dev));  
    
  /*为设备分配内存*/  
  for (i=0; i < MEMDEV_NR_DEVS; i++)   
  {  
        mem_devp[i].size = MEMDEV_SIZE;  
        mem_devp[i].data = kmalloc(MEMDEV_SIZE, GFP_KERNEL);  
        memset(mem_devp[i].data, 0, MEMDEV_SIZE);  
    
      /*初始化等待队列*/  
     init_waitqueue_head(&(mem_devp[i].inq));  
     //init_waitqueue_head(&(mem_devp[i].outq));  
  }  
     
  return 0;  
  
  fail_malloc:   
  unregister_chrdev_region(devno, 1);  
    
  return result;  
}  
  
/*模块卸载函数*/  
static void memdev_exit(void)  
{  
  cdev_del(&cdev);   /*注销设备*/  
  kfree(mem_devp);     /*释放设备结构体内存*/  
  unregister_chrdev_region(MKDEV(mem_major, 0), 2); /*释放设备号*/  
}  
  
MODULE_AUTHOR("David Xie");  
MODULE_LICENSE("GPL");  
  
module_init(memdev_init);  
module_exit(memdev_exit);  

#include <stdio.h>  
  
int main()  
{  
    FILE *fp = NULL;  
    char Buf[128];  
      
      
    /*打开设备文件*/  
    fp = fopen("/dev/memdev0","r+");  
    if (fp == NULL)  
    {  
        printf("Open Dev memdev Error!\n");  
        return -1;  
    }  
      
    /*写入设备*/  
    strcpy(Buf,"memdev is char dev!");  
    printf("Write BUF: %s\n",Buf);  
    fwrite(Buf, sizeof(Buf), 1, fp);  
      
    sleep(5);  
    fclose(fp);  
      
    return 0;      
  
}  

#include <stdio.h>  
#include <stdlib.h>  
#include <unistd.h>  
#include <sys/ioctl.h>  
#include <sys/types.h>  
#include <sys/stat.h>  
#include <fcntl.h>  
#include <sys/select.h>  
#include <sys/time.h>  
#include <errno.h>  
  
int main()  
{  
    int fd;  
    fd_set rds;  
    int ret;  
    char Buf[128];  
      
    /*初始化Buf*/  
    strcpy(Buf,"memdev is char dev!");  
    printf("BUF: %s\n",Buf);  
      
    /*打开设备文件*/  
    fd = open("/dev/memdev0",O_RDWR);  
      
    FD_ZERO(&rds);  
    FD_SET(fd, &rds);  
  
    /*清除Buf*/  
    strcpy(Buf,"Buf is NULL!");  
    printf("Read BUF1: %s\n",Buf);  
  
    ret = select(fd + 1, &rds, NULL, NULL, NULL);  
    if (ret < 0)   
    {  
        printf("select error!\n");  
        exit(1);  
    }  
    if (FD_ISSET(fd, &rds))   
        read(fd, Buf, sizeof(Buf));              
      
    /*检测结果*/  
    printf("Read BUF2: %s\n",Buf);  
      
    close(fd);  
      
    return 0;      
}