struct file_operations {
  struct module *owner;
  loff_t (*llseek) (struct file *, loff_t, int);
  ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
  ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
  ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
  ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
  int (*readdir) (struct file *, void *, filldir_t);
  unsigned int (*poll) (struct file *, struct poll_table_struct *);
  long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
  long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
  int (*mmap) (struct file *, struct vm_area_struct *);
  int (*open) (struct inode *, struct file *);
  int (*flush) (struct file *, fl_owner_t id);
  int (*release) (struct inode *, struct file *);
  int (*fsync) (struct file *, loff_t, loff_t, int datasync);
  int (*aio_fsync) (struct kiocb *, int datasync);
  int (*fasync) (int, struct file *, int);
  int (*lock) (struct file *, int, struct file_lock *);
  ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
  unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
  int (*check_flags)(int);
  int (*flock) (struct file *, int, struct file_lock *);
  ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
  ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
  int (*setlease)(struct file *, long, struct file_lock **);
  long (*fallocate)(struct file *file, int mode, loff_t offset,
     loff_t len);
  int (*show_fdinfo)(struct seq_file *m, struct file *f);
};

struct file_operations_vfs
{
  /* The device driver open method differs from the mountpoint open method */
  int     (*open)(FAR struct file *filep);
  /* The following methods must be identical in signature and position because
   * the struct file_operations and struct mountp_operations are treated like
   * unions.
   */
  int     (*close)(FAR struct file *filep);
  ssize_t (*read)(FAR struct file *filep, FAR char *buffer, size_t buflen);
  ssize_t (*write)(FAR struct file *filep, FAR const char *buffer, size_t buflen);
  off_t   (*seek)(FAR struct file *filep, off_t offset, int whence);
  int     (*ioctl)(FAR struct file *filep, int cmd, unsigned long arg);
  int     (*mmap)(FAR struct file* filep, struct VmMapRegion *region);
  /* The two structures need not be common after this point */
#ifndef CONFIG_DISABLE_POLL
  int     (*poll)(FAR struct file *filep, poll_table *fds);
#endif
  int     (*unlink)(FAR struct inode *inode);
};

static struct file_operations hello_drv = {
  .owner  = THIS_MODULE,
  .open    = hello_drv_open,
  .read    = hello_drv_read,
  .write   = hello_drv_write,
  .release = hello_drv_close,
};
int major = register_chrdev(0, "hello", &hello_drv);  /* /dev/hello */
static struct class *hello_class = class_create(THIS_MODULE, "hello_class");
device_create(hello_class, NULL, MKDEV(major, 0), NULL, "hello"); /* /dev/hello */

static const struct file_operations_vfs g_helloDevOps = {
    .open   = hello_open,
    .close  = hello_close,
    .read   = hello_read,
    .write  = NULL,
    .seek   = NULL,
    .ioctl  = NULL,
    .mmap   = NULL,
    .unlink = NULL,
};
int ret = register_driver("/dev/hello", &g_helloDevOps, 0666, NULL);

int main(int argc, char **argv)
{
  int fd;
  char buf[1024];
  int len;
  /* 1. 判断参数 */
  if (argc < 2) 
  {
  printf("Usage: %s -w <string>\n", argv[0]);
  printf("       %s -r\n", argv[0]);
  return -1;
  }
  /* 2. 打开文件 */
  fd = open("/dev/hello", O_RDWR);
  if (fd == -1)
  {
  printf("can not open file /dev/hello\n");
  return -1;
  }
  /* 3. 写文件或读文件 */
  if ((0 == strcmp(argv[1], "-w")) && (argc == 3))
  {
  len = strlen(argv[2]) + 1;
  len = len < 1024 ? len : 1024;
  write(fd, argv[2], len);
  }
  else
  {
  len = read(fd, buf, 1024);  
  buf[1023] = '\0';
  printf("APP read : %s\n", buf);
  }
  close(fd);
  return 0;
}