系统调用read的过程

<1>

asmlinkage ssize_t sys_read(unsigned int fd, char __user * buf, size_t count)
{
 struct file *file;
 ssize_t ret = -EBADF;
 int fput_needed;

 file = fget_light(fd, &fput_needed);
 if (file) {
  loff_t pos = file_pos_read(file);
  ret = vfs_read(file, buf, count, &pos);
  file_pos_write(file, pos);
  fput_light(file, fput_needed);
 }

 return ret;
}

<2>

ssize_t vfs_read(struct file *file, char __user *buf, size_t count, loff_t *pos)
{
 ssize_t ret;

 if (!(file->f_mode & FMODE_READ))
  return -EBADF;
 if (!file->f_op || (!file->f_op->read && !file->f_op->aio_read))
  return -EINVAL;
 if (unlikely(!access_ok(VERIFY_WRITE, buf, count)))
  return -EFAULT;

 ret = rw_verify_area(READ, file, pos, count);
 if (!ret) {
  ret = security_file_permission (file, MAY_READ);
  if (!ret) {
   if (file->f_op->read)
    ret = file->f_op->read(file, buf, count, pos);
   else
    ret = do_sync_read(file, buf, count, pos);
   if (ret > 0) {
    fsnotify_access(file->f_dentry);
    current->rchar += ret;
   }
   current->syscr++;
  }
 }

 return ret;
}

这样系统就调用的你所写的驱动程序中的read函数！

其他的调用跟这个差不多！