Linux Device Drivers, Second Edition

Seeking a Device

The difficult part of the chapter is over; now we’ll quickly detail the llseek method, which is useful and easy to implement.

The llseek Implementation

The llseek method implements the lseek and llseek system calls. We have already stated that if the llseek method is missing from the device’s operations, the default implementation in the kernel performs seeks from the beginning of the file and from the current position by modifying filp->f_pos, the current reading/writing position within the file. Please note that for the lseek system call to work correctly, the read and write methods must cooperate by updating the offset item they receive as argument (the argument is usually a pointer to filp->f_pos).

You may need to provide your own llseek method if the seek operation corresponds to a physical operation on the device or if seeking from end-of-file, which is not implemented by the default method, makes sense. A simple example can be seen in the scull driver:

loff_t scull_llseek(struct file *filp, loff_t off, int whence)
{
  Scull_Dev *dev = filp->private_data;
  loff_t newpos;

  switch(whence) {
   case 0: /* SEEK_SET */
    newpos = off;
    break;

   case 1: /* SEEK_CUR */
    newpos = filp->f_pos + off;
    break;

   case 2: /* SEEK_END */
    newpos = dev->size + off;
    break;

   default: /* can't happen */
    return -EINVAL;
  }
  if (newpos<0) return -EINVAL;
  filp->f_pos = newpos;
  return newpos;
}

The only device-specific operation here is retrieving the file length from the device. In scull