Character Device Basics (Livehacking Screenplay)

Major and Minor Numbers

• register_chrdev_region() in init(), and unregister_chrdev_region() in exit()

  • Caveat: kernel development just happens. The following methods predate the cdev API; still used by old drivers.

    • Not register_chrdev() - that does cdev_add() implicitly

    • Not unregister_chrdev() - that leaves the major number registered, at least.

  • Hardcode major and minor numbers to (1,1) initially ⟶ error in logs (KERN_ERR)

  • Emphasize on error handling (return -EINVAL)

  • Take (42,1), finally works

    • See how major shows up in /proc/devices

Character Device

• cdev_init(), cdev_add() (in init())

• cdev_del() (in exit())

• Empty file_operations ⟶ later

• Create node …

  # mknod my_driver_henkel c 42 0
  

  • cat my_driver_henkel ⟶ EINVAL

  • strace cat my_driver_henkel ⟶ Aha