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