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DEVELOPMENT.md

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Core Processes

  • karabiner_grabber
    • Run with root privilege.
    • Seize the input devices and modify events then post events using Karabiner-VirtualHIDDevice.
  • karabiner_console_user_server
    • Run with console user privilege.
    • Monitor system preferences values (key repeat, etc) and notify them to karabiner_grabber.
    • Monitor a karabiner configuration file and notify changes to karabiner_grabber.
    • karabiner_grabber seizes devices only when karabiner_console_user_server is running.

program sequence

start up

karabiner_grabber

  1. Run karabiner_grabber.
  2. karabiner_grabber opens grabber server unix domain socket.
  3. karabiner_grabber start polling the session state.
  4. When session state is changed, karabiner_grabber changes the unix domain socket owner to console user.

device grabbing

  1. Run karabiner_console_user_server.
  2. Try to open console_user_server unix domain socket.
  3. grabber seizes input devices.

Other notes

IOHIDSystem requires the process is running with the console user privilege. Thus, karabiner_grabber cannot send events to IOHIDSystem directly.

The difference of event grabbing methods

IOKit

IOKit allows you to read raw HID input events from kernel.
The highest layer is IOHIDQueue which provides us the HID values.

karabiner_grabber uses this method.

CGEventTapCreate

CGEventTapCreate is a limited approach.
It does not work with Secure Keyboard Entry even if we use kCGHIDEventTap and root privillege.
Thus, it does not work in Terminal.
You can confirm this behavior in appendix/eventtap.

However, we should modify mouse event's modifier flags manually. Thus, karabiner_grabber uses this method to modify mouse events.

The difference of event posting methods

IOKit call IOHIKeyboard::dispatchKeyboardEvent in kext

It requires posting HID usage page and usage. karabiner_grabber uses this method by using Karabiner-VirtualHIDDevice.

IOKit device report in kext

It requires posting HID events.
The IOHIKeyboard processes the reports by passing reports to handleReport.

IOHIDPostEvent

It requires posting coregraphics events.

IOHIDPostEvent will be failed if the process is not running in the current session user. (The root user is also forbidden.)

CGEventPost

It requires posting coregraphics events.

CGEventPost does not support some key events in OS X 10.12.

  • Mission Control key
  • Launchpad key
  • Option-Command-Escape

Thus, karabiner_grabber does not use CGEventPost.

IOKit device value in kext

It requires posting HID events.
We have to make a complete set of virtual devices to post the IOHIDValue.

Modifier flags handling in kernel

The modifier flag events are handled in the following sequence in macOS 10.12.

  1. Receive HID reports from device.
  2. Treat reports in the keyboard device driver.
  3. Treat flags in accessibility functions. (eg. sticky keys, zoom)
  4. Treat flags in mouse events.
  5. Treat flags in IOHIDSystem.
  6. Treat flags in Coregraphics.

Thus, IOHIDPostEvent will be ignored in accessibility functions and mouse events.

About hid reports

We can get hid reports from devices via IOHIDDeviceRegisterInputReportCallback.
The hid report contains a list of pressed keys, so it seems suitable information to observe.

But karabiner_grabber does not use it in order to reduce the device dependancy.

The limitation of device reports

Apple devices reports

Apple keyboards does not use generic HID keyboard report descriptor.
Thus, we have to handle them by separate way.

Generic HID keyboard report descriptor

uint8_t modifiers;
uint8_t reserved;
uint8_t keys[6];
modifiers bit:
0x1 << 0 : left control
0x1 << 1 : left shift
0x1 << 2 : left option
0x1 << 3 : left command
0x1 << 4 : right control
0x1 << 5 : right shift
0x1 << 6 : right option
0x1 << 7 : right command

Apple HID keyboard report descriptor

uint8_t record_id;
uint8_t modifiers;
uint8_t reserved;
uint8_t keys[6];
uint8_t extra_modifiers; // fn