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Document for Developers

The functions of Karabiner by Developer's perspective

  • Change the key event. (For example, "Change Right Option to Enter")
  • Change the consumer key event. (The consumer key is the functional key such as "Volume Down")
  • Change the pointing event. (For example, "MiddleClick to Expose")
  • The key repeat function.
  • Add private setting dynamically by private.xml.
  • Filtering by application or input source or specific device, etc.
  • Support to "Fast User Switching".
  • Check software updates automatically

How to build


Main components

  • Karabiner.kext (Kernel Extension)
  • (Userspace Application)

Source code dependencies

+- src/bridge/generator/keycode
  +- src/bridge/include/bridge.h
    +- src/lib/xml_compiler
      +- src/bin/dump_xml_compiler_result
        +- src/bridge/generator/config
          +- src/core/kext
          +- src/core/server

("src/core/server" depends on "src/bridge/generator/config", ...)

About event handling (kext)

To prevent leakage of the input event, the modification of the input event is perfomed inside kernel entirely.

We set a hook function in the middle of hardware driver. And we modify the input event in the hook function.


The sequence of event handling

Karabiner and Seil will modify input events at different timing.

Karabiner: Seil:

  1. Device sends the input event.
  2. Modify the input event by Seil.
  3. Modify the input event by "System Preferences > Keyboard > Modifier Keys..."
  4. Modify the input event by Karabiner.
  5. Send the input event to Applications.

Threads and Lock (kext)

We use a global lock to guard the status of modifier flags, the status of buttons, and other global variables. All threads use this global lock. The only one thread runs at the same time.


  • Timer callback (*_timer_callback)
  • hooked function (EventInputQueue::push_*)
  • gIOMatchedNotification and gIOTerminatedNotification callback of input devices (Core::*Notification_callback)
  • IOUserClient callback (UserClient_kext::callback_*)

Files and Directories Overview

A selected list to orient you...

  • shell script to create a signed .dmg file.
  • version: current version number string
  • Tests/: tests based on gtest (Google C++ Testing Framework).
  • files/: files and configurations for making OS/X applications
    • files/extra: shell scripts for packaging used by
  • pkginfo/: more package tools, including setting the version number.
  • src/:
    • src/vendor/blueutil: command line control of Bluetooth
    • src/core/server/: main server code, what you see as the UI.
      • .../Frameworks/Sparkle.framework: modified Sparkle upgrader
      • .../Resources/include/checkbox: long ChangeKey list items
    • src/core/kext: main kernal extension magic
  • util/uncrustify: code beautifier