Keymap Overview

QMK keymaps are defined inside a C source file. The data structure is an array of arrays. The outer array is a list of layer arrays while the inner layer array is a list of keys. Most keyboards define a KEYMAP() macro to help you create this array of arrays.

Anatomy Of A `keymap.c`

For this example we will walk through the default Clueboard keymap. You'll find it helpful to open that file in another browser window so you can look at everything in context.

There are 3 main sections of a keymap.c file you'll want to concern yourself with:

The Definitions
The Layer/Keymap Datastructure
Custom Functions, if any

Definitions

At the top of the file you'll find this:

#include "clueboard.h"

// Helpful defines
#define GRAVE_MODS  (MOD_BIT(KC_LSHIFT)|MOD_BIT(KC_RSHIFT)|MOD_BIT(KC_LGUI)|MOD_BIT(KC_RGUI)|MOD_BIT(KC_LALT)|MOD_BIT(KC_RALT))
#define _______ KC_TRNS

// Each layer gets a name for readability.
// The underscores don't mean anything - you can
// have a layer called STUFF or any other name.
// Layer names don't all need to be of the same 
// length, and you can also skip them entirely
// and just use numbers.
#define _BL 0
#define _FL 1
#define _CL 2

These are some handy definitions we can use when building our keymap and our custom function. The GRAVE_MODS definition will be used later in our custom function. The _______ define makes it easier to see what keys a layer is overriding, while the _BL, _FL, and _CL defines make it easier to refer to each of our layers.

Layers and Keymaps

The main part of this file is the keymaps[] definition. This is where you list your layers and the contents of those layers. This part of the file begins with this definition:

const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {

After this you'll find a list of KEYMAP() macros. A KEYMAP() is simply a list of keys to define a single layer. Typically you'll have one or more "base layers" (such as QWERTY, Dvorak, or Colemak) and then you'll layer on top of that one or more "function" layers. Due to the way layers are processed you can't overlay a "lower" layer on top of a "higher" layer.

keymaps[][MATRIX_ROWS][MATRIX_COLS] in QMK holds the 16 bit action code (sometimes referred as the quantum keycode) in it. For the keycode representing typical keys, its high byte is 0 and its low byte is the USB HID usage ID for keyboard.

TMK from which QMK was forked uses const uint8_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] instead and holds the 8 bit keycode. Some keycode values are reserved to induce execution of certain action codes via the fn_actions[] array.

Base Layer

Here is an example of the Clueboard's base layer:

  /* Keymap _BL: Base Layer (Default Layer)
   */
[_BL] = KEYMAP(
  F(0),    KC_1,    KC_2,   KC_3,   KC_4,   KC_5,   KC_6,   KC_7,   KC_8,   KC_9,    KC_0,     KC_MINS,  KC_EQL,   KC_GRV,  KC_BSPC,          KC_PGUP, \
  KC_TAB,  KC_Q,    KC_W,   KC_E,   KC_R,   KC_T,   KC_Y,   KC_U,   KC_I,   KC_O,    KC_P,     KC_LBRC,  KC_RBRC,  KC_BSLS,                   KC_PGDN, \
  KC_CAPS, KC_A,    KC_S,   KC_D,   KC_F,   KC_G,   KC_H,   KC_J,   KC_K,   KC_L,    KC_SCLN,  KC_QUOT,  KC_NUHS,  KC_ENT,                             \
  KC_LSFT, KC_NUBS, KC_Z,   KC_X,   KC_C,   KC_V,   KC_B,   KC_N,   KC_M,   KC_COMM, KC_DOT,   KC_SLSH,  KC_RO,    KC_RSFT,          KC_UP,            \
  KC_LCTL, KC_LGUI, KC_LALT, KC_MHEN,          KC_SPC,KC_SPC,                        KC_HENK,  KC_RALT,  KC_RCTL,  MO(_FL), KC_LEFT, KC_DOWN, KC_RGHT),

Some interesting things to note about this:

From a C source point of view it's only a single array, but we have embedded whitespace to more easily visualize where each key is on the physical device.
Plain keyboard scancodes are prefixed with KC_, while "special" keys are not.
The upper left key activates custom function 0 (F(0))
The "Fn" key is defined with MO(_FL), which moves to the _FL layer while that key is being held down.

Function Overlay Layer

Our function layer is, from a code point of view, no different from the base layer. Conceptually, however, you will build that layer as an overlay, not a replacement. For many people this distinction does not matter, but as you build more complicated layering setups it matters more and more.

[_FL] = KEYMAP(
  KC_GRV,  KC_F1,   KC_F2,  KC_F3,  KC_F4,  KC_F5,  KC_F6,  KC_F7,  KC_F8,  KC_F9,   KC_F10,   KC_F11,   KC_F12,   _______, KC_DEL,           BL_STEP, \
  _______, _______, _______,_______,_______,_______,_______,_______,KC_PSCR,KC_SLCK, KC_PAUS,  _______,  _______,  _______,                   _______, \
  _______, _______, MO(_CL),_______,_______,_______,_______,_______,_______,_______, _______,  _______,  _______,  _______,                           \
  _______, _______, _______,_______,_______,_______,_______,_______,_______,_______, _______,  _______,  _______,  _______,          KC_PGUP,         \
  _______, _______, _______, _______,        _______,_______,                        _______,  _______,  _______,  MO(_FL), KC_HOME, KC_PGDN, KC_END),

Some interesting things to note:

We have used our _______ definition to turn KC_TRNS into _______. This makes it easier to spot the keys that have changed on this layer.
While in this layer if you press one of the _______ keys it will activate the key in the next lowest active layer.

Custom Functions

At the bottom of the file we've defined a single custom function. This function defines a key that sends KC_ESC when pressed without modifiers and KC_GRAVE when modifiers are held. There are a couple pieces that need to be in place for this to work, and we will go over both of them.

`fn_actions[]`

We define the fn_actions[] array to point to custom functions. F(N) in a keymap will call element N of that array. For the Clueboard's that looks like this:

const uint16_t PROGMEM fn_actions[] = {
  [0] = ACTION_FUNCTION(0),  // Calls action_function()
};

In this case we've instructed QMK to call the ACTION_FUNCTION callback, which we will define in the next section.

This fn_actions[] interface is mostly for backward compatibility. In QMK, you don't need to use fn_actions[]. You can directly use ACTION_FUNCTION(N) or any other action code value itself normally generated by the macro in keymaps[][MATRIX_ROWS][MATRIX_COLS]. N in F(N) can only be 0 to 31. Use of the action code directly in keymaps unlocks this limitation.

`action_function()`

To actually handle the keypress event we define an action_function(). This function will be called when the key is pressed, and then again when the key is released. We have to handle both situations within our code, as well as determining whether to send/release KC_ESC or KC_GRAVE.

void action_function(keyrecord_t *record, uint8_t id, uint8_t opt) {
  static uint8_t mods_pressed;

  switch (id) {
    case 0:
      /* Handle the combined Grave/Esc key
       */
      mods_pressed = get_mods()&GRAVE_MODS; // Check to see what mods are pressed

      if (record->event.pressed) {
        /* The key is being pressed.
         */
        if (mods_pressed) {
          add_key(KC_GRV);
          send_keyboard_report();
        } else {
          add_key(KC_ESC);
          send_keyboard_report();
        }
      } else {
        /* The key is being released.
         */
        if (mods_pressed) {
          del_key(KC_GRV);
          send_keyboard_report();
        } else {
          del_key(KC_ESC);
          send_keyboard_report();
        }
      }
      break;
  }
}

Nitty Gritty Details

This should have given you a basic overview for creating your own keymap. For more details see the following resources:

https://github.com/qmk/qmk_firmware/blob/master/doc/keymap.md (If you see static const uint8_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS], it is still the TMK code example code.)
https://github.com/qmk/qmk_firmware/wiki/Keycodes
https://github.com/qmk/qmk_firmware/wiki/FAQ-Keymap
https://github.com/qmk/qmk_firmware/wiki/Keymap-examples

We are actively working to improve these docs. If you have suggestions for how they could be made better please file an issue!

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