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.

Keymap and layers

In QMK, const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] holds multiple layers of keymap information in 16 bit data holding the action code. You can define 32 layers at most.

For trivial key definitions, the higher 8 bits of the action code are all 0 and the lower 8 bits holds the USB HID usage code generated by the key as keycode.

Respective layers can be validated simultaneously. Layers are indexed with 0 to 31 and higher layer has precedence.

Keymap: 32 Layers                   Layer: action code matrix
-----------------                   ---------------------
stack of layers                     array_of_action_code[row][column]
       ____________ precedence               _______________________
      /           / | high                  / ESC / F1  / F2  / F3   ....
  31 /___________// |                      /-----/-----/-----/-----
  30 /___________// |                     / TAB /  Q  /  W  /  E   ....
  29 /___________/  |                    /-----/-----/-----/-----
   :   _:_:_:_:_:__ |               :   /LCtrl/  A  /  S  /  D   ....
   :  / : : : : : / |               :  /  :     :     :     :
   2 /___________// |               2 `--------------------------
   1 /___________// |               1 `--------------------------
   0 /___________/  V low           0 `--------------------------

Sometimes, the action code stored in keymap may be referred as keycode in some documents due to the TMK history.

Keymap layer status

Keymap layer has its state in two 32 bit parameters:

default_layer_state indicates a base keymap layer(0-31) which is always valid and to be referred.
layer_state () has current on/off status of the layer on its each bit.

Keymap has its state in two parameter default_layer indicates a base keymap layer(0-31) which is always valid and to be referred, keymap_stat is 16bit variable which has current on/off status of layers on its each bit. Keymap layer '0' is usually default_layer and which is the only valid layer and other layers is initially off after boot up firmware, though, you can configured them in config.h. To change default_layer will be useful when you switch key layout completely, say you want Colmak instead of Qwerty.

Initial state of Keymap          Change base layout              
-----------------------          ------------------              

  31                               31
  30                               30
  29                               29
   :                                :
   :                                :   ____________
   2   ____________                 2  /           /
   1  /           /              ,->1 /___________/
,->0 /___________/               |  0
|                                |
`--- default_layer = 0           `--- default_layer = 1
     layer_state   = 0x00000001       layer_state   = 0x00000002

On the other hand, you shall change layer_state to overlay base layer with some layers for feature such as navigation keys, function key(F1-F12), media keys or special actions.

Overlay feature layer
---------------------      bit|status
       ____________        ---+------
  31  /           /        31 |   0
  30 /___________// -----> 30 |   1
  29 /___________/  -----> 29 |   1
   :                        : |   :
   :   ____________         : |   :
   2  /           /         2 |   0
,->1 /___________/  ----->  1 |   1
|  0                        0 |   0
|                                 +
`--- default_layer = 1            |
     layer_state   = 0x60000002 <-'

Layer Precedence and Transparency

Note that higher layer has higher priority on stack of layers, namely firmware falls down from top layer to bottom to look up keycode. Once it spots keycode other than KC_TRNS(transparent) on a layer it stops searching and lower layers aren't referred.

You can place KC_TRANS on overlay layer changes just part of layout to fall back on lower or base layer. Key with KC_TRANS (KC_TRNS and _______ are the alias) doesn't has its own keycode and refers to lower valid layers for keycode, instead.

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;
  }
}