Key Overrides
Key overrides allow you to override modifier-key combinations to send a different modifier-key combination or perform completely custom actions. Don't want shift + 1 to type ! on your computer? Use a key override to make your keyboard type something different when you press shift + 1. The general behavior is like this: If modifiers w + key x are pressed, replace these keys with modifiers y + key z in the keyboard report.
You can use key overrides in a similar way to momentary layer/fn keys to activate custom keycodes/shortcuts, with a number of benefits: You completely keep the original use of the modifier keys, while being able to save space by removing fn keys from your keyboard. You can also easily configure combinations of modifiers to trigger different actions than individual modifiers, and much more. The possibilities are quite vast and this documentation contains a few examples for inspiration throughout.
A few more examples to get started: You could use key overrides to...
    Send brightness up/down when pressing ctrl + volume up/down.
    Send delete when pressing shift + backspace.
    Create custom shortcuts or change existing ones: E.g. Send ctrl+shift+z when ctrl+y is pressed.
    Run custom code when ctrl + alt + esc is pressed.

Setup

To enable this feature, you need to add KEY_OVERRIDE_ENABLE = yes to your rules.mk.
Then, in your keymap.c file, you'll need to define the array key_overrides, which defines all key overrides to be used. Each override is a value of type key_override_t. The array key_overrides is NULL-terminated and contains pointers to key_override_t values (const key_override_t **).

Creating Key Overrides

The key_override_t struct has many options that allow you to precisely tune your overrides. The full reference is shown below. Instead of manually creating a key_override_t value, it is recommended to use these dedicated initializers:

ko_make_basic(modifiers, key, replacement)

Returns a key_override_t, which sends replacement (can be a key-modifer combination), when key and modifiers are all pressed down. This override still activates if any additional modifiers not specified in modifiers are also pressed down. See ko_make_with_layers_and_negmods to customize this behavior.

ko_make_with_layers(modifiers, key, replacement, layers)

Additionally takes a bitmask layers that defines on which layers the override is used.

ko_make_with_layers_and_negmods(modifiers, key, replacement, layers, negative_mods)

Additionally takes a bitmask negative_mods that defines which modifiers may not be pressed for this override to activate.

ko_make_with_layers_negmods_and_options(modifiers, key, replacement, layers, negative_mods, options)

Additionally takes a bitmask options that specifies additional options. See ko_option_t for available options.
For more customization possibilities, you may directly create a key_override_t, which allows you to customize even more behavior. Read further below for details and examples.

Simple Example

This shows how the mentioned example of sending delete when shift + backspace are pressed is realized:
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const key_override_t delete_key_override = ko_make_basic(MOD_MASK_SHIFT, KC_BSPACE, KC_DELETE);
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// This globally defines all key overrides to be used
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const key_override_t **key_overrides = (const key_override_t *[]){
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&delete_key_override,
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NULL // Null terminate the array of overrides!
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};
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Intermediate Difficulty Examples

Media Controls & Screen Brightness

In this example a single key is configured to control media, volume and screen brightness by using key overrides.
    The key is set to send play/pause in the keymap.
The following key overrides will be configured:
    Ctrl + play/pause will send next track.
    Ctrl + Shift + play/pause will send previous track.
    Alt + play/pause will send volume up.
    Alt + Shift + play/pause will send volume down.
    Ctrl + Alt + play/pause will send brightness up.
    Ctrl + Alt + Shift + play/pause will send brightness down.
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const key_override_t next_track_override =
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ko_make_with_layers_negmods_and_options(
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MOD_MASK_CTRL, // Trigger modifiers: ctrl
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KC_MPLY, // Trigger key: play/pause
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KC_MNXT, // Replacement key
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~0, // Activate on all layers
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MOD_MASK_SA, // Do not activate when shift or alt are pressed
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ko_option_no_reregister_trigger); // Specifies that the play key is not registered again after lifting ctrl
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const key_override_t prev_track_override = ko_make_with_layers_negmods_and_options(MOD_MASK_CS, KC_MPLY,
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KC_MPRV, ~0, MOD_MASK_ALT, ko_option_no_reregister_trigger);
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const key_override_t vol_up_override = ko_make_with_layers_negmods_and_options(MOD_MASK_ALT, KC_MPLY,
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KC_VOLU, ~0, MOD_MASK_CS, ko_option_no_reregister_trigger);
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const key_override_t vol_down_override = ko_make_with_layers_negmods_and_options(MOD_MASK_SA, KC_MPLY,
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KC_VOLD, ~0, MOD_MASK_CTRL, ko_option_no_reregister_trigger);
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const key_override_t brightness_up_override = ko_make_with_layers_negmods_and_options(MOD_MASK_CA, KC_MPLY,
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KC_BRIU, ~0, MOD_MASK_SHIFT, ko_option_no_reregister_trigger);
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const key_override_t brightness_down_override = ko_make_basic(MOD_MASK_CSA, KC_MPLY, KC_BRID);
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// This globally defines all key overrides to be used
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const key_override_t **key_overrides = (const key_override_t *[]){
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&next_track_override,
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&prev_track_override,
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&vol_up_override,
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&vol_down_override,
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&brightness_up_override,
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&brightness_down_override,
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NULL
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};
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Flexible macOS-friendly Grave Escape

The Grave Escape feature is limited in its configurability and has bugs when used on macOS. Key overrides can be used to achieve a similar functionality as Grave Escape, but with more customization and without bugs on macOS.
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// Shift + esc = ~
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const key_override_t tilde_esc_override = ko_make_basic(MOD_MASK_SHIFT, KC_ESC, S(KC_GRAVE));
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// GUI + esc = `
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const key_override_t grave_esc_override = ko_make_basic(MOD_MASK_GUI, KC_ESC, KC_GRAVE);
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const key_override_t **key_overrides = (const key_override_t *[]){
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&tilde_esc_override,
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&grave_esc_override,
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NULL
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};
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In addition to not encountering unexpected bugs on macOS, you can also change the behavior as you wish. Instead setting GUI + ESC = ` you may change it to an arbitrary other modifier, for example Ctrl + ESC = ` .

Advanced Examples

Modifiers as Layer Keys

Do you really need a dedicated key to toggle your fn layer? With key overrides, perhaps not. This example shows how you can configure to use rGUI + rAlt (right GUI and right alt) to access a momentary layer like an fn layer. With this you completely eliminate the need to use a dedicated layer key. Of course the choice of modifier keys can be changed as needed, rGUI + rAlt is just an example here.
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// This is called when the override activates and deactivates. Enable the fn layer on activation and disable on deactivation
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bool momentary_layer(bool key_down, void *layer) {
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if (key_down) {
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layer_on((uint8_t)(uintptr_t)layer);
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} else {
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layer_off((uint8_t)(uintptr_t)layer);
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}
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return false;
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}
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const key_override_t fn_override = {.trigger_mods = MOD_BIT(KC_RGUI) | MOD_BIT(KC_RCTL), //
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.layers = ~(1 << LAYER_FN), //
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.suppressed_mods = MOD_BIT(KC_RGUI) | MOD_BIT(KC_RCTL), //
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.options = ko_option_no_unregister_on_other_key_down, //
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.negative_mod_mask = (uint8_t) ~(MOD_BIT(KC_RGUI) | MOD_BIT(KC_RCTL)), //
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.custom_action = momentary_layer, //
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.context = (void *)LAYER_FN, //
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.trigger = KC_NO, //
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.replacement = KC_NO, //
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.enabled = NULL};
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Keycodes

You can enable, disable and toggle all key overrides on the fly.
Keycode
Description
Function Equivalent
KEY_OVERRIDE_ON
Turns on Key Override feature
key_override_on(void)
KEY_OVERRIDE_OFF
Turns off Key Override feature
key_override_off(void)
KEY_OVERRIDE_TOGGLE
Toggles Key Override feature on and off
key_override_toggle(void)

Reference for key_override_t

Advanced users may need more customization than what is offered by the simple ko_make initializers. For this, directly create a key_override_t value and set all members. Below is a reference for all members of key_override_t.
Member
Description
uint16_t trigger
The non-modifier keycode that triggers the override. This keycode, and the necessary modifiers (trigger_mods) must be pressed to activate this override. Set this to the keycode of the key that should activate the override. Set to KC_NO to require only the necessary modifiers to be pressed and no non-modifier.
uint8_t trigger_mods
Which mods need to be down for activation. If both sides of a modifier are set (e.g. left ctrl and right ctrl) then only one is required to be pressed (e.g. left ctrl suffices). Use the MOD_MASK_XXX and MOD_BIT() macros for this.
layer_state_t layers
This is a BITMASK (!), defining which layers this override applies to. To use this override on layer i set the ith bit (1 << i).
uint8_t negative_mod_mask
Which modifiers cannot be down. It must hold that (active_modifiers & negative_mod_mask) == 0, otherwise the key override will not be activated. An active override will be deactivated once this is no longer true.
uint8_t suppressed_mods
Modifiers to 'suppress' while the override is active. To suppress a modifier means that even though the modifier key is held down, the host OS sees the modifier as not pressed. Can be used to suppress the trigger modifiers, as a trivial example.
uint16_t replacement
The complex keycode to send as replacement when this override is triggered. This can be a simple keycode, a key-modifier combination (e.g. C(KC_A)), or KC_NO (to register no replacement keycode). Use in combination with suppressed_mods to get the correct modifiers to be sent.
ko_option_t options
Options controlling the behavior of the override, such as what actions are allowed to activate the override.
bool (*custom_action)(bool activated, void *context)
If not NULL, this function will be called right before the replacement key is registered, along with the provided context and a flag indicating whether the override was activated or deactivated. This function allows you to run some custom actions for specific key overrides. If you return false, the replacement key is not registered/unregistered as it would normally. Return true to register and unregister the override normally.
void *context
A context that will be passed to the custom action function.
bool *enabled
If this points to false this override will not be used. Set to NULL to always have this override enabled.

Reference for ko_option_t

Bitfield with various options controlling the behavior of a key override.
Value
Description
ko_option_activation_trigger_down
Allow activating when the trigger key is pressed down.
ko_option_activation_required_mod_down
Allow activating when a necessary modifier is pressed down.
ko_option_activation_negative_mod_up
Allow activating when a negative modifier is released.
ko_option_one_mod
If set, any of the modifiers in trigger_mods will be enough to activate the override (logical OR of modifiers). If not set, all the modifiers in trigger_mods have to be pressed (logical AND of modifiers).
ko_option_no_unregister_on_other_key_down
If set, the override will not deactivate when another key is pressed down. Use only if you really know you need this.
ko_option_no_reregister_trigger
If set, the trigger key will never be registered again after the override is deactivated.
ko_options_default
The default options used by the ko_make_xxx functions

For Advanced Users: Inner Workings

This section explains how a key override works in detail, explaining where each member of key_override_t comes into play. Understanding this is essential to be able to take full advantage of all the options offered by key overrides.

Activation

When the necessary keys are pressed (trigger_mods + trigger), the override is 'activated' and the replacement key is registered in the keyboard report (replacement), while the trigger key is removed from the keyboard report. The trigger modifiers may also be removed from the keyboard report upon activation of an override (suppressed_mods). The override will not activate if any of the negative_modifiers are pressed.
Overrides can activate in three different cases:
    1.
    The trigger key is pressed down and necessary modifiers are already down.
    2.
    A necessary modifier is pressed down, while the trigger key and other necessary modifiers are already down.
    3.
    A negative modifier is released, while all necessary modifiers and the trigger key are already down.
Use the option member to customize which of these events are allowed to activate your overrides (default: all three).
In any case, a key override can only activate if the trigger key is the last non-modifier key that was pressed down. This emulates the behavior of how standard OSes (macOS, Windows, Linux) handle normal key input (to understand: Hold down a, then also hold down b, then hold down shift; B will be typed but not A).

Deactivation

An override is 'deactivated' when one of the trigger keys (trigger_mods, trigger) is lifted, another non-modifier key is pressed down, or one of the negative_modifiers is pressed down. When an override deactivates, the replacement key is removed from the keyboard report, while the suppressed_mods that are still held down are re-added to the keyboard report. By default, the trigger key is re-added to the keyboard report if it is still held down and no other non-modifier key has been pressed since. This again emulates the behavior of how standard OSes handle normal key input (To understand: hold down a, then also hold down b, then also shift, then release b; A will not be typed even though you are holding the a and shift keys). Use the option field ko_option_no_reregister_trigger to prevent re-registering the trigger key in all cases.

Key Repeat Delay

A third way in which standard OS-handling of modifier-key input is emulated in key overrides is with a 'key repeat delay'. To explain what this is, let's look at how normal keyboard input is handled by mainstream OSes again: If you hold down a, followed by shift, you will see the letter a is first typed, then for a short moment nothing is typed and then repeating As are typed. Take note that, although shift is pressed down just after a is pressed, it takes a moment until A is typed. This is caused by the aforementioned key repeat delay, and it is a feature that prevents unwanted repeated characters from being typed.
This applies equally to releasing a modifier: When you hold shift, then press a, the letter A is typed. Now if you release shift first, followed by a shortly after, you will not see the letter a being typed, even though for a short moment of time you were just holding down the key a. This is because no modified characters are typed until the key repeat delay has passed.
This exact behavior is implemented in key overrides as well: If a key override for shift + a = b exists, and a is pressed and held, followed by shift, you will not immediately see the letter b being typed. Instead, this event is deferred for a short moment, until the key repeat delay has passed, measured from the moment when the trigger key (a) was pressed down.
The duration of the key repeat delay is controlled with the KEY_OVERRIDE_REPEAT_DELAY macro. Define this value in your config.h file to change it. It is 500ms by default.

Difference to Combos

Note that key overrides are very different from combos. Combos require that you press down several keys almost at the same time and can work with any combination of non-modifier keys. Key overrides work like keyboard shortcuts (e.g. ctrl + z): They take combinations of multiple modifiers and one non-modifier key to then perform some custom action. Key overrides are implemented with much care to behave just like normal keyboard shortcuts would in regards to the order of pressed keys, timing, and interacton with other pressed keys. There are a number of optional settings that can be used to really fine-tune the behavior of each key override as well. Using key overrides also does not delay key input for regular key presses, which inherently happens in combos and may be undesirable.
Last modified 1mo ago