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# How to Customize Your Keyboard's Behavior
For a lot of people a custom keyboard is about more than sending button presses to your computer. You want to be able to do things that are more complex than simple button presses and macros. QMK has hooks that allow you to inject code, override functionality, and otherwise customize how your keyboard behaves in different situations.
This page does not assume any special knowledge about QMK, but reading [Understanding QMK](understanding_qmk.md) will help you understand what is going on at a more fundamental level.
## A Word on Core vs Keyboards vs Keymap
We have structured QMK as a hierarchy:
* Core (`_quantum`)
* Keyboard/Revision (`_kb`)
* Keymap (`_user`)
Each of the functions described below can be defined with a `_kb()` suffix or a `_user()` suffix. We intend for you to use the `_kb()` suffix at the Keyboard/Revision level, while the `_user()` suffix should be used at the Keymap level.
When defining functions at the Keyboard/Revision level it is important that your `_kb()` implementation call `_user()` before executing anything else- otherwise the keymap level function will never be called.
# Custom Keycodes
By far the most common task is to change the behavior of an existing keycode or to create a new keycode. From a code standpoint the mechanism for each is very similar.
## Defining a New Keycode
The first step to creating your own custom keycode(s) is to enumerate them. This means both naming them and assigning a unique number to that keycode. Rather than limit custom keycodes to a fixed range of numbers QMK provides the `SAFE_RANGE` macro. You can use `SAFE_RANGE` when enumerating your custom keycodes to guarantee that you get a unique number.
Here is an example of enumerating 2 keycodes. After adding this block to your `keymap.c` you will be able to use `FOO` and `BAR` inside your keymap.
```c
enum my_keycodes {
FOO = SAFE_RANGE,
BAR
};
```
## Programming the Behavior of Any Keycode
When you want to override the behavior of an existing key, or define the behavior for a new key, you should use the `process_record_kb()` and `process_record_user()` functions. These are called by QMK during key processing before the actual key event is handled. If these functions return `true` QMK will process the keycodes as usual. That can be handy for extending the functionality of a key rather than replacing it. If these functions return `false` QMK will skip the normal key handling, and it will be up to you to send any key up or down events that are required.
These function are called every time a key is pressed or released.
### Example `process_record_user()` Implementation
This example does two things. It defines the behavior for a custom keycode called `FOO`, and it supplements our Enter key by playing a tone whenever it is pressed.
```c
bool process_record_user(uint16_t keycode, keyrecord_t *record) {
switch (keycode) {
case FOO:
if (record->event.pressed) {
// Do something when pressed
} else {
// Do something else when release
}
return false; // Skip all further processing of this key
case KC_ENTER:
// Play a tone when enter is pressed
if (record->event.pressed) {
PLAY_NOTE_ARRAY(tone_qwerty);
}
return true; // Let QMK send the enter press/release events
default:
return true; // Process all other keycodes normally
}
}
```
### `process_record_*` Function Documentation
* Keyboard/Revision: `bool process_record_kb(uint16_t keycode, keyrecord_t *record)`
* Keymap: `bool process_record_user(uint16_t keycode, keyrecord_t *record)`
The `keycode` argument is whatever is defined in your keymap, eg `MO(1)`, `KC_L`, etc. You should use a `switch...case` block to handle these events.
The `record` argument contains information about the actual press:
```c
keyrecord_t record {
keyevent_t event {
keypos_t key {
uint8_t col
uint8_t row
}
bool pressed
uint16_t time
}
}
```
# LED Control
QMK provides methods to read the 5 LEDs defined as part of the HID spec:
* `USB_LED_NUM_LOCK`
* `USB_LED_CAPS_LOCK`
* `USB_LED_SCROLL_LOCK`
* `USB_LED_COMPOSE`
* `USB_LED_KANA`
These five constants correspond to the positional bits of the host LED state.
There are two ways to get the host LED state:
* by implementing `led_set_user()`
* by calling `host_keyboard_leds()`
## `led_set_user()`
This function will be called when the state of one of those 5 LEDs changes. It receives the LED state as a parameter.
Use the `IS_LED_ON(usb_led, led_name)` and `IS_LED_OFF(usb_led, led_name)` macros to check the LED status.
!> `host_keyboard_leds()` may already reflect a new value before `led_set_user()` is called.
### Example `led_set_user()` Implementation
```c
void led_set_user(uint8_t usb_led) {
if (IS_LED_ON(usb_led, USB_LED_NUM_LOCK)) {
PORTB |= (1<<0);
} else {
PORTB &= ~(1<<0);
}
if (IS_LED_ON(usb_led, USB_LED_CAPS_LOCK)) {
PORTB |= (1<<1);
} else {
PORTB &= ~(1<<1);
}
if (IS_LED_ON(usb_led, USB_LED_SCROLL_LOCK)) {
PORTB |= (1<<2);
} else {
PORTB &= ~(1<<2);
}
if (IS_LED_ON(usb_led, USB_LED_COMPOSE)) {
PORTB |= (1<<3);
} else {
PORTB &= ~(1<<3);
|
