# PX4 Board Configuration (Kconfig)

The PX4 Autopilot firmware can be configured at build time to adapt it for specialized applications (fixed-wing, multicopter, rover or more), to enable new and experimental features (such as Cyphal) or to save flash & RAM usage by disabling some drivers and subsystems.
This configuration is handled through _Kconfig_, which is the same [configuration system used by NuttX](../hardware/porting_guide_nuttx.md#nuttx-menuconfig-setup).

The configuration options (often referred as "symbols" by the _kconfig_ language) are defined in `Kconfig` files under the **/src** directory.

## PX4 Kconfig Symbol Naming Convention

By convention, symbols for modules/drivers are named based on the module folder path.
For example, the symbol for the ADC driver at `src/drivers/adc/board_adc` must be named `DRIVERS_ADC_BOARD_ADC`.

To add symbols for driver/module specific options, the naming convention is the module name followed by the option name.
For example `UAVCAN_V1_GNSS_PUBLISHER` which is an option `GNSS_PUBLISHER` for the `UAVCAN_V1` module.
The options have to be guarded behind an `if` statement to ensure that the options are only visible when the module itself is enabled.

Full example:

```
menuconfig DRIVERS_UAVCAN_V1
    bool "UAVCANv1"
    default n
    ---help---
        Enable support for UAVCANv1

if DRIVERS_UAVCAN_V1
    config UAVCAN_V1_GNSS_PUBLISHER
        bool "GNSS Publisher"
        default n
endif #DRIVERS_UAVCAN_V1
```

::: info
Builds will silently ignore any missing or miss-spelled modules in the `*.px4board` configuration file.
:::

## PX4 Kconfig Label Inheritance

Each PX4 board must have a `default.px4board` configuration and can have an optional `bootloader.px4board configuration`.
However you can add also separate configurations under a different label e.g. `cyphal.px4board`.
Note that by default the configuration of `cyphal.px4board` inherits all settings set in `default.px4board`.
When changing the `cyphal.px4board` it only stores the delta of the Kconfig keys that are different compared to `default.px4board`, this is useful to simplify configurations management

::: info
When modifying a Kconfig key in `default.px4board` it will be modified in all derivative configurations of the same board that had the same config as well.
:::

## PX4 Menuconfig Setup

The [menuconfig](https://pypi.org/project/kconfiglib/#menuconfig-interfaces) tool is used to modify the PX4 board configuration, adding/removing modules, drivers, and other features.

There are command line and GUI variants, both of which can be launched using the PX4 build shortcuts:

```
make px4_fmu-v5_default boardconfig
make px4_fmu-v5_default boardguiconfig
```

::: info
_Kconfiglib_ and _menuconfig_ come with the _kconfiglib_ python package, which is installed by the normal [ubuntu.sh](https://github.com/PX4/PX4-Autopilot/blob/main/Tools/setup/ubuntu.sh) installation script.
If _kconfiglib_ is not installed, you can do so using the command: `pip3 install kconfiglib`
:::

The command line and GUI interfaces are shown below.

### menuconfig GUI Interface

![menuconfig GUI interface](../../assets/hardware/kconfig-menuconfig.png)

### menuconfig Command Line Interface

![menuconfig command line interface](../../assets/hardware/kconfig-guiconfig.png)

## Fortified Toolchain Compatibility

Some toolchains define `_FORTIFY_SOURCE` by default. Those toolchains generally require some optimization, which means PX4 configurations that use `-O0` may fail.

PX4 keeps the default debug optimization unchanged unless you explicitly opt in. To switch `PX4_DEBUG_OPT_LEVEL` from `-O0` to `-Og`, enable:

- `Toolchain > Fortified toolchain support`

This is the Kconfig symbol:

```sh
CONFIG_BOARD_SUPPORT_FORTIFIED_TOOLCHAIN=y
```

You can set it either in `boardconfig`/`boardguiconfig` or directly in your board's `*.px4board` file.
