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Kconfig in RIOT

The objective of using Kconfig in RIOT is to configure software modules at compile-time. This means having a standard way of:

  • Exposing configurable parameters
  • Assigning application and user-specific configurations
  • Verifying these parameters
    • Check possible values
    • Check valid configuration considering inter-dependencies
  • Applying the selected configuration



Modules in RIOT expose their configurable parameters via Kconfig files (for more information on Kconfig syntax check the specification). In these files documentation, restrictions, default values and dependencies can be expressed.

Kconfig files are structured through the file system mirroring the current module distribution. In time, all modules will have Kconfig files to make themselves configurable through this system.


The user can assign values to the exposed parameters, either by manually writing '.config' files or using an interface such as Menuconfig. Parameters with no assigned values will take the default ones. For a detailed distinction between Kconfig and '.config' files see Appendix B.

Verification and application

Using '.config' and Kconfig files the build system takes care of doing the necessary checks on the values according to the parameter definition. After that, the autoconf.h header file is generated, it contains all the configurations in the form of (CONFIG_ prefixed) macros.

User's guide to configure with Kconfig

Configure using menuconfig

In order to use the graphical interface menuconfig to configure the application, run make menuconfig in the application's folder. All available configurations (based on the used modules) for the particular platform will be presented. By default, the configuration of a module via Kconfig is not enabled. In order to activate the configuration via Kconfig the corresponding option should be selected. That will enable the configuration of all inner options, if available.

Once the desired configuration is achieved save the configuration to the default proposed path and exit. The saved configuration will be applied when the code is compiled (make all).

If the current configuration will be used in the future it can be saved in the application's folder as user.config, using the 'Save' option in menuconfig. This way it will be persistent after cleaning the application directory (make clean).

Configure using '.config' files

The second way to configure the application is by directly writing '.config' files. Two files will be sources of configuration during the generation of the final header file: app.config and user.config, which should be placed inside the application's folder. app.config sets default configuration values for the particular application, the user can override them by setting them in user.config. Additionally, further .config files can be added to the variable KCONFIG_ADD_CONFIG, which will be applied after default CPU and board configurations, app.config and user.config. This means that they will have priority.

Let's say that the SOCK_UTIL_SCHEME_MAXLEN symbol in sock_util module needs to be configured. The user.config file could look like:

# activate configuration of sock_util using Kconfig
# change scheme part length

In this case, there is no need for using menuconfig. It's enough just to call make all in the application folder, as this configuration will be read and applied. Note that if any dependency issue occurs, warnings will be generated (e.g. not enabling the configuration of a module via Kconfig).

Application configuration with Kconfig

To expose application-specific configuration options a Kconfig file can be placed in the application's folder. For an example of this you can check the tests/build_system/kconfig application.

Configuration via environment variables

For easy debugging of configuration or testing new modules by compiling them into existing applications, one can also use environment variables prefixed by RIOT_CONFIG_. To achieve the same configuration exemplified in Configure using '.config' files, e.g., you could also use


All the checks that apply for .config files also are done with this approach.

Mind that this is only meant to be used during development. In production, please set the configuration via .config files.

A note on the usage of CFLAGS

When a certain module is being configured via Kconfig the configuration macro will no longer be overridable by means of CFLAGS (e.g. set on the compilation command or on a Makefile). Consider this if you are getting a 'redefined warning'.

Integration into the build system

The integration of Kconfig into the build system is mainly done in makefiles/kconfig.mk.

Steps during the build process

Output of every step of the build process

0. Module dependency resolution

The resolution of module dependencies is performed by the build system where all the used modules and packages end up listed in the USEMODULE or USEPKG make variables.


  • Makefiles.


  • USEMODULE and USEPKG variables.

1. Module listing

The list of modules needed for the particular build is dumped into the $ (GENERATED_DIR)/Kconfig.dep file, where each module is translated into a Kconfig symbol as documented in Appendix A.


  • USEMODULE and USEPKG variables


  • $ (GENERATED_DIR)/Kconfig.dep file

2. Merging all configuration sources

In this step configuration values are taken from multiple sources and merged into a single out.config configuration file. This file is temporary and is removed on clean. If the user needs to save a particular configuration set, a backup has to be saved (this can be done using the menuconfig interface) so it can be loaded later in this step.

To accomplish merging of multiple input files, the genconfig script is used. Note that the order matters: existing configuration values are merged in the order expressed in the input section, where the last value assigned to a parameter has the highest priority. If no configuration files are available all default values will be applied.

out.config is the only configuration input for the autoconf.h in the generation step.

Additionally this step generates a file out.config.d which holds the information of all the used Kconfig files in Makefile format. This file is included by the build system and allows to re-trigger the generation of out.conf whenever a Kconfig file is modified.


  • Optional:
    • $ (APPDIR)/app.config: Application specific default configurations.
    • $ (APPDIR)/user.config: Configurations saved by user.


  • $ (GENERATED_DIR)/out.config file.

3. Menuconfig execution (optional)

Menuconfig is a graphical interface for software configuration. It is used for the configuration of the Linux kernel. This section explains the process that occurs when RIOT is being configured using the menuconfig interface.

The main Kconfig file is used in this step to show the configurable parameters of the system. Kconfig will filter inapplicable parameters (i.e. parameters exposed by modules that are not being used) based on the file $ (GENERATED_DIR)/Kconfig.dep generated in step 1.

Note that if Kconfig is not used to configure a module, the corresponding header files default values will be used.

out.config is one of the inputs for menuconfig. This means that any configuration that the application defines in the app.config or a backup configuration from the user in user.config are taken into account on the first run (see Appendix C).

In this step the user chooses configuration values (or selects the minimal configuration) and saves it to the out.config file. Here the user can choose to save a backup configuration file for later at a different location (e.g. a user.config file in the application folder).

If any changes occur to out.config, the generation of autoconf.h is executed automatically.


  • /Kconfig file.
  • Optional:
    • $ (APPDIR)/app.config
    • $ (APPDIR)/user.config
    • $ (GENERATED_DIR)/out.config


  • Updated $ (GENERATED_DIR)/out.config file.
  • $ (GENERATED_DIR)/out.config.old backup file.

4. Generation of the autoconf.h header

With the addition of Kconfig a dependency has been added to the build process: the $ (GENERATED_DIR)/autoconf.h header file. This header file is the main output from the Kconfig configuration system. It holds all the macros that should be used to configure modules in RIOT: CONFIG_<module>_<parameter>.

In order to generate the autoconf.h file the genconfig script is used. Inputs for this script are the main Kconfig file and out.config configuration file, which holds the selected values for the exposed parameters.


  • $ (GENERATED_DIR)/out.config file.
  • Main Kconfig file exposing configuration of modules.


  • $ (GENERATED_DIR)/autoconf.h configuration header file.
  • Optional:
    • $ (GENERATED_DIR)/deps/*/*.h header files that allow incremental builds

Summary of files

These files are defined in kconfig.mk.

File Description
Kconfig Defines configuration options of modules.
Kconfig.dep Holds a list of the modules that are being compiled.
app.config Holds default application configuration values.
user.config Holds configuration values applied by the user.
out.config Configuration file containing all the symbols defined in autoconf.h.
out.config.d Dependency file of out.config containing the list of Kconfig files used to generate it.
autoconf.h Header file containing the macros that applied the selected configuration.

Kconfig symbols in Makefiles

As '.config' files have Makefile syntax they can be included when building, which allows to access the applied configuration from the build system.

During migration this is also useful, as it gives the ability to check if a parameter is being configured via Kconfig or a default value via CFLAGS could be injected. For example:


Symbols will have the same name as the configuration macros (thus will always have the CONFIG_ prefix). As the configuration file is loaded in Makefile.include care should be taken when performing checks in the application's Makefile. The symbols will not be defined until after including Makefile.include.

Transition phase

Making configuration via Kconfig optional

During transition to the usage of Kconfig as the main configuration tool for RIOT, the default behavior will be the traditional one: expose configuration options in header files and use CFLAGS as inputs. To allow optional configuration via Kconfig, a convention will be used when writing Kconfig files.

Modules should be contained in their own menu entries, this way the user can choose to enable the configuration via Kconfig for an specific module. These entries should define a dependency on the module they configure (see Appendix A to see how to check if a module is being used).

The module configuration must be enabled either via make dependency modelling.

Modelling CPUs and boards

CPUs and boards are being modelled in Kconfig. The following is a guide on how to organize and name the symbols.


The proposed hierarchy for the classification of CPUs is as follows:

More Specific | CPU_MODEL |
+ +------------+
| +------------+
| | CPU_FAM |
| +------------+
| +------------+
| | CPU_CORE |
| +------------+
v +------------+
Less Specific | CPU_ARCH |

Where each hierarchy is defined as:

  • CPU_MODEL: The specific identifier of the used CPU, used for some CPU implementations to differentiate between different memory layouts.
  • CPU_FAM: An intermediate identifier between CPU and CPU_MODEL that represents a sub-group of a Manufacturers CPU's.
  • CPU_CORE: The specific identifier of the core present in the CPU.
  • CPU_ARCH: The specific identifier of the architecture of the core defined in CPU_CORE.

In order to model the hierarchies, a hidden boolean symbol must be declared for each. The name of the symbol must begin with the correspondent prefix and must be followed by the specific value. For instance, the 'samd21' family symbol is named CPU_FAM_SAMD21.

In addition, a default value to the correspondent common symbol must be defined. The default value must be guarded by the boolean symbol correspondent to the hierarchy.

Features may be provided by any hierarchy symbol. Usually symbols are selected from more specific to less specific. This means that a CPU_MODEL_<model> symbol usually would select the correspondent CPU_FAM_<family> symbol, which would in turn select the CPU_CORE_<core>. This may change in some cases where CPU_COMMON_ symbols are defined to avoid repetition. For convenience and if it makes sense within a CPU vendor family, it's also allowed to use intermediate grouping levels, like CPU_LINE_<xxx> used for STM32.

In addition to the symbols of the hierarchy described above, a default value to the CPU symbol should be assigned, which will match the value of the CPU Makefile variable in the build system.

The declaration of the symbols should be placed in a Kconfig file in the folder that corresponds to the hierarchy. When the symbols are scattered into multiple files, it is responsibility of file containing the most specific symbols to source the less specific. Keep in mind that only the file located in /cpu/<CPU>/Kconfig will be included by the root /Kconfig file.


# This is the most specific symbol (selected by the board)
# The CPU model selects the family it belongs to
select CPU_FAM_SAMD21
# In this case the family selects a common 'sam0' symbol (which provides some
# features), and the core it has (cortex-m0+)
config CPU_FAM_SAMD21
# The value of the common value depends on the selected model
config CPU_MODEL
default "samd21e18a" if CPU_MODEL_SAMD21E18A
default "samd21g18a" if CPU_MODEL_SAMD21G18A
default "samd21j18a" if CPU_MODEL_SAMD21J18A
default "samr21e18a" if CPU_MODEL_SAMR21E18A
default "samr21g18a" if CPU_MODEL_SAMR21G18A
config CPU_FAM
default "samd21" if CPU_FAM_SAMD21


Boards must be modelled as hidden boolean symbols with the prefix BOARD_ which default to y and are placed in /boards/<BOARD>/Kconfig. This file will be sourced from the main /Kconfig file. The board symbol must select the CPU_MODEL_<model> symbol that corresponds to the CPU model present on the board. The board symbol must also select the symbols that correspond to the features it provides.

In the same Kconfig file a default value must be assigned to the common BOARD symbol. It must be guarded by the board's symbol, so it only applies in that case.

There are cases when grouping common code for multiple boards helps to avoid unnecessary repetition. In the case features are provided in a common board folder (e.g. /boards/common/arduino-atmega) a symbol should be declared to model this in Kconfig. Symbols for common boards must have the BOARD_COMMON_ prefix, and must select the common provided features.


The samr21-xpro has a samr21g18a CPU and provides multiple features. Its symbol is modelled as following:

# /boards/samr21-xpro/Kconfig
config BOARD
default "samr21-xpro" if BOARD_SAMR21_XPRO
default y

Default configurations

Boards, common board directories, CPUs and common CPU directories may need to override default configuration values. Visible configuration symbols are configurable by the user and show on the menuconfig interface. .config files are used to set their values. To allow multiple sources of .config files, there are two Makefile variables developers should use: KCONFIG_CPU_CONFIG for sources added by the CPU or common CPU directories, and KCONFIG_BOARD_CONFIG for sources added by the board or common board directories. This ensures the correct priority of the configurations.

The Makefile.features infrastructure is used to populate the configuration sources. As the order in which .config files are merged matters, configuration sources should be ordered from more generic to more specific. Because board's Makefile.features is included before CPU's Makefile.features it is important to utilize two different lists of configuration sources. For instance, if cpu/cortexm_common adds its configuration, cpu/stm32 should add its configuration after it, and boards/stm32f769i-disco after it.

include $(RIOTCPU)/cortexm_common/Makefile.features
# Add stm32 configs after including cortexm_common so stm32 takes precedence
KCONFIG_CPU_CONFIG += $(RIOTCPU)/stm32/stm32.config

Summary of reserved Kconfig prefixes

The following symbol prefixes have been assigned particular semantics and are reserved for the cases described below:

Prefix Description
BOARD_ Models a board
BOARD_COMMON_ Used for common symbols used by multiple boards
CPU_ARCH_ Models a CPU architecture
CPU_COMMON_ Used for common symbols used by multiple CPUs
CPU_CORE_ Models a CPU core
CPU_FAM_ Models a family of CPUs
CPU_MODEL_ Models a particular model of CPU
USEMODULE_ Models a RIOT module. Generated from USEMODULE variable
USEPKG_ Models an external package. Generated from USEPKG variable


Appendix A: Check if a module or package is used

In order to show only the relevant configuration parameters to the user with respect to a given application and board selection, Kconfig needs knowledge about all modules and packages to be used for a compilation. The dependency handling among modules is performed by the build system (via Makefile.dep files). The interface defined to declared the used modules and packages is the $ (GENERATED_DIR)/Kconfig.dep file.

There will be a symbol for every used module (i.e. every module in USEMODULE make variable) and package. The names in the symbols will be uppercase and separated by _. Based on these symbols configurability is decided.

The following is an example of how to use these symbols in Kconfig files to configure compile-time configurations with USEMODULE dependencies:

menu "Configure Sock Utilities"
int "Maximum length of the scheme part for sock_urlsplit"
default 16
endmenu # Configure Sock Utilities

Appendix B: Difference between 'Kconfig' and '.config' files

Kconfig files describe a configuration database, which is a collection of configuration options organized in a tree structure. Configuration options may have dependencies (among other attributes), which are used to determine their visibility.

Kconfig files are written in Kconfig language defined in the Linux kernel. Configuration options have attributes such as types, prompts and default values.

Kconfig file

menu "Buffer Sizes"
int "Request or response buffer size"
default 128

On the other hand configuration files contain assignment of values to configuration options and use Makefile syntax. They can also be used to save a set of configuration values as backup.

'.config' file

# enable Kconfig configuration for gcoap
# set the value

In other words: Kconfig files describe configuration options and '.config' files assign their values.

Appendix C: Pitfall when using different configuration interfaces

In the current configuration flow the user can choose to configure RIOT using the menuconfig graphical interface or writing '.config' files by hand.

As explained in the 'Configuration sources merging step' of the configuration process, configuration from multiple sources are loaded to create a single out.config file, and the order of merging matters: last file has priority.

While editing values directly via '.config' files out.config will be re-built. The user can also use menuconfig interface to modify the configuration file (this is the recommended way, as it gives access to much more information regarding dependencies and default values of the symbols). Menuconfig will change out.config directly (a backup file out.config.old will be kept).

It is recommended to save backups of the configurations, as any change on the configuration sources would re-trigger the merging process and overwrite out.config.

Appendix D: A few key aspects while exposing a macro to Kconfig

A macro that holds a 0 or 1 is modelled in Kconfig as a bool symbol. References to this macro can then make use of IS_ACTIVE macro from kernel_defines.h with C conditionals for conditional compilation. FXOS8700 driver exposure to Kconfig can be considered as an example. If the macro is defined as TRUE by default, a new symbol gets introduced to invert the semantics. The recommended practice is to add a new symbol and expose it to Kconfig while the old one is tagged to be deprecated. The process is documented in this commit

There may be cases where a macro is expected to hold only specific values, e.g. 'GNRC_IPV6_MSG_QUEUE_SIZE' expressed as the power of two. These may be modelled in such a way that a new macro is introduced to hold the restricted figures while operators are added to arrive at the desired value. The process is documented in this pull request.

Useful references