To create your own application you need to create a directory containing one or multiple C file(s) with your source code and a Makefile. A template Makefile is available in the dist folder of the RIOT repository.

The main function

After the board is initialized, RIOT starts two threads: the idle thread and the main thread. The idle thread has the lowest priority and will run whenever no other thread is ready to run. It will automatically use the lowest possible power mode for the device. The main thread - configured with a default priority that is right in the middle between the lowest and the highest available priorities - is the first thread that runs and calls the main() function. This function needs to be defined in the source code of the application (typically located in main.c).

#include <stdio.h>
 
int main(void)
{
    puts("Hello World!");
    return 0;
}

The application's Makefile

The minimal Makefile

At minimum the Makefile of an application (see Getting started) needs to define the following macros:

APPLICATION: should contain the name of your application
RIOTBASE: specifies the path to your copy of the RIOT repository (note that you may want to use $(CURDIR) here, to give a relative path)

The BOARD macro is also required and recommended to be set to native by default, but is recommended to be overridable with the ?= operator. Additionally, it is required to include the Makefile.include from the RIOTBASE.

# Set the name of your application:
APPLICATION = foobar
 
# If no BOARD is found in the environment, use this default:
BOARD ?= native
 
# This has to be the absolute path to the RIOT base directory:
RIOTBASE ?= $(CURDIR)/../../RIOT
 
include $(RIOTBASE)/Makefile.include

How to handle unsupported boards?

Sometimes it is necessary to exclude boards because they don't provide a required functionality or don't have sufficient memory. RIOT's build system looks for the macros BOARD_BLACKLIST, BOARD_WHITELIST, and BOARD_INSUFFICIENT_MEMORY. Any board name that is not included in BOARD_WHITELIST will issue a message that one has to expect errors if they build the application for the board referred by that name. The list can also be used by a CI system to not build the application for this board at all. A board that is included in BOARD_BLACKLIST will show the same behavior. The build system evaluates BOARD_WHITELIST first and then BOARD_BLACKLIST. The BOARD_INSUFFICIENT_MEMORY macro is similar to BOARD_BLACKLIST but will build in any case. A CI system can use the information provided by the BOARD_INSUFFICIENT_MEMORY macro to skip the linking step in the build process, since some linkers will issue an error if the code won't fit the target board's flash memory or RAM.

Including modules

By default a RIOT application comprises only of the applications' code itself, the kernel, and platform specific code. In order to use additional modules, such as a particular device driver or a system library (including networking capabilities), you have to append the modules' names to the USEMODULE variable. For example, to build an application using the SHT11 temperature sensor and UDP/IPv6 functionalities of the GNRC network stack, your Makefile needs to contain these lines:

USEMODULE += sht11
USEMODULE += gnrc_ipv6_default
USEMODULE += gnrc_udp

Modules typically pull in all required dependencies.

Including source files in subfolders

By default, all source files in an application's (or any RIOT module's) directory are automatically compiled as part of the application. In order to organize source code in a directory structure, two different approaches can be used:

Make each subdirectory a separate RIOT module with a unique name inside its Makefile, either by adding the directory's path to DIRS or with the out-of-tree module support.
Add the source files within subdirectories to SRC, either explicitly or with Makefile wildcards.

Both approaches are illustrated and explained in examples/subfolders.

Helper tools

To help you start writing an application within RIOT, the build system provides the generate-example generate-test make targets. These targets are wrappers around the riotgen command line tool and are helpful when starting to implement an application: all required files are generated with copyright headers, doxygen groups, etc, so you can concentrate on the module implementation. For applications, the Makefile is generated with the dependencies (modules, packages, required features) included.

Usage:

To generate an example application, e.g in the examples directory, from the RIOT base directory, run:

make generate-example

To generate a test application, e.g in the tests directory, from the RIOT base directory, run:

make generate-test

Then answer a few questions about the application:

Application name: enter a name for your application. It will be used as both the name of the application directory under examples or tests and by the build system module (set in the APPLICATION variable).
Application brief description: Describe in one line what is this application about.
Target board: select the default target board. The value is set to native by default.
Modules: give the list of dependency modules, separated by commas. For example: ztimer,fmt
Packages: give the list of dependency packages, separated by commas.
Features required: give the list of CPU features (periph_*, etc) required by the application, all separated by commas.

Other global information (author name, email, organization) should be retrieved automatically from your git configuration.

Once completed, the application files are either located in examples/<application name> or tests/<application name> depending on the target used.

Testrunner: when using the make generate-test, you can also automatically add a testrunner Python script. Just answer 'y' when prompted.

Creating an out of tree application structure

Applications written for RIOT do not have to reside in the RIOT tree. Out of tree applications, modules and boards are supported.

For a full application with custom board and modules, the following directory tree can be used:

├── apps
│   └── my_app
│       └── Makefile
├── boards
│   └── my_board
├── modules
│   └── my_module
│       ├── include
│       │   └── my_module.h
│       ├── Makefile
│       ├── Makefile.include
│       └── my_module.c
└── RIOT

In this example tree, the apps directory contains a collection of applications for the project. The modules directory could contain extra modules for the applications.

The make file inside the application needs at least the following as bare minimum:

APPLICATION = my_app
PROJECT_BASE ?= $(CURDIR)/../..
RIOTBASE ?= $(PROJECT_BASE)/RIOT
 
# Optionally, provide paths to where external boards and/or modules
# reside, so that they can be included in the app
EXTERNAL_MODULE_DIRS += $(PROJECT_BASE)/modules
EXTERNAL_BOARD_DIRS += $(PROJECT_BASE)/boards
 
include $(RIOTBASE)/Makefile.include

The RIOTBASE variable tells the build system where to find the RIOT source tree and to need to point to the RIOT source tree used for the application for the application to work.

The RIOT directory contains the sources of RIOT here. This can be either a direct checkout of the sources or a git submodule, whichever has your preference.

If your project has separate modules or separate boards, these can be contained inside a modules os boards directory. The RIOT build system has both EXTERNAL_MODULE_DIRS and EXTERNAL_BOARD_DIRS variables to specify directories that contain extra modules and extra boards.

External Boards

External boards can be ported in an identical way as porting a regular board to RIOT, see Porting boards.

One approach can be to copy over an existing board and modify it to suit the needs. Boards in the RIOT tree can be included and used as dependency in the custom boards. In case you connect additional hardware to an upstream board (such as e.g. an Arduino shield) or you require a different hardware configuration (e.g. configuring some of the pins configured as ADC as additional PWM outputs instead) a copy of the upstream board that is then customized to the application needs is the best course of action.

External Modules

Similar to the external boards, external modules can be written in a similar way as regular in-tree modules.

One modification is the include directory inside the module directory. For this include directory to be added to the include path during compilation, the following snippet is required in the modules Makefile.include:

USEMODULE_INCLUDES_my_module := $(LAST_MAKEFILEDIR)/include

USEMODULE_INCLUDES += $(USEMODULE_INCLUDES_my_module)

Note that the make variable (here USEMODULE_INCLUDES_my_module) must be unique for every module to make this work. Including the module name here is usually sufficient.

Extra Makefile Scaffolding

A bit of extra, but optional, Makefile scaffolding can help to keep the project easy to maintain. An extra Makefile.include in the root directory of the project that sets the necessary variables can help to deduplicate settings. This includes the RIOTBASE variable and the include to the RIOT Makefile.include:

EXTERNAL_MODULE_DIRS += $(PROJECT_BASE)/modules
EXTERNAL_BOARD_DIRS += $(PROJECT_BASE)/boards
RIOTBASE ?= $(PROJECT_BASE)/RIOT
include $(RIOTBASE)/Makefile.include

Applications then just have to set the PROJECT_BASE variable and include this makefile and don't have to each add the external board and module directories.

The application makefile would then look like this:

APPLICATION = my_app
PROJECT_BASE ?= $(CURDIR)/../..
include $(PROJECT_BASE)/Makefile.include