---
title: Creating an Application
description: How to create your own application for RIOT
---

import FileTree from '@components/FileTree.astro';

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](https://github.com/RIOT-OS/RIOT).

## 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`).

```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
[Creating a Project](/c_tutorials/create_project/)) 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`.

```makefile
# 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](https://doc.riot-os.org/group__drivers.html)
or [a system library](https://doc.riot-os.org/group__sys.html)
(including [networking capabilities](https://doc.riot-os.org/group__net.html)),
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:

```makefile
USEMODULE += sht11
USEMODULE += gnrc_ipv6_default
USEMODULE += gnrc_udp
```

Modules typically pull in all required dependencies themselves.

### Configuring an Application

Many modules in RIOT offer configuration options that will be considered during
compile-time. They are modeled as macros that can be overridden by the user.
Currently there are three ways of doing this: Using `CFLAGS`, `pseudomodules`
and [Kconfig](https://guide.riot-os.org/build-system/kconfig) (the last one is
currently only possible for a subset of modules).

#### CFLAGS

`CFLAGS` can be set in your `Makefile` to configure the values of `defines`.
Make sure to not override the `CFLAGS` variable and only extend it using
the `+=` operator.

For example, to increase the stack size of the main thread,
your Makefile needs to contain this line:

```makefile
CFLAGS += -DTHREAD_STACKSIZE_MAIN=2048
```

For a list of all compile time configurations check the
[API Documentation](https://doc.riot-os.org/group__config.html).

#### Kconfig

To learn how to use [Kconfig](https://guide.riot-os.org/build-system/kconfig)
in RIOT, please refer to the
[User Guide to Configure with Kconfig](https://guide.riot-os.org/build-system/kconfig/#user-guide-to-configure-with-kconfig).

#### Pseudomodules

When devices have a common access interface, having a default configuration to
enable them across platforms, without having to explicitly specify which modules
to include, comes in handy. For this, `pseudomodules` are defined:

##### SAUL Default

Will enable all the drivers of sensors and actuators that are
present in the target platform.

```makefile
USEMODULE += saul_default
```

##### Netdev Default

Will enable all the drivers of network devices
present in the target platform.

```makefile
USEMODULE += netdev_default
```

## 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:

1. 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](#external-modules).
2. Add the source files within subdirectories to `SRC`, either explicitly or with
Makefile wildcards.

Both approaches are illustrated and explained in `examples/basic/subfolders`.

## Setting Board-specific Dependencies

Required dependencies of applications may change depending on the
target board or architecture. This is especially
relevant for networking applications where multiple hardware implementations
exist and the appropriate implementation has to be chosen for the given board
or architecture.
To achieve this task elegantly, a `Makefile.board.dep` file can be
created in the application folder, which is automatically included and
evaluated by RIOT build system during the dependency resolution phase.
This ensures that all the relevant variables such as `FEATURES_USED` or the
`USEMODULE` list are fully populated.

```makefile
ifneq (,$(filter arch_esp,$(FEATURES_USED)))
  USEMODULE += esp_wifi
endif

ifneq (,$(filter native native32 native64,$(BOARD)))
  USEMODULE += netdev_default
endif
```

## Helper Tools

To help you start writing an application within RIOT, the build system provides
the `generate-example` and `generate-test` make targets. These targets are wrappers
around the [riotgen](https://pypi.org/project/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.

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

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

<FileTree>

- apps
    - my_app
        - Makefile
- boards
  - my_board/
- modules
    - my_module
        - include
            -my_module.h
        - Makefile
        - Makefile.include
        - my_module.c
- RIOT/

</FileTree>

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 Makefile inside the application needs at least the following as bare minimum:

```makefile
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](/advanced_tutorials/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`:

```makefile
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`:

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

### Unit Tests for External Modules

When writing external modules, it can be helpful to make use of the existing
unit test infrastructure in `tests/unittests` to run both upstream unit tests
as well as downstream project-specific unit tests in a single app. For this the
environment variable `EXTERNAL_UNITTEST_DIRS` can be used to list directories
(separated by a space) to additional unit tests.
