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Application Development


In this document, we’ll assume your application directory is <home>/app, and that its build directory is <home>/app/build. (These terms are defined in the following Overview.) On Linux/macOS, <home> is equivalent to ~, whereas on Windows it’s %userprofile%.


Zephyr’s build system is based on CMake.

The build system is application-centric, and requires Zephyr-based applications to initiate building the kernel source tree. The application build controls the configuration and build process of both the application and Zephyr itself, compiling them into a single binary.

Zephyr’s base directory hosts Zephyr’s own source code, its kernel configuration options, and its build definitions.

The files in the application directory link Zephyr with the application. This directory contains all application-specific files, such as configuration options and source code.

An application in its simplest form has the following contents:

├── CMakeLists.txt
├── prj.conf
└── src
    └── main.c

These contents are:

  • CMakeLists.txt: This file tells the build system where to find the other application files, and links the application directory with Zephyr’s CMake build system. This link provides features supported by Zephyr’s build system, such as board-specific kernel configuration files, the ability to run and debug compiled binaries on real or emulated hardware, and more.

  • Kernel configuration files: An application typically provides a configuration file (usually called prj.conf) that specifies application-specific values for one or more kernel configuration options. These application settings are merged with board-specific settings to produce a kernel configuration.

    The Kconfig Configuration section below goes over application configuration in detail.

  • Application source code files: An application typically provides one or more application-specific files, written in C or assembly language. These files are usually located in a sub-directory called src.

Once an application has been defined, you can use CMake to create project files for building it from a directory where you want to host these files. This is known as the build directory. Application build artifacts are always generated in a build directory; Zephyr does not support “in-tree” builds.


You must create a build directory yourself, and call cmake from there. The Zephyr build system does not create build directories for you.

The following sections describe how to create, build, and run Zephyr applications, followed by more detailed reference material.

Source Tree Structure

Understanding the Zephyr source tree can be helpful in locating the code associated with a particular Zephyr feature.

At the top of the tree there are several files that are of importance:

The top-level file for the CMake build system, containing a lot of the logic required to build Zephyr.
The top-level Kconfig file, which refers to the file Kconfig.zephyr also found at the top-level directory.
The West (Zephyr’s meta-tool) manifest, listing the external repositories managed by the west command-line tool.

The Zephyr source tree also contains the following top-level directories, each of which may have one or more additional levels of subdirectories which are not described here.


Architecture-specific kernel and system-on-chip (SoC) code. Each supported architecture (for example, x86 and ARM) has its own subdirectory, which contains additional subdirectories for the following areas:

  • architecture-specific kernel source files
  • architecture-specific kernel include files for private APIs
SoC related code and configuration files.
Board related code and configuration files.
Zephyr technical documentation source files and tools used to generate the web content.
Device driver code.
Device tree source (.dts) files used to describe non-discoverable board-specific hardware details previously hard coded in the OS source code.
Externally created code that has been integrated into Zephyr from other sources and that must live inside the zephyr repository unlike external projects
Include files for all public APIs, except those defined under lib.
Architecture-independent kernel code.
Library code, including the minimal standard C library.
Miscellaneous code that doesn’t belong to any of the other top-level directories.
Sample applications that demonstrate the use of Zephyr features.
Various programs and other files used to build and test Zephyr applications.
Additional build scripts needed to build Zephyr.

Subsystems of Zephyr, including:

  • USB device stack code.
  • Networking code, including the Bluetooth stack and networking stacks.
  • File system code.
  • Bluetooth host and controller
Test code and benchmarks for Zephyr features.

Creating an Application

Follow these steps to create a new application directory. (Refer to Samples and Demos for existing applications provided as part of Zephyr.)

  1. Create an application directory on your workstation computer, outside of the Zephyr base directory. Usually you’ll want to create it somewhere under your user’s home directory.

    For example, in a Unix shell or Windows cmd.exe prompt, navigate to where you want to create your application, then enter:

    mkdir app


    Building Zephyr or creating an application in a directory with spaces anywhere on the path is not supported. So the Windows path C:\Users\YourName\app will work, but C:\Users\Your Name\app will not.

  2. It’s recommended to place all application source code in a subdirectory named src. This makes it easier to distinguish between project files and sources.

    Continuing the previous example, enter:

    cd app
    mkdir src
  3. Place your application source code in the src sub-directory. For this example, we’ll assume you created a file named src/main.c.

  4. Create a file named CMakeLists.txt in the app directory with the following contents:

    # Boilerplate code, which pulls in the Zephyr build system.
    cmake_minimum_required(VERSION 3.13.1)
    include($ENV{ZEPHYR_BASE}/cmake/app/boilerplate.cmake NO_POLICY_SCOPE)
    # Add your source file to the "app" target. This must come after
    # the boilerplate code, which defines the target.
    target_sources(app PRIVATE src/main.c)

    The boilerplate code sets the minimum CMake version and pulls in the Zephyr build system, which creates a CMake target named app. Adding sources to this target is how you include them in the build.


    cmake_minimum_required() is also invoked from boilerplate.cmake. The most recent of the two versions will be enforced by CMake.

  5. Set any Kconfig values needed by your application. Zephyr uses the same Kconfig system as the Linux kernel, but with its own database of configuration options.

    For example, create a file named prj.conf in the app directory, and enable or disable Kconfig features as needed. You can use existing Samples and Demos to get started with Kconfig variables you are interested in. See Kconfig Configuration for more details, and Configuration Options for a complete list of available options.

  6. Optionally, you can also configure any Device Tree overlays needed by your application. Zephyr uses the same Device Tree system as the Linux kernel, but with its own definitions.

    This is usually not necessary; see Device Tree Overlays below for details.

Important Build System Variables

You can control the Zephyr build system using many variables. This section describes the most important ones that every Zephyr developer should know about.


The variables BOARD, CONF_FILE, and DTC_OVERLAY_FILE can be supplied to the build system in 3 ways (in order of precedence):

  • As a parameter to the cmake invocation via the -D command-line switch
  • As an environment variables (export on Linux/macOS and set on Windows)
  • As a set(<VARIABLE> <VALUE>) statement in your CMakeLists.txt
  • ZEPHYR_BASE: Sets the path to the directory containing Zephyr, which is needed by the build system’s boilerplate file. This is an environment variable set by the script on Linux/macOS or zephyr-env.cmd on Windows, as you learned when getting started with Zephyr in Build and Run an Application. You can also set ZEPHYR_BASE explicitly, but then you won’t get the other features provided by those scripts.
  • BOARD: Selects the board that the application’s build will use for the default configuration. See Supported Boards for built-in boards, and Board Porting Guide for information on adding board support.
  • CONF_FILE: Indicates the name of one or more configuration fragment files. Multiple filenames can be separated with either spaces or semicolons. Each file includes Kconfig configuration values that override the default configuration values.
  • DTC_OVERLAY_FILE: Indicates the name of one or more Device Tree overlay files. Multiple filenames can be separated with either spaces or semicolons. Each file includes Device Tree values that override the default DT values.
  • ZEPHYR_MODULES: A CMake list containing absolute paths of additional directories with source code, Kconfig, etc. that should be used in the application build. See Modules (External projects) below for details.

Build an Application

The Zephyr build system compiles and links all components of an application into a single application image that can be run on simulated hardware or real hardware.

As described in A Brief Note on the Zephyr Build System, on Linux and macOS you can choose between the make and ninja generators, whereas on Windows you need to use ninja. For simplicity we will use ninja throughout this guide.


  1. Navigate to the application directory <home>/app.

  2. Enter the following commands to build the application’s zephyr.elf image for the board specified in the command-line parameters:

    # On Linux/macOS
    mkdir build && cd build
    # On Windows
    mkdir build & cd build
    # Use cmake to configure a Ninja-based build system:
    cmake -GNinja -DBOARD=<board> ..
    # Now run ninja on the generated build system:

    If desired, you can build the application using the configuration settings specified in an alternate .conf file using the CONF_FILE parameter. These settings will override the settings in the application’s .config file or its default .conf file. For example:

    cmake -GNinja -DBOARD=<board> -DCONF_FILE=prj.alternate.conf ..

    As described in the previous section, you can instead choose to permanently set the board and configuration settings by either exporting BOARD and CONF_FILE environment variables or by setting their values in your CMakeLists.txt using set() statements.

Build Directory Contents

When using the Ninja generator a build directory looks like this:

├── CMakeCache.txt
├── CMakeFiles
├── cmake_install.cmake
└── zephyr

The most notable files in the build directory are:

  •, which can be invoked to build the application.
  • A zephyr directory, which is the working directory of the generated build system, and where most generated files are created and stored.

After running ninja, the following build output files will be written to the zephyr sub-directory of the build directory. (This is not the Zephyr base directory, which contains the Zephyr source code etc. and is described above.)

  • .config, which contains the configuration settings used to build the application.


    The previous version of .config is saved to .config.old whenever the configuration is updated. This is for convenience, as comparing the old and new versions can be handy.

  • Various object files (.o files and .a files) containing compiled kernel and application code.

  • zephyr.elf, which contains the final combined application and kernel binary. Other binary output formats, such as .hex and .bin, are also supported.

Rebuilding an Application

Application development is usually fastest when changes are continually tested. Frequently rebuilding your application makes debugging less painful as the application becomes more complex. It’s usually a good idea to rebuild and test after any major changes to the application’s source files, CMakeLists.txt files, or configuration settings.


The Zephyr build system rebuilds only the parts of the application image potentially affected by the changes. Consequently, rebuilding an application is often significantly faster than building it the first time.

Sometimes the build system doesn’t rebuild the application correctly because it fails to recompile one or more necessary files. You can force the build system to rebuild the entire application from scratch with the following procedure:

  1. Open a terminal console on your host computer, and navigate to the build directory <home>/app/build.

  2. Enter the following command to delete the application’s generated files, except for the .config file that contains the application’s current configuration information.

    ninja clean

    Alternatively, enter the following command to delete all generated files, including the .config files that contain the application’s current configuration information for those board types.

    ninja pristine
  3. Rebuild the application normally following the steps specified in Build an Application above.

Run an Application

An application image can be run on a real board or emulated hardware.

Running on a Board

Most boards supported by Zephyr let you flash a compiled binary using the CMake flash target to copy the binary to the board and run it. Follow these instructions to flash and run an application on real hardware:

  1. Build your application, as described in Build an Application.

  2. Make sure your board is attached to your host computer. Usually, you’ll do this via USB.

  3. Run this console command from the build directory, <home>/app/build, to flash the compiled Zephyr binary and run it on your board:

    ninja flash

The Zephyr build system integrates with the board support files to use hardware-specific tools to flash the Zephyr binary to your hardware, then run it.

Each time you run the flash command, your application is rebuilt and flashed again.

In cases where board support is incomplete, flashing via the Zephyr build system may not be supported. If you receive an error message about flash support being unavailable, consult your board’s documentation for additional information on how to flash your board.


When developing on Linux, it’s common to need to install board-specific udev rules to enable USB device access to your board as a non-root user. If flashing fails, consult your board’s documentation to see if this is necessary.

Running in an Emulator

The kernel has built-in emulator support for QEMU (on Linux/macOS only, this is not yet supported on Windows). It allows you to run and test an application virtually, before (or in lieu of) loading and running it on actual target hardware. Follow these instructions to run an application via QEMU:

  1. Build your application for one of the QEMU boards, as described in Build an Application.

    For example, you could set BOARD to:

    • qemu_x86 to emulate running on an x86-based board
    • qemu_cortex_m3 to emulate running on an ARM Cortex M3-based board
  2. Run this console command from the build directory, <home>/app/build, to flash the compiled Zephyr binary and run it in QEMU:

    ninja run
  3. Press Ctrl A, X to stop the application from running in QEMU.

    The application stops running and the terminal console prompt redisplays.

Each time you execute the run command, your application is rebuilt and run again.


The run target will use the QEMU binary available from the Zephyr SDK by default. To use an alternate version of QEMU, for example the version installed on your host or a custom version, set the environment variable QEMU_BIN_PATH to the alternate path.

Custom Board and SOC Definitions

In cases where the board or platform you are developing for is not yet supported by Zephyr, you can add the board and SOC definition to your application and build for this board or SOC without having to add them to the Zephyr tree.

The structure needed to support out-of-tree board and SOC development is similar to how boards and SOCs are maintained in the Zephyr tree. By using this structure, it will be much easier to upstream your platform related work into the Zephyr tree after your initial development is done.

Add the custom board to your application or a dedicated repository using the following structure:


where the boards directory hosts the board you are building for:

├── boards
│   └── x86
│       └── my_custom_board
│           ├── doc
│           │   └── img
│           └── support
└── src

and the soc directory hosts any SOC code. You can also have boards that are supported by a SOC that is available in the Zephyr tree.


Use the proper architecture folder name (e.g., x86, arm, etc.) under boards for my_custom_board. (See Supported Boards for a list of board architectures.)

Documentation (under doc/) and support files (under support/) are optional, but will be needed when submitting to Zephyr.

The contents of my_custom_board should follow the same guidelines for any Zephyr board, and provide the following files:


Once the board structure is in place, you can build your application targeting this board by specifying the location of your custom board information with the -DBOARD_ROOT parameter to the CMake build system:

cmake -DBOARD=<board name> -DBOARD_ROOT=<path to boards> ..

This will use your custom board configuration and will generate the Zephyr binary into your application directory.

You can also define the BOARD_ROOT variable in the application CMakeLists.txt file.

SOC Definitions

Similar to board support, the structure is similar to how SOCs are maintained in the Zephyr tree, for example:

└── arm
    └── st_stm32
            ├── common
            └── stm32l0

The paths to any Kconfig files inside the structure needs to prefixed with $(SOC_DIR) to make Kconfig aware of the location of the Kconfig files related to the custom SOC.

In the soc directory you will need a top-level Kconfig file pointing to the custom SOC definitions:

        prompt "SoC/CPU/Configuration Selection"

source "$(SOC_DIR)/$(ARCH)/\*/Kconfig.soc"


menu "Hardware Configuration"
osource "$(SOC_DIR)/$(ARCH)/\*/Kconfig"


Once the SOC structure is in place, you can build your application targeting this platform by specifying the location of your custom platform information with the -DSOC_ROOT parameter to the CMake build system:

cmake -DBOARD=<board name> -DSOC_ROOT=<path to soc> -DBOARD_ROOT=<path to boards> ..

This will use your custom platform configurations and will generate the Zephyr binary into your application directory.

You can also define the SOC_ROOT variable in the application CMakeLists.txt file.

Modules (External projects)

Zephyr relies on the source code of several externally maintained projects in order to avoid reinventing the wheel and to reuse as much well-established, mature code as possible when it makes sense. In the context of Zephyr’s build system those are called modules. These modules must be integrated with the Zephyr build system, which is described in more detail in other sections on this page.

There are several categories of external projects that Zephyr depends on, including:

  • Debugger integration
  • Silicon vendor Hardware Abstraction Layers (HALs)
  • Cryptography libraries
  • Filesystems
  • Inter-Process Communication (IPC) libraries

The build system variable ZEPHYR_MODULES is a CMake list of absolute paths to the directories containing Zephyr modules. These modules contain CMakeLists.txt and Kconfig files describing how to build and configure them, respectively. Module CMakeLists.txt files are added to the build using CMake’s add_subdirectory() command, and the Kconfig files are included in the build’s Kconfig menu tree.

If you have west installed, you don’t need to worry about how this variable is defined unless you are adding a new module. The build system knows how to use west to set ZEPHYR_MODULES. You can add additional modules to this list by setting the ZEPHYR_EXTRA_MODULES CMake variable. This can be useful if you want to keep the list of modules found with west and also add your own.

Finally, you can also specify the list of modules yourself in various ways, or not use modules at all if your application doesn’t need them.

Module Initialization Using West

If west is installed and ZEPHYR_MODULES is not already set, the build system finds all the modules in your west installation and uses those. It does this by running west list to get the paths of all the projects in the installation, then filters the results to just those projects which have the necessary module metadata files.

Each project in the west list output is tested like this:

  • If the project contains a file named zephyr/module.yml, then its contents should look like this:

      cmake: <cmake-directory>
      kconfig: <directory>/Kconfig

    The cmake: <cmake-directory> part specifies that <cmake-directory> contains the CMakeLists.txt to use. The kconfig: <directory>/Kconfig part specifies the Kconfig file to use. Neither is required: cmake defaults to zephyr, and kconfig defaults to zephyr/Kconfig.

    Here is an example module.yml file referring to CMakeLists.txt and Kconfig files in the root directory of the module:

      cmake: .
      kconfig: Kconfig
  • Otherwise (i.e. if the project has no zephyr/module.yml), then the build system looks for zephyr/CMakeLists.txt and zephyr/Kconfig files in the project. If both are present, the project is considered a module, and those files will be added to the build.

  • If neither of those checks succeed, the project is not considered a module, and is not added to ZEPHYR_MODULES.

Module Initialization Without West

If you don’t have west installed or don’t want the build system to use it to find Zephyr modules, you can set ZEPHYR_MODULES yourself using one of the following options. Each of the directories in the list must contain either a zephyr/module.yml file or the files zephyr/CMakeLists.txt and Kconfig, as described in the previous section.

  1. At the CMake command line, like this:

    cmake -DZEPHYR_MODULES=<path-to-module1>[;<path-to-module2>[...]] ...
  2. At the top of your application’s top level CMakeLists.txt, like this:

    set(ZEPHYR_MODULES <path-to-module1> <path-to-module2> [...])
    include($ENV{ZEPHYR_BASE}/cmake/app/boilerplate.cmake NO_POLICY_SCOPE)

    If you choose this option, make sure to set the variable before including the boilerplate file, as shown above.

  3. In a separate CMake script which is pre-loaded to populate the CMake cache, like this:

    # Put this in a file with a name like "zephyr-modules.cmake"
    set(ZEPHYR_MODULES <path-to-module1> <path-to-module2>
      CACHE STRING "pre-cached modules")

    You can tell the build system to use this file by adding -C zephyr-modules.cmake to your CMake command line.

Not Using Modules

If you don’t have west installed and don’t specify ZEPHYR_MODULES yourself, then no additional modules are added to the build. You will still be able to build any applications that don’t require code or Kconfig options defined in an external repository.

Application Debugging

This section is a quick hands-on reference to start debugging your application with QEMU. Most content in this section is already covered in QEMU and GNU_Debugger reference manuals.

In this quick reference, you’ll find shortcuts, specific environmental variables, and parameters that can help you to quickly set up your debugging environment.

The simplest way to debug an application running in QEMU is using the GNU Debugger and setting a local GDB server in your development system through QEMU.

You will need an Executable and Linkable Format (ELF) binary image for debugging purposes. The build system generates the image in the build directory. By default, the kernel binary name is zephyr.elf. The name can be changed using a Kconfig option.

We will use the standard 1234 TCP port to open a GDB server instance. This port number can be changed for a port that best suits the development environment.

You can run QEMU to listen for a “gdb connection” before it starts executing any code to debug it.

qemu -s -S <image>

will setup Qemu to listen on port 1234 and wait for a GDB connection to it.

The options used above have the following meaning:

  • -S Do not start CPU at startup; rather, you must type ‘c’ in the monitor.
  • -s Shorthand for -gdb tcp::1234: open a GDB server on TCP port 1234.

To debug with QEMU and to start a GDB server and wait for a remote connect, run the following inside the build directory of an application:

ninja debugserver

The build system will start a QEMU instance with the CPU halted at startup and with a GDB server instance listening at the TCP port 1234.

Using a local GDB configuration .gdbinit can help initialize your GDB instance on every run. In this example, the initialization file points to the GDB server instance. It configures a connection to a remote target at the local host on the TCP port 1234. The initialization sets the kernel’s root directory as a reference.

The .gdbinit file contains the following lines:

target remote localhost:1234


Substitute ZEPHYR_BASE for the current kernel’s root directory.

Execute the application to debug from the same directory that you chose for the gdbinit file. The command can include the --tui option to enable the use of a terminal user interface. The following commands connects to the GDB server using gdb. The command loads the symbol table from the elf binary file. In this example, the elf binary file name corresponds to zephyr.elf file:

..../path/to/gdb --tui zephyr.elf


The GDB version on the development system might not support the –tui option. Please make sure you use the GDB binary from the SDK which corresponds to the toolchain that has been used to build the binary.

If you are not using a .gdbinit file, issue the following command inside GDB to connect to the remote GDB server on port 1234:

(gdb) target remote localhost:1234

Finally, the command below connects to the GDB server using the Data Displayer Debugger (ddd). The command loads the symbol table from the elf binary file, in this instance, the zephyr.elf file.

The DDD may not be installed in your development system by default. Follow your system instructions to install it. For example, use sudo apt-get install ddd on an Ubuntu system.

ddd --gdb --debugger "gdb zephyr.elf"

Both commands execute the gdb. The command name might change depending on the toolchain you are using and your cross-development tools.

Eclipse Debugging


CMake supports generating a project description file that can be imported into the Eclipse Integrated Development Environment (IDE) and used for graphical debugging.

The GNU MCU Eclipse plug-ins provide a mechanism to debug ARM projects in Eclipse with pyOCD, Segger J-Link, and OpenOCD debugging tools.

The following tutorial demonstrates how to debug a Zephyr application in Eclipse with pyOCD in Windows. It assumes you have already installed the GCC ARM Embedded toolchain and pyOCD.

Set Up the Eclipse Development Environment

  1. Download and install Eclipse IDE for C/C++ Developers.
  2. In Eclipse, install the GNU MCU Eclipse plug-ins by opening the menu Window->Eclipse Marketplace..., searching for GNU MCU Eclipse, and clicking Install on the matching result.
  3. Configure the path to the pyOCD GDB server by opening the menu Window->Preferences, navigating to MCU, and setting the Global pyOCD Path.

Generate and Import an Eclipse Project

  1. At a command line, configure your environment to use the GCC ARM Embedded compiler as shown in 3rd Party Toolchains.

  2. Navigate to a folder outside of the Zephyr tree to build your application.

    # On Windows
    cd %userprofile%


    If the build directory is a subdirectory of the source directory, as is usually done in Zephyr, CMake will warn:

    “The build directory is a subdirectory of the source directory.

    This is not supported well by Eclipse. It is strongly recommended to use a build directory which is a sibling of the source directory.”

  3. Configure your application with CMake and build it with ninja. Note the different CMake generator specified by the -G"Eclipse CDT4 - Ninja" argument. This will generate an Eclipse project description file, .project, in addition to the usual ninja build files.

    # On Windows
    mkdir build && cd build
    cmake -G"Eclipse CDT4 - Ninja" -DBOARD=frdm_k64f %ZEPHYR_BASE%\samples\synchronization
  4. In Eclipse, import your generated project by opening the menu File->Import... and selecting the option Existing Projects into Workspace. Browse to your application build directory in the choice, Select root directory:. Check the box for your project in the list of projects found and click the Finish button.

Create a Debugger Configuration

  1. Open the menu Run->Debug Configurations....

  2. Select GDB PyOCD Debugging, click the New button, and configure the following options:

    • In the Main tab:

    • In the Debugger tab:

      • pyOCD Setup
        • Executable path: $pyocd_path\$pyocd_executable
        • Uncheck “Allocate console for semihosting”
      • Board Setup
        • Bus speed: 8000000 Hz
        • Uncheck “Enable semihosting”
      • GDB Client Setup
        • Executable path: C:\gcc-arm-none-eabi-6_2017-q2-update\bin\arm-none-eabi-gdb.exe
    • In the SVD Path tab:

      • File path: <zephyr base>\ext\hal\nxp\mcux\devices\MK64F12\MK64F12.xml


      This is optional. It provides the SoC’s memory-mapped register addresses and bitfields to the debugger.

  3. Click the Debug button to start debugging.

RTOS Awareness

Support for Zephyr RTOS awareness is implemented in pyOCD v0.11.0 and later. It is compatible with GDB PyOCD Debugging in Eclipse, but you must enable CONFIG_OPENOCD_SUPPORT=y in your application.

CMake Details


CMake is used to build your application together with the Zephyr kernel. A CMake build is done in two stages. The first stage is called configuration. During configuration, the CMakeLists.txt build scripts are executed. After configuration is finished, CMake has an internal model of the Zephyr build, and can generate build scripts that are native to the host platform.

CMake supports generating scripts for several build systems, but only Ninja and Make are tested and supported by Zephyr. After configuration, you begin the build stage by executing the generated build scripts. These build scripts can recompile the application without involving CMake following most code changes. However, after certain changes, the configuration step must be executed again before building. The build scripts can detect some of these situations and reconfigure automatically, but there are cases when this must be done manually.

Zephyr uses CMake’s concept of a ‘target’ to organize the build. A target can be an executable, a library, or a generated file. For application developers, the library target is the most important to understand. All source code that goes into a Zephyr build does so by being included in a library target, even application code.

Library targets have source code, that is added through CMakeLists.txt build scripts like this:

target_sources(app PRIVATE src/main.c)

In the above CMakeLists.txt, an existing library target named app is configured to include the source file src/main.c. The PRIVATE keyword indicates that we are modifying the internals of how the library is being built. Using the keyword PUBLIC would modify how other libraries that link with app are built. In this case, using PUBLIC would cause libraries that link with app to also include the source file src/main.c, behavior that we surely do not want. The PUBLIC keyword could however be useful when modifying the include paths of a target library.

Application CMakeLists.txt

Every application must have a CMakeLists.txt file. This file is the entry point, or top level, of the build system. The final zephyr.elf image contains both the application and the kernel libraries.

This section describes some of what you can do in your CMakeLists.txt. Make sure to follow these steps in order.

  1. If you only want to build for one board, add the name of the board configuration for your application on a new line. For example:

    set(BOARD qemu_x86)

    Refer to Supported Boards for more information on available boards.

    The Zephyr build system determines a value for BOARD by checking the following, in order (when a BOARD value is found, CMake stops looking further down the list):

    • Any previously used value as determined by the CMake cache takes highest precedence. This ensures you don’t try to run a build with a different BOARD value than you set during the build configuration step.
    • Any value given on the CMake command line using -DBOARD=YOUR_BOARD will be checked for and used next.
    • If an environment variable BOARD is set, its value will then be used.
    • Finally, if you set BOARD in your application CMakeLists.txt as described in this step, this value will be used.
  2. If your application uses a configuration file or files other than the usual prj.conf (or prj_YOUR_BOARD.conf, where YOUR_BOARD is a board name), add lines setting the CONF_FILE variable to these files appropriately. If multiple filenames are given, separate them by a single space or semicolon. CMake lists can be used to build up configuration fragment files in a modular way when you want to avoid setting CONF_FILE in a single place. For example:

    set(CONF_FILE "fragment_file1.conf")
    list(APPEND CONF_FILE "fragment_file2.conf")

    More details are available below in Kconfig Configuration.

  3. If your application uses a Device Tree overlay file or files other than the usual <board>.overlay, add lines setting the DTC_OVERLAY_FILE variable to these files appropriately.

    More details are available below in Device Tree Overlays.

  4. If your application has its own kernel configuration options, create a Kconfig file in the same directory as your application’s CMakeLists.txt.

    An (unlikely) advanced use case would be if your application has its own unique configuration options that are set differently depending on the build configuration.

    If you just want to set application specific values for existing Zephyr configuration options, refer to the CONF_FILE description above.

    Structure your Kconfig file like this:

    # SPDX-License-Identifier: Apache-2.0
    mainmenu "Your Application Name"
    # Your application configuration options go here
    # Sources Kconfig.zephyr in the Zephyr root directory.
    # Note: All 'source' statements work relative to the Zephyr root directory (due
    # to the $srctree environment variable being set to $ZEPHYR_BASE). If you want
    # to 'source' relative to the current Kconfig file instead, use 'rsource' (or a
    # path relative to the Zephyr root).
    source "Kconfig.zephyr"


    Environment variables in source statements are expanded directly, so you do not need to define an option env="ZEPHYR_BASE" Kconfig “bounce” symbol. If you use such a symbol, it must have the same name as the environment variable.

    See the Kconfig extensions section in the Board Porting Guide for more information.

    The Kconfig file is automatically detected when placed in the application directory, but it is also possible for it to be found elsewhere if the CMake variable KCONFIG_ROOT is set with an absolute path.

  5. Now include the mandatory boilerplate that integrates the application with the Zephyr build system on a new line, after any lines added from the steps above:

    include($ENV{ZEPHYR_BASE}/cmake/app/boilerplate.cmake NO_POLICY_SCOPE)
  6. Now add any application source files to the ‘app’ target library, each on their own line, like so:

    target_sources(app PRIVATE src/main.c)

Below is a simple example CMakeList.txt:

set(BOARD qemu_x86)

include($ENV{ZEPHYR_BASE}/cmake/app/boilerplate.cmake NO_POLICY_SCOPE)

target_sources(app PRIVATE src/main.c)

The Cmake property HEX_FILES_TO_MERGE leverages the application configuration provided by Kconfig and CMake to let you merge externally built hex files with the hex file generated when building the Zephyr application. For example:



CMake uses a CMakeCache.txt file as persistent key/value string storage used to cache values between runs, including compile and build options and paths to library dependencies. This cache file is created when CMake is run in an empty build folder.

For more details about the CMakeCache.txt file see the official CMake documentation runningcmake .

Application Configuration

Kconfig Configuration

The initial configuration for an application is produced by merging configuration settings from three sources:

  1. A BOARD-specific configuration file, stored in boards/ARCHITECTURE/BOARD/BOARD_defconfig in the Zephyr base directory.
  2. Any CMakeCache entries entries that are prefixed with CONFIG_.
  3. One or more application-specific configuration files.

The application-specific configuration file(s) can be specified in any of the following ways. The simplest option is to just have a single prj.conf file.

  1. If CONF_FILE is set in CMakeLists.txt (before including the boilerplate.cmake file), or is present in the CMake variable cache, the configuration files specified in it are merged and used as the application-specific settings.
  2. Otherwise (if (1.) does not apply), if a file prj_BOARD.conf exists in the application directory, where BOARD is the BOARD value set earlier, the settings in it are used.
  3. Otherwise (if (2.) does not apply), if a file boards/BOARD.conf exists in the application directory, where BOARD is the BOARD value set earlier, the settings in it are merged with prj.conf and used.
  4. Otherwise, if a file prj.conf exists in the application directory, the settings in it are used.

Configuration settings that have not been specified fall back on their default value, as given in the Kconfig files.

The merged configuration is saved in zephyr/.config in the build directory.

As long as zephyr/.config exists and is up-to-date (is newer than the BOARD and application configuration files), it will be used in preference to producing a new merged configuration. This can be used during development, as described below in Overriding the Default Configuration.

For more information on Zephyr’s Kconfig configuration scheme, see the Setting configuration values section in the Board Porting Guide. For some tips and general recommendations when writing Kconfig files, see the Kconfig - Tips and Best Practices page.

For information on available kernel configuration options, including inter-dependencies between options, see the Configuration Options.


Dependencies between options can also be viewed in the interactive configuration interface, which is explained in the Overriding the Default Configuration section. It will have the most up-to-date dependencies, and also shows which dependencies are currently unsatisfied.

To view the dependencies of an option in the configuration interface, jump to it with / and press ?. For each unsatisfied dependency, jump to it in turn to check its dependencies.

Setting Application Configuration Values

This section describes how to edit Zephyr configuration (.conf) files.

  • Add each configuration entry on a new line.

  • Enable or disable a boolean option by setting its value to y or n:



    Another way to set a boolean symbol to n is with a comment with the following format:

    # CONFIG_SOME_OTHER_BOOL is not set

    This style is accepted for a technical reason: Kconfig configuration files can be parsed as makefiles (though Zephyr doesn’t use this). Having n-valued symbols correspond to unset variables simplifies tests in Make.

  • You can set integer and string options as well, like this:

    CONFIG_SOME_STRING="the best value ever"
  • Ensure that each entry setting an option contains no spaces (including on either side of the = sign).

  • Use a # followed by a space to comment a line:

    # This is a comment.

The example below shows a comment line and an override setting CONFIG_PRINTK to y:

# Enable printk for debugging

Overriding the Default Configuration

An interactive configuration interface is available for making temporary changes to the configuration. This can be handy during development.


The configuration can also be changed by editing zephyr/.config in the application build directory by hand. Using the configuration interface is safer, as it correctly handles dependencies between configurations symbols.

To make a setting permanent, you should set it in a .conf file, as described above in Setting Application Configuration Values.

The steps below will run the interactive configuration interface:

  1. Create a build directory <home>/app/build inside your application directory and generate build files inside it with CMake, as follows:

    # On Linux/macOS
    cd ~/app
    # On Windows
    cd %userprofile%\app
    mkdir build && cd build
    cmake -GNinja ..
  2. Run the following command from the build directory (<home>/app/build) to start the configuration interface:

    ninja menuconfig

    The configuration interface is shown below:

    Main Configuration Menu
  3. Change configuration symbols to their desired values as follows:

    • Use the arrow keys to navigate the menu.


      Common Vim key bindings are supported as well.

    • Press Enter or Space to enter submenus and choices, which appear with ---> next to them. Press ESC returns to the parent menu.

    • Press Space to toggle or configure a symbol value. Boolean configuration symbols are shown with [ ] brackets, while numeric and string-valued configuration symbols are shown with ( ) brackets.


      You can also press Y or N to set a boolean configuration symbol, to the corresponding value.

    • Press ? to display information about the currently selected symbol. Press ESC or Q to return from the information display to the menu.

  4. After configuring the kernel options for your application, press Q to bring up the save-and-quit dialog:

    Save and Quit Dialog
  5. Press Y to save the kernel configuration options to the default filename (zephyr/.config).

    Typically, you will save to the default filename unless you are experimenting with various configuration scenarios.


    At present, the configuration file used during building is always zephyr/.config. If you have another saved configuration that you want to build with, copy it to zephyr/.config. Make sure to back up your original configuration file.

    Also note that filenames starting with . are not listed by ls by default on Linux and macOS. Use the -a flag to see them.

Finding a symbol in the menu tree and navigating to it can be tedious. To jump directly to a symbol, press the / key. This brings up the following dialog, where you can search for symbols by name and jump to them:

Menuconfig Search Dialog

If you jump to a symbol that isn’t currently visible (e.g., due to having unsatisfied dependencies) then show-all mode will be enabled. In show-all mode, all symbols are displayed, including currently invisible symbols. To disable show-all mode, press A.


Show-all mode can’t be disabled if there are no visible items in the menu.

To figure out why a symbol you jumped to isn’t visible, inspect its dependencies by pressing ?. If you discover that the symbol depends on another symbol that isn’t enabled, you can jump to that symbol, in turn, to see if it can be enabled.

Device Tree Overlays

As described in Device Tree, Zephyr uses Device Tree to describe the hardware it runs on. This section describes how you can modify an application build’s device tree using overlay files. For additional information regarding the relationship between Device Tree and Kconfig see Device Tree vs Kconfig. In some cases the information contained in Device Tree files is closely connected to the software and might need to be modified using the overlay file concept. This can be relevant for many of the different Device Tree nodes, but is particularly useful for certain types of nodes.

Overlay files, which customarily have the .overlay extension, contain device tree fragments which add to or modify the device tree used while building a Zephyr application. To add an overlay file or files to the build, set the CMake variable DTC_OVERLAY_FILE to a whitespace-separated list of your overlay files.

The Zephyr build system begins creation of a device tree by running the C preprocessor on a file which includes the following:

  1. Configuration options from Kconfig.

  2. The board’s device tree source file, which by default is the Zephyr file boards/<ARCHITECTURE>/<BOARD>/<BOARD>.dts. (This location can be overridden by setting the DTS_SOURCE CMake variable.)

  3. Any “common” overlays provided by the build system. Currently, this is just the file dts/common/common.dts. (The common overlays can be overridden by setting the DTS_COMMON_OVERLAYS CMake variable.)

    The file common.dts conditionally includes device tree fragments based on Kconfig settings. For example, it includes a fragment for MCUboot chain-loading, located at dts/common/mcuboot.overlay, if CONFIG_BOOTLOADER_MCUBOOT is set.

  4. Any file or files given by the DTC_OVERLAY_FILE CMake variable.

The Zephyr build system determines DTC_OVERLAY_FILE as follows:

  • Any value given to DTC_OVERLAY_FILE in your application CMakeLists.txt (before including the boilerplate.cmake file), passed to the the CMake command line, or present in the CMake variable cache, takes precedence.
  • The environment variable DTC_OVERLAY_FILE is checked next. This mechanism is now deprecated; users should set this variable using CMake instead of the environment.
  • If the file BOARD.overlay exists in your application directory, where BOARD is the BOARD value set earlier, it will be used.

If DTC_OVERLAY_FILE specifies multiple files, they are included in order by the C preprocessor.

After running the preprocessor, the final device tree used in the build is created by running the device tree compiler, dtc, on the preprocessor output.

Application-Specific Code

Application-specific source code files are normally added to the application’s src directory. If the application adds a large number of files the developer can group them into sub-directories under src, to whatever depth is needed.

Application-specific source code should not use symbol name prefixes that have been reserved by the kernel for its own use. For more information, see Naming Conventions.

Support for building third-party library code

It is possible to build library code outside the application’s src directory but it is important that both application and library code targets the same Application Binary Interface (ABI). On most architectures there are compiler flags that control the ABI targeted, making it important that both libraries and applications have certain compiler flags in common. It may also be useful for glue code to have access to Zephyr kernel header files.

To make it easier to integrate third-party components, the Zephyr build system has defined CMake functions that give application build scripts access to the zephyr compiler options. The functions are documented and defined in cmake/extensions.cmake and follow the naming convention zephyr_get_<type>_<format>.

The following variables will often need to be exported to the third-party build system.

  • ARCH and BOARD, together with several variables that identify the Zephyr kernel version.

samples/application_development/external_lib is a sample project that demonstrates some of these features.