Application Development Primer


The Zephyr Kernel’s build system is based on the Kbuild system used in the Linux kernel.

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

The Zephyr Kernel’s base directory hosts the kernel source code, the configuration options, and the kernel build definitions.

The files in the application directory links the kernel with the application. It hosts the definitions of the application, for example, application-specific configuration options and the application’s source code.

An application in the simplest form has the following structure:

  • 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.
  • Kernel configuration files: An application typically provides a configuration file (.conf) that specifies values for one or more kernel configuration options. If omitted, the application’s existing kernel configuration option values are used; if no existing values are provided, the kernel’s default configuration values are used.
  • Makefile: This file tells the build system where to find the files mentioned above, as well as the desired target board configuration.

Once the application has been defined, it can be built with a single make call. The results of the build process are located in a sub-directory called outdir/BOARD. This directory contains the files generated by the build process, the most notable of which are listed below.

  • The .config file that contains the configuration settings used to build the application.
  • The various object files (.o files and .a files) containing custom-built kernel and application-specific code.
  • The zephyr.elf file that contains the final combined application and kernel binary.

Application Structure

Create one directory for your application and a sub-directory for the application’s source code; this makes it easier to organize directories and files in the structure that the kernel expects.

  1. Create an application directory structure outside of the kernel’s installation directory tree. Often this is your workspace directory.

  2. In a console terminal, navigate to a location where you want your application to reside.

  3. Create the application’s directory, enter:

    $ mkdir app


    This directory and the path to it, are referred to in the documentation as ~/app.

  4. Create a source code directory in your ~/app, enter:

    $ cd app
    $ mkdir src

    The source code directory ~/app/src is created.

    -- app
       |-- src

Application Definition

An application is integrated into the build system by including the file provided.

include $(ZEPHYR_BASE)/

The following predefined variables configure the development project:

  • ZEPHYR_BASE: Sets the path to the kernel’s base directory. This variable is usually set by the script. It can be used to get the kernel’s base directory, as used in the inclusion above, or it can be overridden to select an specific instance of the kernel.
  • PROJECT_BASE: Provides the developer’s application project directory path. It is set by the file.
  • SOURCE_DIR: Overrides the default value for the application’s source code directory. The developer source code directory is set to $(PROJECT_BASE)/src/ by default. This directory name should end with slash ‘/’.
  • BOARD: Selects the board that the application’s build will use for the default configuration.
  • CONF_FILE: Indicates the name of a configuration fragment file. This file includes the kconfig configuration values that override the default configuration values.
  • O: Optional. Indicates the output directory that Kconfig uses. The output directory stores all the files generated during the build process. The default output directory is the $(PROJECT_BASE)/outdir directory.



The build system defines a set of conventions for the correct use of Makefiles in the kernel source directories. The correct use of Makefiles is driven by the concept of recursion.

In the recursion model, each Makefile within a directory includes the source code and any subdirectories to the build process. Each subdirectory follows the same principle. Developers can focus exclusively in their own work. They integrate their module with the build system by adding a very simple Makefile following the recursive model.

Makefile Conventions

The following conventions restrict how to add modules and Makefiles to the build system. These conventions ensure the correct implementation of the recursive model.

  • Each source code directory must contain a single Makefile. Directories without a Makefile are not considered source code directories.
  • The scope of every Makefile is restricted to the contents of that directory. A Makefile can only make a direct reference to files and subdirectories on the same level or below.
  • Makefiles list the object files that are included in the link process. The build system finds the source file that generates the object file by matching the object file name to the source file.
  • Parent directories add their child directories into the recursion model.
  • The root Makefile adds the directories in the kernel base directory into the recursion model.

Adding Source Files

The Makefile must refer the source build indirectly, specifying the object file that results from the source file using the obj-y variable. For example, if the file that we want to add is a C file named <file>.c the following line should be added in the Makefile:

obj-y += <file>.o


The same method applies for assembly files with the .S extension.

Source files can be added conditionally using configuration options. For example, if the option CONFIG_VAR is set and it implies that a source file must be added in the compilation process, then the following line adds the source code conditionally:

obj-$(CONFIG_VAR) += <file>.o

Adding Directories

Add a subdirectory to the build system by editing the Makefile in its directory. The subdirectory is added using the obj-y variable. The correct syntax to add a subdirectory into the build queue is:

obj-y += <directory_name>/

The backslash at the end of the directory name signals the build system that a directory, and not a file, is being added to the build queue.

The conventions require us to add only one directory per line and never to mix source code with directory recursion in a single obj-y line. This helps keep the readability of the Makefile by making it clear when an item adds an additional lever of recursion.

Directories can also be conditionally added:

obj-y-$(CONFIG_VAR) += <directory_name>/

The subdirectory must contain its own Makefile following the rules described in Makefile Conventions.

Application Makefile

Create an application Makefile to define basic information, such as the board configuration used by the application. The build system uses the Makefile to build a zephyr.elf image that contains both the application and the kernel libraries.

  1. Open the Makefile and add the following mandatory entries using any standard text editor.


    Ensure that there is a space before and after each =.

  2. Add the name of the default board configuration for your application on a new line:

    BOARD = board_configuration_name

    The supported boards can be found in Supported Boards.

  3. Add the name of the default kernel configuration file for your application on a new line:

    CONF_FILE ?= kernel_configuration_name
  4. Include the mandatory Makefile on a new line:

    include ${ZEPHYR_BASE}/
  5. Save and close the Makefile.

Below is an example Makefile:

BOARD = qemu_x86
CONF_FILE = prj.conf

include ${ZEPHYR_BASE}/

Application Configuration

The application’s kernel is configured using a set of configuration options that can be customized for application-specific purposes. The Zephyr build system takes a configuration option’s value from the first source in which it is specified.

The available sources are (in order):

  1. The application’s current configuration. (i.e. The .config file.)
  2. The application’s default configuration. (i.e. The .conf file.)
  3. The board configuration used by the application. (i.e. The board’s .defconfig file.)
  4. The kernel’s default configuration. (i.e. One of the kernel’s Kconfig files.)

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

Default Board Configuration

An application’s .conf file defines its default kernel configuration. The settings in this file override or augment the board configuration settings.

The board configuration settings can be viewed LENGTHlWRONGEPHY _BASE/boards/ARCHITECTURE/BOARD/BOARD_defconfig`.


When the default board configuration settings are sufficient for your application, a .conf file is not needed. Skip ahead to Overriding Default Configuration.

  1. Navigate to the app, and create the prj.conf file. Enter:

    $ vim prj.conf

    The default name is prj.conf. The filename must match the CONF_FILE entry in the application Makefile.

  2. Edit the file and add the appropriate configuration entries.

    1. Add each configuration entry on a new line.
    2. Begin each entry with CONFIG_.
    3. Ensure that each entry contains no spaces (including on either side of the = sign).
    4. Use a # followed by a space to comment a line.

    The example below shows a comment line and a board configuration override in the prj.conf.

    # Enable printk for debugging
  3. Save and close the file.

Overriding Default Configuration

Override the default board and kernel configuration to temporarily alter the application’s configuration, perhaps to test the effect of a change.


If you want to permanently alter the configuration you should revise the .conf file.

Configure the kernel options using a menu-driven interface. While you can add entries manually, using the configuration menu is a preferred method.

  1. Run the make menuconfig rule to launch the menu-driven interface.

    1. In a terminal session, navigate to the application directory (~/app).

    2. Enter the following command:

      $ make [BOARD=<type>] menuconfig

      A question-based menu opens that allows you to set individual configuration options.

      Main Configuration Menu
  2. Set kernel configuration values using the following key commands:

    • Use the arrow keys to navigate within any menu or list.

    • Press Enter to select a menu item.

    • Type an upper case Y or N in the

      square brackets [ ] to enable or disable a kernel configuration option.

    • Type a numerical value in the round brackets ( ).

    • Press Tab to navigate the command menu at the bottom of the display.


      When a non-default entry is selected for options that are non-numerical, an asterisk * appears between the square brackets in the display. There is nothing added added the display when you select the option’s default.

  3. For information about any option, select the option and tab to <Help > and press Enter.

    Press Enter to return to the menu.

  4. After configuring the kernel options for your application, tab to < Save > and press Enter.

    The following dialog opens with the < Ok > command selected:

    Save Configuration Dialog
  5. Press Enter to save the kernel configuration options to the default file name; alternatively, type a file name and press Enter.

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

    An outdir directory is created in the application directory. The outdir directory contains symbolic links to files under $ZEPHYR_BASE.


    At present, only a .config file can be built. If you have saved files with different file names and want to build with one of these, change the file name to .config. To keep your original .config, rename it to something other than .config.

    Kernel configuration files, such as the .config file, are saved as hidden files in outdir. To list all your kernel configuration files, enter ls -a at the terminal prompt.

    The following dialog opens, displaying the file name the configuration was saved to.

    Saved Configuration Name Dialog
  6. Press Enter to return to the options menu.

  7. To load any saved kernel configuration file, tab to < Load > and press Enter.

    The following dialog opens with the < Ok > command selected:

    Configuration File Load Dialog
  8. To load the last saved kernel configuration file, press < Ok >, or to load another saved configuration file, type the file name, then select < Ok >.

  9. Press Enter to load the file and return to the main menu.

  10. To exit the menu configuration, tab to < Exit > and press Enter.

    The following confirmation dialog opens with the < Yes > command selected.

    Exit Dialog
  11. Press Enter to retire the menu display and return to the console command line.

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. The developer must supply a Makefile for the src directory, as well as for each sub-directory under src.


These Makefiles are distinct from the top-level application Makefile that appears in ~/app.

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.

The following requirements apply to all Makefiles in the src directory, including any sub-directories it may have.

  • During the build process, Kbuild identifies the source files to compile into object files by matching the file names identified in the application’s Makefile(s).


    A source file that is not referenced by any Makefile is not included when the application is built.

  • A Makefile cannot directly reference source code. It can only reference object files (.o files) produced from source code files.

  • A Makefile can only reference files in its own directory or in the sub-directories of that directory.

  • A Makefile may reference multiple files from a single-line entry. However, a separate line must be used to reference each directory.

  1. Create a directory structure for your source code in src and add your source code files to it.

  2. Create a Makefile in the src directory. Then create an additional Makefile in each of the sub-directories under the src directory, if any.

    1. Use the following syntax to add file references:

      obj-y += file1.o file2.o
    2. Use the following syntax to add directory references:

      obj-y += directory_name/**

This example is taken from the Dining Philosophers. To examine this file in context, navigate to: $ZEPHYR_BASE/samples/philosophers/src.

obj-y = main.o

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 includes a special build target, outputexports, that takes a number of critical variables from the Zephyr build system and copies them into Makefile.export. This allows the critical variables to be included by wrapper code for use in a third-party build system.

The following variables are recommended for use within the third-party build (see Makefile.export for the complete list of exported variables):

  • CROSS_COMPILE, together with related convenience variables to call the cross-tools directly (including AR, AS, CC, CXX, CPP and LD).
  • ARCH and BOARD, together with several variables that identify the Zephyr kernel version.
  • KBUILD_CFLAGS, NOSTDINC_FLAGS and ZEPHYRINCLUDE all of which should normally be added, in that order, to CFLAGS (or CXXFLAGS).
  • All kconfig variables, allowing features of the library code to be enabled/disabled automatically based on the Zephyr kernel configuration.

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

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.

  1. Navigate to the application directory ~/app.

  2. Enter the following command to build the application’s zephyr.elf image using the configuration settings for the board type specified in the application’s Makefile.

    $ make

    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:

    $ make CONF_FILE=prj.alternate.conf

    If desired, you can build the application for a different board type than the one specified in the application’s Makefile using the BOARD parameter. For example:

    $ make BOARD=arduino_101

    Both the CONF_FILE and BOARD parameters can be specified when building the application.

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, Makefiles, 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. Navigate to the application directory ~/app.

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

    $ make [BOARD=<type>] clean

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

    $ make 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 real or emulated hardware. The kernel has built-in emulator support for QEMU. It allows you to run and test an application virtually, before (or in lieu of) loading and running it on actual target hardware.

  1. Open a terminal console and navigate to the application directory ~/app.

  2. Enter the following command to build and run the application using a QEMU-supported board configuration, such as qemu_cortex_m3 or qemu_x86.

    $ make [BOARD=<type> ...] run

    The Zephyr build system generates a zephyr.elf image file and then begins running it in the terminal console.

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

    The application stops running and the terminal console prompt redisplays.

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 on QEMU and GNU_Debugger reference manuals.

In this quick reference you 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 ELF binary image for debugging purposes. The build system generates the image in the output 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 an application:

make BOARD=qemu_x86 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:

$ gdb --tui zephyr.elf


The GDB version on the development system might not support the –tui option.

If you are not using a .gdbinit file, issue the following command inside GDB to connect to the remove 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.

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.