zephyr/doc/application/application.rst

.. _application:

Application Development Primer
##############################

.. note::

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

Overview
********

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:

.. code-block:: none

   <home>/app
   ├── 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 (ending in :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.

* **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 :file:`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.

.. important::

   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.

Creating an Application
***********************

Follow these steps to create a new application directory. (Refer to
:ref:`samples-and-demos` for existing applications provided as part of Zephyr.)

#. 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, navigate to a location where you want your
   application to reside, then enter:

   .. code-block:: console

      mkdir app

#. It's recommended to place all application source code in a subdirectory
   named :file:`src`.  This makes it easier to distinguish between project
   files and sources.

   Continuing the Unix shell example from the previous step, enter:

   .. code-block:: console

      cd app
      mkdir src

#. Create an empty :file:`CMakeLists.txt` file in your application directory.

#. Include the :file:`boilerplate.cmake` to pull in the Zephyr build system:

   .. code-block:: cmake

      include($ENV{ZEPHYR_BASE}/cmake/app/boilerplate.cmake NO_POLICY_SCOPE)
      project(NONE)

#. Place your application source code in the :file:`src` sub-directory. For
   this example, we'll assume you created a file named :file:`src/main.c`.

#. Add your source code files to the ``app`` target in your application
   directory's :file:`CMakeLists.txt`. For example, to add :file:`src/main.c`,
   add the following line to your :file:`CMakeLists.txt`:

   .. code-block:: cmake

      target_sources(app PRIVATE src/main.c)

#. Configure features used by your application. Zephyr's configuration uses
   the same Kconfig and Device Tree systems used by the Linux kernel, but with
   its own configuration trees. Usually, you just create a file named
   :file:`prj.conf` in your application directory, where you enable or disable
   features provided by Zephyr's Kconfig configuration system. Optionally you
   can also configure any Device Tree overlays needed by your application
   (this is usually not necessary; see :ref:`application_dt` below for more
   details). You can use existing samples to get started. For more information,
   see :ref:`application_configuration` below.

Applications integrate with the Zephyr build system using the boilerplate code
shown above in :file:`CMakeLists.txt`. The following important variables
configure the Zephyr build system:

* :makevar:`ZEPHYR_BASE`: Sets the path to the Zephyr base directory.  This is
  usually an environment variable set by the :file:`zephyr-env.sh` script on
  Linux/macOS or manually on Windows, as you learned when getting started
  with Zephyr in :ref:`getting_started_run_sample`. You can also set
  :makevar:`ZEPHYR_BASE`: explicitly on Linux and macOS if you want to.

* :makevar:`BOARD`: Selects the board that the application's build will use for
  the default configuration. This can be defined in the environment, in your
  application's :file:`CMakeLists.txt` file, or in the ``cmake`` command line.

* :makevar:`CONF_FILE`: Indicates the name of one or more configuration
  fragment files.  Each file includes kconfig configuration values that
  override the default configuration values.  Like :makevar:`BOARD`, this can
  also be defined in the environment, in your application's
  :file:`CMakeLists.txt` file, or in the ``cmake`` command line.

* :makevar:`DTC_OVERLAY_FILE`: Indicates the name of one or more Device Tree
  overlay files.  Each file includes Device Tree values that
  override the default DT values.  Like :makevar:`CONF_FILE`, this
  can also be defined in the environment, in your application's
  :file:`CMakeLists.txt` file, or in the ``cmake`` command line.

.. _build_an_application:

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 :ref:`getting_started_cmake`, 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.

Basics
======

#. Navigate to the application directory :file:`<home>/app`.

#. Enter the following commands to build the application's
   :file:`zephyr.elf` image using the configuration settings for the
   board type specified in the application's :file:`CMakeLists.txt`.

   .. code-block:: console

       mkdir build
       cd build
       cmake -GNinja ..
       ninja

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

   .. code-block:: console

       # On Linux/macOS
       export CONF_FILE=prj.alternate.conf
       # On Windows
       set CONF_FILE=prj.alternate.conf

       cmake -GNinja ..
       ninja

   If desired, you can generate project files for a different board
   type than the one specified in the application's
   :file:`CMakeLists.txt` by defining the environment variable
   :code:`BOARD`.

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

Build Directory Contents
========================

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

.. code-block:: none

   <home>/app/build
   ├── build.ninja
   ├── CMakeCache.txt
   ├── CMakeFiles
   ├── cmake_install.cmake
   ├── rules.ninja
   └── zephyr

The most notable files in the build directory are:

* :file:`build.ninja`, which can be invoked to build the application.

* A :file:`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 :file:`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.)

* :file:`.config`, which contains the configuration settings
  used to build the application.

* Various object files (:file:`.o` files and :file:`.a` files) containing
  compiled kernel and application code.

* :file:`zephyr.elf`, which contains the final combined application and
  kernel binary. Other binary output formats, such as :file:`.hex` and
  :file:`.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.

.. important::

    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:


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

#. Enter the following command to delete the application's generated
   files, except for the :file:`.config` file that contains the
   application's current configuration information.

   .. code-block:: console

       ninja clean

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

   .. code-block:: console

       ninja pristine

#. Rebuild the application normally following the steps specified
   in :ref:`build_an_application` above.


.. _application_run:

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:

#. Build your application, as described in :ref:`build_an_application`.

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

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

   .. code-block:: console

      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 :ref:`your board's documentation <boards>`
for additional information on how to flash your board.

.. note:: 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:

#. Build your application for one of the QEMU boards, as described in
   :ref:`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

#. Run this console command from the build directory, :file:`<home>/app/build`,
   to flash the compiled Zephyr binary and run it in QEMU:

   .. code-block:: console

      ninja run

#. Press :kbd:`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.

.. _application_debugging:

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.

.. _QEMU: http://wiki.qemu.org/Main_Page

.. _GNU_Debugger: http://www.gnu.org/software/gdb

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
:file:`zephyr.elf`. The name can be changed using a Kconfig option.

We will use the standard 1234 TCP port to open a :abbr:`GDB (GNU Debugger)`
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.

.. code-block:: bash

   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 :literal:`-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:

.. code-block:: bash

   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 :file:`.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 :file:`.gdbinit` file contains the following lines:

.. code-block:: bash

   target remote localhost:1234
   dir ZEPHYR_BASE

.. note::

   Substitute ZEPHYR_BASE for the current kernel's root directory.

Execute the application to debug from the same directory that you chose for
the :file:`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 :file:`gdb`. The command loads the symbol
table from the elf binary file. In this example, the elf binary file name
corresponds to :file:`zephyr.elf` file:

.. code-block:: bash

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

.. note::

   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:

.. code-block:: bash

   (gdb) target remote localhost:1234

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

The :abbr:`DDD (Data Displayer Debugger)` 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.

.. code-block:: bash

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


Both commands execute the :abbr:`gdb (GNU Debugger)`. The command name might
change depending on the toolchain you are using and your cross-development
tools.

CMake Details
*************

Overview
========

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:

.. code-block:: cmake

   target_sources(app PRIVATE src/main.c)

In the above :file:`CMakeLists.txt`, an existing library target named ``app``
is configured to include the source file :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 :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 :file:`CMakeLists.txt` file. This file is the
entry point, or top level, of the build system. The final :file:`zephyr.elf`
image contains both the application and the kernel libraries.

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

#. 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:

   .. code-block:: cmake

      set(BOARD qemu_x86)

   Refer to :ref:`boards` for more information on available boards.

   The Zephyr build system determines a value for :makevar:`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
     :makevar:`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 :file:`CMakeLists.txt`
     as described in this step, this value will be used.

#. If your application uses a configuration file or files other than
   the usual :file:`prj.conf` (or :file:`prj_YOUR_BOARD.conf`, where
   ``YOUR_BOARD`` is a board name), add lines setting the
   :makevar:`CONF_FILE` variable to these files appropriately.

   More details are available below in :ref:`application_kconfig`.

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

   More details are available below in :ref:`application_dt`.

#. If your application has its own kernel configuration options, add a
   line setting the location of the Kconfig file that defines them.

   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 :makevar:`CONF_FILE` description above.

   For example, if you have a file named :file:`Kconfig` in the same directory
   as your application's :file:`CMakeLists.txt`, add the following line:

   .. code-block:: cmake

      set(KCONFIG_ROOT ${CMAKE_CURRENT_SOURCE_DIR}/Kconfig)

   Make sure to include the following lines in your :file:`Kconfig` file before
   any application-specific configuration options:

   .. literalinclude:: application-kconfig.include

   .. important::

      The indented lines above must use tabs, not spaces.

#. 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**:

   .. code-block:: cmake

      include($ENV{ZEPHYR_BASE}/cmake/app/boilerplate.cmake NO_POLICY_SCOPE)
      project(NONE)

#. Now add any application source files to the 'app' target
   library, each on their own line, like so:

   .. code-block:: cmake

      target_sources(app PRIVATE src/main.c)

Below is a simple example :file:`CMakeList.txt`:

.. code-block:: cmake

   set(BOARD qemu_x86)

   include($ENV{ZEPHYR_BASE}/cmake/app/boilerplate.cmake NO_POLICY_SCOPE)
   project(NONE)

   target_sources(app PRIVATE src/main.c)

.. _application_configuration:

Application Configuration
*************************

.. _application_kconfig:

Kconfig Configuration
=====================

The application is configured using a set of 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, taken from the
following available sources, in order:

#. An application's current configuration (i.e. the file
   :file:`zephyr/.config` in the build directory). This can be used
   during development as described below in :ref:`override_kernel_conf`.

#. The application's configuration file(s) given by the
   :makevar:`CONF_FILE` variable, either as set explicitly by the user
   or using one of the default values detailed below.

#. The board's default configuration for the current :makevar:`BOARD`
   setting (i.e. the :file:`boards/ARCHITECTURE/BOARD/BOARD_defconfig`
   file in the Zephyr base directory).

#. The kernel's default configuration settings (i.e. the default value given to
   the option in one of Zephyr's :file:`Kconfig` files).

The Zephyr build system determines a value for :makevar:`CONF_FILE` by
checking the following until one is found, in order:

- Any value given to :makevar:`CONF_FILE` in your application
  :file:`CMakeLists.txt` (**before including the boilerplate.cmake file**),
  passed to the the CMake command line, or present in the CMake variable cache,
  takes precedence.

- If a CMake command, macro, or function ``set_conf_file`` is defined, it
  will be invoked and must set :makevar:`CONF_FILE`.

- If the file :file:`prj_BOARD.conf` exists in your application directory,
  where ``BOARD`` is the BOARD value set earlier, it will be used.

- Finally, if :file:`prj.conf` exists in your application directory, it will
  be used.

If :makevar:`CONF_FILE` specifies multiple files, they will be merged in order.

For information on available kernel configuration options, including
inter-dependencies between options, see the :ref:`configuration`.

.. _application_set_conf:

Setting Application Configuration Values
----------------------------------------

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

- Add each configuration entry on a new line.

- Enable a boolean option by setting its value to ``y``:

  .. code-block:: none

     CONFIG_SOME_BOOL=y

  To ensure that a boolean configuration option is not set, add a line
  like this instead (including the leading ``#`` symbol):

  .. code-block:: none

     # CONFIG_SOME_BOOL is not set

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

  .. code-block:: none

     CONFIG_SOME_INT=42
     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:

  .. code-block:: none

     # This is a comment.

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

.. code-block:: c

    # Enable printk for debugging
    CONFIG_PRINTK=y

.. _override_kernel_conf:

Overriding Default Configuration
--------------------------------

Follow these steps to override an application's configuration
temporarily, perhaps to test the effect of a change.

.. note::

   If you want to permanently alter the configuration you should set
   the new value in a :file:`.conf` file, as described above in
   :ref:`application_set_conf`.

The steps below describe how to configure your application using a
menu-driven configurator interface. While you can edit your
application's :file:`.config` manually, using a configurator tool is
preferred, since it correctly handles dependencies between options.


#. Generate a Make build system, and use it to run ``ninja
   menuconfig`` as follows.

   a) Using CMake, create a build directory (:file:`<home>/app/build`) from
      your application directory (:file:`<home>/app`).

      For example, on a shell or command prompt:

      .. code-block:: bash

         # On Linux/macOS
         cd ~/app
         # On Windows
         cd %userprofile%\app

         mkdir build && cd build
         cmake -GNinja ..

   b) Run ``ninja menuconfig`` from the build directory
      (:file:`<home>/app/build`).

      Continuing the above Unix shell example:

      .. code-block:: bash

          ninja menuconfig

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

      .. image:: figures/app_kernel_conf_1.png
           :width: 600px
           :align: center
           :alt: Main Configuration Menu

#. Set kernel configuration values using the following
   key commands:

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

   * Press :kbd:`Enter` to select a menu item.

   * Type an upper case :kbd:`Y` or :kbd:`N` in the
      square brackets :guilabel:`[ ]` to
      enable or disable a kernel configuration option.

   * Type a numerical value in the parentheses :guilabel:`( )`.

   * Press :kbd:`Tab` to navigate the command menu at the bottom of the
     display.

     .. note::

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

#. For information about any option, select the option and tab to
   :guilabel:`<Help >` and press :kbd:`Enter`.

   Press :kbd:`Enter` to return to the menu.

#. Press :kbd:`/` to bring up a search menu to look for a particular option.

#. After configuring the kernel options for your application, tab to
   :guilabel:`< Save >` and press :kbd:`Enter`.

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

   .. image:: figures/app_kernel_conf_2.png
      :width: 400px
      :align: center
      :height: 100px
      :alt: Save Configuration Dialog


#. Press :kbd:`Enter` to save the kernel configuration options to the default
   file name; alternatively, type a file name and press :kbd:`Enter`.

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

   A :file:`zephyr` directory will have been created in the build
    directory.

   .. note::

      At present, only a :file:`.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 :file:`.config`. To keep your original
      :file:`.config`, rename it to something other than :file:`.config`.

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

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

   .. image:: figures/app_kernel_conf_3.png
         :width: 400px
         :align: center
         :height: 150px
         :alt: Saved Configuration Name Dialog

#. Press :kbd:`Enter` to return to the options menu.

#. To load any saved kernel configuration file, tab to :guilabel:`< Load >` and
   press :kbd:`Enter`.

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

   .. image:: figures/app_kernel_conf_4.png
      :width: 400px
      :align: center
      :height: 175px
      :alt: Configuration File Load Dialog

#. To load the last saved kernel configuration file, press :guilabel:`< Ok >`,
   or to load another saved configuration file, type the file name, then select
   :guilabel:`< Ok >`.

#. Press :kbd:`Enter` to load the file and return to the main menu.

#. To exit the menu configuration, tab to :guilabel:`< Exit >` and press
   :kbd:`Enter`.

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

   .. image:: figures/app_kernel_conf_5.png
      :width: 400px
      :align: center
      :height: 100px
      :alt: Exit Dialog

#. Press :kbd:`Enter` to retire the menu display and return to the console
   command line.

.. _application_dt:

Device Tree Overlays
====================

Zephyr uses Device Tree to describe the hardware it runs on. Sometimes it is
necessary to modify the definitions in the :file:`.dts` files provided in the
board folder in order to customize settings for a particular application.
For more information on Device Tree see :ref:`device-tree`.

The Zephyr build system takes a Device Tree configuration
option's value from the first source in which it is specified, taken from the
following available sources, in order:

#. An application's current DT configuration (i.e. the file
   :file:`zephyr/<board>.dts.pre.tmp` in the build directory).

#. The application's DT configuration file(s) given by the
   :makevar:`DTC_OVERLAY_FILE` variable, either as set explicitly by the user
   or using one of the default values detailed below.

#. Common overlays conditionally included by :file:`dts/common/common.dts`.

#. The board's default DT configuration for the current :makevar:`BOARD`
   setting (i.e. the :file:`boards/ARCHITECTURE/BOARD/BOARD.dts`
   file in the Zephyr base directory).

#. The kernel's default DT configuration settings (i.e. the default value given
   to the option in one of Zephyr's :file:`dtsi` files).

The Zephyr build system determines a value for :makevar:`DTC_OVERLAY_FILE` by
checking the following until one is found, in order:

- Any value given to :makevar:`DTC_OVERLAY_FILE` in your application
  :file:`CMakeLists.txt` (**before including the boilerplate.cmake file**),
  passed to the the CMake command line, or present in the CMake variable cache,
  takes precedence

- If the file :file:`<board>.overlay` exists in your application directory,
  where ``BOARD`` is the BOARD value set earlier, it will be used.

If :makevar:`DTC_OVERLAY_FILE` specifies multiple files, they will be merged in
order.

The build system will automatically prepend :file:`dts/common/common.dts` to
:makevar:`DTC_OVERLAY_FILE`, which conditionally includes certain overlays that
are considered common to all applications. This includes the required overlay
for MCUboot chain-loading (located in :file:`dts/comon/mcuboot.overlay`)
whenever the :option:`CONFIG_BOOTLOADER_MCUBOOT` is set using Kconfig.

Application-Specific Code
*************************

Application-specific source code files are normally added to the
application's :file:`src` directory. If the application adds a large
number of files the developer can group them into sub-directories
under :file:`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
<https://github.com/zephyrproject-rtos/zephyr/wiki/Naming-Conventions>`_.

Support for building third-party library code
=============================================

It is possible to build library code outside the application's :file:`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 :file:`$ZEPHYR_BASE/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.

* ``CMAKE_C_COMPILER``, ``CMAKE_AR``.

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

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