Building, Flashing and Debugging

Zephyr provides several west extension commands for building, flashing, and interacting with Zephyr programs running on a board: build, flash, debug, debugserver and attach.

These use information stored in the CMake cache [1] to flash or attach a debugger to a board supported by Zephyr. The exception is starting a clean build (i.e. with no previous artifacts) which will in fact run CMake thus creating the corresponding cache. The CMake build system commands with the same names (i.e. all but build) directly delegate to West.

Building: west build

Tip

Run west build -h for a quick overview.

The build command helps you build Zephyr applications from source. You can use west config to configure its behavior.

Its default behavior tries to “do what you mean”:

  • If there is a Zephyr build directory named build in your current working directory, it is incrementally re-compiled. The same is true if you run west build from a Zephyr build directory.
  • Otherwise, if you run west build from a Zephyr application’s source directory and no build directory is found, a new one is created and the application is compiled in it.

Basics

The easiest way to use west build is to go to an application’s root directory (i.e. the folder containing the application’s CMakeLists.txt) and then run:

west build -b <BOARD>

Where <BOARD> is the name of the board you want to build for. This is exactly the same name you would supply to CMake if you were to invoke it with: cmake -DBOARD=<BOARD>.

Tip

You can use the west boards command to list all supported boards.

A build directory named build will be created, and the application will be compiled there after west build runs CMake to create a build system in that directory. If west build finds an existing build directory, the application is incrementally re-compiled there without re-running CMake. You can force CMake to run again with --cmake.

You don’t need to use the --board option if you’ve already got an existing build directory; west build can figure out the board from the CMake cache. For new builds, the --board option, BOARD environment variable, or build.board configuration option are checked (in that order).

Examples

Here are some west build usage examples, grouped by area.

Setting a Default Board

To configure west build to build for the reel_board by default:

west config build.board reel_board

(You can use any other board supported by Zephyr here; it doesn’t have to be reel_board.)

Setting Source and Build Directories

To set the application source directory explicitly, give its path as a positional argument:

west build -b <BOARD> path/to/source/directory

To set the build directory explicitly, use --build-dir (or -d):

west build -b <BOARD> --build-dir path/to/build/directory

To change the default build directory from build, use the build.dir-fmt configuration option. This lets you name build directories using format strings, like this:

west config build.dir-fmt "build/{board}/{app}"

With the above, running west build -b reel_board samples/hello_world will use build directory build/reel_board/hello_world. See Configuration Options for more details on this option.

Controlling the Build System

There are several ways to control the build system generated and used by west build.

To specify the build system target to run, use --target (or -t).

For example, on host platforms with QEMU, you can use the run target to build and run the Hello World sample for the emulated qemu_x86 board in one command:

west build -b qemu_x86 -t run samples/hello_world

As another example, to use -t to list all build system targets:

west build -t help

As a final example, to use -t to run the pristine target, which deletes all the files in the build directory:

west build -t pristine

To have west build run the pristine target before re-running CMake to generate a build system, use the --pristine (or -p) option. For example, to switch board and application (which requires a pristine build directory) in one command:

west build -b qemu_x86 samples/philosophers
west build -p -b reel_board samples/hello_world

To let west decide for you if a pristine build is needed, use -p auto:

west build -p auto -b reel_board samples/hello_world

Tip

You can run west config build.pristine auto to make this setting permanent.

To add additional arguments to the CMake invocation performed by west build, pass them after a -- at the end of the command line.

For example, to use the Unix Makefiles CMake generator instead of Ninja (which west build uses by default), run:

west build -b reel_board -- -G'Unix Makefiles'

Note

Passing additional CMake arguments like this forces west build to re-run CMake, even if a build system has already been generated.

As another example, to use Unix Makefiles and enable the CMAKE_VERBOSE_MAKEFILE option:

west build -b reel_board -- -G'Unix Makefiles' -DCMAKE_VERBOSE_MAKEFILE=ON

Notice how the -- only appears once, even though multiple CMake arguments are given. All command-line arguments to west build after a -- are passed to CMake.

As a final example, to merge the file.conf Kconfig fragment into your build’s .config:

west build -- -DOVERLAY_CONFIG=file.conf

To force a CMake re-run, use the --cmake (or --c) option:

west build -c

Configuration Options

You can configure west build using these options.

Option Description
build.board String. If given, this the board used by west build when --board is not given and BOARD is unset in the environment.
build.board_warn Boolean, default true. If false, disables warnings when west build can’t figure out the target board.
build.dir-fmt

String, default build. The build folder format string, used by west whenever it needs to create or locate a build folder. The currently available arguments are:

  • board: The board name
  • source_dir: The relative path from the current working directory to the source directory. If the current working directory is inside the source directory this will be set to an empty string.
  • app: The name of the source directory.
build.generator String, default Ninja. The CMake Generator to use to create a build system. (To set a generator for a single build, see the above example)
build.guess-dir

String, instructs west whether to try to guess what build folder to use when build.dir-fmt is in use and not enough information is available to resolve the build folder name. Can take these values:

  • never (default): Never try to guess, bail out instead and require the user to provide a build folder with -d.
  • runners: Try to guess the folder when using any of the ‘runner’ commands. These are typically all commands that invoke an external tool, such as flash and debug.
build.pristine

String. Controls the way in which west build may clean the build folder before building. Can take the following values:

  • never (default): Never automatically make the build folder pristine.
  • auto: west build will automatically make the build folder pristine before building, if a build system is present and the build would fail otherwise (e.g. the user has specified a different board or application from the one previously used to make the build directory).
  • always: Always make the build folder pristine before building, if a build system is present.

Flashing: west flash

Tip

Run west flash -h for additional help.

Basics

From a Zephyr build directory, re-build the binary and flash it to your board:

west flash

Without options, the behavior is the same as ninja flash (or make flash, etc.).

To specify the build directory, use --build-dir (or -d):

west flash --build-dir path/to/build/directory

If you don’t specify the build directory, west flash searches for one in build, then the current working directory. If you set the build.dir-fmt configuration option (see Setting Source and Build Directories), west flash searches there instead of build.

Choosing a Runner

If your board’s Zephyr integration supports flashing with multiple programs, you can specify which one to use using the --runner (or -r) option. For example, if West flashes your board with nrfjprog by default, but it also supports JLink, you can override the default with:

west flash --runner jlink

See Implementation Details below for more information on the runner library used by West. The list of runners which support flashing can be obtained with west flash -H; if run from a build directory or with --build-dir, this will print additional information on available runners for your board.

Configuration Overrides

The CMake cache contains default values West uses while flashing, such as where the board directory is on the file system, the path to the kernel binaries to flash in several formats, and more. You can override any of this configuration at runtime with additional options.

For example, to override the HEX file containing the Zephyr image to flash (assuming your runner expects a HEX file), but keep other flash configuration at default values:

west flash --kernel-hex path/to/some/other.hex

The west flash -h output includes a complete list of overrides supported by all runners.

Runner-Specific Overrides

Each runner may support additional options related to flashing. For example, some runners support an --erase flag, which mass-erases the flash storage on your board before flashing the Zephyr image.

To view all of the available options for the runners your board supports, as well as their usage information, use --context (or -H):

west flash --context

Important

Note the capital H in the short option name. This re-runs the build in order to ensure the information displayed is up to date!

When running West outside of a build directory, west flash -H just prints a list of runners. You can use west flash -H -r <runner-name> to print usage information for options supported by that runner.

For example, to print usage information about the jlink runner:

west flash -H -r jlink

Debugging: west debug, west debugserver

Tip

Run west debug -h or west debugserver -h for additional help.

Basics

From a Zephyr build directory, to attach a debugger to your board and open up a debug console (e.g. a GDB session):

west debug

To attach a debugger to your board and open up a local network port you can connect a debugger to (e.g. an IDE debugger):

west debugserver

Without options, the behavior is the same as ninja debug and ninja debugserver (or make debug, etc.).

To specify the build directory, use --build-dir (or -d):

west debug --build-dir path/to/build/directory
west debugserver --build-dir path/to/build/directory

If you don’t specify the build directory, these commands search for one in build, then the current working directory. If you set the build.dir-fmt configuration option (see Setting Source and Build Directories), west debug searches there instead of build.

Choosing a Runner

If your board’s Zephyr integration supports debugging with multiple programs, you can specify which one to use using the --runner (or -r) option. For example, if West debugs your board with pyocd-gdbserver by default, but it also supports JLink, you can override the default with:

west debug --runner jlink
west debugserver --runner jlink

See Implementation Details below for more information on the runner library used by West. The list of runners which support debugging can be obtained with west debug -H; if run from a build directory or with --build-dir, this will print additional information on available runners for your board.

Configuration Overrides

The CMake cache contains default values West uses for debugging, such as where the board directory is on the file system, the path to the kernel binaries containing symbol tables, and more. You can override any of this configuration at runtime with additional options.

For example, to override the ELF file containing the Zephyr binary and symbol tables (assuming your runner expects an ELF file), but keep other debug configuration at default values:

west debug --kernel-elf path/to/some/other.elf
west debugserver --kernel-elf path/to/some/other.elf

The west debug -h output includes a complete list of overrides supported by all runners.

Runner-Specific Overrides

Each runner may support additional options related to debugging. For example, some runners support flags which allow you to set the network ports used by debug servers.

To view all of the available options for the runners your board supports, as well as their usage information, use --context (or -H):

west debug --context

(The command west debugserver --context will print the same output.)

Important

Note the capital H in the short option name. This re-runs the build in order to ensure the information displayed is up to date!

When running West outside of a build directory, west debug -H just prints a list of runners. You can use west debug -H -r <runner-name> to print usage information for options supported by that runner.

For example, to print usage information about the jlink runner:

west debug -H -r jlink

Implementation Details

The flash and debug commands are implemented as west extension commands: that is, they are west commands whose source code lives outside the west repository. Some reasons this choice was made are:

  • Their implementations are tightly coupled to the Zephyr build system, e.g. due to their reliance on CMake cache variables.
  • Pull requests adding features to them are almost always motivated by a corresponding change to an upstream board, so it makes sense to put them in Zephyr to avoid needing pull requests in multiple repositories.
  • Many users find it natural to search for their implementations in the Zephyr source tree.

The extension commands are a thin wrapper around a package called runners (this package is also in the Zephyr tree, in scripts/west_commands/runners).

The central abstraction within this library is ZephyrBinaryRunner, an abstract class which represents runner objects, which can flash and/or debug Zephyr programs. The set of available runners is determined by the imported subclasses of ZephyrBinaryRunner. ZephyrBinaryRunner is available in the runners.core module; individual runner implementations are in other submodules, such as runners.nrfjprog, runners.openocd, etc.

Hacking and APIs

Developers can add support for new ways to flash and debug Zephyr programs by implementing additional runners. To get this support into upstream Zephyr, the runner should be added into a new or existing runners module, and imported from runner/__init__.py.

Note

The test cases in scripts/west_commands/tests add unit test coverage for the runners package and individual runner classes.

Please try to add tests when adding new runners. Note that if your changes break existing test cases, CI testing on upstream pull requests will fail.

API Documentation for the runners.core module can be found in West APIs.

Doing it By Hand

If you prefer not to use West to flash or debug your board, simply inspect the build directory for the binaries output by the build system. These will be named something like zephyr/zephyr.elf, zephyr/zephyr.hex, etc., depending on your board’s build system integration. These binaries may be flashed to a board using alternative tools of your choice, or used for debugging as needed, e.g. as a source of symbol tables.

By default, these West commands rebuild binaries before flashing and debugging. This can of course also be accomplished using the usual targets provided by Zephyr’s build system (in fact, that’s how these commands do it).

Footnotes

[1]The CMake cache is a file containing saved variables and values which is created by CMake when it is first run to generate a build system. See the cmake(1) manual for more details.