Building, Flashing and Debugging

West provides 5 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 additional help.

The build command allows you to build any source tree from any directory in your file system, placing the build results in a folder of your choice.

In its simplest form, the command can be run by navigating to the root folder (i.e. the folder containing a CMakeLists.txt file) of the Zephyr application of your choice and running:

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>. A build folder named build (default name) will be created and the build output will be placed in it.

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

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

Since the build directory defaults to build, if you do not specify a build directory but a folder named build is present, that will be used, allowing you to incrementally build from outside the build folder with no additional parameters.

Note

The -b <BOARD> parameter is only required when there is no CMake cache present at all, usually when you are building from scratch or creating a brand new build directory. After that you can rebuild (even with additional parameters) without having to specify the board again. If you’re unsure whether -b is required, just try leaving it out. West will print an error if the option is required and was not given.

Specify the source directory path as the first positional argument:

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

Additionally you can specify the build system target using the --target (or -t) option. For example, to run the clean target:

west build -t clean

You can list all targets with ninja help (or west build -t help) inside the build folder.

Finally, you can add additional arguments to the CMake invocation performed by west build by supplying additional parameters (after a --) to the command. For example, to use the Unix Makefiles CMake generator instead of Ninja (which west build uses by default), run:

west build -b reel_board samples/hello_world -- -G'Unix Makefiles'

As another example, and assuming you have already built a sample, the following command adds the file.conf Kconfig fragment to the files which are merged into your final build configuration:

west build -- -DOVERLAY_CONFIG=file.conf

Passing additional CMake arguments like this forces west build to re-run CMake, even if a build system has already been generated. You can also force a CMake re-run using the -c (or --cmake) option:

west build -c

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

Since the build directory defaults to build, if you do not specify a build directory but a folder named build is present, that will be used, allowing you to flash from outside the build folder with no additional parameters.

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

Since the build directory defaults to build, if you do not specify a build directory but a folder named build is present, that will be used, allowing you to debug from outside the build folder with no additional parameters.

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.