Writing Snippets

Basics 

Snippets are defined using YAML files named snippet.yml.

A snippet.yml file contains the name of the snippet, along with additional build system settings, like this:

name: snippet-name
# ... build system settings go here ...

Build system settings go in other keys in the file as described later on in this page.

You can combine settings whenever they appear under the same keys. For example, you can combine a snippet-specific devicetree overlay and a .conf file like this:

name: foo
append:
  EXTRA_DTC_OVERLAY_FILE: foo.overlay
  EXTRA_CONF_FILE: foo.conf

Namespacing 

When writing devicetree overlays in a snippet, use snippet_<name> or snippet-<name> as a namespace prefix when choosing names for node labels, node names, etc. This avoids namespace conflicts.

For example, if your snippet is named foo-bar, write your devicetree overlays like this:

chosen {
        zephyr,baz = &snippet_foo_bar_dev;
};

snippet_foo_bar_dev: device@12345678 {
        /* ... */
};

Where snippets are located 

The build system looks for snippets in these places:

In directories configured by the SNIPPET_ROOT CMake variable. This always includes the zephyr repository (so snippets/ is always a source of snippets) and the application source directory (so <app>/snippets is also).

Additional directories can be added manually at CMake time.

The variable is a whitespace- or semicolon-separated list of directories which may contain snippet definitions.

For each directory in the list, the build system looks for snippet.yml files underneath a subdirectory named snippets/, if one exists.

For example, if SNIPPET_ROOT is set to /foo;/bar, the build system will look for snippet.yml files underneath the following subdirectories:
- /foo/snippets/
- /bar/snippets/
The snippet.yml files can be nested anywhere underneath these locations.
In any module whose module.yml file provides a snippet_root setting.

For example, in a zephyr module named baz, you can add this to your module.yml file:
```
settings:
  snippet_root: .
```
And then any snippet.yml files in baz/snippets will automatically be discovered by the build system, just as if the path to baz had appeared in SNIPPET_ROOT.

Processing order 

Snippets are processed in the order they are listed in the SNIPPET variable, or in the order of the -S arguments if using west.

To apply bar after foo:

cmake -Sapp -Bbuild -DSNIPPET="foo;bar" [...]
cmake --build build

The same can be achieved with west as follows:

west build -S foo -S bar [...] app

When multiple snippets set the same configuration, the configuration value set by the last processed snippet ends up in the final configurations.

For instance, if foo sets CONFIG_FOO=1 and bar sets CONFIG_FOO=2 in the above example, the resulting final configuration will be CONFIG_FOO=2 because bar is processed after foo.

This principle applies to both Kconfig fragments (.conf files) and devicetree overlays (.overlay files).

Devicetree overlays (`.overlay`)

This snippet.yml adds foo.overlay to the build:

name: foo
append:
  EXTRA_DTC_OVERLAY_FILE: foo.overlay

The path to foo.overlay is relative to the directory containing snippet.yml.

`.conf` files 

This snippet.yml adds foo.conf to the build:

name: foo
append:
  EXTRA_CONF_FILE: foo.conf

The path to foo.conf is relative to the directory containing snippet.yml.

Board-specific settings 

You can write settings that only apply to some boards.

The settings described here are applied in addition to snippet settings that apply to all boards. (This is similar, for example, to the way that an application with both prj.conf and boards/foo.conf files will use both .conf files in the build when building for board foo, instead of just boards/foo.conf)

By name 

name: ...
boards:
  bar: # settings for board "bar" go here
    append:
      EXTRA_DTC_OVERLAY_FILE: bar.overlay
  baz: # settings for board "baz" go here
    append:
      EXTRA_DTC_OVERLAY_FILE: baz.overlay

The above example uses bar.overlay when building for board bar, and baz.overlay when building for baz.

By regular expression 

You can enclose the board name in slashes (/) to match the name against a regular expression in the CMake syntax. The regular expression must match the entire board name.

For example:

name: foo
boards:
  /my_vendor_.*/:
    append:
      EXTRA_DTC_OVERLAY_FILE: my_vendor.overlay

The above example uses devicetree overlay my_vendor.overlay when building for either board my_vendor_board1 or my_vendor_board2. It would not use the overlay when building for either another_vendor_board or x_my_vendor_board.