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Kconfig extensions

Zephyr uses the Kconfiglib implementation of Kconfig, which includes some Kconfig extensions:

  • Default values can be applied to existing symbols without weakening the symbols dependencies through the use of configdefault.

    config FOO
        bool "FOO"
        depends on BAR
    configdefault FOO
        default y if FIZZ

    The statement above is equivalent to:

    config FOO
        bool "Foo"
        default y if FIZZ
        depends on BAR

    configdefault symbols cannot contain any fields other than default, however they can be wrapped in if statements. The two statements below are equivalent:

    configdefault FOO
        default y if BAR
    if BAR
    configdefault FOO
        default y
    endif # BAR
  • Environment variables in source statements are expanded directly, meaning no “bounce” symbols with option env="ENV_VAR" need to be defined.


    option env has been removed from the C tools as of Linux 4.18 as well.

    The recommended syntax for referencing environment variables is $(FOO) rather than $FOO. This uses the new Kconfig preprocessor. The $FOO syntax for expanding environment variables is only supported for backwards compatibility.

  • The source statement supports glob patterns and includes each matching file. A pattern is required to match at least one file.

    Consider the following example:

    source "foo/bar/*/Kconfig"

    If the pattern foo/bar/*/Kconfig matches the files foo/bar/baz/Kconfig and foo/bar/qaz/Kconfig, the statement above is equivalent to the following two source statements:

    source "foo/bar/baz/Kconfig"
    source "foo/bar/qaz/Kconfig"

    If no files match the pattern, an error is generated.

    The wildcard patterns accepted are the same as for the Python glob module.

    For cases where it’s okay for a pattern to match no files (or for a plain filename to not exist), a separate osource (optional source) statement is available. osource is a no-op if no file matches.


    source and osource are analogous to include and -include in Make.

  • An rsource statement is available for including files specified with a relative path. The path is relative to the directory of the Kconfig file that contains the rsource statement.

    As an example, assume that foo/Kconfig is the top-level Kconfig file, and that foo/bar/Kconfig has the following statements:

    source "qaz/Kconfig1"
    rsource "qaz/Kconfig2"

    This will include the two files foo/qaz/Kconfig1 and foo/bar/qaz/Kconfig2.

    rsource can be used to create Kconfig “subtrees” that can be moved around freely.

    rsource also supports glob patterns.

    A drawback of rsource is that it can make it harder to figure out where a file gets included, so only use it if you need it.

  • An orsource statement is available that combines osource and rsource.

    For example, the following statement will include Kconfig1 and Kconfig2 from the current directory (if they exist):

    orsource "Kconfig[12]"
  • def_int, def_hex, and def_string keywords are available, analogous to def_bool. These set the type and add a default at the same time.