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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.

Note

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.

Note

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.