Re: [PATCH] Documentation: warn against using int, hex, string options as expressions in Kconfig

[Randy Dunlap <rdunlap@infradead.org>]

Hi,
I haven't had time to really look at this yet, but...


On 7/30/26 7:01 AM, Julian Braha wrote:
> I've been working on type checking Kconfig, and found some instances of
> dead code due to conditions that always evaluate to false.
> 
> For example:
> 
> config FOO
>   int
> 
> config BAR
>   bool
>   default 'y' if FOO
> 
> This 'if FOO' will always be false, regardless of FOO's value (even when
> FOO=1). The same goes for strings and hex.

Does it help to say
	default y if FOO > 0
or
	default y if FOO != 0

Oh, I think that's what you suggested below. :)
Sorry, I just read that part.

This is unfortunate IMO but yes, let's at least document it.
Thanks.

> See also this related patch:
> https://lore.kernel.org/all/20260729210546.394392-1-julianbraha@gmail.com/
> 
> Let's warn users against this usage.
> 
> Signed-off-by: Julian Braha <julianbraha@gmail.com>
> ---
>  Documentation/kbuild/kconfig-language.rst | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst
> index ab2822d7ca1c..aee84b294a56 100644
> --- a/Documentation/kbuild/kconfig-language.rst
> +++ b/Documentation/kbuild/kconfig-language.rst
> @@ -320,6 +320,17 @@ An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
>  respectively for calculations). A menu entry becomes visible when its
>  expression evaluates to 'm' or 'y'.
>  
> +Note that options of int, hex, or string type should not be used directly
> +as expressions; these will always evaluate to 'n'. Instead, to check if
> +one of these options is in use, consider using one of these heuristics::
> +
> +  <symbol> != 0
> +  <symbol> != 0x0
> +  <symbol> != ""
> +
> +because int, hex, and string options will take these values of '0', '0x0',
> +and the empty string, respectively, when the dependencies are unmet.
> +
>  There are two types of symbols: constant and non-constant symbols.
>  Non-constant symbols are the most common ones and are defined with the
>  'config' statement. Non-constant symbols consist entirely of alphanumeric

-- 
~Randy