Re: [PATCH v1 1/2] args: Add DEFARG() to implement macro default arguments

[Alejandro Colomar <alx@kernel.org>]

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> Date: 2026-11-03 14:27:11+0100
> From: Alejandro Colomar <alx@kernel.org>
>
> It needs __VA_OPT__, which is a C23 feature, but it's provided by GCC
> and Clang in GNU-C mode as an extension even without C23.  __VA_OPT__
> was implemented in GCC 9 and Clang 6, I think, so it should be widely

Actually, it seems it was implemented in Clang 12, not Clang 6.
I got that 6 from llvmorg-6.0.0-rc1, but that version number seems to
not be the same as the Clang version number.  Anyway, Clang 12 should be
fine, I think.


Cheers,
Alex

> available.
> 
> Here's a small example program showing how this works:
> 
> 	alx@devuan:~/tmp$ cat d.c
> 	#define D__(x, ...)  (x)
> 	#define D_(x, ...)   D__(__VA_ARGS__ __VA_OPT__(,) x)
> 	#define D(a, b)      D_(a, b)
> 	D(1, 2)
> 	D(1, )
> 	D(, 2)
> 	D(,)
> 	alx@devuan:~/tmp$ gcc -E -Wall -Wextra d.c | grep '^[^#]'
> 	(2)
> 	(1)
> 	(2)
> 	()
> 
> Signed-off-by: Alejandro Colomar <alx@kernel.org>
> ---
>  include/linux/args.h | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/include/linux/args.h b/include/linux/args.h
> index 0562dc51435e..5886011cf791 100644
> --- a/include/linux/args.h
> +++ b/include/linux/args.h
> @@ -3,6 +3,11 @@
>  #ifndef _LINUX_ARGS_H
>  #define _LINUX_ARGS_H
>  
> +/* Expand to 'arg' if non-empty, or to 'def' if 'arg' is empty.  */
> +#define ___DEFARG(x, ...)   (x)
> +#define __DEFARG(def, ...)  ___DEFARG(__VA_ARGS__ __VA_OPT__(,) def)
> +#define DEFARG(def, arg)    __DEFARG(def, arg)
> +
>  /*
>   * How do these macros work?
>   *
> -- 
> 2.55.0
> 



-- 
<https://www.alejandro-colomar.es>

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