netdev - Re: [PATCH v2 bpf-next] libbpf: make DECLARE_LIBBPF_OPTS macro strictly a variable declaration

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Date:   Tue, 22 Oct 2019 19:42:01 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Andrii Nakryiko <andriin@...com>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, ast@...com, daniel@...earbox.net
Cc:     andrii.nakryiko@...il.com, kernel-team@...com,
        Andrii Nakryiko <andriin@...com>
Subject: Re: [PATCH v2 bpf-next] libbpf: make DECLARE_LIBBPF_OPTS macro strictly a variable declaration

Andrii Nakryiko <andriin@...com> writes:

> LIBBPF_OPTS is implemented as a mix of field declaration and memset
> + assignment. This makes it neither variable declaration nor purely
> statements, which is a problem, because you can't mix it with either
> other variable declarations nor other function statements, because C90
> compiler mode emits warning on mixing all that together.
>
> This patch changes LIBBPF_OPTS into a strictly declaration of variable
> and solves this problem, as can be seen in case of bpftool, which
> previously would emit compiler warning, if done this way (LIBBPF_OPTS as
> part of function variables declaration block).
>
> This patch also renames LIBBPF_OPTS into DECLARE_LIBBPF_OPTS to follow
> kernel convention for similar macros more closely.
>
> v1->v2:
> - rename LIBBPF_OPTS into DECLARE_LIBBPF_OPTS (Jakub Sitnicki).
>
> Signed-off-by: Andrii Nakryiko <andriin@...com>

Acked-by: Toke Høiland-Jørgensen <toke@...hat.com>


> +#define DECLARE_LIBBPF_OPTS(TYPE, NAME, ...)				    \
> +	struct TYPE NAME = ({ 						    \
> +		memset(&NAME, 0, sizeof(struct TYPE));			    \
> +		(struct TYPE) {						    \
> +			.sz = sizeof(struct TYPE),			    \
> +			__VA_ARGS__					    \
> +		};							    \
> +	})

Found a reference with an explanation of why this works, BTW; turns out
it's a GCC extension:

http://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html#Statement-Exprs

-Toke