netdev - Re: [iproute PATCH RFC] libnetlink: introduce DECLARE

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Date:	Thu, 26 Nov 2015 14:56:30 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Phil Sutter <phil@....cc>, Stephen Hemminger <shemming@...cade.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [iproute PATCH RFC] libnetlink: introduce DECLARE_NLREQ

Phil Sutter <phil@....cc> writes:

> This macro aims to simplify most netlink users' pattern to prepare a
> request, which is to create an unnamed struct and initialize it:
>
> | struct {
> |	struct nlmsghdr n;
> |	struct whatever foo;
> |	char buf[arbitrary number];
> | } req;
> |
> | memset(&req, 0, sizeof(req));
> | req.n.nlmsg_len = NLMSG_LENGTH(sizeof(struct whatever));
> | req.n.nlmsg_flags = NLM_F_REQUEST;
>
> Having this patch applied, the above can be replaced by a static
> initializer like so:
>
> | DECLARE_NLREQ(req, n, struct whatever foo, arbitrary number);
>
> There is an added benefit, as well: Due to explicit alignment, the
> requested tailroom is really as big as requested no matter what size
> struct whatever really is.
>
> Signed-off-by: Phil Sutter <phil@....cc>
> ---
> This patch is RFC because I want to wait for peer review and upstream
> acceptance before sending in the big refactoring patch itself.
> ---
>  include/libnetlink.h | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/include/libnetlink.h b/include/libnetlink.h
> index 2280c39c670a4..7e12a7b12b55d 100644
> --- a/include/libnetlink.h
> +++ b/include/libnetlink.h
> @@ -196,5 +196,16 @@ int rtnl_from_file(FILE *, rtnl_listen_filter_t handler,
>   * messages from dump file */
>  #define NLMSG_TSTAMP	15
>  
> +/* declare a netlink request with initial payload */
> +#define DECLARE_NLREQ(name, hdrname, payload, tailroom)                       \
> +	struct {                                                              \
> +		struct nlmsghdr hdrname;                                      \
> +		payload;                                                      \
> +		char __b[tailroom] __attribute__((aligned(NLMSG_ALIGNTO)));   \
> +	} name = { .hdrname = {                                               \
> +		.nlmsg_len = (unsigned long)&name.__b - (unsigned long)&name, \

offsetof(typeof(name), __b) ?

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