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Date: Mon, 17 May 2004 12:17:44 +0900
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From: JINMEI Tatuya / =?ISO-2022-JP?B?GyRCP0BMQEMjOkgbKEI=?=
	<jinmei@isl.rdc.toshiba.co.jp>
To: "George V. Neville-Neil" <gnn@neville-neil.com>
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cc: net@freebsd.org
Subject: Re: RFC 2292 CMSG_FIRSTHDR macro...
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>>>>> On Wed, 28 Apr 2004 17:09:30 -0700, 
>>>>> "George V. Neville-Neil" <gnn@neville-neil.com> said:

> 	This should be a minor nit but, is there any reason we
> 	implement this this way:

> #define	CMSG_FIRSTHDR(mhdr)	((struct cmsghdr *)(mhdr)->msg_control)

> instead of this way:

> #define CMSG_FIRSTHDR(mhdr) \
>           ( (mhdr)->msg_controllen >= sizeof(struct cmsghdr) ? \
>            (struct cmsghdr *)(mhdr)->msg_control : \
>            (struct cmsghdr *)NULL )

> as suggested in RFC 2292?

> A coworker of mine asked about this and then I got to wondering
> myself.

I see no special reason.  In fact, KAME snapshots have this check as
suggested in RFC2292 (or 3542):

/*
 * RFC 2292 requires to check msg_controllen, in case that the kernel returns
 * an empty list for some reasons.
 */
#define	CMSG_FIRSTHDR(mhdr) \
	((mhdr)->msg_controllen >= sizeof(struct cmsghdr) ? \
	 (struct cmsghdr *)(mhdr)->msg_control : \
	 (struct cmsghdr *)NULL)

					JINMEI, Tatuya
					Communication Platform Lab.
					Corporate R&D Center, Toshiba Corp.
					jinmei@isl.rdc.toshiba.co.jp