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Date: Wed, 10 Aug 2011 15:20:01 +0200
From: Dimitry Andric <dim@FreeBSD.org>
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To: Alexander Best <arundel@freebsd.org>
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Cc: svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org,
	src-committers@FreeBSD.org, Jonathan Anderson <jonathan@FreeBSD.org>,
	Bruce Evans <brde@optusnet.com.au>
Subject: Re: svn commit: r224721 - head/sys/sys
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On 2011-08-10 12:38, Alexander Best wrote:
...
> any reason {TIMEVAL,TIMESPEC}_TO_{TIMESPEC,TIMEVAL}()s code is being executed
> in a
>
> do { ... } while (0)
>
> conditional loop? both macros are also defined in crypto/openssh/defines.h and
> don't seem to need that extra one-time-loop.

It's a very common C idiom.  See:
http://stackoverflow.com/questions/154136/why-are-there-sometimes-meaningless-do-while-and-if-else-statements-in-c-c-macr