linux-kernel - Re: [PATCH 2.6.20-rc1] ib_verbs: Use explicit if-else statements to avoid errors with do-while macros

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Date:	Thu, 14 Dec 2006 06:56:24 +0000
From:	Al Viro <viro@....linux.org.uk>
To:	Ben Collins <ben.collins@...ntu.com>
Cc:	Linus Torvalds <torvalds@...l.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2.6.20-rc1] ib_verbs: Use explicit if-else statements to avoid errors with do-while macros

On Thu, Dec 14, 2006 at 06:44:30AM +0000, Al Viro wrote:
> On Wed, Dec 13, 2006 at 10:10:05PM -0500, Ben Collins wrote:
> > At least on PPC, the "op ? op : dma" construct causes a compile failure
> > because the dma_* is a do{}while(0) macro.
> > 
> > This turns all of them into proper if/else to avoid this problem.
> 
> NAK.
> 
> Proper fix is to kill stupid do { } while (0) mess.  It's supposed
> to behave like a function returning void, so it should be ((void)0).

BTW, even though the original patch is already merged, I think that
we ought to get rid of do-while in such stubs, exactly to avoid such
problems in the future.  Probably even add to CodingStyle - it's not
the first time such crap happens.

IOW, do ; while(0) / do { } while (0)  is not a proper way to do a macro
that imitates a function returning void.

Objections?
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