linux-kernel - Re: do { } while (0) question

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Date:	Tue, 01 Aug 2006 11:59:59 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Jiri Slaby <jirislaby@...il.com>
Cc:	Hua Zhong <hzhong@...il.com>,
	'Heiko Carstens' <heiko.carstens@...ibm.com>,
	'Andrew Morton' <akpm@...l.org>, linux-kernel@...r.kernel.org,
	'Martin Schwidefsky' <schwidefsky@...ibm.com>
Subject: Re: do { } while (0) question

On Tue, 2006-08-01 at 11:45 +0159, Jiri Slaby wrote:
> Peter Zijlstra wrote:
> > On Tue, 2006-08-01 at 02:03 -0700, Hua Zhong wrote:
> >>> #if KILLER == 1
> >>> #define MACRO
> >>> #else
> >>> #define MACRO do { } while (0)
> >>> #endif
> >>>
> >>> {
> >>> 	if (some_condition)
> >>> 		MACRO
> >>>
> >>> 	if_this_is_not_called_you_loose_your_data();
> >>> }
> >>>
> >>> How do you want to define KILLER, 0 or 1? I personally choose 0.
> >> Really? Does it compile?
> > 
> > No, and that is the whole point.
> > 
> > The empty 'do {} while (0)' makes the missing semicolon a syntax error.
> 
> Bulls^WNope, it was a bad example (we don't want to break the compilation, just 
> not want to emit a warn or an err).

It was a perfectly good example why 'do {} while (0)' is useful. The
perhaps mistakenly forgotten ';' after MACRO will not stop your example
from compiling if KILLER == 1. Even worse, it will compile and do
something totally unexpected.

If however you use KILLER != 1, the while(0) will require a ';' and this
example will fail to compile.

Not compiling when you made a coding error (forgetting ';' is one of the
most common) is a great help.



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