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

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Date:	Tue, 01 Aug 2006 12:02:12 +0159
From:	Jiri Slaby <jirislaby@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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

Peter Zijlstra wrote:
> 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.

That's what I'm trying to say. It was a _bad_ piece of code. It doesn't 
demonstrate I want it to demonstrate.

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

regards,
-- 
<a href="http://www.fi.muni.cz/~xslaby/">Jiri Slaby</a>
faculty of informatics, masaryk university, brno, cz
e-mail: jirislaby gmail com, gpg pubkey fingerprint:
B674 9967 0407 CE62 ACC8  22A0 32CC 55C3 39D4 7A7E
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