linux-kernel - Re: [PATCH] Documentation: Explain a second alternative for multi-line macros.

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Date:	Mon, 01 Jan 2007 08:14:21 -0800
From:	Randy Dunlap <randy.dunlap@...cle.com>
To:	"Robert P. J. Day" <rpjday@...dspring.com>
CC:	Christoph Hellwig <hch@...radead.org>,
	Linux kernel mailing list <linux-kernel@...r.kernel.org>,
	trivial@...nel.org
Subject: Re: [PATCH] Documentation: Explain a second alternative for multi-line
 macros.

Robert P. J. Day wrote:
> On Mon, 1 Jan 2007, Christoph Hellwig wrote:
> 
>> On Sun, Dec 31, 2006 at 02:32:25PM -0500, Robert P. J. Day wrote:
>>> + (a) Enclose those statements in a do - while block:
>>> +
>>> +	#define macrofun(a, b, c) 		\
>>> +		do {				\
>>> +			if (a == 5)		\
>>> +				do_this(b, c);	\
>>> +		} while (0)
>> nitpick, please don't add an indentaion level for the do {.  Do this
>> should look like:
>>
>> 	#define macrofun(a, b, c) 	\
>> 	do {				\
>> 		if (a == 5)		\
>> 			do_this(b, c);	\
>> 	} while (0)
> 
> the former is the way it's presented in CodingStyle currently, it
> wasn't my personal opinion on the subject.  i was just reproducing
> what was already there.
> 
>>> + (b) Use the gcc extension that a compound statement enclosed in
>>> +     parentheses represents an expression:
>>> +
>>> +	#define macrofun(a, b, c) ({		\
>>>  		if (a == 5)			\
>>>  			do_this(b, c);		\
>>> -	} while (0)
>>> +	})
>> I'd rather document to not use this - it makes the code far less
>> redable.  And it's a non-standard extension, something we only
>> use if it provides us a benefit which it doesn't here.
> 
> it might be a bit late to put a cork in *that* bottle:
> 
>   $ grep -r "#define.*({" *

We aren't trying to prevent its past use.  We just aren't encouraging
the use of gcc extensions if there are reasonable & better ways to
do something.

-- 
~Randy
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