linux-kernel - Re: kernel coding style for if ... else which cross #ifdef

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Date:	Sat, 24 May 2008 16:39:50 +0200
From:	Willy Tarreau <w@....eu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Sam Ravnborg <sam@...nborg.org>, "H. Peter Anvin" <hpa@...or.com>,
	Steve French <smfrench@...il.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: kernel coding style for if ... else which cross #ifdef

On Sat, May 24, 2008 at 03:35:58PM +0100, Jeremy Fitzhardinge wrote:
> Sam Ravnborg wrote:
> >I assume you wanted to say:
> >  
> >>   #undef  CONFIG_FOO
> >>   #define CFG_FOO   1
> >>   #define CONFIG_FOO_MODULE
> >>   #define CFG_FOO_MODULE 1
> >>    
> >Because then the CONFIG_* is not changed
> >and we do not want to change that.
> >  
> 
> Yeah, I didn't intend to change the meaning of CONFIG_FOO.
> 
> >I'm not fully convinced about:
> >  
> >>   #define CFG_FOO   1
> >>    
> >But on the other hand it is only in odd
> >cases we distingush between built-in and module.
> >So it makes most sense.
> >  
> 
> I think CONFIG_ and CFG_ should be exact parallels, so if CONFIG_FOO is 
> undefined, CFG_FOO should be 0.
> 
> >>Not sure what CFG_* should be for string/numeric options.  Probably "1" 
> >>if the value is defined, "0" if not, with CONFIG_* being the actual 
> >>value (so a CONFIG_ value of 0 is distinguishable from not defined).
> >>    
> >For non-boolean/tristate values we simply skip CFG_ values - thats
> >the most simple approach.
> 
> I suppose, but it might be useful to know whether a constant is present:
> 
> 	if (CFG_THINGY_LIMIT && x > CONFIG_THINGY_LIMIT) {...}
> 
> (which fails if CONFIG_THINGY_LIMIT is undefined, so I guess it still 
> doesn't work very well).

You still have the possibility to use the "-0" trick :

 	if (CFG_THINGY_LIMIT && x > (CONFIG_THINGY_LIMIT-0)) {...}

Willy

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