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

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Date:	Fri, 23 May 2008 23:49:12 +0300
From:	Adrian Bunk <bunk@...nel.org>
To:	Willy Tarreau <w@....eu>
Cc:	Steve French <smfrench@...il.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: kernel coding style for if ... else which cross #ifdef

On Fri, May 23, 2008 at 10:42:28PM +0200, Willy Tarreau wrote:
> On Fri, May 23, 2008 at 02:11:43PM -0500, Steve French wrote:
> > A question splitting "else" and "if" on distinct lines vs. using an
> > extra line and extra #else came up as I was reviewing a proposed cifs
> > patch.   Which is the preferred style?
> >
> > #ifdef CONFIG_SOMETHING
> >    if (foo)
> >       something ...
> >    else
> > #endif
> >    if ((mode & S_IWUGO) == 0)
> > 
> > or alternatively
> > 
> > #ifdef CONFIG_SOMETHING
> >    if (foo)
> >       something ...
> >    else if ((mode & S_IWUGO) == 0)
> > #else
> >    if ((mode & S_IWUGO) == 0)
> > #endif
> 
> The second one is dangerous because if code evolves, chances are that
> only one of the two identical lines will be updated.
> 
> At least the first one is clearly readable.

I would consider the first one much harder to read since you can _very_ 
easily miss that the "if" is in an "else" clause and completely misread 
the code.

> But if you have tons of
> places with the same construct, it's better to create a macro which
> will inhibit the if branch, which gcc will happily optimize away.
> For instance :
> 
> #ifdef CONFIG_FOO
> #define FOO_ENABLED 1
> #else
> #define FOO_ENABLED 0
> #endif
> 
> if (FOO_ENABLED && foo)
>     something
> else if ((mode & S_IWUGO) == 0)
>    ...
>...

I'd also say that's the best solution.

> Regards,
> Willy

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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