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

[Willy Tarreau <w@1wt.eu>]

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. 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)
   ...

One variant includes :

#ifdef CONFIG_FOO
#define FOO_COND(x) (x)
#else
#define FOO_COND(x) 0
#endif

if (FOO_COND(foo))
    something
else if ((mode & S_IWUGO) == 0)
   ...

Regards,
Willy