linux-kernel - Re: Strange code in include/linux/cpumask.h

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Message-Id: <200903251541.12180.rusty@rustcorp.com.au>
Date:	Wed, 25 Mar 2009 15:41:11 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Nikanth Karthikesan <knikanth@...ell.com>
Cc:	nikanth@...il.com, linux-kernel@...r.kernel.org
Subject: Re: Strange code in include/linux/cpumask.h

On Wednesday 25 March 2009 15:21:13 Nikanth Karthikesan wrote:
> Hi Rusty
> 
> I do not understand this code.

Hi Nikanth,

   That's OK, it's a little tricky.

> The conditional operator would always evaluates to true and return bitmap. So 
> all it seems to does is
> #define to_cpumansk(bitmap) (struct cpumask *)(bitmap)

Yes, except that this insists that bitmap be an unsigned long * or you'll get
a warning.  Otherwise the macro could be used on anything.  And it needs to be
a macro to use it as a static initializer.

As the NR_CPUS bit arrays vanish, this macro will be used less and less; but
some of them will probably take a while.

Thanks for the question,
Rusty.
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