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

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Date:	Wed, 25 Mar 2009 10:21:13 +0530
From:	Nikanth Karthikesan <knikanth@...ell.com>
To:	rusty@...tcorp.com.au
Cc:	nikanth@...il.com, linux-kernel@...r.kernel.org
Subject: Strange code in include/linux/cpumask.h

Hi Rusty

I do not understand this code.

/**
 * to_cpumask - convert an NR_CPUS bitmap to a struct cpumask *
 * @bitmap: the bitmap
 *
 * There are a few places where cpumask_var_t isn't appropriate and
 * static cpumasks must be used (eg. very early boot), yet we don't
 * expose the definition of 'struct cpumask'.
 *
 * This does the conversion, and can be used as a constant initializer.
 */
#define to_cpumask(bitmap)                                              \
        ((struct cpumask *)(1 ? (bitmap)                                \
                            : (void *)sizeof(__check_is_bitmap(bitmap))))

static inline int __check_is_bitmap(const unsigned long *bitmap)
{
        return 1;
}

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)

Even If it would return (void *)sizeof(__check_is_bitmap(bitmap)), wouldn't it 
be always
(struct cpumask *) (sizeof(1))!?

Is this some magic for type safety?

Thanks
Nikanth
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