linux-kernel - typecheck code

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Date:	Mon, 31 Jan 2011 12:03:37 -0500
From:	Sri Ram Vemulpali <sri.ram.gmu06@...il.com>
To:	Kernel-newbies <kernelnewbies@...linux.org>,
	linux-kernel-mail <linux-kernel@...r.kernel.org>
Subject: typecheck code

Hi all,

/*
 * Check at compile time that something is of a particular type.
 * Always evaluates to 1 so you may use it easily in comparisons.
 */
 #define typecheck(type,x) \
 ({      type __dummy; \
        typeof(x) __dummy2; \
        (void)(&__dummy == &__dummy2); \
        1; \
 })

#define typecheck_fn(type,function) \
({      typeof(type) __tmp = function; \
       (void)__tmp; \
})

Can anyone help me, explain the above code typecheck. How does
(void)(&__dummy == &__dummy2) evaluates to 1

I appreciate any explain.

-- 
Regards,
Sri.
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