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From: Manish Katiyar <mkatiyar@gmail.com>
To: Sri Ram Vemulpali <sri.ram.gmu06@gmail.com>
Cc: Kernel-newbies <kernelnewbies@nl.linux.org>,
	linux-kernel-mail <linux-kernel@vger.kernel.org>
Subject: Re: typecheck code
Date: Mon, 31 Jan 2011 10:03:36 -0800	[thread overview]
Message-ID: <AANLkTimTQxDCJ004VOFCdo3rz01tHsj9R4Ce1OGb75sH@mail.gmail.com> (raw)
In-Reply-To: <AANLkTimjz6t9ziPajW1wJdUE8ytWy=FbCr-Zg71PiBgf@mail.gmail.com>

On Mon, Jan 31, 2011 at 9:03 AM, Sri Ram Vemulpali
<sri.ram.gmu06@gmail.com> wrote:
> 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

Infact I think it will never return 1, since the addresses of __dummy1
and __dummy2 have to be different (off by 4 or 8). As pointed out it
is the next line that always returns 1. The purpose of this line is to
throw away warnings like "Incompatible pointer comparison" or
something like that (haven't tried :-)) incase there is a mismatch.

-- 
Thanks -
Manish

  parent reply	other threads:[~2011-01-31 18:13 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-01-31 17:03 Sri Ram Vemulpali
2011-01-31 17:20 ` Jamie Iles
2011-01-31 17:22 ` Geert Uytterhoeven
2011-01-31 17:44 ` Rajat Sharma
2011-01-31 18:03 ` Manish Katiyar [this message]
2011-01-31 20:23   ` Sri Ram Vemulpali
2011-01-31 20:26 ` Dave Hylands
2011-01-31 21:42   ` julie Sullivan

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