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Date: Thu, 03 May 2012 13:35:18 +0300
From: Sakari Ailus <sakari.ailus@iki.fi>
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To: Mauro Carvalho Chehab <mchehab@redhat.com>
CC: Andy Walls <awalls@md.metrocast.net>,
	Hans Verkuil <hverkuil@xs4all.nl>, linux-media@vger.kernel.org,
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Subject: Re: [RFC v3 1/2] v4l: Do not use enums in IOCTL structs
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Hi Mauro,

Mauro Carvalho Chehab wrote:
> Em 02-05-2012 21:42, Andy Walls escreveu:
>> On Wed, 2012-05-02 at 19:17 -0300, Mauro Carvalho Chehab wrote:
>>
>>> We can speed-up the conversions, with something like:
>>>
>>> enum foo {
>>> 	BAR
>>> };
>>>
>>> if (sizeof(foo) != sizeof(u32))
>>> 	call_compat_logic().
>>>
>>> I suspect that sizeof() won't work inside a macro.
>>
>> sizeof() is evaluated at compile time, after preprocessing.
>> It should work inside of a macro.
>
> I tried to compile this small piece of code:
>
> enum foo { BAR };
> #if sizeof(foo) != sizeof(int)
> void main(void) { printf("different sizes\n"); }
> #else
> void main(void) { printf("same size\n"); }
> #endif
>
> It gives an error:
>
> /tmp/foo.c:2:11: error: missing binary operator before token "("
>
> So, either this doesn't work, because sizeof() is evaluated too late,
> or some trick is needed.
>
> Weird enough, cpp generates the error, but the expression is well-evaluated:
>
> $ cpp /tmp/foo.c
> # 1 "/tmp/foo.c"
> # 1 "<built-in>"
> # 1 "<command-line>"
> # 1 "/tmp/foo.c"
> /tmp/foo.c:2:11: error: missing binary operator before token "("
> enum foo { BAR };

sizeof() is processed by C compiler while #if is preprocessor directive, 
and its arguments have to be evaluable by the preprocessor, which is the 
problem here.

The C compiler can also optimise away things like that but it's more 
difficult to see whether that takes place or not; one would need to look 
at the resulting assembly code.

Regards,

-- 
Sakari Ailus
sakari.ailus@iki.fi