linux-kernel - Re: macros: "do-while" versus "({ })" and a compile-time error

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Message-ID: <Pine.LNX.4.61.0701100715330.16104@chaos.analogic.com>
Date:	Wed, 10 Jan 2007 07:16:55 -0500
From:	"linux-os \(Dick Johnson\)" <linux-os@...logic.com>
To:	"Robert P. J. Day" <rpjday@...dspring.com>
Cc:	"Stefan Richter" <stefanr@...6.in-berlin.de>,
	"Linux kernel mailing list" <linux-kernel@...r.kernel.org>
Subject: Re: macros:  "do-while" versus "({ })" and a compile-time error


On Wed, 10 Jan 2007, Robert P. J. Day wrote:

> On Tue, 9 Jan 2007, linux-os (Dick Johnson) wrote:
>
>>
>> On Tue, 9 Jan 2007, Stefan Richter wrote:
>>
>>> Robert P. J. Day wrote:
>>>>   just to stir the pot a bit regarding the discussion of the two
>>>> different ways to define macros,
>>>
>>> You mean function-like macros, right?
>>>
>>>> i've just noticed that the "({ })"
>>>> notation is not universally acceptable.  i've seen examples where
>>>> using that notation causes gcc to produce:
>>>>
>>>>   error: braced-group within expression allowed only inside a function
>>>
>>> And function calls and macros which expand to "do { expr; } while (0)"
>>> won't work anywhere outside of functions either.
>>>
>>>> i wasn't aware that there were limits on this notation.  can someone
>>>> clarify this?  under what circumstances *can't* you use that notation?
>>>> thanks.
>>>
>>> The limitations are certainly highly compiler-specific.
>>
>> I don't think so. You certainly couldn't write working 'C' code like
>> this:
>>
>>  	do { a = 1; } while(0);
>>
>> This _needs_ to be inside a function. In fact any runtime operations
>> need to be inside functions. It's only in assembly that you could
>> 'roll your own' code like:
>>
>> main:
>>  	ret 0
>>
>>
>> Most of these errors come about as a result of changes where a macro
>> used to define a constant. Later on, it was no longer a constant in
>> code that didn't actually get compiled during the testing.
>
> just FYI, the reason i brought this up in the first place is that i
> noticed that the ALIGN() macro in kernel.h didn't verify that the
> alignment value was a power of 2, so i thought -- hmmm, i wonder if
> there are any invocations where that's not true, so i (temporarily)
> rewrote ALIGN to incorporate that check, and the build blew up
> including include/net/neighbour.h, which contains the out-of-function
> declaration:
>
> struct neighbour
> {
>        ...
>        unsigned char           ha[ALIGN(MAX_ADDR_LEN, sizeof(unsigned long))];
>        ...
>
> so it's not a big deal, it was just me goofing around and breaking
> things.
>
> rday


Hmmm, in that case you would be trying to put code inside a structure!
Neat --if you could do it!


Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.24 on an i686 machine (5592.72 BogoMips).
New book: http://www.AbominableFirebug.com/
_
..

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