linux-kernel - Re: [PATCH] Documentation: Explain a second alternative for multi-line macros.

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Date:	Sun, 31 Dec 2006 20:30:42 -0800
From:	Randy Dunlap <randy.dunlap@...cle.com>
To:	Segher Boessenkool <segher@...nel.crashing.org>
CC:	Muli Ben-Yehuda <muli@...ibm.com>,
	Linux kernel mailing list <linux-kernel@...r.kernel.org>,
	"Robert P. J. Day" <rpjday@...dspring.com>, trivial@...nel.org
Subject: Re: [PATCH] Documentation: Explain a second alternative for multi-line
 macros.

Segher Boessenkool wrote:
>>>>   #define setcc(cc) ({ \
>>>>     partial_status &= ~(SW_C0|SW_C1|SW_C2|SW_C3); \
>>>>     partial_status |= (cc) & (SW_C0|SW_C1|SW_C2|SW_C3); })
>>> This _does_ return a value though, bad example.
>>
>> Where does it return a value?
> 
> partial_status |=

as I expected (or suspected).
I also suspect that it wasn't intended, but this is old code
and I wasn't around Linux when it was written, so I don't know
about it for sure.

>> I don't see any uses of it
> 
> Ah, that's a separate thing -- it returns a value, it's just
> never used.

Ack.

>> And with a small change to put it inside a do-while block
>> instead of ({ ... }), it at least builds cleanly.
> 
> Well please replace it then, statement expressions should be
> avoided where possible (to start with, they don't have well-
> defined semantics).

We should probably avoid gcc extensions when possible.

I'll send a separate email for the patch.

-- 
~Randy
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