linux-kernel - Re: macro _set_base - "do

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Message-ID: <20080102114956.GB8489@uranus.ravnborg.org>
Date:	Wed, 2 Jan 2008 12:49:56 +0100
From:	Sam Ravnborg <sam@...nborg.org>
To:	Abdel <draconux@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: macro _set_base - "do - while(0)" question

On Wed, Jan 02, 2008 at 12:45:39PM +0100, Abdel wrote:
> Hi,
> 
> In file include/asm-i386/system.h,  _set_base and _set_limit use an
> useless do ... while(0)
> 
> Why is this needed ?
> 
> exemple with _set_base from linux-2.6.23
> 
> #define _set_base(addr,base) do { unsigned long __pr; \
> __asm__ __volatile__ ("movw %%dx,%1\n\t" \
> 	"rorl $16,%%edx\n\t" \
> 	"movb %%dl,%2\n\t" \
> 	"movb %%dh,%3" \
> 	:"=&d" (__pr) \
> 	:"m" (*((addr)+2)), \
> 	 "m" (*((addr)+4)), \
> 	 "m" (*((addr)+7)), \
>          "0" (base) \
>         ); } while(0)
> 
Without the do {} while (0)

then
1) you could not define the variable __pr
2) You would be fooled when you did:

	if (foo != bar)
		set_base(addr,base);


	Sam
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