Re: macro _set_base - "do - while(0)" question

Sam Ravnborg
2008-01-02T11:50:05Z

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