On 1/21/09, debian developer <[email protected]> wrote:
> I don't seem to understand this patch. What is swap(a, b) returning here?
> and where is it returning from?
It is not "returning" but substituting, cause it's a macro, not a
function. It swaps the values of "a" and "b".
>
> What advantage does using a do{}while(0); have on this?
Because the macro is formed by more than a line, the do-while(0)
thing, prevents from errors when using something like this:
if(condition)
swap(a,b)
With do-while, even in the case of the missing "{" and "}" _all_ the
lines in the macro will be executed if "condition" is true.
>
> On Fri, Jan 16, 2009 at 3:15 PM, Peter Zijlstra <[email protected]>
> wrote:
> >
> > On Fri, 2009-01-16 at 01:38 -0800, Andrew Morton wrote:
> > > On Fri, 16 Jan 2009 10:26:34 +0100 Peter Zijlstra <[email protected]>
> wrote:
> > >
> > > > Hi,
> > > >
> > > > Where did this patch come from? -- I can't seem to find it on lkml at
> all..
> > >
> > > That's a damn good question. I received:
> >
> > Hehe, one of the mysteries of life then :-)
> >
> >
> > > > The reason I ask it that I wonder why swap() has a return value?
> > >
> > > Accident, I guess.
> > >
> > > > +#define swap(a, b) ({ typeof(a) __tmp = (a); (a) = (b); (b) = __tmp;
> })
> > >
> > > You think it should use the do{}while(0) thing?
> >
> > That was what I was thinking indeed. Non of the current users appear to
> > make use of the (somewhat arbitrary) return value, so lets change it
> > before someone gets creative ;-)
> >
> > Signed-off-by: Peter Zijlstra <[email protected]>
> > ---
> >
> > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > index 343df9e..7fa3718 100644
> >
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -480,7 +480,8 @@ static inline char *pack_hex_byte(char *buf, u8 byte)
> > /*
> >
> > * swap - swap value of @a and @b
> > */
> >
> > -#define swap(a, b) ({ typeof(a) __tmp = (a); (a) = (b); (b) = __tmp; })
> > +#define swap(a, b) \
> > + do { typeof(a) __tmp = (a); (a) = (b); (b) = __tmp; } while (0)
> >
> >
> > /**
> > * container_of - cast a member of a structure out to the containing
> structure
> >
> >
> >
> >
> >
> > --
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> >
>
>
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