Re: [Xen-devel] question regarding gnu-isms

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

OK, now I'm curious too...

> > same file:
> >
> >     #define xen_create_contiguous_region(vstart, order, address_bits)
> > ({0;})
>
<snip parts of explanation>
> But clearly you cannot use return for this (and you cannot avoid making
> a statement here).  The GNU ({}) syntax allows you to have statements
> within an expression and the value of the very last statement in the
> block becomes the value of the expression.  The above could be written:

But why do that, if you just wanted to return a constant?  why not 

#define xen_create_contiguous_region(vstart, order, address_bits) 0

for instance?

Cheers,
Mark

>
> #define min(a, b) ({int lhs = a; int rhs = b; (lhs < rhs) ? lhs : rhs;})
>
> All of these are documented in the GCC Info page (see the section on C
> Extensions).
>
> Regards,
>
> Anthony Liguori
>
> >     This looks like the usual ({...}) construction but seems
> >     pointless.  The nearest explanation I could gather for this
> >     applies only to C++...  surely I'm missing something. :-)
> >
> > Thanks,
> > Aron
> >
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>
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-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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