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

To: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-devel] question regarding gnu-isms
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Fri, 28 Apr 2006 00:08:46 +0100
Cc: aron@xxxxxx
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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?


> #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
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
> _______________________________________________
> Xen-devel mailing list
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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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