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

[Anthony Liguori <aliguori@xxxxxxxxxx>]

Mark Williamson wrote:
> 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?
>   
I thought the same exact thing myself.  Of course, I can't find it in my 
tree (xen_create_contiguous_region is a function) so perhaps someone has 
already fixed it.
Regards,

Anthony Liguori

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

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