Re: question regarding gnu-isms

[Anthony Liguori <aliguori@us.ibm.com>]

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
>>>
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>
>