Re: question regarding gnu-isms

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

Aron Griffis wrote:
> Hi Anthony,
>
> Thanks for the explanation of the struct initialization.  Regarding
> the second one, though...
>
> Anthony Liguori wrote: [Thu Apr 27 2006, 07:04:01PM EDT]
>   
>> Aron Griffis wrote:
>>     
>>>    #define xen_create_contiguous_region(vstart, order, address_bits) 
>>>    ({0;})
>>>       
>> This is a really common one that lets you make statements into 
>> expressions.
>>     
> ...
>   
>> #define min(a, b) ({int lhs = a; int rhs = b; (lhs < rhs) ? lhs : rhs;})
>>     
>
> That makes sense for the example you gave, but how does it apply to
> the definition in question?  Is there any difference between ({0;})
> and (0) or even 0?
>   

Can you point me to where you saw this (and in what version of Xen)?

There is no difference between (0) and 0 of course.  I don't *think* 
there's a difference between ({0;}) and (0) but of course I've seen 
stranger things before.  My guess is that it started it's life as a more 
complex set of statements and overtime was reduced to just that.

Regards,

Anthony Liguori

>> All of these are documented in the GCC Info page (see the section on
>> C Extensions).
>>     
>
> :-)  I read that before posting, so I was familiar with the purpose of
> enclosing compound statements in expressions.  But I'll reread in case
> there's something I missed regarding the interpretation of ({0;})
>
> Regards,
> Aron
>