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

To: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] question regarding gnu-isms
From: Anthony Liguori <aliguori@xxxxxxxxxx>
Date: Thu, 27 Apr 2006 18:41:58 -0500
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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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