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

To: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-devel] question regarding gnu-isms
From: Anthony Liguori <aliguori@xxxxxxxxxx>
Date: Thu, 27 Apr 2006 18:04:01 -0500
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Aron Griffis wrote:
I understand most of the gcc extensions that I find in xen, for
example ({...}) in a #define.  However I've come across a couple that
I'm not familiar with.  Does somebody mind explaining the point of

from linux-2.6-xen-sparse/include/asm-ia64/hypervisor.h:

    #define __pte_ma(_x)    ((pte_t) {(_x)})

This is structure initialization assignment. pte_t is a struct. It can be initialized like:

pte_t pte = {0};

If you want to assign to it with a similar structure, you cannot do:

pte_t pte = {0};
pte = {3};

The GNU extension allows you to do this by doing:

pte_t pte = {0};
pte = (pte_t){3};

    What's the point of the braces here?

same file:

    #define xen_create_contiguous_region(vstart, order, address_bits) ({0;})

This is a really common one that lets you make statements into expressions. Say you wanted to implement a min function over int's as a macro, to do it properly, you have to do something like:

#define min(a, b) {int lhs = a; int rhs = b; return (lhs < rhs) ? lhs : rhs; }

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:

#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).


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. :-)


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