Why do gcc support empty struct extension for?

[Michael Meissner]

On Thu, Oct 16, 2008 at 10:02:24AM +0800, tiande wrote:
> In C++ context, I understand that it might be used as base class.
> But I don't understand, what does it for in C context? What is the
> reason we used empty struct in C.
> 
> (This is the first time I post in this mailing list and I have search
> through comp.lang.c, gcc mailing list, google to make sure that this
> is not a repeated post)
> 
> e.g.
> 
> struct foo{};
> 
> or just
> 
> struct bar;

Using struct bar without a definition has always been part of the C language
(and is part of the C90/C99 standard).  Usually you want to use it to declare
pointers to data that is only visible in private modules, i.e.

	 struct bletch {
		int a;
		struct bar *private_ptr;
		int c;
	};

The ISO standard does require that all pointers to structures be the same size
and format, just to allow this usage.

-- 
Michael Meissner, IBM
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