Re: weird structure definition in header file

[Steve Graegert <graegerts@gmail.com>]

On 9/6/05, nhorman@tuxdriver.com <nhorman@tuxdriver.com> wrote:
> On Tue, Sep 06, 2005 at 03:13:48PM -0400, Robert P. J. Day wrote:
> >
> >   i'm looking at some legacy code and, in a header file, i find the
> > following (paraphrased for brevity):
> >
> > typedef struct {
> >       ... stuff ...
> > } Widgets ;
> >
> > extern Widgets Widget ;
> >
> >
> >   huh?  i can see why a header file would want to define a structure
> > but i'm confused why the *header* file would then refer to an external
> > object of that type.  that's a new one on me -- typically, i'd expect
> > a *source* file to define such a thing and other *source* files to
> > contain the "extern" declaration.
> >
> >   is this some subtle programming cleverness of which i am unaware?
> > thanks.
> >
> This is done quite frequently when a data structure needs to be referenced from
> multiple locations.  They're not necessicarily consecutive like that, but its
> rather a common practice to extern a instance of a type in a header file.  The
> kernel source tree does this very frequently, check include/net/ipv4 for lots of
> examples.

That's correct.  It is also used to implement some kind of inheritance
in C as can be seen in the Motif code.  Here, widgets are organized in
a "class hierarchy" and every widget inherits features of it "super
class" by simply defining new "parts" in separate structures that are
combined with the super class' part structure into a record which is
then defined extern for the reasons Neil just mentioned.  This leads
to these weird structure definitions you encountered.
 
Regards

	\Steve

--

Steve Graegert <graegerts@gmail.com>
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