Re: weird structure definition in header file

["Robert P. J. Day" <rpjday@mindspring.com>]

On Tue, 6 Sep 2005, 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.

ok, so what it represents is just a shortcut so that each source file
doesn't have to have its own extern, it just comes along with the
header file.  all right, i guess i can live with that although it
strikes me as just a little bit sleazy.  oh, well.

rday