weird structure definition in header file

weird structure definition in header file

[Robert P. J. Day]

  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.

rday

Re: weird structure definition in header file

[nhorman]

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.

Regards
Neil

> rday
> -
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> the body of a message to majordomo@vger.kernel.org
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-- 
/***************************************************
 *Neil Horman
 *Software Engineer
 *gpg keyid: 1024D / 0x92A74FA1 - http://pgp.mit.edu
 ***************************************************/

Re: weird structure definition in header file

[Robert P. J. Day]

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

Re: weird structure definition in header file

[Robert P. J. Day]

On Tue, 6 Sep 2005, Robert P. J. Day wrote:

> 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.

um, still a couple questions on this.  first, in terms of examples,
there *is* no include/net/ipv4 directory in the kernel source tree
anymore, it's just net/ipv4.  and there's only a single header file in
there, which has no example of such a practise.  were you thinking of
a different directory?  (i'm in the 2.6.12.5 source tree.)

also, i'm assuming that some source file must eventually include an
actual definition of a "Widget" for the purposes of linking, as in:

  #include "widgets.h"
  ...
  Widgets Widget ;

however, given the inclusion of the header file, doesn't this give me
both a referencing declaration and a defining declaration of that
object in the same file?  is there no problem with that?  i was under
the impression that common practise was to have a single defining
declaration and all the *remaining* be referencing declarations.

rday

Re: weird structure definition in header file

[nhorman]

On Tue, Sep 06, 2005 at 03:45:01PM -0400, Robert P. J. Day wrote:
> On Tue, 6 Sep 2005, Robert P. J. Day wrote:
> 
> > 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.
> 
> um, still a couple questions on this.  first, in terms of examples,
> there *is* no include/net/ipv4 directory in the kernel source tree
> anymore, it's just net/ipv4.  and there's only a single header file in
> there, which has no example of such a practise.  were you thinking of
> a different directory?  (i'm in the 2.6.12.5 source tree.)
> 
I think you have a bad kernel tree:
http://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=tree
There is pretty clearly an include subdirectory there.

> also, i'm assuming that some source file must eventually include an
> actual definition of a "Widget" for the purposes of linking, as in:
> 
>   #include "widgets.h"
>   ...
>   Widgets Widget ;
> 
> however, given the inclusion of the header file, doesn't this give me
> both a referencing declaration and a defining declaration of that
> object in the same file?  is there no problem with that?  i was under
> the impression that common practise was to have a single defining
> declaration and all the *remaining* be referencing declarations.
> 
No.  You've got a defining declaration (the typedef struct {...} widgets;) and a
declaration which specifically defines an instance of the type widgets (named
widget) as being instatiated externally to the scope of this file.  Theres
nothing wrong with doing that.  Its exactly the same as placing an extern
directive in each C file that includes the header.  Its not at all problematic
(or sleazy :) ).
Neil

> rday
> 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

-- 
/***************************************************
 *Neil Horman
 *Software Engineer
 *gpg keyid: 1024D / 0x92A74FA1 - http://pgp.mit.edu
 ***************************************************/

Re: weird structure definition in header file

[Steve Graegert]

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>
Software Consultancy {C/C++ && Java && .NET}
Mobile: +49 (176)  21248869
Office: +49 (9131) 7126409

Re: weird structure definition in header file

[Ronaldo.Afonso]

I'm not sure if I understood your doubt but  just think a little bit, you
can put all the variables declaration inside a header file, be those
variables extern or not, and your source.c file (or source.c files) could
just "include" that header file. Beside of this, you could have other
modules where those variables are "really" declared. A extern variable
informs to the compiler that this variable is not is that module, just
that. The link must do the hard work.
I hope this information worths for you.
Good luck.

Ronaldo Z. Afonso
Projetista de Software
Cyclades Brasil
ronaldo.afonso@cyclades.com.br

Phone: 55 11 5033-3361
Fax: 55 11 5033-3388
www.cyclades.com.br


                                                                           
             "Robert P. J.                                                 
             Day"                                                          
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             09/06/2005 04:13          header file                         
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  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.

rday
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Re: weird structure definition in header file

[Glynn Clements]

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.

Yep.

> all right, i guess i can live with that although it strikes me as
> just a little bit sleazy. oh, well.

Huh? That's what header files are for. Or do you have stuff like:

	extern int strcmp(const char *s1, const char *s2);

at the top of all of your source files?

-- 
Glynn Clements <glynn@gclements.plus.com>

Re: weird structure definition in header file

[Glynn Clements]

Robert P. J. Day wrote:

> also, i'm assuming that some source file must eventually include an
> actual definition of a "Widget" for the purposes of linking, as in:
> 
>   #include "widgets.h"
>   ...
>   Widgets Widget ;

Correct.

> however, given the inclusion of the header file, doesn't this give me
> both a referencing declaration and a defining declaration of that
> object in the same file?

It gives you both a declaration and a definition in the same
compilation unit.

> is there no problem with that?

No.

If there was, you wouldn't be able to have forward declarations, and
thus wouldn't be able to define mutually-recursive functions.

>  i was under
> the impression that common practise was to have a single defining
> declaration and all the *remaining* be referencing declarations.

You shouldn't define a symbol more than once in any compilation unit,
but you can declare it as many times as you like.

-- 
Glynn Clements <glynn@gclements.plus.com>