Re: [Qemu-devel] [RFC] HACKING: Document 'struct' keyword usage

[Eduardo Habkost <ehabkost@redhat.com>]

On Wed, Jul 31, 2019 at 10:35:31AM +0200, Thomas Huth wrote:
> On 30/07/2019 23.07, Eduardo Habkost wrote:
> > Sometimes we use the 'struct' keyword to help us reduce
> > dependencies between header files.  Document that practice.
> > 
> > Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> > ---
> > I wonder if this is too terse?  Should we give examples?
> > ---
> >  HACKING | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/HACKING b/HACKING
> > index 0fc3e0fc04..112685bdaf 100644
> > --- a/HACKING
> > +++ b/HACKING
> > @@ -101,6 +101,8 @@ it points to, or it is aliased to another pointer that is.
> >  
> >  2.3. Typedefs
> >  Typedefs are used to eliminate the redundant 'struct' keyword.
> > +However, the 'struct' keyword may be sometimes used in header
> > +files to avoid unnecessary dependencies between headers.
> 
> See also the discussion earlier this year:
> 
> https://www.mail-archive.com/qemu-devel@nongnu.org/msg586180.html

Nice, it adds even more information than this patch.

For reference, this is the patch at the URL above:

-Typedefs are used to eliminate the redundant 'struct' keyword.
+Typedefs can be used to eliminate the redundant 'struct' keyword. This is
+especially helpful for common types that are used all over the place. Since
+certain C compilers choke on duplicated typedefs, you should avoid them and
+declare a typedef only in one header file. For common types, you can use
+"include/qemu/typedefs.h" for example. Note that it is also perfectly fine to
+use forward struct definitions without typedefs for references in headers
+to avoid the problem with duplicated typedefs.

I don't agree with the first two sentences, and I agree with what Paolo said
here:
  https://www.mail-archive.com/qemu-devel@nongnu.org/msg586214.html

("I agree 100% with the wording after 'Since'.  However, I think the first
part should be made stronger, not weaker.")

Paolo sent the following proposal:

| Typedefs are use to eliminate the redundant 'struct' keyword, since type
| names have a different style than other identifiers ("CamelCase" versus
| "snake_case").  Each struct should have a CamelCase name and a
| corresponding typedef.
| 
| Since certain C compilers choke on duplicated typedefs, you should avoid
| them and declare a typedef only in one header file.  For common types,
| you can use "include/qemu/typedefs.h" for example.  However, as a metter
| of convenience it is also perfectly fine to use forward struct
| definitions instead of typedefs in headers and function prototypes; this
| avoids problems with duplicated typedefs and reduces the need to include
| headers from other headers.

It seems perfect to me.

Paolo, do I have your signed-off-by to send that in a patch?

> 
> ... and we should merge HACKING and CODING_STYLE finally (that was on my
> private TODO list, but I never found the time to do it).

Agreed, but I prefer to fix one problem at a time.

-- 
Eduardo