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

[Paolo Bonzini <pbonzini@redhat.com>]

On 01/08/19 20:50, Eduardo Habkost wrote:
> 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?

Sure.

Paolo