Re: [Xen-devel] [PATCH] CODING_STYLE: clarify function argument indentation

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 31/07/2019 18:49, Volodymyr Babchuk wrote:
> Hi Andrew,
>
> Andrew Cooper writes:
>
>> On 31/07/2019 17:24, Volodymyr Babchuk wrote:
>>> There are coding style rules that are widely accepted by community,
>>> but newer were formalized in the document. Notable example is the
>>> question on how function arguments and parameters should be indented
>>> when they do not fit into one line.
>>>
>>> This question was raised multiple times lately, mostly because of
>>> ongoing efforts to create Xen coding style formatting tool and because
>>> of new community members, who are not aware of such unwritten rules.
>>>
>>> Actually, this rule is already implicitly defined in the document by
>>> defining emacs coding style: 'c-file-style: "BSD"'. In this mode emacs
>>> lines up function arguments under the first argument. Naturally, most
>>> of Xen code is written in this style.
>>>
>>> So, lets state the obvious and fix this rule explicitly.
>>>
>>> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
>>> ---
>>>  CODING_STYLE | 14 ++++++++++++++
>>>  1 file changed, 14 insertions(+)
>>>
>>> diff --git a/CODING_STYLE b/CODING_STYLE
>>> index 6cc5b774cf..6479215a15 100644
>>> --- a/CODING_STYLE
>>> +++ b/CODING_STYLE
>>> @@ -53,6 +53,20 @@ Line Length
>>>  Lines should be less than 80 characters in length.  Long lines should
>>>  be split at sensible places and the trailing portions indented.
>>>
>>> +For multiline function declaration and call each new line should be
>>> +aligned with the first the parameter or argument. e.g.:
>>> +
>>> +void my_function_with_long_name(struct lengthy_struct_name *struct1,
>>> +                                struct lengthy_struct_name *struct2,
>>> +                                struct lengthy_struct_name *struct3);
>>> +
>>> +or
>>> +
>>> +function_with_so_many_params(wordy_parameter1, wordy_parameter2,
>>> +                             wordy_parameter3, wordy_parameter4);
>>> +
>>> +The same applies for macros.
>> For very wordy functions, or ones with silly quantities of parameters,
>> the following is also acceptable
>>
>> void my_function_with_long_and_silly_name(
>>  struct lengthy_struct_name *struct1, unsigned int womble, unsigned
>> int whatsit,
>>  struct lengthy_struct_name *struct2, bool yes, bool no, bool maybe,
>>  bool file_not_found, struct lengthy_struct_name *struct3, struct
>> lengthy_struct_name *struct4);
>>
>> which you will find in a few places throughout the code, because the
>> above doesn't waste enough vertical space to fit several functions in,
>> and push all the relevant details to the RHS.
> Excuse me, what it RHS?

Right Hand Side.

Sorry - I was being lazy when typing.

>
>> Per the above rules, the result would be this:
>>
>> void my_function_with_long_and_silly_name(struct lengthy_struct_name
>> *struct1,
>>  unsigned int womble,
>>  unsigned int whatsit,
>>  struct lengthy_struct_name
>> *struct2,
>>  bool yes, bool no, bool maybe,
>>  bool file_not_found,
>>  struct lengthy_struct_name
>> *struct3,
>>  struct lengthy_struct_name
>> *struct4);
>>
>> Of course, this is also a sign that maybe the function signature wants
>> changing anyway, but that may not be possible/sensible at the time.
>>
>> As with everything, the coding style is a set of guidelines which are
>> applicable to 98% of cases, but there are cases where aren't
>> appropriate, and common sense is the only reasonable deciding factor.
> I totally agree with you. Probably we should either add a generic clause
> like "This coding style rules may be violated if they produce weird
> results".

We should have a general clause (rather than specific ones), but I'd be
hesitant to word it like that.

How about:

These guidelines are expected to be applicable to all circumstances.  If
the result looks weird, consider whether this is the wisest way to solve
the problem in the first place, or whether an exception may be warranted.

The advantage here is if we see the same kind of exceptions being
requested repeatedly, then perhaps this is a hint that the coding style
should be modified.

> Or we can add clarification to this particular rule: "Do not break
> parameter definition to multiple lines. If parameters are too long,
> decrease indentation, but try to line them up. e.g.:
>
> void my_function_with_long_and_silly_name(
>         struct lengthy_struct_name *struct1,
>         unsigned int womble,
>         unsigned int whatsit,
>         struct lengthy_struct_name *struct2,
>         bool yes, bool no, bool maybe,
>         bool file_not_found,
>         struct lengthy_struct_name *struct3,
>         struct lengthy_struct_name *struct4);
> "
>
> What do you think?

The specific example I gave used exactly 4 spaces, consistent with the
rest of the style.

At the point that we are trying to reclaim space, reclaiming as much as
possible is the obvious move.

The above case is actually easy to spot in an automated fashion
(function declaration/call, open bracket, newline, indentation by one
block, subsequent lines on at the same indentation), and while it is
something which I wouldn't expect an automated tool to recommend, all
that matters is that it leaves it alone if it finds it.

~Andrew

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