Re: [PATCH] range-diff: fix some 'hdr-check' and sparse warnings

[Johannes Sixt <j6t@kdbg.org>]

Am 13.07.19 um 12:44 schrieb Johannes Sixt:
> Am 12.07.19 um 18:44 schrieb Junio C Hamano:
>> Johannes Sixt <j6t@kdbg.org> writes:
>>
>>> Am 12.07.19 um 00:03 schrieb Ramsay Jones:
>>>> diff --git a/range-diff.c b/range-diff.c
>>>> index ba1e9a4265..0f24a4ad12 100644
>>>> --- a/range-diff.c
>>>> +++ b/range-diff.c
>>>> @@ -102,7 +102,7 @@ static int read_patches(const char *range, struct string_list *list)
>>>>  		}
>>>>  
>>>>  		if (starts_with(line, "diff --git")) {
>>>> -			struct patch patch = { 0 };
>>>> +			struct patch patch = { NULL };
>>>
>>> There is nothing wrong with 0 here. IMHO, zero-initialization should
>>> *always* be written as = { 0 } and nothing else. Changing 0 to NULL to
>>> pacify sparse encourages a wrong style.
>>
>> Hmm, care to elaborate a bit?  Certainly, we have a clear preference
>> between these two:
>>
>> 	struct patch patch;
>> 	patch.new_name = 0;
>> 	patch.new_name = NULL;
>>
>> where the "char *new_name" field is the first one in the structure.
>> We always try to write the latter, even though we know they ought to
>> be equivalent to the language lawyers.
> 
> I'm not questioning this case; the latter form is clearly preferable.
> 
> Using only = { 0 } for zero-initialization makes the code immune to
> rearrangements of the struct members. That is not the case with = { NULL
> } because it requires that the first member is a pointer; if
> rearrangement makes the first member a non-pointer, the initializations
> must be adjusted.
> 
> On the other hand, I'm not arguing that
> 
>   struct string_list dup_it = { NULL, 0, 0, 1, NULL };
> 
> should be written as
> 
>   struct string_list dup_it = { 0, 0, 0, 1, 0 };
> 
> I'm only complaining about the single-initializer = { 0 } "please

Of course, I'm not "complaining about" it; I'm "arguing for" it...

> initialize this whole struct with zero values" form.
> 
>> Is the reason why you say 0 is fine here because we consider
>>
>> 	struct patch patch, *dpatch;
>> 	memset(&patch, 0, sizeof(patch));
>> 	dpatch = xcalloc(1, sizeof(patch));
>>
>> are perfectly good way to trivially iniitialize an instance of the
>> struct?
> 
> Absolutely not. Both forms are evildoing as far as struct initialization
> is concerned because they ignore the types of the members. The memset
> form should always be replaced by = { 0 }. The correct replacement for
> the xcalloc form would be
> 
> 	struct patch zero = { 0 };
> 	struct patch *dpatch = xmalloc(sizeof(*dpatch));
> 	*dpatch = zero;
> 
> but I do understand that this transformation is unacceptably verbose.
> 
>> Do we want to talk to sparse folks about this?
> 
> I've no idea which camp they are in. How would they respond to an
> exceptional case that is also very much a matter of taste?
> 
> -- Hannes
>