Re: [dpdk-dev] [PATCH] ixgbe: fix build with gcc 4.4

[Vlad Zolotarov <vladz@cloudius-systems.com>]

On 04/14/15 17:17, Thomas Monjalon wrote:
> 2015-04-14 16:38, Vlad Zolotarov:
>> On 04/14/15 16:06, Ananyev, Konstantin wrote:
>>> From: Vlad Zolotarov [mailto:vladz@cloudius-systems.com]
>>>> On 04/14/15 12:31, Thomas Monjalon wrote:
>>>>> -	struct rte_eth_dev_info dev_info = { 0 };
>>>>> +	struct rte_eth_dev_info dev_info = { .max_rx_queues = 0 };
>>>> Hmmm... Unless I miss something this and one above would zero only a
>>>> single field - "max_rx_queues"; and would leave the rest uninitialized.
>>>> The original code intend to zero the whole struct. The alternative to
>>>> the original lines could be usage of memset().
>>> As I understand, in that case compiler had to set all non-explicitly initialised members to 0.
>>> So I think we are ok here.
>> Yeah, I guess it does zero-initializes the rest
>> (https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html) however I
>> don't understand how the above change fixes the error if it complains
>> about the dev_info.driver_name?
> As only 1 field is required, I chose the one which should not be removed
> from this structure in the future.
>
>> What I'm trying to say - the proposed fix is completely unclear and
>> confusing. Think of somebody reading this line in a month from today -
>> he wouldn't get a clue why is it there, why to explicitly set
>> max_rx_queues to zero and leave the rest be zeroed automatically... Why
>> to add such artifacts to the code instead of just zeroing the struct
>> with a memset() and putting a good clear comment above it explaining why
>> we use a memset() and not and initializer?
> We can make it longer yes.
> I think you agree we should avoid extra lines if not needed.
> In this case, when reading "= { .field = 0 }", it seems clear our goal
> is to zero the structure (it is to me).

I'm sorry but it's not clear to me at all since the common C practice 
for zeroing the struct would be

struct st a = {0};

Like in the lines u are changing. The lines as above are clearly should 
not be commented and are absolutely clear.
The lines u are adding on the other hand are absolutely unclear and 
confusing outside the gcc bug context. Therefore it should be clearly 
stated so in a form of comment. Otherwise somebody (like myself) may see 
this and immediately fix it back (as it should be).

> I thought it is a basic C practice.

I doubt that. ;) Explained above.

>
> You should try "git grep '\.[^ ]\+ *= *0 *}'" to be convinced that we are
> not going to comment each occurence of this coding style.
> But it must be explained in the coding style document. Agree?

OMG! This is awful! I think everybody agrees that this is a workaround 
and has nothing to do with a codding style (it's an opposite to a style 
actually). I don't know where this should be explained, frankly.

Getting back to the issue - I'm a bit surprised since I use this kind of 
initializer ({0}) in a C code for quite a long time - long before 2012. 
I'd like to understand what is a problem with this specific gcc version. 
This seems to trivial. I'm surprised CentOS has a gcc version with this 
kind of bugs.