Re: [PATCH 1/3] struct packed: Use gcc_struct on Windows

Pedro Alves
2022-07-21T18:30:41Z

On 2022-07-21 7:23 p.m., Eli Zaretskii wrote:
>> Cc: gdb-patches@sourceware.org
>> From: Pedro Alves <pedro@palves.net>
>> Date: Thu, 21 Jul 2022 19:15:17 +0100
>>
>>>> No. The attribute only changes the layout of that particular structure.
>>>
>>> And we are 110% sure that structure will never be passed to any other
>>> code?
>>
>> What other code are you talking about? struct packed is used in GDB's internal
>> structures. Nothing outside GDB ever sees it. GDB doesn't export a C api.
>> And if it did, we probably wouldn't use struct packed in exported structures.
>
> If this is supposed to be based on our vigilance and manual prevention
> of exporting it, I think it's fragile and not very reliable. If we
> forget or miss something, we get a subtly broken build.
>

I'm sorry, but this isn't making a lot of sense. If we ever exposed a public
C API, we'd have to be very careful with _all_ the types we expose, wrt to ABI stability.

>>>>> Can you tell why we must have the regular GCC layout of bitfields
>>>>> here?
>>>>
>>>> Because without it the struct won't really be packed.
>>>
>>> Can you tell why is that necessary?
>
> Can you answer this question, please?

Yes, sorry, I missed it before. I replied in another email.

>
> What I'm actually asking is whether there's any alternative which
> would avoid overriding the defaults in this matter.
>
>>> In any case, I'm very uneasy about changes that break ABI
>>> compatibility between parts of a program.
>>
>> But no ABI compatibility is broken.
>
> But it could be, even if today it isn't.
>

I could also write a ton of other bugs. What concern is this?