From: David Brown <david@westcontrol.com>
To: gcc@gcc.gnu.org
Subject: Re: [wish] Flexible array members in unions
Date: Fri, 12 May 2023 14:32:22 +0200 [thread overview]
Message-ID: <u3lbkm$ovo$1@ciao.gmane.io> (raw)
In-Reply-To: <CAFiYyc1JRm6gm8aw2ZQmcycN3AF3hu5wjwGeUgDCN6gRN7UbbA@mail.gmail.com>
On 12/05/2023 08:16, Richard Biener via Gcc wrote:
> On Thu, May 11, 2023 at 11:14 PM Kees Cook via Gcc <gcc@gcc.gnu.org> wrote:
>>
>> On Thu, May 11, 2023 at 08:53:52PM +0000, Joseph Myers wrote:
>>> On Thu, 11 May 2023, Kees Cook via Gcc wrote:
>>>
>>>> On Thu, May 11, 2023 at 06:29:10PM +0200, Alejandro Colomar wrote:
>>>>> On 5/11/23 18:07, Alejandro Colomar wrote:
>>>>> [...]
>>>>>> Would you allow flexible array members in unions? Is there any
>>>>>> strong reason to disallow them?
>>>>
>>>> Yes please!! And alone in a struct, too.
>>>>
>>>> AFAICT, there is no mechanical/architectural reason to disallow them
>>>> (especially since they _can_ be constructed with some fancy tricks,
>>>> and they behave as expected.) My understanding is that it's disallowed
>>>> due to an overly strict reading of the very terse language that created
>>>> flexible arrays in C99.
>>>
>>> Standard C has no such thing as a zero-size object or type, which would
>>> lead to problems with a struct or union that only contains a flexible
>>> array member there.
>>
>> Ah-ha, okay. That root cause makes sense now.
>
> Hmm. but then the workaround
>
> struct X {
> int n;
> union u {
> char at_least_size_one;
> int iarr[];
> short sarr[];
> };
> };
>
> doesn't work either. We could make that a GNU extension without
> adverse effects?
>
> Richard.
>
I would like and use an extension like that (for C and C++) - the
flexible arrays would act as though they were the same size as the
size-specific part of the union, rounding up in this case to make the
alignments correct.
I regularly want something like :
union ProtocolBuffer {
struct {
header ...
data fields ...
}
uint8_t raw8[];
uint32_t raw32[];
}
The "raw" arrays would be used to move data around, or access it from
communication drivers. As C (and C++) is defined, I have to split this
up so that the "raw" arrays can use "sizeof(ProtocolTelegram) / 4" or
similar expressions for their size. If flexible arrays in unions were
allowed here, it could make my code a little neater and use more
anonymous unions and structs to reduce unhelpful verbosity.
>> Why are zero-sized objects missing in Standard C? Or, perhaps, the better
>> question is: what's needed to support the idea of a zero-sized object?
>>
>> --
>> Kees Cook
>
next prev parent reply other threads:[~2023-05-12 12:32 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-11 16:07 Alejandro Colomar
2023-05-11 16:29 ` Alejandro Colomar
2023-05-11 19:07 ` Kees Cook
2023-05-11 20:53 ` Joseph Myers
2023-05-11 21:13 ` Kees Cook
2023-05-11 21:43 ` Joseph Myers
2023-05-11 22:16 ` Kees Cook
2023-05-11 22:52 ` Joseph Myers
2023-05-12 0:25 ` Alejandro Colomar
2023-05-12 7:49 ` Jonathan Wakely
2023-05-12 6:16 ` Richard Biener
2023-05-12 12:32 ` David Brown [this message]
2023-05-15 19:58 ` Qing Zhao
2023-05-18 16:25 ` Martin Uecker
2023-05-18 20:59 ` Qing Zhao
2023-05-19 12:08 ` Martin Uecker
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