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
