Re: [edk2-devel] [Question] using Flexible Array Member in Structure.

["Pedro Falcato" <pedro.falcato@gmail.com>]

On Fri, Apr 5, 2024 at 7:43 AM levi.yun <yeoreum.yun@arm.com> wrote:
>
> Hi Michael! Thanks for answer.
>
> > Converting these to a flexible array member would not be
> a backwards compatible change.
>
> That's the point. But at least when I see the compiler used in tool_def.txt,
> there's no compiler which doesn't support to flexible array member.
>
> Do we still need to consider the case building edk2 with lower version of comipler
> which isn't manifested in tool_def.txt?

That's not in question. What Mike is referring to here is, e.g:

typedef struct {
BOOLEAN Supported;
UINT64 KeyCount;
UINT64 CapabilityCount;
EFI_BLOCK_IO_CRYPTO_CAPABILITY Capabilities [1];
} EFI_BLOCK_IO_CRYPTO_CAPABILITIES1;

typedef struct {
BOOLEAN Supported;
UINT64 KeyCount;
UINT64 CapabilityCount;
EFI_BLOCK_IO_CRYPTO_CAPABILITY Capabilities [];
} EFI_BLOCK_IO_CRYPTO_CAPABILITIES2;

sizeof(EFI_BLOCK_IO_CRYPTO_CAPABILITIES1) !=
sizeof(EFI_BLOCK_IO_CRYPTO_CAPABILITIES2), so it's an ABI break.
And unfortunately many of these structs need to be ABI-stable (I took
this one off the spec, you can find many such cases by ctrl+F'ing
[1]).

Basically, it comes down to the ABI. If it breaks the ABI, you need to
figure out *if* it matters (e.g is it part of a protocol, or is it an
internal lib struct that does not need to be stable, is it ABI to some
external component).
Also, worth noting that the "[1]" pattern is UB, and only works
because GCC (et al) don't want to break a bunch of code. If you're
defining a new struct, there's no reason to use [1] as a flexible
array.

-- 
Pedro


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