Re: [edk2-devel] [PATCH v1 0/5] EDK2 Code First: PI Specification: Update EFI_MM_COMMUNICATE_HEADER

["Laszlo Ersek" <lersek@redhat.com>]

On 06/18/21 11:37, Marvin Häuser wrote:
> On 16.06.21 22:58, Kun Qin wrote:
>> On 06/16/2021 00:02, Marvin Häuser wrote:

>>> 2) Is it feasible yet with the current set of supported compilers to
>>> support flexible arrays?
>> My impression is that flexible arrays are already supported (as seen
>> in UnitTestFrameworkPkg/PrivateInclude/UnitTestFrameworkTypes.h).
>> Please correct me if I am wrong.
>>
>> Would you mind letting me know why this is applicable here? We are
>> trying to seek ideas on how to catch developer mistakes caused by this
>> change. So any input is appreciated.
> 
> Huh, interesting. Last time I tried I was told about incompatibilities
> with MSVC, but I know some have been dropped since then (2005 and 2008
> if I recall correctly?), so that'd be great to allow globally.

I too am surprised to see
"UnitTestFrameworkPkg/PrivateInclude/UnitTestFrameworkTypes.h". The
flexible array member is a C99 feature, and I didn't even know that we
disallowed it for the sake of particular VS toolchains -- I thought we
had a more general reason than just "not supported by VS versions X and Y".

The behavior of OFFSET_OF() would be interesting -- the OFFSET_OF()
macro definition for non-gcc / non-clang:

#define OFFSET_OF(TYPE, Field) ((UINTN) &(((TYPE *)0)->Field))

borders on undefined behavior as far as I can tell, so its behavior is
totally up to the compiler. It works thus far okay on Visual Studio, but
I couldn't say if it extended correctly to flexible array members.

Thanks
Laszlo

> I feel
> like if the structure is modified anyway, it should probably get a
> trailing flexible array over the 1-sized hack. What do you think?