Re: Add compiler option -fstrict-flex-arrays=1

On 19.10.26 15:47, David Geier wrote:
> Hi Peter!
>
>> Ideally, we'd want to use the maximum level -fstrict-flex-arrays=3,
>> which would only allow flexible array members declared with the C
>> standard "[]".  But we do need to permit flexible array members declared
>> with "[1]" in a few cases because standard C does not allow flexible
>> array members as the only element in a struct.  Affected are:
>>
>> - struct RevmapContents
>> - struct ReplicationSlotCtlData
>> - union pgresult_data
>>
>> (Some of these have comments "It should be declared
>> [FLEXIBLE_ARRAY_MEMBER], but for some reason you can't do that in an
>> otherwise-empty struct.")
>
> I think that's because sizeof any struct in C must be > 0. If a struct
> contains no other member than an [] array, sizeof would be 0.

Yes, that's probably why.

> Can we maybe use typedefs instead of structs for the cases you
> mentioned? At least it's valid C to write typedef char Foo[].

Yeah, maybe some of this code could be rewritten to avoid this, but it
doesn't seem very urgent.

>> There is also:
>>
>> - struct sqlda_struct
>>
>> which, as I understand it, is bound by external API specification, so
>> not easy to change.
>>
>> If we turned on -fstrict-flex-arrays=3, these would become undefined
>> behavior and trigger the ubsan sanitizer.
>
> The compiler setting can be changed on a per-struct basis via
>
> __attribute__((strict_flex_arrays(N)));

This only works with gcc, not clang, so it's not a viable solution.