Phillip Wood <phillip.wood123@xxxxxxxxx> writes:
>> the end of a struct isn't triggering a compile error. But having a field
>> _after_ that empty flex array _does_ trigger a compile error:
>> struct MyStruct {
>> int x;
>> int flexA[];
>> char string[256];
>> };
>> Note the added `string` field.
Ick.
If we use the "safer but a bit wasteful" alternative in such a case,
it will become just wrong. We'll have a single-member array there,
and extra allocation for an instance of MyStruct will only lengthen
the .string member, without allowing the .flexA member to have more
elements.
> Having a field after the flex array is a violation of the C
> standard. Section 6.7.2.1:
> ... the last member of a structure with more than one named member
> may have incomplete array type, such a structure (and any union
> containing, possibly recursively, a member that is such a structure)
> shall not be a member of a structure or an element of an array.
>
> Note that the wording also forbids
>
> struct A {
> int x;
> char flex[];
> };
>
> struct B {
> struct A a; /* This is forbidden */
> };
>
> There was a proposal a few years ago to relax that restriction [1] but
> it does not seem to be in the latest draft standard.
>
> None of this helps fix the problem, but it does explain why MSVC complains.
Thanks.
In short, the code is wrong, and the compiler is complaining to
avoid us using a wrong code. If the same code is given to other
compilers that support ANCI C style flexible array member, we are
likely to see the same error.
Which makes the whole thing (including my "let's make the
conditionals to set up FLEX_ARRAY for various compilers easier to
read") much less urgent.
Thanks for a well-written summary.
