Re: [PATCH v8 1/5] Provide counted_by attribute to flexible array member field (PR108896)

[Martin Uecker <uecker@tugraz.at>]

Am Mittwoch, dem 10.04.2024 um 17:35 +0000 schrieb Joseph Myers:
> On Fri, 29 Mar 2024, Qing Zhao wrote:
> 
> > +  /* Issue error when there is a counted_by attribute with a different
> > +     field as the argument for the same flexible array member field.  */
> 
> There's another case of this to consider, though I'm not sure where best 
> to check for it (Martin might have suggestions) - of course this case will 
> need testcases as well.
> 
> Suppose, as allowed in C23, a structure is defined twice in the same 
> scope, but the two definitions of the structure use inconsistent 
> counted_by attributes.  I'd say that, when the declarations are in the 
> same scope (thus required to be consistent), it should be an error for the 
> two definitions of what is meant to be the same structure to use 
> incompatible counted_by attributes (even though the member declarations 
> are otherwise the same).

I think the right place could be comp_types_attributes in
attributes.cc.  It may be sufficient to set the
affects_type_identify flag.

This should then give a redefinition error as it should do for
"packed".

> 
> In C23 structures defined with the same tag in different scopes are 
> compatible given requirements including compatible types for corresponding 
> elements.  It would seem most appropriate to me for such structures with 
> incompatible counted_by attributes to be considered *not* compatible types 
> (but it would be valid to define structures with the same tag, different 
> scopes, and elements the same except for counted_by - just not to use them 
> in any way requiring them to be compatible).

Another option might be to warn about the case when those types
are then used together in a way where they are required to
be compatible.  Then comp_types_attributes would have to return 2.


Martin

> 
> > +The @code{counted_by} attribute may be attached to the C99 flexible array
> > +member of a structure.  It indicates that the number of the elements of the
> > +array is given by the field "@var{count}" in the same structure as the
> 
> As noted previously, the "" quotes should be removed there (or replaced by 
> ``'' quotes).
>