https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896
--- Comment #16 from Bill Wendling <isanbard at gmail dot com> --- (In reply to Martin Uecker from comment #15) > Am Freitag, dem 03.03.2023 um 20:27 +0000 schrieb isanbard at gmail dot com: > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896 > > > > --- Comment #14 from Bill Wendling <isanbard at gmail dot com> --- > > (In reply to Martin Uecker from comment #9) > > > > > Considering that the GNU extensions is rarely used, one could consider > > > > > redefining the meaning of > > > > > > > > > > int n = 1; > > > > > struct { > > > > > int n; > > > > > char buf[n]; > > > > > }; > > > > > > > > > > so that the 'n' refers to the member. Or we add a new syntax similar > > > > > to > > > > > designators (which intuitively makes sense to me). > > > > designator might be better IMO. > > > > > > > > a question here is: > > > > > > > > for the following nested structure: > > > > > > > > struct object { > > > > ... > > > > char items; > > > > ... > > > > struct inner { > > > > ... > > > > int flex[]; > > > > }; > > > > } *ptr; > > > > > > > > what kind of syntax is good to represent the upper bound of "flex" in > > > > the inner > > > > struct with "items" in the outer structure? any suggestion? > > > > > > I would disallow it. At least at first. It also raises some > > > questions: For example, one could form a pointer to the inner > > > struct, and then it is not clear how 'items' could be accessed > > > anymore. > > > > > > > That would be limiting its use in the Linux kernel. It seems that there are > > ways to refer to struct members already using something like "offsetof": > > > > struct object { > > ... > > char items; > > ... > > struct inner { > > ... > > int flex[] __attribute__((__element_count__(offsetof(struct object, > > items)))); > > }; > > } *ptr; > > This seems to be something completely different. offsetof > computes the offset from the type given in its argument. > But it would not access the value of the member of the > enclosing struct. But it would not work in your example, > because the struct object is incomplete at this point. > > So no, you can not use offsetof to reference a member > of an enclosing struct. > "offsetof(struct foo, count)" is a fancy wrapper for "((struct foo *)0)->count", which is resolved during sema, where it does have the full structure definition. For instance, this compiles in C++: struct a { int count; int y = ((struct a *)0)->count; } x; void foo(struct a *); > > > > The object referenced by "offsetof" would be from the containing structure > > (see > > "container_of" macro in Linux). > > > > It has the benefit of not needing to extend C's syntax to allow for > > designated > > initializers outside of initialization lists. > > Yes, but that syntax would be intuitive which I would see > as an advantage. > Sure, but you have a similar issue to your objection above because you're referencing members of a struct before it's completely defined. :-) > But I am not saying we shouldn't have the attribute first. > I personally prefer using an attribute than the suggestion to use some other syntax, like: struct foo { int fam[.count]; }; It becomes less intuitive what's going on here. And might conflict with VLA's in structures. > > It also has the benefit of > > allowing one to reference a variable not in the structure: > > > > const int items; > > struct object { > > ... > > char items; > > ... > > struct inner { > > ... > > int flex[] __attribute__((__element_count__(items))); /* References > > global "items" */ > > }; > > } *ptr; > > Whether you allow this or not has nothing to do with the syntax. > > The question is what semantics you attach to this and this is > also a question in your example. > > If you define > > struct inner* a = ... > > What does it say for a->flex ? > I need to point out that I used "offsetof" only as an example. It's possible to create something more robust which will carry along type information, etc., which the current offsetof macro throws away. I should have made that clear. The sanitizers that see "a->flex" will try to find the correct variable. If they can't, then they won't generate a check. In the case of it referencing a non-field member, it'll use that if it's within scope. If it refers to a field member of a parent container that's not within scope, it'll also not generate a check. It's unfortunate that these checks are done as a "best effort," but it could lead to software changes to improve security checks (like passing a parent structure into a function rather than an inner structure.
