On Tue, 21 Jan 2025, Qing Zhao wrote:
> So, even after we introduce the designator syntax for counted_by attribute,
> arbitrary expressions as:
>
> counted_by (.len1 + const)
> counted_by (.len1 + .len2)
>
> Still cannot be supported?
Indeed. Attempting to use ".len1" inside something that looks like an
expression is fundamentally ambiguous, as I've noted in previous
discussions of such syntax. Consider
counted_by ((struct s) { .len1 = x }.len1)
where you now have an ambiguity of whether ".len1 = x" is a designated
initializer for the compound literal of type struct s, or an assignment to
".len1" within the structure referred to in counted_by.
> If not, how should we support simple expressions for counted_by attribute?
First, I should point out that the proposed standard feature in this area
(which received along-the-lines support at the WG14 meeting in Strasbourg
last year, though with a very large number of issues with the proposed
wording that would need to be resolved for any such feature to go in, and
without support for another paper needed for the feature to be useful) was
intentionally limited; it didn't try for general expressions, just for
.IDENTIFIER, where IDENTIFIER named a *const-qualified* structure member
(that thus had to be set when the structure was initialized and not
modified thereafter, so avoiding various of the complications we have with
counted_by of defining exactly when the value applies in relation to
accesses to different structure members).
But if you want a less-limited feature that allows for expressions, you
need some syntax for referring to a structure member that's not ambiguous.
For example, some new notation such as __self__.len1 to refer to a member
of the closest enclosing structure definition when in counted_by (while
being invalid except in counted_by inside a structure definition).
(That's just one example of how you might define syntax that avoids
ambiguity.)
--
Joseph S. Myers
josmy...@redhat.com
