Am Donnerstag, dem 23.01.2025 um 17:39 +0000 schrieb Qing Zhao:
>
> > On Jan 22, 2025, at 12:20, Martin Uecker <[email protected]> wrote:
> >
> > Am Mittwoch, dem 22.01.2025 um 18:11 +0100 schrieb Martin Uecker:
> > > Am Mittwoch, dem 22.01.2025 um 16:37 +0000 schrieb Qing Zhao:
> > > >
> > > > > On Jan 22, 2025, at 11:22, Martin Uecker <[email protected]> wrote:
> > > > >
> > > > >
> > > > > Hello Michael,
> > > > >
> > > > > Am Mittwoch, dem 22.01.2025 um 16:54 +0100 schrieb Michael Matz:
> > > > > > On Wed, 22 Jan 2025, Martin Uecker wrote:
> > > > > >
> > > > > > > > > So you do not need to look further. But maybe I am missing
> > > > > > > > > something
> > > > > > > > > else.
> > > > > > > >
> > > > > > > > Like ...
> > > > > > > >
> > > > > > > > > > Note further that you may have '{ .y[1][3].z }', which is
> > > > > > > > > > still not a
> > > > > > > > > > designation, but an expression under your proposal, whereas
> > > > > > > > > > '{ .y[1][3].z = 1 }' would remain a designation. This
> > > > > > > > > > shows that you
> > > > > > > > > > now need arbitrary look-ahead to disambiguate the two. A
> > > > > > > > > > Very Bad Idea.
> > > > > > > >
> > > > > > > > ... this?
> > > > > > >
> > > > > > > In .y[1][3].z after .y you can decide whether y is a member of
> > > > > > > the
> > > > > > > struct being initialized. If it is, it is a designator and if not
> > > > > > > it must be an expression.
> > > > > >
> > > > > > If y is not a member it must be an expression, true. But if it's a
> > > > > > member
> > > > > > you don't know, it may be a designation or an expression.
> > > > >
> > > > > In an initializer I know all the members.
> > > >
> > > > I am not familiar with the parser, so, I am a little confused about the
> > > > following:
> > > >
> > > > Suppose we have:
> > > >
> > > > struct foo {
> > > > int z;
> > > > float f;
> > > > }
> > > >
> > > > struct bar {
> > > > char *array __attribute__ ((counted_by (.y[1][3].z + 4)));
> > > > struct foo y[5][10];
> > > > }
> > > >
> > > > So, in the above, when parsing the above expression inside counted_by,
> > > > can the
> > > > current parser be easily to be extended to parse it?
> > >
> > > No, I don't think this can be done easily. The issue is that you do
> > > not know the declaration for y because it hasn't been parsed yet.
> > >
> > > If you forward reference some struct member, you have several
> > > possibilities:
> > >
> > > - use it only in limited contexts where you do not need to know
> > > the type (e.g. this works for goto labels) or for a basic
> > > counted_by attribute that only takes an identifier as we have it now.
> > >
> > > - simply assume it has a certain type (size_t as is proposed in the
> > > WG14 paper Joseph mentioned) and fail later if it does not.
> > >
> > >
> > > Both options would rule the construct above (but there could be
> > > workarounds).
> >
> > One of the workarounds could be to instead call a function (which could
> > be inlined later) and that function takes a pointer to the member.
> > Then it does not need to now anything about any member, e.g.:
> >
> >
> > struct foo {
> > int z;
> > float f;
> > }
> >
> > size_t bar_count(struct bar *);
> >
> > struct bar {
> > char *array __attribute__ ((counted_by (bar_count(__self__))));
> > struct foo y[5][10];
> > }
> >
> > size_t bar_count(struct bar *p)
> > {
> > return p->y[1][3].z +4;
> > }
> >
> >
> In this workaround, we also need to introduce a new key word “__self__”,
> Is this the same as the “__self__” Joseph mentioned in a previous email
> (I copied Joseph’s word in below for easy reference):
I think it could be same, although Joseph uses it with a member access.
One could also use designator syntax (without __self__) and pass a reference
to the member only (which also seems appropriate for counted_by).
struct foo {
int z;
float f;
}
size_t bar_count(struct foo (*yp)[5][10]);
struct bar {
char *array __attribute__ ((counted_by (bar_count(&.y))));
struct foo y[5][10];
}
size_t bar_count(struct foo (*yp)[5][10])
{
return (*yp)[1][3].z +4;
}
I still prefer the version without __self__ and will respond to Michael
why I think so, but I am also fine *with* __self__. I just think the
delayed parsing version used in the Apple prototype should be avoided,
because it is very confusing (and because it needs delayed parsing).
>
>
> "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.)”
>
> So, for the following two approaches to represent expression as argument of
> “counted_by” attribute:
>
> A. Allowing function call as the argument, and the new key word “__self__”
> is passed to this function to
> reference members of the closest enclosing structure.
>
> > struct bar {
> > char *array __attribute__ ((counted_by (bar_count(__self__))));
> > struct foo y[5][10];
> > }
>
>
> B. Allowing expression with __self__ as the argument:
>
> > struct bar {
> > char *array __attribute__ ((counted_by (__self__.y[1][3].z + 4)));
> > struct foo y[5][10];
> > }
>
>
> My question:
>
> Which is better, A or B?
B does not work, because you can not parse the expression using the
existing parser because the type of y is unknown.
I also do not really like A because I think we should not allow function
calls, so I tend to favour the version with the cast at the moment.
Martin
>
>
> Thanks a lot for your help.
>
> Qing
>
> >
> > > Other alternatives are:
> > >
> > > - you have same kind of forward declaration (as we have for
> > > parameters as GNU extension). In the context of C, this is the
> > > cleanest solution but either requires forward declaring the
> > > full struct (which can be done in C23) or new syntax for only
> > > forward declaring the member.
> >
> > A possible C23 workaround could be:
> >
> > struct foo {
> > int z;
> > float f;
> > }
> >
> > struct bar {
> > char *array __attribute__ ((counted_by (*)));
> > // star indicates missing size exppression
> > struct foo y[5][10];
> > }
> >
> > struct bar { // redeclare with known size
> > char *array __attribute__ ((counted_by (.y[1][3].z + 4)));
> > struct foo y[5][10];
> > }
> >
> >
> > Martin
> >
> >
> > >
> > > - or you use some delayed parsing where you store away the tokens
> > > and parse it later when all structure members are done. I think
> > > this is a highly problematic approach for a variety of reasons.
> > >
> > >
> > > Martin
> > >
> > >
> > > >
> > > > thanks.
> > > >
> > > > Qing
> > > > >
> > > > > Martin
> > > > >
> > > > >
> > > >
> > >
> >
>
