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;
}
> 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
> > >
> > >
> >
>
