Am Freitag, dem 27.10.2023 um 14:32 +0000 schrieb Qing Zhao:
>
> > On Oct 27, 2023, at 3:21 AM, Martin Uecker <uec...@tugraz.at> wrote:
> >
> > Am Donnerstag, dem 26.10.2023 um 19:57 +0000 schrieb Qing Zhao:
> > > I guess that what Kees wanted, ""fill the array without knowing the
> > > actual final size" code pattern”, as following:
> > >
> > > > > struct foo *f;
> > > > > char *p;
> > > > > int i;
> > > > >
> > > > > f = alloc(maximum_possible);
> > > > > f->count = 0;
> > > > > p = f->buf;
> > > > >
> > > > > for (i; data_is_available() && i < maximum_possible; i++) {
> > > > > f->count ++;
> > > > > p[i] = next_data_item();
> > > > > }
> > >
> > > actually is a dynamic array, or more accurately, Bounded-size dynamic
> > > array: ( but not a dynamic allocated array as we discussed so far)
> > >
> > > https://en.wikipedia.org/wiki/Dynamic_array
> > >
> > > This dynamic array, also is called growable array, or resizable array,
> > > whose size can
> > > be changed during the lifetime.
> > >
> > > For VLA or FAM, I believe that they are both dynamic allocated array,
> > > i.e, even though the size is not know at the compilation time, but the
> > > size
> > > will be fixed after the array is allocated.
> > >
> > > I am not sure whether C has support to such Dynamic array? Or whether
> > > it’s easy to provide dynamic array support in C?
> >
> > It is possible to support dynamic arrays in C even with
> > good checking, but not safely using the pattern above
> > where you derive a pointer which you later use independently.
> >
> > While we could track the connection to the original struct,
> > the necessary synchronization between the counter and the
> > access to the buffer is difficult. I do not see how this
> > could be supported with reasonable effort and cost.
> >
> >
> > But with this restriction in mind, we can do a lot in C.
> > For example, see my experimental (!) container library
> > which has vector type.
> > https://github.com/uecker/noplate/blob/main/test.c
> > You can get an array view for the vector (which then
> > also can decay to a pointer), so it interoperates nicely
> > with C but you can get good bounds checking.
> >
> >
> > But once you derive a pointer and pass it on, it gets
> > difficult. But if you want safety, you just have to
> > to simply avoid this in code.
>
> So, for the following modified code: (without the additional pointer “p”)
>
> struct foo
> {
> size_t count;
> char buf[] __attribute__((counted_by(count)));
> };
>
> struct foo *f;
> int i;
>
> f = alloc(maximum_possible);
> f->count = 0;
>
> for (i; data_is_available() && i < maximum_possible; i++) {
> f->count ++;
> f->buf[i] = next_data_item();
> }
>
> The support for dynamic array should be possible?
With the design we discussed this should work because
__builtin_with_access (or whatever) it reads:
f = alloc(maximum_possible);
f->count = 0;
for (i; data_is_available() && i < maximum_possible; i++) {
f->count ++;
__builtin_with_access(f->buf, f->count)[i] = next_data_item();
}
>
>
> >
> > What we could potentially do is add restrictions so
> > that the access to buf always has to go via x->buf
> > or you get at least a warning.
>
> Are the following two restrictions to the user enough:
>
> 1. The access to buf should always go via x->buf,
> no assignment to another independent pointer
> and access buf through this new pointer.
Yes, maybe. One could also try to be smarter.
For example, one warn only when &f->buf is
assigned to another pointer and one of the
following conditions is fulfilled:
- the pointer escapes from the local context
- there is a store to f->counter in the
local context that does not dominate &f->buf.
Then Kees' example would work too in most cases.
But I would probably wait until we have some
initial experience with this feature.
Martin
> 2. User need to keep the synchronization between
> the counter and the access to the buffer all the time.
>
>
