bwendling wrote:
@apple-fcloutier:
> think that there's room to allow `__counted_by` on incomplete types so that a
> TU where it's complete could use it (and we have use cases where that would
> be handy), but our implementation doesn't support it at this time. This can
> be added without disruptions at a later time. FWIW, the point of
> bounds-checking an index is moot at least while the type is incomplete, since
> it's an error to dereference a pointer to an incomplete type. Aside from
> indexing, operations that would cast the pointer (such as `memset(x->foo, 0,
> some_value)`, assuming `memset(void *__sized_by(n), int, size_t n)`) would
> still have to be illegal when `struct foo` is incomplete because there's no
> way to know how many bytes we have at x->foo without `sizeof(struct foo)`.
What I'm thinking is something like:
```c
a.c:
struct foo;
struct bar {
size_t count;
struct foo *ptr __counted_by(count);
};
struct bar *alloc_bar(size_t num_elems) {
struct bar *p = malloc(sizeof(bar));
p->count = num_elems;
p->ptr = malloc(sizeof(struct foo *) * num_elems);
return p;
}
/* later on, or in another TU */
struct foo {
/* fields */
};
void bork(size_t num_elems) {
struct bar *p = alloc_bar(num_elems);
for (size_t i = 0; i < num_elems; ++i) {
p->ptr[i] = alloc_foo();
}
/* some use of p */
}
}
```
I was incorrect about `void *`. It's valid to cast any pointer to `void *` and
back to the original type.
> For structs with a flexible array member, the reason is that sizeof(struct
> bar) is a lie. How would we reconcile, for instance, a count of 1 (implying
> there's like 4 bytes at x->bar) with a x->bar->a of 10 (implying that
> x->bar->fam[8] is in bounds)? How do we feel that x->bar[0]->fam[0] aliases
> with x->bar[1]->a?
It's not a lie, because the contents of a pointer don't contribute to the size
of the struct containing that pointer.
https://github.com/llvm/llvm-project/pull/90786
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