Requirements on the target platform C implementation #46
Conversation
gnzlbg
changed the title
|
|
||
| * Some platforms do not have any pointer value that compares equal to `NULL`. | ||
| Should we support these platforms? For example, an OS kernel like Linux that | ||
| wants to use the `0x0` address. Rust is used for these platforms already. |
There was a problem hiding this comment.
I think Rust should be able to target these platforms but Rust programs can't reference any object at address zero.
| * This interacts which the bit-patterns: | ||
|
|
||
| * for which `&T` and `&mut T` have niches, and therefore `Option<&T>` is | ||
| `None` |
There was a problem hiding this comment.
Regardless of the value of NULL (zero or otherwise), it should be straightforward for the Rust compiler to translate between a pointer and Option<&T> if the definition of std::ptr::NonNull were made more flexible; currently its documentation conflates zero and NULL..
OTOH, I think that would be a breaking change so I think it would be OK to restrict the current edition of Rust to platforms where NULL is zero since there's probably a lot of things in Rust that assume that already.
| `None` | ||
| * `<*[mut,const] T>::is_null(self)` returns `true` - note that `NonNull` | ||
| does not convey any information about the exact representation of its | ||
| niche, that is, it is different from `NonZero`. |
There was a problem hiding this comment.
The documentation for NonNull says "*mut T but non-zero and covariant" (emphasis mine). Later on the documentation says "non-null"; i.e."non-zero" and "non-null" are used interchangeably.
There was a problem hiding this comment.
The docs might need fixing.
There was a problem hiding this comment.
The docs might need fixing.
I agree. However, somebody may have already read the documentation and made the assumption that NULL == 0 based on them, and may then be relying on that, so it should be announced loudly if that change is made. If it isn't too late, it would be good to do it at the same time Rust 2018 edition ships and maybe consider it part of Rust 2018.
There was a problem hiding this comment.
It is unclear at this point what we are going to require about the niches of &T, that probably will be part of the discussions about "validity", which are not taking place yet (hence why this is an unresolved question).
|
So i'm closing this, since everything is in their respective chapters, I think its better to continue doing so for the time being, and like @gankro recommended once we are done with that, try to "independently" gather all those parts in a single document about the C platform. I think that all points made here should go in the section about layout / validity representation of integers and floating points. |
No description provided.