Re: [RFC PATCH v2 2/4] rust: io_uring: introduce rust abstraction for io-uring cmd

[Daniel Almeida <daniel.almeida@collabora.com>]

Hi Benno,

> On 2 Aug 2025, at 07:52, Benno Lossin <lossin@kernel.org> wrote:
> 
> On Fri Aug 1, 2025 at 3:48 PM CEST, Daniel Almeida wrote:
>>> On 27 Jul 2025, at 12:03, Sidong Yang <sidong.yang@furiosa.ai> wrote:
>>> +    #[inline]
>>> +    pub fn pdu(&mut self) -> &mut MaybeUninit<[u8; 32]> {
>> 
>> Why MaybeUninit? Also, this is a question for others, but I don’t think
>> that `u8`s can ever be uninitialized as all byte values are valid for `u8`.
> 
> `u8` can be uninitialized. Uninitialized doesn't just mean "can take any
> bit pattern", but also "is known to the compiler as being
> uninitialized". The docs of `MaybeUninit` explain it like this:
> 
>    Moreover, uninitialized memory is special in that it does not have a
>    fixed value (“fixed” meaning “it won’t change without being written
>    to”). Reading the same uninitialized byte multiple times can give
>    different results.
> 
> But the return type probably should be `&mut [MaybeUninit<u8>; 32]`
> instead.


Right, but I guess the question then is why would we ever need to use
MaybeUninit here anyways.

It's a reference to a C array. Just treat that as initialized.

— Daniel