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

[Sidong Yang]

On Wed, Aug 06, 2025 at 03:38:24PM +0200, Benno Lossin wrote:
> On Wed Aug 6, 2025 at 2:38 PM CEST, Daniel Almeida wrote:
> > Hi Benno,
> >
> >> On 2 Aug 2025, at 07:52, Benno Lossin <lossin@xxxxxxxxxx> 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@xxxxxxxxxx> 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.
> 
> AFAIK C uninitialized memory also is considered uninitialized in Rust.
> So if this array is not properly initialized on the C side, this would
> be the correct type. If it is initialized, then just use `&mut [u8; 32]`.

pdu field is memory chunk for driver can use it freely. The driver usually
saves a private data and read or modify it on the other context. using
just `&mut [u8;32]` would be simple and easy to use.

> 
> ---
> Cheers,
> Benno