lkml.org 
[lkml]   [2023]   [Jan]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 2/5] rust: types: introduce `ForeignOwnable`
On Sat, Jan 28, 2023 at 11:53:45AM -0300, Martin Rodriguez Reboredo wrote:
[...]
> > + /// Borrows a foreign-owned object.
> > + ///
> > + /// # Safety
> > + ///
> > + /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
> > + /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
> > + /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow_mut`]
> > + /// for this object must have been dropped.
> > + unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> Self::Borrowed<'a>;
> > +
> > + /// Mutably borrows a foreign-owned object.
> > + ///
> > + /// # Safety
> > + ///
> > + /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
> > + /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
> > + /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] and
> > + /// [`ForeignOwnable::borrow_mut`] for this object must have been dropped.
> > + unsafe fn borrow_mut<T: ForeignOwnable>(ptr: *const core::ffi::c_void) -> ScopeGuard<T, fn(T)> {
> > + // SAFETY: The safety requirements ensure that `ptr` came from a previous call to
> > + // `into_foreign`.
> > + ScopeGuard::new_with_data(unsafe { T::from_foreign(ptr) }, |d| {
> > + d.into_foreign();
> > + })
> > + }
>
> Could these three methods have a borrowing equivalent? When I was
> working on some features for the USB module I've stumbled upon the case
> of having to encode a pointer (with a pivot) and I cannot do it without
> taking ownership of the pointer.
>

*const T is Copy, so you can still use it after pass it to a function or
a new binding, e.g.

pub fn use_ptr(ptr: *const i32) { .. }

let p: *const i32 = some_func();

let q = p;

// q is just a copy of p
use_ptr(p);
// passing to a function parameter is just copying
use_ptr(p);

maybe I'm missing something subtle, but if you have an example I can
help take a look.

Regards,
Boqun

> > +
> > + /// Converts a foreign-owned object back to a Rust-owned one.
> > + ///
> > + /// # Safety
> > + ///
> > + /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
> > + /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
> > + /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] and
> > + /// [`ForeignOwnable::borrow_mut`] for this object must have been dropped.
> > + unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self;
> > +}
> > +
> > /// Runs a cleanup function/closure when dropped.
> > ///
> > /// The [`ScopeGuard::dismiss`] function prevents the cleanup function from running.
> > --
> > 2.34.1
>
> Aside from these comments I observe that there's a possibility to make
> ForeignOwnable a const trait and have non const implementors. Otherwise
> if these things are out of scope, no problem whatsoever and this has my
> OK.
>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>

\
 
 \ /
  Last update: 2023-03-27 00:00    [W:0.142 / U:0.564 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site