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From:   Alice Ryhl <aliceryhl@google.com>
Date:   Wed, 29 Nov 2023 18:14:24 +0100
Message-ID: <CAH5fLgi6n6WiueLkzvZ7ywt5hXWAJFAyseRr3O=KRAHUQ=hNrQ@mail.gmail.com>
Subject: Re: [PATCH 4/7] rust: file: add `FileDescriptorReservation`
To:     brauner@kernel.org
Cc:     a.hindborg@samsung.com, alex.gaynor@gmail.com, arve@android.com,
        benno.lossin@proton.me, bjorn3_gh@protonmail.com,
        boqun.feng@gmail.com, cmllamas@google.com,
        dan.j.williams@intel.com, dxu@dxuuu.xyz, gary@garyguo.net,
        gregkh@linuxfoundation.org, joel@joelfernandes.org,
        keescook@chromium.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, maco@android.com, ojeda@kernel.org,
        peterz@infradead.org, rust-for-linux@vger.kernel.org,
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        viro@zeniv.linux.org.uk, wedsonaf@gmail.com, willy@infradead.org
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On Wed, Nov 29, 2023 at 5:55=E2=80=AFPM Alice Ryhl <aliceryhl@google.com> w=
rote:

> >> +    pub fn commit(self, file: ARef<File>) {
> >> +        // SAFETY: `self.fd` was previously returned by `get_unused_f=
d_flags`, and `file.ptr` is
> >> +        // guaranteed to have an owned ref count by its type invarian=
ts.
> >> +        unsafe { bindings::fd_install(self.fd, file.0.get()) };
> >
> > Why file.0.get()? Where did that come from?
>
> This gets a raw pointer to the C type.
>
> The `.0` part is a field access. `ARef` struct is a tuple struct, so its
> fields are unnamed. However, the fields can still be accessed by index.

Oh, sorry, this is wrong. Let me try again:

This gets a raw pointer to the C type. The `.0` part accesses the
field of type `Opaque<bindings::file>` in the Rust wrapper. Recall
that File is defined like this:

pub struct File(Opaque<bindings::file>);

The above syntax defines a tuple struct, which means that the fields
are unnamed. The `.0` syntax accesses the first field of a tuple
struct [1].

The `.get()` method is from the `Opaque` struct, which returns a raw
pointer to the C type being wrapped.

Alice

[1]: https://doc.rust-lang.org/std/keyword.struct.html#:~:text=3DTuple%20st=
ructs%20are%20similar%20to,with%20regular%20tuples%2C%20namely%20foo.