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The previous link anchor was broken in rust 1.77, because the
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Change the link to refer to the new section in the rust documentation.
Signed-off-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Link: a146142fe1
[ Fixed commit authorship. - Benno ]
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
236 lines
9.2 KiB
Markdown
236 lines
9.2 KiB
Markdown
[](https://crates.io/crates/pin-init)
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[](https://docs.rs/pin-init/)
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[](https://deps.rs/repo/github/Rust-for-Linux/pin-init)
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[](#nightly-only)
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# `pin-init`
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<!-- cargo-rdme start -->
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Library to safely and fallibly initialize pinned `struct`s using in-place constructors.
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[Pinning][pinning] is Rust's way of ensuring data does not move.
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It also allows in-place initialization of big `struct`s that would otherwise produce a stack
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overflow.
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This library's main use-case is in [Rust-for-Linux]. Although this version can be used
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standalone.
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There are cases when you want to in-place initialize a struct. For example when it is very big
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and moving it from the stack is not an option, because it is bigger than the stack itself.
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Another reason would be that you need the address of the object to initialize it. This stands
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in direct conflict with Rust's normal process of first initializing an object and then moving
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it into it's final memory location. For more information, see
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<https://rust-for-linux.com/the-safe-pinned-initialization-problem>.
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This library allows you to do in-place initialization safely.
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### Nightly Needed for `alloc` feature
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This library requires the [`allocator_api` unstable feature] when the `alloc` feature is
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enabled and thus this feature can only be used with a nightly compiler. When enabling the
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`alloc` feature, the user will be required to activate `allocator_api` as well.
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[`allocator_api` unstable feature]: https://doc.rust-lang.org/nightly/unstable-book/library-features/allocator-api.html
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The feature is enabled by default, thus by default `pin-init` will require a nightly compiler.
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However, using the crate on stable compilers is possible by disabling `alloc`. In practice this
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will require the `std` feature, because stable compilers have neither `Box` nor `Arc` in no-std
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mode.
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### Nightly needed for `unsafe-pinned` feature
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This feature enables the `Wrapper` implementation on the unstable `core::pin::UnsafePinned` type.
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This requires the [`unsafe_pinned` unstable feature](https://github.com/rust-lang/rust/issues/125735)
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and therefore a nightly compiler. Note that this feature is not enabled by default.
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## Overview
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To initialize a `struct` with an in-place constructor you will need two things:
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- an in-place constructor,
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- a memory location that can hold your `struct` (this can be the [stack], an [`Arc<T>`],
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[`Box<T>`] or any other smart pointer that supports this library).
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To get an in-place constructor there are generally three options:
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- directly creating an in-place constructor using the [`pin_init!`] macro,
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- a custom function/macro returning an in-place constructor provided by someone else,
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- using the unsafe function [`pin_init_from_closure()`] to manually create an initializer.
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Aside from pinned initialization, this library also supports in-place construction without
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pinning, the macros/types/functions are generally named like the pinned variants without the
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`pin_` prefix.
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## Examples
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Throughout the examples we will often make use of the `CMutex` type which can be found in
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`../examples/mutex.rs`. It is essentially a userland rebuild of the `struct mutex` type from
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the Linux kernel. It also uses a wait list and a basic spinlock. Importantly the wait list
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requires it to be pinned to be locked and thus is a prime candidate for using this library.
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### Using the [`pin_init!`] macro
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If you want to use [`PinInit`], then you will have to annotate your `struct` with
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`#[`[`pin_data`]`]`. It is a macro that uses `#[pin]` as a marker for
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[structurally pinned fields]. After doing this, you can then create an in-place constructor via
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[`pin_init!`]. The syntax is almost the same as normal `struct` initializers. The difference is
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that you need to write `<-` instead of `:` for fields that you want to initialize in-place.
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```rust
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use pin_init::{pin_data, pin_init, InPlaceInit};
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#[pin_data]
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struct Foo {
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#[pin]
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a: CMutex<usize>,
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b: u32,
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}
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let foo = pin_init!(Foo {
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a <- CMutex::new(42),
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b: 24,
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});
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```
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`foo` now is of the type [`impl PinInit<Foo>`]. We can now use any smart pointer that we like
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(or just the stack) to actually initialize a `Foo`:
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```rust
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let foo: Result<Pin<Box<Foo>>, AllocError> = Box::pin_init(foo);
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```
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For more information see the [`pin_init!`] macro.
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### Using a custom function/macro that returns an initializer
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Many types that use this library supply a function/macro that returns an initializer, because
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the above method only works for types where you can access the fields.
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```rust
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let mtx: Result<Pin<Arc<CMutex<usize>>>, _> = Arc::pin_init(CMutex::new(42));
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```
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To declare an init macro/function you just return an [`impl PinInit<T, E>`]:
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```rust
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#[pin_data]
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struct DriverData {
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#[pin]
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status: CMutex<i32>,
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buffer: Box<[u8; 1_000_000]>,
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}
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impl DriverData {
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fn new() -> impl PinInit<Self, Error> {
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try_pin_init!(Self {
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status <- CMutex::new(0),
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buffer: Box::init(pin_init::zeroed())?,
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}? Error)
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}
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}
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```
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### Manual creation of an initializer
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Often when working with primitives the previous approaches are not sufficient. That is where
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[`pin_init_from_closure()`] comes in. This `unsafe` function allows you to create a
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[`impl PinInit<T, E>`] directly from a closure. Of course you have to ensure that the closure
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actually does the initialization in the correct way. Here are the things to look out for
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(we are calling the parameter to the closure `slot`):
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- when the closure returns `Ok(())`, then it has completed the initialization successfully, so
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`slot` now contains a valid bit pattern for the type `T`,
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- when the closure returns `Err(e)`, then the caller may deallocate the memory at `slot`, so
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you need to take care to clean up anything if your initialization fails mid-way,
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- you may assume that `slot` will stay pinned even after the closure returns until `drop` of
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`slot` gets called.
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```rust
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use pin_init::{pin_data, pinned_drop, PinInit, PinnedDrop, pin_init_from_closure};
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use core::{
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ptr::addr_of_mut,
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marker::PhantomPinned,
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cell::UnsafeCell,
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pin::Pin,
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mem::MaybeUninit,
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};
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mod bindings {
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#[repr(C)]
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pub struct foo {
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/* fields from C ... */
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}
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extern "C" {
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pub fn init_foo(ptr: *mut foo);
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pub fn destroy_foo(ptr: *mut foo);
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#[must_use = "you must check the error return code"]
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pub fn enable_foo(ptr: *mut foo, flags: u32) -> i32;
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}
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}
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/// # Invariants
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///
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/// `foo` is always initialized
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#[pin_data(PinnedDrop)]
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pub struct RawFoo {
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#[pin]
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_p: PhantomPinned,
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#[pin]
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foo: UnsafeCell<MaybeUninit<bindings::foo>>,
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}
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impl RawFoo {
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pub fn new(flags: u32) -> impl PinInit<Self, i32> {
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// SAFETY:
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// - when the closure returns `Ok(())`, then it has successfully initialized and
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// enabled `foo`,
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// - when it returns `Err(e)`, then it has cleaned up before
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unsafe {
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pin_init_from_closure(move |slot: *mut Self| {
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// `slot` contains uninit memory, avoid creating a reference.
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let foo = addr_of_mut!((*slot).foo);
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let foo = UnsafeCell::raw_get(foo).cast::<bindings::foo>();
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// Initialize the `foo`
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bindings::init_foo(foo);
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// Try to enable it.
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let err = bindings::enable_foo(foo, flags);
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if err != 0 {
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// Enabling has failed, first clean up the foo and then return the error.
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bindings::destroy_foo(foo);
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Err(err)
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} else {
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// All fields of `RawFoo` have been initialized, since `_p` is a ZST.
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Ok(())
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}
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})
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}
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}
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}
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#[pinned_drop]
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impl PinnedDrop for RawFoo {
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fn drop(self: Pin<&mut Self>) {
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// SAFETY: Since `foo` is initialized, destroying is safe.
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unsafe { bindings::destroy_foo(self.foo.get().cast::<bindings::foo>()) };
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}
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}
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```
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For more information on how to use [`pin_init_from_closure()`], take a look at the uses inside
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the `kernel` crate. The [`sync`] module is a good starting point.
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[`sync`]: https://rust.docs.kernel.org/kernel/sync/index.html
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[pinning]: https://doc.rust-lang.org/std/pin/index.html
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[structurally pinned fields]: https://doc.rust-lang.org/std/pin/index.html#projections-and-structural-pinning
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[stack]: https://docs.rs/pin-init/latest/pin_init/macro.stack_pin_init.html
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[`impl PinInit<Foo>`]: https://docs.rs/pin-init/latest/pin_init/trait.PinInit.html
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[`impl PinInit<T, E>`]: https://docs.rs/pin-init/latest/pin_init/trait.PinInit.html
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[`impl Init<T, E>`]: https://docs.rs/pin-init/latest/pin_init/trait.Init.html
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[Rust-for-Linux]: https://rust-for-linux.com/
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<!-- cargo-rdme end -->
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<!-- These links are not picked up by cargo-rdme, since they are behind cfgs... -->
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[`Arc<T>`]: https://doc.rust-lang.org/stable/alloc/sync/struct.Arc.html
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[`Box<T>`]: https://doc.rust-lang.org/stable/alloc/boxed/struct.Box.html
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