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//! Atomic References //! //! These types act similarially to the Atomic types from std::sync::atomic, //! Except that instead of containing an integer type or a pointer, they contain //! an `Option<&'a T>` value. //! //! Like other option values, these types present operations which, when used //! correctly, synchronize updates between threads. This type is a form of //! interior mutability, like `Cell<T>`, `RefCell<T>`, or `Mutex<T>`. //! //! To store an atomic reference in a static variable, a the macro //! `static_atomic_ref!` must be used. A static initializer like //! `ATOMIC_REF_INIT` is not possible due to the need to be generic over any //! reference target type. //! //! This type in static position is often used for lazy global initialization. //! //! `AtomicRef` may only contain `Sized` types, as unsized types have wide //! pointers which cannot be atomically written to or read from. //! //! //! # Examples //! //! Static logger state //! //! ``` //! #[macro_use] //! extern crate atomic_ref; //! use atomic_ref::AtomicRef; //! use std::sync::atomic::Ordering; //! use std::io::{stdout, Write}; //! //! // Define the idea of a logger //! trait Logger { //! fn log(&self, msg: &str) {} //! } //! struct LoggerInfo { //! logger: &'static (Logger + Sync) //! } //! //! // The methods for working with our currently defined static logger //! static_atomic_ref! { //! static LOGGER: AtomicRef<LoggerInfo>; //! } //! fn log(msg: &str) -> bool { //! if let Some(info) = LOGGER.load(Ordering::SeqCst) { //! info.logger.log(msg); //! true //! } else { //! false //! } //! } //! fn set_logger(logger: Option<&'static LoggerInfo>) { //! LOGGER.store(logger, Ordering::SeqCst); //! } //! //! // Defining the standard out example logger //! struct StdoutLogger; //! impl Logger for StdoutLogger { //! fn log(&self, msg: &str) { //! stdout().write(msg.as_bytes()); //! } //! } //! static STDOUT_LOGGER: LoggerInfo = LoggerInfo { logger: &StdoutLogger }; //! //! fn main() { //! let res = log("This will fail"); //! assert!(!res); //! set_logger(Some(&STDOUT_LOGGER)); //! let res = log("This will succeed"); //! assert!(res); //! } //! ``` use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT, Ordering}; use std::marker::PhantomData; use std::fmt; use std::default::Default; /// A mutable Option<&'a, T> type which can be safely shared between threads. #[repr(C)] pub struct AtomicRef<'a, T: 'a> { data: AtomicUsize, _marker: PhantomData<&'a T>, } /// You will probably never need to use this type. It exists mostly for internal /// use in the `static_atomic_ref!` macro. /// /// Unlike `AtomicUsize` and its ilk, we cannot have an `ATOMIC_REF_INIT` const /// which is initialized to `None`, as constants cannot be generic over a type /// parameter. This is the same reason why `AtomicPtr` does not have an /// `ATOMIC_PTR_INIT` const. /// /// Instead, we have a single const for `&'static u8`, and take advantage of the /// fact that all AtomicRef types have identical layout to implement the /// `static_atomic_ref!` macro. /// /// Please use `static_atomic_ref!` instead of this constant if you need to /// implement a static atomic reference variable. pub const ATOMIC_U8_REF_INIT: AtomicRef<'static, u8> = AtomicRef { data: ATOMIC_USIZE_INIT, _marker: PhantomData, }; /// A macro to define a statically allocated `AtomicRef<'static, T>` which is /// initialized to `None`. /// /// # Examples /// /// ``` /// # #[macro_use] /// # extern crate atomic_ref; /// use std::sync::atomic::Ordering; /// /// static_atomic_ref! { /// static SOME_REFERENCE: AtomicRef<i32>; /// pub static PUB_REFERENCE: AtomicRef<u64>; /// } /// /// fn main() { /// let a: Option<&'static i32> = SOME_REFERENCE.load(Ordering::SeqCst); /// assert_eq!(a, None); /// } /// ``` #[macro_export] macro_rules! static_atomic_ref { ($(#[$attr:meta])* static $N:ident : AtomicRef<$T:ty>; $($t:tt)*) => { static_atomic_ref!(@PRIV, $(#[$attr])* static $N : $T; $($t)*); }; ($(#[$attr:meta])* pub static $N:ident : AtomicRef<$T:ty>; $($t:tt)*) => { static_atomic_ref!(@PUB, $(#[$attr])* static $N : $T; $($t)*); }; (@$VIS:ident, $(#[$attr:meta])* static $N:ident : $T:ty; $($t:tt)*) => { static_atomic_ref!(@MAKE TY, $VIS, $(#[$attr])*, $N); impl ::std::ops::Deref for $N { type Target = $crate::AtomicRef<'static, $T>; #[allow(unsafe_code)] fn deref<'a>(&'a self) -> &'a $crate::AtomicRef<'static, $T> { unsafe { ::std::mem::transmute(&self._ref) } } } static_atomic_ref!($($t)*); }; (@MAKE TY, PUB, $(#[$attr:meta])*, $N:ident) => { #[allow(missing_copy_implementations)] #[allow(non_camel_case_types)] #[allow(dead_code)] $(#[$attr])* pub struct $N { _ref: $crate::AtomicRef<'static, u8> } #[doc(hidden)] pub static $N: $N = $N { _ref: $crate::ATOMIC_U8_REF_INIT }; }; (@MAKE TY, PRIV, $(#[$attr:meta])*, $N:ident) => { #[allow(missing_copy_implementations)] #[allow(non_camel_case_types)] #[allow(dead_code)] $(#[$attr])* struct $N { _ref: $crate::AtomicRef<'static, u8> } #[doc(hidden)] static $N: $N = $N { _ref: $crate::ATOMIC_U8_REF_INIT }; }; () => (); } /// An internal helper function for converting `Option<&'a T>` values to usize /// for storing in the `AtomicUsize`. fn from_opt<'a, T>(p: Option<&'a T>) -> usize { match p { Some(p) => p as *const T as usize, None => 0, } } /// An internal helper function for converting `usize` values stored in the /// `AtomicUsize` back into `Option<&'a T>` values. unsafe fn to_opt<'a, T>(p: usize) -> Option<&'a T> { (p as *const T).as_ref() } impl<'a, T> AtomicRef<'a, T> { /// Creates a new `AtomicRef`. /// /// # Examples /// /// ``` /// use atomic_ref::AtomicRef; /// /// static VALUE: i32 = 10; /// let atomic_ref = AtomicRef::new(Some(&VALUE)); /// ``` pub fn new(p: Option<&'a T>) -> AtomicRef<'a, T> { AtomicRef { data: AtomicUsize::new(from_opt(p)), _marker: PhantomData, } } /// Loads the value stored in the `AtomicRef`. /// /// `load` takes an `Ordering` argument which describes the memory ordering of this operation. /// /// # Panics /// /// Panics if `order` is `Release` or `AcqRel`. /// /// # Examples /// /// ``` /// use std::sync::atomic::Ordering; /// use atomic_ref::AtomicRef; /// /// static VALUE: i32 = 10; /// /// let some_ref = AtomicRef::new(Some(&VALUE)); /// assert_eq!(some_ref.load(Ordering::Relaxed), Some(&10)); /// ``` pub fn load(&self, ordering: Ordering) -> Option<&'a T> { unsafe { to_opt(self.data.load(ordering)) } } /// Stores a value into the `AtomicRef`. /// /// `store` takes an `Ordering` argument which describes the memory ordering of this operation. /// /// # Panics /// /// Panics if `order` is `Acquire` or `AcqRel`. /// /// # Examples /// /// ``` /// use std::sync::atomic::Ordering; /// use atomic_ref::AtomicRef; /// /// static VALUE: i32 = 10; /// /// let some_ptr = AtomicRef::new(None); /// some_ptr.store(Some(&VALUE), Ordering::Relaxed); /// ``` pub fn store(&self, ptr: Option<&'a T>, order: Ordering) { self.data.store(from_opt(ptr), order) } /// Stores a value into the `AtomicRef`, returning the old value. /// /// `swap` takes an `Ordering` argument which describes the memory ordering of this operation. /// /// # Examples /// /// ``` /// use std::sync::atomic::Ordering; /// use atomic_ref::AtomicRef; /// /// static VALUE: i32 = 10; /// static OTHER_VALUE: i32 = 20; /// /// let some_ptr = AtomicRef::new(Some(&VALUE)); /// let value = some_ptr.swap(Some(&OTHER_VALUE), Ordering::Relaxed); /// ``` pub fn swap(&self, p: Option<&'a T>, order: Ordering) -> Option<&'a T> { unsafe { to_opt(self.data.swap(from_opt(p), order)) } } /// Stores a value into the `AtomicRef` if the current value is the "same" as /// the `current` value. /// /// The return value is always the previous value. If it the "same" as /// `current`, then the value was updated. /// /// This method considers two `Option<&'a T>`s to be the "same" if they are /// both `Some` and have the same pointer value, or if they are both `None`. /// This method does not use `Eq` or `PartialEq` for comparison. /// /// `compare_and_swap` also takes an `Ordering` argument which describes the /// memory ordering of this operation. /// /// # Examples /// /// ``` /// use std::sync::atomic::Ordering; /// use atomic_ref::AtomicRef; /// /// static VALUE: i32 = 10; /// static OTHER_VALUE: i32 = 20; /// /// let some_ptr = AtomicRef::new(Some(&VALUE)); /// let value = some_ptr.compare_and_swap(Some(&OTHER_VALUE), None, Ordering::Relaxed); /// ``` pub fn compare_and_swap(&self, current: Option<&'a T>, new: Option<&'a T>, order: Ordering) -> Option<&'a T> { unsafe { to_opt(self.data.compare_and_swap(from_opt(current), from_opt(new), order)) } } /// Stores a value into the `AtomicRef` if the current value is the "same" as /// the `current` value. /// /// The return value is a result indicating whether the new value was /// written, and containing the previous value. On success this value is /// guaranteed to be the "same" as `new`. /// /// This method considers two `Option<&'a T>`s to be the "same" if they are /// both `Some` and have the same pointer value, or if they are both `None`. /// This method does not use `Eq` or `PartialEq` for comparison. /// /// `compare_exchange` takes two `Ordering` arguments to describe the memory /// ordering of this operation. The first describes the required ordering if /// the operation succeeds while the second describes the required ordering /// when the operation fails. The failure ordering can't be `Release` or /// `AcqRel` and must be equivalent or weaker than the success ordering. /// /// # Examples /// /// ``` /// use std::sync::atomic::Ordering; /// use atomic_ref::AtomicRef; /// /// static VALUE: i32 = 10; /// static OTHER_VALUE: i32 = 20; /// /// let some_ptr = AtomicRef::new(Some(&VALUE)); /// let value = some_ptr.compare_exchange(Some(&OTHER_VALUE), None, /// Ordering::SeqCst, Ordering::Relaxed); /// ``` pub fn compare_exchange(&self, current: Option<&'a T>, new: Option<&'a T>, success: Ordering, failure: Ordering) -> Result<Option<&'a T>, Option<&'a T>> { unsafe { match self.data.compare_exchange(from_opt(current), from_opt(new), success, failure) { Ok(p) => Ok(to_opt(p)), Err(p) => Err(to_opt(p)), } } } /// Stores a value into the pointer if the current value is the same as the `current` value. /// /// Unlike `compare_exchange`, this function is allowed to spuriously fail even when the /// comparison succeeds, which can result in more efficient code on some platforms. The /// return value is a result indicating whether the new value was written and containing the /// previous value. /// /// `compare_exchange_weak` takes two `Ordering` arguments to describe the memory /// ordering of this operation. The first describes the required ordering if the operation /// succeeds while the second describes the required ordering when the operation fails. The /// failure ordering can't be `Release` or `AcqRel` and must be equivalent or weaker than the /// success ordering. /// /// # Examples /// /// ``` /// use std::sync::atomic::Ordering; /// use atomic_ref::AtomicRef; /// /// static VALUE: i32 = 10; /// static OTHER_VALUE: i32 = 20; /// /// let some_ptr = AtomicRef::new(Some(&VALUE)); /// /// let mut old = some_ptr.load(Ordering::Relaxed); /// loop { /// match some_ptr.compare_exchange_weak(old, Some(&VALUE), /// Ordering::SeqCst, Ordering::Relaxed) { /// Ok(_) => break, /// Err(x) => old = x, /// } /// } /// ``` pub fn compare_exchange_weak(&self, current: Option<&'a T>, new: Option<&'a T>, success: Ordering, failure: Ordering) -> Result<Option<&'a T>, Option<&'a T>> { unsafe { match self.data.compare_exchange_weak(from_opt(current), from_opt(new), success, failure) { Ok(p) => Ok(to_opt(p)), Err(p) => Err(to_opt(p)), } } } } impl<'a, T: fmt::Debug> fmt::Debug for AtomicRef<'a, T> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { f.debug_tuple("AtomicRef").field(&self.load(Ordering::SeqCst)).finish() } } impl<'a, T> Default for AtomicRef<'a, T> { fn default() -> AtomicRef<'a, T> { AtomicRef::new(None) } } #[cfg(test)] mod tests { use std::sync::atomic::Ordering; static_atomic_ref! { static FOO: AtomicRef<i32>; } static A: i32 = 10; #[test] fn it_works() { assert!(FOO.load(Ordering::SeqCst) == None); FOO.store(Some(&A), Ordering::SeqCst); assert!(FOO.load(Ordering::SeqCst) == Some(&A)); assert!(FOO.load(Ordering::SeqCst).unwrap() as *const _ == &A as *const _); } }