library/std/src/sync/mod.rs

//! Useful synchronization primitives.
//!
//! ## The need for synchronization
//!
//! Conceptually, a Rust program is a series of operations which will
//! be executed on a computer. The timeline of events happening in the
//! program is consistent with the order of the operations in the code.
//!
//! Consider the following code, operating on some global static variables:
//!
//! ```rust
//! // FIXME(static_mut_refs): Do not allow `static_mut_refs` lint
//! #![allow(static_mut_refs)]
//!
//! static mut A: u32 = 0;
//! static mut B: u32 = 0;
//! static mut C: u32 = 0;
//!
//! fn main() {
//!     unsafe {
//!         A = 3;
//!         B = 4;
//!         A = A + B;
//!         C = B;
//!         println!("{A} {B} {C}");
//!         C = A;
//!     }
//! }
//! ```
//!
//! It appears as if some variables stored in memory are changed, an addition
//! is performed, result is stored in `A` and the variable `C` is
//! modified twice.
//!
//! When only a single thread is involved, the results are as expected:
//! the line `7 4 4` gets printed.
//!
//! As for what happens behind the scenes, when optimizations are enabled the
//! final generated machine code might look very different from the code:
//!
//! - The first store to `C` might be moved before the store to `A` or `B`,
//!   _as if_ we had written `C = 4; A = 3; B = 4`.
//!
//! - Assignment of `A + B` to `A` might be removed, since the sum can be stored
//!   in a temporary location until it gets printed, with the global variable
//!   never getting updated.
//!
//! - The final result could be determined just by looking at the code
//!   at compile time, so [constant folding] might turn the whole
//!   block into a simple `println!("7 4 4")`.
//!
//! The compiler is allowed to perform any combination of these
//! optimizations, as long as the final optimized code, when executed,
//! produces the same results as the one without optimizations.
//!
//! Due to the [concurrency] involved in modern computers, assumptions
//! about the program's execution order are often wrong. Access to
//! global variables can lead to nondeterministic results, **even if**
//! compiler optimizations are disabled, and it is **still possible**
//! to introduce synchronization bugs.
//!
//! Note that thanks to Rust's safety guarantees, accessing global (static)
//! variables requires `unsafe` code, assuming we don't use any of the
//! synchronization primitives in this module.
//!
//! [constant folding]: https://en.wikipedia.org/wiki/Constant_folding
//! [concurrency]: https://en.wikipedia.org/wiki/Concurrency_(computer_science)
//!
//! ## Out-of-order execution
//!
//! Instructions can execute in a different order from the one we define, due to
//! various reasons:
//!
//! - The **compiler** reordering instructions: If the compiler can issue an
//!   instruction at an earlier point, it will try to do so. For example, it
//!   might hoist memory loads at the top of a code block, so that the CPU can
//!   start [prefetching] the values from memory.
//!
//!   In single-threaded scenarios, this can cause issues when writing
//!   signal handlers or certain kinds of low-level code.
//!   Use [compiler fences] to prevent this reordering.
//!
//! - A **single processor** executing instructions [out-of-order]:
//!   Modern CPUs are capable of [superscalar] execution,
//!   i.e., multiple instructions might be executing at the same time,
//!   even though the machine code describes a sequential process.
//!
//!   This kind of reordering is handled transparently by the CPU.
//!
//! - A **multiprocessor** system executing multiple hardware threads
//!   at the same time: In multi-threaded scenarios, you can use two
//!   kinds of primitives to deal with synchronization:
//!   - [memory fences] to ensure memory accesses are made visible to
//!   other CPUs in the right order.
//!   - [atomic operations] to ensure simultaneous access to the same
//!   memory location doesn't lead to undefined behavior.
//!
//! [prefetching]: https://en.wikipedia.org/wiki/Cache_prefetching
//! [compiler fences]: crate::sync::atomic::compiler_fence
//! [out-of-order]: https://en.wikipedia.org/wiki/Out-of-order_execution
//! [superscalar]: https://en.wikipedia.org/wiki/Superscalar_processor
//! [memory fences]: crate::sync::atomic::fence
//! [atomic operations]: crate::sync::atomic
//!
//! ## Higher-level synchronization objects
//!
//! Most of the low-level synchronization primitives are quite error-prone and
//! inconvenient to use, which is why the standard library also exposes some
//! higher-level synchronization objects.
//!
//! These abstractions can be built out of lower-level primitives.
//! For efficiency, the sync objects in the standard library are usually
//! implemented with help from the operating system's kernel, which is
//! able to reschedule the threads while they are blocked on acquiring
//! a lock.
//!
//! The following is an overview of the available synchronization
//! objects:
//!
//! - [`Arc`]: Atomically Reference-Counted pointer, which can be used
//!   in multithreaded environments to prolong the lifetime of some
//!   data until all the threads have finished using it.
//!
//! - [`Barrier`]: Ensures multiple threads will wait for each other
//!   to reach a point in the program, before continuing execution all
//!   together.
//!
//! - [`Condvar`]: Condition Variable, providing the ability to block
//!   a thread while waiting for an event to occur.
//!
//! - [`mpsc`]: Multi-producer, single-consumer queues, used for
//!   message-based communication. Can provide a lightweight
//!   inter-thread synchronisation mechanism, at the cost of some
//!   extra memory.
//!
//! - [`mpmc`]: Multi-producer, multi-consumer queues, used for
//!   message-based communication. Can provide a lightweight
//!   inter-thread synchronisation mechanism, at the cost of some
//!   extra memory.
//!
//! - [`Mutex`]: Mutual Exclusion mechanism, which ensures that at
//!   most one thread at a time is able to access some data.
//!
//! - [`Once`]: Used for a thread-safe, one-time global initialization routine.
//!   Mostly useful for implementing other types like `OnceLock`.
//!
//! - [`OnceLock`]: Used for thread-safe, one-time initialization of a
//!   variable, with potentially different initializers based on the caller.
//!
//! - [`LazyLock`]: Used for thread-safe, one-time initialization of a
//!   variable, using one nullary initializer function provided at creation.
//!
//! - [`RwLock`]: Provides a mutual exclusion mechanism which allows
//!   multiple readers at the same time, while allowing only one
//!   writer at a time. In some cases, this can be more efficient than
//!   a mutex.
//!
//! [`Arc`]: crate::sync::Arc
//! [`Barrier`]: crate::sync::Barrier
//! [`Condvar`]: crate::sync::Condvar
//! [`mpmc`]: crate::sync::mpmc
//! [`mpsc`]: crate::sync::mpsc
//! [`Mutex`]: crate::sync::Mutex
//! [`Once`]: crate::sync::Once
//! [`OnceLock`]: crate::sync::OnceLock
//! [`RwLock`]: crate::sync::RwLock

#![stable(feature = "rust1", since = "1.0.0")]

#[unstable(feature = "exclusive_wrapper", issue = "98407")]
pub use core::sync::Exclusive;
#[stable(feature = "rust1", since = "1.0.0")]
pub use core::sync::atomic;

#[stable(feature = "rust1", since = "1.0.0")]
pub use alloc_crate::sync::{Arc, Weak};

#[stable(feature = "rust1", since = "1.0.0")]
pub use self::barrier::{Barrier, BarrierWaitResult};
#[stable(feature = "rust1", since = "1.0.0")]
pub use self::condvar::{Condvar, WaitTimeoutResult};
#[stable(feature = "lazy_cell", since = "1.80.0")]
pub use self::lazy_lock::LazyLock;
#[unstable(feature = "mapped_lock_guards", issue = "117108")]
pub use self::mutex::MappedMutexGuard;
#[stable(feature = "rust1", since = "1.0.0")]
pub use self::mutex::{Mutex, MutexGuard};
#[stable(feature = "rust1", since = "1.0.0")]
#[allow(deprecated)]
pub use self::once::{ONCE_INIT, Once, OnceState};
#[stable(feature = "once_cell", since = "1.70.0")]
pub use self::once_lock::OnceLock;
#[stable(feature = "rust1", since = "1.0.0")]
pub use self::poison::{LockResult, PoisonError, TryLockError, TryLockResult};
#[unstable(feature = "reentrant_lock", issue = "121440")]
pub use self::reentrant_lock::{ReentrantLock, ReentrantLockGuard};
#[unstable(feature = "mapped_lock_guards", issue = "117108")]
pub use self::rwlock::{MappedRwLockReadGuard, MappedRwLockWriteGuard};
#[stable(feature = "rust1", since = "1.0.0")]
pub use self::rwlock::{RwLock, RwLockReadGuard, RwLockWriteGuard};

#[unstable(feature = "mpmc_channel", issue = "126840")]
pub mod mpmc;
pub mod mpsc;

mod barrier;
mod condvar;
mod lazy_lock;
mod mutex;
pub(crate) mod once;
mod once_lock;
mod poison;
mod reentrant_lock;
mod rwlock;