Module std::sync::atomic

Atomic types

Atomic types provide primitive shared-memory communication between threads, and are the building blocks of other concurrent types.

This module defines atomic versions of a select number of primitive types, including AtomicBool, AtomicIsize, AtomicUsize, AtomicI8, AtomicU16, etc. Atomic types present operations that, when used correctly, synchronize updates between threads.

Atomic variables are safe to share between threads (they implement Sync) but they do not themselves provide the mechanism for sharing and follow the threading model of Rust. The most common way to share an atomic variable is to put it into an Arc (an atomically-reference-counted shared pointer).

Atomic types may be stored in static variables, initialized using the constant initializers like AtomicBool::new. Atomic statics are often used for lazy global initialization.

Memory model for atomic accesses

Rust atomics currently follow the same rules as C++20 atomics, specifically atomic_ref. Basically, creating a shared reference to one of the Rust atomic types corresponds to creating an atomic_ref in C++; the atomic_ref is destroyed when the lifetime of the shared reference ends. (A Rust atomic type that is exclusively owned or behind a mutable reference does not correspond to an “atomic object” in C++, since it can be accessed via non-atomic operations.)

Each method takes an Ordering which represents the strength of the memory barrier for that operation. These orderings are the same as the C++20 atomic orderings. For more information see the nomicon.

Since C++ does not support mixing atomic and non-atomic accesses, or non-synchronized different-sized accesses to the same data, Rust does not support those operations either. Note that both of those restrictions only apply if the accesses are non-synchronized.

use std::sync::atomic::{AtomicU16, AtomicU8, Ordering};
use std::mem::transmute;
use std::thread;

let atomic = AtomicU16::new(0);

thread::scope(|s| {
    // This is UB: mixing atomic and non-atomic accesses
    s.spawn(|| atomic.store(1, Ordering::Relaxed));
    s.spawn(|| unsafe { atomic.as_ptr().write(2) });
});

thread::scope(|s| {
    // This is UB: even reads are not allowed to be mixed
    s.spawn(|| atomic.load(Ordering::Relaxed));
    s.spawn(|| unsafe { atomic.as_ptr().read() });
});

thread::scope(|s| {
    // This is fine, `join` synchronizes the code in a way such that atomic
    // and non-atomic accesses can't happen "at the same time"
    let handle = s.spawn(|| atomic.store(1, Ordering::Relaxed));
    handle.join().unwrap();
    s.spawn(|| unsafe { atomic.as_ptr().write(2) });
});

thread::scope(|s| {
    // This is UB: using different-sized atomic accesses to the same data
    s.spawn(|| atomic.store(1, Ordering::Relaxed));
    s.spawn(|| unsafe {
        let differently_sized = transmute::<&AtomicU16, &AtomicU8>(&atomic);
        differently_sized.store(2, Ordering::Relaxed);
    });
});

thread::scope(|s| {
    // This is fine, `join` synchronizes the code in a way such that
    // differently-sized accesses can't happen "at the same time"
    let handle = s.spawn(|| atomic.store(1, Ordering::Relaxed));
    handle.join().unwrap();
    s.spawn(|| unsafe {
        let differently_sized = transmute::<&AtomicU16, &AtomicU8>(&atomic);
        differently_sized.store(2, Ordering::Relaxed);
    });
});

Portability

All atomic types in this module are guaranteed to be lock-free if they’re available. This means they don’t internally acquire a global mutex. Atomic types and operations are not guaranteed to be wait-free. This means that operations like fetch_or may be implemented with a compare-and-swap loop.

Atomic operations may be implemented at the instruction layer with larger-size atomics. For example some platforms use 4-byte atomic instructions to implement AtomicI8. Note that this emulation should not have an impact on correctness of code, it’s just something to be aware of.

The atomic types in this module might not be available on all platforms. The atomic types here are all widely available, however, and can generally be relied upon existing. Some notable exceptions are:

• PowerPC and MIPS platforms with 32-bit pointers do not have AtomicU64 or AtomicI64 types.
• ARM platforms like armv5te that aren’t for Linux only provide load and store operations, and do not support Compare and Swap (CAS) operations, such as swap, fetch_add, etc. Additionally on Linux, these CAS operations are implemented via operating system support, which may come with a performance penalty.
• ARM targets with thumbv6m only provide load and store operations, and do not support Compare and Swap (CAS) operations, such as swap, fetch_add, etc.

Note that future platforms may be added that also do not have support for some atomic operations. Maximally portable code will want to be careful about which atomic types are used. AtomicUsize and AtomicIsize are generally the most portable, but even then they’re not available everywhere. For reference, the std library requires AtomicBools and pointer-sized atomics, although core does not.

The #[cfg(target_has_atomic)] attribute can be used to conditionally compile based on the target’s supported bit widths. It is a key-value option set for each supported size, with values “8”, “16”, “32”, “64”, “128”, and “ptr” for pointer-sized atomics.

Atomic accesses to read-only memory

In general, all atomic accesses on read-only memory are Undefined Behavior. For instance, attempting to do a compare_exchange that will definitely fail (making it conceptually a read-only operation) can still cause a segmentation fault if the underlying memory page is mapped read-only. Since atomic loads might be implemented using compare-exchange operations, even a load can fault on read-only memory.

For the purpose of this section, “read-only memory” is defined as memory that is read-only in the underlying target, i.e., the pages are mapped with a read-only flag and any attempt to write will cause a page fault. In particular, an &u128 reference that points to memory that is read-write mapped is not considered to point to “read-only memory”. In Rust, almost all memory is read-write; the only exceptions are memory created by const items or static items without interior mutability, and memory that was specifically marked as read-only by the operating system via platform-specific APIs.

As an exception from the general rule stated above, “sufficiently small” atomic loads with Ordering::Relaxed are implemented in a way that works on read-only memory, and are hence not Undefined Behavior. The exact size limit for what makes a load “sufficiently small” varies depending on the target:

target_arch	Size limit
x86, arm, mips, mips32r6, powerpc, riscv32, sparc, hexagon	4 bytes
x86_64, aarch64, loongarch64, mips64, mips64r6, powerpc64, riscv64, sparc64, s390x	8 bytes

Atomics loads that are larger than this limit as well as atomic loads with ordering other than Relaxed, as well as all atomic loads on targets not listed in the table, might still be read-only under certain conditions, but that is not a stable guarantee and should not be relied upon.

If you need to do an acquire load on read-only memory, you can do a relaxed load followed by an acquire fence instead.

Examples

A simple spinlock:

use std::sync::Arc;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::{hint, thread};

fn main() {
    let spinlock = Arc::new(AtomicUsize::new(1));

    let spinlock_clone = Arc::clone(&spinlock);

    let thread = thread::spawn(move|| {
        spinlock_clone.store(0, Ordering::Release);
    });

    // Wait for the other thread to release the lock
    while spinlock.load(Ordering::Acquire) != 0 {
        hint::spin_loop();
    }

    if let Err(panic) = thread.join() {
        println!("Thread had an error: {panic:?}");
    }
}

Keep a global count of live threads:

use std::sync::atomic::{AtomicUsize, Ordering};

static GLOBAL_THREAD_COUNT: AtomicUsize = AtomicUsize::new(0);

// Note that Relaxed ordering doesn't synchronize anything
// except the global thread counter itself.
let old_thread_count = GLOBAL_THREAD_COUNT.fetch_add(1, Ordering::Relaxed);
// Note that this number may not be true at the moment of printing
// because some other thread may have changed static value already.
println!("live threads: {}", old_thread_count + 1);

Structs

• AtomicBool
  A boolean type which can be safely shared between threads.
• AtomicI8
  An integer type which can be safely shared between threads.
• AtomicI16
  An integer type which can be safely shared between threads.
• AtomicI32
  An integer type which can be safely shared between threads.
• AtomicI64
  An integer type which can be safely shared between threads.
• AtomicIsize
  An integer type which can be safely shared between threads.
• AtomicPtr
  A raw pointer type which can be safely shared between threads.
• AtomicU8
  An integer type which can be safely shared between threads.
• AtomicU16
  An integer type which can be safely shared between threads.
• AtomicU32
  An integer type which can be safely shared between threads.
• AtomicU64
  An integer type which can be safely shared between threads.
• AtomicUsize
  An integer type which can be safely shared between threads.

Enums

• Ordering
  Atomic memory orderings

Constants

• ATOMIC_BOOL_INITDeprecated
  An AtomicBool initialized to false.
• ATOMIC_ISIZE_INITDeprecated
  An AtomicIsize initialized to 0.
• ATOMIC_USIZE_INITDeprecated
  An AtomicUsize initialized to 0.

Functions

• compiler_fence
  A compiler memory fence.
• fence
  An atomic fence.
• spin_loop_hintDeprecated
  Signals the processor that it is inside a busy-wait spin-loop (“spin lock”).