Struct Futex 

pub struct Futex { /* private fields */ }

A safe wrapper around zx_futex_t, generally called as part of higher-level synchronization primitives.

Implementations

impl Futex

pub const fn new(value: i32) -> Self

pub fn wake(&self, wake_count: u32)

See https://fuchsia.dev/reference/syscalls/futex_wake.

pub fn wake_all(&self)

Wakes the maximum number of waiters possible.

See https://fuchsia.dev/reference/syscalls/futex_wake.

pub fn wake_single_owner(&self)

See https://fuchsia.dev/reference/syscalls/futex_wake_single_owner.

pub fn wake_handle_close_thread_exit( &self, wake_count: u32, new_value: i32, to_close: Handle, ) -> !

See https://fuchsia.dev/reference/syscalls/futex_wake_handle_close_thread_exit.

pub fn requeue( &self, wake_count: u32, current_value: i32, requeue_to: &Self, requeue_count: u32, new_requeue_owner: Option<&Thread>, ) -> Result<(), Status>

See https://fuchsia.dev/reference/syscalls/futex_requeue.

pub fn requeue_single_owner( &self, current_value: i32, requeue_to: &Self, requeue_count: u32, new_requeue_owner: Option<&Thread>, ) -> Result<(), Status>

See https://fuchsia.dev/reference/syscalls/futex_requeue_single_owner.

pub fn wait( &self, current_value: i32, new_owner: Option<&Thread>, deadline: MonotonicInstant, ) -> Result<(), Status>

See https://fuchsia.dev/reference/syscalls/futex_wait.

pub fn get_owner(&self) -> Option<Koid>

See https://fuchsia.dev/reference/syscalls/futex_get_owner.

Methods from Deref<Target = zx_futex_t>

pub fn load(&self, order: Ordering) -> i32

Loads a value from the atomic integer.

load takes an Ordering argument which describes the memory ordering of this operation. Possible values are SeqCst, Acquire and Relaxed.

Panics

Panics if order is Release or AcqRel.

Examples

use std::sync::atomic::{AtomicI32, Ordering};

let some_var = AtomicI32::new(5);

assert_eq!(some_var.load(Ordering::Relaxed), 5);

pub fn store(&self, val: i32, order: Ordering)

Stores a value into the atomic integer.

store takes an Ordering argument which describes the memory ordering of this operation. Possible values are SeqCst, Release and Relaxed.

Panics

Panics if order is Acquire or AcqRel.

Examples

use std::sync::atomic::{AtomicI32, Ordering};

let some_var = AtomicI32::new(5);

some_var.store(10, Ordering::Relaxed);
assert_eq!(some_var.load(Ordering::Relaxed), 10);

pub fn swap(&self, val: i32, order: Ordering) -> i32

Stores a value into the atomic integer, returning the previous value.

swap takes an Ordering argument which describes the memory ordering of this operation. All ordering modes are possible. Note that using Acquire makes the store part of this operation Relaxed, and using Release makes the load part Relaxed.

Note: This method is only available on platforms that support atomic operations on i32.

Examples

use std::sync::atomic::{AtomicI32, Ordering};

let some_var = AtomicI32::new(5);

assert_eq!(some_var.swap(10, Ordering::Relaxed), 5);

pub fn compare_and_swap(&self, current: i32, new: i32, order: Ordering) -> i32

👎Deprecated since 1.50.0: Use compare_exchange or compare_exchange_weak instead

Stores a value into the atomic integer if the current value is the same as the current value.

The return value is always the previous value. If it is equal to current, then the value was updated.

compare_and_swap also takes an Ordering argument which describes the memory ordering of this operation. Notice that even when using AcqRel, the operation might fail and hence just perform an Acquire load, but not have Release semantics. Using Acquire makes the store part of this operation Relaxed if it happens, and using Release makes the load part Relaxed.

Note: This method is only available on platforms that support atomic operations on i32.

Migrating to compare_exchange and compare_exchange_weak

compare_and_swap is equivalent to compare_exchange with the following mapping for memory orderings:

Original	Success	Failure
Relaxed	Relaxed	Relaxed
Acquire	Acquire	Acquire
Release	Release	Relaxed
AcqRel	AcqRel	Acquire
SeqCst	SeqCst	SeqCst

compare_and_swap and compare_exchange also differ in their return type. You can use compare_exchange(...).unwrap_or_else(|x| x) to recover the behavior of compare_and_swap, but in most cases it is more idiomatic to check whether the return value is Ok or Err rather than to infer success vs failure based on the value that was read.

During migration, consider whether it makes sense to use compare_exchange_weak instead. compare_exchange_weak is allowed to fail spuriously even when the comparison succeeds, which allows the compiler to generate better assembly code when the compare and swap is used in a loop.

Examples

use std::sync::atomic::{AtomicI32, Ordering};

let some_var = AtomicI32::new(5);

assert_eq!(some_var.compare_and_swap(5, 10, Ordering::Relaxed), 5);
assert_eq!(some_var.load(Ordering::Relaxed), 10);

assert_eq!(some_var.compare_and_swap(6, 12, Ordering::Relaxed), 10);
assert_eq!(some_var.load(Ordering::Relaxed), 10);

pub fn compare_exchange( &self, current: i32, new: i32, success: Ordering, failure: Ordering, ) -> Result<i32, i32>

Stores a value into the atomic integer 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 equal to current.

compare_exchange takes two Ordering arguments to describe the memory ordering of this operation. success describes the required ordering for the read-modify-write operation that takes place if the comparison with current succeeds. failure describes the required ordering for the load operation that takes place when the comparison fails. Using Acquire as success ordering makes the store part of this operation Relaxed, and using Release makes the successful load Relaxed. The failure ordering can only be SeqCst, Acquire or Relaxed.

Note: This method is only available on platforms that support atomic operations on i32.

Examples

use std::sync::atomic::{AtomicI32, Ordering};

let some_var = AtomicI32::new(5);

assert_eq!(some_var.compare_exchange(5, 10,
                                     Ordering::Acquire,
                                     Ordering::Relaxed),
           Ok(5));
assert_eq!(some_var.load(Ordering::Relaxed), 10);

assert_eq!(some_var.compare_exchange(6, 12,
                                     Ordering::SeqCst,
                                     Ordering::Acquire),
           Err(10));
assert_eq!(some_var.load(Ordering::Relaxed), 10);

Considerations

compare_exchange is a compare-and-swap operation and thus exhibits the usual downsides of CAS operations. In particular, a load of the value followed by a successful compare_exchange with the previous load does not ensure that other threads have not changed the value in the interim! This is usually important when the equality check in the compare_exchange is being used to check the identity of a value, but equality does not necessarily imply identity. This is a particularly common case for pointers, as a pointer holding the same address does not imply that the same object exists at that address! In this case, compare_exchange can lead to the ABA problem.

pub fn compare_exchange_weak( &self, current: i32, new: i32, success: Ordering, failure: Ordering, ) -> Result<i32, i32>

Stores a value into the atomic integer if the current value is the same as the current value.

Unlike AtomicI32::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. success describes the required ordering for the read-modify-write operation that takes place if the comparison with current succeeds. failure describes the required ordering for the load operation that takes place when the comparison fails. Using Acquire as success ordering makes the store part of this operation Relaxed, and using Release makes the successful load Relaxed. The failure ordering can only be SeqCst, Acquire or Relaxed.

Note: This method is only available on platforms that support atomic operations on i32.

Examples

use std::sync::atomic::{AtomicI32, Ordering};

let val = AtomicI32::new(4);

let mut old = val.load(Ordering::Relaxed);
loop {
    let new = old * 2;
    match val.compare_exchange_weak(old, new, Ordering::SeqCst, Ordering::Relaxed) {
        Ok(_) => break,
        Err(x) => old = x,
    }
}

Considerations

compare_exchange is a compare-and-swap operation and thus exhibits the usual downsides of CAS operations. In particular, a load of the value followed by a successful compare_exchange with the previous load does not ensure that other threads have not changed the value in the interim. This is usually important when the equality check in the compare_exchange is being used to check the identity of a value, but equality does not necessarily imply identity. This is a particularly common case for pointers, as a pointer holding the same address does not imply that the same object exists at that address! In this case, compare_exchange can lead to the ABA problem.

pub fn fetch_add(&self, val: i32, order: Ordering) -> i32

Adds to the current value, returning the previous value.

This operation wraps around on overflow.

fetch_add takes an Ordering argument which describes the memory ordering of this operation. All ordering modes are possible. Note that using Acquire makes the store part of this operation Relaxed, and using Release makes the load part Relaxed.

Note: This method is only available on platforms that support atomic operations on i32.

Examples

use std::sync::atomic::{AtomicI32, Ordering};

let foo = AtomicI32::new(0);
assert_eq!(foo.fetch_add(10, Ordering::SeqCst), 0);
assert_eq!(foo.load(Ordering::SeqCst), 10);

pub fn fetch_sub(&self, val: i32, order: Ordering) -> i32

Subtracts from the current value, returning the previous value.

This operation wraps around on overflow.

fetch_sub takes an Ordering argument which describes the memory ordering of this operation. All ordering modes are possible. Note that using Acquire makes the store part of this operation Relaxed, and using Release makes the load part Relaxed.

Note: This method is only available on platforms that support atomic operations on i32.

Examples

use std::sync::atomic::{AtomicI32, Ordering};

let foo = AtomicI32::new(20);
assert_eq!(foo.fetch_sub(10, Ordering::SeqCst), 20);
assert_eq!(foo.load(Ordering::SeqCst), 10);

pub fn fetch_and(&self, val: i32, order: Ordering) -> i32

Bitwise “and” with the current value.

Performs a bitwise “and” operation on the current value and the argument val, and sets the new value to the result.

Returns the previous value.

fetch_and takes an Ordering argument which describes the memory ordering of this operation. All ordering modes are possible. Note that using Acquire makes the store part of this operation Relaxed, and using Release makes the load part Relaxed.

Note: This method is only available on platforms that support atomic operations on i32.

Examples

use std::sync::atomic::{AtomicI32, Ordering};

let foo = AtomicI32::new(0b101101);
assert_eq!(foo.fetch_and(0b110011, Ordering::SeqCst), 0b101101);
assert_eq!(foo.load(Ordering::SeqCst), 0b100001);

pub fn fetch_nand(&self, val: i32, order: Ordering) -> i32

Bitwise “nand” with the current value.

Performs a bitwise “nand” operation on the current value and the argument val, and sets the new value to the result.

Returns the previous value.

fetch_nand takes an Ordering argument which describes the memory ordering of this operation. All ordering modes are possible. Note that using Acquire makes the store part of this operation Relaxed, and using Release makes the load part Relaxed.

Note: This method is only available on platforms that support atomic operations on i32.

Examples

use std::sync::atomic::{AtomicI32, Ordering};

let foo = AtomicI32::new(0x13);
assert_eq!(foo.fetch_nand(0x31, Ordering::SeqCst), 0x13);
assert_eq!(foo.load(Ordering::SeqCst), !(0x13 & 0x31));

pub fn fetch_or(&self, val: i32, order: Ordering) -> i32

Bitwise “or” with the current value.

Performs a bitwise “or” operation on the current value and the argument val, and sets the new value to the result.

Returns the previous value.

fetch_or takes an Ordering argument which describes the memory ordering of this operation. All ordering modes are possible. Note that using Acquire makes the store part of this operation Relaxed, and using Release makes the load part Relaxed.

Note: This method is only available on platforms that support atomic operations on i32.

Examples

use std::sync::atomic::{AtomicI32, Ordering};

let foo = AtomicI32::new(0b101101);
assert_eq!(foo.fetch_or(0b110011, Ordering::SeqCst), 0b101101);
assert_eq!(foo.load(Ordering::SeqCst), 0b111111);

pub fn fetch_xor(&self, val: i32, order: Ordering) -> i32

Bitwise “xor” with the current value.

Performs a bitwise “xor” operation on the current value and the argument val, and sets the new value to the result.

Returns the previous value.

fetch_xor takes an Ordering argument which describes the memory ordering of this operation. All ordering modes are possible. Note that using Acquire makes the store part of this operation Relaxed, and using Release makes the load part Relaxed.

Note: This method is only available on platforms that support atomic operations on i32.

Examples

use std::sync::atomic::{AtomicI32, Ordering};

let foo = AtomicI32::new(0b101101);
assert_eq!(foo.fetch_xor(0b110011, Ordering::SeqCst), 0b101101);
assert_eq!(foo.load(Ordering::SeqCst), 0b011110);

pub fn fetch_update<F>( &self, set_order: Ordering, fetch_order: Ordering, f: F, ) -> Result<i32, i32>

where F: FnMut(i32) -> Option<i32>,

Fetches the value, and applies a function to it that returns an optional new value. Returns a Result of Ok(previous_value) if the function returned Some(_), else Err(previous_value).

Note: This may call the function multiple times if the value has been changed from other threads in the meantime, as long as the function returns Some(_), but the function will have been applied only once to the stored value.

fetch_update takes two Ordering arguments to describe the memory ordering of this operation. The first describes the required ordering for when the operation finally succeeds while the second describes the required ordering for loads. These correspond to the success and failure orderings of AtomicI32::compare_exchange respectively.

Using Acquire as success ordering makes the store part of this operation Relaxed, and using Release makes the final successful load Relaxed. The (failed) load ordering can only be SeqCst, Acquire or Relaxed.

Note: This method is only available on platforms that support atomic operations on i32.

Considerations

This method is not magic; it is not provided by the hardware, and does not act like a critical section or mutex.

It is implemented on top of an atomic compare-and-swap operation, and thus is subject to the usual drawbacks of CAS operations. In particular, be careful of the ABA problem if this atomic integer is an index or more generally if knowledge of only the bitwise value of the atomic is not in and of itself sufficient to ensure any required preconditions.

Examples

use std::sync::atomic::{AtomicI32, Ordering};

let x = AtomicI32::new(7);
assert_eq!(x.fetch_update(Ordering::SeqCst, Ordering::SeqCst, |_| None), Err(7));
assert_eq!(x.fetch_update(Ordering::SeqCst, Ordering::SeqCst, |x| Some(x + 1)), Ok(7));
assert_eq!(x.fetch_update(Ordering::SeqCst, Ordering::SeqCst, |x| Some(x + 1)), Ok(8));
assert_eq!(x.load(Ordering::SeqCst), 9);

pub fn try_update( &self, set_order: Ordering, fetch_order: Ordering, f: impl FnMut(i32) -> Option<i32>, ) -> Result<i32, i32>

🔬This is a nightly-only experimental API. (atomic_try_update)

Fetches the value, and applies a function to it that returns an optional new value. Returns a Result of Ok(previous_value) if the function returned Some(_), else Err(previous_value).

See also: update.

Note: This may call the function multiple times if the value has been changed from other threads in the meantime, as long as the function returns Some(_), but the function will have been applied only once to the stored value.

try_update takes two Ordering arguments to describe the memory ordering of this operation. The first describes the required ordering for when the operation finally succeeds while the second describes the required ordering for loads. These correspond to the success and failure orderings of AtomicI32::compare_exchange respectively.

Using Acquire as success ordering makes the store part of this operation Relaxed, and using Release makes the final successful load Relaxed. The (failed) load ordering can only be SeqCst, Acquire or Relaxed.

Note: This method is only available on platforms that support atomic operations on i32.

Considerations

This method is not magic; it is not provided by the hardware, and does not act like a critical section or mutex.

It is implemented on top of an atomic compare-and-swap operation, and thus is subject to the usual drawbacks of CAS operations. In particular, be careful of the ABA problem if this atomic integer is an index or more generally if knowledge of only the bitwise value of the atomic is not in and of itself sufficient to ensure any required preconditions.

Examples

#![feature(atomic_try_update)]
use std::sync::atomic::{AtomicI32, Ordering};

let x = AtomicI32::new(7);
assert_eq!(x.try_update(Ordering::SeqCst, Ordering::SeqCst, |_| None), Err(7));
assert_eq!(x.try_update(Ordering::SeqCst, Ordering::SeqCst, |x| Some(x + 1)), Ok(7));
assert_eq!(x.try_update(Ordering::SeqCst, Ordering::SeqCst, |x| Some(x + 1)), Ok(8));
assert_eq!(x.load(Ordering::SeqCst), 9);

pub fn update( &self, set_order: Ordering, fetch_order: Ordering, f: impl FnMut(i32) -> i32, ) -> i32

🔬This is a nightly-only experimental API. (atomic_try_update)

Fetches the value, applies a function to it that it return a new value. The new value is stored and the old value is returned.

See also: try_update.

Note: This may call the function multiple times if the value has been changed from other threads in the meantime, but the function will have been applied only once to the stored value.

update takes two Ordering arguments to describe the memory ordering of this operation. The first describes the required ordering for when the operation finally succeeds while the second describes the required ordering for loads. These correspond to the success and failure orderings of AtomicI32::compare_exchange respectively.

Using Acquire as success ordering makes the store part of this operation Relaxed, and using Release makes the final successful load Relaxed. The (failed) load ordering can only be SeqCst, Acquire or Relaxed.

Note: This method is only available on platforms that support atomic operations on i32.

Considerations

This method is not magic; it is not provided by the hardware, and does not act like a critical section or mutex.

It is implemented on top of an atomic compare-and-swap operation, and thus is subject to the usual drawbacks of CAS operations. In particular, be careful of the ABA problem if this atomic integer is an index or more generally if knowledge of only the bitwise value of the atomic is not in and of itself sufficient to ensure any required preconditions.

Examples

#![feature(atomic_try_update)]
use std::sync::atomic::{AtomicI32, Ordering};

let x = AtomicI32::new(7);
assert_eq!(x.update(Ordering::SeqCst, Ordering::SeqCst, |x| x + 1), 7);
assert_eq!(x.update(Ordering::SeqCst, Ordering::SeqCst, |x| x + 1), 8);
assert_eq!(x.load(Ordering::SeqCst), 9);

pub fn fetch_max(&self, val: i32, order: Ordering) -> i32

Maximum with the current value.

Finds the maximum of the current value and the argument val, and sets the new value to the result.

Returns the previous value.

fetch_max takes an Ordering argument which describes the memory ordering of this operation. All ordering modes are possible. Note that using Acquire makes the store part of this operation Relaxed, and using Release makes the load part Relaxed.

Note: This method is only available on platforms that support atomic operations on i32.

Examples

use std::sync::atomic::{AtomicI32, Ordering};

let foo = AtomicI32::new(23);
assert_eq!(foo.fetch_max(42, Ordering::SeqCst), 23);
assert_eq!(foo.load(Ordering::SeqCst), 42);

If you want to obtain the maximum value in one step, you can use the following:

use std::sync::atomic::{AtomicI32, Ordering};

let foo = AtomicI32::new(23);
let bar = 42;
let max_foo = foo.fetch_max(bar, Ordering::SeqCst).max(bar);
assert!(max_foo == 42);

pub fn fetch_min(&self, val: i32, order: Ordering) -> i32

Minimum with the current value.

Finds the minimum of the current value and the argument val, and sets the new value to the result.

Returns the previous value.

fetch_min takes an Ordering argument which describes the memory ordering of this operation. All ordering modes are possible. Note that using Acquire makes the store part of this operation Relaxed, and using Release makes the load part Relaxed.

Note: This method is only available on platforms that support atomic operations on i32.

Examples

use std::sync::atomic::{AtomicI32, Ordering};

let foo = AtomicI32::new(23);
assert_eq!(foo.fetch_min(42, Ordering::Relaxed), 23);
assert_eq!(foo.load(Ordering::Relaxed), 23);
assert_eq!(foo.fetch_min(22, Ordering::Relaxed), 23);
assert_eq!(foo.load(Ordering::Relaxed), 22);

If you want to obtain the minimum value in one step, you can use the following:

use std::sync::atomic::{AtomicI32, Ordering};

let foo = AtomicI32::new(23);
let bar = 12;
let min_foo = foo.fetch_min(bar, Ordering::SeqCst).min(bar);
assert_eq!(min_foo, 12);

pub fn as_ptr(&self) -> *mut i32

Returns a mutable pointer to the underlying integer.

Doing non-atomic reads and writes on the resulting integer can be a data race. This method is mostly useful for FFI, where the function signature may use *mut i32 instead of &AtomicI32.

Returning an *mut pointer from a shared reference to this atomic is safe because the atomic types work with interior mutability. All modifications of an atomic change the value through a shared reference, and can do so safely as long as they use atomic operations. Any use of the returned raw pointer requires an unsafe block and still has to uphold the requirements of the memory model.

Examples

use std::sync::atomic::AtomicI32;

extern "C" {
    fn my_atomic_op(arg: *mut i32);
}

let atomic = AtomicI32::new(1);

// SAFETY: Safe as long as `my_atomic_op` is atomic.
unsafe {
    my_atomic_op(atomic.as_ptr());
}

Trait Implementations

impl Debug for Futex

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Deref for Futex

type Target = AtomicI32

The resulting type after dereferencing.

fn deref(&self) -> &Self::Target

Dereferences the value.