Struct AtomicPtr

#[repr(C, align(8))]
pub struct AtomicPtr<T> { /* private fields */ }

A raw pointer type which can be safely shared between threads.

This type has the same in-memory representation as a *mut T.

If the compiler and the platform support atomic loads and stores of pointers, this type is a wrapper for the standard library’s AtomicPtr. If the platform supports it but the compiler does not, atomic operations are implemented using inline assembly.

Implementations

impl<T> AtomicPtr<T>

pub const fn new(p: *mut T) -> Self

Creates a new AtomicPtr.

Examples

use portable_atomic::AtomicPtr;

let ptr = &mut 5;
let atomic_ptr = AtomicPtr::new(ptr);

pub unsafe fn from_ptr<'a>(ptr: *mut *mut T) -> &'a Self

Creates a new AtomicPtr from a pointer.

Safety

• ptr must be aligned to align_of::<AtomicPtr<T>>() (note that on some platforms this can be bigger than align_of::<*mut T>()).
• ptr must be valid for both reads and writes for the whole lifetime 'a.
• If this atomic type is lock-free, non-atomic accesses to the value behind ptr must have a happens-before relationship with atomic accesses via the returned value (or vice-versa).
  • In other words, time periods where the value is accessed atomically may not overlap with periods where the value is accessed non-atomically.
  • This requirement is trivially satisfied if ptr is never used non-atomically for the duration of lifetime 'a. Most use cases should be able to follow this guideline.
  • This requirement is also trivially satisfied if all accesses (atomic or not) are done from the same thread.
• If this atomic type is not lock-free:
  • Any accesses to the value behind ptr must have a happens-before relationship with accesses via the returned value (or vice-versa).
  • Any concurrent accesses to the value behind ptr for the duration of lifetime 'a must be compatible with operations performed by this atomic type.
• This method must not be used to create overlapping or mixed-size atomic accesses, as these are not supported by the memory model.

pub fn is_lock_free() -> bool

Returns true if operations on values of this type are lock-free.

If the compiler or the platform doesn’t support the necessary atomic instructions, global locks for every potentially concurrent atomic operation will be used.

Examples

use portable_atomic::AtomicPtr;

let is_lock_free = AtomicPtr::<()>::is_lock_free();

pub const fn is_always_lock_free() -> bool

Returns true if operations on values of this type are lock-free.

If the compiler or the platform doesn’t support the necessary atomic instructions, global locks for every potentially concurrent atomic operation will be used.

Note: If the atomic operation relies on dynamic CPU feature detection, this type may be lock-free even if the function returns false.

Examples

use portable_atomic::AtomicPtr;

const IS_ALWAYS_LOCK_FREE: bool = AtomicPtr::<()>::is_always_lock_free();

pub fn get_mut(&mut self) -> &mut *mut T

Returns a mutable reference to the underlying pointer.

This is safe because the mutable reference guarantees that no other threads are concurrently accessing the atomic data.

Examples

use portable_atomic::{AtomicPtr, Ordering};

let mut data = 10;
let mut atomic_ptr = AtomicPtr::new(&mut data);
let mut other_data = 5;
*atomic_ptr.get_mut() = &mut other_data;
assert_eq!(unsafe { *atomic_ptr.load(Ordering::SeqCst) }, 5);

pub const fn into_inner(self) -> *mut T

Consumes the atomic and returns the contained value.

This is safe because passing self by value guarantees that no other threads are concurrently accessing the atomic data.

This is const fn on Rust 1.56+.

Examples

use portable_atomic::AtomicPtr;

let mut data = 5;
let atomic_ptr = AtomicPtr::new(&mut data);
assert_eq!(unsafe { *atomic_ptr.into_inner() }, 5);

pub fn load(&self, order: Ordering) -> *mut T

Loads a value from the pointer.

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 portable_atomic::{AtomicPtr, Ordering};

let ptr = &mut 5;
let some_ptr = AtomicPtr::new(ptr);

let value = some_ptr.load(Ordering::Relaxed);

pub fn store(&self, ptr: *mut T, order: Ordering)

Stores a value into the pointer.

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 portable_atomic::{AtomicPtr, Ordering};

let ptr = &mut 5;
let some_ptr = AtomicPtr::new(ptr);

let other_ptr = &mut 10;

some_ptr.store(other_ptr, Ordering::Relaxed);

pub fn swap(&self, ptr: *mut T, order: Ordering) -> *mut T

Stores a value into the pointer, 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.

Examples

use portable_atomic::{AtomicPtr, Ordering};

let ptr = &mut 5;
let some_ptr = AtomicPtr::new(ptr);

let other_ptr = &mut 10;

let value = some_ptr.swap(other_ptr, Ordering::Relaxed);

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

Stores a value into the pointer 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.

Panics

Panics if failure is Release, AcqRel.

Examples

use portable_atomic::{AtomicPtr, Ordering};

let ptr = &mut 5;
let some_ptr = AtomicPtr::new(ptr);

let other_ptr = &mut 10;

let value = some_ptr.compare_exchange(ptr, other_ptr, Ordering::SeqCst, Ordering::Relaxed);

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

Stores a value into the pointer if the current value is the same as the current value.

Unlike AtomicPtr::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.

Panics

Panics if failure is Release, AcqRel.

Examples

use portable_atomic::{AtomicPtr, Ordering};

let some_ptr = AtomicPtr::new(&mut 5);

let new = &mut 10;
let mut old = some_ptr.load(Ordering::Relaxed);
loop {
    match some_ptr.compare_exchange_weak(old, new, Ordering::SeqCst, Ordering::Relaxed) {
        Ok(_) => break,
        Err(x) => old = x,
    }
}

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

where F: FnMut(*mut T) -> Option<*mut T>,

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 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.

Panics

Panics if fetch_order is Release, AcqRel.

Considerations

This method is not magic; it is not provided by the hardware. It is implemented in terms of compare_exchange_weak, and suffers from the same drawbacks. In particular, this method will not circumvent the ABA Problem.

Examples

use portable_atomic::{AtomicPtr, Ordering};

let ptr: *mut _ = &mut 5;
let some_ptr = AtomicPtr::new(ptr);

let new: *mut _ = &mut 10;
assert_eq!(some_ptr.fetch_update(Ordering::SeqCst, Ordering::SeqCst, |_| None), Err(ptr));
let result = some_ptr.fetch_update(Ordering::SeqCst, Ordering::SeqCst, |x| {
    if x == ptr {
        Some(new)
    } else {
        None
    }
});
assert_eq!(result, Ok(ptr));
assert_eq!(some_ptr.load(Ordering::SeqCst), new);

pub fn fetch_ptr_add(&self, val: usize, order: Ordering) -> *mut T

Offsets the pointer’s address by adding val (in units of T), returning the previous pointer.

This is equivalent to using wrapping_add to atomically perform the equivalent of ptr = ptr.wrapping_add(val);.

This method operates in units of T, which means that it cannot be used to offset the pointer by an amount which is not a multiple of size_of::<T>(). This can sometimes be inconvenient, as you may want to work with a deliberately misaligned pointer. In such cases, you may use the fetch_byte_add method instead.

fetch_ptr_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.

Examples

use portable_atomic::{AtomicPtr, Ordering};
use sptr::Strict; // stable polyfill for strict provenance

let atom = AtomicPtr::<i64>::new(core::ptr::null_mut());
assert_eq!(atom.fetch_ptr_add(1, Ordering::Relaxed).addr(), 0);
// Note: units of `size_of::<i64>()`.
assert_eq!(atom.load(Ordering::Relaxed).addr(), 8);

pub fn fetch_ptr_sub(&self, val: usize, order: Ordering) -> *mut T

Offsets the pointer’s address by subtracting val (in units of T), returning the previous pointer.

This is equivalent to using wrapping_sub to atomically perform the equivalent of ptr = ptr.wrapping_sub(val);.

This method operates in units of T, which means that it cannot be used to offset the pointer by an amount which is not a multiple of size_of::<T>(). This can sometimes be inconvenient, as you may want to work with a deliberately misaligned pointer. In such cases, you may use the fetch_byte_sub method instead.

fetch_ptr_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.

Examples

use portable_atomic::{AtomicPtr, Ordering};

let array = [1i32, 2i32];
let atom = AtomicPtr::new(array.as_ptr().wrapping_add(1) as *mut _);

assert!(core::ptr::eq(atom.fetch_ptr_sub(1, Ordering::Relaxed), &array[1]));
assert!(core::ptr::eq(atom.load(Ordering::Relaxed), &array[0]));

pub fn fetch_byte_add(&self, val: usize, order: Ordering) -> *mut T

Offsets the pointer’s address by adding val bytes, returning the previous pointer.

This is equivalent to using wrapping_add and cast to atomically perform ptr = ptr.cast::<u8>().wrapping_add(val).cast::<T>().

fetch_byte_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.

Examples

use portable_atomic::{AtomicPtr, Ordering};
use sptr::Strict; // stable polyfill for strict provenance

let atom = AtomicPtr::<i64>::new(core::ptr::null_mut());
assert_eq!(atom.fetch_byte_add(1, Ordering::Relaxed).addr(), 0);
// Note: in units of bytes, not `size_of::<i64>()`.
assert_eq!(atom.load(Ordering::Relaxed).addr(), 1);

pub fn fetch_byte_sub(&self, val: usize, order: Ordering) -> *mut T

Offsets the pointer’s address by subtracting val bytes, returning the previous pointer.

This is equivalent to using wrapping_sub and cast to atomically perform ptr = ptr.cast::<u8>().wrapping_sub(val).cast::<T>().

fetch_byte_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.

Examples

use portable_atomic::{AtomicPtr, Ordering};
use sptr::Strict; // stable polyfill for strict provenance

let atom = AtomicPtr::<i64>::new(sptr::invalid_mut(1));
assert_eq!(atom.fetch_byte_sub(1, Ordering::Relaxed).addr(), 1);
assert_eq!(atom.load(Ordering::Relaxed).addr(), 0);

pub fn fetch_or(&self, val: usize, order: Ordering) -> *mut T

Performs a bitwise “or” operation on the address of the current pointer, and the argument val, and stores a pointer with provenance of the current pointer and the resulting address.

This is equivalent to using map_addr to atomically perform ptr = ptr.map_addr(|a| a | val). This can be used in tagged pointer schemes to atomically set tag bits.

Caveat: This operation returns the previous value. To compute the stored value without losing provenance, you may use map_addr. For example: a.fetch_or(val).map_addr(|a| a | val).

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.

This API and its claimed semantics are part of the Strict Provenance experiment, see the module documentation for ptr for details.

Examples

use portable_atomic::{AtomicPtr, Ordering};
use sptr::Strict; // stable polyfill for strict provenance

let pointer = &mut 3i64 as *mut i64;

let atom = AtomicPtr::<i64>::new(pointer);
// Tag the bottom bit of the pointer.
assert_eq!(atom.fetch_or(1, Ordering::Relaxed).addr() & 1, 0);
// Extract and untag.
let tagged = atom.load(Ordering::Relaxed);
assert_eq!(tagged.addr() & 1, 1);
assert_eq!(tagged.map_addr(|p| p & !1), pointer);

pub fn fetch_and(&self, val: usize, order: Ordering) -> *mut T

Performs a bitwise “and” operation on the address of the current pointer, and the argument val, and stores a pointer with provenance of the current pointer and the resulting address.

This is equivalent to using map_addr to atomically perform ptr = ptr.map_addr(|a| a & val). This can be used in tagged pointer schemes to atomically unset tag bits.

Caveat: This operation returns the previous value. To compute the stored value without losing provenance, you may use map_addr. For example: a.fetch_and(val).map_addr(|a| a & val).

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.

This API and its claimed semantics are part of the Strict Provenance experiment, see the module documentation for ptr for details.

Examples

use portable_atomic::{AtomicPtr, Ordering};
use sptr::Strict; // stable polyfill for strict provenance

let pointer = &mut 3i64 as *mut i64;
// A tagged pointer
let atom = AtomicPtr::<i64>::new(pointer.map_addr(|a| a | 1));
assert_eq!(atom.fetch_or(1, Ordering::Relaxed).addr() & 1, 1);
// Untag, and extract the previously tagged pointer.
let untagged = atom.fetch_and(!1, Ordering::Relaxed).map_addr(|a| a & !1);
assert_eq!(untagged, pointer);

pub fn fetch_xor(&self, val: usize, order: Ordering) -> *mut T

Performs a bitwise “xor” operation on the address of the current pointer, and the argument val, and stores a pointer with provenance of the current pointer and the resulting address.

This is equivalent to using map_addr to atomically perform ptr = ptr.map_addr(|a| a ^ val). This can be used in tagged pointer schemes to atomically toggle tag bits.

Caveat: This operation returns the previous value. To compute the stored value without losing provenance, you may use map_addr. For example: a.fetch_xor(val).map_addr(|a| a ^ val).

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.

This API and its claimed semantics are part of the Strict Provenance experiment, see the module documentation for ptr for details.

Examples

use portable_atomic::{AtomicPtr, Ordering};
use sptr::Strict; // stable polyfill for strict provenance

let pointer = &mut 3i64 as *mut i64;
let atom = AtomicPtr::<i64>::new(pointer);

// Toggle a tag bit on the pointer.
atom.fetch_xor(1, Ordering::Relaxed);
assert_eq!(atom.load(Ordering::Relaxed).addr() & 1, 1);

pub fn bit_set(&self, bit: u32, order: Ordering) -> bool

Sets the bit at the specified bit-position to 1.

Returns true if the specified bit was previously set to 1.

bit_set 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.

This corresponds to x86’s lock bts, and the implementation calls them on x86/x86_64.

Examples

use portable_atomic::{AtomicPtr, Ordering};
use sptr::Strict; // stable polyfill for strict provenance

let pointer = &mut 3i64 as *mut i64;

let atom = AtomicPtr::<i64>::new(pointer);
// Tag the bottom bit of the pointer.
assert!(!atom.bit_set(0, Ordering::Relaxed));
// Extract and untag.
let tagged = atom.load(Ordering::Relaxed);
assert_eq!(tagged.addr() & 1, 1);
assert_eq!(tagged.map_addr(|p| p & !1), pointer);

pub fn bit_clear(&self, bit: u32, order: Ordering) -> bool

Clears the bit at the specified bit-position to 1.

Returns true if the specified bit was previously set to 1.

bit_clear 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.

This corresponds to x86’s lock btr, and the implementation calls them on x86/x86_64.

Examples

use portable_atomic::{AtomicPtr, Ordering};
use sptr::Strict; // stable polyfill for strict provenance

let pointer = &mut 3i64 as *mut i64;
// A tagged pointer
let atom = AtomicPtr::<i64>::new(pointer.map_addr(|a| a | 1));
assert!(atom.bit_set(0, Ordering::Relaxed));
// Untag
assert!(atom.bit_clear(0, Ordering::Relaxed));

pub fn bit_toggle(&self, bit: u32, order: Ordering) -> bool

Toggles the bit at the specified bit-position.

Returns true if the specified bit was previously set to 1.

bit_toggle 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.

This corresponds to x86’s lock btc, and the implementation calls them on x86/x86_64.

Examples

use portable_atomic::{AtomicPtr, Ordering};
use sptr::Strict; // stable polyfill for strict provenance

let pointer = &mut 3i64 as *mut i64;
let atom = AtomicPtr::<i64>::new(pointer);

// Toggle a tag bit on the pointer.
atom.bit_toggle(0, Ordering::Relaxed);
assert_eq!(atom.load(Ordering::Relaxed).addr() & 1, 1);

pub const fn as_ptr(&self) -> *mut *mut T

Returns a mutable pointer to the underlying pointer.

Returning an *mut pointer from a shared reference to this atomic is safe because the atomic types work with interior mutability. Any use of the returned raw pointer requires an unsafe block and has to uphold the safety requirements. If there is concurrent access, note the following additional safety requirements:

• If this atomic type is lock-free, any concurrent operations on it must be atomic.
• Otherwise, any concurrent operations on it must be compatible with operations performed by this atomic type.

This is const fn on Rust 1.58+.

Trait Implementations

impl<T> Debug for AtomicPtr<T>

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

Formats the value using the given formatter.

impl<T> Default for AtomicPtr<T>

fn default() -> Self

Creates a null AtomicPtr<T>.

impl<T> From<*mut T> for AtomicPtr<T>

fn from(p: *mut T) -> Self

Converts to this type from the input type.

impl<T> Pointer for AtomicPtr<T>

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

Formats the value using the given formatter.

impl<T> RefUnwindSafe for AtomicPtr<T>