Swift Package Index

C11 atomics for Swift

  • The latest stable release is 6.5.1. Released about 1 month ago.
  • The last commit to main was about 1 month ago.

Swift Version Compatibility

  • 6.5.1 and main
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Platform Compatibility

  • 6.5.1 and main
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CAtomics Build Status

Some atomic functions made available to Swift 3.1 and up, including Swift 5.3

This project bridges a subset of Clang's C11 atomics support to Swift, in a manner compatible with SE-0282. This package has been the underlying guts of the long-standing (but now deprecated) SwiftAtomics package since 2017.

Module CAtomics

CAtomics provides atomic operations to Swift using a C-style interface. CAtomics implements the following types:

  • AtomicRawPointer, AtomicMutableRawPointer, AtomicOpaquePointer, and versions for Optional pointers (AtomicOptionalRawPointer, etc.);
  • AtomicInt and AtomicUInt, as well as signed and unsigned versions of the 8-bit, 16-bit, 32-bit and 64-bit integer types;
  • AtomicBool

Functions defined in CAtomics are prefixed with CAtomics. All types define CAtomicsLoad, CAtomicsStore, CAtomicsExchange, CAtomicsCompareAndExchangeWeak and CAtomicsCompareAndExchangeStrong. Additionally, the integer types define CAtomicsAdd, CAtomicsSubtract, CAtomicsBitwiseAnd, CAtomicsBitwiseOr and CAtomicsBitwiseXor. Finally, AtomicBool defines CAtomicsAnd, CAtomicsOr, and CAtomicsXor.

The memory ordering (from <stdatomic.h>) can be set by using the order: MemoryOrder parameter on each method.

public enum MemoryOrder : UInt32 {
  case relaxed, acquire, release, acqrel, sequential

Note that memory_order_consume has no equivalent in this module, as (as far as I can tell) clang silently upgrades that ordering to memory_order_acquire, making it impossible (at the moment) to test whether an algorithm can properly use memory_order_consume. This also means nothing is lost by its absence.

Notes on atomics and the law-of-exclusivity:

Atomic types are useful as synchronization points between threads, and therefore have an interesting relationship with Swift's exclusivity checking. They should be initialized at fixed memory locations, such as members of reference types, captured by closures, or in heap allocations. This being said, although the first two work as expected with current Swift, only the latter (heap allocations) is compatible with the thread sanitizer.

In order to use atomics in a way that is acceptable to the thread sanitizer, one must allocate memory for atomic variables using UnsafeMutablePointer. Then, pass that pointer to the functions defined in the CAtomics module, as needed.

import CAtomics

class Example {
  private var counter = UnsafeMutablePointer<AtomicInt>.allocate(capacity: 1)
  init() {
    CAtomicsInitialize(counter, 0)

  deinit {

  func increment(by value: Int = 1) {
    CAtomicsAdd(counter, value, .relaxed)


This library requires Swift 3.1 or later. On Linux, it also requires Clang 3.6 or later.