How do I atomically increment a variable in Swift?

swift concurrency

From Low-Level Concurrency APIs:

There’s a long list of OSAtomicIncrement and OSAtomicDecrement functions that allow you to increment and decrement an integer value in an atomic way – thread safe without having to take a lock (or use queues). These can be useful if you need to increment global counters from multiple threads for statistics. If all you do is increment a global counter, the barrier-free OSAtomicIncrement versions are fine, and when there’s no contention, they’re cheap to call.

These functions work with fixed-size integers, you can choose the 32-bit or 64-bit variant depending on your needs:

class Counter {
    private (set) var value : Int32 = 0
    func increment () {
        OSAtomicIncrement32(&value)
    }
}

(Note: As Erik Aigner correctly noticed, OSAtomicIncrement32 and friends are deprecated as of macOS 10.12/iOS 10.10. Xcode 8 suggests to use functions from <stdatomic.h> instead. However that seems to be difficult, compare Swift 3: atomic_compare_exchange_strong and https://openradar.appspot.com/27161329. Therefore the following GCD-based approach seems to be the best solution now.)

Alternatively, one can use a GCD queue for synchronization. From Dispatch Queues in the "Concurrency Programming Guide":

... With dispatch queues, you could add both tasks to a serial dispatch queue to ensure that only one task modified the resource at any given time. This type of queue-based synchronization is more efficient than locks because locks always require an expensive kernel trap in both the contested and uncontested cases, whereas a dispatch queue works primarily in your application’s process space and only calls down to the kernel when absolutely necessary.

In your case that would be

// Swift 2:
class Counter {
    private var queue = dispatch_queue_create("your.queue.identifier", DISPATCH_QUEUE_SERIAL)
    private (set) var value: Int = 0

    func increment() {
        dispatch_sync(queue) {
            value += 1
        }
    }
}

// Swift 3:
class Counter {
    private var queue = DispatchQueue(label: "your.queue.identifier") 
    private (set) var value: Int = 0

    func increment() {
        queue.sync {
            value += 1
        }
    }
}

See Adding items to Swift array across multiple threads causing issues (because arrays aren't thread safe) - how do I get around that? or GCD with static functions of a struct for more sophisticated examples. This thread What advantage(s) does dispatch_sync have over @synchronized? is also very interesting.


Queues are an overkill in this case. You can use a DispatchSemaphore introduced in Swift 3 for this purpose like so:

import Foundation

public class AtomicInteger {

    private let lock = DispatchSemaphore(value: 1)
    private var value = 0

    // You need to lock on the value when reading it too since
    // there are no volatile variables in Swift as of today.
    public func get() -> Int {

        lock.wait()
        defer { lock.signal() }
        return value
    }

    public func set(_ newValue: Int) {

        lock.wait()
        defer { lock.signal() }
        value = newValue
    }

    public func incrementAndGet() -> Int {

        lock.wait()
        defer { lock.signal() }
        value += 1
        return value
    }
}

The latest version of the class is available over here.


I know this question is already a little bit older, but I just recently stumbled upon the same problem. After researching a little and reading posts like http://www.cocoawithlove.com/blog/2016/06/02/threads-and-mutexes.html I came up with this solution for an atomic counter. Maybe it will also help others.

import Foundation

class AtomicCounter {

  private var mutex = pthread_mutex_t()
  private var counter: UInt = 0

  init() {
    pthread_mutex_init(&mutex, nil)
  }

  deinit {
    pthread_mutex_destroy(&mutex)
  }

  func incrementAndGet() -> UInt {
    pthread_mutex_lock(&mutex)
    defer {
      pthread_mutex_unlock(&mutex)
    }
    counter += 1
    return counter
  }
}

Details

  • Xcode 10.1 (10B61)
  • Swift 4.2

Solution

import Foundation

struct AtomicInteger<Type>: BinaryInteger where Type: BinaryInteger {

    typealias Magnitude = Type.Magnitude
    typealias IntegerLiteralType = Type.IntegerLiteralType
    typealias Words = Type.Words
    fileprivate var value: Type

    private var semaphore = DispatchSemaphore(value: 1)
    fileprivate func _wait() { semaphore.wait() }
    fileprivate func _signal() { semaphore.signal() }

    init() { value = Type() }

    init(integerLiteral value: AtomicInteger.IntegerLiteralType) {
        self.value = Type(integerLiteral: value)
    }

    init<T>(_ source: T) where T : BinaryInteger {
        value = Type(source)
    }

    init(_ source: Int) {
        value = Type(source)
    }

    init<T>(clamping source: T) where T : BinaryInteger {
        value = Type(clamping: source)
    }

    init?<T>(exactly source: T) where T : BinaryInteger {
        guard let value = Type(exactly: source) else { return nil }
        self.value = value
    }

    init<T>(truncatingIfNeeded source: T) where T : BinaryInteger {
        value = Type(truncatingIfNeeded: source)
    }

    init?<T>(exactly source: T) where T : BinaryFloatingPoint {
        guard let value = Type(exactly: source) else { return nil }
        self.value = value
    }

    init<T>(_ source: T) where T : BinaryFloatingPoint {
        value = Type(source)
    }
}

// Instance Properties

extension AtomicInteger {
    var words: Type.Words {
        _wait(); defer { _signal() }
        return value.words
    }
    var bitWidth: Int {
        _wait(); defer { _signal() }
        return value.bitWidth
    }
    var trailingZeroBitCount: Int {
        _wait(); defer { _signal() }
        return value.trailingZeroBitCount
    }
    var magnitude: Type.Magnitude {
        _wait(); defer { _signal() }
        return value.magnitude
    }
}

// Type Properties

extension AtomicInteger {
    static var isSigned: Bool { return Type.isSigned }
}

// Instance Methods

extension AtomicInteger {

    func quotientAndRemainder(dividingBy rhs: AtomicInteger<Type>) -> (quotient: AtomicInteger<Type>, remainder: AtomicInteger<Type>) {
        _wait(); defer { _signal() }
        rhs._wait(); defer { rhs._signal() }
        let result = value.quotientAndRemainder(dividingBy: rhs.value)
        return (AtomicInteger(result.quotient), AtomicInteger(result.remainder))
    }

    func signum() -> AtomicInteger<Type> {
        _wait(); defer { _signal() }
        return AtomicInteger(value.signum())
    }
}


extension AtomicInteger {

    fileprivate static func atomicAction<Result, Other>(lhs: AtomicInteger<Type>,
                                                        rhs: Other, closure: (Type, Type) -> (Result)) -> Result where Other : BinaryInteger {
        lhs._wait(); defer { lhs._signal() }
        var rhsValue = Type(rhs)
        if let rhs = rhs as? AtomicInteger {
            rhs._wait(); defer { rhs._signal() }
            rhsValue = rhs.value
        }
        let result = closure(lhs.value, rhsValue)
        return result
    }

    fileprivate static func atomicActionAndResultSaving<Other>(lhs: inout AtomicInteger<Type>,
                                                               rhs: Other, closure: (Type, Type) -> (Type)) where Other : BinaryInteger {
        lhs._wait(); defer { lhs._signal() }
        var rhsValue = Type(rhs)
        if let rhs = rhs as? AtomicInteger {
            rhs._wait(); defer { rhs._signal() }
            rhsValue = rhs.value
        }
        let result = closure(lhs.value, rhsValue)
        lhs.value = result
    }
}

// Math Operator Functions

extension AtomicInteger {

    static func != <Other>(lhs: AtomicInteger, rhs: Other) -> Bool where Other : BinaryInteger {
        return atomicAction(lhs: lhs, rhs: rhs) { $0 != $1 }
    }

    static func != (lhs: AtomicInteger, rhs: AtomicInteger) -> Bool {
        return atomicAction(lhs: lhs, rhs: rhs) { $0 != $1 }
    }

    static func % (lhs: AtomicInteger, rhs: AtomicInteger) -> AtomicInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 % $1 }
        return self.init(value)
    }

    static func %= (lhs: inout AtomicInteger, rhs: AtomicInteger) {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 % $1 }
    }

    static func & (lhs: AtomicInteger, rhs: AtomicInteger) -> AtomicInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 & $1 }
        return self.init(value)
    }

    static func &= (lhs: inout AtomicInteger, rhs: AtomicInteger) {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 & $1 }
    }

    static func * (lhs: AtomicInteger, rhs: AtomicInteger) -> AtomicInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 * $1 }
        return self.init(value)
    }

    static func *= (lhs: inout AtomicInteger, rhs: AtomicInteger) {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 * $1 }
    }

    static func + (lhs: AtomicInteger, rhs: AtomicInteger) -> AtomicInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 + $1 }
        return self.init(value)
    }
    static func += (lhs: inout AtomicInteger, rhs: AtomicInteger) {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 + $1 }
    }

    static func - (lhs: AtomicInteger, rhs: AtomicInteger) -> AtomicInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 - $1 }
        return self.init(value)
    }

    static func -= (lhs: inout AtomicInteger, rhs: AtomicInteger) {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 - $1 }
    }

    static func / (lhs: AtomicInteger, rhs: AtomicInteger) -> AtomicInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 / $1 }
        return self.init(value)
    }

    static func /= (lhs: inout AtomicInteger, rhs: AtomicInteger) {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 / $1 }
    }
}


// Shifting Operator Functions

extension AtomicInteger {
    static func << <RHS>(lhs:  AtomicInteger<Type>, rhs: RHS) -> AtomicInteger where RHS : BinaryInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 << $1 }
        return self.init(value)
    }

    static func <<= <RHS>(lhs: inout AtomicInteger, rhs: RHS) where RHS : BinaryInteger {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 << $1 }
    }

    static func >> <RHS>(lhs: AtomicInteger, rhs: RHS) -> AtomicInteger where RHS : BinaryInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 >> $1 }
        return self.init(value)
    }

    static func >>= <RHS>(lhs: inout AtomicInteger, rhs: RHS) where RHS : BinaryInteger {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 >> $1 }
    }
}

// Comparing Operator Functions

extension AtomicInteger {

    static func < <Other>(lhs: AtomicInteger<Type>, rhs: Other) -> Bool where Other : BinaryInteger {
        return atomicAction(lhs: lhs, rhs: rhs) { $0 < $1 }
    }

    static func <= (lhs: AtomicInteger, rhs: AtomicInteger) -> Bool {
        return atomicAction(lhs: lhs, rhs: rhs) { $0 <= $1 }
    }

    static func == <Other>(lhs: AtomicInteger, rhs: Other) -> Bool where Other : BinaryInteger {
        return atomicAction(lhs: lhs, rhs: rhs) { $0 == $1 }
    }

    static func > <Other>(lhs: AtomicInteger, rhs: Other) -> Bool where Other : BinaryInteger {
        return atomicAction(lhs: lhs, rhs: rhs) { $0 > $1 }
    }

    static func > (lhs: AtomicInteger, rhs: AtomicInteger) -> Bool {
        return atomicAction(lhs: lhs, rhs: rhs) { $0 > $1 }
    }

    static func >= (lhs: AtomicInteger, rhs: AtomicInteger) -> Bool {
        return atomicAction(lhs: lhs, rhs: rhs) { $0 >= $1 }
    }

    static func >= <Other>(lhs: AtomicInteger, rhs: Other) -> Bool where Other : BinaryInteger {
        return atomicAction(lhs: lhs, rhs: rhs) { $0 >= $1 }
    }
}

// Binary Math Operator Functions

extension AtomicInteger {

    static func ^ (lhs: AtomicInteger, rhs: AtomicInteger) -> AtomicInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 ^ $1 }
        return self.init(value)
    }

    static func ^= (lhs: inout AtomicInteger, rhs: AtomicInteger) {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 ^ $1 }
    }

    static func | (lhs: AtomicInteger, rhs: AtomicInteger) -> AtomicInteger {
        let value = atomicAction(lhs: lhs, rhs: rhs) { $0 | $1 }
        return self.init(value)
    }

    static func |= (lhs: inout AtomicInteger, rhs: AtomicInteger) {
        atomicActionAndResultSaving(lhs: &lhs, rhs: rhs) { $0 | $1 }
    }

    static prefix func ~ (x: AtomicInteger) -> AtomicInteger {
        x._wait(); defer { x._signal() }
        return self.init(x.value)
    }
}

// Hashable

extension AtomicInteger {

    var hashValue: Int {
        _wait(); defer { _signal() }
        return value.hashValue
    }

    func hash(into hasher: inout Hasher) {
        _wait(); defer { _signal() }
        value.hash(into: &hasher)
    }
}

// Get/Set

extension AtomicInteger {

    // Single  actions

    func get() -> Type {
        _wait(); defer { _signal() }
        return value
    }

    mutating func set(value: Type) {
        _wait(); defer { _signal() }
        self.value = value
    }

    // Multi-actions

    func get(closure: (Type)->()) {
        _wait(); defer { _signal() }
        closure(value)
    }

    mutating func set(closure: (Type)->(Type)) {
        _wait(); defer { _signal() }
        self.value = closure(value)
    }
}

Usage

// Usage Samples
let numA = AtomicInteger<Int8>(0)
let numB = AtomicInteger<Int16>(0)
let numC = AtomicInteger<Int32>(0)
let numD = AtomicInteger<Int64>(0)

var num1 = AtomicInteger<Int>(0)
num1 += 1
num1 -= 1
num1 = 10
num1 = num1/2

var num2 = 0
num2 = num1.get()
num1.set(value: num2*5)

// lock num1 to do several actions
num1.get { value in
    //...
}

num1.set { value in
    //...
    return value
}

Full Sample

import Foundation

var x = AtomicInteger<Int>(0)
let dispatchGroup = DispatchGroup()
private func async(dispatch: DispatchQueue, closure: @escaping (DispatchQueue)->()) {
    for _ in 0 ..< 100 {
        dispatchGroup.enter()
        dispatch.async {
            print("Queue: \(dispatch.qos.qosClass)")
            closure(dispatch)
            dispatchGroup.leave()
        }
    }
}

func sample() {
    let closure1: (DispatchQueue)->() = { _ in x += 1 }
    let closure2: (DispatchQueue)->() = { _ in x -= 1 }
    async(dispatch: .global(qos: .userInitiated), closure: closure1) // result: x += 100
    async(dispatch: .global(qos: .utility), closure: closure1) // result: x += 100
    async(dispatch: .global(qos: .background), closure: closure2) // result: x -= 100
    async(dispatch: .global(qos: .default), closure: closure2) // result: x -= 100
}

sample()
dispatchGroup.wait()
print(x) // expected result x = 0

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