Are Swift variables atomic?

Question

In Objective-C you have a distinction between atomic and nonatomic properties:

@property (nonatomic, strong) NSObject *nonatomicObject;
@property (atomic, st         


        

Answer 1

It is probably to early to answer this question. Currently swift lacks access modifiers, so there is not obvious way to add code which manages concurrency around a properties getter / setter. Furthermore, the Swift Language doesn't seem to have any information about concurrency yet! (It also lacks KVO etc ...)

I think the answer to this question will become clear in future releases.

Answer 2

Details

• Xcode 9.1, Swift 4
• Xcode 10.2.1 (10E1001), Swift 5

Links

• apple.developer.com Dispatch
• Grand Central Dispatch (GCD) and Dispatch Queues in Swift 3
• Creating Thread-Safe Arrays in Swift
• Mutexes and closure capture in Swift

Implemented types

• QueueSafeValue - a universal wrapper that provides safe multi-threaded access to values
• AtomicArray
• AtomicInteger
• AtomicValue

Main Idea

class Example {
    
    private lazy var semaphore = DispatchSemaphore(value: 1)
    
    func executeThreadSafeFunc1() {
        // Lock access. Only first thread can execute code below.
        // Other threads will wait until semaphore.signal() will execute
        semaphore.wait()
        // your code
        semaphore.signal()         // Unlock access
    }
    
    func executeThreadSafeFunc2() {
        // Lock access. Only first thread can execute code below.
        // Other threads will wait until semaphore.signal() will execute
        semaphore.wait()
        DispatchQueue.global(qos: .background).async {
            // your code
            self.semaphore.signal()         // Unlock access
        }
    }
}

Sample of atomic access

class Atomic {
    
    let dispatchGroup = DispatchGroup()
    private var variable = 0
    
    // Usage of semaphores
    
    func semaphoreSample() {
        
        // value: 1 - number of threads that have simultaneous access to the variable
        let atomicSemaphore = DispatchSemaphore(value: 1)
        variable = 0
        
        runInSeveralQueues { dispatchQueue  in
            // Only (value) queqes can run operations betwen atomicSemaphore.wait() and atomicSemaphore.signal()
            // Others queues await their turn
            atomicSemaphore.wait()            // Lock access until atomicSemaphore.signal()
            self.variable += 1
            print("\(dispatchQueue), value: \(self.variable)")
            atomicSemaphore.signal()          // Unlock access
        }
        
        notifyWhenDone {
            atomicSemaphore.wait()           // Lock access until atomicSemaphore.signal()
            print("variable = \(self.variable)")
            atomicSemaphore.signal()         // Unlock access
        }
    }
    
    // Usage of sync of DispatchQueue
    
    func dispatchQueueSync() {
        let atomicQueue = DispatchQueue(label: "dispatchQueueSync")
        variable = 0
        
        runInSeveralQueues { dispatchQueue  in
            
            // Only queqe can run this closure (atomicQueue.sync {...})
            // Others queues await their turn
            atomicQueue.sync {
                self.variable += 1
                print("\(dispatchQueue), value: \(self.variable)")
            }
        }
        
        notifyWhenDone {
            atomicQueue.sync {
                print("variable = \(self.variable)")
            }
        }
    }
    
    // Usage of objc_sync_enter/objc_sync_exit
    
    func objcSync() {
        variable = 0
        
        runInSeveralQueues { dispatchQueue  in
            
            // Only one queqe can run operations betwen objc_sync_enter(self) and objc_sync_exit(self)
            // Others queues await their turn
            objc_sync_enter(self)                   // Lock access until objc_sync_exit(self).
            self.variable += 1
            print("\(dispatchQueue), value: \(self.variable)")
            objc_sync_exit(self)                    // Unlock access
        }
        
        notifyWhenDone {
            objc_sync_enter(self)                   // Lock access until objc_sync_exit(self)
            print("variable = \(self.variable)")
            objc_sync_exit(self)                    // Unlock access
        }
    }
}

// Helpers

extension Atomic {

    fileprivate func notifyWhenDone(closure: @escaping ()->()) {
        dispatchGroup.notify(queue: .global(qos: .utility)) {
            closure()
            print("All work done")
        }
    }
    
    fileprivate func runInSeveralQueues(closure: @escaping (DispatchQueue)->()) {
        
        async(dispatch: .main, closure: closure)
        async(dispatch: .global(qos: .userInitiated), closure: closure)
        async(dispatch: .global(qos: .utility), closure: closure)
        async(dispatch: .global(qos: .default), closure: closure)
        async(dispatch: .global(qos: .userInteractive), closure: closure)
    }
    
    private func async(dispatch: DispatchQueue, closure: @escaping (DispatchQueue)->()) {
        
        for _ in 0 ..< 100 {
            dispatchGroup.enter()
            dispatch.async {
                let usec = Int(arc4random()) % 100_000
                usleep(useconds_t(usec))
                closure(dispatch)
                self.dispatchGroup.leave()
            }
        }
    }
}

Usage

Atomic().semaphoreSample()
//Atomic().dispatchQueueSync()
//Atomic().objcSync()

Result

Answer 3

From Swift 5.1 you can use property wrappers to make specific logic for your properties. This is atomic wrapper implementation:

@propertyWrapper
struct atomic<T> {
    private var value: T
    private let lock = NSLock()

    init(wrappedValue value: T) {
        self.value = value
    }

    var wrappedValue: T {
      get { getValue() }
      set { setValue(newValue: newValue) }
    }

    func getValue() -> T {
        lock.lock()
        defer { lock.unlock() }

        return value
    }

    mutating func setValue(newValue: T) {
        lock.lock()
        defer { lock.unlock() }

        value = newValue
    }
}

How to use:

class Shared {
    @atomic var value: Int
...
}

Answer 4

Swift has no language constructs around thread safety. It is assumed that you will be using the provided libraries to do your own thread safety management. There are a large number of options you have in implementing thread safety including pthread mutexes, NSLock, and dispatch_sync as a mutex mechanism. See Mike Ash's recent post on the subject: https://mikeash.com/pyblog/friday-qa-2015-02-06-locks-thread-safety-and-swift.html So the direct answer to your question of "Can I read and write to this variable in parallel safely?" is No.

Answer 5

It's very early to assume as no low-level documentation is available, but you can study from assembly. Hopper Disassembler is a great tool.

@interface ObjectiveCar : NSObject
@property (nonatomic, strong) id engine;
@property (atomic, strong) id driver;
@end

Uses objc_storeStrong and objc_setProperty_atomic for nonatomic and atomic respectively, where

class SwiftCar {
    var engine : AnyObject?    
    init() {
    }
}

uses swift_retain from libswift_stdlib_core and, apparently, does not have thread safety built in.

We can speculate that additional keywords (similar to @lazy) might be introduced later on.

Update 07/20/15: according to this blogpost on singletons swift environment can make certain cases thread safe for you, i.e.:

class Car {
    static let sharedCar: Car = Car() // will be called inside of dispatch_once
}

private let sharedCar: Car2 = Car2() // same here
class Car2 {

}

Update 05/25/16: Keep an eye out for swift evolution proposal https://github.com/apple/swift-evolution/blob/master/proposals/0030-property-behavior-decls.md - it looks like it is going to be possible to have @atomic behavior implemented by yourself.

Answer 6

Here is the atomic property wrapper that I use extensively. I made the actual locking mechanism a protocol, so I could experiement with different mechanisms. I tried semaphores, DispatchQueues, and the pthread_rwlock_t. The pthread_rwlock_t was chosen because it appears to have the lowest overhead, and a lower chance of a priority inversion.

/// Defines a basic signature that all locks will conform to. Provides the basis for atomic access to stuff.
protocol Lock {
    init()
    /// Lock a resource for writing. So only one thing can write, and nothing else can read or write.
    func writeLock()
    /// Lock a resource for reading. Other things can also lock for reading at the same time, but nothing else can write at that time.
    func readLock()
    /// Unlock a resource
    func unlock()
}

final class PThreadRWLock: Lock {
    private var rwLock = pthread_rwlock_t()

    init() {
        guard pthread_rwlock_init(&rwLock, nil) == 0 else {
            preconditionFailure("Unable to initialize the lock")
        }
    }

    deinit {
        pthread_rwlock_destroy(&rwLock)
    }

    func writeLock() {
        pthread_rwlock_wrlock(&rwLock)
    }

    func readLock() {
        pthread_rwlock_rdlock(&rwLock)
    }

    func unlock() {
        pthread_rwlock_unlock(&rwLock)
    }
}

/// A property wrapper that ensures atomic access to a value. IE only one thing can write at a time.
/// Multiple things can potentially read at the same time, just not during a write.
/// By using `pthread` to do the locking, this safer then using a `DispatchQueue/barrier` as there isn't a chance
/// of priority inversion.
@propertyWrapper
public final class Atomic<Value> {

    private var value: Value
    private let lock: Lock = PThreadRWLock()

    public init(wrappedValue value: Value) {
        self.value = value
    }

    public var wrappedValue: Value {
        get {
            self.lock.readLock()
            defer { self.lock.unlock() }
            return self.value
        }
        set {
            self.lock.writeLock()
            self.value = newValue
            self.lock.unlock()
        }
    }

    /// Provides a closure that will be called synchronously. This closure will be passed in the current value
    /// and it is free to modify it. Any modifications will be saved back to the original value.
    /// No other reads/writes will be allowed between when the closure is called and it returns.
    public func mutate(_ closure: (inout Value) -> Void) {
        self.lock.writeLock()
        closure(&value)
        self.lock.unlock()
    }
}