Sync

Useful synchronization primitives in Swift.

Mutex<T>

A fair mutual exclusion primitive useful for protecting shared data

This mutex will block threads waiting for the lock to become available. The mutex can also be statically initialized or created via a new constructor. Each mutex has a type parameter which represents the data that it is protecting. The data can only be accessed through the access handle passed to the callback of lock and tryLock, which guarantees that the data is only ever accessed when the mutex is locked.

Note: The implementation is based on pthread_mutex_t (64 bytes).

Minimal Example

let mutex = try Mutex(0)

try! mutex.read { value in 
    print(value)
}

try! mutex.write { access in
    access { value in
        value += 42
    }
}

let mutex = try Mutex(0)

let count: Int = 1000

let queue = DispatchQueue(
    label: #function,
    attributes: .concurrent
)

let group = DispatchGroup()

for _ in 0..<count {
    group.enter()

    queue.async {
        defer {
            group.leave()
        }
        try! mutex.write { value in
            value += 2
        }
    }
}

group.wait()

let value = try! mutex.unwrap()

XCTAssertEqual(value, 2 * count)

UnfairMutex<T>

An unfair mutual exclusion primitive useful for protecting shared data

This mutex will block threads waiting for the lock to become available. The mutex can also be statically initialized or created via a new constructor. Each mutex has a type parameter which represents the data that it is protecting. The data can only be accessed through the access handle passed to the callback of lock and tryLock, which guarantees that the data is only ever accessed when the mutex is locked.

Note: The implementation is based on os_unfair_lock_s (4 bytes).

Minimal Example

let mutex = try Mutex(0)

try! mutex.read { value in 
    print(value)
}

try! mutex.write { access in
    access { value in
        value += 42
    }
}

let unfairMutex = try UnfairMutex(0)

let count: Int = 1000

let queue = DispatchQueue(
    label: #function,
    attributes: .concurrent
)

let group = DispatchGroup()

for _ in 0..<count {
    group.enter()

    queue.async {
        defer {
            group.leave()
        }
        try! unfairMutex.write { value in
            value += 2
        }
    }
}

group.wait()

let value = try! unfairMutex.unwrap()

XCTAssertEqual(value, 2 * count)

RWLock<T>

A reader-writer lock

This type of lock allows a number of readers or at most one writer at any point in time. The write portion of this lock typically allows modification of the underlying data (exclusive access) and the read portion of this lock typically allows for read-only access (shared access).

In comparison, a Mutex does not distinguish between readers or writers that acquire the lock, therefore blocking any threads waiting for the lock to become available. An RWLock will allow any number of readers to acquire the lock as long as a writer is not holding the lock.

Note: The implementation is based on pthread_rwlock_t (200 bytes).

Important: RWLock does not support priority inversion avoidance.

Minimal Example

let rwlock = try RWLock(0)

try! rwlock.read { value in 
    print(value)
}

try! rwlock.write { access in
    access { value in
        value += 42
    }
}

let rwlock = try RWLock(0)

let count: Int = 1000

let queue = DispatchQueue(
    label: #function,
    attributes: .concurrent
)

let group = DispatchGroup()

for _ in 0..<count {
    group.enter()

    queue.async {
        defer {
            group.leave()
        }
        try! rwlock.write { value in
            value += 2
        }
    }
}

group.wait()

let value = try! rwlock.unwrap()

XCTAssertEqual(value, 2 * count)

License

This project is licensed under the MPL-2.0 – see the LICENSE.md file for details.