https://stackoverflow.com/q/55544393/1271826

Factorial.c

// consider

void process(long long * array, int arraySizeInBytes) {
    long long value = 1;

    for (long long i = 0; i < arraySizeInBytes / sizeof(long long); i++) {
        value *= i + 1;
        NSLog(@"%lld", array[i]);
        array[i] = value;
    }
}

This works:

class ViewController: NSViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        DispatchQueue.global().async {
            var a = [Int64](repeating: -1, count: 5)
            process(&a, Int32(a.count * MemoryLayout<Int64>.stride))

            print(a)
        }
    }

}

As does this:

class ViewController: NSViewController {

    var a = [Int64]()

    override func viewDidLoad() {
        super.viewDidLoad()

        DispatchQueue.global().async {
            self.a = [Int64](repeating: -1, count: 5)
            process(&self.a, Int32(self.a.count * MemoryLayout<Int64>.stride))

            print(self.a)
        }
    }

}

They both produce

2019-04-05 18:48:37.993089-0700 MyApp[5422:664763] -1
2019-04-05 18:48:37.993116-0700 MyApp[5422:664763] -1
2019-04-05 18:48:37.993125-0700 MyApp[5422:664763] -1
2019-04-05 18:48:37.993131-0700 MyApp[5422:664763] -1
2019-04-05 18:48:37.993138-0700 MyApp[5422:664763] -1
[1, 2, 6, 24, 120]