Threading with Swift on Ubuntu 18.04 - swift

Question

I've been trying to reproduce example from on NSCondition here using my Ubuntu 18.04 machine and it seems that threads won't start even though thread_object.start() is called.
Code of the example
import Foundation
let cond = NSCondition()
var available = false
var SharedString = ""
class WriterThread : Thread {
override func main(){
for _ in 0..<5 {
cond.lock()
SharedString = "😅"
available = true
cond.signal() // Notify and wake up the waiting thread/s
cond.unlock()
}
}
}
class PrinterThread : Thread {
override func main(){
for _ in 0..<5 { //Just do it 5 times
cond.lock()
while(!available){ //Protect from spurious signals
cond.wait()
}
Thread.sleep(forTimeInterval:100)
print(SharedString)
SharedString = ""
available = false
cond.unlock()
}
}
}
let writet = WriterThread()
let printt = PrinterThread()
printt.start()
writet.start()
What have I done:
inited project with swift init --type=executable
amended code in Sources/Project/main.swift with the code above
used swift build without any errors
run the compiled object using .build/x86_64-unknown-linux-gnu/debug/Project
And the output is empty, which seems like thread won't start, even if I put print statement as first command inside the threads main. Is there any problem with running threading with Swift on Ubuntu, or do I do something wrong?
The Swift version is 5.5.3