GCD - Critical Section/Mutex - objective-c

Question

Can somebody answer with short example:
How to correctly Lock code part with condition: if this part is locked by some thread don't hold other threads just skip this part by other threads and keep going.

Answer 1

ok, here is the working example (credit goes to #KenThomases ...)
import Dispatch
let semaphore = DispatchSemaphore(value: 1)
let printQueue = DispatchQueue(label: "print queue")
let group = DispatchGroup()
func longRuningTask(i: Int) {
printQueue.async(group: group) {
print(i,"GREEN semaphore")
}
usleep(1000) // cca 1 milisecond
printQueue.async(group: group) {
print(i,"job done")
}
}
func shortRuningTask(i: Int) {
group.enter()
guard semaphore.wait(timeout: .now() + 0.001) == .success else { // wait for cca 1 milisecond from now
printQueue.async(group: group) {
print(i,"RED semaphore, job not done")
}
group.leave()
return
}
longRuningTask(i: i)
semaphore.signal()
group.leave()
}
printQueue.async(group: group) {
print("running")
}
DispatchQueue.concurrentPerform(iterations: 10, execute: shortRuningTask )
group.wait()
print("all done")
and its printout
running
0 GREEN semaphore
2 RED semaphore, job not done
1 RED semaphore, job not done
3 RED semaphore, job not done
0 job done
4 GREEN semaphore
5 RED semaphore, job not done
6 RED semaphore, job not done
7 RED semaphore, job not done
4 job done
8 GREEN semaphore
9 RED semaphore, job not done
8 job done
all done
Program ended with exit code: 0