Adding to the array in parallel

Question

I use Grand Central Dispatch to convert elements from one array to another. I call dispatch_applyin the source array, convert it to zero or more elements, and then add them to the target array. This is a simplified example:

let src = Array(0..<1000)
var dst = [UInt32]()

let queue = dispatch_queue_create("myqueue", DISPATCH_QUEUE_CONCURRENT)
dispatch_apply(src.count, queue) { i in
    dst.append(arc4random_uniform(UInt32(i))) // <-- potential error here
}

print(dst)

I sometimes get an error in the line append. The error is always one of:

1. malloc: *** error for object 0x107508f00: pointer being freed was not allocated
2. fatal error: UnsafeMutablePointer.destroy with negative count
3. fatal error: Can't form Range with end < start

I assume this is due to the fact that appendit is not thread safe. What have I done wrong and how to fix it?

Answer 1

, Swift . append, . , - . dst:

let src = Array(0..<1000)
var dst = [UInt32]()

let queue = dispatch_queue_create("myqueue", DISPATCH_QUEUE_CONCURRENT)
let serialQueue = dispatch_queue_create("mySerialQueue", DISPATCH_QUEUE_SERIAL)
let serialGroup = dispatch_group_create()

dispatch_apply(src.count, queue) { i in
    let result = arc4random_uniform(UInt32(i)) // Your long-running transformation here
    dispatch_group_async(serialGroup, serialQueue) {
        dst.append(result)
    }
}

// Wait until all append operations are complete
dispatch_group_wait(serialGroup, DISPATCH_TIME_FOREVER)