[swift-users] Is this really a race condition?

[Edward Connell]

Ahh! thank you. That makes sense.

On Wed, Mar 1, 2017 at 3:29 PM, Guillaume Lessard <
glessard at tffenterprises.com> wrote:

>
> > On Mar 1, 2017, at 3:21 PM, Edward Connell via swift-users <
> swift-users at swift.org> wrote:
> >
> > The thread sanitizer on Linux is reporting that I have race conditions
> in libswiftcore. I eliminated enough code down to this trivial example. Is
> there really a race condition here or are these bogus errors?
> >
> >       let count = 1000
> >       var items = [[UInt8]?](repeating: nil, count: count)
> >
> >       DispatchQueue.concurrentPerform(iterations: count) {
> >               items[$0] = [UInt8](repeating: 7, count: 10)
> >       }
> >
> > My real scenario is retrieving data asynchronously, so I just threw in a
> buffer assignment.
>
> The assignments to array elements are where the race lies.
>
> I don’t know about the libswiftcore part, but: assigning to a shared Array
> concurrently from multiple threads won't work, because of Array's
> copy-on-write behaviour. You could do
>
> let items = UnsafeMutablePointer<[UInt8]?>.allocate(capacity: 1)
> items.initialize(to: nil, count: count)
>
> DispatchQueue.concurrentPerform(iterations: count) {
>   items[$0].initialize([UInt8](repeating: 7, count: 10))
> }
>
> // you’ll be able to see here that they’re all initialized
>
> items.deallocate(capacity: count)
>
> Cheers,
> Guillaume Lessard
>
>
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