<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Dec 4, 2016, at 4:53 PM, Andrew Trick via swift-users <<a href="mailto:swift-users@swift.org" class="">swift-users@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Nov 30, 2016, at 5:40 AM, Anders Ha via swift-users <<a href="mailto:swift-users@swift.org" class="">swift-users@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=us-ascii" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div dir="auto" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div dir="auto" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div dir="auto" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div dir="auto" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">Hi guys</div><div class=""><br class=""></div><div class="">I have recently started adopting lock-free atomics with memory fences, but it seems Swift at this moment does not have any native instruments.</div><div class=""><br class=""></div><div class="">Then I read a thread in the Apple Developer Forum (<a href="https://forums.developer.apple.com/thread/49334" class="">https://forums.developer.apple.com/thread/49334</a>), which an Apple staff claimed that all imported atomic operations are "not guaranteed to be atomic". But for my tests with all optimizations enabled (-Owholemodule and -O), the OSAtomic primitives and stdatomic fences do not seem going wild.</div><div class=""><br class=""></div><div class="">Is these `atomic_*` and `OSAtomic*` primitives really unsafe in Swift as claimed? It doesn't seem like the Swift compiler would reorder memory accesses around a C function call that it wouldn't be able to see through.</div></div></div></div></div></div></div></blockquote><div class=""><br class=""></div><div class="">Did you get an answer to this? I’m not sure what led you to believe the primitives are unsafe in Swift. Importing them doesn’t change their semantics.</div></div></div></div></blockquote><br class=""></div><div>If you apply them to memory you allocated manually with malloc/free on UnsafeMutablePointer's allocation methods, then yeah, they should work as they do in C. That's the safest way to use these functions today. Passing a Swift `var` inout to one of these functions does not guarantee that accesses to that var will maintain atomicity, since there may be bridging or reabstracting conversions happening under the hood.</div><div><br class=""></div><div>-Joe</div></body></html>