The function:
public func OSAtomicCompareAndSwapPtr(_ __oldValue: UnsafeMutableRawPointer!, _ __newValue: UnsafeMutableRawPointer!, ___theValue: UnsafeMutablePointer<UnsafeMutableRawPointer?>!) -> Bool
requires a type of UnsafeMutablePointer<UnsafeMutableRawPointer?> for the a parameter theValue but I'm having trouble figuring out how to make one from a type UnsafeMutablePointer<T?>
Any thoughts on how I could do this?
EDIT:
for the curious about why I am trying to do this, I am trying to create a generic wrapper around this to create a bounded MPMC queue in swift as outlined in this blog post. Here is my wrapper so far
func compareAndSwap<T: AnyObject>(current: T?, future: T?, toPtr: UnsafeMutablePointer<T?>) -> Bool {
let currentPtr = current.map(Unmanaged.passUnretained)?.toOpaque()
let futurePtr = future.map(Unmanaged.passRetained)?.toOpaque()
if OSAtomicCompareAndSwapPtr(currentPtr, futurePtr, ????) {
return true
}
return false
In Swift 3, use .withMemoryRebound to reinterpret-cast that "T**" pointer to a "void**" pointer:
return toPtr.withMemoryRebound(to: (UnsafeMutableRawPointer?).self, capacity: 1) {
OSAtomicCompareAndSwapPtr(currentPtr, futurePtr, $0)
}
Note that
atomic_compare_and_exchange_strong, which unfortunately you cannot use in Swift. This is a known issue.There is also a thin wrapper library timehop/Atomics which hides all the dangerous casts for you (it is not generic, however).
var toPtr = HOPAtomicReference(object: ...)
...
return toPtr.compareTo(current, andSetObject: future)
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