How to create UnsafeMutablePointer<UnsafeMutablePointer> in swift

Question

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

Why do you need to call this function? Please show us the code that uses this. — kennytm
– kennytm, Commented Mar 3, 2017 at 17:42

Community · Accepted Answer · 2017-05-23 12:09:14Z

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

OSAtomicCompareAndSwapPtr has been deprecated in favor of the C11 functions like atomic_compare_and_exchange_strong, which unfortunately you cannot use in Swift. This is a known issue.
The blog post's atomic operations uses "acquire", "release" and also "relaxed" memory order. The OSAtomic libraries provides only the "seq_cst" and "relaxed" order (the Barrier vs non-Barrier variant of each function). So your Swift translation cannot be a 1:1 functional copy of the C++ code.

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)

thanks for the help, is there a place you found the info on OSAtomic or is that just from looking at the source?
You could check the manpage. man 3 atomic_deprecated. The header file is more detailed though.

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