Thanks very much John, that's what I expected and hoped for as it's enough to
call kevent() for a (kevent_t, kevent_t) used as struct kevent[2] .
> On 29 Apr 2017, at 00:28, John McCall <rjmcc...@apple.com> wrote:
>
>> On Apr 28, 2017, at 7:03 AM, Johannes Weiss via swift-dev
>> <swift-...@swift.org> wrote:
>> Hi swift-users,
>>
>> (sorry for the cross post to swift-dev, but wasn't sure where it belongs)
>>
>> I tried to find guarantees about the memory layout Swift tuples but couldn't
>> find anything. The reason I ask is because I'd like to use them as fixed
>> sized (stack allocated) arrays. I'm pretty sure they're actually not
>> guaranteed to be stack allocated but highly likely I assume :).
>
> Tuples are guaranteed to use a standard C-style layout wherever that layout
> is ABI-observable, e.g. when you construct an UnsafePointer to one.
>
> Note that that layout isn't ABI-observable when the tuple is, say, just
> stored in a struct. The compiler is allowed to break up the tuple and store
> its components however it likes. Of course, if the compiler does that, and
> you turn around and construct an UnsafePointer to that property, then the
> compiler has to reconstitute the tuple into an ABI-compliant temporary that
> it can give you a pointer to; this is yet another reason why you can't
> maintain permanent unsafe pointers to components of a struct.
>
> John.
>
>>
>> Am I correct in assuming that
>>
>> let swift_events: (kevent, kevent) = ...
>>
>> has the same memory layout as
>>
>> struct kevent c_events[2] = ...
>>
>> ? In other words, is this legal:
>>
>> var events = (kevent(), kevent())
>> withUnsafeMutableBytes(of: &events) { event_ptr in
>> precondition(MemoryLayout<kevent>.size * 2 == event_ptr.count)
>> if let ptr = event_ptr.baseAddress?.bindMemory(to: kevent.self,
>> capacity: 2) {
>> return kevent(someFileDescriptor, ptr, 2, ptr, 2, nil)
>> }
>> }
>>
>> I'm assuming yes but I'd like to make sure.
>>
>> Many thanks,
>> Johannes
>>
>> _______________________________________________
>> swift-dev mailing list
>> swift-...@swift.org
>> https://lists.swift.org/mailman/listinfo/swift-dev
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