[swift-users] UnsafeMutablePointer on the stack?

[Dave Abrahams]

on Sun Oct 02 2016, Mike Ferenduros <swift-users-AT-swift.org> wrote:

> You can use a local like this:
>     var x: UInt32 = 0
>     withUnsafePointer(to: x) { randomWordPT in
>         //your existing code here
>     }
>
> Or I'm not sure if small arrays go on the heap, but

They do.

>     var bytes: [UInt8] = [0,0,0,0]
>     _ = SecRandomCopyBytes(kSecRandomDefault, bytes.count, &bytes)
>     return bytes.reduce(0) { ($0 << 8) | UInt32($1) }
>
> reads a little nicer.
>
> Personally I would be surprised if the malloc caused an actual measurable
> performance hit tbh.
>
> On Mon, 3 Oct 2016 at 02:15 Jean-Denis Muys via swift-users <
> swift-users at swift.org> wrote:
>
>> Hi,
>>
>> I have some issues using the new raw memory API. For instance, let's
>> suppose I want to call the `SecRandomCopyBytes` API to generate a
>> cryptographically secure random 32-bit number. The difficulty is its 3rd
>> argument, which is declared as UnsafeMutablePointer<UInt8>. Here is a
>> function that does that:
>>
>> func entropicRandom() -> UInt32 {
>>
>>     let randomWordPT = UnsafeMutablePointer<UInt32>.allocate(capacity: 1)
>>
>>
>>     let _ = randomWordPT.withMemoryRebound(to: UInt8.self, capacity: 4) {
>> (p: UnsafeMutablePointer<UInt8>) -> Int32 in
>>
>>         let result = SecRandomCopyBytes(kSecRandomDefault, MemoryLayout<
>> UInt32>.size, p)
>>
>>         return result
>>
>>     }
>>
>>     let randomInt32 = randomWordPT[0]
>>
>>     randomWordPT.deallocate(capacity: 1)
>>
>>     return randomInt32
>>
>> }
>>
>> apparently, the calls to allocate and then deallocate suggest that there
>> is some heap allocation happening behind the scene here, possibly
>> malloc/free. Is that correct?
>>
>> If so, this is quite wasteful. Is there a way to use a local variable on
>> the stack to achieve the same result?
>>
>> Thanks,
>>
>> Jean-Denis
>>
>>
>>
>> _______________________________________________
>> swift-users mailing list
>> swift-users at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-users
>>
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users
>

-- 
-Dave