Swift array as c function buffer

Question

I'm using swift array as output buffer for a function that takes a pointer and fills it, like this:

var buffer : [Int32] = ...
tehFillFunc(UnsafeMutablePointer<Int32>(buffer))

This works fine, the problem is that compiler is complaining that Variable 'buffer' was never mutated; consider changing to 'let' constant, which I don't want to do as I'm pretty sure it was mutated in my fill function.

So, is there a way to silence it? (I could just do some dummy set, but I'd prefer to do it properly).

Edit: As requested complete example code that shows the problem (c is not even necessary):

class ViewController: UIViewController {

    func fill(sth: UnsafeMutablePointer<Int32>) {
        sth[0] = 7
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        var array = [Int32].init(count: 10, repeatedValue: 0)
        fill(UnsafeMutablePointer<Int32>(array))
        print("\(array)")
    }
}

However, the solution was already posted. In simplest form:

        fill(&array)

@MartinR You are right, I wonder how the original C function is declared. — Sulthan
– Sulthan, Commented Jun 14, 2016 at 10:34
Please update your question with a (small) self-contained example demonstrating the problem. — Martin R
– Martin R, Commented Jun 14, 2016 at 11:12
And it seems to be independent on the c/swift as source of the function - just the swift declaration form mattered. — Rychu
– Rychu, Commented Jun 14, 2016 at 11:32

Yury · Accepted Answer · 2016-06-14 10:31:39Z

2

Usually you need to have specify buffer size. And in this case I prefer following solution:

let bufferSize = 1000
var buffer = [Int32](count: bufferSize, repeatedValue: 0)
tehFillFunc(&buffer)

answered Jun 14, 2016 at 10:31

Yury

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1 Comment

Rychu Over a year ago

Well, this is the closest to what I need. Looks like it was simpler than I thought.

Nikolai Ruhe · Accepted Answer · 2016-06-14 10:32:06Z

1

There's a method on Array for this purpose:

var buffer : [Int32] = ...

buffer.withUnsafeMutableBufferPointer {
    tehFillFunc($0.baseAddress)
}

I makes sure the array is alive at least until withUnsafeMutableBufferPointer returns. As it takes an inout parameter, buffer has to be a var.

answered Jun 14, 2016 at 10:32

Nikolai Ruhe

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6 Comments

Martin R Over a year ago

That's what I also thought until some time ago, but as I understand forums.developer.apple.com/thread/47085, using withUnsafeMutableBufferPointer is not necessary for a single function invocation: "Swift guarantees that pointers you generate this way will remain valid for the duration of the function call."

Nikolai Ruhe Over a year ago

@MartinR The good thing about using it that it correctly behaves in the mutating case. If, for example, there's a didSet observer on buffer it would correctly be called once after withUnsafeMutableBufferPointer returns.

Martin R Over a year ago

I see, that is a good point. (But I still wonder where the compiler warning should come from, I cannot reproduce it.)

Nikolai Ruhe Over a year ago

@MartinR I was wondering if the function was actually declared tehFillFunc(const int *ptr);. Another reason could be an outdated Swift version... But I agree, it would be great if the OP would fill in this missing information.

Martin R Over a year ago

In that case the function could not assign new values to the pointed-to elements.

|

Dr. Teeth · Accepted Answer · 2023-07-20 06:04:21Z

1

How about this one?

let buffer = [Int32](unsafeUninitializedCapacity: bufferSize) { buffer, initializedCount in
    tehFillFunc(buffer.baseAddress)
    initializedCount = bufferSize
}

answered Jul 20, 2023 at 6:04

Dr. Teeth

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1 Comment

Rychu Over a year ago

I don't even work on that project anymore, but this looks much better

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