UnsafeMutablePointer Warning with Swift 5

Question

I had this:

let alphaPtr = UnsafeMutablePointer<vImagePixelCount>(mutating: alpha) as UnsafeMutablePointer<vImagePixelCount>?

Which now I get the warning:

Initialization of 'UnsafeMutablePointer' (aka 'UnsafeMutablePointer') results in a dangling pointer

Detailed warning consists of:

Implicit argument conversion from '[vImagePixelCount]' (aka 'Array') to 'UnsafePointer' (aka 'UnsafePointer') produces a pointer valid only for the duration of the call to 'init(mutating:)'

Use the 'withUnsafeBufferPointer' method on Array in order to explicitly convert argument to buffer pointer valid for a defined scope

Is there a way around this?

You just need to follow the instructions shown in the Detailed warning. — OOPer
– OOPer, Commented Mar 26, 2020 at 15:23

Rajasekhar Pasupuleti · Accepted Answer · 2020-05-27 09:09:17Z

11

Try this

var bytes = [UInt8]()
let uint8Pointer = UnsafeMutablePointer<UInt8>.allocate(capacity: bytes.count)
uint8Pointer.initialize(from: &bytes, count: bytes.count)

answered May 27, 2020 at 9:09

Rajasekhar Pasupuleti

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3 Comments

ScottyBlades Over a year ago

Where is alpha, the variable mentioned in the question's code?

twohyjr Over a year ago

Legit this worked for me! Thank you. btw Scotty, uint8Pointer == alphaPtr.

Vahida Havaldar Over a year ago

awesome! I was struggling with crash due to UnsafeMutablePointer ! your answer helped me thank you @rajasekhar-pasupuleti I had an array pixelBuffer - [Float32] so I did this let floatPointer = UnsafeMutablePointer<Float32>.allocate(capacity:pixelBuffer.count) floatPointer.initialize(from: &pixelBuffer, count: pixelBuffer.count)

Rob Napier · Accepted Answer · 2020-03-27 02:01:03Z

7

It was never safe to do this, and the compiler now is warning you more aggressively.

let alphaPtr = UnsafeMutablePointer ...

At the end of this line, alphaPtr is already invalid. There is no promise that what it points to is still allocated memory.

Instead, you need to nest whatever usage you need into a withUnsafeMutablePointer() (or withUnsafePointer()) block. If you cannot nest it into a block (for example, if you were storing the pointer or returning it), there is no way to make that correct. You'll have to redesign your data management to not require that.

answered Mar 27, 2020 at 2:01

Rob Napier

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Comments

Denis Oliveira · Accepted Answer · 2020-03-27 17:55:25Z

2

Do you need use the withUnsafeBufferPointer method from Array as

var alphaPtr: UnsafeBufferPointer = alpha.withUnsafeBufferPointer { $0 }

that's command produce a pointer optional if you need working with a specific type could you you use bindMemory(to:) or other function that match with you requirements.

Sometimes use a &alpha if you need a UnsafeRawPointer as a function parameter.

edited Mar 27, 2020 at 17:55

answered Mar 27, 2020 at 0:29

Denis Oliveira

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2 Comments

Rob Napier Over a year ago

This is not any more correct than the original code. There is no promise that alphaPtr is valid after the closing brace. If you use alphaPtr in any way it's undefined behavior, which is what the warning is telling you.

ScottyBlades Over a year ago

that gets me: Cannot convert value of type 'UnsafeBufferPointer<UInt8>' to closure result type 'UnsafePointer<UInt8>?'

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