Converting an UnsafePointer with length to a Swift Array type

17,287

Solution 1

You can simply initialize a Swift Array from an UnsafeBufferPointer:

func convert(length: Int, data: UnsafePointer<Int8>) -> [Int8] {

    let buffer = UnsafeBufferPointer(start: data, count: length);
    return Array(buffer)
}

This creates an array of the needed size and copies the data.

Or as a generic function:

func convert<T>(count: Int, data: UnsafePointer<T>) -> [T] {

    let buffer = UnsafeBufferPointer(start: data, count: count);
    return Array(buffer) 
}

where length is the number of items that the pointer points to.

If you have a UInt8 pointer but want to create an [T] array from the pointed-to data, then this is a possible solution:

// Swift 2:
func convert<T>(length: Int, data: UnsafePointer<UInt8>, _: T.Type) -> [T] {

    let buffer = UnsafeBufferPointer<T>(start: UnsafePointer(data), count: length/strideof(T));
    return Array(buffer) 
}

// Swift 3:
func convert<T>(length: Int, data: UnsafePointer<UInt8>, _: T.Type) -> [T] {
    let numItems = length/MemoryLayout<T>.stride
    let buffer = data.withMemoryRebound(to: T.self, capacity: numItems) {
        UnsafeBufferPointer(start: $0, count: numItems)
    }
    return Array(buffer) 
}

where length now is the number of bytes. Example:

let arr  = convert(12, data: ptr, Float.self)

would create an array of 3 Floats from the 12 bytes pointed to by ptr.

Solution 2

extension NSData {

    public func convertToBytes() -> [UInt8] {
        let count = self.length / sizeof(UInt8)
        var bytesArray = [UInt8](count: count, repeatedValue: 0)
        self.getBytes(&bytesArray, length:count * sizeof(UInt8))
        return bytesArray
    }
}

You can convert row data to byts (Uint8)

Copy Extension and use it..

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17,287
Ephemera
Author by

Ephemera

Updated on June 02, 2022

Comments

  • Ephemera
    Ephemera over 1 year

    I'm looking for the simplest ways to achieve reasonable C interoperability in Swift, and my current block is converting an UnsafePointer<Int8> (which was a const char *), into an [Int8] array.

    Currently, I have a naïve algorithm that can take an UnsafePointer and a number of bytes and converts it to an array, element by element:

    func convert(length: Int, data: UnsafePointer<Int8>) {
    
        let buffer = UnsafeBufferPointer(start: data, count: length);
        var arr: [Int8] = [Int8]()
        for (var i = 0; i < length; i++) {
            arr.append(buffer[i])
        }
    }
    

    The loop itself can be sped up by using arr.reserveCapacity(length), however that does not remove the issue of the loop itself.

    I'm aware of this SO question which covers how to convert UnsafePointer<Int8>to String, however String is a different beast entirely to [T]. Is there a convenient Swift way of copying length bytes from an UnsafePointer<T> into a [T]? I'd prefer pure Swift methods, without passing through NSData or similar. If the above algorithm is really the only way to do it, I'm happy to stick with that.

  • Alexander
    Alexander over 7 years
    Does this cause a memory leak? I can't find any indication that UnsafeBufferPointer will free its memory
  • Martin R
    Martin R over 7 years
    @AMomchilov: UnsafeBufferPointer is just a pointer, it does not allocate memory, so there is nothing to free. The Array is then managed by Swift.
  • Alexander
    Alexander over 7 years
    Yes, but the pointer passed into convert has to be freed, right? That should be made explicit, IMO
  • Martin R
    Martin R over 7 years
    @AMomchilov: You are right, if the memory has been allocated then it must be freed eventually. But we don't know where that pointer comes from, it could point to static memory. As I see it, that is independent of the convert function.
  • kabiroberai
    kabiroberai about 7 years
    In Swift 3, the let buffer line should be let buffer = data.withMemoryRebound(to: T.self, capacity: 1) {UnsafeBufferPointer(start: $0, count: length/MemoryLayout<T>.stride)}
  • Martin R
    Martin R about 7 years
    @kabiroberai: Thanks, updated! (The capacity should be the number of target items as well.)
  • kabiroberai
    kabiroberai about 7 years
    @MartinR welcome 😀 may I ask though, why does capacity need to be the number of items? It seems to work even when capacity is set to 1 (since $0 only refers to the start and not the entire data).
  • Martin R
    Martin R about 7 years
    @kabiroberai: Well, that is how I understand the description "Rebinds memory at self to type T with capacity to hold count adjacent T values while executing the body closure. ". I must admit that I don't know what the consequences of a different value would be.
  • dugla
    dugla almost 7 years
    This is insanely helpful. My C <-> Swift interop anxiety is just melting away.
  • tBug
    tBug almost 6 years
    Nice to hear that ^^