Safely manage pointers in Swift

5:44 - Images: undefined behavior can lead to data loss

struct Image {
    // elided...
}

// Undefined behavior can lead to data loss…
struct Collage {
    var imageData: UnsafeMutablePointer<Image>?
    var imageCount: Int = 0
}

// C-style API expects a pointer-to-Int
func addImages(_ countPtr: UnsafeMutablePointer<UInt32>) -> UnsafeMutablePointer<Image> {
    // ...
    let imageData = UnsafeMutablePointer<Image>.allocate(capacity: 1)
    imageData[0] = Image()
    countPtr.pointee += 1
    return imageData
}

func saveImages(_ imageData: UnsafeMutablePointer<Image>, _ count: Int) {
    // Arbitrary function body...
    print(count)
}

var collage = Collage()
collage.imageData = withUnsafeMutablePointer(to: &collage.imageCount) {
    addImages(UnsafeMutableRawPointer($0).assumingMemoryBound(to: UInt32.self))
}
saveImages(collage.imageData!, collage.imageCount) // May see imageCount == 0

10:06 - Direct memory allocation

func directAllocation<T>(t: T, count: Int) {
    let tPtr = UnsafeMutablePointer<T>.allocate(capacity: count)
    tPtr.initialize(repeating: t, count: count)
    tPtr.assign(repeating: t, count: count)
    tPtr.deinitialize(count: count)
    tPtr.deallocate()
}

14:24 - Using a raw pointer to read from Foundation Data

import Foundation

func readUInt32(data: Data) -> UInt32 {
    data.withUnsafeBytes { (buffer: UnsafeRawBufferPointer) in
        buffer.load(fromByteOffset: 4, as: UInt32.self)
    }
}

let data = Data(Array<UInt8>([0, 0, 0, 0, 1, 0, 0, 0]))
print(readUInt32(data: data))

14:37 - Raw allocation

func rawAllocate<T>(t: T, numValues: Int) -> UnsafeMutablePointer<T> {
    let rawPtr = UnsafeMutableRawPointer.allocate(
            byteCount: MemoryLayout<T>.stride * numValues,
            alignment: MemoryLayout<T>.alignment)
    let tPtr = rawPtr.initializeMemory(as: T.self, repeating: t, count: numValues)
    // Must use the typed pointer ‘tPtr’ to deinitialize.
    return tPtr
}

15:43 - Contiguous allocation

func contiguousAllocate<Header>(header: Header, numValues: Int) -> (UnsafeMutablePointer<Header>, UnsafeMutablePointer<Int32>) {
    let offset = MemoryLayout<Header>.stride
    let byteCount = offset + MemoryLayout<Int32>.stride * numValues
    assert(MemoryLayout<Header>.alignment >= MemoryLayout<Int32>.alignment)
    let bufferPtr = UnsafeMutableRawPointer.allocate(
            byteCount: byteCount, alignment: MemoryLayout<Header>.alignment)
    let headerPtr = bufferPtr.initializeMemory(as: Header.self, repeating: header, count: 1)
    let elementPtr = (bufferPtr + offset).initializeMemory(as: Int32.self, repeating: 0, count: numValues)
    return (headerPtr, elementPtr)
}

18:03 - Using assumingMemoryBound(to:) to recover a typed pointer

func takesIntPointer(_: UnsafePointer<Int>) { /* elided */ }

struct RawContainer {
    var rawPtr: UnsafeRawPointer
    var pointsToInt: Bool
}

func testContainer(numValues: Int) {
    let intPtr = UnsafeMutablePointer<Int>.allocate(capacity: numValues)
    let rc = RawContainer(rawPtr: intPtr, pointsToInt: true)
    // ...
    if rc.pointsToInt {
        takesIntPointer(rc.rawPtr.assumingMemoryBound(to: Int.self))
    }
}

18:40 - Calling pthread_create

// Use assumingMemoryBound to recover a pointer type from a (void *) C callback.
/*
func pthread_create(_ thread: UnsafeMutablePointer<pthread_t?>!,
    _ attr: UnsafePointer<pthread_attr_t>?,
    _ start_routine: (UnsafeMutableRawPointer) -> UnsafeMutableRawPointer?,
    _ arg: UnsafeMutableRawPointer?) -> Int32
*/
import Darwin

struct ThreadContext { /* elided */ }

func testPthreadCreate() {
    let contextPtr = UnsafeMutablePointer<ThreadContext>.allocate(capacity: 1)
    contextPtr.initialize(to: ThreadContext())
    var pthread: pthread_t?
    let result = pthread_create(
            &pthread, nil,
            { (ptr: UnsafeMutableRawPointer) in
                let contextPtr = ptr.assumingMemoryBound(to: ThreadContext.self)
                // ... The rest of the thread start routine
                return nil
            },
            contextPtr)
}

19:26 - Pointing to tuple elements

func takesIntPointer(_: UnsafePointer<Int>) { /* elided */ }

func testPointingToTuple() {
    let tuple = (0, 1, 2)
    withUnsafePointer(to: tuple) { (tuplePtr: UnsafePointer<(Int, Int, Int)>) in
        takesIntPointer(UnsafeRawPointer(tuplePtr).assumingMemoryBound(to: Int.self))
    }
}

20:26 - Pointing to struct properties

func takesIntPointer(_: UnsafePointer<Int>) { /* elided */ }

struct MyStruct {
    var status: Bool
    var value: Int
}

func testPointingToStructProperty() {
    let myStruct = MyStruct(status: true, value: 0)
    withUnsafePointer(to: myStruct) { (ptr: UnsafePointer<MyStruct>) in
        let rawValuePtr =
                (UnsafeRawPointer(ptr) + MemoryLayout<MyStruct>.offset(of: \MyStruct.value)!)
        takesIntPointer(rawValuePtr.assumingMemoryBound(to: Int.self))
    }
}

21:17 - bindMemory(to:capacity:) invalidates pointers

func testBindMemory() {
    let uint16Ptr = UnsafeMutablePointer<UInt16>.allocate(capacity: 2)
    uint16Ptr.initialize(repeating: 0, count: 2)
    let int32Ptr = UnsafeMutableRawPointer(uint16Ptr).bindMemory(to: Int32.self, capacity: 1)
    // Accessing uint16Ptr is now undefined
    int32Ptr.deallocate()
}

23:13 - withMemoryRebound(to:capacity:) API

func takesUInt8Pointer(_: UnsafePointer<UInt8>) { /* elided */ }

func testWithMemoryRebound(int8Ptr: UnsafePointer<Int8>, count: Int) {
    int8Ptr.withMemoryRebound(to: UInt8.self, capacity: count) {
        (uint8Ptr: UnsafePointer<UInt8>) in
        // int8Ptr cannot be used within this closure
        takesUInt8Pointer(uint8Ptr)
    }
    // uint8Ptr cannot be used outside this closure
}

25:49 - BufferView: Layering types on top of raw memory

struct UnsafeBufferView<Element>: RandomAccessCollection {
    let rawBytes: UnsafeRawBufferPointer
    let count: Int

    init(reinterpret rawBytes: UnsafeRawBufferPointer, as: Element.Type) {
        self.rawBytes = rawBytes
        self.count = rawBytes.count / MemoryLayout<Element>.stride
        precondition(self.count * MemoryLayout<Element>.stride == rawBytes.count)
        precondition(Int(bitPattern: rawBytes.baseAddress).isMultiple(of: MemoryLayout<Element>.alignment))
    }

    var startIndex: Int { 0 }

    var endIndex: Int { count }

    subscript(index: Int) -> Element {
        rawBytes.load(fromByteOffset: index * MemoryLayout<Element>.stride, as: Element.self)
    }
}

func testBufferView() {
    let array = [0,1,2,3]
    array.withUnsafeBytes {
        let view = UnsafeBufferView(reinterpret: $0, as: UInt.self)
        for val in view {
            print(val)
        }
    }
}