Unsafe reminiscence pointers in Swift

Pointers in Swift

What’s is a pointer? A pointer is a variable that shops the reminiscence handle of a referenced object. As I discussed this in my earlier article, in regards to the reminiscence format of assorted objects in Swift, a reminiscence handle is only a hexadecimal illustration of an information situated someplace within the reminiscence. You employ cases of assorted unsafe pointer sorts to entry information of a selected kind in reminiscence.

Why can we need to use these form of pointers on the first place? By default you do not have to jot down unsafe Swift code, and in many of the instances you’ll be able to dwell with out unsafe reminiscence pointers. These pointers come useful if it’s important to interoperate with different “unsafe” languages, corresponding to C. There are different low stage or legacy APIs that require the usage of handbook reminiscence administration, however you should not be afraid, when you get acquainted with unsafe Swift pointer sorts you will know much more about how reminiscence works and you will see how skinny is the layer between C libraries and Swift. 😱

What sort of pointers are there? As a way to perceive pointer sorts higher in Swift, let’s get again to C only for a second. Take into account the next C code instance:

int principal () {

    int x = 20;
    int* xPointer = &x;

    printf("x handle: `%p`n", &x);
    printf("x worth: `%u`n", x);
    printf("pointer handle: `%p`n", &xPointer);
    printf("pointer reference: `%p`n", xPointer); // equals the handle of x
    printf("pointer reference worth: `%u`n", *xPointer);

    *xPointer = 420;
    printf("x worth: `%u`n", x);
    printf("pointer reference worth: `%u`n", *xPointer);

    x = 69;
    printf("x worth: `%u`n", x);
    printf("pointer reference worth: `%u`n", *xPointer);

    return 0;
}

It can save you this code snippet utilizing the principal.c title, then compile & run it utilizing the clang principal.c -o principal && ./principal command. It’s going to present a fairly similiar output.

$ clang principal.c -o principal && ./principal
x handle: `0x16b0c7a38`
x worth: `20`
pointer handle: `0x16b0c7a30`
pointer reference: `0x16b0c7a38`
pointer reference worth: `20`
pointer worth `20`
[email protected]~: clang principal.c -o principal && ./principal
x handle: `0x16d52fa38`
x worth: `20`
pointer handle: `0x16d52fa30`
pointer reference: `0x16d52fa38`
pointer reference worth: `20`
x worth: `420`
pointer reference worth: `420`
x worth: `69`
pointer reference worth: `69`

So what is going on on right here? Effectively, we merely created an integer variable and a pointer variable with an integer kind. We used the handle of our x variable (&x) to affiliate our pointer with the reminiscence handle of x. Now each variables factors to the identical reminiscence handle.

We are able to affirm this by logging the reminiscence handle of each variables. We are able to additionally alter the worth of x by updating the referenced worth of the pointer (we will use the * character for this) or go along with the same old make x = one thing line. We have merely logged the modified values to verify that the pointer worth replace additionally modified the worth of x. Let’s imagine that xPointer is only a reference to x.

Now how can we obtain the identical factor in Swift? First we now have to learn to outline a pointer kind. This is a fast record of all the unsafe pointer objects out there within the Swift customary library:

You may need observed a sample right here: Unsafe|[Mutable][Raw][Buffer]Pointer[<T>].

Unsafe pointers are simply direct reminiscence addresses. Every thing that’s mutable might be modified, in different phrases you’ll be able to write to that handle. Uncooked signifies that there isn’t a related (generic, T) kind to the given pointer, it is only a blob of uncooked bytes. Buffers are batches (collections) of pointers.

Don’t fret if these sorts are fairly complicated for you proper now, it will all make sense in a couple of minutes. Let’s get again to our unique C pattern code and port it to Swift actual fast.

var x: Int = 20
var xPointer: UnsafeMutablePointer<Int> = .init(&x)

print("x handle:", UnsafeRawPointer(&x));
print("x worth:", x);
print("pointer handle:", UnsafeRawPointer(&xPointer));
print("pointer reference:", xPointer);
print("pointer reference worth:", xPointer.pointee);


xPointer.pointee = 420;
print("x worth:", x);
print("pointer reference worth:", xPointer.pointee);

x = 69;
print("x worth:", x);
print("pointer reference worth:", xPointer.pointee);

We have created an UnsafeMutablePointer<Int> reference to our x worth, that is mainly an int* kind if we return to the C instance. We are able to use the identical ampersand (&) character to create pointers from variables. We have created a typed mutable pointer, since we would like to vary the worth of the referenced integer object (by means of the pointee property) in a while.

To print the reminiscence handle of a variable we will merely use an UnsafeRawPointer kind, as a result of we do not actually care in regards to the underlying “pointee” worth, however we simply want the handle of the reference. If you happen to print a pointer kind the debug description will comprise the underlying reminiscence handle of the referenced object. On this case the handle of x and xPointer. 🤔

Working with typed pointers in Swift

How can we retailer some values at “unsafe” reminiscence addresses in Swift? The most straightforward manner is that we begin with a generic mutable pointer. We are able to allocate pointers utilizing the required capability, since we’re working with unsafe reminiscence, we additionally need to deallocate reminiscence after we have completed utilizing it. We additionally need to manually initialize pointer reference values, unsafe pointers can already comprise some type of leftover information, so the protected strategy is to initialize them with a brand new default worth.

let numbers = [4, 8, 15, 16, 23, 42]

let pointer = UnsafeMutablePointer<Int>.allocate(capability: numbers.depend)
pointer.initialize(repeating: 0, depend: numbers.depend)
defer {
    pointer.deinitialize(depend: numbers.depend)
    pointer.deallocate()
}

for (index, worth) in numbers.enumerated() {
    pointer.superior(by: index).pointee = worth
}

print(pointer.superior(by: 5).pointee); 

let bufferPointer = UnsafeBufferPointer(begin: pointer, depend: numbers.depend) 
for (index, worth) in bufferPointer.enumerated() {
    print(index, "-", worth)
}


let bufferPointer = UnsafeMutableBufferPointer(begin: pointer, depend: numbers.depend)
for (index, _) in bufferPointer.enumerated() {
    bufferPointer[index] = index + 1
}

After we now have the allotted reminiscence storage, we will set the suitable pointee values, since we have allotted the pointer with a capability of six integer values, we will retailer as much as 6 numbers utilizing this pointer. You need to use the superior(by:) technique (pointer arithmetic (pointer + 5).pointee = 42) works as nicely) to maneuver to the subsequent handle and set the pointee worth of it.

The very last item I might wish to let you understand is that you should use a typed buffer pointer to iterate by means of these quantity references. You possibly can consider buffer pointers as an array of pointer references. It’s potential to enumerate by means of pointer values and indexes instantly. You possibly can replace buffer pointer values through the use of the subscript syntax on a mutable buffer pointer. 💡

We already talked in regards to the UnsafePointer, UnsafeMutablePointer, UnsafeRawPointer, UnsafeBufferPointer and UnsafeMutableBufferPointer kind let’s dive in to uncooked pointers.

Reminiscence administration utilizing uncooked pointers

Typed pointers present some form of security if it involves pointers, however how can we work with uncooked pointers? We have already seen how straightforward is to print out an handle of a given worth kind utilizing an UnsafeRawPointer reference, now it is time to join the dots and allocate some unsafe uncooked reminiscence. If you want to know extra about reminiscence format in Swift, please learn my earlier article.

To start with, we’ll must understand how a lot reminiscence to allocate. We are able to use the MemoryLayout struct to get information a couple of worth kind. We are able to use the stride and the variety of objects to depend how a lot byte is required to retailer our information kind utilizing a uncooked reminiscence storage.

let numbers = [4, 8, 15, 16, 23, 42]

let stride = MemoryLayout<Int>.stride
let alignment = MemoryLayout<Int>.alignment
let byteCount = stride * numbers.depend

let pointer = UnsafeMutableRawPointer.allocate(byteCount: byteCount, alignment: alignment)
defer {
    pointer.deallocate()
}
  
for (index, worth) in numbers.enumerated() {
    pointer.superior(by: stride * index).storeBytes(of: worth, as: Int.self)
}
  


let bufferPointer = UnsafeRawBufferPointer(begin: pointer, depend: byteCount)
for index in 0..&lt;numbers.depend {
    let worth = bufferPointer.load(fromByteOffset: stride * index, as: Int.self)
    print(index, "-", worth)
}

After we have allotted the required house, we will use the pointer and the superior(by:) technique to retailer byte values of a given kind (storeBytes(of:as:)) as uncooked bytes. We are able to load a given kind utilizing the load(as:) technique. It’s value to say that if the reminiscence doesn’t comprise a price that may be represented because the given kind, incompatible worth sorts can crash your app. ☠️

Anyway, in case you saved a number of values utilizing a pointer you should use the uncooked buffer assortment to iterate by means of them and cargo again the categories as values from a given byte offset. If you happen to enumerate by means of a uncooked byte buffer you can too print the byte illustration for the pointer.

If you wish to know extra about find out how to Safely handle pointers in Swift, I extremely advocate watching the linked WWDC video. It is a recent one, the pattern code is appropriate with Swift 5. 💪

Reminiscence binding might be harmful

You need to use the bindMemory and the asssumingMemoryBound strategies to transform a uncooked pointer to a typed pointer. The primary will really bind the reminiscence to a given kind, however the second operate simply returns a referenced pointer assuming it is already sure to the desired kind. You possibly can learn extra about the important thing variations right here or test the unique UnsafeRawPointer API proposal.

let stride = MemoryLayout<Int>.stride
let alignment = MemoryLayout<Int>.alignment
let depend = 1
let byteCount = stride * depend

let rawPointer = UnsafeMutableRawPointer.allocate(byteCount: byteCount, alignment: alignment)
defer {
    rawPointer.deallocate()
}
let pointer = rawPointer.bindMemory(to: Int.self, capability: depend)

pointer.initialize(repeating: 0, depend: depend)
defer {
    pointer.deinitialize(depend: depend)
}

pointer.pointee = 42
print(rawPointer.load(as: Int.self))
rawPointer.storeBytes(of: 69, toByteOffset: 0, as: Int.self)
print(pointer.pointee)

Binding reminiscence might be harmful, there are a few guidelines that it is best to comply with:

• By no means return the pointer from a withUnsafeBytes name
• Solely bind to at least one kind at a time
• Watch out with off-by-one errors

This text lists the problems that may occur in case you re-bind a reminiscence handle.

let badPointer = rawPointer.bindMemory(to: Bool.self, capability: depend)
print(badPointer.pointee) 
 
pointer.withMemoryRebound(to: Bool.self, capability: depend) { boolPointer in
    print(boolPointer.pointee) 
}


withUnsafeBytes(of: &pointer.pointee) { pointer -> Void in
    for byte in pointer {
        print(byte)
    }
    
}


let bufferPointer = UnsafeRawBufferPointer(begin: pointer, depend: byteCount + 1)
for byte in bufferPointer {
    print(byte) 
}

I additionally advocate checking this text about reminiscence administration and byte computation in Swift. It’s also potential to repeat or transfer a reminiscence to a given vacation spot utilizing the assign(from:depend:) or moveAssign(from:depend:) strategies. You possibly can learn extra about these features right here.

Opaque and managed Swift pointers

If unsafe pointers weren’t simply sufficient, it is best to know that Swift has a number of different pointer sorts.

As Vadim Bulavin describes this in his article, with the assistance of the Unmanaged kind you’ll be able to bypass Automated Reference Counting (ARC) that’s in any other case enforced to each Swift class. The opposite case is to transform objects between opaque pointers forwards and backwards.

class MyPoint {

    let x: Int
    let y: Int

    init(x: Int, y: Int) {
        self.x = x
        self.y = y
    }

    deinit {
        print("deinit", x, y)
    }
}

let unmanaged = Unmanaged.passRetained(MyPoint(x: 4, y: 20))
unmanaged.launch()

_ = Unmanaged.passUnretained(MyPoint(x: 6, y: 9))

let opaque = Unmanaged.passRetained(MyPoint(x: 1, y: 0)).toOpaque()
Unmanaged<MyPoint>.fromOpaque(opaque).launch()

Opaque pointers are used when it’s important to work with incomplete C information buildings which can’t be represented in Swift. For instance when you have a struct that accommodates a pointer kind, that variable goes to be imported to Swift as an OpaquePointer kind when interacting with C code.

ManagedBufferPointer and the ManagedBuffer kind permits you to implement your personal copy-on-write information construction. This manner you’ll be able to obtain the very same habits because the built-in array, set or dictionary sorts have. Russ Bishop has an incredible publish associated to this subject.

AutoreleasingUnsafeMutablePointer is a pointer that factors to an Goal-C reference that does not personal its goal. you’ll be able to learn extra about it right here by Keith Harrison

The CVaListPointer is an easy wrapper round a C va_list pointer.