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 structure of varied objects in Swift, a reminiscence handle is only a hexadecimal illustration of an information situated someplace within the reminiscence. You employ cases of varied unsafe pointer sorts to entry information of a selected kind in reminiscence.

Why will we need to use these type of pointers on the first place? By default you do not have to put in writing unsafe Swift code, and in a lot of the circumstances you may dwell with out unsafe reminiscence pointers. These pointers come helpful if you must 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 may 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. Think about the next C code instance:

#embrace <stdio.h>

int most important () {

    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 most important.c title, then compile & run it utilizing the clang most important.c -o most important && ./most important command. It can present a fairly comparable output.

$ clang most important.c -o most important && ./most important
x handle: `0x16b0c7a38`
x worth: `20`
pointer handle: `0x16b0c7a30`
pointer reference: `0x16b0c7a38`
pointer reference worth: `20`
pointer worth `20`
tib@~: clang most important.c -o most important && ./most important
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? Nicely, 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 will verify this by logging the reminiscence handle of each variables. We will additionally alter the worth of x by updating the referenced worth of the pointer (we are able to use the * character for this) or go together with the standard 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. Lets say that xPointer is only a reference to x.

Now how will we obtain the identical factor in Swift? First we’ve to discover ways to outline a pointer kind. Here is a fast record of the entire unsafe pointer objects obtainable within the Swift commonplace library:

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

Unsafe pointers are simply direct reminiscence addresses. All the pieces that’s mutable might be modified, in different phrases you may write to that handle. Uncooked implies 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 be concerned if these sorts are fairly complicated for you proper now, it’s going to 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 will use the identical ampersand (&) character to create pointers from variables. We have created a typed mutable pointer, since we might like to alter the worth of the referenced integer object (by the pointee property) in a while.

To print the reminiscence handle of a variable we are able to 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 include 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 will allocate pointers utilizing the required capability, since we’re working with unsafe reminiscence, we additionally should deallocate reminiscence after we have completed utilizing it. We additionally should manually initialize pointer reference values, unsafe pointers can already include some kind of leftover information, so the secure 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’ve the allotted reminiscence storage, we are able to set the suitable pointee values, since we have allotted the pointer with a capability of six integer values, we are able to retailer as much as 6 numbers utilizing this pointer. You need to use the superior(by:) methodology (pointer arithmetic (pointer + 5).pointee = 42) works as nicely) to maneuver to the subsequent handle and set the pointee worth of it.

The very final thing I might prefer to let you recognize is that you should use a typed buffer pointer to iterate by these quantity references. You may consider buffer pointers as an array of pointer references. It’s attainable to enumerate by pointer values and indexes straight. You may replace buffer pointer values by utilizing 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 type of security if it involves pointers, however how will 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 should know extra about reminiscence structure in Swift, please learn my earlier article.

To start with, we’ll have to know the way a lot reminiscence to allocate. We will use the MemoryLayout struct to get information a couple of worth kind. We will use the stride and the variety of gadgets 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 are able to use the pointer and the superior(by:) methodology to retailer byte values of a given kind (storeBytes(of:as:)) as uncooked bytes. We will load a given kind utilizing the load(as:) methodology. It’s value to say that if the reminiscence doesn’t include a price that may be represented because the given kind, incompatible worth sorts can crash your app. ☠️

Anyway, for those who saved a number of values utilizing a pointer you should use the uncooked buffer assortment to iterate by them and cargo again the kinds as values from a given byte offset. If you happen to enumerate by a uncooked byte buffer you may as well print the byte illustration for the pointer.

If you wish to know extra about methods 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 truly bind the reminiscence to a given kind, however the second operate simply returns a referenced pointer assuming it is already sure to the required kind. You may 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 1 kind at a time
• Watch out with off-by-one errors

This text lists the problems that may occur for those who 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 attainable to repeat or transfer a reminiscence to a given vacation spot utilizing the assign(from:depend:) or moveAssign(from:depend:) strategies. You may learn extra about these capabilities right here.

Opaque and managed Swift pointers

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

As Vadim Bulavin describes this in his article, with the assistance of the Unmanaged kind you may 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 you must work with incomplete C information constructions which can’t be represented in Swift. For instance when you have a struct that comprises 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 may obtain the very same conduct 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 may learn extra about it right here by Keith Harrison

The CVaListPointer is a straightforward wrapper round a C va_list pointer.