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Unsafe Swift: Using Pointers and Interacting With C

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Unsafe Swift: Using Pointers and Interacting With C
In this tutorial, you’ll learn how to use unsafe Swift to directly access memory through a variety of pointer types.

By default, Swift is memory safe: It prevents direct access to memory and makes sure you’ve initialized everything before you use it. The key phrase is “by default.” You can also use unsafe Swift, which lets you access memory directly through pointers.

This tutorial will take you on a whirlwind tour of the so-called unsafe features of Swift.

Unsafe doesn’t mean dangerously bad code that might not work. Instead, it refers to code that needs extra care because it limits how the compiler can protect you from making mistakes.

These features are useful if you interoperate with an unsafe language such as C, need to gain additional runtime performance or simply want to explore the internals of Swift. In this tutorial, you’ll learn how to use pointers and interact with the memory system directly.

Note

While this is an advanced topic, you’ll be able to follow along if you have reasonable competency in Swift. If you need to brush up on your skills, please check out our iOS and Swift for Beginners series. C experience is beneficial but not necessary.

Getting Started

Download the begin project by clicking the [Download Materials] button at the top or bottom of the tutorial.

This tutorial consists of three empty Swift playgrounds:

Exploring Memory Layout With Unsafe Swift

Start by opening the UnsafeSwift playground. Since all the code in this tutorial is platform-agnostic, you may select any platform.

Sample memory addresses
Sample memory addresses

Unsafe Swift works directly with the memory system. You can visualize memory as a series of boxes — billions of boxes, actually — each containing a number.

Each box has a unique memory address. The smallest addressable unit of storage is a byte, which usually consists of eight bits.

Eight-bit bytes can store values from 0-255. Processors can also efficiently access words of memory, which are typically more than one byte.

On a 64-bit system, for example, a word is 8 bytes or 64 bits. To see this in action, you’ll use MemoryLayout to tell you the size and alignment of components of some native Swift types.

Add the following to your playground:

  • In the first playground, you’ll use several short snippets of code to explore memory layout. You’ll also give unsafe pointers a try.
  • In the second, you’ll take a low-level C API that performs streaming data compression and wrap it with a Swifty interface.
  • In the final playground, you’ll create a platform-independent alternative to arc4random to generate random numbers. It uses unsafe Swift, but hides that detail from users.
import Foundation

MemoryLayout<Int>.size          // returns 8 (on 64-bit)
MemoryLayout<Int>.alignment     // returns 8 (on 64-bit)
MemoryLayout<Int>.stride        // returns 8 (on 64-bit)

MemoryLayout<Int16>.size        // returns 2
MemoryLayout<Int16>.alignment   // returns 2
MemoryLayout<Int16>.stride      // returns 2

MemoryLayout<Bool>.size         // returns 1
MemoryLayout<Bool>.alignment    // returns 1
MemoryLayout<Bool>.stride       // returns 1

MemoryLayout<Float>.size        // returns 4
MemoryLayout<Float>.alignment   // returns 4
MemoryLayout<Float>.stride      // returns 4

MemoryLayout<Double>.size       // returns 8
MemoryLayout<Double>.alignment  // returns 8
MemoryLayout<Double>.stride     // returns 8

MemoryLayout<Type> is a generic type evaluated at compile time. It determines the size, alignment and stride of each specified Type and returns a number in bytes.

For example, an Int16 is two bytes in size and has an alignment of two as well. That means it has to start on even addresses — that is, addresses divisible by two.

For example, it’s legal to allocate an Int16 at address 100, but not at 101 — an odd number violates the required alignment.

When you pack a bunch of Int16s together, they pack at an interval of stride. For these basic types, the size is the same as the stride.

Examining Struct Layouts

Next, look at the layout of some user-defined structs by adding the following to the playground:

struct EmptyStruct {}

MemoryLayout<EmptyStruct>.size      // returns 0
MemoryLayout<EmptyStruct>.alignment // returns 1
MemoryLayout<EmptyStruct>.stride    // returns 1

struct SampleStruct {
  let number: UInt32
  let flag: Bool
}

MemoryLayout<SampleStruct>.size       // returns 5
MemoryLayout<SampleStruct>.alignment  // returns 4
MemoryLayout<SampleStruct>.stride     // returns 8

The empty structure has a size of zero. It can exist at any address since alignment is one and all numbers are evenly divisible by one.

The stride, curiously, is one. That’s because each EmptyStruct you create has to have a unique memory address, even though its size is zero.

For SampleStruct, the size is five but the stride is eight. That’s because its alignment requires it to be on 4-byte boundaries. Given that, the best Swift can do is pack at an interval of eight bytes.

To see how the layout differs for class versus struct, add the following:

class EmptyClass {}

MemoryLayout<EmptyClass>.size      // returns 8 (on 64-bit)
MemoryLayout<EmptyClass>.stride    // returns 8 (on 64-bit)
MemoryLayout<EmptyClass>.alignment // returns 8 (on 64-bit)

class SampleClass {
  let number: Int64 = 0
  let flag = false
}

MemoryLayout<SampleClass>.size      // returns 8 (on 64-bit)
MemoryLayout<SampleClass>.stride    // returns 8 (on 64-bit)
MemoryLayout<SampleClass>.alignment // returns 8 (on 64-bit)

Classes are reference types, so MemoryLayout reports the size of a reference: Eight bytes.

If you want to explore memory layout in greater detail, check out Mike Ash’s excellent talk, Exploring Swift Memory Layout.

Using Pointers in Unsafe Swift

A pointer encapsulates a memory address.

Types that involve direct memory access get an unsafe prefix, so the pointer type name is UnsafePointer.

The extra typing may seem annoying, but it reminds you that you’re accessing memory that the compiler isn’t checking. When done incorrectly, this could lead to undefined behavior, not just a predictable crash.

Swift doesn’t offer just a single UnsafePointer type that accesses memory in an unstructured way, like C’s char * does. Swift contains almost a dozen pointer types, each with different capabilities and purposes.

You always want to use the most appropriate pointer type for your purpose. This communicates intent better, is less error-prone and avoids undefined behavior.

Unsafe Swift pointers use a predictable naming scheme that describes the pointers’ traits: mutable or immutable, raw or typed, buffer style or not. In total, there are eight pointer combinations. You’ll learn more about them in the following sections.

Guide to unsafe swift pointers
Guide to unsafe swift pointers

Using Raw Pointers

In this section, you’ll use unsafe Swift pointers to store and load two integers. Add the following code to your playground:

// 1
let count = 2
let stride = MemoryLayout<Int>.stride
let alignment = MemoryLayout<Int>.alignment
let byteCount = stride * count

// 2
do {
  print("Raw pointers")
  
  // 3
  let pointer = UnsafeMutableRawPointer.allocate(
    byteCount: byteCount,
    alignment: alignment)
  // 4
  defer {
    pointer.deallocate()
  }
  
  // 5
  pointer.storeBytes(of: 42, as: Int.self)
  pointer.advanced(by: stride).storeBytes(of: 6, as: Int.self)
  pointer.load(as: Int.self)
  pointer.advanced(by: stride).load(as: Int.self)
  
  // 6
  let bufferPointer = UnsafeRawBufferPointer(start: pointer, count: byteCount)
  for (index, byte) in bufferPointer.enumerated() {
    print("byte \(index): \(byte)")
  }
}

Here’s what’s going on:

  • Count holds the number of integers to store.
  • Stride holds the stride of type Int.
  • Alignment holds the alignment of type Int.
  • ByteCount holds the total number of bytes needed.
  1. These constants hold frequently used values:
  2. A do block adds a scope level, so you can reuse the variable names in upcoming examples.
  3. UnsafeMutableRawPointer.allocate allocates the required bytes. This method returns an UnsafeMutableRawPointer. The name of that type tells you the pointer can load and store, or mutate, raw bytes.
  4. A defer block makes sure you deallocate the pointer properly. ARC isn’t going to help you here — you need to handle memory management yourself! You can read more about defer statements in the official Swift documentation.
  5. storeBytes and load, unsurprisingly, store and load bytes. You calculate the memory address of the second integer by advancing the pointer stride bytes. Since pointers are Strideable, you can also use pointer arithmetic like: (pointer+stride).storeBytes(of: 6, as: Int.self).
  6. An UnsafeRawBufferPointer lets you access memory as if it were a collection of bytes. This means you can iterate over the bytes and access them using subscripting. You can also use cool methods like filter, map and reduce. You initialize the buffer pointer using the raw pointer.

Even though UnsafeRawBufferPointer is unsafe, you can still make it safer by constraining it to specific types.

Using Typed Pointers

You can simplify the previous example by using typed pointers. Add the following code to your playground:

do {
  print("Typed pointers")
  
  let pointer = UnsafeMutablePointer<Int>.allocate(capacity: count)
  pointer.initialize(repeating: 0, count: count)
  defer {
    pointer.deinitialize(count: count)
    pointer.deallocate()
  }
  
  pointer.pointee = 42
  pointer.advanced(by: 1).pointee = 6
  pointer.pointee
  pointer.advanced(by: 1).pointee
  
  let bufferPointer = UnsafeBufferPointer(start: pointer, count: count)
  for (index, value) in bufferPointer.enumerated() {
    print("value \(index): \(value)")
  }
}

Notice the following differences:

  • You allocate memory using UnsafeMutablePointer.allocate. The generic parameter lets Swift know you’re using the pointer to load and store values of type Int.
  • You must initialize typed memory before use and deinitialize it after use. You do this using the initialize and deinitialize methods, respectively. Deinitialization is only required for non-trivial types. However, including deinitialization is a good way to future-proof your code in case you change to something non-trivial. It usually doesn’t cost anything since the compiler will optimize it out.
  • Typed pointers have a pointee property that provides a type-safe way to load and store values.
  • When advancing a typed pointer, you can simply state the number of values you want to advance. The pointer can calculate the correct stride based on the type of values it points to. Again, pointer arithmetic also works. You can also say (pointer+1).pointee = 6
  • The same holds true for typed buffer pointers: They iterate over values instead of bytes.

Next, you’ll learn how to go from unconstrained UnsafeRawBufferPointer to safer, type constrained UnsafeRawBufferPointer.

Converting Raw Pointers to Typed Pointers

You don’t always need to initialize typed pointers directly. You can derive them from raw pointers as well.

Add the following code to your playground:

do {
  print("Converting raw pointers to typed pointers")
  
  let rawPointer = UnsafeMutableRawPointer.allocate(
    byteCount: byteCount,
    alignment: alignment)
  defer {
    rawPointer.deallocate()
  }
  
  let typedPointer = rawPointer.bindMemory(to: Int.self, capacity: count)
  typedPointer.initialize(repeating: 0, count: count)
  defer {
    typedPointer.deinitialize(count: count)
  }

  typedPointer.pointee = 42
  typedPointer.advanced(by: 1).pointee = 6
  typedPointer.pointee
  typedPointer.advanced(by: 1).pointee
  
  let bufferPointer = UnsafeBufferPointer(start: typedPointer, count: count)
  for (index, value) in bufferPointer.enumerated() {
    print("value \(index): \(value)")
  }
}

This example is similar to the previous one, except that it first creates a raw pointer. You create the typed pointer by binding the memory to the required type Int.

By binding memory, you can access it in a type-safe way. Memory binding goes on behind the scenes when you create a typed pointer.

The rest of this example is also the same as the previous one. Once you’re in typed pointer land, you can make use of pointee, for example.

Getting the Bytes of an Instance

Often, you have an existing instance of a type and you want to inspect the bytes that form it. You can achieve this using a method called withUnsafeBytes(of:).

To do so, add the following code to your playground:

do {
  print("Getting the bytes of an instance")
  
  var sampleStruct = SampleStruct(number: 25, flag: true)

  withUnsafeBytes(of: &sampleStruct) { bytes in
    for byte in bytes {
      print(byte)
    }
  }
}

This prints out the raw bytes of the SampleStruct instance.

withUnsafeBytes(of:) gives you access to an UnsafeRawBufferPointer that you can use inside the closure.

withUnsafeBytes is also available as an instance method on Array and Data.

Computing a Checksum

Using withUnsafeBytes(of:), you can return a result. For example, you might use this to compute a 32-bit checksum of the bytes in a structure.

Add the following code to your playground:

do {
  print("Checksum the bytes of a struct")
  
  var sampleStruct = SampleStruct(number: 25, flag: true)
  
  let checksum = withUnsafeBytes(of: &sampleStruct) { (bytes) -> UInt32 in
    return ~bytes.reduce(UInt32(0)) { $0 + numericCast($1) }
  }
  
  print("checksum", checksum) // prints checksum 4294967269
}

The reduce call adds the bytes, then ~ flips the bits. While not the most robust error detection, it shows the concept.

Now that you know how to use unsafe Swift, it’s time to learn some things you should absolutely not do with it.

Three Rules of the Unsafe Club

Don’t Return the Pointer From withUnsafeBytes!

Only Bind to One Type at a Time!

Don’t Walk Off the End… Whoops!

Unsafe Swift Example 1: Compression

Unsafe Swift Example 2: Random Generator

Where to Go From Here?

Congratulations on finishing this tutorial! You can download the completed project files at the top or bottom of this tutorial using the [Download Materials].

There are many additional resources you can explore to learn more about using unsafe Swift:

I hope you’ve enjoyed this tutorial. If you have questions or experiences you would like to share, feel free to share them in the forums!