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 Brody Eller.

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Update note: Brody Eller updated this tutorial for Swift 5.1. Ray Fix wrote the original.

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

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