Unsafe Swift: Using Pointers and Interacting With C

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

Be careful to avoid undefined behavior when writing unsafe code. Here are a few examples of bad code: