Swift System is Now Open Source

Michael Ilseman is an engineer on the Swift Standard Library team at Apple.

In June, Apple introduced Swift System, a new library for Apple platforms that provides idiomatic interfaces to system calls and low-level currency types. Today, I’m excited to announce that we’re open-sourcing System and adding Linux support! Our vision is for System to eventually act as the single home for low-level system interfaces for all supported Swift platforms.

Goodbye Imported C Interfaces

Most operating systems today support some flavor of system interfaces written in C that have existed for decades. While it is possible to use these APIs directly from Swift, these weakly-typed system interfaces imported from C can be error-prone and unwieldy. For example, the open system call (available on UNIX-like operating systems such as Linux and Apple platforms) imports as a pair of global functions:

func open(_ path: UnsafePointer<CChar>, _ oflag: Int32) -> Int32
func open(_ path: UnsafePointer<CChar>, _ oflag: Int32, _ mode: mode_t) -> Int32

These weakly-typed functions suffer from several shortcomings and fail to utilize the expressivity and type safety of Swift:

None of these semantic rules are captured in the API’s signature, preventing the programming language from guiding the user towards correct usage of the API.

Hello Idiomatic Swift Interfaces

The System module brings various language features to bear to improve expressivity and eliminate these opportunities for error. For example, System defines the open system call as a static function with defaulted arguments in the FileDescriptor namespace:

extension FileDescriptor {
  /// Opens or creates a file for reading or writing.
  /// - Parameters:
  ///  - path: The location of the file to open.
  ///  - mode: The read and write access to use.
  ///  - options: The behavior for opening the file.
  ///  - permissions: The file permissions to use for created files.
  ///  - retryOnInterrupt: Whether to retry the open operation
  ///    if it throws `Errno.interrupted`.
  ///    The default is `true`.
  ///    Pass `false` to try only once and throw an error upon interruption.
  /// - Returns: A file descriptor for the open file
  /// The corresponding C function is `open`.
  public static func open(
    _ path: FilePath,
    _ mode: FileDescriptor.AccessMode,
     options: FileDescriptor.OpenOptions = FileDescriptor.OpenOptions(),
     permissions: FilePermissions? = nil,
     retryOnInterrupt: Bool = true
  ) throws -> FileDescriptor

When one compares this version of open to the original version from C, several significant differences stand out:

The result is code that reads and behaves like idiomatic Swift. For example, this code creates a file path from a string literal and uses it to open and append to a log file:

let message: String = "Hello, world!" + "\n"
let path: FilePath = "/tmp/log"
let fd = try FileDescriptor.open(
  path, .writeOnly, options: [.append, .create], permissions: .ownerReadWrite)
try fd.closeAfter {
  _ = try fd.writeAll(message.utf8)

A Multi-platform Library

System is a multi-platform library, not a cross-platform one. It provides a separate set of APIs and behaviors on every supported platform, closely reflecting the underlying OS interfaces. A single import will pull in the native platform interfaces specific for the targeted OS.

Our immediate goal is to simplify building cross-platform libraries and applications such as SwiftNIO and the Swift Package Manager. System does not eliminate the need for #if os() conditionals to implement cross-platform abstractions, but it does make it safer and more expressive to fill out the platform-specific parts.

What’s Next?

System is only in its infancy—it currently includes a small number of system calls, currency types, and convenience functionality. As part of the effort to increase the API coverage, we’ll be working to adopt System in the Swift Package Manager. This will include enhancements to FilePath and adding support for the recently announced Swift on Windows.

There’s a ton of exciting work left to do. System (especially the forthcoming Windows support!) is a fantastic opportunity to get involved in the Swift project and help it grow into a strong, vibrant, cross-platform ecosystem.

Get Involved

Your experience, feedback, and contributions are greatly encouraged!


Please feel free to ask questions about this post in the associated thread on the Swift forums.