Swift Embedded

Swift for Embedded Systems ✨
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This project aims to bring Swift to the world of embedded systems and IoT. It enables using Swift on microcontrollers with no operating system and with minimal resources available.

What version of Swift does it support?

Swift 5.1... enjoy the latest features of Swift on bare metal! 🚀

Are all features of Swift available?

Yes, except for full unicode support. To save some memory, it includes simplified support only – you can use unicode characters in your strings, but they are ignored by operations like .uppercased() ("žluťoučký".uppercased() returns "žLUťOUčKý" instead of "ŽLUŤOUČKÝ").

Any limitations?

Code size. A "hello world" application has a little bit over one megabyte because it includes a big part of the Swift standard library. However, it is a fixed cost (does not grow proportionally with your program) and you can fit quite a lot on a microcontroller with 2MB of flash!

What boards are supported?

Short answer: NUCLEO-F439ZI, but adding support for any other STM32F4-based board with 2MB of flash memory should require only few lines of code.

The toolchain itself should be able to target any microcontroller using the thumbv7-m or thumbv7-em architecture. However, any practical embedded application is going to require a package providing access to hardware peripherals. I am currently focusing on supporting the STM32F4 family of microcontrollers – the stm32 Swift package provides access to the basic hardware peripherals of those microcontrollers, such as UART, SPI or GPIO.

To make building an embedded application as simple as possible, I have created a small cross command-line utility. It is a wrapper around swift build that handles all the things such as setting up a linker script or using the right compiler flags, making compiling an app a simple one-liner: cross build.

This project is in an early phase, and there is still a lot to work on. Also, if you want to know more about the process of porting Swift to embedded systems, feel free to check out my thesis Swift for Embedded Systems.

Getting Started

Installing the toolchain

  1. Download the latest build of the toolchain from here and put the .xctoolchain file to either /Library/Developer/Toolchains/ or ~/Library/Developer/Toolchains/:

    $ mkdir -p /Library/Developer/Toolchains
    $ sudo xattr -dr com.apple.quarantine <downloaded file>.xctoolchain # required on macOS Catalina only
    $ mv <downloaded file>.xctoolchain ~/Library/Developer/Toolchains

    Why the xattr -dr ... on macOS Catalina? It makes sure you won't get the "XYZ can’t be opened because Apple cannot check it for malicious software" message. The toolchain is properly signed, but it is currently not notarized, as Apple newly requires (starting from Feb 2020) the hardened runtime feature to be enabled and this seems to break the toolchain. I hope to resolve this soon!

  2. Activate the toolchain with:

    $ export TOOLCHAINS=baremetal.YYYYMMDD
  3. Install the cross utility

    $ brew install swift-embedded/formulae/cross
  4. Check that swift and other command-line utilities now reference the newly downloaded toolchain.

    $ xcrun -f swift

    You need to have Xcode installed. Otherwise, xcrun won't find the toolchain.

Running an example

  1. Checkout this repository:

    $ git clone https://github.com/swift-embedded/swift-embedded
  2. Go to a directory with some example app and compile it:

    $ cd swift-embedded/examples/Blinky
    $ cross build
  3. Flash and run the application. One option is using the openocd (brew install openocd):

    In a terminal, run openocd (it connects to your board and starts a gdb server)

    $ openocd -f board/st_nucleo_f4.cfg

    In an another terminal, load your application to the board and start it:

    $ xcrun arm-none-eabi-gdb .build/debug/Blinky -ex 'tar ext :3333' -ex 'load'

    Always make sure you have set the TOOLCHAINS environment variable, so you are using the right toolchain! Or use something like direnv, so you don't have to think about it 😏.

Using the Swift Package Manager and the cross utility

The Swift Package Manager is fully supported and is part of the pre-built baremetal toolchain. Furthermore, it should be possible to use any existing package for your bare-metal application, as long as it does not depend on some unsupported library (e. g. Foundation).

One thing to keep in mind is that running swift build builds your application for the computer at which you are running the command. To cross-compile the application for some bare-metal device, you would have to create a destination.json file specifying all the cross-compilation settings and run swift build --destination destination.json.

Creating the destination.json file is not a trivial task, and always having to add --destination destination.json gets tedious quite quickly. Both those things are solved by the cross utility. It works as follows:

  1. Create a Cross.toml next to your Package.swift file. Its content can be as simple as:

    target = "STM32F439ZI"
  2. Running cross build then a) automatically creates the required destination.json file in the build directory and b) invokes swift build with the proper --destination flag.

What IDEs are supported?

  • Xcode is not (it does not support the baremetal platform introduced by this project and its extensibility is very limited).
  • However, the toolchain contains modified sourcekit-lsp, so you should be able to use any editor with LSP support!
    • Visual Studio Code

      • For autocompletion, install sourcekit-lsp extension and set the toolchain's and sourcekit-lsp's paths in settings:

        "sourcekit-lsp.serverPath": "/Path/to/toolchain/swift-LOCAL-2020-01-04-a.xctoolchain/usr/bin/sourcekit-lsp",
        "sourcekit-lsp.toolchainPath": "/Path/to/toolchain/swift-LOCAL-2020-01-04-a.xctoolchain",
      • To integrate openocd and arm-none-eabi-gdb into vscode, you can use the Cortex-Debug extension. Example launch configuration:

            "cwd": "${workspaceRoot}",
            "executable": "./.build/debug/Blinky",
            "name": "Debug Microcontroller",
            "request": "launch",
            "type": "cortex-debug",
            "servertype": "openocd",
            "configFiles": ["board/st_nucleo_f4.cfg"]

        arm-none-eabi-gdb does not understand Swift, so you will not be able to read Swift variables etc.

    • Vim

      • I use coc.nvim with the following settings:

            "languageserver": {
                "swift": {
                    "command": "xcrun",
                    "args": [
                    "filetypes": [
                    "initializationOptions": {},
                    "settings": {},
                    "rootPatterns": [
                    "requireRootPattern": true

        Make sure to have the TOOLCHAINS environment variable set!


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