What is Kotlin/Native?

Kotlin/Native is a technology capable of converting your Kotlin code into standalone binaries that can run on Android, iOS, macOS, Windows, Linux, embedded systems, and more.

It uses LLVM to achieve this. LLVM stands for Low Level Virtual Machine. The name is unfortunate, as it has little to do with virtual machines. It’s actually a collection of modular and reusable compiler and toolchain technologies. Through a series of steps, the Kotlin/Native compiler transforms Kotlin code into LLVM intermediate representation, or IR for short.

IR is a strongly typed RISC instruction set which abstracts away the underlying platform. Here’s a simple Hello, World in LLVM IR:

@.str = private unnamed_addr constant [15 x i8] c"Hello, World!\0A\00", align 1

; Function Attrs: noinline nounwind optnone ssp uwtable
define i32 @main() #0 {
  %1 = alloca i32, align 4
  store i32 0, i32* %1, align 4
  %2 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([15 x i8], [15 x i8]* @.str, i32 0, i32 0))
  ret i32 0

declare i32 @printf(i8*, ...) #1

The LLVM compilers, which understand IR, then create binaries for the desired platforms. Perhaps the name makes a bit more sense now - while LLVM needs no virtual machine, the compilation process through IR resembles one.

Kotlin/Native has no problem interacting with C, Objective C, and Swift. In fact, it provides a rich set of libraries with platform-specific APIs such as POSIX, OpenGL, Core Foundation, and more. To get a device name using Apple’s UIKit framework, for example, this is all it takes:

import platform.UIKit.UIDevice

fun deviceName(): String = UIDevice.currentDevice.name

There are currently no plans to support C++.

Kotlin/Native also comes with a standard library which is almost the same as the one on the JVM. The platform-specific constructs and objects, e.g. File, are not included, but you can use POSIX, Core Foundation, or other libraries provided out-of-the-box, to overcome this.

Finally, Kotlin/Native features fully automatic memory management, which means you don’t have to worry too much about memory. However, when allocating objects via platform-specific means, for example a C library, you need to clean up manually. The standard library offers nice helper functions to keep this as painless as possible.

Some examples of Kotlin/Native in the wild are Kotlin Coroutines, the JetBrains KotlinConf app, and more.