Why Is macOS Software Sometimes Labeled “Darwin”?

If you’re a Mac user and a fan of open source software, you’ve probably seen certain apps with the “Darwin” label. But why do the macOS versions of apps carry this name?

Because macOS is, along with iOS and tvOS, powered by a piece of open source, BSD-based software called Darwin. Like many open source operating systems, Darwin even has a mascot: Hexley the Platypus.

Get it? Because the BSD mascot is a devil? Oh, never mind.

This isn’t some gimmick: Apple takes the open source thing seriously. You can download all of the Darwin source code, right now, at opensource.apple.com. You’ll find different downloads for every version of macOS.

It’s because of this legacy that macOS software is sometimes labelled “Darwin,” particularly by open source enthusiasts.

Wait, Open Source? Does That Mean I Can Use macOS For Free?

No.

Well…mostly no. While Darwin itself is open source, most of the things you think of when you picture macOS are not. The Aqua user interface and the Cocoa API are both closed source, for example, and no macOS software can run without those things.

So while you can download Darwin’s source code, free of charge, and you could compile it if you had the right skills, you’d never get macOS software working on it—including, ironically, many of those labeled “darwin” (unless you want to spend a few years and/or decades reverse-engineering the proprietary portions of macOS). Darwin is just the basic foundation upon which the rest of macOS is built.

But that doesn’t mean you can’t get anything to run on Darwin. And there are third party versions of Darwin you can get running relatively easily, notably PureDarwin. This volunteer-built operating system uses Darwin as its core, and you can even get open source user interfaces running on it. Here’s what that looks like:

Not exactly macOS Sierra, is it? You can also get PureDarwin working in a virtual machine, if you’re willing to put in the effort. Just don’t expect to run Mac software on it.

Why Does This Even Exist?

When Apple fired Steve Jobs in 1985, he kept busy. He helped spin Graphics Group off from LucasFilm, for example. That company was re-named Pixar.

Around the same time, Jobs also founded NeXT, which made high-end computers. These devices, aimed primarily at academics, seriously impressed computer scientists with its high specs and its object-oriented, UNIX-inspired operating system: NeXTSTEP. This operating system, while mostly closed source, used some open source code, notably from BSD.

While NeXT devices never sold particularly well, they were were influential: Tim Berners-Lee designed the first web browser in NeXTSTEP, for example.

In 1997, Apple bought NeXT. This brought Steve Jobs back to the company, but also meant Apple owned the NeXTSTEP operating system. Parts of that operating system were re-worked to form the basis for Darwin, and the UNIX legacy meant those parts ended up being open source.

Other NeXTSTEP features, including the Cocoa API, also became part of macOS. Even basic things like the Dock and the .app extension for applications can be traced back to NeXTSTEP, as this video shows.

NeXTSTEP features live on to this day on the Mac, but every Apple device is part of this legacy. Darwin powers the iPhone, the Apple Watch, and the Apple TV. Most Apple users will never know it, and they really don’t need to, but everyone once and a while you’ll run into a package with “Darwin” in the name. Now you know why.

Justin Pot is a staff writer for How-To Geek, and a technology enthusiast who lives in Hillsboro, Oregon. Follow him on Twitter and Facebook, if you want. You don't have to.