May 4, 2021

OpenSSL on Apple Watch

I needed OpenSSL library for the project I've been meaning to start for Apple Watch. So I've checked in all the usual places, and found out that one wasn't made available yet, so I thought I'd take that on myself.

That meant compiling it for watchOS platform, and four different architectures: armv7k and arm64_32 for device, and x86_64 and arm64 for the Simulator. That last one is a new addition in Xcode 12 to support new M1 Macs. I don't have one yet, but it would help to prepare for the future.

And that presents first problem. Earlier, before M1 arrived, it was possible to combine a library built for, say, iOS platform on arm64 architecture with library built for iOS Simulator on x86_64 into single Mach-O universal binary (or "fat library"), and Xcode was able to figure out which one goes where, even though universal binary does not identify platforms for each included architecture. Now with both M1 and iOS devices running on arm64 architecture, we have two platforms with the same architecture, and it is not possible to create such universal binary at all.

So the solution is to create separate "fat libraries" for each individual platform. That means separate library for:

  • iOS,
  • iOS Simulator,
  • tvOS,
  • tvOS Simulator,
  • watchOS,
  • watchOS Simulator,
  • ... and Catalyst, of course.

This presents second problem. Previously, you'd just drop single libcrypto.a (and libssl.a, if needed) into your project, and call it a day, but now you have to figure out how to make Xcode use one file, libcrypto-iOS.a, on device, and another, libcrypto-iOS-Sim.a, on Simulator.

Solution to the second problem is new XCFramework framework format. It combines libraries for all of these platforms (and corresponding header files) into single directory, and then Xcode magically picks just the right library automatically. 🎩 It can be easily created using xcodebuild by passing it a few parameters.

Resulting script is available on GitHub, and there is also a pull request against parent repository. And below is requisite screen shot of OpenSSL library running on watchOS as a proof that it works:


Posted by Vadim at 6:59 PM | Comments (0) | TrackBack