Incorrect value of CMAKE_SYSTEM_PROCESSOR when using cibuildwheel and scikit-build-core on GitHub macos-14 runner

If you’re reading this, chances are you’ve stumbled upon the frustrating error of incorrect value of CMAKE_SYSTEM_PROCESSOR when trying to build your Python package using cibuildwheel and scikit-build-core on a GitHub macos-14 runner. Don’t worry, you’re not alone! This error has puzzled many a developer, but fear not, for we’re about to embark on a journey to conquer this beast and get your package building like a charm.

What is CMAKE_SYSTEM_PROCESSOR and why does it matter?

Before we dive into the solution, let’s take a step back and understand what CMAKE_SYSTEM_PROCESSOR is and why it’s crucial for our build process. CMAKE_SYSTEM_PROCESSOR is a CMake variable that determines the processor architecture of the system. It’s used to configure the build process, ensuring that the generated binaries are compatible with the target platform.

In the context of GitHub Actions and cibuildwheel, CMAKE_SYSTEM_PROCESSOR is especially important. When you run your build workflow on a macos-14 runner, it’s essential to ensure that the processor architecture is correctly identified. If not, you’ll encounter errors, and your build will fail.

The incorrect value of CMAKE_SYSTEM_PROCESSOR error

So, what happens when CMAKE_SYSTEM_PROCESSOR is set to an incorrect value? Well, it’s quite straightforward: your build process will fail. You’ll see an error message like this:

CMake Error at /Users/runner/hostedtoolcache/Python/3.9.7/x64/lib/python3.9/site-packages/cmake/data/share/cmake-3.22/Modules/CMakeSystemSpecificInformation.cmake:6 (include):
  included from /Users/runner/hostedtoolcache/Python/3.9.7/x64/lib/python3.9/site-packages/cmake/data/share/cmake-3.22/Modules/CMakeGenericSystem.cmake:7
  included from CMakeLists.txt:2 (include)

  /Users/runner/hostedtoolcache/Python/3.9.7/x64/lib/python3.9/site-packages/cmake/data/share/cmake-3.22/Modules/Platform/Darwin.cmake:196 (CMAKE_SYSTEM_PROCESSOR):
  Incorrect value of CMAKE_SYSTEM_PROCESSOR: x86_64

This error message indicates that the value of CMAKE_SYSTEM_PROCESSOR is set to x86_64, which is incorrect for a macos-14 runner. The correct value should be arm64.

The hacky solution: override CMAKE_SYSTEM_PROCESSOR

One quick fix is to override the CMAKE_SYSTEM_PROCESSOR variable in your CMakeLists.txt file. You can do this by adding the following lines of code:

if(CMAKE_SYSTEM_PROCESSOR STREQUAL "x86_64")
  set(CMAKE_SYSTEM_PROCESSOR "arm64" CACHE STRING "Override CMAKE_SYSTEM_PROCESSOR")
endif()

This code snippet checks if the CMAKE_SYSTEM_PROCESSOR is set to x86_64 and, if so, overrides it with the correct value of arm64. This solution works, but it’s not the most elegant or maintainable approach.

The better solution: configure cibuildwheel and scikit-build-core correctly

Rather than hacking CMAKE_SYSTEM_PROCESSOR, let’s take a step back and configure cibuildwheel and scikit-build-core correctly. The key to success lies in specifying the correct platform and architecture in your ci script.

Here’s an updated example of a ci script that sets the platform and architecture correctly:

steps:
  - uses: actions/checkout@v2
  - uses: cibuildwheel/manylinux@v2
    with:
      platform: macOS
      architecture: arm64
  - uses: scikit-build-core/setup-python@v1
    with:
      python-version: 3.9.7
  - run: |
    pip install .
    cibuildwheel --platform=macOS --architecture=arm64 .

In this script, we’ve specified the platform as macOS and architecture as arm64, ensuring that cibuildwheel and scikit-build-core use the correct settings for the GitHub macos-14 runner.

Additional tips and tricks

Here are some additional tips to keep in mind when building your Python package using cibuildwheel and scikit-build-core:

• Make sure you’re using the latest versions of cibuildwheel and scikit-build-core.
• Use the correct Python version for your build process. In this example, we’re using Python 3.9.7.
• Specify the correct platform and architecture for your target system.
• Avoid using hardcoded values for CMAKE_SYSTEM_PROCESSOR or other variables. Instead, let cibuildwheel and scikit-build-core handle the configuration for you.

Conclusion

There you have it – a comprehensive guide to resolving the incorrect value of CMAKE_SYSTEM_PROCESSOR error when using cibuildwheel and scikit-build-core on a GitHub macos-14 runner. By following these simple steps and configuring your ci script correctly, you’ll be able to build your Python package with ease.

Remember, the key to success lies in specifying the correct platform and architecture for your target system. Don’t be afraid to experiment and try different approaches until you find the one that works best for your project.

Platform	Architecture
macOS	arm64
Linux	x86_64
Windows	x86_64

Happy building, and may the odds be ever in your favor!

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