Building with MinGW on Windows fails

[ThoreWietzke]

Installing e.g. the pybind11 example https://github.com/pybind/scikit_build_example with only MinGW (GCC) installed on Windows fails with a cryptic error message:

  × Building wheel for scikit_build_example (pyproject.toml) did not run successfully.
  │ exit code: 1
  ╰─> [23 lines of output]
      2024-09-14 09:09:38,528 - scikit_build_core - INFO - RUN: C:\Program Files\CMake\bin\cmake.EXE -E capabilities
      2024-09-14 09:09:38,544 - scikit_build_core - INFO - CMake version: 3.29.2
      *** scikit-build-core 0.10.6 using CMake 3.29.2 (wheel)
      2024-09-14 09:09:38,557 - scikit_build_core - INFO - Build directory: C:\Users\Thore\AppData\Local\Temp\tmpcpenilot\build
      *** Configuring CMake...
      2024-09-14 09:09:38,634 - scikit_build_core - WARNING - Can't find a Python library, got libdir=None, ldlibrary=None, multiarch=None, masd=None
      2024-09-14 09:09:38,643 - scikit_build_core - INFO - RUN: C:\Program Files\CMake\bin\cmake.EXE -S. -BC:\Users\Thore\AppData\Local\Temp\tmpcpenilot\build -CC:\Users\Thore\AppData\Local\Temp\tmpcpenilot\build\CMakeInit.txt -DCMAKE_INSTALL_PREFIX=C:\Users\Thore\AppData\Local\Temp\tmpcpenilot\wheel\platlib
      loading initial cache file C:\Users\Thore\AppData\Local\Temp\tmpcpenilot\build\CMakeInit.txt
      -- Building for: NMake Makefiles
      CMake Error at CMakeLists.txt:7 (project):
        Running
      
         'nmake' '-?'
     
        failed with:
     
         no such file or directory
     
     
      CMake Error: CMAKE_CXX_COMPILER not set, after EnableLanguage
      -- Configuring incomplete, errors occurred!
     
      *** CMake configuration failed
      [end of output]

  note: This error originates from a subprocess, and is likely not a problem with pip.
  ERROR: Failed building wheel for scikit_build_example
Failed to build scikit_build_example
ERROR: ERROR: Failed to build installable wheels for some pyproject.toml based projects (scikit_build_example)

Building with MSVC works fine.
I know that GCC is not recommended for building extensions on Windows. Thus, a descriptive error message should hint the user at this problem.

[henryiii]

Scikit-build-core should try to build with the compiler used to compile Python. So if you used an MSVC-compiled Python, you'll likely see the above. If you used a Python built with MinGW, it should work out of the box.

Though I think the actual problem here is CMake can't find MSVC, so it's falling back on NMake as a default generator. Can you set CMAKE_GENERATOR to Ninja (for example) and try again?

[henryiii]

If you used a Python built with MinGW, it should work out of the box.

We test this, actually:

scikit-build-core/.github/workflows/ci.yml

Line 294 in ca529d6

python:p python-pip:p gcc:p cmake:p

Though MSVC might still be detectable, so maybe we should make sure it's not used.

[ThoreWietzke]

Scikit-build-core should try to build with the compiler used to compile Python. So if you used an MSVC-compiled Python, you'll likely see the above. If you used a Python built with MinGW, it should work out of the box.

Though I think the actual problem here is CMake can't find MSVC, so it's falling back on NMake as a default generator. Can you set CMAKE_GENERATOR to Ninja (for example) and try again?

That's good to know. It can't find MSVC, because it was not installed on that machine.
I've encountered this problem when using a pre-compiled Python for Windows (Which are compiled with MSVC, I presume) with a machine with only GCC available.

Can scikit-build-core detect this? For me it's no problem to install MSVC, but it took 2 hours with a colleague to find the culprit (Missing MSVC on his machine).

[ThoreWietzke]

Actually, sk-build is able to identify such a situation:

2024-09-14 09:09:38,634 - scikit_build_core - WARNING - Can't find a Python library, got libdir=None, ldlibrary=None, multiarch=None, masd=None

Maybe make it more verbose and hint on this error?

[LecrisUT]

Actually, sk-build is able to identify such a situation:

2024-09-14 09:09:38,634 - scikit_build_core - WARNING - Can't find a Python library, got libdir=None, ldlibrary=None, multiarch=None, masd=None

Maybe make it more verbose and hint on this error?

Unfortunately that may not be desired. There are projects made in skbuild-core without python bindings like cmake or ninja. Maybe there could be a find_python like option that would control finding python-devel files and error if it couldn't.

I've encountered this problem when using a pre-compiled Python for Windows (Which are compiled with MSVC, I presume) with a machine with only GCC available.

Can scikit-build-core detect this?

Interesting idea, I think there could be a design for that if skbuild-core has a find_package(scikit-build-core) I'll open an issue for that.

[ThoreWietzke]

The problem I see here is, that the build process hints at the CMakeLists.txt file as the source of error, whereas the different compilers for Python and the bindings are the problem.