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Merge pull request #424 from YannickJadoul/docs-vc++-runtime

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Add section to the docs' FAQ about VC++ runtimes and VCRUNTIME140_1.dll on VS2019
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Yannick Jadoul
2020-08-29 20:17:12 +02:00
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@@ -61,3 +61,23 @@ python3 -m cibuildwheel --output-dir wheelhouse
python3 -m pip install twine
python3 -m twine upload wheelhouse/*.whl
```
### 'ImportError: DLL load failed: The specific module could not be found' error on Windows
Visual Studio and MSVC link the compiled binary wheels to the Microsoft Visual C++ Runtime. Normally, these are included with Python, but when compiling with a newer version of Visual Studio, it is possible users will run into problems on systems that do not have these runtime libraries installed. The solution is to ask users to download the corresponding Visual C++ Redistributable from the [Microsoft website](https://support.microsoft.com/en-us/help/2977003/the-latest-supported-visual-c-downloads) and install it. Since a Python installation normally includes these VC++ Redistributable files for [the version of the MSVC compiler used to compile Python](https://wiki.python.org/moin/WindowsCompilers), this is typically only a problem when compiling a Python 2.7 C extension with a newer compiler, e.g. to support a modern C++ standard (see [the section on modern C++ standards for Python 2.7](cpp_standards.md#windows-and-python-27) for more details).
Additionally, Visual Studio 2019 started linking to an even newer DLL, `VCRUNTIME140_1.dll`, besides the `VCRUNTIME140.dll` that is included with recent Python versions (starting from Python 3.5; see [here](https://wiki.python.org/moin/WindowsCompilers) for more details on the corresponding Visual Studio & MSVC versions used to compile the different Python versions). To avoid this extra dependency on `VCRUNTIME140_1.dll`, the [`/d2FH4-` flag](https://devblogs.microsoft.com/cppblog/making-cpp-exception-handling-smaller-x64/) can be added to the MSVC invocations (check out [this issue](https://github.com/joerick/cibuildwheel/issues/423) for details and references).
To add the `/d2FH4-` flag to a standard `setup.py` using `setuptools`, the `extra_compile_args` option can be used:
```python
ext_modules=[
Extension(
'c_module',
sources=['extension.c'],
extra_compile_args=['/d2FH4-'] if sys.platform == 'win32' else []
)
],
```
To investigate the dependencies of a C extension (i.e., the `.pyd` file, a DLL in disguise) on Windows, [Dependency Walker](http://www.dependencywalker.com/) is a great tool.