How do I build a C Python Extension for Windows using Cygwin64?

Question

To preface: my code works as I expect when compiling and running on Linux. However, this library needs to be compiled for use on a Windows machine. I looked in to a couple different options, and decided that using Cygwin to compile for Windows seemed to be the correct choice. I'm using a setup.py file with the distutils.core library and compiling using python setup.py install. When compiling on Windows in Cygwin, it fails to find pthread.h, arpa/inet.h, netinet/in.h, and sys/socket.h. I was under the impression that Cygwin came prepackaged with these headers, which is why I chose to use it. The alternative to Cygwin is putting preprocessor commands everywhere and using Windows specific libraries such as winsock2.h, which I want to avoid if at all possible. Is it possible to compile for Windows using Cygwin? If so, what have I done wrong to cause Cygwin to not recognize these headers?

Doug Henderson · Answer

Most important, I think, is to follow the instructions in the Python help or on the Python doc web site for "Extending and Embedding the Python Interpreter" for the version you are building the extension for. For windows, the build instructions identify the build environment used to create the binary package that you download from python.org, usually something like VS2013 or VS2017. (As an aside, I think the Community editions have everything you need, and I don't think you actually have to use the Visual Studio GUI when you build using nmake from the CMD.EXE terminal.)
To build in Cygwin for use in a Windows version of Python, you may need to install and then use the x86_64-w64-mingw32-gcc, etc., cygwin packages to cross-compile non-cygwin (i.e. pure windows) executables and DLLs from Cygwin.
Binary extensions must be built using the source tree for a specific Python major.minor version, and bitness. For windows, you will need to build multiple versions of the extension, one for each major.minor, bitness version of Python that will import it, e.g. 3.6, 3.7, 3.8, 3.9, 32-bit, 64-bit. The extension code may not require changes between versions, but it still needs to be compiled with the right compiler and linked against exactly the same shared libraries (in this case .DLL files) as used by the Python executable. For instance, it must use exactly the same version of Microsoft's C run time library DLL as the Python executable does. This is a bit more sensitive and restrictive than on Linux, where you can rebuild the python executable and your extension with the same toolchain from your distro more easily.

matzeri · Answer

You need to install the proper headers
$ cygcheck -p usr/include/pthread.h
Found 9 matches for usr/include/pthread.h
cygwin-devel-3.0.7-1 - cygwin-devel: Core development files
..
cygwin-devel-3.1.6-1 - cygwin-devel: Core development files
...

so install the cygwin-devel package
To check all the shared libraries needed by the built dll, you can use cygcheck
$ cygcheck /usr/lib/python3.8/site-packages/Cython/Compiler/FlowControl.cpython-38-x86_64-cygwin.dll
D:cygwin64libpython3.8site-packagesCythonCompilerFlowControl.cpython-38-x86_64-cygwin.
dll
  D:cygwin64bincygwin1.dll
    C:WINDOWSsystem32KERNEL32.dll
      C:WINDOWSsystem32ntdll.dll
      C:WINDOWSsystem32KERNELBASE.dll
  D:cygwin64binlibpython3.8.dll
    D:cygwin64bincygintl-8.dll
      D:cygwin64bincygiconv-2.dll
    D:cygwin64bincyggcc_s-seh-1.dll

As was built with Cygwin Python, you need also to transfer the cygwin python...

How do I build a C Python Extension for Windows using Cygwin64?

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