Building a ctypes-“based” C library with distutils

The distutils documentation here states that:

A C extension for CPython is a shared library (e.g. a .so file on Linux, .pyd on Windows), which exports an initialization function.

So the only difference regarding a plain shared library seems to be the initialization function (besides a sensible file naming convention I don’t think you have any problem with). Now, if you take a look at distutils.command.build_ext you will see it defines a get_export_symbols() method that:

Return the list of symbols that a shared extension has to export. This either uses ‘ext.export_symbols’ or, if it’s not provided, “PyInit_” + module_name. Only relevant on Windows, where the .pyd file (DLL) must export the module “PyInit_” function.

So using it for plain shared libraries should work out-of-the-box except in
Windows. But it’s easy to also fix that. The return value of get_export_symbols() is passed to distutils.ccompiler.CCompiler.link(), which documentation states:

‘export_symbols’ is a list of symbols that the shared library will export. (This appears to be relevant only on Windows.)

So not adding the initialization function to the export symbols will do the trick. For that you just need to trivially override build_ext.get_export_symbols().

Also, you might want to simplify the module name. Here is a complete example of a build_ext subclass that can build ctypes modules as well as extension modules:

from distutils.core import setup, Extension
from distutils.command.build_ext import build_ext


class build_ext(build_ext):

    def build_extension(self, ext):
        self._ctypes = isinstance(ext, CTypes)
        return super().build_extension(ext)

    def get_export_symbols(self, ext):
        if self._ctypes:
            return ext.export_symbols
        return super().get_export_symbols(ext)

    def get_ext_filename(self, ext_name):
        if self._ctypes:
            return ext_name + '.so'
        return super().get_ext_filename(ext_name)


class CTypes(Extension): pass


setup(name="testct", version='1.0',
      ext_modules=[CTypes('ct', sources=['testct/ct.c']),
                   Extension('ext', sources=['testct/ext.c'])],
      cmdclass={'build_ext': build_ext})