# 4. Building C and C++ Extensions

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

To be importable, the shared library must be available on `PYTHONPATH`, and must be named after the module name, with an appropriate extension. When using setuptools, the correct filename is generated automatically.

The initialization function has the signature:

PyObject *PyInit_modulename(void)

It returns either a fully initialized module, or a `PyModuleDef` instance. See Initializing C modules for details.

For modules with ASCII-only names, the function must be named `PyInit_<name>`, with `<name>` replaced by the name of the module. When using Multi-phase initialization, non-ASCII module names are allowed. In this case, the initialization function name is `PyInitU_<name>`, with `<name>` encoded using Python’s punycode encoding with hyphens replaced by underscores. In Python:

    def initfunc_name(name):
        try:
            suffix = b'_' + name.encode('ascii')
        except UnicodeEncodeError:
            suffix = b'U_' + name.encode('punycode').replace(b'-', b'_')
        return b'PyInit' + suffix

It is possible to export multiple modules from a single shared library by defining multiple initialization functions. However, importing them requires using symbolic links or a custom importer, because by default only the function corresponding to the filename is found. See the “Multiple modules in one library” section in PEP 489 for details.

## 4.1. Building C and C++ Extensions with setuptools

Python 3.12 and newer no longer come with distutils. Please refer to the `setuptools` documentation at https://setuptools.readthedocs.io/en/latest/setuptools.html to learn more about how build and distribute C/C++ extensions with setuptools.