helium.def

Module definition file Reference

A .def file is a module definition file for the Windows linker. The plain-text file names the exported functions and other attributes of a DLL. The build reads the file, thus the source code needs no export declarations. The format comes from early Windows development. Projects with native code, including Python extension modules, still use it.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.