Файл DEF

.def files are associated with Windows and OS/2 development environments, used alongside C, C++, and other compiled languages.

Files with the .def extension store data in plain text format. .def files are sometimes referred to as module definition files and are used for generating DLL libraries. A .def file stores a special set of linker directives - statements such as LIBRARY, EXPORTS, IMPORTS, SECTIONS, HEAPSIZE, and STACKSIZE - used to define key aspects of a DLL library, including which functions and data symbols are exposed to callers and what ordinal numbers they carry.

The file is passed to the linker at build time using the /DEF:name.def switch (MSVC) or equivalent flags in MinGW/GCC and LLVM lld-link. It is a build input, not a compiled artifact; the resulting binary is a .dll or .exe.

A DLL is a library file that stores executable code and other related data. DLL files are reusable - they can be accessed by multiple applications simultaneously - and loaded dynamically when needed. DLL files are also a means of saving disk space and operational memory due to their dynamic nature. Updating software is also easier due to the fact that DLL files are shared among multiple applications.

Modern C/C++ compilers allow developers to mark exported symbols directly in source code using __declspec(dllexport), largely reducing the need for a separate .def file. However, .def files remain supported by the current MSVC linker (link.exe) and LLVM lld-link, and are still preferred when a stable, explicitly ordered export table is required.