Efficient mediators with closures for handling dynamic interfaces in an imperative language

Abstract

Mediators are well-known software components in the construction of distributed systems and applications, with clear advantages when adding functionality to legacy code. However, mediators that must handle dynamic interfaces (i.e. those that may change at run-time such as callback functions) are not easy to build in most imperative languages such as C, due to the many variants implied by the dynamic interfaces. We call this kind of mediators intermediators. We propose a systematic implementation method based on the concept of closures to implement intermediators using the Tempo run-time specializer for C programs. To illustrate this method, we implemented a unified user authentication intermediator for Unix and Windows 2000 called GINA-IM. GINA-IM gets the password entries from a Unix NIS server and performs user authentication based on the Graphical Identification and Authentication (GINA) model of Windows 2000. The code of GINA-IM is a quarter of the code size of a conventional component written without our tools. GINA-IM is in production use at authors' university by two thousands of freshmen in class and several thousands of students daily.