Understanding Program Dependencies

This paper reports on an experiment in trying to understand anunfamiliar program of some complexity and to record the authors’ under-standing of it. The goal was to simulate a practicing programmer in aprogram maintenance environment using the techniques of program designadapted to program understanding and documentation; that is, given aprogram, a specification and correctness proof were developed for the program.The approach points out the value of correctness proof ideas in guiding thediscovery process. Toward this end, a variety of techniques were used: directcognition for smaller parts, discovering and verifying loop invariants forlarger program parts, and functions determined by additional analysis forlarger program parts. An indeterminate bounded variable was introducedinto the program documentation to summarize the effect of several programvariables and simplify the proof of correctness. Basili90Basili, Victor. “Viewing Maintenance as Reuse-Oriented SoftwareDevelopment.” IEEE Software 7, 1 (Jan. 1990), 19-25.